Thermal Spray Coating Processes

4.10

Thermal Spray Coating Processes

P Vuoristo, Tampere University of Technology, Tampere, Finland Ó 2014 Elsevier Ltd. All rights reserved.

4.10.1 4.10.2 4.10.2.1 4.10.2.2 4.10.2.3 4.10.2.4 4.10.3 4.10.3.1 4.10.3.2 4.10.3.2.1 4.10.3.2.2 4.10.3.2.3 4.10.3.3 4.10.3.4 4.10.3.4.1 4.10.3.4.2 4.10.3.4.3 4.10.3.5 4.10.3.5.1 4.10.3.5.2 4.10.3.5.3 4.10.3.5.4 4.10.3.5.5 4.10.3.6 4.10.3.6.1 4.10.3.6.2 4.10.3.7 4.10.4 4.10.4.1 4.10.4.2 4.10.4.3 4.10.4.4 4.10.5 4.10.5.1 4.10.5.1.1 4.10.5.1.2 4.10.5.1.3 4.10.5.1.4 4.10.5.1.5 4.10.5.1.6 4.10.5.2 4.10.5.2.1 4.10.5.2.2 4.10.5.2.3 4.10.5.2.4 4.10.5.2.5 4.10.5.2.6 4.10.5.2.7 4.10.6 4.10.6.1 4.10.6.2 4.10.6.3 4.10.6.4 4.10.6.5

Introduction Spray Materials Powders Wires and Rods Suspensions and Solutions Substrate Materials Thermal Spray Processes Classification of Processes Flame Spraying Powder Flame Spraying Wire Flame Spraying Spray and Fuse Process Electric Arc Spraying Plasma Spraying Atmospheric Plasma Spraying Low-Pressure Plasma Spraying High-Power and Special Plasma Spray Processes High-Velocity Flame Spraying HVOF Spraying Warm Spraying HVAF Spraying High-Velocity Combustion Arc Spraying Detonation Gun Spraying Cold Spraying High-Pressure Cold Spraying Low-Pressure Cold Spraying Suspension and Solution Spraying Formation of Sprayed Coatings Coatings from Molten Droplets Formation of Sprayed and Fused Coatings Solid-State Formation in Cold Spraying Posttreatments Coating Materials and Properties Coating Materials Pure Metals Metal Alloys Hard Metals Oxide Ceramics Polymers Composites and Other Materials Coating Properties Microstructure, Phase Structure, and Density Mechanical Properties Corrosion Properties Wear Properties Friction Properties High-Temperature Properties Biomedical Properties Industrial Applications Mechanical Engineering Corrosion Protection in Infrastructures and Steel Structures Paper and Pulp Gas Turbines Textile Industry

Comprehensive Materials Processing, Volume 4

http://dx.doi.org/10.1016/B978-0-08-096532-1.00407-6

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Thermal Spray Coating Processes

4.10.6.6 4.10.6.7 4.10.6.8 4.10.6.9 4.10.6.10 4.10.6.11 4.10.7 4.10.8 References

4.10.1

Printing Industry Electrical Industry Steel Industry Biomedical Industry Power Generation Plants Miscellaneous Applications Development Trends Concluding Remarks

270 272 272 272 273 273 275 275 275

Introduction

Thermal spraying includes a group of coating processes in which metallic and nonmetallic materials are spray deposited as fine particles in a molten or semimolten condition or even in fully solid state to form a coating. Figure 1 presents the principle of thermal spray coating process. In the process, a heat source, which can be a combustion flame, an electric arc, or an ionized gas, i.e., plasma, is used to melt the feedstock material (powder, wire, stick, suspension/solution) and accelerate the particles toward the solid surface of the substrate, which is the material or component to be coated. Other thermal or kinetic energy sources used today are, for instance, laser beams in laser coating and inert gas flow jets in the cold kinetic spray process. The sprayed coatings are formed from flattened and solidified particles. Thermal spraying originated a long time ago when Dr M. U. Schoop from Switzerland built his first metal spray gun in about 1910 (1). History says that he got the idea either from the trace that a lead bullet left when impacting on the surface of a stone wall or from a method to prepare lead powder. Dr Schoop was working for some time in the laboratory of Thomas A. Edison in New Jersey, where he developed a new manufacturing method for lead powder pigment used in paints. After returning to Switzerland, Schoop filed a patent application for a method in which molten lead was poured into a fast flowing compressed air jet. After several trials and experiments, he finished his first commercialized wire spray guns – thermal spraying was actually called during the first years the Schoop spray method. Just prior to the First World War, he sold the technique to a German company, Metallizator. This equipment was sold and rented in Europe and in the United States in the early 1920s. A British company, Metallisation Ltd., was established during these years. The first thermal spray guns in the United States were manufactured by the Metallizing Company of America in Los Angeles during the great depression, starting in 1929. The company still exists with the name Mogul. In 1931, Schoop’s patent expired, which was followed by the establishment of other manufacturing companies and coating producers. However, the thermal spray method was at that time not yet convincing enough, making commercial activities still difficult. One of the new companies resulting from Mogul in the mid-1930s was Metallizing Engineering Company Inc., later known as Metco. In the beginning of the1940s, during the Second World War, the US Army successfully started several workshops for repair of heavy military equipment by thermal spraying in China, Burma, India, and Italy. From those years forward thermal spraying has become a noteworthy coating technology among the many other processes, although it has been still fairly unknown by lay people. During the past 20 years, thermal spraying has developed much more than during the whole history of the process before. Coating materials in thermal spraying include pure metals, metal alloys, hard metals (carbides), oxide ceramics, plastics, cermets, composites, and blended materials (2). Materials suitable for thermal spraying include practically all materials, except compounds that decompose irreversibly during melting and the ones that do not have a stable melt state and vaporize excessively in the spray process. Some materials, e.g., silicon ceramics and magnesia, are difficult to spray thermally. The deposition rate, i.e., coating production rate, in thermal spraying is high compared with many other deposition technologies. Depending on the applications, thermally sprayed coatings have thickness ranging from some micrometers up to several millimeters. Due to the versatility of thermal spraying regarding coating materials, thermal spray processes, coatings properties and in many case also cost efficiency, thermally sprayed coatings are industrially used very widely, e.g., in mechanical engineering applications, aviation and

Figure 1

Principle of the thermal spray coating process. Source: Tampere University of Technology.

Thermal Spray Coating Processes

231

energy technologies, paper and printing industries, infrastructure applications, biomedical applications, electronics, etc. The flexibility of thermal spraying is probably the highest among all different deposition technologies. This is due to several reasons, particularly because of the following features of the thermal spray technology: l l l l l l l l

Very large amounts of different types of spray materials and compositions are available, including pure metals, metal alloys, hard metals (carbides/cermets), ceramics, polymers, and combinations of these materials. Coating properties, for instance, wear resistance, hardness, and adhesion strength are clearly adequate for many industrial applications. Various properties and functionalities can be obtained by thermally sprayed coatings. The production rate of thermal spraying is remarkably high and is suitable also for large-area surfaces. The heat input to the components to be coated by thermal spraying is noticeably low; no metallurgical changes or distortion of components is present. An obvious exception is the spray and fuse process for self-fluxing alloys. Coating manufacturing costs are acceptable for most industrial applications. Thermal spraying is regarded as green technology; particularly in comparison with hard chromium plating. Novel variations of thermal spray technologies have been developed, e.g., cold kinetic spraying and suspension and solution spraying, allowing new possibilities in totally new application areas.

In this chapter, thermal spray technology is reviewed. The presentation includes a review of spray materials, thermal spray processes, the basics of coating formation, types and properties of coating materials, and industrial applications and a summary of recent developments in thermal spray technology. For more detailed information about the topic, several good books are also available for further reading (2–7). Thermal spray coating- and processing-related chapters are published in such refereed journals as Journal of Thermal Spray Technology (ASM International), Surface and Coatings Technology (Elsevier), Surface Engineering (Maney), and several other journals, and in the proceedings of some scientific conferences such as International Thermal Spray Conference (ITSC), RIPT conference, International Conference on Metallurgical Coatings and Thin Films. Societies such as ASM Thermal Spray Society (ASM/TSS) and European Thermal Spray Association (ETSA) are both active in promoting thermal spray science and technology worldwide.

4.10.2

Spray Materials

Thermal spray coating processes use powder, wire, or rod-shaped feedstock materials for coating manufacturing. All these types are used and depend highly on the particular thermal spray process equipment. Some of the spray processes, such as plasma, highvelocity oxygen-fuel (HVOF), and powder flame spray systems, use only powder materials, and some processes, e.g., electric wire arc and wire flame spraying, are based on wire materials as feedstock. Some exceptions to this may exist; also under research and in the early stages of use are liquid-type feedstock materials, e.g., suspensions, which are liquids containing nanometer- or micrometer-sized solid particles, and fully liquid materials containing specific chemicals and metallic salts for coating formation. It is notable that the number of different spray materials is extremely high; several hundreds or even up to the level of one thousand different materials in different forms of feedstock are available. Thermal spray technology is clearly the most versatile coating manufacturing technology among all the deposition technologies when the comparison is done according to the availability of different coating materials and structures and the flexibility of using many of these coating materials in the same particular spray equipment. Both powder and wire materials show advantages and disadvantages in materials selection for the coating manufacturer, as well as the end user and the final application. The main advantage of wires is simple handling, but material variety is somewhat limited and significantly less than that available as spray powders. Selection of the form of the coating material is usually determined by the availability of appropriate spray equipment and by the required coating structures and coating properties. Wires used in thermal spray processes, particularly in electric arc spraying, are usually either thinner (1.6 and 2.5 mm) solid or cored wires similar to those in arc welding or wires with higher diameter (1.4–4.8 mm) used particularly in the flame spray process. Moreover, relatively thick, flexible powder-containing filled wires (flexicords) with polymer cover and thick solid ceramic rods or sticks are used in flame spraying. Particularly for thermal spray powders, powder characteristics and coating properties obtained are remarkably determined by the powder-production methods. Based on weight, the annual consumption of wires is notably higher than that of spray powders. However, coatings prepared from powder materials typically result in improved coating properties and are also used in more demanding industrial applications, e.g., in aerospace, energy, and mechanical engineering applications. Besides powders and wires, suspensions and even liquids without solid particles are used in thermal spray processes. So far, suspension and liquid spraying are not yet widely used; instead these types of feedstock materials are in the research and development stage in several laboratories. Some evaluations and attempts for industrial use are seen for these types of novel materials.

4.10.2.1

Powders

Thermal spray powders are highly developed products with tailored compositions and carefully adjusted particle sizes and size distributions. The most important spray powder types and their manufacturing processes are (2,3,7): l l

gas and water atomized powders, fused and crushed powders,

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l

sintered and crushed powders, clad/composite powders and dense/porous coated powders, l agglomerated and sintered/spray-dried powders, as well as l special powders, e.g., sol–gel and mechanically milled/alloyed powders. l

For metallic powders, gas and water atomizing process has the highest importance. Inert gas atomizing is more popular for the production of metal and alloy powders for flame spray, plasma spray, and high-velocity flame spray processes. In the gas atomizing process, the metal is induction melted and the molten metal is injected as a stream to a vacuum chamber, where the molten material is spheroidized and collected by separators and cyclones as fine-sized spherical powders. In the water atomizing process, a water jet is used for the spheroidization. In atomizing processes, it is notable that melting of the raw material can occur under a protective gas or in air, depending on the tolerated oxygen content of the spray powder produced. The atomizing medium, either inert gas (argon, nitrogen, etc.) or water, has a significant influence on the gas content (oxygen) as well as the particle shape of the spray powder. Inert gas-atomized powders are typically highly spherical and have significantly lower gas contents than water-atomized powders. Figure 2 presents some state-of-the-art inert gas-atomized powders for flame spray and HVOF thermal spray processes. Fusing or sintering with subsequent crushing has been used traditionally for production of several different type of materials, including hard metals, metals, and oxide ceramic powders. These powder-production technologies are still in use, particularly for the production of ceramic powders for plasma spraying. Other powder manufacturing techniques, e.g., spray drying and sintering, are gathering more use, particularly in the production of multicomponent composite powders based on hard metals and mixed oxides. Figures 3 and 4 illustrate examples of sintered and crushed WC–10%Co–4%Cr powder for HVOF and spraying and fused and crushed ceramic oxide powders for different plasma spray processes, respectively. Sintered or fused and subsequently mechanically crushed powders show dense microstructures and blocky morphologies. Agglomeration by the spray drying process with a subsequent sintering step is one of the most modern production methods for spray powders, particularly for hard metal and oxide ceramic spray powders. The production process is usually followed by sintering in furnaces or spheroidizing by plasma treatment in order to compact and densify the powder. The spray drying process readily results in spherical particles and the sintering step then prevents the agglomerated powder from being destroyed during powder transport from the feeder to the spray gun, or in the combustion flame or plasma used to melt the material. The main benefits of spray dried and sintered powders are the spherical particle morphology ensuring excellent powder flowability from the powder feeder to the thermal spray gun and good homogeneity of various elemental components in the powder particles. In hard metal

Figure 2 Morphologies of gas-atomized powders for different thermal spray processes: (a) coarse NiCrBSi powder for flame spray and fusing process and (b) fine-sized stainless steel AISI 316 powder for HVOF spray process. Source: Tampere University of Technology.

Figure 3 Morphology (a) and cross-sectional microstructure (b) of sintered and crushed WC–10%Co–4%Cr powder for HVOF spraying. Source: Tampere University of Technology.

Thermal Spray Coating Processes

233

Figure 4 Morphologies of fused- and crushed-type ceramic spray powders for plasma spraying: (a) ZrO2-based thermal barrier coating (TBC) powder, (b) very coarse Cr2O3 powder for water-stabilized plasma spray process. Source: Tampere University of Technology.

powders, such as WC–Co(Cr) and Cr3C2–NiCr, good control of carbon content allows production of microstructures in which small micrometer- or even nanometer-sized hard carbides (WC) are embedded uniformly in the soft metallic binder phase. Figure 5 shows the morphology and cross-sectional microstructure of an agglomerated and sintered WC–10%Co–4%Cr powder for HVOF spraying. Spray drying and sintering is also a widely used powder manufacturing method for micrometer- and nanometer-sized multicomponent ceramic oxide powders, such as various Cr2O3–25%TiO2, ZrO2–8%Y2O3, and Al2O3–(3–40)%TiO2 compositions. There exist also other useful powder manufacturing technologies, which are widely used due to their capabilities of producing specific powder compositions and properties. Clad powder is one important example. In clad powders, coating of the so-called primary grains with fine secondary grains and an additional organic binder is the method for producing such composite powders. Also, relatively rough primary grains can be electroplated with nickel or cobalt. Nickel/graphite is typical for this group of materials, which are used as abradable coatings in gas turbine applications. In such powders, the core of the powder particle is graphite, which is then protected from burning during spraying by a nickel cladding layer. Powder production by mixing two or more different powder types and compositions is one more possibility for widening the technical applications of thermal spray coatings; such types of powder blends are still widely used and some examples of compositions are, for instance, WC/NiCrBSi and Mo/NiCrBSi blends. When selecting spray materials, attention should be paid to the fact that the properties of sprayed coatings can be completely different from the properties of cast or wrought corresponding materials. Not only density and porosity but also internal stresses and oxide content of sprayed coatings differ significantly from bulk materials. Tables 1–3 summarize the importance of powder properties on sprayability and coating properties, characterization techniques for thermal spray powders, and typical appropriate particle sizes for different thermal spray processes, respectively. It is obvious that the size distribution and shape of powders determines such important requirements as good flowability and melting behavior in specific spray processes. A wide range of compositions of thermal spray powders is standardized and presented, e.g., in EN 1274 and other standards. For highly demanding industrial applications, significantly more strict powder specifications are prepared by powder manufacturers and advanced thermal spray coating suppliers and manufacturers.

4.10.2.2

Wires and Rods

The majority of thermal spraying feedstock materials on the basis of weight are used as wire materials. This is because practically all corrosion protection materials based on cathodic protection are zinc and aluminum. Wire flame spraying and electric arc spraying

Figure 5 Morphology (a) and cross-sectional microstructure (b) of agglomerated (spray dried) and sintered WC–10%Co–4%Cr powder for HVOF spraying. Source: Tampere University of Technology.

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Table 1

General requirements for thermal spray powders

Characteristic

Consequence

Particle size

l l l

Spherical shape of powder particles

l l l

Narrow particle size range

l l l

Homogeneous alloying

l l

Low interparticle porosity

l

Manufacturing by mass production processes

l

Suitability to particular spray process Reproducible coating properties Improved flowability Uniform powder feeding and clogging-free spraying Uniform melting of powder particles Uniform melting of powder particles Reproducible coating properties High deposition efficiency Homogeneous microstructure of coating Homogeneous coating properties Uniform melting characteristics of powder Dense coating structure, particularly in low-temperature processes Appropriate powder price for industrial use

Source: Tampere University of Technology.

