Design and evaluation of tribological coatings

Wear 246 (2000) 20–33

Design and evaluation of tribological coatings Sture Hogmark, Staffan Jacobson∗ , Mats Larsson The Ångström Laboratory, Tribomaterials Group, Uppsala University, Box 534, SE 751-21 Uppsala, Sweden Received 24 September 1999; received in revised form 29 November 1999; accepted 29 November 1999

Abstract The use of coatings to improve the tribological properties of components such as tools for metal cutting and forming, and machine elements e.g. sliding bearings, seals and valves is constantly increasing. This paper presents tribological design considerations of coating composites and recommends methods and techniques for their evaluation. It is aimed both for those involved in scientific development of coatings and for the practising tribology engineer. Current design concepts of coatings for tools and machine elements are exemplified together with successfully coated components. Techniques for evaluation of some of the more important fundamental and tribological properties will be presented and several examples are given mainly with thin physical vapour deposition (PVD) coatings. The evaluation techniques can be utilised both in coatings selection and in development of new coatings. Some recent trends and speculations about the future end the paper. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Tribological coatings; Optimum topography; Tribological response in field tests

1. Introduction 1.1. Coating composites In the development of modern materials, the functionality is often improved by combining materials of different properties into composites. Many classes of composites exist, most of which are addressing improved mechanical properties such as stiffness, strength, toughness and resistance to fatigue. Coating composites (i.e. surface engineered materials) are designed to specifically improve functions such as tribological, electrical, optical, electronic, chemical and magnetic, see Fig. 1. It is thus natural to select the bulk of a component to meet the demands for stiffness, strength, formability, cost, etc. and then modify or add an other material as a thin surface layer. This surface layer or coating is the carrier of virtually all other functional properties. Application of coatings on tools and machine elements is, therefore, a very efficient way of improving their friction and wear resistance properties. The obvious aim of applying tribological coatings is to obtain an increased lifetime. There are, however, several other positive effects. • The improved wear resistance of coated metal cutting tools is usually utilised to increase the cutting speed and ∗

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thereby the productivity, rather than to give a prolonged tool life. • Reduced friction often means reduced energy consumption. In some cases, a lowered friction may permit the exclusion of lubrication or of cooling stages. • Increased or controlled friction may be a beneficial effect in other applications such as brakes, bolted joints and safety connectors. • Reduced tendency to sticking and material pick up from the counter surface is crucial to the performance of forming tools and many sliding applications. Anti sticking agents may be omitted in forming tool applications. • Components of reduced weight can be designed by application of coatings. Reduced weight means e.g. an increased ratio of power to weight of car engines, which in turn may give lower fuel consumption. In combination with the rapid development of new coating technologies, this has led to an accelerating increase in the use of coated components. This paper is primarily restricted to thin (1–10 ␮m) physical vapour deposited (PVD) and chemically vapour deposited (CVD) coatings. 1.2. Coating damage in tribology It is important to realise that the mechanical contact between solids is localised to microscale contact spots that together form the real contact area.

0043-1648/00/$ – see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 ( 0 0 ) 0 0 5 0 5 - 6

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Fig. 1. Types of composite materials and the functional improvements addressed.

Irrespective of the nominal (or geometrical) contact area, a good estimate of the real contact area is in many situations obtained by dividing the normal load with the hardness of the softer of the two mating surfaces. In other words, the contact pressure in these tiny areas is of the order of their hardness irrespective of the nominal contact pressure. Bearing this in mind, it is not surprising that damage can occur even in apparently very mildly loaded tribological contacts. One such example is the damage occurring on ceramic seals in sliding contact despite the nominal contact pressure being a hundred thousand times lower than their hardness [1]. A tribological coating can fail prematurely due to detachment, delamination, cracking and/or spalling of the coating material. Similar damage mechanisms rarely occur for homogeneous materials. On the other hand, the typical mechanisms for gradual wear are the same for coatings as for homogeneous materials. This paper emphasises situations where the properties of the coating, the substrate and the interface play significant roles. The coating shall be selected to match the tribological situation and the life limiting surface damage mechanisms of the intended application. A classification of these mechanisms into three categories is convenient [2]: • Damage without exchange of material. • Damage with loss of material, i.e. wear. • Damage with material pick up. 1.2.1. Damage without exchange of material This category basically involves permanent changes in component geometry and/or in surface topography. Decisive parameters for a change in geometry are Young’s modulus and hardness of coating and substrate, and coating toughness, cp. Section 4.3. Tiny scratches or cracks may disqualify a forming tool used to e.g. press compact discs. Coating hardness is the crucial parameter for scratch resistance, and coating toughness or fracture resistance for the resistance to surface cracking.

