Paper XV (i) The inter-relationship between coating microstructure and the tribological performance of PVD coatings

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Paper XV (i)

The inter-relationship between coating microstructure and the tribological performance of PVD coatings S.J. Bull and D.S. Rickerby

It is well known that the microstructure of physical vapour deposited (PVD) coatings controls many of the properties (hardness, adhesion, residual stress) which are important in tribological applications. However, it is not generally recognised that the coating microstructure dictates the operating wear mechanism, both indirectly through determining the range of properties of a particular coating, and directly through its promotion of wear mechanisms that depend purely on the morphology of the coating. In this paper the effects of microstructure on hardness, internal stress and adhesion are briefly reviewed and the relevance of these properties to wear applications is discussed for titanium nitride coatings.

1. INTRODUCTION

2. MICROSTRUCTURE AND PROPERTIES

The microstructure of a material exerts a considerable influence on its properties, and indeed the subject of materials science is basically concerned with the investigation of the interrelationship between microstructure and macroscopic properties. For vapour-deposited materials the coating is comprised of columns which are aligned perpendicular to the coating/substrate interface, and it is this columnar microstructure which affects the physical properties of the film [l].Due to this aligned growth (texture) the structure of the coating is highly anisotropic and this leads to anisotropy in many of the physical properties of the film [2]. However, a certain amount of microstructural control is possible since manipulation of the processing parameters can result in changes in the microstructure of the deposited coating (for example by variations in the packing density of the columns and the grain boundary strength which binds them together), and thus a knowledge of the inter-relationship between microstructure and properties allows engineering of the surface to produce optimum coating properties for a particular application. In this paper the factors affecting the microstructure of vapour deposited coatings are briefly reviewed and the relationship between microstructure and those physical properties that are important in wear applications is introduced. Finally the wear behaviour of titanium nitride coated steel substrates is discussed in terms of the microstructure and physical properties of the coatings.

2.1 Fundamentals of Physical Vauour Deposition (PVD): Structure Zone Models (SZMs)

In physical vapour deposition processes coatings are formed from a flux of atoms that approaches the substrate from a limited range of directions and as a result of this the microstructure of the coating is columnar in nature. There are a number of ways in which the coating flux may be produced, but these break down essentially into two groups, sputtering and evaporation. In both cases a columnar microstructure is produced and many of the methods for changing the structure may be applied equally to both deposition technologies, but there are some differences in structure which are a function of the source of coating flux. An example of this is discussed in section 2.4. Mochvan and Demchishin [31 were the first to classify thin film microstructures and identified three distinct structural zones as a function of the homologous deposition temperature. The low temperature zone 1 microstructure consists of tapered columns with domed tops and is determined by conditions of low adatom mobility. As the surface temperature is increased, surface diffusion becomes more important and the microstructure of the coating consists of parallelsided columnar regions which have a smooth surface topography (zone 2 ) . At the highest temperatures, bulk diffusion also becomes important and the zone 3 microstructure consists of equiaxed grains. Later work by Thornton 141 suggested that the presence of a sputtering gas

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E D C

BombardmentInduced Mobility

ThermalInduced MobiIity

Characteristic size A = 1to3nm B = 5to20nm C = 20to40nm D = 50to200nm E = 200to400nm

Figure 1 Structure zone model for P V D coatings after Messier (51. would modify the Mochvan and Demchishin model and a further region was identified (called zone T) which consists of poorly defined fibrous grains. 2.2 The Effects of Ion Bombardment: Ion Plating

As an alternative to increasing the deposition temperature, zone 1 microstructures can be overcome by bombardment of the films with particles of sufficient energy to cause the filling of the voided boundaries by coating atoms which will lead to a denser structure of the the zone T type. This filling can take place by the knock-on of already deposited atoms, but is mainly due to bombardment-induced mobility of these atoms. Messier et a1 [51 have suggested improvements to the SZMs which take into account the evolution of the microstructure with increasing film thickness and the effects of thermal and bombardmentinduced mobility of coating atoms. This model shows that ion bombardment promotes a dense structure of the zone T type, but also that the atomic movements responsible for the formation of the film can have either a thermal or a bombardment induced origin: see Figure 1. Such bombardment of the coating during growth can be provided by the application of a suitable bias to the substrate, providing that the coating flux is sufficiently ionised. For sputtering this is usually the case but for coating fluxes produced by evaporation it is necessary to increase the degree of ionisation of the flux, often by the use