Table 2

Characterization techniques for thermal spray powders

Property/characteristic

Analysis/research method

Example: WC/Co 88/12

Chemical composition

All relevant analytical methods, including spectroscopic techniques and X-ray fluorescence Sieve analysis; techniques based on X-ray absorption and laser scattering SEM or optical micrographs Scott method, EN 23923-2 Hall test, EN ISO 4490 Mercury porosimeter. Nitrogen absorption Cross-sectional specimen from powder particles X-ray diffraction analysis, microprobe Differential thermal analysis

12%Co, 3.8%C, max. 2%Fe, bal. W, also light elements and impurities
90 þ 45 mm

Particle size and size distribution Production method/particle shape Apparent density Hall flow behavior Porosity Microstructure Phase structure and composition Melting behavior (powder for spray and fuse process)

Fused, crushed/blocky 6.8 g cm
3 14 s 50 g
1 – SEM images Phase structure: WC, W2C, Co–W–C mixed phases –

SEM, scanning electron microscope. Source: Tampere University of Technology.

Table 3

Typical particle sizes of powders used in different thermal spray processes

Spray process

Nominal particle sizes (mm)

Comments

Flame spraying Powder welding Plasma spraying HVOF spraying HVAF spraying Cold spraying PTA weld surfacing Laser spraying/cladding

30–125 20–90 10–70 10–60 10–45 10–30 50–150 50–150

– – Different size ranges used Typical 15–45 mm Typical 10–30 mm Fine powders in LPCS; coarser in HPCS Coarse powders Typically PTA grade powders used

LPCS, low-pressure cold spraying; HPSC, high-pressure cold spraying; PTA, plasma transferred arc; HVAF, high-velocity air-fuel. Source: Tampere University of Technology.

are the processes that use only wire materials. The wires are either solid wires or tubular wires containing various additives inside, called cored wires, see Figure 6. Solid wires are manufactured by drawing from materials that will enable such a wire manufacturing process. The use of cored wires has become more common in recent years. These wires can be made of highly alloyed iron-, nickel-, or cobalt-based alloys. The principle is that a thin and ductile metal, e.g., stainless-steel strip, is used to prepare the tubular wire and this metallic tube is packed with alloying elements and/or with hard particles, such as WC or other carbides, and the strip is then processed to a cored wire. Table 4 shows a summary of the different types of thermal spraying wires for wear protection. Wire-type feedstock materials are standardized, for example, in EN 14919. Wires for thermal spray processes, such as wire flame spray and electric arc spraying, are basically based on the following material groups: l

zinc and its alloys (pure Zn, Zn–15%Al); aluminum and its alloys (Al 99.0%, Al–5%Mg, Al–12%Si); l lead alloys (Pb–12.5%Sb–9.2%Sn–0.5%Cu); l

Thermal Spray Coating Processes

Figure 6

235

Different types of thermal spray wires and production method of cored wires. Source: Tampere University of Technology. Table 4

Wire feedstock materials for wear and tribological applications

Material class

Example of composition

Coating hardness

Chromium steels Stainless steels Carbon steels Nickel-based alloys Molybdenum Aluminum bronze Brass Babbitt metals

Fe þ 13%Cr þ 0.35%C Fe þ 17%Cr þ 12%Ni þ 2.5%Mo Fe þ 0.1–1.1%C Ni þ 20%Al þ 20%Fe þ 4%Cr Mo 99.9% Cu þ 9%Al þ 1%Fe Cu þ 40%Zn Sn þ 7.5%Sb þ 3.5%Cu þ 0.25%Pb

360–400 HB 140–200 HB 150–550 HBa 300–350 HB 37–60 HRC 180–200 HB 60–100 HB –

a

Depends on carbon content and other alloying.

Source: Tampere University of Technology. l l l l l l l

tin and its alloys (Sn 99.9%, Sn–7.5%Sb–3.5%Cu–0.25%Pb); copper and its alloys (Cu 99.8%, Cu–9%Al–1%Fe, Cu–40%Zn); iron and its alloys (Fe–13%Cr–0.35%C, Fe–18%Cr–%8Ni, Fe 29%Cr–3.7%B–1.25%Si); nickel and its alloys (Ni–20%Al, Ni–30%Cu); molybdenum (Mo 99.9%); other metals (Ag–38%Cu–32%Zn); and oxide ceramics (rigid stick and flexicord wires) (Cr2O3, Al2O3–TiO2, and other compositions).

Spray wire based on filled wires, i.e., the cord wires, are also widely available. By adding alloying elements and/or hard particles into the cord wires, coatings can be produced containing high alloying levels in metals. For example, most of the cobalt-based hardfacing wires, the so-called Stellite compositions, particularly grades other than Stellite 21, are available only as cord wires for electric arc spraying. Cord wires may also contain hard carbide particles, which are known to produce wear-resistant coatings.

4.10.2.3

Suspensions and Solutions

The use of suspensions and liquids/solutions as feedstock materials is a relatively new approach in thermal spray technology and has been under more active research and development by various researchers only during the past 10–15 years (8–10). The motivation for using suspensions or solutions in thermal spraying is the possibility to: l

prepare thin and nanostructured coatings (8–10); prepare new materials for specific applications, e.g., for SOFC and photocatalytic applications (10–12); and l simplify and shorten the feedstock material production chain for thermal spray coating manufacturing by use of solution precursor spraying (12). l

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Table 5

Thermal Spray Coating Processes

Suitability of various substrate materials in thermal spray coating processing

Substrate material group

Suitability to thermal spraying; remarks

Mild steels, low alloyed, and steel alloys Hardened steels Stainless steels, nickel-based alloys Copper and Cu alloys Aluminum and Al alloys Titanium and magnesium Polymers and polymer composites

Excellent materials to be coated, prevent oxidation by avoiding heating above 150  C Can be coated, high substrate hardness lowers efficiency of grit blasting, for steels <50 HRC Can be thermally spray coated, high thermal expansion coefficient must be noted, preheat to max. 100  C Can be coated, high thermal expansion coefficient must be noted, no preheating due to oxidation sensitivity Can be coated, high thermal expansion coefficient must be noted, no preheating due to oxidation sensitivity Can be coated, no preheating Can be coated but requires low heat input, low temperature resistance, low bond strength, buffer/intermediate layers recommended

Source: Tampere University of Technology.

The suspensions are liquids including fine solid particles, which may be nanosized or up to a few micrometers. Both water- and alcohol-based suspensions are used. Suspensions contain also additives such as plasticizer, as well as dispersing agents. Commonly used suspension-sprayed coating materials are, for example, Al2O3, TiO2, ZrO2, Cr2O3, and other ceramic materials (8). Also, WC–12%Co nanostructured coatings have been prepared from suspensions (13). So far, suspension and liquid spraying are not widely used industrially; instead these types of feedstock materials are undergoing much research and development in several laboratories. Some evaluations and attempts for industrial use are seen for these types of novel materials.

4.10.2.4

Substrate Materials

Since thermal spraying is in most cases a deposition method that does not increase much the temperature of the substrate during processing, it allows the use of many different substrate materials other than metals and alloys, even polymers and their composites. Table 5 summarizes the use of different materials as substrates in thermal spray coating.

4.10.3

Thermal Spray Processes

Thermal spray technology involves several different deposition processes that all use concentrated heat sources to melt the feedstock materials and to project the melted particles at different levels of kinetic energies toward the surface to be coated. Thermal spray processes can be based on thermal energies obtained from burning of combustible gases, such as hydrocarbons or hydrogen, or liquid fuels or can be obtained from electric discharges. Besides actual thermal spray processes, such as flame spray with powder or wire material, electric arc spraying, plasma arc spraying, and HVOF spraying, processes entirely based on deposition by kinetic energies are also classified under the general term thermal spray processes. In the following sections, each of the thermal spray processes will be presented in more detail after a more general classification and comparison of these processes. Thermal sprayrelated processes such as plasma transferred arc (PTA) surfacing, laser spraying or laser cladding, and melt spraying are excluded from this chapter and readers are recommended to read about these techniques from elsewhere, e.g., Refs. (14–16).

4.10.3.1

Classification of Processes

Thermal spray processes are usually classified according to the type of energy source used to melt the feedstock material, as is presented in Figure 7. The most typical energy sources in thermal spraying are thermal (and/or kinetic) energy obtained from combustion of gases, typically hydrocarbon or hydrogen, or liquids; from thermal energy obtained from electric discharges such as electric arcs or ionized plasma gases; or from purely kinetic energy sources in cold kinetic spray process. In addition to these, energy from molten liquids or high-power laser beams may be used in thermal spray-like deposition of materials. Depending on the type of energy source, thermal spray processes can be further classified according to the spray gun principle or design, type of feedstock material used in the process, type of fuel (gas or liquid), type of deposition atmosphere (atmospheric, low/high pressure, inert gas, under water, etc.), type of oxidizer in combustion, etc. It is notable that in spite of the availability of several more or less differing techniques in thermal spray processes, they all have their own important niches and only minor overlaps in performance, investment and operation cost levels, and coating material processability and coating properties exist. Table 6 is a very general comparison of different thermal spray processes and gives some most important but very generalized numerical data and information for each one. It should be noted that all values and numbers presented in the table are based on general interpretation of data from several different sources and partly based on the experience of the author in the topic. The values and numbers may be highly material, parameter, and process/equipment sensitive and act therefore only as comparative data for general comparison of the different thermal spray processes. It is obvious that processability of different coatings to obtain high quality plays a large role in selection of an appropriate thermal spray process. Selection and use of an optimal thermal spray process for particular material and application may look highly complex and demanding. This is partly true, but understanding the technological and economical advantages and limitations of different processes allows successful use of these in varying technical applications. A highly simplified expression of the information from Table 6 allows us to draw conclusions from its contents.

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237

Figure 7 Classification of thermal spray coating processes according to the type of energy source used in the process. Source: Tampere University of Technology. Table 6

Comparison of characteristics of coatings deposited by different thermal spray processes

Spray method

Temperature ( C)a

Particle velocity (m s
1)

Adhesion (MPa)b

Oxide content (%)c

Porosity (%)

Spray rate (kg h
1)

Relative cost d, low ¼ 1 high ¼ 5

Typical coating thickness (mm)

Flame Electric arc HVOF HVAF

3000 4000 3000 2000–3000

40 100 600–800 600–1200e

8 12 >70 >70

10–15 10–20 1–5 ***f

10–15 10 1–2 0–0.2

1 2 3 2

0.1–15 0.1–15 0.1–2 0.1–12

Detonation APS LPPS/VPS LPCS HPCS

4000 12 000 12 000 200–650 500–1000

800–1200 200–400 400–600 300–500 400–800

>70 10–70 >70 5–30 10–40

1–5 1–3 0 0 0

1–2 1–5 <0.5 <0.5 <0.5

2–6 10–25 2–8 Hard metals: 2–30; metals: 2–23g 0.5–2 2–10 2–6 0.5–3 4–12

4 4 5 1 4

0.05–0.3 0.1–1 0.1–1 0.2–2 0.3–4

a

Temperature of the heat source. Depends on deposit material. c Oxide content in metallic deposits. d Investment cost of process. e Depends on equipment type; e.g., M2 AC-HVAF, 600–700 m s
1; M3 SAP, 800–1200 m s
1. f Oxide content is approximately 1.5–2 times the oxide content of the feedstock. g Depends on gun type; M2 AC-HVAF spray gun; M3 supersonic spray gun. Flame – flame spraying, wire and powder method; electric arc – electric arc wire spraying; detonation – detonation gun spraying; HVOF – high-velocity oxygen-fuel spraying; HVAF – high-velocity air-fuel spraying; LPPS/VPS – low-pressure/vacuum plasma spraying; LPCS – low-pressure cold spraying; HPCS – high-pressure cold spraying/kinetic spraying; APS – atmospheric plasma spraying. Source: Tampere University of Technology. b

The temperature of the heat source is important; refractory materials, i.e., high-melting-point ceramics and refractory metals can be melted only by plasma spray-based processes. Easily oxidizing metals may require spray atmospheres in which oxygen has been eliminated, i.e., inert gas shroud or low-pressure plasma spray (LPPS) or vacuum plasma spray (VPS) processes. Cold spraying may be a clear exception, because in this process the powder material does not melt at all, and therefore can be processed to coatings even in air atmosphere. Particle velocities in different spray processes will clearly have an impact on the coating properties, such as adhesion to the substrate (bond strength), coating density, and other technical properties including hardness, wear, and corrosion resistance. The residual porosity in thermal spray coatings is usually lowest in the high-velocity spray processes. Such features as

238

Thermal Spray Coating Processes

Figure 8 Overview of particle temperatures and velocities for different spray processes; process names from TopGun to K2 refer to various HVOF systems. Source: H.C. Starck GmbH.

spray rate may act as economic factors influencing the cost of the process and coated part. Investment and operation costs of different processes definitely have a large influence on the possibility of using such a process in the specific application; other surface treatments and coating methods may in such cases be more economical. Some industrial applications may allow the use of only very conventional spray processes, such as flame spraying, due to cost level limitations. Maximum coating thickness available in different processes may also recommend some techniques instead of others. Some thermal spray processes are more ideal for sprayspecific coating materials. The high-velocity flame spray processes (HVOF, high-velocity air-fuel spraying (HVAF), and detonation spraying) are more suitable for spraying hard metal powders, such as WC–Co(Cr) and Cr3C2–NiCr, due to their high particle velocities and more suitable lower flame temperatures. Different thermal spray processes can also be located to specific characteristic places in the process temperature vs particle velocity charts or maps, as is done in Figure 8. The arrow indicates development trends in which process temperatures are continuously decreased, while particle velocities show simultaneous increase is a general trend in the development of novel thermal spray processes, particularly for manufacturing dense and/or hard coatings with improved mechanical and corrosion properties. The following chapters describing different thermal spray processes in more detail will give further information about their applicability. The formation of thermally sprayed coatings, examples of coating materials, their microstructures and properties, and areas of applications are presented in the following chapters.

4.10.3.2

Flame Spraying

Conventional flame spray was the first thermal spray process developed. Both powder and wire feedstock materials are used in flame spraying. In spite of their long history, these techniques are still widely used particularly due to the low cost of equipment and processing. However, the importance of flame spray technologies in more demanding applications has significantly reduced as more advanced processes such as plasma spraying and HVOF spraying have become more predominant. Modern flame spray torches have changed fairly little since the 1950s. Flame spraying process and other technologies such as HVOF use the chemical energy of combusting fuel gases to generate heat and consequently accelerate the molten particles toward the substrate. Oxygen acetylene torches are the most common, using acetylene (C2H2) as the main fuel in combination with oxygen to generate the highest combustion temperature of approximately 3160  C. Other gases in use are propane (C3H8), propylene (C3H6), hydrogen (H2), and ethane (C2H4) (14). Powders, wires, or rods are introduced axially through the rear of the nozzle into the flame at the nozzle exit. The feedstock materials are melted and the particles/droplets accelerated toward the substrate surface by the expanding gas flow and in some cases also by air jets. Externally combusted, or open-flame, jet temperatures are generally above 2600  C and are controlled by the combustion temperatures of the fuel/oxygen mixtures and the mixing patterns of the combustion gases with the surrounding air. The temperature curves for fuel gases are parabolic and centered roughly around the stoichiometric ratio with oxygen. Adjusting the fuel/oxygen ratio, either side of stoichiometry, will cool the flame. The reason behind the adjustment is to make the flame either oxidizing or reducing as the feedstock material and application dictate. Flames are set to be reducing (fuel-rich) for most metal spraying in order to minimize oxidation. Generally, flame spray torches are designed to scavenge part of the oxygen needed for combustion from the atomizing air or cooling air jets. This design further helps produce metal coatings with lower oxides. Flame spray is capable of depositing a wide range of materials, ranging from polymers to ceramics and refractory metals. The different components of typical flame spray systems are (2): l l l l l l l l

gas supplies (acetylene and oxygen); air supply (compressed air); gas hoses; gas regulators for oxygen, fuel, and air (if used); rotameters for gas flow controls; flashback arrestors at the gun and regulators; flame spray torch comprising a torch body, nozzle, and atomizing air cap; and feedstock delivery system comprising a separate powder feeder, powder hopper, and air turbine drive for wires and rods.

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239

Powder is fed into flame spray torches in one of two ways: by carrier gas or by gravity. Gravity-fed devices have powder canisters or bottles mounted directly to, and on top of, the torch. Powder feed rate is controlled by a pinch valve that meters powder into the body of the torch, where it is aspirated by the gases flowing through the torch. Carrier-gas-fed units use externally mounted powder feeders. External powder feeders use a carrier gas (typically nitrogen, air is also used) stream to transport the powder from the feeder through a hose to the spray torch. Wire- and rod-fed devices use air turbines built into the torch that power the drive rolls, which pull feedstock from the source and push it through the nozzle.

4.10.3.2.1

Powder Flame Spraying

Figure 9 illustrates schematically the structure of a powder flame spray torch. In flame spray torches, a mixture of acetylene and oxygen is combusted at the front of the torch, external to the nozzle. In some designs, a constricting flow of air is fed through a concentric set of holes at the nozzle exit and is used to pinch the expanding gases, generating a higher temperature combustion zone. The flame spray torch consists of gas distribution passages through the nozzle and mixing orifices at the nozzle end. Nozzles are used to shape the spray pattern. Powder flame spray processes are used widely due to the low equipment and processing cost. Typical materials sprayed are iron, nickel, cobalt, copper, and some other pure metals and their alloys. Due to relatively low particle velocities, the coatings show only moderate properties, which are definitely adequate for repair and restoration and also for various new components. In addition, ceramic powders are sprayed by powder flame spraying, but the resulting coating properties again do not reach those of plasma-sprayed coatings. Powder flame spraying is the most used process to spray nickel- and cobalt-based selffluxing alloy powders, which are not used in the as-sprayed state, but instead posttreated by a fusing/sintering process. These alloys are typically NiCrBSi with varying amounts of chromium and carbon, and the self-fluxing additives boron and silicon. The methods used in posttreatment are fusing by oxyacetylene torch, furnace sintering, induction fusion, or other techniques such as laser treatment.

