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Selection of surfacing treatments
Selection of surfacing treatments R.F. Smart* Satisfactory performance of surfaced components depends upon careful selection procedures, as well as good surfacing practice. After a review of available surfacing treatments and a comparison of their principal characteristics, a systematic approach to the problems of selection against wear is discussed, and the ancillary factors that must also be considered are described; design aspects are included Wear resistance in tribological components may be obtained by the use of appropriate bulk materials but this is often expensive or inconvenient. Product performance and economics can generally be improved by the application of a suitable surfacingt treatment to the wear faces, leaving the substrate to be chosen for its structural properties and ease of manufacture I . Recent concern with the efficient use of materials has stimulated an already-growing interest in surfacing but, to obtain optimum performance, it is necessary to select the treatment with care; failure to do this can give unsatisfactory (or even disastrous) results. Selection, however, is difficult, partly because of the complex nature of wear and partly because of the intrusion of other factors that influence, or more generally limit, the choice. Selection is consequently far from an exact science; an empirical approach, based on common sense, experience and intuition, is still the norm. It is the aim of this paper to outline a systematic approach to the problems of selection, so that no obvious factors are overlooked.
The appr(:,ach to selection A pre-requisite to the specification of a surfacing treatment is an understanding of the many surfacing processes that are available and of the characteristics of the surfaced layers they produce. The variety of processes, or process variants, commercially available or under development can be confusing, particularly because of the use of proprietary designations for many treatments. The primary requirement in the selection procedure will normally be to identify the type of wear to which the component will be exposed in service, taking into account the presence of elevated temperatures or corrosive media, etc. However, wear is a difficult phenomenon to analyse and prescribe against, since industrial wear situations may involve more than one type of wear ~ and confirmation of the analyses can only be obtained by examination of the worn surfaces. The selection based on wear must be complemented by considering other relevant factors, eg the cost and *Mater~ls Engineering Department, Associated Engineering Developments Ltd., Cawston House, Cawston, Rugby, Warwickshire CV22 7SA, UK "/'The term "surfacing' is used as a general term," "surface treatment" is used for processes that do not involve the build up o f a deposit on the surface, and 'surface coating'for those in which a coating is deposited
availability of the preferred surfacing material, the surfacing equipment that is available, the composition of the substrate and the geometry of the component, the nature of the mating surface, the service environment generally, etc. A typical list of factors that should be considered most conveniently in a 'checklist' fashion - is shown in Table 13 ,4. In some applications, the ancillary factors assume overwhelming importance and selection may best be approached by what has been described a as 'the technique of progressive elimination'. Despite the difficulties imposed by lack of information, the overall aim of any selection procedure should be the specification of an optimum treatment, based ripen technical and economic considerations, for the particular component and application.
Surfacing processes Surfacing treatments broadly fall into three categories and may be produced by mechanical, physical, or chemical processes, or by combinations of these. Table 2 lists treatments of tribological interest s . Considerable variation is observed among the processes, as Table 3 illustrates z ,a ,6. The practical thickness that can be applied varies with both process and material. With sprayed deposits, very thick coatings can be produced but residual stresses, arising from thermal mismatch between coating and substrate, build up and may cause spalling: in practice, therefore, the thickness is usually limited. Electrodeposits are also limited by residual stresses (usually due to hydrogen inclusions), as well as by the economics of the relatively slow deposition rate. Thicker coatings can usually be applied with safety by weld deposition or by spray fusion, while thermal and thermochemical treatments can be chosen to cover a range of thicknesses. Most welding processes, and detonation spraying, can be used only on fairly simple substrate shapes. The other spraying techniques can cope with a much greater range of component geometries and tend to be limited only by the need to present the torch normal, or at a slight angle, to the substrate. Plating techniques are also versatile, provided suitably shaped electrodes are available. Spraying and welding processes are essentially line-of-sight techniques, whereas electrodeposition has a limited ability to deposit beyond this (expressed as the throwing power). One of the interesting features of plasma assisted vapour deposition (eg sputtering and ion plating)
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Table 1 Typical checklist to aid surfacing selection* Process selection
Materials selection
General factors
General factors
1. What processes are available 2. Available forms of surfacing materials 3. Quantity of coating required 4. Areas to be protected 5. Quantities required 6. Where is work to be done 7. Labour available 8. Possibilities of mechanisation 9. Services available 10. Costs
1. Knowledge of similar applications 2. Surfacing processes 3. Surface preparation 4. Future repairs 5. Are test pieces needed 6. Cost Operating environment
1. 2. 3. 4.
Wear type(s) Temperatures Corrosion problems Mating surfaces and adjacent material 5. Lubrication
The job
1. Size, weight and shape 2. Location and accessibility of area for production 3. Surface cleanliness 4. Limitations of adjacent areas 5. Hardness and melting point of substrate 6. Machinability 7. Component strength at temperature
Properties required
1. Wear Resistance 2. Chemistry 3. Adhesion 4. Porosity 5. Hardness 6. Microstructure 7. Wear Resistance 8. Thermal Properties 9. Machinability Substrate
Surface preparation
1. 2. 3. 4.
Methods feasible Equipment available Tolerances Register and datum point requirements 5. Undercutting 6. Presetting possibilities
Finishing
1. Surface function 2. Finish required 3. Dimensional accuracy and distortion limits 4. Equipment available 5. Post surfacing actions
1. Substrate composition 2. Previous surface treatments 3. Stresses in component 4. Importance of metallurgical condition 5. Surface hardness 6. Thermal expansion 7. Size and shape 8. Surface condition 9. Temperature sensitivity
*,,4dap ted from James3
is that they have sufficient throwing power to deposit on back surfaces 7 . The maximum rate of deposition of different coating techniques depends upon the nature of the process: with mechanical processes, the amount of material deposited per hour depends upon the size and power of the equipment;with chemical processes, rate is determined by chemical kinetics, while in physical vapour deposition, the rate is determined by energy input and extraction 2. In general, surface coating processes are faster than surface treatments, which are diffusion controlled; arc spraying and welding methods can give particularly high rates of deposition.
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Coating costs are a factor of prime importance but they are difficult to discuss sensibly in the abstract, since the cost of treating an arbitrary (simple) shape gives little. guidance to the cost for a component. In general, electrodeposits are economic, especially on small components or areas, if thin coatings are required. Thermal deposition (apart from the very expensive detonation spraying) tends to be attractive for somewhat thicker deposits over large areas; the fusing operation can make spray fusing expensive, while welding tends not to be particularly cheap. Standard thermaland thermochemical (eg nitriding or carburising) treatments tend to be relatively cheap. The characteristics of the surfaced layers, which are shown in Table 4, also vary among the processes. Weld deposits have essentially cast structures whereas sprayed coatings (unless subsequently fused) show a much finer structure, with flattened particles, representing the build up by deposition of overlapping layers; they are characterised by marked directionality of properties, non-equilibrium structures, and possibly the presence of oxides. Electrodeposits are usually fine structures which, with alloy plates, frequently exhibit metastable phases; inclusions may also be present. The structures of thermally hardened steels usually comprise tempered martensites reflecting the heat treatment of the steel or cast iron. A wide variety of structural features is obtained with thermochemical processes, depending on the composition of the bulk material and the nature of the process used. Most of the processes aim at essentially pore-free deposits but thermal spraying almost always yields porous deposits, the amount varying with process and operating conditions. This can be sealed if necessary, although it can act as a useful reservoir for lubricant in wear applications, provided the reduction in strength caused by the porosity can be accepted. Values for bond strength of adhesion to the siabstrate should be regarded with caution - because of inherent difficulties in testing s - but, in general, overlay coatings, which have been deposited on a cold substrate and/or which have not been subsequently fused, have limited adhesion. Electrodeposition, spraying, and vacuum evaporation give modest adhesion values; plasma assisted vapour deposition appears to yield higher bond strengths due to the plasma conditions and good substrate cleaning that is possible, while chemical vapour deposition produces a high bond strength because of the elevated temperatures of the substrates. In welding type processes, the bond strength should be as good as that of the substrate; equally this is so in thermal and thermochemical treatments, where the treated layer is an integral part of the component. The effect on the substrate varies for the different processes. Thus, in welding and post spray fusing, distortion of the component may result, whereas this is not a problem with electroplating or thermal spraYing; however, the latter does require abrasive blast pre-treatment which may prove troublesome with thin gauge parts. With some deposits, reduction in base-material fatigue strength may result from surfacing. Changes in the composition of the substrate occur with thermochemical treatments but are not usually significant
Table 2 Classification of surface treatmentst
Type 1 Material added to surface A. Welding AI. Gas A2. Arc A3. Plasma
• C. Cladding
C1. Brazing C2. Explosive bonding
Type 3 Surfacemicrostructure altered
H. Interstitial hardening
K. Mechanical treatment K1. Peening K2. Rolling K3. Machining
E. Electrodeposition
El. Electrolysis (Aqueous) E2. Metalliding E3. Anodising E4. Electrophoresis
B. Thermal spraying B1. Flame
B2. Arc B3. Plasma B4. Detonation
Type 2 Surface chemistry altered HI. H2. H3. H4. H5. H6.