1.2.2. Wear (damage with loss of material) The wear resistance of a coated component is mainly determined by the coating as long as it covers the contact area. As soon as the coating is partly worn through, or the substrate is exposed due to adhesive failure or cracking and spalling, the wear resistance of the substrate material becomes important. An in-depth description of the large number of wear mechanisms found in the applications of coated components is given in [3]. However, two main categories can be distinguished: wear dominated by coating detachment and wear caused by gradual removal of coating material. The latter often involves mild wear due to abrasion, erosion, chemical dissolution, etc., and does not deviate from the mechanisms causing wear of homogeneous materials. Suitable evaluation techniques will be given in Section 4.4. Generally, tribological applications put higher demands on the coating adhesion than any other area of application, although the demands may differ substantially from one situation to the other. It is instructive to distinguish between the actual adhesive forces (the strength of the physical adhesion or atomic bonding which acts between coating and substrate) and the practical adhesion. The practical adhesion is the ability of the coating composite to resist interfacial failure in its practical application. Naturally, the upper limit of the practical adhesion is correlated to the atomic bonding strength, but this parameter cannot be directly measured, cp. Section 4.2. 1.2.3. Damage with material pick up Work material locally adhered to e.g. the surface of a sheet forming tool used in the automotive industry will inevitably produce indentations or scratches in the surface of the product. Material transfer between the contact surfaces of sliding machine elements is a similar problem often named galling, scuffing or seizure. Material pick up of material from the counter surface is; again, not unique to coating composites. It is generally

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Fig. 2. Typical values of coating thickness and process temperature (temperature at the substrate surface) of today’s tribological coating methods.

reduced or avoided by giving the surface a smooth topography and making sure that the chemical affinity to the counter surface is low. This is often accomplished by applying a proper coating. Tribochemical layers may form, e.g. when machining certain work materials at high cutting speeds. They are the results of mechanical smearing of or chemical reactions with constituents in the work material, and may have the positive effect to protect the coating from excessive damage [4,5]. 1.3. Tribological coatings of today Surface coating of tribological applications is associated with deposition temperatures ranging from room temperature to over 1000◦ C, see Fig. 2. The coating thicknesses range from microns to several millimetres. Typically, the atomistic methods produce the thinnest coatings. Some methods involve high deposition temperatures that may

give undesired phase transformations, softening or shape changes of the coated component. An important benefit of PVD and CVD processes is the high flexibility as to composition and structure of the coatings, and these processes are today successfully utilised to coat a large variety of mechanical components, cp. Fig. 3. The most common PVD and CVD coating materials are nitrides (TiN, CrN, etc.), carbides (TiC, CrC, W2 C, WC/C, etc.), oxides (e.g. alumina) or combinations of these. In addition to these material groups, molybdenum disulphide (MoS2 ), diamond like carbon (DLC) and diamond have also become very successful. WC/C, MoS2 and DLC can be classified as low friction coatings since they often display a friction coefficient ranging from 0.05 to 0.25 in dry sliding [3,6]. Amorphous DLC forms a large group of coatings. They can be doped with metals, nitrides and carbides to further improve the mechanical and tribological properties and to enhance the adhesion

Fig. 3. Limitations in heat resistance often exclude materials from different applications and coating processes. This figure illustrates typical temperature limits of potential substrate materials compared to typical working temperatures of applications and deposition temperatures of PVD and CVD coating processes.

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Fig. 4. (a) Comparative friction values recorded during dry sliding in a ball-on-disk geometry [8]. A water lubricated diamond/Al value is added for comparison; (b) intrinsic abrasive wear resistance of diamond and TiC coatings obtained by micro abrasion using diamond abrasives, cp. Section 4.4.2 [8]. The wear resistance of CC and HSS is also shown.

to the substrate. Under suitable conditions, DLC can provide a combination of good wear and corrosion resistance and a low friction. The main limitation is poor thermal stability; DLC coatings decompose above 300◦ C [6]. The wear resistance of the low friction coatings is generally inferior to that of the nitrides, carbides and oxides. These wear resistant coatingstypically exhibit friction values between 0.4 and 0.9 in dry sliding. A very important exception to this simple classification is the CVD diamond coating which in many applications combines an ultra-low friction with very high wear resistance, see Fig. 4. Friction values below 0.05 have been recorded for smooth diamond coatings in non-conforming dry sliding. A further friction reduction down to 0.02 can be obtained by water lubrication. This makes diamond coating a very potent candidate for demanding applications, in particular where also environmental considerations have to be met [7]. Fig. 3 indicates that limitations in the heat resistance of substrate materials severely restrict the number of possible applications and deposition processes. The most heat resistant materials, such as ceramics and cemented carbides (CC) can be used also in very hot applications and all types of PVD and CVD processes can be used to deposit coatings. The possibilities to use PVD or CVD to coat low alloy steels, copper-based alloys and light metals as aluminium and magnesium are still very limited.

2. Design of tribological coatings A tribological coating composite is primarily designed to offer two types of functions: • A specified friction behaviour (including low, high or just stable friction level). • A high wear resistance. To fulfil these functional demands, a sufficient adhesion between coating and substrate plus a sufficient load carrying capacity are both necessary. The load carrying capacity is the ability of the composite to resist tribological loads without subsurface plastic deformation or premature failure due to cracking or delamination of the coating.