of a plasma as in plasma assisted PVD [6]. The general term for all those processes where a substrate bias is used to promote the formation of dense coatings is ion plating. The application of a substrate bias during deposition of a coating has a profound effect on the growth and microstructure of PVD thin films [7-101. Figure 2 shows the microstructure of titanium nitride films deposited onto stainless steel by sputter ion plating under two different substrate bias conditions. The unbiased film (Figure 2a) shows an open columnar microstructure (zone 1 type) whilst the biased film (Figure 2b) appears more dense, the individual columns being less well defined (zone T). Coatings with these microstructures will have substantially different physical properties due predominantly to changes in the packing density of the columns which comprise the microstructure [ll]. 2.3 Microstructure/Propertv Relationships

The changes in properties which can be expected on going from zone 1 to zone T microstructures are presented in Figure 3 which shows the variation in internal stress, hardness and critical load for coating detachment as determined from the scratch test (a qualitative measure of coating/substrate adhesion) as a function of substrate bias voltage for sputter deposited titanium nitride coatings on a stainless steel substrate. For zone 1 microstructures the open columnar boundaries ensure that any residual stresses in the deposit are kept to a minimum [2, 11-13]. By virtue

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Figure 2

Scanning electron fractographs (secondary electron imaging) of ( a ) unbiased and (b) -6OV biased titanium nitride coatings deposited onto a stainless steel substrate by sputter ion plating.

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Variation in coating hardness, internal stress and critical load for coating detachment with substrate bias voltage for titanium nitride coatings deposited onto austenitic stainless steel by the sputter ion plating process.

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of the open microstructure and the subsequent reduced load-bearing capacity of the surface, the hardness of the coating is dramatically reduced over that of the more dense zone T microstructures formed at higher bias voltages. The hardness of these high bias coatings is comparable to that of bulk titanium nitride. However, it should be realised that the hardness of the individual columnar grains will be identical for the zone 1 and zone T microstructures. A direct consequence of the denser microstructures formed at higher bias voltages is that substantial stresses can be generated within the coating. Indeed the apparent increase in density of PVD coatings when moving from zone 1 to zone T microstructures can also be correlated with the levels of residual stress present in the coatings [ll].

Variations in the levels of elastic strain energy stored within the coating accompany these microstructural changes and with increasing coating thickness it eventually becomes favourable for the system to minimise its stored elastic energy by coating spallation. Thus the presence of internal stresses will limit the maximum thickness to which a coating can be deposited. As the substrate bias increases (that is the coating becomes denser), the effective adhesion of the coating is reduced due to the build-up of residual stress and the critical load for coating detachment, Lc, as determined from the scratch test, is also reduced [14-151. Although the Young's modulus of the individual columns will be the same for zone 1 or zone T microstructures, the effective modulus of the coating will increase as a function of bias as the coating density increases.

Figure 4 (a) A n etched metallographic section of a FeCrAlY coating showing a zone 1 type microstructure; (b) - (d) schematic variations in (b) hardness, (c) internal stress and ( d ) density with distance from the coatinglsubstrate interface for zone 1 microstructures (dotted line) and zone 2 microstructures (solid line).

34 1

Zone 1, zone T and zone 2 microstructures are associated with the preferred orientation of the growing columns and this development of texture in PVD films has interesting consequences when it comes to understanding the inter-relationship between microstructure and properties. For titanium nitride, the most commonly reported texture is (111) although (200) and (220) textures have been observed [6, 151. The development of texture in PVD coatings occurs in three stages [21:(a) Nucleation - crystallites are nucleated on the substrate from the vapour phase. The distribution and size of these will depend on the nature of the substrate. (b) Competitive growth - certain favourably oriented nucleii will grow faster than the remainder of the crystallites. These may not constitute the majority of the nucleii population. (c) Steady growth - once a preferred orientation has achieved dominance, steady state growth will occur. The detailed deposition conditions of any particular PVD process can change all of these stages and it is this which explains the variations in texture and physical properties reported for nominally identical films. These stages in coating growth will result in changes in the microstructure of titanium nitride coatings with increasing thickness. Initially at the interface region a very fine grain size is established and little or no preferred growth is observed [171. With increasing thickness there is an increase in average column size and a tendency towards a (Ill) preferred orientation in the outer region of the coating. Figure 4 illustrates that for PVD films the levels of internal stress and hardness will thus vary through the coating thickness. For zone 1 and to a lesser extent zone T microstructures (the dotted bands in Figure 4) both hardness and internal stress decrease with distance from the interface. The expected variations for zone 2 microstructures (solid bands) show much less variation. Due to the increase in average grain size the yield strength of the coating will decrease towards its outer surface [171 and thus the stresses supported by the outer regions of the coating will be much smaller than those that can be supported by the interfacial regions. This behaviour is modified by the changes in density produced by the competitive growth process. Figure 5a shows a fracture section through a 6pm thick titanium nitride coating on a stainless steel substrate. In the interface region the coating is dense, but the density is much reduced at about 34pm from the interface where the coating begins to adopt the {111)texture. As the coating thickness is further increased, the density increases again as the (111)texture is established and steady-state growth begins. Figure 5b shows the variation in coating hardness with thickness for titanium nitride on stainless steel. The coating hardness is the best-fit value determined from the application of the volume law-of-mixtures hardness model and is independent of the hardness of the substrate [15, 181.