F. Vapour deposition
F1. Physical F2. Chemical G. Chemical deposition G 1. Chemical plating G2. Phosphating G3. Chromating
I. Diffusion treatment 11. Siliconising 12. Aluminising
Spark hardening Powder coating Organic finishes Painting
L. Thermal treatment
L1. Flame hardening L2. Induction hardening L3. Chill casting M. Thermo-mechanical treatment
M1. Martensitic work hardening
13. Chromising 14. Sulphurising J. Chemical treatment
D. Miscellaneous
D1. D2. D3, D4.
Carburising Nitriding Carbonising Cyaniding Sulphonitriding Boriding
C. Cladding
J1. Etching J2. Oxidation
C3. Diffusion bonding D. Miscellaneous D. Miscellaneous
D6. Impacting abrasives
D5. Hot dipping Multiple processes e.g. Plating followed by diffusion treatment - PVD followed by peening treatment ?After James, Smart and Reynolds s
with coating processes, except in the case of dilution with weld deposition. Taken as a group, surfacing processes offer great versatility and almost any material can be used for surfacing, provided the appropriate process is chosen. The main materials used in commercial work are listed in Table 5, most of these having been introduced empirically9" 11.
Wear T y p e
Wear M e c h a n i s m
M i l d a n d severe wear, scuffing ADHESION -
-
Fretting
-
-
Low stress abrasion
-
-
Machining wear
Selection for wear resistance
The various individual types of wear have been described in the previous paper in this issue. The mechanisms that result in the removal of material from the surface of a component are mainly adhesion, abrasion, and fatigue12; Fig 1, based on a classification due to Wells et al 2, shows how the different types can be related to these three basic mechanisms. Adhesive wear, in the presence of lubrication, is best resisted by the use of soft, dissimilar metal coatings with little tendency to mutual solubility. Where lubrication is marginal or absent, harder surfaces are required, the precise hdrdness being dictated by the contact pressure and the hardness of the contacting surface; with steel parts a hardness greater than 550 Hv is generally beneficial. It may be noted that the wear is greater with rough surfaces, so that a smooth surface is desirable. Scuffing is a particular form of severe adhesive wear that is observed during 'running-in' and requires transient protection. The different varieties of abrasive wear tend to present slightly different materials requirements and an attempt should be made to identify the particular type of abrasive. In general, a high shear stress in the surfaced material is necessary and this is commonly translated into
ABRASION
- -
Gouging Grinding (three-body)
Particle impact erosion
FATIGUE
-
-
C o n t a c t fatigue
-
-
P e r c u s s i v e wear
-
-
Cavitation
Fig I Classification of wear types a hardness requirement of 0.5 - 1.3 times that of the abrasive particles. A common rule-of-thumb is to aim at a hardness greater than 800 Hv, the hardness of the common abrasive silica sand. Harder materials tend to be selected for small, hard abrasives, with tougher materials more suited for larger abrasives, or where ancillary impact loads are significant. The thickness of the surfaced deposit will be related to the likely depth of abrasive penetration. Where low angle particle impact erosion is involved, hard brittle materials give good resistance; with
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CD ',,,d
0
0 r0 c) -< 5"
O0
0
3> 0.5
Versatile
Limited by plating bath
Electrical conductors
3> 50
Chemical cleaning and etching
Stress relieve
Deposition rate, kg/h
Component geometry
Component size
Substrate
Substrate temperature
Pre-treatment
Post treatment
None
Grit blasting
100
Almost limitless
Limited by handling equipment; no limit for hand spraying
Versatile (except detonation spraying)
Up to 10
0.1 -- 1.0
Thermal spraying
100
Care with edges
Good
Bond Strength, MPa
Integrity
Tolerances
*NA = n o t applicable: integral with substrate
Nil
Porosity, %
Electrodeposition
Table 4 Comparison of properties of surfaced layers
Fairly good
Mechanical bond: susceptible to edge chipping
20 - 140
1 - 15
Thermal spraying
*Surfaced layer formation typically O. 1 mrn/h for carburising and 0.01 m m / h for nitriding
0.02 - 0.5
Thickness, mm
Electrodeposition
Table 3 Comparison of surfacing process characteristics
0.2 - 2 (carburising) 0.5 - 0.75 (nitriding)
1-6
2 - 20
Thermochemical treatment Nil NA NA
Good with due all owa nces
Thermal treatment Nil NA NA
Good
Welding Nil NA High deposit integrity Poor
Nil NA* Metallurgical bond Moderate
Stress relieve
Mechanical cleaning
1400
925 (carburising) 525 (nitriding)
Fe Fe 900
Equipment limitation
Few
No limit
Usually Cu, Fe super-alloys
Versatile
Versatile
Simple Shapes
3 - 50
Thermochemical treatment
Thermal treatment
Welding
Spray fusing
None
Grit blasting
1050
A b i l i t y to withstand heat
No limit by hand
Versatile
Up to 10
0.5 - 1.5
Spray fusing
high angle impact, however, materials that are tough and ductile, and can absorb substantial amounts of energy before fracture, would be preferred. If cavitation erosion is present, it is desirable to use a material with a high value of ultimate resilience (ie ½ (ultimate tensile strength) 2/elastic modulus), provided the corrosion can be contained. In contact fatigue, the surface is subjected to repeated cyclic stresses due to impact or rolling motion, and these can cause surface cracking. The need, in this case, is for a material with high fatigue strength (broadly equated to high yield strength and hardness) but with adequate toughness. 'Care is necessary in selecting'suitable core or substrate conditions in surfaced parts, to ensure that subsurface cracking, or cracking at the interface between treated layer and substrate, is avoided. Table 5 Typical materials used for surface coatings for wear applications Material
Hardness, Hv
Remarks
1000 400 - 700 400
Hard chromium
Cobalt Alloys Lead/Ti n Nickel or Chromium Composite
500 10 400 - 450
Hardened with sulphur Running-in aid
330 170 390 300 100 - 150 300 - 700 1300 - 1800 1100 2200 2400
Spray fused deposits Nickel + BiSi Nickel-Chromium + BiSi Cobalt-Chromium + BiSi (W) Cr-Ni + BiSi + WC Cr-Ni + BiSi + NbC
220 - 300 300 - 600
450 - 700
Welded deposits Iron base with < 20% additions Iron base with > 20% additions Nickel or Cobalt base Carbides Copper Alloys
250 - 650
Group 1
500 - 750
Group 2
300 - 720 > 900
I~ HIGH LOW
I
~1~I${I}
Mortlms ~t~e~;I¢linlesS
HEAT AND CORROSION RESISTANCE [ st~le] 8t8 C,-Ni (2} $#oinl.s I.