2.1. Stresses in tribology of coatings Two types of contact stress patterns have to be considered in the design of coatings: • The Hertzian contact stresses related to the macroscopic contact geometry. • The stresses associated with the microscopic asperity contacts. In static contact, rolling contact or sliding contact with very low friction the maximum shear stresses occur at a depth roughly equal to half the Hertzian contact radius. For a ball bearing with 10 mm balls, this typically corresponds to 0.1–0.3 mm. The maximum shear stress may thus appear well below the typical PVD coating thickness. In sliding contact, the location of the maximum shear stress approaches the surface when the friction increases, and for a friction coefficient of about 0.3 or higher, it is confined to the contact surface [9]. Examples of components which may fail by surface fatigue due to excessively high cyclic Hertzian stresses are ball bearings, gears, and other machine elements with non-conforming contact surfaces. In sliding contact between conformal surfaces, such as those of journal bearings, face seals, piston/cylinders, and similar components, the highest contact stresses are restricted to the locations of asperity contact. The highest stresses usually occur within a few microns from the interface, cp. Fig. 5. Here, the prognosis for improving the friction and wear properties by applying a thin coating is a lot better. High residual stresses are typical features of PVD and CVD coatings, see Section 4.3.2. It has been shown that reasonably high compressive stresses act beneficially not only on the cohesion and fracture toughness but also on the wear resistance of the coating [10]. In the case of CVD coatings, e.g. TiC, TiN or Al2 O3 on CC substrates, the residual stress is typically tensile. 2.2. Topography There are several aspects on the optimum topography of thin hard coatings. To minimise the maximum contact stresses on the asperity level, the coating surface should be as

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Fig. 5. Normal stress distribution at different levels below the contact surface. The discrete localised elastic and plastic normal load distributions associated to asperity contacts are gradually smoothened below the surface. The pressure peaks at the coating interface may lead to local plastic deformation of the substrate, despite the average stress level being rather low.

smooth as possible. Since most thin coatings inherit the substrate topography, the final step in substrate surface preparation should be a careful polishing or a very mild blasting. This is also recommended to increase the practical adhesion of coatings with high residual stresses, Section 4.3.2. However, all coating processes introduce some surface irregularities, and it may be necessary to polish also the coating or use a superficial layer with good running-in properties, cf. Section 3.2. In most tooling applications the surface finish of the tool is replicated on the product, and the requirement on the coating topography is related to that of the product. In many metal cutting operations, however, constituents in the work material form protective tribochemical layers on the tool [4,5]. This mechanism may be facilitated by a reasonably rough topography. A certain roughness may also be beneficial for oil retention of e.g. sliding bearings.

on cemented carbide inserts is one of the largest industrial applications. Like CVD TiN, alumina is normally deposited on an intermediate layer of TiC. In some critical applications, several successive layers of different composition are applied to form a sandwich coating. Metal cutting inserts for machining austenitic stainless steel are typically CVD coated with three layers of TiC–Al2 O3 –TiN as seen from the substrate. TiC accounts for good bonding to the CC material, Al2 O3 provides a good wear resistance at elevated temperatures, and, in addition to looking good, TiN gives a clear visual indication of which edges of the insert that have been used. In addition, TiN often reduces the sticking of the work material. A graded coating composition or structure improves the load carrying capacity by offering smoother transitions in mechanical properties from those of the hard and stiff coating to those of the softer and more flexible substrate. In this

2.3. Current designs of coating structures Today, three types of coating structures are frequently found; single layer coatings, sandwich coatings and graded coatings, see Fig. 6. Most commercial CVD and PVD coatings are made up of one single layer often containing one structure phase. Among the most frequent are TiC, TiN, CrN, alumina (Al2 O3 ) and diamond like carbon (DLC). They are usually applied directly on to the surface of a homogeneous substrate material. Consequently, to provide a high load carrying capacity of the coating composite the substrate must possess a high hardness and a high Young’s modulus. Thin coatings of alumina have been manufactured for more than 20 years using CVD, and today CVD alumina

Fig. 6. Possible structures of tribological coatings.

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way, the contact load can be distributed over larger areas, which reduces the maximum contact stresses and the stress at the coating/substrate interface. Successful graded surface layers on steel have been produced by nitriding or carbonising since many decades, and the examples of forming tools and machine components taking advantage of these treatments are abundant [11,12]. Today, graded coatings are utilised also in commercial PVD coatings such as Ti(CN) [13].

2.4.3. Duplex coatings Since many of the wear resistant PVD and CVD coatings are relatively brittle, they can only be successfully applied to hard and stiff substrate materials such as hardened steel, cemented carbides or structural ceramics. On softer substrates an intermediate layer acting as a mechanical support for the coating is usually required, cf. Section 3.2. For steel and titanium alloys this support is preferably achieved by nitriding [11,12,19]. This combination of nitriding and PVD coating of steel is denoted duplex coating.

2.4. Coatings of tomorrow

2.4.4. Multilayered coatings Multilayered coatings are distinguished from sandwich layers by their periodically repeated structure of lamellae of two or more materials. The lamella thickness can vary between a few nanometer to a few tenths of a micrometer. Coatings made of multilayered structures have proved to be harder and significantly tougher than homogeneous coatings of the same materials since the lamellae structure obstructs dislocation glide and crack propagation, cp. Section 4.3.4 [20,21].