4.10.3.2.2

Wire Flame Spraying

Figure 10 shows schematically the structure of a wire flame spray torch. Basically the gun is similar to a powder flame spray torch, but in wire flame spraying, special air caps are added to these torches, producing a concentrated air jet to atomize the tip of the molten wire or rod. The wires and the rods are mechanically fed through the center of the nozzle into the heating zone. Oxide and metal rods and wires can be used as coating feedstock. One significant advantage of wires and rods over powders is that the degree of melting is significantly higher than in powder flame spraying, producing denser coatings. All material that is emitted from the hot wire tip is fully molten. In powder flame spraying process, some powder particles, particularly the largest ones, do not necessarily melt fully and may cause porosity formation in the coating and may also lower the deposition efficiency (D.E). D.E. in thermal spraying is the ratio of coating deposited divided by the amount of feedstock material fed to the spray torch (excluding off-sprayed coating material).

Figure 9

Figure 10

Schematic presentation of a flame spray gun using powder feedstock material. Source: Tampere University of Technology.

Schematic presentation of a flame spray gun using wire as feedstock material. Source: Tampere University of Technology.

240

Thermal Spray Coating Processes

In the wire flame spraying process, the atomizing air can produce finer droplets, which in turn may produce finer and smoother coatings. In flame spray processes, fuel/oxygen ratio and total gas flow rates are adjusted to produce the desired thermal output needed to melt the specific feedstock material. Optional air jets, downstream of the combustion zone, may also further adjust the thermal profile of the flame. Spray gas speeds typically are below 100 m s
1, generating particle speeds up to approximately 80 m s
1 before impact.

4.10.3.2.3

Spray and Fuse Process

Spraying with a postheat treatment by sintering/fusing can be carried out to obtain dense coatings with metallurgical bonding (diffusion bonds). This thermal spray processing consisting of two separate stages – flame spraying and postfusion – is clearly different from other thermal spray processes, which are one-step processes and in which the coating adherence to the substrate material is mainly of the type of mechanical anchoring and bonding to the substrate surface asperities created by grit blasting of the surface prior to thermal spray coating. The fusing is done with an oxyacetylene flame, inductive heating, or in a furnace (gas furnaces or electric furnaces with protective gas atmosphere). The fusing occurs at a temperature where only deposited material can melt to close interparticle gaps and where fluxing, i.e., removing of the oxides occurs. The fluxing elements, particularly boron and silicon, have direct influence on the wetting behavior of the practically molten coating material on the substrate. When the fusing stage is complete, the surface has a highly reflective, glassy look. These types of coatings are homogeneous with very good bonding strength, 350–500 MPa. Because of the high fusing temperature (approximately 1050  C), there is a risk of deformation and structural transformation of the substrate. One type of fused layer is made by simultaneous fusing, so-called spray welding. The bond strength is high and comparable to brazing. Figure 11 presents the fusing stage of a flame spray-coated sleeve with oxyacetylene torch.

4.10.3.3

Electric Arc Spraying

Figure 12 presents an electric arc two-wire spray gun schematically. Arc spraying is used to apply only electrically conductive materials including different metals, metal alloys, and metal–metal oxide or metal–carbide mixtures (cord wires). In arc spraying, an electric arc between two wires is used to melt the coating feedstock material. Compressed gas, usually air, is used to atomize and propel the molten material to the substrate. Sometimes air is replaced with nitrogen to lower the degree of oxidation of the metallic coating material. An electric arc spray system consists of a high-power direct current (DC) power supply, two wire coils (which can

Figure 11

Postfusing operation for flame-sprayed NiCrBSi self-fluxing alloy coating with oxyacetylene torch. Source: Kuopion Konepaja Oy.

Figure 12 Schematic presentation of an electric arc wire spray gun using two electrically conducting wires as feedstock material. Source: Tampere University of Technology.

Thermal Spray Coating Processes

241

be different material), wire feeding mechanisms, wire feeding hoses, electrical conducting cables, and the arc spray gun. The two wires are continuously fed to the gun at a uniform speed. A low-voltage (18–40 V) DC power supply is used, with one wire serving as the cathode and the other as the anode. Coating quality and properties can be controlled by varying the atomization pressure, air nozzle shape, power, wire feed rate, traverse speed, and spray distance (standoff distance). Arc spraying is generally the most economical thermal spray process for applying corrosion-resistant metal coatings, including zinc, aluminum, and their alloys. Energy costs are lower and production rates are significantly higher than they are in competing methods such as wire flame spray.

4.10.3.4

Plasma Spraying

Plasma spraying process is a widely used method of manufacturing very different types of coatings. The process due to its hightemperature heat source, a thermal plasma, makes it possible to melt practically all possible material including all ceramic materials and refractory metals. A thermal plasma, often called the fourth state of matter, normally consists of neutral atoms, positive ions, and free electrons. Plasma is produced by transferring energy into a gas until the energy level is sufficient to ionize the gas, allowing the electrons and ions to act independent of one another. The plasma state is achieved when, under an electric field, currents can be sustained as the free electrons move through the ionized gas. Once the energy input is removed, the electrons and ions recombine, releasing heat and light energy. Figure 13 shows the enthalpy vs temperature behavior of common plasma-forming gases in thermal spraying. Monoatomic gases such as argon (Ar) and helium (He) ionize directly from atomic state, whereas diatomic gases such as nitrogen (N2) and hydrogen (H2) first dissociate and then become ionized. Argon and nitrogen are used in plasma spraying as the primary plasma gases and hydrogen and helium as secondary gases. The combination of argon/hydrogen is common; argon ionizes easily and hydrogen significantly increases the enthalpy enabling efficient melting capacity of the plasma torch. Hydrogen addition, typically only up to 25% to prevent electrode and nozzle wear, also increases the thermal conductivity of the gas mixture and allows increase of the arc voltage for higher plasma power levels.

4.10.3.4.1

Atmospheric Plasma Spraying

Plasma spray torches are the most flexible of the thermal spray processes with respect to materials that can be sprayed, because of the inert gases, high gas velocity, and extremely high temperatures. Plasma spray processes operating in ambient atmospheres are called atmospheric (or air) plasma spraying (APS) processes. A schematic structure of a typical DC plasma gun is illustrated in Figure 14. Such plasma torches consist of a circular anode/nozzle made of copper and a cathode made of thoriated tungsten. Powder feeding is

Figure 13

Enthalpy vs temperature behavior of common plasma-forming gases in plasma. Source: Tampere University of Technology.

Figure 14 Schematic presentation of a plasma spray gun using DC power for plasma forming. The powder is injected radially to the plasma jet. Source: Tampere University of Technology.

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Thermal Spray Coating Processes

Figure 15 Mixing of the surrounding air with the plasma gas as function of distance from plasma gun nozzle exit. Reproduced from Matejka, D.; Benko, B. Plasma Spraying of Metallic and Ceramic Materials; John Wiley & Sons, 1989; p 280.

usually realized by pneumatically injecting fine spray powder radially to the plasma jet. The electric arc discharge, supported by a powerful DC generator (80 kW and above) through the connectors heats up by ionization the plasma forming gases. When the ionized gas returns back to atoms/molecules by recombining, a high amount of energy is released. The gas expands in the atmosphere and forms an extremely hot and high-velocity gas jet. The particles of the powder after being melted and accelerated in the jet impact the substrate and form the coating. DC plasma guns are intensely water cooled to prevent melting and to minimize cathode and nozzle wear. The plasma forming gases are introduced through the rear of the gun. Typically, these gases enter the arc chamber through a gas ring (gas injector) that imparts a spin or vortex flow to the gas. The vortex stabilizes the arc at the cathode tip in the low-pressure region of the vortex and rotates the arc attachment or arc root at the anode. This is called vortex stabilization. The plasma forming gases are heated as they pass around and through the arc. The heated gases expand radially and axially, accelerating as they expand and exit through the nozzle. Depending on the pressure ratio between the upstream arc region and the downstream nozzle exit of the gun, the gases (plasma jet) either expand subsonically or supersonically. Many gun designs use converging/ diverging de Laval nozzles to achieve supersonic expansion, which increases significantly the particle velocity, thus improving the coating properties. Most plasma spray equipment operates in atmospheric conditions, i.e., some air is entrapped into the hot plasma jet causing some interaction between the fast-flying hot spray powder particles and the surrounding gas atmosphere. Figure 15 shows this behavior schematically. Therefore, it is apparent that sprayed coatings also tend to react with oxygen from the entrapped air in the otherwise inert gas atmosphere of plasma spraying. In order to totally prevent the interaction of the molten spray particles and oxygen from air, special plasma spray processes called vacuum or low-pressure plasma spraying has been developed. Atmospheric plasma spraying has some separate modifications from the conventional 40-kW level plasma spray processes, e.g., high-power plasma spray (HPPS), axial plasma spray, and water-stabilized plasma spray (WSP) processes. These technologies will be reviewed later in this chapter.

4.10.3.4.2

Low-Pressure Plasma Spraying

LPPS/VPS processes were developed in 1974 by E. Muehlberger. This new process demonstrated that very clean coatings with practically no oxide inclusions could be produced. The main difference of LPPS/VPS processes compared with APS is that these processes are carried out inside a vacuum chamber, which fully eliminates the interaction of molten spray particles with oxygen. Main uses of LPPS/VPS are in manufacturing oxidation-sensitive coating materials such as MCrAlYs (M ¼ Ni, Co, NiCo, CoNi; sometimes Fe) and reactive metals such as titanium (Ti) and tantalum (Ta) coating. Such coatings when sprayed in chambers have porosities significantly below 1% and are free of oxide inclusions, with clean substrate/coating interfaces. The LPPS/VPS spray equipment consists of a conventional plasma spray gun equipped with a nozzle modified for higher pressure expansion ratios. These high ratios are achievable because the plasma jet exhausts into a chamber at below atmosphere pressure, typically at the level of 100 mbar. The closed environment of the chambers necessitates remote manipulation of the spray guns, for example, with an industrial robot, and a door in the chamber or load-lock systems to transfer parts into the chamber. Other unique aspects of chamber spraying include vacuum pumping, exhaust cooling, filtration, and chamber wall cooling. LPPS results in the following process and coating characteristics relative to conventional APS (2): l

broader and longer spray jets; cleaner interfaces, made possible by reverse transferred arc sputter cleaning; l oxide-free coatings; l high coating densities, close to theoretical; l

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243

l

low residual stresses; increased deposition thickness capability (>25 mm); and l high substrate/deposit preheating capability: greater than 1000  C prior to and during coating. l

4.10.3.4.3

High-Power and Special Plasma Spray Processes

Besides APS and LPPS/VPS plasma-based processes, there exist some variants and developments. In order to increase the powder feeding rates for more efficient and productive plasma spraying, technologies such as HPPS (17), axial plasma spraying (2), and WSP (18) have been developed during the past years. The main driving forces in developing such new processes have been high-rate spraying obtainable by high powder throughputs (high feeding rates), high D.E. to reach high powder usage, and high process stability for uniform melting of the spray material. The solutions have been primarily the use of high power levels up to 200 kW (normal APS has 40-kW gun power), developing improved stability of the plasma arc by new gun designs, and getting optimal powder utilization by new powder injection principles, e.g., axial powder feeding by special multielectrode gun designs. Figure 16 is a schematic presentation of an HPPS torch, in which nitrogen is the primary gas (argon can not be used in this process) and hydrogen is the secondary plasma gas; the gas flow rates are high (300–400 l min
1; in conventional APS approximately 50 l min
1), the DC power level is high (165–220 kW), and the powder spray rates are increased from those of conventional APS (100–200 g min
1 vs 40 g min
1). In a very specific plasma spray process, the WSP using water as plasma-forming media, the cathode is made of graphite and the anode is an internally water-cooled rotating copper disk located in front of the gun nozzle. The main differences of WSP compared with APS are the plasma-forming media used (gas vs water), significantly different torch design, plasma temperature, power level in process (160 kW vs 40 kW), and powder feeding rates. A comparison of these processes is presented in Table 7 (19). There exist basically three different variants of conventional plasma spray torch, where the plasma guns are based on using more than one electrode; typically three electrodes (cathodes) are used in plasma spray torches such as Axial III and Triplex. The first one is based on using three pairs of separate cathodes and anodes, which are located symmetrically around the center powder feeding nozzle, allowing powder feeding axially directly to the plasma plume. Deposition efficiencies of the order of 75–90% (conventional APS has typically 40–60%) combined with high powder feeding rates makes such technologies attractive for industrial plasma spraying (Table 8). It is also evident that in all multielectrode plasma torches (Axial, Triplex) the stability of the plasma arc discharge is much improved from conventional single-electrode plasma torches enhancing more uniform powder melting in the process; the single-electrode plasma torch may show significant instabilities of the plasma arc, particularly when the

Figure 16 Schematic presentation of a high-power plasma spray torch (Plazjet), which consists of an extended nozzle/anode. Source: Praxair Surface Technologies Inc. Table 7 Comparison of conventional gas-stabilized and water-stabilized plasma spray processes

Plasma forming media Arc current (A) Arc voltage (V) Plasma power (kW) Plasma temperature (K) Spray distance (mm) Injection distance (mm) Powder feed rate (kg h
1)

WSP

Conventional plasma

Water 500 300–320 160 30 000 350 30 20–50

Gas (Ar, H2, N2, He) 380–400 68–75 40 10 000–15 000 100 6 3–6

Modified from Sampath, S.; Gansert, R.; Heramn, H. Research Summary: Plasma-spray Forming Ceramics and Layered Composites. JOM (USA) 1995, 47 (10), 30–33.

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Table 8

Deposition efficiencies and powder feeding rates obtainable with axial powder feeding plasma torch

Material

Feed rate (kg h
1)

Feed rate (g min
1)

Deposition efficiency, D.E. (%)

Hard metals (cermets) WC–17%Co WC–12%Co WC–Cr3C2–Ni Cr3C2–NiCr

9–13.5 9–13.5 7.2–11.4 3.6–6.9

150–225 150–225 120–190 60–115

75–90 65–80 75–90 65–90

Ceramics (oxides) Al2O3 Al2O3–13%TiO2 ZrO2–8%Y2O3 Cr2O3

1.8–4.5 2.7–5.4 2.1–3.6 1.8–5.4

30–75 45–90 35–60 30–90

75–90 75–90 60–85 45–75

Abradables AlSi–Polyester Al–Si–Graphite

1.8–3.6 1.8–2.7

30–60 30–45

70–90 45–50

Metals and alloys AISI 316L stainless steel Titanium (c.p.) NiAlMo NiCoCrAlY Copper Molybdenum

4.5–11.4 1.8–4.5 8.1–9.9 4.5–9 9–10.8 9–18

75–190 30–75 135–165 75–150 150–180 150–300

70–90 75–90 75–90 75–90 75–90 75–90

Source: Northwest Mettech Corp.

electrode pairs are already worn by arc erosion, which may lead to nonuniform melting of the spray powder. This will lead to lowered D.E. and changes in the coating quality. The conventional technique to ionize gases for plasma spraying is the use of DC plasma configurations. However, induction coils powered with high frequency can also be used to ionize gases for plasma spraying and other material processing. Besides the type of power supply, some other significant deviations from DC plasma torches exist in inductively coupled plasma torches. The gas velocity in induction plasma is significantly lower than that in DC plasma torches; larger powder particles have to be used in induction plasma processes. The torch design is quite different. Induction plasma spraying systems are not used to such an extent as DC plasma torches for coating production, due to bulky equipment and the lower coating properties achievable. Examples of applications are powder spheroidization and production of coatings for highly specific applications.