H'gh steelsS,O" {L, 2 )
H rr~r Au:s,enitic lll~t© onO items ( 2 ) 1
COboll * end ~ickel- bcl~e 0 t |Oy${ 3) HIGH--~
IMPACT RESISTANCE - -
] I
HIGH
-z-LOW
Fig 2 Comparison of the properties of weld surfacing alloys. Figures in parenthesis indicate the alloy group
The above considerations must be modified if elevated temperature or corrosive conditions are likely to be experienced.
Table 5 includes some hardness data for the common surfacing materials. Wells et al 2 have recently published information on hardness, maximum service temperature, and corrosion rating for a fairly wide selection of surfacing materials while Avery, in an early paper 9, reports useful data on hot hardness and abrasion resistance for hardfacing materials. Comprehensive property data, however, are not easily available. It should be emphasised that the properties in the form of surfaced layers may differ significantly from the values for nominally similar materials in bulk form. The surface layer may not be in equilibrium, so that the phases present are different to those in cast or forged counterparts. Hardness measurements on thin coatings can be affected by the thickness of the deposit, the hardness of the substrate, and the presence of porosity. The latter, together with inclusions in coatings, affect mechanical properties, as does the presence of residual stresses. Particular care is therefore necessary in the extrapolation of property values for bulk materials.
Thermally sprayed deposits 13% Steel 18/8 Stainless Steel Molybdenum NiAI AI Bronze Stellite WC/Cobalt Cr3 C2/N ickelChromium AI 2 03 Cr2 O3 Zr 02
ABRASION RESISTANCE
Materials properties and structure
Electrodeposits Chromium Iron Alloys Nickel
LOW ~
Group 3 Group 4 For bearing overlays
In general, there will be a requirement in the surface layers for strength, which may be equated, approximately, with hardnesS, and toughness. However, high hardness tends to be associated with low impact resistance, notch sensitivity, and poor resistance to crack propagation. Thus, selection against wear is frequently a compromise between hardness and toughness; there must be sufficient strength to resist the imposed loads but enough toughness to avoid failure due to surface imperfections or incipient fatigue cracks. If there is any doubt, the balance should be tilted in favour of extra toughness; a higher than optimum wear rate will ultimately reduce component life but less rapidly than complete failure of the comPonent. While surfacing materials are not generally arranged systematically in terms of impact resistance and toughness, the hardfacing alloys form a series of gradually varying properties (Fig 2) which can be helpful in materials selection 1°. It should be emphasised that, except with simple microstructures (eg hardened and tempered carbon steels), wear
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resistance tends only to be generally related to hardness. Especially under abrasive wear conditions, hardness will not be the sole criterion of wear performance, and any successful material must be able to undergo a significant amount of work hardening TM 14. Structure is also important in the ability to withstand abrasive wear. Except for austenitic manganese steels, which have a continuously work hardening matrix, most surfaced layers derive their wear resistance from the presence of carbides or nitrides; the type, amQunt, shape and distribution of these, as well as their support within the matrix, affecf the actual wear performance. Under pure abrasion, a coarse carbide seems desirable but where there is some impact a finer, well distributed carbide is probably preferable. The matrix structure is also important. The ability to predict the wear resistance of a potential surfacing material from its known composition or microstructure has so far received little study. Typically, such an analysis is not fundamental but attempts to relate relative wear (compared to a known standard material) to composition or microstructure constituents. One example gives as ~r = 204 - 70(C) - 4(S) - 15~/(Cr) - 80~(Mo) where ~r = wear index of test materiat relative to standard die steel, C -- % total carbon, S = % sum of all elements in solid solution, Cr = % chromium of carbide, and Mo = % molybdenum equivalent (strong carbide formers) = % M o + ½%W+ 2%V + % N b Alternatively, the relative volume of each structure element, and its relative wear resistance, may be incorporated in a more complex series of equations 16. None of these analyses, however, is yet near commercial usefulness.
Guide to selection Various studies to classify industrial wear have been reported 17 and, while the detailed results differ slightly, there seems to be general agreement that about half of all industrial wear situations are essentially abrasive in nature, about 15% involve adhesive wear and the remaining 35% comprise a combination of fatigue, fretting erosion and cavitation, and chemical attack and oxidation. These types are thus the modes of wear most likely to be encountered. In adhesive wear situations, provided there is lubrication, thermally sprayed deposits of chromium or molybdenum, for example, are useful, while electrodeposited hard chromium is well established; surface coatings with some inherent porosity have the ability to retain oil and this can be useful, in case of temporary breakdown of lubrication. However, when lubrication is generally marginal or absent, harder coatings such as plasma sprayed ceramics or carbides (with metallic binder) would be needed. Electroless r~ickel has proved useful under adhesive wear conditions, as has, more recently, vapour deposited titanium carbide. For scuffing, short life treatments such as phosphating or nitrocarburising are useful. Under conditions of fretting, sprayed copper alloys, anodised aluminium (or possibly titanium), plasma sprayed oxides or carbide cermets, or nitrocarburising may be used. Turning to abrasive wear, low stress abrasion and machining wear are best combated by hard coatings, chromium
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carbide often being the most economical solution; in grinding abrasion, tungsten carbide is frequently specified. For these types of abrasion, martensitic irons may be used as an alternative; this gives a smooth wear surface (which is advantageous) as well as resisting well metal-tometal wear and light impact. Thermal and thermochemical treatments may also be used in these cases. For resistance to gouging, weld deposits would normally be specified. The choice would thus tend to be carbides for hard small abrasives, through the martensitic alloy steels, through to the 14% manganese and 14% manganese-14% chromium steels a~ the need for toughness with large particle abrasion increased. With particle impact erosion, weld deposits or plasma sprayed metals or carbides would be favoured, if the impact were at high angles; hard coatings in general will withstand low angle impact provided they are harder than the abrading particles themselves. In contact fatigue situations, thermal and thermochemical treatments, weld deposits and spray fused coatings are all used. If the stresses are low, nitrocarburising may be an economic solution but as the applied stress increases, nitrided or carburised cases would be needed. If percussive wear is involved, a range of treatments is possible, including weld deposits, sprayed carbide/cobalt alloys, composite electrodeposits such as Co/Cr3C ~ , and thermal and thermochemical treatments. The cast cobalt alloys of the stellite type are resistant to cavitation erosion as are bronze, nickel and tit~mium alloys; hard chromium plating is also used to withstand these conditions. If operation is to be at elevated temperature it should be noted that softening of thermal and thermochemical treatments above 200°C will occur while above ~500°C the the choice is largely restricted to chromium and cobalt containing weld or sprayed deposits. Many metallic materials form a naturally protective film or a passivated reaction layer, but this will tend to be removed by the mechanical action of wear, so that bare metal is continuously exposed to the media, which will thus appear unusually aggressive. With increasing temperature, the corrosion rate increases and, above ~500°C, the process will tend to become one of gross oxidation. The principal aim in materials selection must be the prevention (or at least the substantial reduction) of the corrosive effect. For more detailed guidance on selection, references such as 2 and 9 should be consulted, or papers on the specific processes.
Ancillary factors in selection Factors other than the ability to withstand wear play some part in the choice of surfacing treatment; the particular weight to be given to these varies with the application, but in all cases they should be considered.