2.4.1. Materials and strengthening structures In modern coatings development, several phases and layers are combined into sandwich coatings, graded coatings, duplex coatings, multilayer, superlattice, nanocrystalline and multi-component coatings, etc., see Fig. 6 [3,14]. Obviously, the strengthening mechanisms known to the metallurgists for many decades are now being introduced to tribological coatings, now with numerous new combination possibilities. 2.4.2. New coating materials Diamond coatings have recently been introduced on inserts for aluminium cutting [15]. Diamond offers a unique combination of high hardness and wear resistance, low friction properties, high thermal conductivity, and environmental friendliness. The latter applies during both processing and application. Diamond will probably be one of the most versatile coating materials once it can be deposited at a more moderate temperature and ways of improving its toughness have been established. Up to now the PVD techniques have not allowed deposition of useful alumina coatings. However, recent advances in process technology have made reactive deposition of good quality alumina coatings possible [16]. Cubic boron nitride (CBN) cutting edges are today produced by conventional hot isostatic pressing and brazed onto the tip of cemented carbide cutting tools. CBN is second to diamond the hardest material, 5200 HV, and it is very effective in cutting hardened steels and other difficult alloys. Applying CBN directly to the tool in the form of a coating would of course be very attractive. The current restriction is that CBN coatings produced by PVD exhibit excessively high compressive stresses [17]. Carbon nitride (C3 N4 ) would theoretically be harder than diamond if it could be given the same structure as Si3 N4 . Although there have been reports of producing crystalline carbon nitride coatings, to date no one has succeeded in producing fully crystalline C3 N4 coatings. The carbon nitrides produced today, often denoted Cx Ny , have shown extreme elastic properties combined with relatively high hardness values (15–60 GPa) [18]. Similar to diamond, Cx Ny is stable in air only up to 600◦ C, above which it rapidly loses nitrogen and softens [17].

2.4.5. Superlattice coatings Multilayered coatings of materials with similar crystal structures tend to form columnar crystals which extend through the whole coating, provided that the thickness of the individual lamellae is sufficiently thin, typically 5–25 nm. Such coatings are referred to as superlattice coatings. One of the first examples of superlattice coatings was obtained by combining TiN/VN and TiN/NbN [22–25]. Several authors have shown that this type of multilayered coating structure can improve the hardness as well as the toughness, compared to single layers of the same materials, cp. Sections 4.3.4 and 4.4.3 [23,26]. Superlattice strengthening is well known from classical metallurgy. By selecting a suitable combination of materials for the multilayered structure it is possible to improve the resistance against wear, corrosion, oxidation, high friction, etc. For example, superlattice coatings with thin lamellae of TiN and TaN, see Fig. 7, have shown very good results in cutting of stainless steel. This is believed to be a result of a very good toughness combined with a low affinity of TaN to the work material [27]. 2.4.6. Nanocrystalline coatings The yield strength, hardness and toughness of polycrystalline materials all generally improve with decreasing grain size according to the well-known Hall–Petch relation. A similar phenomenon seems to be valid for thin coatings down to nanometer sized grains. In addition to improved mechanical properties, nanocrystalline materials can exhibit higher thermal expansion, lower thermal conductivity, unique optical, magnetic and electronic properties, etc. [28,29]. Currently, nanocrystalline structures of bulk materials as well as thin coatings are explored for tribological applications [30,31].

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Fig. 7. A cross-section of TiN/TaN polycrystalline superlattice coating on HSS: (a) fine grained columnar structure revealed by SEM; (b) close-up of (a); (c) the superlattice structure revealed by TEM.

2.4.7. Multi-component coatings Multi-component coatings are composed of two or more constituents in the form of grains, particles or fibres. Although many of today’s single layer coatings have a multicomponent structure, this is still a rather unexplored means for coating strengthening.

3. Design of coated components 3.1. Cutting tools Cutting tools will probably continue to be the leading application of modern tribological coatings. They are relatively small so that batch coating can keep the cost within reasonable limits even for high-tech coatings of e.g. nanocrystalline and nanolayered multi-component layers. Today, there are two obvious trends in cutting tool developments. Dry machining is desirable to avoid the extra costs and environmental problems associated to cutting fluids. High speed machining of hardened steel has the potential of giving sufficiently high quality of the machined surface to make finishing operations such as grinding or polishing unnecessary. In both cases, the heat generation along the tool surfaces will be even more intense than with today’s contact conditions, and consequently the tools must possess further improved hot hardness, thermal and chemical stability, etc.