4.10.3.5

High-Velocity Flame Spraying

High-velocity flame spraying (HVFS), also more frequently called HVOF spraying involves nowadays several different combustion flame spray processes in which a gaseous or liquid fuel is combusted in a combustion chamber of the spray gun with oxygen or air as oxidizer. Particularly typical of these processes is the very high gas velocity leading to high particle velocities of the order of 500–800 m s
1, depending on the type of spray gun and moderate particle temperatures. In such spray processes, the thermal energy of the process is partly replaced with the high kinetic energy, i.e., increased particle velocities. This means that the particle temperatures are not excessively high as in many other spray processes but are kept as low as possible at a level in which deposition still occurs with acceptable D.E. The particle velocities in HVOF processes are markedly higher than in other conventional spray processes, e.g., in plasma spraying. High particle velocities combined with moderate particle temperatures result in coatings that are dense, well-adhered to the substrate, and show a low amount of oxidation in the case of metallic and hard metal coatings. Since HVFS processes were primarily developed to spray tungsten carbide-based cermet (WC–Co), another obvious benefit was the high amount of retained carbides (WC) in the structure resulting in markedly improved mechanical and wear properties versus, for example, plasma sprayed coatings. HVFS involves techniques such as high-velocity oxygen-fuel (HVOF) spraying, high-pressure high-velocity oxygen-fuel (HP-HVOF) spraying, HVAF spraying, high-velocity impact-fusion spraying, and other novel techniques, e.g., the warm spraying process. Basically all techniques are similar, but they differ from each other in the gun design, type of fuel (gas or liquid), type of oxidizer (oxygen or air), particle temperature and velocities, and in other less important characteristics. HVFS plays today very important role in the thermal spray process palette and the most important spray techniques are HVOF, HP-HVOF, and HVAF. In the next section, the HVOF (includes here also HP-HVOF), HVAF, and warm spraying are described in more detail due to such factors as the importance and the novelty of the processes. Also, a hybrid method composed of HVFS and electric arc spraying is described separately in its another chapter. The detonation gun spray process (DGS or D-Gun), a traditional high-velocity flame spray process known already for a long time, is also described due to its similarity with the later developed spray techniques. Table 9 presents some important properties, such as maximum temperature when combusted with oxygen and the corresponding heat of combustion for different fuel gases and a liquid fuel frequently used in HVOF and similar high-velocity flame

Thermal Spray Coating Processes

Table 9

245

Properties of different fuel gases and a liquid fuel used in HVOF and other similar processes

Fuel

Maximum flame temperature ( C)

Heat of combustion (MJ m
3)

Oxygen/fuel ratio for max. flame temperature

Oxygen/fuel ratio for neutral flame

Oxygen/fuel ratio for HVOF applications

Propane Propylene Hydrogen Ethylene Acetylene Kerosene

2828 2896 2856 2924 3160 ca. 2900

93.2 87.6 10.8 59.5 56.4 37.3 MJ l
1

4.5 3.7 0.42 2.4 1.5 2.9

5.0 4.5 0.5 3.0 2.5 3.4a

3.0–8.0 3.5–7.0 0.3–0.6 2.0–5.0 1.3–4.0 2.8–4.8a

a

Standardized mass ratio oxygen/kerosene.

Reproduced from Kreye, H.; Gärtner, F.; Kirsten, A.; Schwetzke, R. High-velocity Oxy-fuel Flame Spraying State of the Art, Prospects and Alternatives. In 5. Colloquium on High Velocity Flame Spraying; GTS e.V.: Erding, Nov. 16–17, 2000.

Figure 17 Influence of oxygen/fuel ratio l on flame temperature (l ¼ 1 for stoichiometric combustion). Reproduced from Kreye, H.; Gärtner, F.; Kirsten, A.; Schwetzke, R. High-velocity Oxy-fuel Flame Spraying State of the Art, Prospects and Alternatives. In 5. Colloquium on High Velocity Flame Spraying; GTS e.V.: Erding, Nov. 16–17, 2000.

spray processes. Figure 17 shows the influence of normalized oxygen–fuel ratio on the temperature of the flame for gaseous and liquid fuels in HVOF spraying (20). It is evident that the choice of fuel type has a marked influence not only on the obtained maximum temperature but also on the cost of HVOF spraying. It should be mentioned that acetylene is seldom used in HVOF spraying, but in turn is the main fuel gas in powder and wire flame spraying processes. Flame temperatures in HVOF processes can be easily adjusted by changing the ratio of the fuel gas and oxygen flow rates. Combustion of liquid fuel, and other hydrocarbon fuel gases, with compressed air in the HVAF process lowers significantly the flame temperature and may result in significant improvements in coating quality. It is known that hard metal coatings, e.g., WC–Co(Cr), and metallic coatings show in HVAFsprayed coatings better properties, such as higher amount of retained carbide phases for improved wear resistance and ductility and lower level of oxidation for improved corrosion characteristics.

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Figure 18

Schematic presentation of an HVOF spray torch. Source: Tampere University of Technology.

Figure 19 Influence of type of fuel gas and spray distance from the HVOF gun exit on measured powder particle temperatures and their average velocities. Reproduced from Nestler, M. C.; Benary, R.; Rusch, W. Innovations in HVOF Coating Solutions. In 5. Colloquium on High Velocity Flame Spraying; GTS e.V.: Erding, Nov. 16–17, 2000.

4.10.3.5.1

HVOF Spraying

Figure 18 shows an HVOF spray gun schematically. HVOF spray guns consist of a fuel gas/oxygen mixer, a combustion chamber, and an expansion nozzle, which is usually in modern HVOF guns a converging/diverging de Laval-type nozzle. Such nozzles lead to high gas velocities, which in turn accelerate the powder particles to very high velocities of the order of 600–800 m s
1 (Figure 19) (21). The HVOF guns may also be either water or sometimes only air cooled. Full HVOF spraying systems consist of different units such as (2): l l l l l l l l l

gas supplies (fuel gas/liquid fuel, oxygen); air supply (compressed air); gas hoses; gas regulators for oxygen, fuel, and air (if used); rotameters or mass flow controllers for gas flow controls; flashback arrestors at the gun and regulators; HVOF spray torch composed of a torch body, gas mixer, combustion chamber, de Laval nozzle, and gun cooling; powder feeding system; and spray gun manipulator/robot (unless manually operated).

4.10.3.5.2

Warm Spraying

Warm spray is a novel method that aims to compete with cold spray and HVAF in depositing dense coatings without changing the phase or chemical compositions of the feedstock. Warm spray is very similar to HVOF with the difference of an inert gas (nitrogen) being introduced to the combustion jet before the powder in a mixing chamber, thus allowing more control over the temperature and velocity of the supersonic jet. The temperature of the jet is lower than in corresponding HVOF processes and the velocities are same as in HVOF processes. The melting temperature of most materials is approximately 800–1500 K, and the warm spray system enables the control of the jet temperature very specifically depending on the material. Warm spraying has been primarily used to spray WC–Co(Cr) and some metallic powders. When spraying carbides, it has been observed that warm spray does induce some decarburization of WC, although significantly less than the HVOF process (22–25).

4.10.3.5.3

HVAF Spraying

HVAF spray process can be regarded as a modification of HVOF (26,27). The HVAF uses a mixture of air and a fuel gas in the combustion chamber and a second mixture of air and fuel in the nozzle to propel the powder feedstock from the gun to high

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247

Figure 20 An illustration of a third-generation HVAF spray torch (M3 type) using hydrocarbon gas and air as process gases. Source: Uniquecoat Technologies Inc.

velocities. Since the HVAF uses air instead of oxygen for the combustion, the operating costs are significantly lower due to safer and more controllable combustion, which also lowers expenses on security, and to the notably lower cost of air compared with oxygen. Additionally, the powder feeding rates are markedly higher than in HVOF systems. The main advantages regarding coating qualities arise from lower flame temperatures, which minimizes the amount of dissolved carbides in WC–Co(Cr) coatings and produces less oxidized metallic coatings. The feedstock used in HVAF guns is powder. In activated combustion high-velocity air-spraying (AC-HVAF), a hot catalyst is used to stabilize the combustion and make it even. Recent types of HVAF torches such as the thirdgeneration equipment that utilize supersonic air-fuel technology can have particle velocities well above those of the previous HVAF torches and other HVOF torches that are still in use. The recognized advantages of this method are high coating density, due to high velocities, and low oxidation, due to lower temperature, of the coating. An illustration of a third-generation HVAF torch is presented in Figure 20.

4.10.3.5.4

High-Velocity Combustion Arc Spraying

High-velocity combustion arc spray (HV-Arc) process is a hybrid thermal spray process, which consists of a special electric arc spray torch integrated in a high-velocity air-fuel combustion chamber and spray nozzle, as is schematically presented in Figure 21. In comparison with the traditional electric arc spray process, the HV-Arc process has higher particle velocities, the molten material is atomized by the fast gas jet into finer droplets, and the coatings consist of densely packed fine splats producing better coating properties. Propane is a typical fuel gas in the HV-Arc spray process (28).

4.10.3.5.5

Detonation Gun Spraying

Detonation gun spray equipment is basically a long water-cooled barrel with inlet valves for gases and powder (Figure 22). Oxygen and fuel gas, acetylene being the most common one, is fed into the barrel along with a charge of powder. A spark plug is used to ignite the gas mixture and the resulting detonation heats and accelerates the powder to supersonic velocity down the barrel. A pulse

Figure 21

Schematic structure of a high-velocity arc spray torch. Source: Uniquecoat Technologies Inc.

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Thermal Spray Coating Processes

Figure 22

Schematic structure of a detonation gun spray equipment. Source: Tampere University of Technology.

Table 10 Temperatures of the detonation products with various gas mixtures. Amounts of components in mixture, % C2H2

O2

N2

Td.p ( C)

38 33 28 24 20

62 55 47 39 31

0 12 25 37 49

2900 2550 2200 1900 1600

Reproduced from Astakhov, E. A.; Klimenko, V. S.; Skadin, V. G.; Zverev, A. T. Sov. Powder Metall. Met. Ceram. 1982, 21, 307–310.

of nitrogen or air is used to purge the barrel after each detonation. This process is repeated many times a second. The high kinetic energy of the hot powder particles on impact with the substrate results in the buildup of a very dense and strong coating. The firing (explosion) frequency of various detonation gun systems can be in the range of 1–50 Hz (shots per second), but is typically in the range of 5–10 Hz (29,30). In detonation gun spraying, the flame temperature is easily controlled by varying the oxygen/fuel gas ratio and/or the amount of diluent gas (nitrogen or air) in the gas mixture. Table 10 shows the influence of a diluent gas (nitrogen) on the flame temperature of an acetylene/oxygen mixture. The development of HVOF technology and particularly the availability of HVOF spray equipment has lowered markedly the industrial importance of DGS. The main drawbacks of the detonation spray technology has been its poor availability, particularly in the 1980s and the 1990s, and low production rates (low powder feed rates) typical to this process.

4.10.3.6

Cold Spraying

Cold spraying was developed in the former Soviet Union in the 1980s. Figure 23 is a schematic presentation of the cold kinetic spray equipment. Cold spraying is a thermal spray process that enables production of metallic and metallic–ceramic coatings with dense (very low porosity level) and pure (low oxygen content) structures. The process differs entirely from all other thermal spray processes, because the powder feedstock material is always unmelted; the process is a solid-state spray method. Cold-sprayed coatings have some specific characteristics compared to other thermal spray processes. Due to solid-state processing combined with high particle velocities, the metallic coatings are pure, i.e., free of oxides, and noticeably dense. In cold spraying, the heat input not only to the spray material but also to the substrate is significantly low, not changing the substrate

Figure 23

Schematic presentation of cold kinetic spray process. Source: Linde AG.

Thermal Spray Coating Processes

249

Table 11 Comparison of the characteristics of high-pressure and low-pressure cold spray processes Parameter

HPCS

LPCS

Process gas Pressure Gas preheat temperature Gas flow rate

N2, He, or mixtures 10–50 bar RT: 800–1000  C 0.85–2.5 m3 min
1 N2 Max 4.2 m3 min
1 He 4.5–13.5 kg h
1 10–50 mm 17–47 kW 1–50 mm

Air (or N2, He) 6–10 bar RT: 650  C 0.3–0.4 m3 min
1

Powder feed rate Spraying distance Electric power Particle size

0.3–3 kg h
1 5–15 mm Max. 3.3 kW 5–30 mm

Source: Tampere University of Technology.

properties. The temperature of the gas is well below the melting point of all sprayed material and thus particles are not melted in the gas flow. Some preheating of the spray powder particles is sometimes beneficial in order to soften the particles for improved deformation and flattening. Furthermore, residual stresses are usually compressive due to the shot peening effect of impinging solid particles. Cold spraying consists of two differing processes, which are high-pressure cold spraying (HPCS) and low-pressure cold spraying (LPCS) divided by the pressure level used in the processes (40 bar vs max. 10 bar). Generally, cold spraying is based on higher particle velocities and lower process temperatures than in other thermal spray processes. The coating is formed in a solid state when powder particles impact on a sprayed surface with high kinetic energy, deform, and adhere to the substrate or to other particles. Therefore, a high level of plastic deformation and adiabatic shear instability are required for a tight bonding between powder particles and for the formation of dense microstructures. Moreover, in cold spraying, many factors, e.g., powder characteristics and compositions, spraying parameters, and posttreatments affect the formation and properties of the coating. Table 11 compares the two types of cold spray processes and shows their main parameters and characteristics. Cold spray processes and coatings have been reviewed in several books, journals, and conference papers. The reader is recommended to search for more detailed information about the topic, e.g., from Refs. (31–38).

4.10.3.6.1

High-Pressure Cold Spraying

Cold spraying is divided into two different processes based on the pressure level used: HPCS and LPCS processes. The HPCS process enables the use of pressure as high as 40 bar and preheating temperature of max. 800  C; some new equipment reach the level of 1000  C. Typically, pure metal and metal alloys are used as powder materials in the HPCS process. Furthermore, the selection of powder materials is larger with the HPCS process compared to the LPCS process. Typically, metals and metal alloys, e.g., Cu, Al, Ta, Ni, Cu–Sn, NiCr, and Ni–Al are used as powder materials. Figure 24 shows a schematic presentation of the HPCS gun. Basically, in the HPCS process, highly pressurized gas (N2, He, or mixture of these gases) is heated and flows to the nozzle to which powder is fed with a carrier gas. Then, the gas–powder flow is fed through the nozzle in which high velocity is generated. After that, particle–gas flow exits the nozzle with high speed and impacts on the sprayed surface. The HPCS process is controlled with a control unit. In HPCS, a converging–diverging nozzle is used to produce supersonic gas flow in which particles achieve supersonic velocity. Supersonic velocity is created in the nozzle throat, in the diverging part of the nozzle. Powder is fed with a carrier gas in the axial direction and then mixed with the main gas forming a particle–gas flow before the nozzle. A de Laval-type converge/diverge nozzle is the most commonly used. The schematic presentation of one cold spray gun with de Laval nozzle is shown in Figure 24. The colors indicate temperatures in different regions of the gun, from warmer (red) input to colder (blue) output.

4.10.3.6.2

Low-Pressure Cold Spraying

In the LPCS process, preheating temperatures of the process gas are between room temperature (RT) and 650  C, and pressures are between 5 and 9 bar. Usually, compressed air is used as process gas to spray powder mixtures. Typically, the LPCS process is a method to spray metallic powders (e.g., Cu, Al, Ni, Zn) with an addition of ceramic powder (Al2O3, SiC) as powder mixtures.

Figure 24 Schematic presentation of cold spray gun. Colors indicate temperatures; in the red areas temperature is the highest and in the blue areas temperature is the lowest. Source: Linde AG.

250

Thermal Spray Coating Processes

Figure 25 Schematic presentation of spraying gun in LPCS process. Powder is fed radially to the diverging part of the converging–diverging-type nozzle. Some LPCS equipment have nearly barrel-type nozzles. Source: Obnisk Centre for Powder Spraying.

The main functions of ceramic powder addition are to activate the sprayed surface and to keep the nozzle of the gun clean. In addition to these, ceramic particles affect the coating by mechanical hammering of the substrate/sprayed layers or by the so-called shot peening via particle impacts. The ceramic particle addition in the powder mixtures has also a compacting effect during the impacts, indicating improved coating properties and D.E. of metallic coatings. The hard phase can also be used for reinforcing and strengthening the metallic matrix in the LPCS process. The main difference between the spray guns used in the HPCS and LPCS processes is that in the LPCS process, powder is fed to the gas flow perpendicularly (radial injection) to the diverging part of the nozzle, as presented in Figure 25. In addition to that, Papyrin et al. (32) have presented the possibility of injecting two powders, e.g., Cu þ Al, and Cu þ SiC, separately to the nozzle (two powder feeders at the same time) to form composite coatings. Additionally, another difference between the LPCS and HPCS processes is that heating of gas is done only in the spraying gun in the LPCS process, whereas in the HPCS process, the gas is preheated in a separate heating unit and also, in addition to that, in the spraying gun. Although powder feed rates are lower in the LPCS process, particle concentration in gas flow can be higher than in the HPCS process.

4.10.3.7

Suspension and Solution Spraying

In addition to cold spray processes, the use of liquids, i.e., suspensions or solutions, is one of the latest thermal spray technologies under active research and evaluation for future industrial use (8). Suspension and liquid spraying is usually done by using a plasma, HVOF, or flame spray torch as the heat source for the process. However, in order to be able to spray suspensions, specific hardware components are needed. These are a suspension feeder and a suspension injector. Nowadays suspension and solution spraying is done with only minor modification of the thermal spray torch itself; i.e., guns initially designed for this purpose do not generally exist. An exception maybe a liquid flame spraying process developed for clearly other purposes than traditional thermal spray coating. Figures 26 and 27 illustrates two different suspension spray processes; one based on plasma spraying and one on HVOF spraying techniques, respectively (8–10). Coatings sprayed using liquids or suspensions have much finer surfaces than those sprayed using powders. They are also thinner and need more combustion or electrical energy to be sprayed. This results from the fact that a considerable part of the energy is used to evaporate the liquid. The most important area of application of solution or suspension thermal spraying are nanostructured coatings that have improved mechanical or electrical properties. An important advantage is the possible cost saving. The smooth surfaces of suspension-sprayed coatings may render it unnecessary to use postspray mechanical finishing. Suspension spraying may be the last step in powder spraying as the spray torch with the sprayed material remains the same. The resulting coatings would have a gradient of crystal grain size.