Availability Any selection must be compatible with the available equipment and with the skills available to control it reliably and reproducibly. The capital costs of different surfacing processes differ greatly and this may be important in choice. In general, however, where equipment for a range of processes is available, it may still be preferable to
select a simple, well-tried technique or material rather than one of more advanced technology. Another factor, more usual with reclamation work, is the necessity in some cases, to take the surfacing equipment to the component, which deafly rules out a number of processes. As far as consumables are concerned, the primary criteria are cost and availability in the correct form; the latter requirement is particularly important for those processes (eg weld deposition or arc spraying) which require the consumable to be presented in wire form, which may not in all cases be easy or economic. Substrate
One of the most important of the ancillary factors is the surface of the component to be protected; this is normally the bulk material although it can be an intermediate layer. Reasonable chemical compatibility between substrate and surfacing material is necessary, to prevent excessive chemical interaction during processing or in service; the higher the temperatures, the faster the rate of interdiffusion of elements in substrate or surfaced,layer3. The ability of the substrate to withstand mechanical stress may also be important in processes (eg thermal spraying) where preparation of the component involves mechanical deformation (grit blasting). The ability to resist heat is necessary in components surfaced by 'hot' processes such as welding or spray fusing; since the adhesion of most surface coatings is improved by heating the substrate, heat resistance is in any case generally desirable. Large differences in the coefficient of thermal expansion between component and proposed surfacing material may prove troublesome, due to the generation of contractional stresses on cooling, and this can limit the choice. Finally, the geometry of the component, must be considered against the throwing power of the chosen surfacing process; in this respect, careful design can be invaluable. Counterface
The mating surface, against which the wear surface will run, is of particular importance in the surfacing selection, since this forms the other component of the wear couple. In many, if not most, cases this will be the reference surface and cannot be arbitrarily changed, so that the only scope for materials changes is via the surfaced layer. Where possible, attempts should be made to select materials for the couple with little mutual solubility; even with lubricated systems, this would be desirable against temporary break-down of the lubricant. Fig 3 gives some broad indications of solubility between materials 1.
materials; it is easy to overlook such a constraint, if there is not familiarity with the environme~atatdetails. Thus, components for use in nuclear reactors cannot usually be surfaced with cobalt-conta~ing materials since a long lived radioactive isotope may result from the exposure of the cobalt to irradiation; the,,use of copper alloys in surfacing may be unac~ptable for parts for aircraft fuel systems, where the presence of copper may crack the fuel and liberate free sulphur; again, possible toxic hazards must be considered in surfacing certain equipment, eg for food processing. It is particularly desirable to give preliminary consideration to such factors before the detailed wear selection is made. Design considerations
Surfacing, if it is to be used, should be considered at the component design stage; if this is done, subsequent processing is usually straightforward. If surfacing is to be used in the reclamation field, this approach will not be possible: where it is becoming widely accepted on original equipment, it is not only possible but should be mandatory. Appropriate standards are available 19 to aid the process but, in general, the guiding principles should be. 1. All surfaces should be accessible for surface preparation and shotlld, wherever possible, permit complete and uniform treatment. 2. There should be cleanliness of design and the elimination of dirt-retaining features. 3. The overall design should facilitate inspection, cleaning and maintenance. Abrupt changes in shape, sharp edges, deep recesses and blind holes should be avoided. While some of the vapour deposition processes have very good 'throwing power', it is still desirable for these, and essential for the traditional surfacing methods, to avoid these features. Apart from the actual surfacing operation, the design should be so arranged that it facilitates any finishing operations; since these tend to be labour intensive, difficulties, or extra rejects, at that stage can weigh heavily against the economics of the whole process.
Conclusion The overall justification for surfacing is an affirmative answer to two questions2°: 1. does it perform the function satisfactorily? 2. is it cheaper than alternative methods of obtaining adequate functional performance? Re WMoBeRhCrCoNi Ire PlCuTi Nb
,n Pb
Lal
Finishing
If it is required to finish machine or dress the component after surfacing, it will be necessary to choose a material that can, in fact, be economically finished. Many of the surfacing deposits can be machined only by grinding. Environmental factors
Consideration must always be given to environmental factors likely to be encountered by the component in the application that would exclude otherwise acceptable
, 3 Z
Z3 I
I
I
I 13131/ I
1141'1/
2 2141/"
Sn 2 3 I 4 I 5 3 21// Mg 2 2 y Cd 3 :3 3 / K e y AI 13 4 3 2 4 4 i / I Twoliquid )hoses, solid solution less thon 0.1% Ag 2 I 2 I// :) Twn lln.id )h0ses,s01id solution (:jreoter than Au 0,1% or one NquJd phase, solid solution less Zn than 0.1% Zr 3 One liquid phase,solid solution between OI % Wh and 1.0 ale Ti 4 One liquid phase, solid solution over 1% Blank boxes indicate insufficient information
Fig 3 Adhesive compatability of metal pairs
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The aim of the selection procedure must be to optimise the choice for the particular application, taking into account both technical performance and cost. The selection of surfacing treatments has tended to be a largely empirical process, based upon a knowledge of what worked well in previous, although possibly slightly different, circumstances. This must continue to be the basis of selection, since insufficient is known of the relationships between wear resistance and materials characteristics, such as composition, structure, etc, to allow a wholly theoretical basis for selection. It has been the aim of this article to outline how, within the empirical context, the approach to selection can be refined. It should be emphasised that it is only by consistently good treatment selection, coupled with good operational practice, that the potential benefits of surfacing will be obtained. It may be sufficient, in many cases, to specify a treatment or material that does not give the component the highest technical capability but which is economic and performs adequately; that there are real dangers in this approach, if taken too far or applied inexpertly, does not invalidate the concept. Equally, it may be desirable to specify a relatively simple surfacing process that is well understood and easily controlled in preference to a technologically more advanced process, which may pose problems of control 2a . Underlying the discussions in the paper is the problem of quality. Quality assurance in surfaced components, especially those with a coating, has traditionally caused concern to the engineer. Reliable non-destructive tests do not generally exist and tests must be conducted on test pieces or scrap components; only simulated service tests, which tend to be expensive and inconvenient, are likely to have any real validity. Quality assurance therefore depends upon simple, but stringent, control of each stage of the processing cycle, rather than upon sophisticated testing. A standard procedure should be followed, based where appropriate on a national specification or code of practice. Faulty materials or design, or incorrect deposition practice, can produce a poor result that may not be realised until the part goes into service. Poor quality cannot be tested out; rather, good quality should be built in, so that inspection then becomes a verification that good practice has been observed 8. It is expected, as materials costs rise and operating conditions become more severe, that opportunities for surfacing will increase and this will be reflected in greater application of such techniques. However, consideration should be given, in any potential wear selection, to avoiding wear by improved design as an alternative to containing the effects by surfacing. Good tribological practice demands
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TRIBOLOGY international April 1978
the use of surfacing with discretion a4. However, where it is specified, all possible steps should be taken to make the selection as efficient as possible.
References 1.
Donovan M. and Sanders J.L. Surface Coatings : 1 - Introduction, Tribology, Vol 5, No 5 (1972) 205-206
2.
Wear Resistant Surfaces (T.C. Wells et al) International Research and Development Co. Ltd., 1977
3.
James D.H. Component Reclamation by Plasma Spraying, Paper to 8th International Metal Spraying Conference, Miami, (1976) 148-155
4.
Donovan M. The Purpose and Scope of Surfacing, 'SurJace Coatings for Savings in Engineering' Welding Institute Seminar (1972) 1-3
5.
Surface Treatments in Engine Component Technology, Wear, Vol 34,
James D.tL, Smart R.F. and Reynolds J.A.
No 3 (1975) 373-382
6.
Graham J.A.S., Jesper A.C, and Wright K,W. Hard Surface Coatings, Selection/Application, Tribology Handbook
7.
Teer D.G. The Ion Plating Technique, Surfacing Journal,
8.