ements cannot resist the currently used deposition temperatures. In addition, these components often have complicated and narrow sections which are difficult or even impossible to coat. Finally, often the high tool cost makes the user restrictive in application of new unexplored coatings. A promising technical solution currently being evaluated is to use plasma assisted PVD processes to produce duplex coatings, see Section 2.4.3. The functional PVD coating is typically CrN or TiN or a wear resistant, low friction coating, e.g. DLC or WC/C [32,33]. The automotive industry encourages research on new concepts of surface engineering, with the general aim to substitute traditional steel components with components made of lighter materials, typically aluminium, titanium and magnesium alloys. The ultimate aim is to reduce fuel consumption. The application of thin wear resistant and/or low friction coatings on inherently soft materials requires a supporting, intermediate layer to provide a sufficient load carrying capacity. Electroless nickel, hard chrome, laser cladding, thermal spraying, nitrided and carburized layers are all candidates for providing this property. The primary design parameters of the supporting layer are thickness and Young’s modulus, and the aim is to avoid plastic deformation and to minimise elastic deformation of the substrate. An additional softer low friction coating may have to be applied on top of the wear resistant PVD coating, see Fig. 8. This

3.2. Forming tools and machine components Application of thin tribological coatings on forming tools and machine components is still in its infancy, but has an enormous potential. Forming tools and machine components constitute much larger industrial sectors than do cutting tools. There are several reasons for the use of coatings in these applications still being relatively scarce. Many forming tools and machine elements are too large to be economically treated by today’s coating processes. Further, the substrate materials of most forming tools and machine el-

Fig. 8. Structure and materials of a multilevel composite coating designed for soft substrate materials.

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layer serves to spread the load, thereby reducing the stress peaks indicated in Fig. 5, and subsequently ensuring a mild running in wear. Modern PVD or CVD coated materials are thus often multilevel composites. The base material of, say a hardened steel, is typically a particle composite, the base material plus coating constitute the coating composite, and finally, the coating is of sandwich type and finally, one or more of its layers may possess various substructures.

4. Evaluation of coating composites 4.1. Important parameters A general theory covering all relevant properties and parameters involved in the design and application of tribological coating composites is very far from being realised. Such a theory would have to treat the long chain of relations ranging from the coating deposition parameters to the tribological response of the coated component [34]. The deposition parameters (substrate temperature, plasma characteristics, etching time, substrate bias, etc.) together with the substrate characteristics (e.g. composition, microstructure, topography) determine the coating characteristics (thickness, chemical composition, microstructure and topography, etc.). Here, the influence from the substrate is primarily related to the nucleation and growth of the coating, and to the coating topography. Consequently, the substrate material and surface preparation is crucial to the coating topography and adhesion, and, in turn, to the performance of the coating composite. Further, the coating characteristics consequently govern the basic coating properties such as chemical, thermal and mechanical. By tribological properties of the coating composite, we may understand friction properties and resistance against surface damage due to deformation, abrasion, erosion, adhesive contact, repeated impact, etc. These properties are given by the basic properties of the coating and, assuming that the coating is thin, also directly by the basic properties of the substrate. Finally, the tribological response of a coated component in operation is estimated from the tribological properties necessary to cope with the actual tribological situation, i.e. conditions such as geometry, contact pressure, sliding velocity, temperature, lubrication, etc. People involved in coatings development and production usually assess some of the relevant coating characteristics and basic properties, whereas end users should focus on the tribological response of the coated component in the actual application. Knowledge of the whole chain of interdependent properties is important for the general understanding of the behaviour of coating composites. Since different applications put different demands on the coating composites, the most decisive parameters will vary from one situation to another.

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4.2. Adhesion to the substrate Obviously, a good adhesion to the substrate is a crucial property of most applications of coated components. Any adhesion test must superimpose an external stress field over the coating/substrate interface to cause a measurable adhesive failure. Since this stress field will depend on the geometry and type of loading (indentation, scratching, sliding, abrasion, impact, etc.) as well as on the elastic and plastic parameters of the coating and substrate, the resulting adhesion value will only be representative of the particular test from which it has been obtained. Since the situation in the test most likely deviates significantly from that of the intended application, the result has to be handled with caution. In fact, the relations between the above parameters are so complicated that no general theory to predict practical adhesion exists. The preferable method to evaluate the practical adhesion of coated components is to use a tribological test with the closest possible resemblance to the actual situation. Such a test will show under which loading conditions the adhesion is a limiting factor, and under which other mechanisms of tribological damage will dominate. 4.3. Intrinsic mechanical properties Basic coating properties such as thickness, composition, structure, morphology and topography are analysed by modern imaging and analytical techniques, see [35]. For example, coating thickness and morphology of thin PVD or CVD coatings are easily revealed by SEM imaging of fractured cross-sections, cf. Fig. 7. Some intrinsic mechanical coating properties of particular interest, the Young’s modulus, residual stress, hardness and toughness or fracture resistance, are presented below. 4.3.1. Young’s modulus The Young’s modulus of the coating (Ec ) is a useful parameter, e.g. for measurements and calculations of the stress state and the cracking and delamination behaviour of coating composites. It is possible to obtain Ec through a number of techniques, where the uniaxial tensile test is the most straightforward [36]. The intrinsic elastic modulus of thin coatings can also be obtained by nanoindentation, cp. Section 4.3.3, vibrating reed tests, bulge tests, beam bending tests, ultrasonic wave propagation, etc. [37–41]. 4.3.2. Residual stresses Tribological PVD and CVD coatings usually display residual stresses (σ res ). Structural misfits in epitactic nucleation and growth, and ion bombardment during growth are two stress origins of intrinsic nature. The stresses induced during cooling from the deposition temperature due to mismatch in thermal expansion between coating and substrate materials, and possible phase transformations occurring