A hardness minimum occurs at about 4pm which corresponds to the position where the coating density is reduced. Thus the coating microstructure exerts a considerable influence on its properties and it is important to know how the coating microstructure evolves during deposition if the properties of a coated component are to be fully understood.

2.4 Effect of Microstructure

Deposition Technology

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Figure 5 (a) Scanning electron fractograph of a 6pm titanium nitride coating on a stainless steel substrate. (b) Variation in coating hardness (as determined from the volume law-of-mixtures hardness model f15, 181) with coating thickness.

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Figure 6

Scanning electron micrographs of the surface topography of titanium nitride coatings deposited by (a) arc-evaporation and (b) sputter ion plating.

produced by a range of deposition technologies are very similar, differences do arise due to variations in processing route and this should always be realised when attempting to make comparisons between identical coatings deposited by different techniques. An example of this can be seen in the microstructures of titanium nitride coatings produced by sputtering and arc-evaporation (Figure 6 ) . The surface topography of the sputtered coating is flat indicating a dense zone T type microstructure whereas the surface finish of the arc-evaporated coating is much rougher, despite the fact that a similar structure would be predicted by the SZMs. Due to microdroplet formation during the deposition process, the arc-evaporated coating contains particles, some of which are pure titanium metal and others of which have a titanium rich core surrounded by titanium nitride [18], and these cause a reduction in the internal stress in the coating since they allow some stress relaxation to occur within the layer [20]. However, the presence of the titanium has little or no effect on the hardness or adhesion of the layer; these properties are very similar to comparable sputtered coatings.

with the complete range of desired properties as often these are inter-related and the properties of the film after deposition are a complex function of the film microstructure, the substrate material and geometry and the physics of the deposition process. In general there is a trade-off between important properties and thus these have to be optimised for a given application. It is important to determine which coating properties dictate its behaviour in any application if an appropriate coating for that application is to be produced. Results from laboratory wear tests can give some indication of the operating wear mechanisms that might be found in in-service conditions and those properties which are important for a particular mechanism can also be identified. However, if coatings are to be optimised for a particular application it will be necessary to look at the components in real service conditions to see if the wear mechanisms predicted in the laboratory tests remain appropriate. Only by such an approach will it be possible to truly engineer coatings for a particular application.

2.5 Engineering with Surface Coatings.

3. PROPERTIES OF COATINGS IN TRIBOLOGICAL APPLICATIONS

By manipulating the processing conditions for a particular deposition technology it is possible to control the microstructure of the film to a certain extent and hence control its properties. There is, however, no simple scheme for producing a film

3.1 Hardness

For cutting tool applications and other situations where abrasive wear is dominant it is

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3.2 Internal Stress

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Zone,l Zone T, low stress Zone T, high stress

0 Number of cycles

Figure 7 The variation in weight loss with number of cycles for uncoated and titanium nitride coated stainless steel tested under mild abrasive wear conditions; %uncoated stainless steel; zone 1 type microstructure; 0 CI zone T type microstructure (low stress); W zone T type microstructure (high stress).