Smart ILF. Quality Control of Sprayed Coatings, Paper to
(M.J. Neale Ed.) Butterworths, London (1973) A42 Vol 8, No 4 (1977) 11-14 8th International Metal Spraying Conference, Miami (1976) 195-202
9.
Avery H.S. Selection of Materials for Wear Resistance, 'Surface Protection Against Wearand Corrosion ' ASM, Cleveland (1954) 10-40
10. 11.
Gregory E.N. Hard Surfacing by Welding for Improved Wear Resistance, Surfacing Journal, Vol 7, No 2 (1976) 5-10 SilmanH. Electroplated Coatings, Electroless Deposition and Anodising, 'Design for Surface Coatings' WeMing Institute Seminar (1976)
12.
Wright K.H.R. Mechanisms of Wear, Tribology Handbook
13.
Hurricks P.L. Some Aspects of the Metallurgy and Wear Resistance of Surface Coatings, Wear, Vol 22, (1972) 291-320 Hurricks P.L. Sprayed Surface Coatings, Industrial Lubrica-
(34..[. Neale Ed) Butterworths, London (1973) F6
14.
tion and Tribology, Nov/Dec (1973) 225-228 and Jan/Feb (1974) 8-12
15. 16.
Thomas A. Wear of Drop Forging Dies, ISI Publ 12.5 (1970) Finkin E.F. What Happens when Parts Wear, Industrial
17.
Lubrication and Tribology, Vol 23, No 9 (1971) 345 WilliamsP.R. The Mechanics of Wear, 'Weld SurJacing ,for Production, Repair and Reclamation in Heavy Engineering '. Welding Institute Seminar (1977) Paper 1
19.
.20.
British Standard 4479, 4495 and 4761; Code of Practice CP 3012, British Standards Institution Waterman N.A. Surfacing Opportunities for Designers, 'Design for Surface Coatings' WeldingInstitute Semhzar (1976)
21.
Hazzard R. Surface Coatings : 2 ; Arc Welded Coatings, Tribology Vol 5, No 5 (1972) 207-214
The appr(:,ach to selection A pre-requisite to the specification of a surfacing treatment is an understanding of the many surfacing processes that are available and of the characteristics of the surfaced layers they produce. The variety of processes, or process variants, commercially available or under development can be confusing, particularly because of the use of proprietary designations for many treatments. The primary requirement in the selection procedure will normally be to identify the type of wear to which the component will be exposed in service, taking into account the presence of elevated temperatures or corrosive media, etc. However, wear is a difficult phenomenon to analyse and prescribe against, since industrial wear situations may involve more than one type of wear ~ and confirmation of the analyses can only be obtained by examination of the worn surfaces. The selection based on wear must be complemented by considering other relevant factors, eg the cost and *Mater~ls Engineering Department, Associated Engineering Developments Ltd., Cawston House, Cawston, Rugby, Warwickshire CV22 7SA, UK "/'The term "surfacing' is used as a general term," "surface treatment" is used for processes that do not involve the build up o f a deposit on the surface, and 'surface coating'for those in which a coating is deposited
availability of the preferred surfacing material, the surfacing equipment that is available, the composition of the substrate and the geometry of the component, the nature of the mating surface, the service environment generally, etc. A typical list of factors that should be considered most conveniently in a 'checklist' fashion - is shown in Table 13 ,4. In some applications, the ancillary factors assume overwhelming importance and selection may best be approached by what has been described a as 'the technique of progressive elimination'. Despite the difficulties imposed by lack of information, the overall aim of any selection procedure should be the specification of an optimum treatment, based ripen technical and economic considerations, for the particular component and application.
Surfacing processes Surfacing treatments broadly fall into three categories and may be produced by mechanical, physical, or chemical processes, or by combinations of these. Table 2 lists treatments of tribological interest s . Considerable variation is observed among the processes, as Table 3 illustrates z ,a ,6. The practical thickness that can be applied varies with both process and material. With sprayed deposits, very thick coatings can be produced but residual stresses, arising from thermal mismatch between coating and substrate, build up and may cause spalling: in practice, therefore, the thickness is usually limited. Electrodeposits are also limited by residual stresses (usually due to hydrogen inclusions), as well as by the economics of the relatively slow deposition rate. Thicker coatings can usually be applied with safety by weld deposition or by spray fusion, while thermal and thermochemical treatments can be chosen to cover a range of thicknesses. Most welding processes, and detonation spraying, can be used only on fairly simple substrate shapes. The other spraying techniques can cope with a much greater range of component geometries and tend to be limited only by the need to present the torch normal, or at a slight angle, to the substrate. Plating techniques are also versatile, provided suitably shaped electrodes are available. Spraying and welding processes are essentially line-of-sight techniques, whereas electrodeposition has a limited ability to deposit beyond this (expressed as the throwing power). One of the interesting features of plasma assisted vapour deposition (eg sputtering and ion plating)
TRIBOLOGY international April 1978
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Table 1 Typical checklist to aid surfacing selection* Process selection
Materials selection
General factors
General factors
1. What processes are available 2. Available forms of surfacing materials 3. Quantity of coating required 4. Areas to be protected 5. Quantities required 6. Where is work to be done 7. Labour available 8. Possibilities of mechanisation 9. Services available 10. Costs
1. Knowledge of similar applications 2. Surfacing processes 3. Surface preparation 4. Future repairs 5. Are test pieces needed 6. Cost Operating environment
1. 2. 3. 4.
Wear type(s) Temperatures Corrosion problems Mating surfaces and adjacent material 5. Lubrication
The job
1. Size, weight and shape 2. Location and accessibility of area for production 3. Surface cleanliness 4. Limitations of adjacent areas 5. Hardness and melting point of substrate 6. Machinability 7. Component strength at temperature
Properties required
1. Wear Resistance 2. Chemistry 3. Adhesion 4. Porosity 5. Hardness 6. Microstructure 7. Wear Resistance 8. Thermal Properties 9. Machinability Substrate
Surface preparation
1. 2. 3. 4.
Methods feasible Equipment available Tolerances Register and datum point requirements 5. Undercutting 6. Presetting possibilities
Finishing
1. Surface function 2. Finish required 3. Dimensional accuracy and distortion limits 4. Equipment available 5. Post surfacing actions
1. Substrate composition 2. Previous surface treatments 3. Stresses in component 4. Importance of metallurgical condition 5. Surface hardness 6. Thermal expansion 7. Size and shape 8. Surface condition 9. Temperature sensitivity
*,,4dap ted from James3
is that they have sufficient throwing power to deposit on back surfaces 7 . The maximum rate of deposition of different coating techniques depends upon the nature of the process: with mechanical processes, the amount of material deposited per hour depends upon the size and power of the equipment;with chemical processes, rate is determined by chemical kinetics, while in physical vapour deposition, the rate is determined by energy input and extraction 2. In general, surface coating processes are faster than surface treatments, which are diffusion controlled; arc spraying and welding methods can give particularly high rates of deposition.