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during cooling are two sources of the external origin. The final stress state is a combination of these components [42–45]. The actual stress during application (σ ) is given by σ = σres + σapp

(1)

where σ app denotes the stress field induced by the application, including external forces and thermal mismatch stresses due to frictional heating. Too high compressive stresses may result in spontaneous coating detachment, e.g. during cooling from the process temperature. In less severe cases, the coating may detach when the coated component becomes loaded externally, see Fig. 9a [46,47]. The risk for detachment is closely related to the geometry and topography of the coating/substrate interface, the smoother the interface the less is the risk. On an uneven surface, the interfacial normal or shear stress generated by the residual coating stresses can exceed 50% of the residual stress level, cp. Fig. 9b. The popular techniques used for residual stress measurements are based either on measurements of the elastic strains

in the film using X-ray diffraction, or on the deflection of thin coated substrates. X-ray techniques can yield information of all strain components in the coating [48,49], and also give information about the strain distribution through the thickness of the coating [50]. To obtain the residual stress using X-rays, the elastic constants of the coating must be known. In the substrate deflection techniques, the coating residual stress is determined through measuring the deflection it causes to the substrate [51–53]. 4.3.3. Hardness Coating developers often use hardness measurement to assess coating quality and to predict the coating performance in various applications. However, the importance of a high intrinsic coating hardness should not be exaggerated. Generally, in pure two-body abrasive wear, the wear resistance is very closely coupled to the hardness, as long as the abrasives or abrasive surface is harder than the wearing surface. Most counter surfaces expected for tribological applications of coated components are softer than 20 GPa, a value exceeded by many of today’s PVD and CVD coatings, cp. Table 1. Intrinsic hardness values of thin hard coatings can be directly measured by conventional microhardness testing if the indentation depth does not exceed some 10% of the coating thickness. Consequently, direct measurements using Vickers indentation are restricted to coatings thicker than about 5 ␮m. It is possible to use microhardness values obtained from thinner coatings by using models which consider the substrate deformation [54–56]. During the last decade, nanoindentation has become the predominant technique to obtain intrinsic mechanical properties of thin coatings. In nanoindentation, the applied load is typically 0.01–5 g as compared to 5–1000 g for microhardness testing. In nanoindentation, the tip displacement, load and time are continuously recorded, see Fig. 10. The hardness and Young’s modulus are obtained from the load/displacement curves using different theoretical approaches, e.g. as proposed by Oliver and Pharr [57]. 4.3.4. Toughness Coating cracking or fracture often precedes damage of PVD and CVD coatings. Thus, the ability of the coating

Table 1 Common PVD and CVD coatings and their hardness PVD

Fig. 9. A compressive residual stress of 4 MPa in a TiN coating on a rough HSS substrate generates “lift off” stresses across the interface. A tensile residual stress state would give similar stresses of opposite signs: (a) coating detachment along a sharp ridge resulting from the combination of high residual stresses and external loading by scratching; (b) maximum tensile stress across the interface, maximum σ n vs. radius R of surface ridges for four coating thicknesses [46].

CVD

Material

Hardness (GPa)

Material

Hardness (GPa)

TiN CrN MoS2 DLC WC/Ca TiAlN

20–24 18–20 0.6–15 5–30 12–18 26–30

TiC Al2 O3 Diamond

28–35 18–23 80–100

a Process combining the chemistry and physics of CVD and PVD normally used for deposition.

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Fig. 10. Nanoindentation curves for three PVD coatings: TiB2 , TiN, and CrN. The hardness values and Young’s moduli (GPa) obtained were 53/566 (TiB2 ), 30/450 (TiN), and 25/330 (CrN), respectively.

composite to accommodate deformation in tension or compression without crack nucleation and propagation is crucial. Critical situations are found in applications of non-conforming sliding or rolling, cp. Sections 4.4.4 and 4.4.5. Several investigators have used beam bending to assess the deformability of coatings and to obtain numerical estimates of their toughness (or fracture resistance) [58–60]. In the device shown in Fig. 11a, the bending load is continuously increased and the critical strain to initiation of the first crack is recorded acoustically or in the SEM. It has been observed that multilayered coatings generally show higher critical strains to fracture than do homogeneous coatings, see Fig. 11b. Since cracking is initiated by tensile stresses, any compressive residual stress has first to be relaxed. Consequently, if the coating initially has a high compressive residual stress, the coated component can take more tensile strain before the coating will fracture. The critical component strain is thus a more important parameter than the critical intrinsic tensile strain of the coating. Note also that the true fracture strain of PVD coatings is very low compared to that of corresponding homogeneous bulk ceramics, which indicates that there is a huge potential for improving their toughness even further. 4.4. Tribological properties In this section, five of the most important tribological properties of the coating composites are identified and simple tests for their assessment are demonstrated. 4.4.1. Scratch resistance Scratch testing has, together with hardness measurements, become the most common way of assessing the mechanical quality of coating composites [61]. Usually, the scratch test utilises a spherical diamond tip of Rockwell C geometry (200 ␮m tip radius). During testing the tip load is continuously increased and a critical load for coating failure is detected. The failure criterion may be occurrence of the first