often assumed that it is the high hardness of the coating that results in improvements in wear resistance provided the adhesion of the coating to the substrate and the cohesive strength of the coating itself are adequate. However, it is not sufficient to consider the hardness of the coating in isolation, it is also important to consider the loadbearing capacity of the coating substrate system if wear behaviour is to be fully understood [ Z l l . Even given this knowledge, hardness is not a good predictor for coating properties because other factors, such as the chemical inertness of the coating, may also be important in determining the measured wear rates. The increase in hardness on going from zone 1 to zone T microstructures does initially lead to an increase in abrasive wear resistance (Figure 7 [ZZ]). With increasing substrate bias, the load support offered by the coating is greater and wear of the coating itself is reduced. However, the corresponding increase in internal stress with bias results in a reduction in wear resistance at high bias voltages d u e to its adverse effect on coating/substrate adhesion 1221. Zone T microstructures which have low levels of internal stress thus offer the most wear resistant coatings, but it is important to realise that the microstructure of the coating dictates the dominant wear mechanism which has an important effect on the wear rates measured in any wear test.

The magnitude and sign of the residual stresses in PVD coatings depends on the properties of both the coating and substrate material. The internal stresses in PVD coatings arise from two sources:Growth Stresses - generated during (a) deposition by the competitive growth of the columns that make up the coating microstructure. These are generally compressive. Thermal Stresses - generated on (b) cooling after deposition due to the thermal expansion mismatch between coating and substrate. These can be tensile or compressive depending on the choice of coating and substrate material. In general compressive stresses are most advantageous since these can act to close throughthickness cracks and densify the microstructure. Indeed the increase in compressive residual stresses as the bias voltage is increased can be directly correlated with coating hardness and thus the improvements in abrasive wear behaviour reported in the previous section. However, at high stress levels the reduction in adhesion produced by the build-up of stored elastic strain energy leads to a reduction in wear resistance. The generation of residual stresses within the coating limits the maximum thickness of coating that can be deposited and this restricts the use of PVD coatings in severe abrasive wear applications where the scale of the wear deformation is greater than the coating thickness. 3.3 Adhesion In many ways this is the most important coating property for most wear applications since spallation of the coating will result in unacceptably high wear rates ,due to the introduction of hard wear debris into the system. Under mild abrasive wear conditions, thin coatings fail by localised detachment of the film at the intersections of scratch tracks produced by the abrasive particles (Figure 8) [21]. Similarly under conditions of erosive wear, a tendency for coating spallation can lead to very high wear rates [231, and the growth of these localised regions of detached coating ultimately determines the wear rate and this depends on both the cohesive strength of the coating and the state of the coating/substrate interface. 3.4 Cohesive Strength Due to their columnar microstructure and relatively weak intercolumnar boundaries, sputtered titanium nitride coatings possess easy fracture paths normal to the surface of the substrate and are thus prone to through-thickness cracking if subjected to tensile stresses. This can happen, for

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Figure 8 The development of regions of detached coating in the abrasive wear of titanium nitride coatings: (a) a pit forms at the intersection of two scratches; (b) the pit at higher magnification showing microcracking at the edge where an abrasive particle has entered it; (c) detail of the microcracking; ( d ) the end of the scratch where the abrasive particle has left the pit, smearing substrate material over the coating.

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compared to those for metal/metal contacts (-0.5) provided that testing (or service) does not occur in vacuum [25]. Titanium nitride coatings have been shown to decrease the friction between surfaces and hence reduce the wear rates compared to uncoated surfaces by a number of workers [26-301. According to Bowden and Tabor [311, friction arises from two separate contributions; a ploughing term which reflects the plastic deformation that takes place during sliding and an adhesion term which reflects the interactions (chiefly chemical) between the slider and counterface. Because of their high hardness, ceramic coatings would be expected to reduce friction by reducing the ploughing term and indeed this does occur on the scale of asperity contacts even if plastic deformation in the coating or substrate is minimal. However, by far the most important factor is the adhesion between slider and counterface since this will be the dominant friction component unless gross plastic deformation occurs. Deposition of a thin metallic layer onto the surface of a sputtered titanium nitride coating has a considerable effect on the wear rates in single pass scratch testing using diamond stylii [14]. The use of a chemically reactive metal, such as titanium, which increases the adhesion between diamond and coating (and hence the measured friction) results in a dramatic reduction in the critical load for coating detachment. In contrast, the use of a non-reactive metal, such as lead, results in a