Figure 26 Schematic presentation of suspension thermal spray torch-based plasma spraying with radial liquid feeding. Source: Technical University of Stuttgart

Thermal Spray Coating Processes

Figure 27 Stuttgart.

4.10.4

251

Schematic presentation of suspension thermal spray torch-based HVOF spraying with axial liquid feeding. Source: Technical University of

Formation of Sprayed Coatings

The formation of thermally sprayed coatings is fairly unique compared with many other deposition technologies. In thermal spraying, the coatings are built up from individual spray material particles that impact on the substrate surface, spread, and solidify rapidly. The particles can be in fully or partly molten state at the moment of impact in all spray processes where the feedstock material (powder or wire) is aimed to be melted or in solid state in all cold spray processes. The thermal history that the spray particles experience when traveling in-flight from the spray torch to the surface of the substrate highly influences the structure and therefore also the properties of the coatings. Such factors as the type of the spray materials and the spray process, parameters of the spray torch, kinematic parameters, and coating procedures all have their marked influences on the resulting coating structures and properties. In the next section, the formation of thermally sprayed coatings for material melting type of processes, solid-state spray processes, and spray processes involved with posttreatments is presented. Figure 28 shows the variety of process parameters in thermal spraying influencing the structure and properties of the coatings. It is evident that thermal spraying process consists of a large number of influencing parameters and factors, which are also in many cases interrelated.

4.10.4.1

Coatings from Molten Droplets

Most thermal pray processes involve melting of the feedstock material and spraying the particles in molten state. Cold spraying, and similar processes, which aim to process the material in solid state, are a clear exception. In thermal spraying with molten particles, the coatings are built up from individual spray material particles that impact on the substrate surface, spread, and solidify rapidly. The particles are primarily in a fully molten state at the moment of impact, thus allowing easy spreading of the particles on the substrate surface or on the previously deposited coating layer. The thermal history the spray particles experience from the spray torch to the surface of the substrate influence is schematically presented in Figure 29. It is evident that the material meets a highly specific thermal history starting from heating of the particles by the energy from the spray torch, melting and acceleration of the particles by the high-temperature high-velocity gas jet in millisecond-scale periods, spreading and solidification of the particles associated with rapid quenching in microseconds, and cooling of the sprayed coating layer in the next several minutes. Figure 30 illustrates the

Figure 28

Summary of various parameters influencing thermal (plasma) spray process. Source: Tampere University of Technology.

252

Thermal Spray Coating Processes

Figure 29 Thermal history of powder particles in a thermal spray process. Reproduced from Pawlowski, L. The Science and Engineering of Thermal Spray Coatings; John Wiley & Sons Ltd., 2008.

Figure 30

Principle of coating formation in thermal spraying processes from molten droplets. Source: Tampere University of Technology.

formation of the coating from molten droplets. The coatings are formed by this type in most of the thermal spraying processes, including powder and wire flame spraying, electric arc spraying, plasma spraying, and in high-velocity methods such as in HVOF and detonation spraying. It is the case even also in suspension spraying process, but, however, in somewhat different size scale. In thermal spraying, it is important to carefully optimize the spray parameters so that adequate melting would occur for high D.E. without excessive evaporation of the material. In the process, solid particles would easily rebound or remain weakly connected to the rest of the coating as unmelted ingredients. At the beginning of the coating buildup, particles impact directly onto the substrate. The phenomena occurring at this stage determine the adhesion of the coating to the substrate. The molten particles deform, become lamellae, and solidify into columnar or fine-grained equiaxial crystals. The spraying torch moves over the substrate and the first layer usually composed of 5–15 lamellae, depending on the processing parameters, such as powder feed rate, spray distance, particles size, and linear velocity of the spray torch, is formed. Molten particles on impact with a substrate (or a previously deposited coating) transform into lamellae. The transformation is associated with the processes of deformation and solidification, in turn depending on such factors as (2): l

particle velocity, size, and phase content (totally liquid, partly liquid, etc.) on impact; particle material properties in the liquid state (viscosity, surface tension, etc.); l ability of wetting the substrate by liquid particles; l temperature of substrate; as well as l substrate roughness. l

The solidification and deformation processes occur more or less simultaneously. The temperature at the particle’s interface with the substrate at impact, called the contact temperature, influences adhesion of the lamellae and, consequently, adhesion of the coating to the substrate. The mechanisms of adhesion can be, most probably, mechanical anchorage and, less probably, metallurgical bonding. The temperature in the particle bodies, which can be at present measured by using high-speed and high-sensitivity sensors, strongly influences the microstructure of the coatings. First, its evolution with time determines material solidification and cooling rates, which, in turn, influences the formation of crystal phases and crystal size.

Thermal Spray Coating Processes

253

The phenomena occurring inside the generated layer determine the crystal structure and size, porosity formation, and formation of microcracks. The torch returns after a little time, depending on the work piece size, to the same spot. Meanwhile, the surface of the layer is subjected to the action of the environment, i.e., oxidation (for metals or alloys) and/or cooling. The cooling leads to generation of residual stresses, which may, eventually, cause the coating to crack. During spraying of one layer of coating, the torch also heats up the previously deposited material. Additional heat fluxes result from solidification of the particles and their cooling down to the temperature of equilibrium. The final coating thickness is reached in a few tens to a few hundred passes of the torch over the substrate. Thereafter, the coating is cooled down to RT. This last stage of buildup is often crucial because of the generation of thermal stresses. The latter in worst case causes detachment of the entire coating from its substrate. Spraying conditions and the particular spray material predominantly influence the properties of spray coatings. Coating structure and the configuration of sprayed coatings determine the characteristics of the coating/substrate system. Spray coatings grow when individual spray particles impact the substrate surface, deform or splatter, and solidify due to heat transfer into the workpiece. Thermal spray coatings usually have lamellar and, depending on the spray technique and material, more or less porous, microcracked, heterogeneous, and anisotropic structure. Furthermore, they contain partially molten particles or particles that solidified before impacting the surface or have reacted with gas from the atmosphere. Figure 31 presents schematically possible features in thermally sprayed coatings. Figure 32 in turn shows a cross-sectional fracture and surface view of plasma-sprayed ceramic coatings. The coating consists of flattened lamellar particles layered on each other. A typical microstructure of an electric arc-sprayed metallic coating is shown in Figure 33. Typical thermally sprayed coating features such as individual flattened particles, interparticle regions, oxides, and some residual porosity is present in the coating. Novel thermal spray technologies, in which the particle velocities are high, result in coating structures largely without such voids and other features, illustrated in Figure 31. This is obtained by replacement of part of the thermal energy with high level of kinetic energy. Figure 34 presents an HVAF-sprayed very dense hard metal coating of good quality. Suspension and solution spraying has been researched actively during the past years. In suspension spraying, the coating formation is based on several stages involving evaporation of the liquid carrier, in-flight sintering of some fine solid particles, melting of the solids and agglomerates, potential evaporation from the melt, and finally, impact of the molten particles to form the coating. Figure 35 illustrates these stages in suspension thermal spraying process (39). The micrographs in Figure 36 show the structure of suspension HVOF-sprayed alumina (Al2O3) coating on a steel substrate. Figure 36(b) is a high-magnification

Figure 31

Schematic illustration of the structure of thermally sprayed coating. Source: Tampere University of Technology.

Figure 32 Microstructure of plasma-sprayed ceramic oxide coating. (a) Cross-sectional fracture structure of plasma-sprayed Al2O3 coating and (b) surface view of plasma-sprayed Al2O3 coating. Source: Tampere University of Technology.

254

Thermal Spray Coating Processes

Figure 33 Cross-sectional microstructure of arc-sprayed NiCrMo (Inconel 625) coating showing individual flattened particles, interparticle regions, oxides, and some residual porosity. Source: Tampere University of Technology.

Figure 34 Cross-sectional microstructure of good-quality HVAF-sprayed WC–10%Co–4%Cr coating showing no voids in the structure. Source: Tampere University of Technology.

Figure 35 Coating formation stages in suspension thermal spraying process. Reproduced from Pawlowski, L. Suspension and Solution Thermal Spray Coatings. Surf. Coat. Technol. June 25, 2009, 203 (19), 2807–2829.

Figure 36 Microstructure of suspension HVOF-sprayed alumina coating on steel substrate as studied by FE-SEM. (a) General view of the coating cross-section and (b) detailed microstructure of the coating. Source: Tampere University of Technology.

Thermal Spray Coating Processes

255

Figure 37 Microstructure of solution precursor HVOF-sprayed MnCo oxide coating. Reproduced from Puranen, J.; Laakso, J.; Kylmälahti, M.; Vuoristo, P. J. Therm. Spray Technol. 2013, 22, 622–630.

field-emission-scanning electron microscopic (FE-SEM) image of the structure of the coating showing areas formed from flattened molten particles with some unmelted or partially melted particles. Besides suspensions, solutions without solid particles can be used to prepare coatings by thermal spraying. Now the coating formation mechanisms may involve chemical reactions during spray processing in the gas phase forming reaction products and chemical vapor deposition or spray pyrolysis-type coating formations on the substrate surface. Figure 37 illustrates the microstructure of high-velocity solution precursor flame-sprayed manganese–cobalt oxide coating.

4.10.4.2

Formation of Sprayed and Fused Coatings

Self-fluxing alloy powders are widely sprayed by flame spray process. These coatings are usually used to a smaller extent in the assprayed state, but are posttreated by fusing the coating at high temperature of the order to 1050–1150  C, depending on the composition. These coating materials are applied by powder flame spraying or welding using commonly available oxygen– acetylene equipment. During thermal spraying or welding, the molten alloy forms a metallurgical bond with the substrate surface. Coatings thus formed are very dense and show bond strength values significantly higher than those obtained with other spray techniques. Both coating and substrate are treated by torch or furnace heating. Self-fluxing alloys contain elements that react with oxygen or oxides to form low-density oxides that float to the surface, thus improving density, bonding, and mechanical properties. In general, these are nickel- or cobalt-based alloys that contain boron or silicon, either singly or in combination, in excess of 1.5%, as well as carbon and iron. Chromium is added to many alloys for increased wear and corrosion resistance. Boron and silicon also promote wetting by reducing oxides of nickel, cobalt, chromium, and iron and control surface tension and fluidity. During fusing, a borosilicate glass is formed that is believed to further reduce oxidation. Boron and silicon are fluxing agents, hence the term self-fluxing. It is evident that the coatings prepared by flame spraying with subsequent fusing are dense and the interface also confirms the metallurgical joining to the substrate material.

4.10.4.3

Solid-State Formation in Cold Spraying

Formation of a cold kinetic-sprayed coating is definitely very different from conventional thermally sprayed coatings. The method is applicable to several pure metals and alloys. Other materials such as ceramic are not typically deposited by cold spraying. Cold spraying is a solid-state process and therefore the coating is formed without melting of the material. However, there are some similarities with other thermal spray processes. In both types of coating formation, the powder particles are accelerated to high velocities by fast gas flow, the particles impact the surface, and they spread; in thermal spraying with molten particles, the melt flows, but in cold spraying, the solid material flows due to high kinetic energy and the particles deform on impact and stick to the previous layers by explosive welding-type mechanism. In cold spraying, the powder particles need a certain critical velocity (vcrit) for successful coating formation. If the particle velocity is lower than the material-dependent critical velocity, the particles tend to rebound and do not form a coating. In turn, when the particle velocities are above the critical velocity, the particles adhere to the surface and build up the coating. Particles are stuck to the surface by plastic deformation or they just penetrate or are embedded into the sprayed surface. Formation of the cold-sprayed coating is schematically presented in Figure 38. The first particles cause the buildup of the coating (adhered particles) and substrate cratering (rebounding particles). Formation of the cold-sprayed coating is based on two steps: buildup of the first layer (particle–substrate interactions) and deposit of the coating (particle–particle interactions). In the first step, the adhesion strength between coating and substrate is defined, whereas step 2 defines the cohesive strength of the coating. In the first stage, particle–substrate interactions occur and are affected by substrate and coating materials as well as surface preparation. After that, the next incoming particles adhere to the previous particles, deform, and realign; now the coating is built up. In the second stage, particles interact with each other and form the coating structure. During the spraying, metallic bonding and void reduction occur and after that, the coating structure is densified and work hardened.

256

Thermal Spray Coating Processes

Figure 38 Coating formation in cold kinetic spraying process. Reproduced from Van Steenkiste, T.; Smith, J.; Teets, R.; Moleski, J.; Gorkiewicz, D.; Tison, R.; Marantz, D.; Kowalsky, K.; Riggs, W.; Zajchowski, P.; Pilsner, B.; McCune, R.; Barnett, K. Surf. Coat. Technol. 1999, 111, 62–71.

It is notable that cold kinetic-sprayed metallic coatings are highly deformed and thus work hardened. The hardness of cold-sprayed coatings is noticeably higher than that of the corresponding wrought metal. The elongation is low and the coating may show brittle behavior. Figure 39 presents the microstructure of a fully dense and oxide-free cold-sprayed metallic tantalum coating. Figure 40 is a higher magnification SEM image of high-pressure cold-sprayed copper coating showing the highly deformed copper particles (etched structure).

Figure 39

Structure of cold-sprayed fully dense and oxide-free tantalum metal coating. Source: Tampere University of Technology.

Figure 40

Structure of high-pressure cold-sprayed copper coating; etched microstructure. Source: Tampere University of Technology.

Thermal Spray Coating Processes

Table 12

257

Properties of several sealants for thermally sprayed coatings

Sealant

Base polymer

Curing parameters

Wetting angle on g-alumina ( )

Viscosity, h (mPa s
1)

Curing shrinkage by weight, %

Curing shrinkage by volume, %

A B C D E F G H I

Inorganic aluminum phosphate Methacrylatea Methacrylatea Phenola Epoxy Methacrylatea Epoxy Furan Vinyl ester

200  C/7 h 60  C/1 h 60  C/1 h RT 80  C/2 h UV 60  C/2 h 60  C/1 h – RT 60  C/1 h – RT

97 15 15 10 39 26 42 67 83

505 6.5 8.6 1.5 162 28 179 626 319

48 3.8 2.6 79.6 0.1 0.5 1 2 0.6

– 17.6 5.3 8 0.2 <2 0.1 2 2.4

a

Commercial sealant.

Reproduced from Knuuttila, J.; Sorsa, P.; Mäntylä, T. J. Therm. Spray Technol. 1999, 8, 249.

Figure 41

4.10.4.4

Laser fusion of thermally sprayed coating. Source: Force Technology.

Posttreatments

Thermally sprayed coatings are sometimes posttreated in order to modify the structure of the coating and to improve coating properties. Such treatments include impregnating the coatings with organic or inorganic sealers, fusing the self-fluxing alloy coatings, or treating the surfaces by laser beams, induction melting, etc. Machining and grinding of thermal spray coatings is also a conventional and widely used process, but will not be discussed here. Thermally sprayed coatings fairly often contain a noticeable amount of residual porosity, which particularly in plasma-sprayed ceramic coatings is also through porosity. Sealants are frequently used to remove the open porosity of the sprayed coatings and to provide better corrosion protection to the coating. Table 12 gives some characteristics of several polymeric sealants and one inorganic sealer for plasma-sprayed ceramic coatings (40). Important properties of the sealants are, for instance, their chemical resistance, capability to fill the pores in the coating, low curing shrinkage, wetting behavior, and thermal treatment required. As was mentioned above, there are also other techniques such as laser beam melting of the sprayed coating. Figure 41 presents such treatment operation for thermally sprayed NiCrBSi coating. In laser surface remelting, it is important to avoid excessive melting of the substrate material, which would in such case be transferred to the molten coating material and interfere with its corrosion properties by dilution with iron from the substrate. The coatings can be only partially melted, i.e., only the surface layer of the coating is treated with the laser beam. The main disadvantages related to laser beam remelting of thermally sprayed coatings are the need of a two-step process, quality of adjacent treated joints, tendency of crack formation in coatings, particularly in hard coatings, and cost of processing.

4.10.5

Coating Materials and Properties

As has been already mentioned, thermal spray technology seems to be clearly the most versatile coating manufacturing technology among all deposition technologies. This claim is particularly relevant when the availability of different coating materials and structures and the flexibility of using many of these coating materials in the same particular spray equipment are compared. Nearly all materials can be processed by thermal spray technologies into sprayed coatings; this includes all pure metals and alloys, all hard

258

Thermal Spray Coating Processes

Main groups of coating materials and their typical properties when sprayed by different common techniques

Table 13 Property

Material

Flame

HVOF

Arc

Plasma

Ferrous alloys Nonferrous alloys Self-fluxing alloys Ceramics Carbides Ferrous alloys Nonferrous alloys Self-fluxing alloys Ceramics Carbides Ferrous alloys Nonferrous alloys Self-fluxing alloys Ceramics Carbides Ferrous alloys Nonferrous alloys Self-fluxing alloys Ceramics Carbides

3000 2–6 Up to 50 14–21 7–34 83þ (fused) 14–34 34–48 0.05–2.0 0.05–5.0 0.15–2.5 0.25–2.0 0.15–0.8 35 20 30–60 40–65 45–55 3–10 3–10 <2 (fused) 5–15 5–15

2600–3000 1–9 Up to 700 48–62 48–62 70–80 – 83þ 0.05–2.5 0.05–2.5 0.05–2.5 – 0.05–5.0 45 55 30–60 – 55–72 <2 <2 <2 – <1

4000 (Arc) 10–25 Approx. 150 28–41 14–48 15–50 – – 0.1–2.5 0.1–5.0 – – – 40 35 – – – 3–10 3–10 – – –

12 000–16 000 2–10 Up to 450 21–34 14–48 – 21–41 55–69 0.4–2.5 0.05–5.0 – 0.1–2.0 0.15–0.8 40 50 30–60 45–65 50–65 2–5 2–5 – 1–2 2–3



Gas temperature ( C) Spray rate (kg h
1) Particle velocity (m s
1) Bond strength (MPa)

Coating thickness (mm)

Hardness (HRC)

Porosity (%)

Source: Sulzer Metco AG.

metal-like materials also named as cermets or carbides (e.g., WC–Co), as well as all ceramic materials with a few exceptions (magnesia and silicon nitride and carbide as pure compounds), all thermoplastics, and mixtures of these. Due to the very large variation of coating materials, it is not reasonable to deal with all these here, but to give an overview of typical thermal spray coating materials.