Fig. 11. (a) A four-point beam bending device suited for operation in an SEM [60] where ‘A’ indicates the test beam, ‘B’ the load cell and ‘C’ the acoustic detector; (b) a representative plot of crack density (䊊) and acoustic emission (line) vs. coating strain and applied strain, respectively, for a 4 ␮m PVD TiN-coating on ASP2030 HSS. The insert shows a coating strained beyond the fracture limit; (c) critical strains of three multilayered TiN/NbN coatings compared with those of the single TiN and NbN layers. The figures denote the individual layer thicknesses in nm. The coating strain is the applied strain plus the residual strain in the coating.

crack or first induced cohesive or interfacial fracture. The failure may be determined by friction and acoustic emission (AE) recording. Optical microscopy or SEM should confirm the results [62]. Scratch testing can also give detailed information about different modes of coating failure and much knowledge of a coating composite can be gained by studying the scratched sample, e.g. in the SEM. A general rule of thumb says that a critical load of 30 N in scratch testing with a Rockwell C diamond tip is sufficient for sliding contact applications. Critical loads of 60–70 N are frequently recorded for PVD coatings on hardened HSS. It should be pointed out that the critical load usually increases with substrate hardness and coating thickness, and decreases with increasing surface roughness [63].

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4.4.2. Resistance to abrasive wear In situations of mild abrasion, the coating material may determine the wear resistance of a coating composite. Standard abrasive wear tests are usually too coarse to be useful for generating the intrinsic wear resistance of thin coatings. However, by the micro abrasion test originally proposed by Kassman et al. [64] and further developed by Rutherford and Hutchings [65,66], and Gåhlin et al [67], it is possible to distinguish the abrasive wear resistance of a thin coating material from that of the substrate, also in situations where the coating is worn through. A small grinding wheel (dimple grinding) or ball (ball cratering) is used to produce a spherical crater in the surface of the coating composite, cf. Fig. 12a. The contact area is surrounded by an abrasive medium. The test is interrupted at regular intervals and the crater volume is estimated either from measuring the diameter using an optical microscope, or directly by using 3D surface profilometry. By assuming Archard’s law to be valid for coating and substrate individually, one arrives at a simple rule of mixture SL =

Vs Vc + κc κs

(2)

where S is the sliding distance, L the applied load, Vc and Vs the wear volumes of coating and substrate, respectively and κ c and κ s are the specific wear rates of the coating and substrate. Intrinsic wear resistance of some PVD coatings obtained as 1/κ c are given in Fig. 12b.

Fig. 12. (a) A schematic view of the dimple grinder; (b) typical values of intrinsic hardness, abrasive and erosive wear resistance of single and multilayered PVD coatings [69]. The figures at the bottom indicate the individual layer thicknesses in nanometer of the multilayered coatings.

In addition to the coating and substrate wear resistances, by subsequent inspection of the test craters in the SEM the micro abrasion test reveals any content of coating defects such as pores and cracks. A poor adhesion is often detected from the presence of spallings in the coating substrate interface. Microabrasion test results must be handled with caution. Two-body abrasive wear of the coated surface prevails if the abrasives adhere to the surface of the grinding wheel or ball. This is likely to be the case if the surface of the wheel or ball is softer than the tested material. If the tested material is softer, the abrasives will become pressed into its surface, resulting in abrasive wear of the rotating ball or wheel. In situations where both surfaces are of equal hardness or the particles form thick layers, the particles may roll rather than slide, and the wear will be dominated by three-body abrasion [68]. Other parameters which must be kept under control is size distribution and volume fraction of the abrasive particles and viscosity and wetting angle of the liquid medium. 4.4.3. Resistance to particle erosion Surface damage caused by impinging hard particles is usually referred to as particle erosion. Generally, resistance to particle erosion requires a combination of hardness and toughness, with the toughness being the dominant parameter, cp. Fig. 12b. For a thin coating to be effective in erosion protection, individual impacts must not plastically deform the substrate material. The extension of plastic strain is controlled by the particle size, velocity and angle of impact. In mild situations, where only the coating is permanently deformed by the impacts, particle erosion can be used to evaluate intrinsic erosion properties of the coating, or as a micro scale toughness test. 4.4.4. Resistance to sliding wear Sliding wear is here referred to as wear in a tribological system where the coated component slides against a relatively smooth counter surface, free from hard particles or hard asperities. Naturally, sliding wear involves a very large group of tribological situations, and the wear may range from very mild chemical wear to severe adhesive wear and coating detachment. After a sliding wear test, the mass loss of typical PVD or CVD coatings is too small to be resolved by weighing. However, the situation has recently been improved by the introduction of accurate surface profilometers and by atomic force microscopes (AFM). These techniques allow very small wear volumes to be mapped and measured [70]. Apart from the small volumes involved, there is no principal difference in evaluating the intrinsic sliding wear resistance of thin coatings and bulk materials. In tooling applications, tribochemical mechanisms often dominate over mechanical wear. Thus, the selection of counter material as well as reproduction of representative temperature and atmospheres are crucial components in simulative tests.