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TRIBOLOGY international April 1978
Coating costs are a factor of prime importance but they are difficult to discuss sensibly in the abstract, since the cost of treating an arbitrary (simple) shape gives little. guidance to the cost for a component. In general, electrodeposits are economic, especially on small components or areas, if thin coatings are required. Thermal deposition (apart from the very expensive detonation spraying) tends to be attractive for somewhat thicker deposits over large areas; the fusing operation can make spray fusing expensive, while welding tends not to be particularly cheap. Standard thermaland thermochemical (eg nitriding or carburising) treatments tend to be relatively cheap. The characteristics of the surfaced layers, which are shown in Table 4, also vary among the processes. Weld deposits have essentially cast structures whereas sprayed coatings (unless subsequently fused) show a much finer structure, with flattened particles, representing the build up by deposition of overlapping layers; they are characterised by marked directionality of properties, non-equilibrium structures, and possibly the presence of oxides. Electrodeposits are usually fine structures which, with alloy plates, frequently exhibit metastable phases; inclusions may also be present. The structures of thermally hardened steels usually comprise tempered martensites reflecting the heat treatment of the steel or cast iron. A wide variety of structural features is obtained with thermochemical processes, depending on the composition of the bulk material and the nature of the process used. Most of the processes aim at essentially pore-free deposits but thermal spraying almost always yields porous deposits, the amount varying with process and operating conditions. This can be sealed if necessary, although it can act as a useful reservoir for lubricant in wear applications, provided the reduction in strength caused by the porosity can be accepted. Values for bond strength of adhesion to the siabstrate should be regarded with caution - because of inherent difficulties in testing s - but, in general, overlay coatings, which have been deposited on a cold substrate and/or which have not been subsequently fused, have limited adhesion. Electrodeposition, spraying, and vacuum evaporation give modest adhesion values; plasma assisted vapour deposition appears to yield higher bond strengths due to the plasma conditions and good substrate cleaning that is possible, while chemical vapour deposition produces a high bond strength because of the elevated temperatures of the substrates. In welding type processes, the bond strength should be as good as that of the substrate; equally this is so in thermal and thermochemical treatments, where the treated layer is an integral part of the component. The effect on the substrate varies for the different processes. Thus, in welding and post spray fusing, distortion of the component may result, whereas this is not a problem with electroplating or thermal spraYing; however, the latter does require abrasive blast pre-treatment which may prove troublesome with thin gauge parts. With some deposits, reduction in base-material fatigue strength may result from surfacing. Changes in the composition of the substrate occur with thermochemical treatments but are not usually significant
Table 2 Classification of surface treatmentst
Type 1 Material added to surface A. Welding AI. Gas A2. Arc A3. Plasma
• C. Cladding
C1. Brazing C2. Explosive bonding
Type 3 Surfacemicrostructure altered
H. Interstitial hardening
K. Mechanical treatment K1. Peening K2. Rolling K3. Machining
E. Electrodeposition
El. Electrolysis (Aqueous) E2. Metalliding E3. Anodising E4. Electrophoresis
B. Thermal spraying B1. Flame
B2. Arc B3. Plasma B4. Detonation
Type 2 Surface chemistry altered HI. H2. H3. H4. H5. H6.
F. Vapour deposition
F1. Physical F2. Chemical G. Chemical deposition G 1. Chemical plating G2. Phosphating G3. Chromating
I. Diffusion treatment 11. Siliconising 12. Aluminising
Spark hardening Powder coating Organic finishes Painting
L. Thermal treatment
L1. Flame hardening L2. Induction hardening L3. Chill casting M. Thermo-mechanical treatment
M1. Martensitic work hardening
13. Chromising 14. Sulphurising J. Chemical treatment
D. Miscellaneous
D1. D2. D3, D4.
Carburising Nitriding Carbonising Cyaniding Sulphonitriding Boriding
C. Cladding
J1. Etching J2. Oxidation
C3. Diffusion bonding D. Miscellaneous D. Miscellaneous
D6. Impacting abrasives
D5. Hot dipping Multiple processes e.g. Plating followed by diffusion treatment - PVD followed by peening treatment ?After James, Smart and Reynolds s
with coating processes, except in the case of dilution with weld deposition. Taken as a group, surfacing processes offer great versatility and almost any material can be used for surfacing, provided the appropriate process is chosen. The main materials used in commercial work are listed in Table 5, most of these having been introduced empirically9" 11.
Wear T y p e
Wear M e c h a n i s m
M i l d a n d severe wear, scuffing ADHESION -
-
Fretting
-
-
Low stress abrasion
-
-
Machining wear
Selection for wear resistance
The various individual types of wear have been described in the previous paper in this issue. The mechanisms that result in the removal of material from the surface of a component are mainly adhesion, abrasion, and fatigue12; Fig 1, based on a classification due to Wells et al 2, shows how the different types can be related to these three basic mechanisms. Adhesive wear, in the presence of lubrication, is best resisted by the use of soft, dissimilar metal coatings with little tendency to mutual solubility. Where lubrication is marginal or absent, harder surfaces are required, the precise hdrdness being dictated by the contact pressure and the hardness of the contacting surface; with steel parts a hardness greater than 550 Hv is generally beneficial. It may be noted that the wear is greater with rough surfaces, so that a smooth surface is desirable. Scuffing is a particular form of severe adhesive wear that is observed during 'running-in' and requires transient protection. The different varieties of abrasive wear tend to present slightly different materials requirements and an attempt should be made to identify the particular type of abrasive. In general, a high shear stress in the surfaced material is necessary and this is commonly translated into
ABRASION
- -
Gouging Grinding (three-body)
Particle impact erosion
FATIGUE
-
-
C o n t a c t fatigue
-
-
P e r c u s s i v e wear
-
-
Cavitation
Fig I Classification of wear types a hardness requirement of 0.5 - 1.3 times that of the abrasive particles. A common rule-of-thumb is to aim at a hardness greater than 800 Hv, the hardness of the common abrasive silica sand. Harder materials tend to be selected for small, hard abrasives, with tougher materials more suited for larger abrasives, or where ancillary impact loads are significant. The thickness of the surfaced deposit will be related to the likely depth of abrasive penetration. Where low angle particle impact erosion is involved, hard brittle materials give good resistance; with
TRIBOLOGY international April 1978
99
CD ',,,d
0
0 r0 c) -< 5"
O0
0
3> 0.5
Versatile
Limited by plating bath
Electrical conductors
3> 50
Chemical cleaning and etching
Stress relieve
Deposition rate, kg/h
Component geometry
Component size
Substrate
Substrate temperature
Pre-treatment
Post treatment
None
Grit blasting
100
Almost limitless
Limited by handling equipment; no limit for hand spraying
Versatile (except detonation spraying)
Up to 10
0.1 -- 1.0
Thermal spraying
100
Care with edges
Good
Bond Strength, MPa
Integrity
Tolerances
*NA = n o t applicable: integral with substrate
Nil
Porosity, %
Electrodeposition
Table 4 Comparison of properties of surfaced layers
Fairly good
Mechanical bond: susceptible to edge chipping
20 - 140
1 - 15
Thermal spraying
*Surfaced layer formation typically O. 1 mrn/h for carburising and 0.01 m m / h for nitriding
0.02 - 0.5
Thickness, mm
Electrodeposition
Table 3 Comparison of surfacing process characteristics
0.2 - 2 (carburising) 0.5 - 0.75 (nitriding)
1-6
2 - 20
Thermochemical treatment Nil NA NA
Good with due all owa nces
Thermal treatment Nil NA NA
Good
Welding Nil NA High deposit integrity Poor
Nil NA* Metallurgical bond Moderate
Stress relieve
Mechanical cleaning
1400
925 (carburising) 525 (nitriding)
Fe Fe 900
Equipment limitation
Few
No limit
Usually Cu, Fe super-alloys
Versatile
Versatile
Simple Shapes
3 - 50
Thermochemical treatment
Thermal treatment
Welding
Spray fusing
None
Grit blasting
1050
A b i l i t y to withstand heat
No limit by hand
Versatile
Up to 10
0.5 - 1.5
Spray fusing
high angle impact, however, materials that are tough and ductile, and can absorb substantial amounts of energy before fracture, would be preferred. If cavitation erosion is present, it is desirable to use a material with a high value of ultimate resilience (ie ½ (ultimate tensile strength) 2/elastic modulus), provided the corrosion can be contained. In contact fatigue, the surface is subjected to repeated cyclic stresses due to impact or rolling motion, and these can cause surface cracking. The need, in this case, is for a material with high fatigue strength (broadly equated to high yield strength and hardness) but with adequate toughness. 'Care is necessary in selecting'suitable core or substrate conditions in surfaced parts, to ensure that subsurface cracking, or cracking at the interface between treated layer and substrate, is avoided. Table 5 Typical materials used for surface coatings for wear applications Material
Hardness, Hv
Remarks
1000 400 - 700 400
Hard chromium
Cobalt Alloys Lead/Ti n Nickel or Chromium Composite
500 10 400 - 450
Hardened with sulphur Running-in aid
330 170 390 300 100 - 150 300 - 700 1300 - 1800 1100 2200 2400
Spray fused deposits Nickel + BiSi Nickel-Chromium + BiSi Cobalt-Chromium + BiSi (W) Cr-Ni + BiSi + WC Cr-Ni + BiSi + NbC
220 - 300 300 - 600
450 - 700
Welded deposits Iron base with < 20% additions Iron base with > 20% additions Nickel or Cobalt base Carbides Copper Alloys
250 - 650
Group 1
500 - 750
Group 2
300 - 720 > 900
I~ HIGH LOW
I
~1~I${I}
Mortlms ~t~e~;I¢linlesS
HEAT AND CORROSION RESISTANCE [ st~le] 8t8 C,-Ni (2} $#oinl.s I.