4.10.5.1

Coating Materials

Table 13 summarizes the main groups of coating materials and their typical properties when sprayed by different common techniques. Typical process temperatures, feedstock feeding rates, particles velocities, coating bond strength numbers, typical coating thickness, hardness, and porosities are presented.

4.10.5.1.1

Pure Metals

Pure metals are available as powders and wire. The most important pure metals in thermal spraying are aluminum (Al), zinc (Zn), copper (Cu), nickel (Ni), molybdenum (Mo), titanium (Ti), tantalum (Ta), and iron in steels. Such materials as titanium and tantalum show high reactivity with oxygen when sprayed in air atmosphere, resulting in highly oxidized coatings with unsatisfactory properties and poor sprayability. These materials require spraying in vacuum or inert gas, by VPS/LPPS techniques or shrouded plasma spraying, respectively. However, the development of cold spray technology has offered possibility to these materials to spray them in atmospheric conditions without any practical oxidation of the coating. All other pure metallic coating materials mentioned above can be processed by all normal thermal spray technologies.

4.10.5.1.2

Metal Alloys

This group of coating materials in thermal spraying is so large that it is not completely discussed here. Metallic alloys used as powders and wires are largely based on alloys of iron, nickel, cobalt, aluminum, zinc, molybdenum, and copper, i.e., metallic alloys commonly used as construction metals, and some more other alloys, which are used primarily only as sprayed coatings, e.g., zinc alloys (Zn–15%Al). Table 14 summarizes some common metal alloys used as coatings in thermal spray processing.

4.10.5.1.3

Hard Metals

Hard metal-like coatings, also referred to as carbides or cermets, are common coating materials for wear prevention (Table 15). These materials are composite materials and consist of a very high volume fraction of hard carbide phase, typically tungsten carbide (WC), chromium carbide (Cr3C2), or titanium carbide (TiC), and a metallic binder phase, typically cobalt (Co) or nickel (Ni). For improved corrosion properties for the metallic binder phase, alloying with chromium (Cr) is commonly done. The spray processes used for these materials are plasma, detonation, HVOF, and HVAF, with coating properties improving also in the same order. The compositions all show excellent wear properties. WC-based compositions can be used in operation temperatures up to 540  C in air, but higher temperatures lead to catastrophic oxidation of WC to WO3. Cr3C2-based hard metals in turn can be used at significantly higher temperatures up to 900  C in air. WC–Ni coatings can be used in mild corrosive applications and in nuclear

Thermal Spray Coating Processes

259

Examples of metal alloys for thermal spraying

Table 14 Group

Compositions

Application

Ferrous Nickel Cobalt Aluminum Zinc Copper Molybdenum

Fe–18%Cr–8%Ni, Fe–13%Cr, FeCrAl NiCrMo alloys (Inconel 625, Hastelloy, Monel 400, Ni–(5–20)%Al, NiCrAlY, etc.) Stellites, triballoys, CoNiCrAlY, etc. Al–Mg, Al–Zn, Al–Si, etc. Zn–15%Al Cu–Al, Cu–Ni, etc. Mo/NiCrBSi, etc. (note: blended powders)

Buildup, wear, corrosion Corrosion, oxidation, bond layers, etc. Wear, corrosion, oxidation Corrosion, etc. Corrosion Corrosion, sliding wear, buildup, etc. Wear, friction, bond layer

Source: Tampere University of Technology.

Table 15

Examples of hard metal-like coatings and typical compositions

Group

Compositions

Application

WC–Co WC–CoCr WC–Ni WC–NiCr WC–Alloy binder Cr3C2–NiCr TiC–Ni/Co

WC–6%Co, WC–12%Co, WC–17%Co WC–10%Co–4%Cr, WC–6%Co–8%Cr WC–12%Ni WC–20%Cr–7%Ni WC–15%Hastelloy, WC–15%FeCrAl Cr3C2–(20–50)%NiCr, e.g., Cr3C2–25%NiCr TiC–(20–40)%Ni or Co

Wear Wear, corrosion Wear, corrosion, nuclear Wear, corrosion Wear, corrosion Wear, corrosion, oxidation Wear and corrosion

Source: Tampere University of Technology.

applications where Co is not allowed to be used. Alloying of the metallic binders with Cr leads to marked improvements in the corrosion resistances in aqueous solutions.

4.10.5.1.4

Oxide Ceramics

Typical ceramic coating materials in thermal spraying are oxides of aluminum, titanium, chromium, and zirconium. Other ceramic materials may also exist as thermally sprayed coatings, such as borides, carbides without metallic binder, and nitrides, but these are not commonly used. Typical oxides for wear and corrosion applications are: l

alumina, Al2O3, as pure oxide or alloyed with other oxides, e.g., titania (2.5–40%TiO2), silica (SiO2), zirconia (20–40%ZrO2), or chromia (Cr2O3); l chromia, Cr2O3, as pure oxide or alloyed with TiO2, SiO2, or Al2O3; as well as l titania as pure oxide. Another noticeable group of ceramic oxide coating materials used in thermal spraying are the TBCs based on zirconia ZrO2. Zirconia is not used as a pure oxide due to its poor properties under thermal changes, but is instead stabilized partially or fully with such other oxides as magnesia (MgO), yttria (Y2O3), calcia (CaO), or ceria (CeO2). Other novel TBC compositions also exist.

4.10.5.1.5

Polymers

Polymeric materials such as thermoplastics can also be processed by thermal spraying. In spraying of polymers it is essential not to degrade the polymer structure by excessive heating, i.e., burning of the material. Powder flame spraying, particularly using propane and air as process gases, is the most suitable spray technique for polymers. Such polymers as polyolefines (polyethylene (PE), polypropylene (PP), copolymers), other low temperature thermoplastics (ethylene vinyl acetate, ethylene-methacrylic acid, polyamide), high-temperature thermoplastics (polyether ether ketone, polyphenylene sulfide), and some fluoropolymers (ethylene chlorotrifluoroethylene, fluorinated ethylene propylene, polyvinylidene fluoride) have been reported to be processable by thermal spraying. Particularly polyolefins such as PE and PP, and polyamides (nylon) can produce dense and corrosion-resistant coatings. Special care should be taken when spraying polymers that emit harmful species (e.g., fluoropolymers).

4.10.5.1.6

Composites and Other Materials

The category of composites is an extremely broad definition of thermal spray coating materials. All mechanical powder mixtures/ blends can be defined as composites in thermal spraying. Also, there exist a number of real composite materials, e.g., in the group of abradable coatings. These may be, for instance, nickel–graphite composites, which consist of graphite core particle clad with a layer of nickel. More information about various composite materials in thermal spraying can be found elsewhere in the literature (2,3,7).

4.10.5.2

Coating Properties

The structure and consequently the technical properties of thermally sprayed coatings are highly dependent on several factors, such as material composition, class, and feedstock material characteristics; particular spray process and process parameters used; coating

260

Thermal Spray Coating Processes

Figure 42

Factors influencing the required properties of thermally sprayed coatings. Source: Tampere University of Technology.

formation, and posttreatment procedures. Figure 42 schematically presents all main factors influencing the needed coating properties of thermally spray materials planned to fill the specific requirements of the intended technical application. Several factors are known to have influence on the structure and properties of thermal sprayed coatings: l l l l l l l l l l l l l

feedstock material type and composition; powder manufacturing methods; powder properties such as particle size, size distribution, presence of small particles, presence of excessive large particles, carbide size, e.g., in WC–Co; wire type and composition; thermal spray method; spray torch parameters, e.g., gas types and flow rates; electrical parameters such as voltage and current (in plasma); state of electrodes/nozzles; kinematic parameters, such as surface velocity, distance between passes/increment, spray distance, degree of overlapping, etc.; substrate temperature and cooling media and configuration; heat load to substrate and temperature cycling during spraying; powder/wire feeding rate and layer thickness per pass; evaporated species from the process; external dust removal and presence air knives and air cooling; as well as stability of power suppliers, powder/wire feeding, and gas flows.

The presence of such a large number of thermal spraying parameters gives huge possibilities for tailoring coating properties and producing excellent coatings with specific properties. However, this also makes the process to some extent sensitive to variations in spray conditions, which may result in deviation of coating quality. Nevertheless, thermal spray technology can be a highly reproducible deposition process if all important parameters are monitored and controlled during spraying.

4.10.5.2.1

Microstructure, Phase Structure, and Density

As was presented already in connection with the coating formation mechanisms, thermally sprayed coatings may contain several different features, which can be: l l l l l

Residual porosity, often through porosity, the amount of which, however, can vary highly depending on the spray process and type of coating material; all variations from highly porous microstructures to fully dense and oxide-free coatings can exist. Oxidation in metallic coatings, particularly at the splat interfaces. Microcracks inside individual lamellae, primarily in ceramic coatings; such microcracks with porosity may improve resistance against thermal shocks. Lack of bonding between overlapping lamellae/splats. Presence of stable crystalline phase and metastable or even amorphous phases due to rapid solidification phenomena during coating formation.

However, is spite of such possible, and to some extent harmful, features, thermally sprayed coatings can show excellent coating structures and properties, which allow their use in very many demanding and specific applications.

4.10.5.2.2

Mechanical Properties

Several thermally sprayed coatings are widely used in many applications that require good mechanical properties, such as high hardness, low friction, high ductility, etc. For instance coating hardness may vary significantly depending on the resulting coating microstructure, porosity, phase structure, and level of residual stress in the coating. Figures 43 and 44 and Table 16 present

Thermal Spray Coating Processes

Figure 43 Comparison of microhardness of hard metal-like coatings deposited by HVOF, D-Gun, APS, and VPS processes. Reproduced from Pawlowski, L. The Science and Engineering of Thermal Spray Coatings; John Wiley & Sons Ltd., 2008.

Figure 44 Comparison of hardness of different ceramic oxide coatings. Reproduced from Gärtner, F.; Voyer, J.; Qi, X.; Kreye, H.; Richter H. J.; Krömmer, W. New Challenges for Wire and Rod Flame Spraying. In Proceedings of the 6th HVOF Colloquium, Nov. 27–28, 2003, Erding.

Table 16

Microhardness of some APS and VPS ceramic oxide coatings

Coating

Atmospheric plasma sprayed

Vacuum plasma sprayed

Al2O3 Al2O3–40%TiO2 Cr2O3 TiO2

720 690 930 730

1310 810 1500 750

Reproduced from Takeuchi, J.; Nakahira, H.; Nagai, J. In Proc. 2nd Plasma Technik Symposium, Vol. 2, Switzerland, 1991.

261

262

Thermal Spray Coating Processes

some useful and comparative microhardness data for different hard metal coatings, ceramic oxide coatings, and for APS- and VPS-processed ceramic oxide coatings, respectively (41). Highest hardness values are usually obtained with high-velocity coating processes, which simultaneously produce dense coating structures and, with hard metal coatings the highest amount of retained carbides in the coating.

4.10.5.2.3

Corrosion Properties

Several factors are known to affect the corrosion properties of thermally sprayed coatings. Primarily the coating material composition is responsible for this, but it is well accepted that in most cases the corrosion properties of thermally sprayed coatings are not equal to those of the corresponding bulk material. This is mainly due to such reason as differences in coating microstructure vs bulk material. Sprayed coatings often contain porosity, which may lead to corrosion of the substrate material or at least corrosion of the coating material itself. The inhomogeneous composition and phase structure together with crevice corrosion tendency will often lead to selective corrosion inside the coating. In particular, the heterogeneous microstructure leads to selective corrosion of specific phases, sensitization, galvanic corrosion, etc. Coating porosity, often through porosity, and the presence of cracks will lead to such internal crevice corrosion, concentrated chlorine ions, low pH, substrate/interface corrosion, and finally delamination of the whole coating from the substrate. Sealing of the open porosity can prevent the electrolyte penetration into the coating structure and substrate/coating interface, thus possibly solving corrosion attacks. Ceramic coatings on mild steel or cast iron substrates are often equipped with corrosion-resistant bond layers, e.g., Ni–20Cr. A thorough presentation of corrosion properties in this context is not possible and therefore only some examples are presented in the next section. Figure 45 compares the extent of corrosion of some HVOF-sprayed hard metal coatings. According to the results, it is evident that addition of Cr to the WC–Co system clearly improves the corrosion resistance of HVOF-sprayed wear-resistant hard metal coatings. Such coatings are widely used in wear applications, in which corrosion attack may also be an issue, e.g., in the paper manufacturing industry. Such thermal spray technologies, which can produce impermeable, i.e., fully dense and unoxidized and homogeneous metallic coatings, can offer interesting possibilities for using the coatings in demanding corrosive applications. Figure 46 presents electrochemical results, which show that e.g., cold-sprayed tantalum coatings can be fully protective against corrosion by, in this case, 3.5% NaCl solutions. Such coatings are also known to be resistant to other very aggressive strongly acidic and basic electrolytes (43,44). As was discussed earlier, thermally sprayed zinc and aluminum coatings can give cathodic protection to steel structures and be used e.g., in applications exposed to the effect of sea water.

4.10.5.2.4

Wear Properties

Wear of materials includes several different types of wear phenomena and mechanisms. Typical wear types are abrasive wear, adhesive wear, erosion wear, fretting, corrosion wear, cavitation wear, etc. Due to the high versatility of thermal spray technology and the wide range of coating materials available, this technology is one of the main candidates for tackling wear problems economically and effectively. Several different coatings with different properties and characteristics exist. Wear-resistant coatings can be e.g., l

hard coatings, e.g., hard metal-like coatings and oxides; soft solid lubricating coatings, e.g., bronze, lead alloys, and tin alloys; and l hard and lubricious metallic coatings, e.g., molybdenum. l

No.

Powder

Type

1

WC–17Co

Aggl./sint.

Jetkote/propane

System/fuel

2

WC–17Co

Aggl./sint.

TopGun/ethylene

3

WC–17Co

Aggl./sint.

DJH-2600/hydrogen

4

WC–12Co

Aggl./sint.

DJH-2700/propane

5

WC–12Co

Sintered

DJH-2700/propane

6

WC–12Co

Fused

DJH-2700/propane

7

WC–10%Co–4%Cr

Sint. crush.

DJH-2700/propane

8

WC–10%Co–4%Cr

Sintered

DJH-2700/propane

9

WC–6Co–8Cr

Sintered

DJH-2700/propane

Figure 45 Comparison of corrosion behavior of some HVOF-sprayed hard metal coatings prepared from different spray powder types. Corrosion test: neutral salt spray test and corrosion test in salt spray with SO2 atmosphere. Reproduced from Schwetzke, R.; Kreye, H. J. Therm. Spray Technol. 1999, 8, 433–439.

Thermal Spray Coating Processes

263

Figure 46 Electrochemical corrosion behavior of fully dense cold-sprayed tantalum coating showing excellent corrosion resistance and protection characteristics of the coating; results for a porous tantalum coating are also included for comparison. Reproduced from Koivuluoto, H.; Näkki, J.; Vuoristo, P. J. Therm. Spray Technol. 2009, 18, 75–82.

Figure 47 Abrasion wear of thermally sprayed hard metal coatings compared with wear of hard chromium plating and mild steel. Sand abrasion test: abrasive 0.1–0.6 mm quartz sand; Fe 52, mild steel; HCr, hard chromium plating; P, plasma sprayed; H, HVOF sprayed; D, detonation sprayed; CrC, Cr3C2–25NiCr; WC, WC–12Co. Source: Tampere University of Technology.

Figure 47 compares the abrasion wear of thermally sprayed hard metal coatings with wear of hard chromium plating and mild steel. It is obvious that the high-velocity spray methods produce excellent coating properties against abrasion wear by quartz sand. Sprayed coatings show generally better properties than hard chromium plating, making thermal spraying a cost-effective alternative to hazardous electrochemical processing. Table 17 is a comparison of wear, hardness, and corrosion properties of some hard metal coatings manufactured by HVOF spraying. Figure 48 adds further information about the abrasive wear properties of several WC–10%Co–4%Cr coatings prepared from five different powders of the same composition and by different thermal spray processes, including plasma spraying, detonation, and HVOF spraying technologies. Comparative wear results of various ceramic oxide coatings are in turn presented in Figure 49.