S. Hogmark et al. / Wear 246 (2000) 20–33

A new test for evaluation of friction and load carrying capacity of coating composites has recently been suggested [71]. Two elongated specimens are slid against each other in a way similar to that of the contact between the edges of a pair of scissors. If the load is gradually increased, as in the scratch test, each contact spot along the wear track will experience a unique load. Unidirectional as well as multipass sliding can be applied, and critical loads for coating failure can be obtained as for the scratch test. The advantage over the scratch test is, however, that the contact situation is very much closer to practical applications of sliding contact. In addition, the counter material as well as the contact geometry (radius of contacting rods) can be selected to represent the intended application. 4.4.5. Resistance to wear in rolling contact The stress distribution in rolling contact between smooth surfaces can be estimated according to Hertz, cf. Fig. 5. Since the maximum shear stress is generated below the contact surface, intrinsic wear resistance of the surface material is usually not the main concern. If a small permanent deformation of the substrate is accumulated in each contact event (ratchetting), the fracture limit of the coating will eventually be reached. Pure rolling usually gives negligible wear. A small proportion of sliding in dry or boundary lubricated rolling contacts may give a mild wear. In slowly rotating roller bearings such mild wear may gradually concentrate the contact pressure to the unworn regions of pure rolling, and eventually cause catastrophic surface fatigue [72]. Applying a wear resistant coating can solve this problem, provided the adhesion is sufficient. Examples of rolling components considered for application of coatings are rolling element bearings. Railway wheels constitute a larger scale of application which, consequently, requires relatively thick coatings for wear protection.

4.5. Tribological response of coated components Generally, the end users of coated components are recommended to make the final evaluation of the tribological response in field tests or in component tests, i.e. tests where the actual component is evaluated under realistic conditions. Simplified laboratory tests often deviate from the actual situation as to nominal and real contact pressure, sliding speed, heat conductivity and capacity, ambient cooling, etc., which makes correlation to the real case hazardous. Cutting tools and other relatively small components with relatively short service lives are well suited for evaluation in field tests while larger and more expensive components with longer lives, may have to be evaluated in simplified tests. Sometimes a compromise may be to design a large component incorporating small easily exchangeable inserts. Such inserts can act as cheaper test surfaces, and if small enough, can be inspected in a microscope and weighted on

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a precision balance to evaluate also small amounts of wear. In carefully designed tests, these inserts will experience the same tribological conditions as the actual component.

5. Visions and conclusions The evolution of surface engineering has just begun. Sandvik Coromant introduced the first tribological CVD coating (TiC) in 1969 to protect CC cutting tools, and the first PVD coating (TiN) was applied to high speed steel in the late 1970s for the same reason. These two are still considered to be among the very best tribological coating materials although new improved PVD and CVD coatings have evolved and proved superior in specific applications. During the last decade the number of new coating materials, structures, combinations and applications has increased exponentially. We have only seen the very beginning of an evolution in the use of coatings which in the future probably will encompass virtually all-mechanical components. We will soon succeed in applying coatings to vital machine components of lightweight materials, thereby reducing the weight of machinery while maintaining or even improving their performance and lifetime. Furthermore, many low friction coatings may offer the same low friction levels as those of boundary lubricated contacts (0.10–0.20). Thus, the need for lubrication of tools and machine components will decrease, which will help to release some of today’s environmental stresses. The current aim to coat light materials such as alloys based on Al, Ti, and Mg was pointed out in Section 3.2. Since these materials cannot resist the temperatures of most of today’s PVD and CVD processes (cp. Fig. 2), there is an urgent need to develop deposition processes which involve considerably lower temperatures. Alloying with metals has successfully been used to improve the toughness of other coating materials and bulk ceramics, and should be explored also for diamond. The columnar structure of current PVD coatings makes them sensitive to stresses parallel to the plane of the coating. There has been some pioneering work to overcome this drawback by introducing second phase particles, as reported by Veprek and Reprich [31]. It would be even more challenging to design coatings with grains elongated to fibres preferably oriented in the plane of the coating. Such a structure should have the potential of being very tough. Finally, there is a need for models to predict the tribological response of actual components made of multi level sandwich composites. This is most urgent for machine components, cp. Section 3.2. Given relevant information of the application, these models should preferentially yield recommendations of the coating thickness and of coating and substrate materials properties in terms of Young’s modulus, hardness and toughness. However, while awaiting analytical tools for coatings selection and design, we will have to rely on selective testing

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[73]. Thus, versatile and reliable techniques for evaluation of coated components will continue to be important for the tribological assessment and development of new coating composites and their applications.

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