H'gh steelsS,O" {L, 2 )
H rr~r Au:s,enitic lll~t© onO items ( 2 ) 1
COboll * end ~ickel- bcl~e 0 t |Oy${ 3) HIGH--~
IMPACT RESISTANCE - -
] I
HIGH
-z-LOW
Fig 2 Comparison of the properties of weld surfacing alloys. Figures in parenthesis indicate the alloy group
The above considerations must be modified if elevated temperature or corrosive conditions are likely to be experienced.
Table 5 includes some hardness data for the common surfacing materials. Wells et al 2 have recently published information on hardness, maximum service temperature, and corrosion rating for a fairly wide selection of surfacing materials while Avery, in an early paper 9, reports useful data on hot hardness and abrasion resistance for hardfacing materials. Comprehensive property data, however, are not easily available. It should be emphasised that the properties in the form of surfaced layers may differ significantly from the values for nominally similar materials in bulk form. The surface layer may not be in equilibrium, so that the phases present are different to those in cast or forged counterparts. Hardness measurements on thin coatings can be affected by the thickness of the deposit, the hardness of the substrate, and the presence of porosity. The latter, together with inclusions in coatings, affect mechanical properties, as does the presence of residual stresses. Particular care is therefore necessary in the extrapolation of property values for bulk materials.
Thermally sprayed deposits 13% Steel 18/8 Stainless Steel Molybdenum NiAI AI Bronze Stellite WC/Cobalt Cr3 C2/N ickelChromium AI 2 03 Cr2 O3 Zr 02
ABRASION RESISTANCE
Materials properties and structure
Electrodeposits Chromium Iron Alloys Nickel
LOW ~
Group 3 Group 4 For bearing overlays
In general, there will be a requirement in the surface layers for strength, which may be equated, approximately, with hardnesS, and toughness. However, high hardness tends to be associated with low impact resistance, notch sensitivity, and poor resistance to crack propagation. Thus, selection against wear is frequently a compromise between hardness and toughness; there must be sufficient strength to resist the imposed loads but enough toughness to avoid failure due to surface imperfections or incipient fatigue cracks. If there is any doubt, the balance should be tilted in favour of extra toughness; a higher than optimum wear rate will ultimately reduce component life but less rapidly than complete failure of the comPonent. While surfacing materials are not generally arranged systematically in terms of impact resistance and toughness, the hardfacing alloys form a series of gradually varying properties (Fig 2) which can be helpful in materials selection 1°. It should be emphasised that, except with simple microstructures (eg hardened and tempered carbon steels), wear
TRIBOLOGY international April 1978
101
resistance tends only to be generally related to hardness. Especially under abrasive wear conditions, hardness will not be the sole criterion of wear performance, and any successful material must be able to undergo a significant amount of work hardening TM 14. Structure is also important in the ability to withstand abrasive wear. Except for austenitic manganese steels, which have a continuously work hardening matrix, most surfaced layers derive their wear resistance from the presence of carbides or nitrides; the type, amQunt, shape and distribution of these, as well as their support within the matrix, affecf the actual wear performance. Under pure abrasion, a coarse carbide seems desirable but where there is some impact a finer, well distributed carbide is probably preferable. The matrix structure is also important. The ability to predict the wear resistance of a potential surfacing material from its known composition or microstructure has so far received little study. Typically, such an analysis is not fundamental but attempts to relate relative wear (compared to a known standard material) to composition or microstructure constituents. One example gives as ~r = 204 - 70(C) - 4(S) - 15~/(Cr) - 80~(Mo) where ~r = wear index of test materiat relative to standard die steel, C -- % total carbon, S = % sum of all elements in solid solution, Cr = % chromium of carbide, and Mo = % molybdenum equivalent (strong carbide formers) = % M o + ½%W+ 2%V + % N b Alternatively, the relative volume of each structure element, and its relative wear resistance, may be incorporated in a more complex series of equations 16. None of these analyses, however, is yet near commercial usefulness.
Guide to selection Various studies to classify industrial wear have been reported 17 and, while the detailed results differ slightly, there seems to be general agreement that about half of all industrial wear situations are essentially abrasive in nature, about 15% involve adhesive wear and the remaining 35% comprise a combination of fatigue, fretting erosion and cavitation, and chemical attack and oxidation. These types are thus the modes of wear most likely to be encountered. In adhesive wear situations, provided there is lubrication, thermally sprayed deposits of chromium or molybdenum, for example, are useful, while electrodeposited hard chromium is well established; surface coatings with some inherent porosity have the ability to retain oil and this can be useful, in case of temporary breakdown of lubrication. However, when lubrication is generally marginal or absent, harder coatings such as plasma sprayed ceramics or carbides (with metallic binder) would be needed. Electroless r~ickel has proved useful under adhesive wear conditions, as has, more recently, vapour deposited titanium carbide. For scuffing, short life treatments such as phosphating or nitrocarburising are useful. Under conditions of fretting, sprayed copper alloys, anodised aluminium (or possibly titanium), plasma sprayed oxides or carbide cermets, or nitrocarburising may be used. Turning to abrasive wear, low stress abrasion and machining wear are best combated by hard coatings, chromium
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TRIBOLOGY international April 1978
carbide often being the most economical solution; in grinding abrasion, tungsten carbide is frequently specified. For these types of abrasion, martensitic irons may be used as an alternative; this gives a smooth wear surface (which is advantageous) as well as resisting well metal-tometal wear and light impact. Thermal and thermochemical treatments may also be used in these cases. For resistance to gouging, weld deposits would normally be specified. The choice would thus tend to be carbides for hard small abrasives, through the martensitic alloy steels, through to the 14% manganese and 14% manganese-14% chromium steels a~ the need for toughness with large particle abrasion increased. With particle impact erosion, weld deposits or plasma sprayed metals or carbides would be favoured, if the impact were at high angles; hard coatings in general will withstand low angle impact provided they are harder than the abrading particles themselves. In contact fatigue situations, thermal and thermochemical treatments, weld deposits and spray fused coatings are all used. If the stresses are low, nitrocarburising may be an economic solution but as the applied stress increases, nitrided or carburised cases would be needed. If percussive wear is involved, a range of treatments is possible, including weld deposits, sprayed carbide/cobalt alloys, composite electrodeposits such as Co/Cr3C ~ , and thermal and thermochemical treatments. The cast cobalt alloys of the stellite type are resistant to cavitation erosion as are bronze, nickel and tit~mium alloys; hard chromium plating is also used to withstand these conditions. If operation is to be at elevated temperature it should be noted that softening of thermal and thermochemical treatments above 200°C will occur while above ~500°C the the choice is largely restricted to chromium and cobalt containing weld or sprayed deposits. Many metallic materials form a naturally protective film or a passivated reaction layer, but this will tend to be removed by the mechanical action of wear, so that bare metal is continuously exposed to the media, which will thus appear unusually aggressive. With increasing temperature, the corrosion rate increases and, above ~500°C, the process will tend to become one of gross oxidation. The principal aim in materials selection must be the prevention (or at least the substantial reduction) of the corrosive effect. For more detailed guidance on selection, references such as 2 and 9 should be consulted, or papers on the specific processes.