Table 17

Comparison of properties some hard metal coatings manufactured by HVOF spraying

Coating

Hardness HV0.3

Weight loss (mg)

Corrosion behavior

WC–12%Co WC–17%Co WC–20%Cr–7%Ni WC–10%Co–4%Cr WC/TiC–17%Ni Mild steel

1171 1112 1129 1064 913 –

2 4 2 2 9 55

Corrosion at 24 h Corrosion at 24 h Uncorroded at 360 h Uncorroded at 120 h Uncorroded at 120 h Severe corrosion

Abrasion wear test: JIS H8615, abrasive is a cylinder coated with SiC emery paper. Corrosion test: JIS Z2371 salt spray test (5% NaCl, temperature 35  C). Source: Tocalo Co. Ltd.

264

Thermal Spray Coating Processes

Figure 48 Comparison of several thermally sprayed WC–10%Co–4%Cr coatings prepared by APS (Ar/H2 and Ar/He gases), two HVOF processes (CDS and DJH-2700), and detonation spraying. The five columns refer to five different powders or similar composition. Source: Tampere University of Technology.

Figure 49 Dry abrasion wear results for different ceramic coatings. Reproduced from Gärtner, F.; Voyer, J.; Qi, X.; Kreye, H.; Richter H. J.; Krömmer, W. New Challenges for Wire and Rod Flame Spraying. In Proceedings of the 6th HVOF Colloquium, Nov. 27–28, 2003, Erding.

Figure 50 Dry abrasion wear results of several different thermal spray coating materials as a function of wear distance. Reproduced from Houdkova, S.; Zahalka, F.; Kasparova, M.; Berger, L. -M. Tribol. Lett. 2011, 43, 139–154.

Figures 50 and 51 summarize and compare abrasive wear properties of several different thermally sprayed coatings with some bulk materials and other coatings (45). It is obvious that thermally sprayed coatings can provide excellent wear properties to surfaces and the results and behavior depends a lot on the specific coating material used.

4.10.5.2.5

Friction Properties

Thermally sprayed coatings can be used not only as low-friction surfaces but also to increase friction. Table 18 presents some results obtained by using a ball-on-disk tribometer, in which the mating ball was either alumina or hardened steel.

Thermal Spray Coating Processes

265

Figure 51 Dry abrasion wear results of several different thermal spray coating materials as a function of wear distance. Reproduced from Houdkova, S.; Zahalka, F.; Kasparova, M.; Berger, L. -M. Tribol. Lett. 2011, 43, 139–154. Table 18

Comparison of friction properties of several coatings in a ball-on-disk tribometer

Coating

Coeff. of wear resistance Kabr (mm3 Nm
1)  10
6

coefficient of friction (COF) for Al2O3 ball

COF for steel ball

WC–Co WC–CoCr WC–Hastelloy Cr3C2–NiCr (Ti)(Mo)(C,N)–NiCo NiCrSiB AISI 316L Cr2O3 Hardened steel Hard chrome

– – – 5.5 0.35 65 182 – 6.2 277

0.369  0.008 0.398  0.009 0.382  0.011 0.549  0.008 0.653  0.016 0.645  0.026 0.644  0.021 0.350  0.032 0.568  0.008 0.704  0.023

0.780  0.011 0.757  0.033 0.811  0.008 0.717  0.093 0.529  0.011 0.622  0.024 0.638  0.030 0.567  0.015 – –

Reproduced from Houdkova, S.; Zahalka, F.; Kasparova, M.; Berger, L. -M. Tribol. Lett. 2011, 43, 139–154.

4.10.5.2.6

High-Temperature Properties

Coatings with high-temperature properties are frequently refractory metals, ceramic materials, or metallic alloys containing various oxidation resistance-improving elements such as chromium, aluminum, and yttrium. The oxidation behavior of thermally sprayed coatings is much dependent on the composition of the coating and the microstructure.

4.10.5.2.7

Biomedical Properties

Some thermal spray coatings materials such as titanium and hydroxyapatite (HAp), are bioinert and have bioactivity properties, respectively. Titanium coatings are usually prepared so that the coating deposits as porous thus allowing the organic tissue to penetrate into the surface of various implants. Similar behavior but with bioactivity occurs with HAp coatings also applied by thermal spray techniques, e.g., plasma spraying.

4.10.6

Industrial Applications

Thermal spray coatings were originally used in the past only for repair and maintenance purposes; the technology indeed was called ‘metal spraying.’ However, soon the technology was adopted to several other application fields and today thermal spray coatings are used also much in new production, i.e., components and equipment are designed to have thermal spray coatings for high performance and durability. Applications in which a thermal spray process is one part of the full manufacturing chain are known. At present, thermal spray coatings are used in very many different industrial applications and in industrial sectors, for instance in: l

refurbishment of worn or incorrectly machined parts; wear protection against abrasion, erosion wear, adhesive wear, etc.; l corrosion protection against atmospheric and wet corrosion; l protection against oxidation and high-temperature corrosion; l thermal insulation at high temperatures, e.g., TBCs; l

266 l l l l l l l

Thermal Spray Coating Processes

electrical insulation; improvement of thermal conduction, e.g., copper coatings; providing electrically conducting layers; modification of surface properties, e.g., lowering of increasing friction properties; abradable coatings to provide tolerance-control coatings; bioinert and bioactive surfaces in human implants in medical technology; as well as decorative coatings.

Table 19 summarizes with examples industrial applications of thermal spray coatings. It is evident that thermal spray technology is the most versatile and most widely used among all coating technologies available. Thermally sprayed coatings can be manufactured by manual spraying, which finds much use due to its versatility and the importance of operator’s skill on quality. In particular, conventional thermal spray techniques such as wire and powder flame and arc spraying may often be operated manually. Figure 52 shows manual spraying in refurbishment of a worn component. Automated spraying using various levels of mechanization and robotics is surely preferred over manual spraying in order to increase coating quality, for reproducibility, and to eliminate potential health and environmental risks. Figure 53 shows an example of an automated mass production system for grit blasting and coating of synchronizer rings by wire flame spraying. The capacity of such installation is approximately 500 rings per hour in three shifts. Large components over 10 m in length, such as rolls and cylinders for paper machines, are frequently coated in large spray booths such as the one presented in Figure 54. The cylinders are loaded into the spray booth through opened roofs. Figure 55 illustrates HVOF spraying of a large paper machine cylinder by HVOF spray process. The spraying is done in a separate spray booth and the spraying procedure is controlled and monitored remotely from the operator’s spray control panel. In the next Table 19

Industrial applications of thermal spray coatings

Application

Material

Corrosion protection of steel structures, steel parts in bridges, ship hulls, exhaust tubes, offshore oil drilling platforms, etc. Corrosion protection of parts, rolls and cylinders, pump shafts, pump housings, propeller shaft, etc. Corrosion and wear protection, sealing surfaces of pumps, sleeves, wear parts, mechanical seals, feeding screws, cylinders, rolls, etc.

Zinc, Zn–15%Al alloys; aluminum, Al–5%Mg alloys

Thermal barrier coatings; combustion chambers, transition ducts, blades and vanes, piston crowns, exhaust valves, etc. Oxidation, hot corrosion, erosion; heat exchangers surfaces in boilers, superheaters, water wall tubes, etc. Fretting wear in gas turbines

Abrasion wear resistance; guider rolls, shafts, piston rings, piston rods, feeding screws, drawing drums Adhesive wear resistance; piston rings, gear guides, synchronizing rings, drawing drums, drawing tools, etc.

Erosion and cavitation resistance; blower blades, wear plates, pump housings, etc.

Nonslip surfaces, friction surfaces, floor plates, rolls, paper feeders, etc.

Electrical conducting layers, heaters, condensators, electrical contacts, etc. Electrically insulating coatings, alternator parts, heaters, corona rolls, welding nozzles, etc. Electromagnetic shielding, housings for electrical devices Clearance surfaces; abradable coatings in gas turbines and turbine compressors Repair of mismachined components, shafts, pump parts, drums, bearing surfaces, etc.

Source: Tampere University of Technology.

Stainless steels, Inconel 625, Hastelloy C 276 Chromium oxide Chromium steels Stainless steels Self-fluxing alloys (NiCrBSi) Zirconium oxide (partly and fully stabilized) FeCrAl alloys Chromium steels Self-fluxing alloys Tungsten carbide (<500  C) Chromium carbide (<815  C) Cu–Ni–In Chromium steels Tungsten carbide Self-fluxing alloys Molybdenum Aluminum bronze Tungsten carbides Babbitt metals Chromium steels Aluminum bronze Tungsten carbide Chromium carbide Self-fluxing alloys Aluminum Chromium steels Tungsten carbide Silver, tin, aluminum, copper, nickel–chromium Aluminum oxide Zinc, aluminum, tin Nickel–graphite, nickel aluminum, aluminum– polyester, nickel–bentonite Chromium steels, stainless steels, special steel grades, aluminum bronze, babbitt metals, selffluxing alloys

Thermal Spray Coating Processes

Figure 52

Manual powder flame spraying in refurbishment. Source: Kuopion Konepaja Oy.

Figure 53

Automated production system for grit blasting and coating of synchronizer rings by wire flame spraying. Source: GTV mbH.

Figure 54

Thermal spraying booth for coating large paper machine rolls and cylinders. Source: Vaahto Group Oy.

267

268

Thermal Spray Coating Processes

Figure 55

Spraying of a large paper machine cylinder by HVOF spray process. Source: Metso Paper Inc.

section, some industrial examples of thermally sprayed coatings are reviewed. The aim is not to present all possible applications, but instead to show the high versatility of thermal spraying in very diverse industrial sectors.

4.10.6.1

Mechanical Engineering

In mechanical engineering sectors, thermally sprayed coatings are widely used in restoration and in providing specific surface properties such are wear and corrosion protection, friction properties, and heat resistance. The application area is extremely wide. Wear resistance of surfaces is a complex issue depending on the whole system, which includes the wear mode present, material properties, and properties of the tribosystem. Besides the material combinations of the mating surfaces, this is influenced by many factors including relative motion of the mating surfaces, strain properties, roughness of surfaces, atmosphere, temperature, etc. The main wear mechanisms that can also occur simultaneously in certain applications are adhesive wear, abrasive wear, erosion wear, surface fatigue, fretting, and corrosion wear. To avoid adhesive wear, metallurgical interactions between friction partners by microwelding have to be prevented. In engine applications, this can be realized by producing thermal spray coatings with a porous structure that picks up and holds the lubricating oil; plasma-sprayed coating based on a mixture of molybdenum and self-fluxing alloy NiCrBSi is one good choice. In abrasive wear applications, the surfaces require thermally sprayed coatings with high and uniform hardness. For this, homogeneous and dense coatings are needed. HVOF spraying and its variants, e.g., HVAF, are optimal for producing such wearresistant coatings. Typical materials are WC–Co and NiCrBSi þ WC–Co mixtures. Coatings used in applications where surface fatigue is a problem have to be uniform and free from pores, cracks, or delaminations. Therefore, very dense coatings produced under protective gas atmosphere in vacuum, by HVOF, or by HVAF are best suited. The coating materials can be hard metallic alloys or carbide-containing coatings. In corrosive wear applications, iron-, cobalt-, or nickel-based alloys or sealed chromium oxide and aluminum oxide ceramic coatings have often be used. Typical industrial applications are, e.g., sealing surfaces of pumps, valves, sleeves, wear parts, mechanical seals, feeding screws, shafts etc. Figure 56 shows HVOF-coated ball valve parts after spraying.

4.10.6.2

Corrosion Protection in Infrastructures and Steel Structures

Thermal spray coatings used for corrosion protection are either coatings providing cathodic protection or coatings that are more noble than the substrate material. In the latter case, the coating needs to be impermeable, either directly in the as-sprayed state or after postsealing treatment. In case of cathodic corrosion protection, the component (substrate) is as a cathode, whereas the thermally sprayed zinc or aluminum coating is used as a sacrificial anode in the electrochemical circuit. Using more noble materials as a coating involves opposite charge of coating and substrate, and therefore formation of an electrochemical circuit must be prevented. The only possibility now is to deposit dense, corrosion-resistant coatings or sealed ceramic coatings. Ceramic coatings definitely often require additional dense Ni–20%Cr or Ni–5%Al bond layers in order to protect the steel or cast iron substrate from corrosion and to guarantee good bond strength for the ceramic coating. Applications in infrastructure are mainly based on using active coating materials such as zinc and aluminum alloys, which provide cathodic corrosion protection to steel structures. Examples of applications are, e.g., corrosion protection of steel structures, steel parts in bridges, ship hulls, exhaust tubes, off-shore oil drilling platforms, etc.

4.10.6.3

Paper and Pulp

Applications of thermally sprayed coatings in the paper and pulp industry are numerous. The surface areas of paper machine rolls are large, which imposes a number of special requirements on the reliability of thermal spray processes. The use of spray materials in

Thermal Spray Coating Processes

Figure 56

269

HVOF-sprayed WC–10Co4Cr coatings on valve parts prior to surface grinding and polishing. Source: Tampere University of Technology.

these applications is also large. Examples of applications in the pulp and paper industry are pulp digesters, blow tanks, suction rolls, calendar rolls, center press rolls, dryer rolls, yankee dryer cylinders, coater blades, and creping blades. Several other applications exist including water removal elements etc. Figures 57–59 presents HVOF spraying of a large center press roll with HVOF, ceramic-coated center press roll, and on-site coating of a large yankee dryer cylinder, respectively.

Figure 57 HVOF spraying of a center press roll with corrosion-resistant metal alloy coating for bond layer of ceramic top coating. Source: Metso Paper Inc.

Figure 58

Paper machine center press roll with a thermally sprayed ceramic coating. Source: Metso Paper Inc.

270

Thermal Spray Coating Processes

Figure 59

4.10.6.4

Thermal spray coating of a yankee dryer cylinder. With permission of Tocalo Co. Ltd.

Gas Turbines

Both aeroengine and industrial gas turbine use various different thermally sprayed coatings. The types of applications are oxidation- and corrosion-resistant coatings on turbine blades and vanes; TBCs in combustion chambers, transition ducts, and blades and vanes; abradable coatings as seals in the compressor and turbine sector; wear- and erosion-resistant coatings; and fretting-resistant coatings. Corrosion protection in elevated and high-temperature applications with metal alloys is only possible by using relatively complex metal alloys that are able to form Al2O3, SiO2, or Cr2O3 surface oxide layers at high-temperature exposure. Often the content of these alloying elements in the coating has to be sufficiently high in order to form the protective oxidic layer several times. Coating materials called MCrAlY alloys are thermal spray coating materials developed specifically for oxidation and hot corrosion-resistant coatings on gas turbine blades and vanes. The basis for these alloys is a nickel, cobalt, or sometimes iron alloy that is adjusted for particular applications by adding a large number of additional alloying elements. The ‘M’ represents one or more of the elements iron, nickel, or cobalt. In the terminology of the business, these then become FeCrAlY, NiCrAlY, CoCrAlY, NiCoCrAlY, etc. These alloys form complex oxides from the aluminium, chromium, and yttrium, which serve as oxygen and corrosion barriers to protect the less-resistant substrate. These coatings are often combined with ceramic top coats, the TBCs. MCrAlY coatings are traditionally applied by VPS/LPPS processes, in which oxidation of the alloying elements such as Cr, Al, and Y is totally avoided. Coated blades and blades are normally heat-treated to create the creep properties of the base material; such heat treatment consists of a solution treatment at high temperatures well above 1100  C followed by a lower temperature precipitation treatment. The treatment procedures depend on the substrate material. Nickel–graphite abradable coatings have been used successfully in turbine and engine applications for many years. After wear-in during start-up of the engine, the coatings prevent performance loss due to friction of expanding components against the housing of the turbine at the operating temperature. The use of TBCs is expanding with increasing use in power generation. Yttria-stabilized zirconia (YSZ) is used as a TBC on components in the hot section, i.e., turbine section, of aircraft engines and industrial gas turbines. The low thermal conductivity of TBC materials facilitates increased performance by lowering component temperatures in the presence of high-temperature combustion. YSZ has a relatively high coefficient of thermal expansion (CTE) and is near that of the nickel- and cobalt-based superalloys used for turbine components. This fortuitous CTE match minimizes stress induced by differential expansion between the coating and its substrate. To a lesser extent, other materials are used as stabilizers, including calcia (CaO), ceria (CeO2), and magnesia (MgO). Stabilization is necessary to eliminate the volumetric change associated with the phase transformation of zirconia from monoclinic to tetragonal during thermal excursions, which occurs as the turbine ramps from ambient to operational temperatures. Figures 60–62 illustrate some thermal spray coating applications in gas turbines.

4.10.6.5

Textile Industry

Various thermally sprayed coatings, particularly plasma-sprayed wear-resistant oxides, are used in machines in textile industry. These coatings are characterized by a precise definition of the morphology produced using various handling methods and the topology of the surface. The surface texture is of particular importance for production components in contact with textile thread. In order to maximize fiber production, coatings of ceramic oxides are used, usually with a nickel-based bond coat that also provides corrosion protection. Some coated parts for textile industry are represented in Figure 63.