Ancillary factors in selection Factors other than the ability to withstand wear play some part in the choice of surfacing treatment; the particular weight to be given to these varies with the application, but in all cases they should be considered.
Availability Any selection must be compatible with the available equipment and with the skills available to control it reliably and reproducibly. The capital costs of different surfacing processes differ greatly and this may be important in choice. In general, however, where equipment for a range of processes is available, it may still be preferable to
select a simple, well-tried technique or material rather than one of more advanced technology. Another factor, more usual with reclamation work, is the necessity in some cases, to take the surfacing equipment to the component, which deafly rules out a number of processes. As far as consumables are concerned, the primary criteria are cost and availability in the correct form; the latter requirement is particularly important for those processes (eg weld deposition or arc spraying) which require the consumable to be presented in wire form, which may not in all cases be easy or economic. Substrate
One of the most important of the ancillary factors is the surface of the component to be protected; this is normally the bulk material although it can be an intermediate layer. Reasonable chemical compatibility between substrate and surfacing material is necessary, to prevent excessive chemical interaction during processing or in service; the higher the temperatures, the faster the rate of interdiffusion of elements in substrate or surfaced,layer3. The ability of the substrate to withstand mechanical stress may also be important in processes (eg thermal spraying) where preparation of the component involves mechanical deformation (grit blasting). The ability to resist heat is necessary in components surfaced by 'hot' processes such as welding or spray fusing; since the adhesion of most surface coatings is improved by heating the substrate, heat resistance is in any case generally desirable. Large differences in the coefficient of thermal expansion between component and proposed surfacing material may prove troublesome, due to the generation of contractional stresses on cooling, and this can limit the choice. Finally, the geometry of the component, must be considered against the throwing power of the chosen surfacing process; in this respect, careful design can be invaluable. Counterface
The mating surface, against which the wear surface will run, is of particular importance in the surfacing selection, since this forms the other component of the wear couple. In many, if not most, cases this will be the reference surface and cannot be arbitrarily changed, so that the only scope for materials changes is via the surfaced layer. Where possible, attempts should be made to select materials for the couple with little mutual solubility; even with lubricated systems, this would be desirable against temporary break-down of the lubricant. Fig 3 gives some broad indications of solubility between materials 1.
materials; it is easy to overlook such a constraint, if there is not familiarity with the environme~atatdetails. Thus, components for use in nuclear reactors cannot usually be surfaced with cobalt-conta~ing materials since a long lived radioactive isotope may result from the exposure of the cobalt to irradiation; the,,use of copper alloys in surfacing may be unac~ptable for parts for aircraft fuel systems, where the presence of copper may crack the fuel and liberate free sulphur; again, possible toxic hazards must be considered in surfacing certain equipment, eg for food processing. It is particularly desirable to give preliminary consideration to such factors before the detailed wear selection is made. Design considerations
Surfacing, if it is to be used, should be considered at the component design stage; if this is done, subsequent processing is usually straightforward. If surfacing is to be used in the reclamation field, this approach will not be possible: where it is becoming widely accepted on original equipment, it is not only possible but should be mandatory. Appropriate standards are available 19 to aid the process but, in general, the guiding principles should be. 1. All surfaces should be accessible for surface preparation and shotlld, wherever possible, permit complete and uniform treatment. 2. There should be cleanliness of design and the elimination of dirt-retaining features. 3. The overall design should facilitate inspection, cleaning and maintenance. Abrupt changes in shape, sharp edges, deep recesses and blind holes should be avoided. While some of the vapour deposition processes have very good 'throwing power', it is still desirable for these, and essential for the traditional surfacing methods, to avoid these features. Apart from the actual surfacing operation, the design should be so arranged that it facilitates any finishing operations; since these tend to be labour intensive, difficulties, or extra rejects, at that stage can weigh heavily against the economics of the whole process.
Conclusion The overall justification for surfacing is an affirmative answer to two questions2°: 1. does it perform the function satisfactorily? 2. is it cheaper than alternative methods of obtaining adequate functional performance? Re WMoBeRhCrCoNi Ire PlCuTi Nb
,n Pb
Lal
Finishing
If it is required to finish machine or dress the component after surfacing, it will be necessary to choose a material that can, in fact, be economically finished. Many of the surfacing deposits can be machined only by grinding. Environmental factors
Consideration must always be given to environmental factors likely to be encountered by the component in the application that would exclude otherwise acceptable
, 3 Z
Z3 I
I
I
I 13131/ I
1141'1/
2 2141/"
Sn 2 3 I 4 I 5 3 21// Mg 2 2 y Cd 3 :3 3 / K e y AI 13 4 3 2 4 4 i / I Twoliquid )hoses, solid solution less thon 0.1% Ag 2 I 2 I// :) Twn lln.id )h0ses,s01id solution (:jreoter than Au 0,1% or one NquJd phase, solid solution less Zn than 0.1% Zr 3 One liquid phase,solid solution between OI % Wh and 1.0 ale Ti 4 One liquid phase, solid solution over 1% Blank boxes indicate insufficient information
Fig 3 Adhesive compatability of metal pairs
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The aim of the selection procedure must be to optimise the choice for the particular application, taking into account both technical performance and cost. The selection of surfacing treatments has tended to be a largely empirical process, based upon a knowledge of what worked well in previous, although possibly slightly different, circumstances. This must continue to be the basis of selection, since insufficient is known of the relationships between wear resistance and materials characteristics, such as composition, structure, etc, to allow a wholly theoretical basis for selection. It has been the aim of this article to outline how, within the empirical context, the approach to selection can be refined. It should be emphasised that it is only by consistently good treatment selection, coupled with good operational practice, that the potential benefits of surfacing will be obtained. It may be sufficient, in many cases, to specify a treatment or material that does not give the component the highest technical capability but which is economic and performs adequately; that there are real dangers in this approach, if taken too far or applied inexpertly, does not invalidate the concept. Equally, it may be desirable to specify a relatively simple surfacing process that is well understood and easily controlled in preference to a technologically more advanced process, which may pose problems of control 2a . Underlying the discussions in the paper is the problem of quality. Quality assurance in surfaced components, especially those with a coating, has traditionally caused concern to the engineer. Reliable non-destructive tests do not generally exist and tests must be conducted on test pieces or scrap components; only simulated service tests, which tend to be expensive and inconvenient, are likely to have any real validity. Quality assurance therefore depends upon simple, but stringent, control of each stage of the processing cycle, rather than upon sophisticated testing. A standard procedure should be followed, based where appropriate on a national specification or code of practice. Faulty materials or design, or incorrect deposition practice, can produce a poor result that may not be realised until the part goes into service. Poor quality cannot be tested out; rather, good quality should be built in, so that inspection then becomes a verification that good practice has been observed 8. It is expected, as materials costs rise and operating conditions become more severe, that opportunities for surfacing will increase and this will be reflected in greater application of such techniques. However, consideration should be given, in any potential wear selection, to avoiding wear by improved design as an alternative to containing the effects by surfacing. Good tribological practice demands
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the use of surfacing with discretion a4. However, where it is specified, all possible steps should be taken to make the selection as efficient as possible.
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