4.10.6.6

Printing Industry

Printing and graphic industries have also been users of thermally sprayed coatings. Plasma-sprayed electrically insulating ceramic coatings based on alumina are used in corona discharge rolls to activate the surface of PE films. Corona rolls are typically approximately 5 m long and the diameter is between 250 and 500 mm.

Thermal Spray Coating Processes

Figure 60

Section of gas turbine with abradable coating for clearance control. Source: Patria Aviation Oy.

Figure 61

Thermal barrier-coated surface in a military aeroengine afterburner. With permission of Patria Aviation Oy.

Figure 62

Robotized plasma spraying of a transition duct of an industrial land-based gas turbine power plant. Source: Patria Aviation.

271

272

Thermal Spray Coating Processes

Figure 63

Wearing parts with plasma-sprayed coatings for textile industry applications. Source: Advanced Coating S.A.

Another application is coating of the anilox rolls. Such roll is a cylinder usually constructed of a steel or aluminum core that is coated by plasma-sprayed chromia and laser engraved to contain millions of very fine small cavities known as cells. The function of the anilox roll is to transfer ink in the printing process.

4.10.6.7

Electrical Industry

Thermally sprayed coatings can also be used in electrical and electronics industry. Zinc coating prepared by electric arc spraying is used as metallization of capacitor ends. Plasma- and HVOF-sprayed coatings can be used for electrical insulation; an example of industrial use is an electrical insulating coating in the alternators of cars. Other applications are the use of electrically conducting coatings for various purposes, including electromagnetic shielding.

4.10.6.8

Steel Industry

Examples of use of thermally sprayed coatings in steel industry are in continuous annealing lines used for the production of sheets and in continuous galvanizing lines in the galvanizing section, which includes sink rolls. The rolls in the high-temperature annealing zone are coated by Cr2C3–NiCr by HVOF and detonation spraying. Cermet coatings including MCrAlY as the metallic part (15–60 vol.%) and oxides or carbides as reinforcements are also used. The sink rolls are made of stainless steel and are coated with WC–Co coatings, and also with oxide coatings such as Al2O3, Al2O3–TiO2, and Al2O3–ZrO2. The preferred deposition techniques are HVOF and detonation spraying. Figures 64 and 65 present examples of thermal spray coating applications in steel industry.

4.10.6.9

Biomedical Industry

Biomedical applications are a relatively novel use for thermal spray processes and coatings. Metallic coatings, particularly titanium and its alloys, are sprayed onto metal prostheses in vacuum processes to provide nonoxidized titanium coatings. The coatings are bioinert surfaces with sufficient roughness and porosity allowing mechanical interlocking of the bone tissue. Bioactive HAp or fluorapatite coatings with bone-type phosphate structures have even better compatibility and performance. These materials allow growth of bone tissue on the surface of the prosthesis. Different human spare parts, e.g., hip, knee, and tooth implants, are today coated by vacuum and atmospheric plasma processes. Hip implants with thermally sprayed coatings are illustrated in Figure 66. Novel approaches in this field of industry are the use of cold spray technology for titanium spraying.

Figure 64

Coated rolls for continuous annealing lines used for the production of sheets. Source: Advanced Coating S.A.

Thermal Spray Coating Processes

Figure 65

Thermally sprayed sink rolls for continuous galvanizing of steel sheet metal. Source: Advanced Coating S.A.

Figure 66

Hip implants with thermally sprayed coatings. Source: Sulzer Metco Inc.

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4.10.6.10 Power Generation Plants Thermally sprayed coatings are also widely used in various power generation plants including fluidized bed combustors, soda boilers, and waste incineration plants. The main functions of the coatings are to provide resistance against erosion wear, oxidation, and high-temperature corrosion. Energy industry uses coatings that are applied by combustion wire, electric arc, plasma, and HVOF spray processes. Wear- and corrosion-resistant coatings are used in surfaces of heat exchanger such as the water walls, superheater tubes, etc. The coating materials used are usually wear- and corrosion-resistant materials such as ironbased alloys, e.g., FeCrAl and FeCrTi; nickel-based alloys such as NiCr, NiCrMo (Inconel 625), and NiCrBSi self-fluxing alloys; and carbide-containing materials such as Cr3C2–NiCr. Several other materials including oxides are also used. Electric arc spraying and HVOF are the most common techniques, but other novel techniques such as HV-Arc and HVAF are also used. Sprayed coatings need to be dense if such coatings are used in very difficult corrosive conditions such as waste boilers. Figure 67 presents manual arc spraying of boiler tubes and Figure 68 mechanized spray with wire flame spray process using multiple torches.

4.10.6.11 Miscellaneous Applications Further examples of special thermal spray coating applications are electrically insulating Al2O3, superconductive YBaCu oxide with perovskite-type structure, oxygen ion conduction in fuel cells, molybdenum and tungsten coatings for high-vacuum technology, and decorative coatings on household products, which are often Al2O3–TiO2 coatings. Figures 69 and 70 present some special applications of thermally sprayed coatings. Ball bearings in Figure 69 have electrically insulating coatings to prevent degradation of the balls by stray current-induced sparking. Figure 70 in turn presents a plasma sprayformed free-standing part for lambda sensors of car engines. The coating material is magnesium aluminate, i.e., spinel oxide ceramic. The current and future applications of thermally sprayed coatings are numerous. More detailed information about applications can be found, e.g., from the books in the references.

274

Thermal Spray Coating Processes

Figure 67

Manual arc spraying of boiler tubes on-site. Source: Telatek Oy.

Figure 68

Mechanized spraying of boiler tubes with three wire flame spray torches. Source: Tocalo Co. Ltd.

Figure 69

Ball bearings with electrically insulating coatings. Source: Tocalo Co. Ltd.

Thermal Spray Coating Processes

275

Figure 70 Plasma spray-formed free-standing part for lambda sensor. Coating material is magnesium aluminate spinel ceramic. Source: Advanced Coating S.A.

4.10.7

Development Trends

Thermal spray technology is continuously developing and new applications are seen for thermally sprayed coating materials and structures. Main development trends are in the following technical areas: l l l l l l l l l

thermal spray business is smoothly growing in all sectors of use; thermal spray processes used are becoming more cost-effective, more robust, less operator dependent, and more productive; coatings are more reproducible and properties are more predictable; new functional properties are used in applications, e.g., electrical properties, photocatalytic properties, etc.; novel spray technologies have been recently taken into research stage and industrial use; examples are HVAF, cold spraying, suspension plasma spraying, high-velocity suspension/liquid spraying, direct write process, etc.; thermal spray coatings are found in new applications, e.g., in fuel cells, as hard chromium alternatives, etc.; novel coatings materials and structures are studied and used; e.g., novel nano/submicron-sized carbides, coatings with nanosized crystals, nanocomposite materials, cost-efficient spray materials, liquids as feedstock materials, suspensions, etc.; hybrid processes are studied and used; these include, for instance, combining thermal spray processes with laser novel laser materials processing; and life cycle assessments, environmental considerations, and sustainable use of spray materials and processes.

4.10.8

Concluding Remarks

The present overview of thermal spray coating processing is a generic summary about the topic trying to comprehensively cover many important related aspects and to give the reader a general knowledge about the technology, formation of the coatings, coating materials with their properties, and industrial applications of the coatings. The numbers of available processes, coating materials and properties, and applications for thermal spray coatings are practically endless and cannot be perfectly covered in the frame of this chapter. Additional information is available from high-level journal papers such as Journal of Thermal Spray Technology (ASM International) and in related conferences, e.g., ITSC and in its proceedings. Societies such as ASM/TSS, German Gesellschaft für Termisches Spritzen, and ETSA promote the thermal spray technology and its science, technology, and application.

References Siegmann, S.; Abert, C. 100 Years of Thermal Spray: About the Inventor Max Ulrich Schoop. Surf. Coat. Technol. 2013, 220, 3–13. Davis, J. R., Ed. Handbook of Thermal Spray Technology; ASM International, 2013. Pawlowski, L. The Science and Engineering of Thermal Spray Coatings; John Wiley & Sons Ltd: Hoboken, NJ, 2008. Matejka, D.; Benko, B. Plasma Spraying of Metallic and Ceramic Materials; John Wiley & Sons: New York, 1989; p 280. Heimann, R. B. Plasma-spray Coating – Principles and Applications; VCH Verlagsgesellschaft mbH: New York, 1996; p 339. Schneider, K. E.; Belashchenko, V.; Dratwinski, M.; Siegmann, S.; Zagorski, A. Thermal Spraying for Power Generation Components; Wiley-VCH: Weinheim, 2006; p 271. Beczkowiak, J. Spray Materials. In Modern Surface Technology; Bach, Fr.-W., Laarmann, A., Wenz, T., Eds.; Wiley-VCH: Weinheim, 2006; pp 137–144. Pawlowski, L. Suspension and Solution Thermal Spray Coatings. Surf. Coat. Technol. 2009, 203, 2807–2829. Toma, F.-L.; Berger, L.-M.; Naumann, T.; Langner, S. Microstructures of Nanostructured Ceramic Coatings Obtained by Suspension Thermal Spraying. Surf. Coat. Technol. 2008, 202, 4343–4348. 10. Bolelli, G.; Cannillo, V.; Gadow, R.; Killinger, A.; Lusvarghi, L.; Rauch, J. Properties of High Velocity Suspension Flame Sprayed (HVSFS) TiO2 Coatings. Surf. Coat. Technol. 2009, 203, 1722–1732. 1. 2. 3. 4. 5. 6. 7. 8. 9.

276

Thermal Spray Coating Processes

11. Monterrubio-Badillo, C.; Ageorges, H.; Chartier, T.; Coudert, J.-F.; Fauchais, P. Preparation of LaMnO3 Perovskite Thin Films by Suspension Plasma Spraying for SOFC Cathodes. Surf. Coat. Technol. 2006, 200, 3743–3756. 12. Puranen, J.; Laakso, J.; Kylmälahti, M.; Vuoristo, P. J. Therm. Spray Technol. 2013, 22, 622–630. 13. Oberste, J.; Marple, B.; Moreau, C. Suspension Plasma Spraying Nanostructured WC–12Co Coatings. J. Therm. Spray Technol. 2006, 15, 676–681. 14. Great Britain. Wear Resistant Surfaces in Engineering – A Guide to Their Production, Properties and Selection; London Her Majesty’s Stationery Office, Department of Trade and Industry: UK, 1985. 15. Burakowsky, T.; Wierzchon, T. Surface Engineering of Metals; CRC Press LLC: Boca Raton, FL, 1999. 16. Toyserkani, E.; Khajepour, A.; Corbin, S. Laser Cladding; CRC Press: Boca Raton, FL, 2005. 17. Quint, M.; Kopech, H. High Energy Plasma Ceramic Coating Optimization. In 23rd Annual Conference on Composites, Advanced Ceramics, Materials, and Structures; Cocoa Beach, FL, USA; 1999; Ceramic Engineering and Science Proceedings. 1999 Ceramic Engineering and Science Proceedings. Vol. 20, Issue 4, pp 335–345. 18. Chráska, P.; Hrabovsky, M. An Overview of Water Stabilized Plasma Guns and Their Applications. In Proceedings of the International Thermal Spray Conference & Exposition; ASM International: Orlando Florida, USA, 28.5.–5.6.1992; pp 81–85. 19. Sampath, S.; Gansert, R.; Heramn, H. Research Summary: Plasma-spray Forming Ceramics and Layered Composites. JOM (USA) 1995, 47 (10), 30–33. 20. Kreye, H.; Gärtner, F.; Kirsten, A.; Schwetzke, R. High-velocity Oxy-fuel Flame Spraying State of the Art, Prospects and Alternatives. In 5. Colloquium on High Velocity Flame Spraying; GTS e.V.: Erding, Nov. 16–17, 2000. 21. Nestler, M. C.; Benary, R.; Rusch, W. Innovations in HVOF Coating Solutions. In 5. Colloquium on High Velocity Flame Spraying; GTS e.V.: Erding, Nov. 16–17, 2000. 22. Kawakita, J.; Katanoda, H.; Watanabe, M.; Yokoyama, K.; Kuroda, S. Warm Spraying: An Improved Spray Process to Deposit Novel Coatings. Surf. Coat. Technol. 2008, 202, 4369–4373. 23. Kuroda, Seiji; Watanabe, Makoto; Kim, KeeHyun; Katanoda, Hiroshi Current Status and Future Prospects of Warm Spray Technology. J. Therm. Spray Technol. June 2011, 20 (4), 653. 24. Kawakita, J.; Kuroda, S.; Fukushima, T.; Katanoda, H.; Matsuo, K.; Fukanuma, H. Dense Titanium Coatings by Modified HVOF Spraying. Surf. Coat. Technol. 2006, 201, 1250–1255. 25. Kuroda, S.; Kawakita, J.; Watanade, M.; Katanoda, H. Sci. Technol. Adv. Mater. 2008, 9, 17. 26. Verstak, A.; Baranovski, V. Deposition of Carbides by Activated Combustion HVAF Spraying. In Proceedings of the International Thermal Spray Conference, Osaka, Japan; 10 May 2004 through 12 May 2004; pp 551–555. 27. Verstak, A.; Baranovski, V. Activated Combustion HVAF Coatings for Protection against Wear and High Temperature Corrosion. In Marple, B. R., Moreau, C., Eds., Vol. 1; ASM International: Orlando, Florida, USA, 5–8 May, 2003; pp 535–541. 28. Verstak, A.; Baranovski, V. HVAF Arc Spraying. Osaka, Japan. In Proc. Int. Thermal Spray Conference; 2004; pp 696–701. 29. Kharlamov, Y. A. Bonding of Detonation-Sprayed Coatings. Thin Solid Films 1978, 54, 271–278. 30. Astakhov, E. A.; Klimenko, V. S.; Skadin, V. G.; Zverev, A. T. Sov. Powder Metall. Met. Ceram. 1982, 21, 307–310. 31. Champagne, V., Ed. The Cold Spray Materials Deposition Process: Fundamentals and Applications; Woodhead Publishing Ltd.: Cambridge, England, 2007; p 362. 32. Papyrin, A.; Kosarev, V.; Klinkov, S.; Alkimov, A.; Fomin, V. Cold Spray Technology, 1st ed.; Elsevier: Netherlands, 2007; p 328. 33. Grujicic, M.; Zhao, C.; Tong, C.; DeRosset, W.; Helfritch, D. Analysis of the Impact Velocity of Powder Particles in the Cold-gas Dynamic-spray Process. Mater. Sci. Eng. A 2004, 368, 222–230. 34. Dykhuizen, R.; Smith, M. Gas Dynamic Principles of Cold Spray. J. Therm. Spray Technol. 1998, 7 (2), 205–212. 35. Borchers, C.; Gärtner, F.; Stoltenhoff, T.; Assadi, H.; Kreye, H. Microstructural and Macroscopic Properties of Cold Sprayed Copper Coatings. J. Appl. Phys. 2003, 93 (12), 10064–10070. 36. Maev, R.; Leshchynsky, V. J. Therm. Spray Technol. 2006, 15, 198–205. 37. Van Steenkiste, T.; Smith, J.; Teets, R.; Moleski, J.; Gorkiewicz, D.; Tison, R.; Marantz, D.; Kowalsky, K.; Riggs, W.; Zajchowski, P.; Pilsner, B.; McCune, R.; Barnett, K. Kinetic Spray Coatings. Surf. Coat. Technol. 1999, 111, 62–71. 38. Klassen, T.; Kliemann, J. O.; Onizava, K.; Donnen, K.; Gutzmann, H.; Binder, K.; Schmidt, T.; Gärtner, F.; Kreye, H. Cold Spraying – New Developments and Application Potential. In Proceedings of the 8th Colloquium of HVOF Spraying, Nov. 5–6, 2009, Erding. 39. Pawlowski, L. Suspension and Solution Thermal Spray Coatings. Surf. Coat. Technol. June 25, 2009, 203 (19), 2807–2829. 40. Knuuttila, J.; Sorsa, P.; Mäntylä, T. Sealing of Thermal Spray Coatings by Impregnation. J. Therm. Spray Technol. 1999, 8, 249. 41. Gärtner, F.; Voyer, J.; Qi, X.; Kreye, H.; Richter H. J.; Krömmer, W. New Challenges for Wire and Rod Flame Spraying. In Proceedings of the 6th HVOF Colloquium, Nov. 27–28, 2003, Erding. 42. Schwetzke, R.; Kreye, H. Microstructure and Properties of Tungsten Carbide Coatings Sprayed with Various High-Velocity Oxygen Fuel Spray Systems. J. Therm. Spray Technol. 1999, 8, 433–439. 43. Koivuluoto, H.; Näkki, J.; Vuoristo, P. Corrosion Properties of Cold-Sprayed Tantalum Coatings. J. Therm. Spray Technol. 2009, 18, 75–82. 44. Koivuluoto, H.; Bolelli, G.; Lusvarghi, L.; Casadei, F.; Vuoristo, P. Corrosion Resistance of Cold-Sprayed Ta Coatings in Very Aggressive Conditions. Surf. Coat. Technol. 2010, 205, 1103–1107. 45. Houdkova, S.; Zahalka, F.; Kasparova, M.; Berger, L.-M. Comparative Study of Thermally Sprayed Coatings Under Different Types of Wear Conditions for Hard Chromium Replacement. Tribol. Lett. 2011, 43, 139–154.