instance, as the coating bends to accommodate a scratch track produced by plastic deformation in the underlying substrate. Such cracking is most apparent for zone 1 microstructures (low bias voltages) where considerable cracking of the coating occurs during abrasive wear testing 1221. The open outermost regions of the coating are easily removed leaving the material in the interfacial region with its small grain size (and consequently better loadbearing capacity) to dominate the wear behaviour. The cohesive strength of the coating is also important for thicker coatings where wear occurs by a micropolishing or chipping mechanism within the coating itself [21]. Similar cohesive failures have been observed for thin high bias coatings, though in this case wear behaviour is dominated by coating/substrate adhesion [221. Thus the cohesive strength of the coating can play an important part in wear. It is basically determined by the choice of coating material, but the strength of intercolumnar boundaries is known to be a function of deposition temperature and so some improvements may be possible [231). 3.5 Friction Hard reduce the because the on ceramic

ceramic coatings are often used to friction between sliding components friction coefficients for ceramic sliders counterfaces can be very low (-0.1)

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Number of cycles

Figure 10 Weight loss as a function of sliding distance (number of cycles) for arc-evaporated and sputtered TI" on stainless steel tested under severe abrasive wear conditions. Best performance is achieved by the 4.lprn sputtered coating. reduction in friction and an increase in critical load. The adhesive effects of metallic titanium are very important in the understanding of the wear behaviour of arc-evaporated titanium nitride coatings where micro-droplet formation during deposition results in regions of unreacted titanium within the coating (see section 2.4). Under conditions of mild abrasive wear, arc coatings perform much better than similar coatings produced by sputtering (Figure 9), but the trend is reversed as the severity of the wear increases (Figure 10) [20]. For the mild abrasive wear conditions the abrasive particles do not penetrate the titanium nitride skin around the titanium micro-droplets and hence abrasive/ coating adhesion and coating wear rate are low. As the abrasive particles break through this skin and contact the titanium core the abrasive/coating adhesion will increase and this is accompanied by an increase in coating wear rate. Thus by adjusting the processing parameters to reduce the amount of metallic titanium in the coating (and hence the friction between the coating and any abrasive) the wear of a coated component would be considerably reduced. Perhaps the simplest way to minimise the effect of adhesion between coating and counterface is to use a suitable lubricant. However, careful choice of coating and control of its microstructure allows components to be used in applications where the presence of a lubricant would cause further problems.

3.6 Thermal Properties

In sliding wear the friction between the sliding surfaces results in the generation of heat which must be dissipated within the system. For metallic components heat may easily be conducted away from the contact due to the high thermal diffusivity of the materials and thus bulk and surface temperature rises are minimal during wear. However, the use of ceramic coatings with much lower thermal diffusivities will modify this behaviour and high surface temperatures may be recorded in sliding wear tests. For the sliding wear of steel spheres against titanium nitride coated discs the wear mechanisms of both sphere and disc vary as a function of substrate bias for this reason [30]. At low bias voltages (zone 1 microstructure) the main wear mechanism is adhesive wear due to the high sphere/coating adhesion (Figure 11). However, as the bias is increased and zone T microstructures form, the amount of transfer of material from the sphere to the flat is reduced and hence the adhesion between sphere and flat is reduced. This changes the main wear mechanism from adhesive wear to oxidative wear, initially of the sphere but also of the titanium nitride coating at the highest bias voltages (Figure 12). Though oxidative wear rates are lower than adhesive wear rates under the same testing conditions, they may be unacceptably high for a particular application and thus it may be necessary to choose a coating material that does not react during wear or use a lubricant to exclude the environment during service.

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Figure 21 Scanning electron micrographs of the worn regions for an uncoated steel sphere sliding on a -2OV bias TiN coated disc: f a ) inside the wear track on the disc, and f b ) flat on the sphere.

Figure 22 Scanning electron micrographs of the worn regions for an uncoated steel sphere sliding on a -230V bias TiN coated disc: ( a ) wear track on the disc, and f b ) flat on the sphere.

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3.7 Microstructure In this paper we have shown that the microstructure of a coating has an important effect of the behaviour of the film in a given application because it controls the properties of the coating material. The particular combination of properties that are determined by the coating microstructure will ultimately dictate the main operating wear mechanism. However, irrespective of the properties of the coating material itself, the microstructure can lead to the operation of other new wear mechanisms. For instance in the sliding wear of titanium nitride coatings with zone 1 microstructures against steel spheres, the open boundaries between the columns can trap metallic debris that has been cut from the sphere by the sharp column tips (see Figure 1 and Figure 11 [301). As the amount of iron trapped in these intercolumnar spaces increases, adhesion between the sphere and the coating and hence the rate of any adhesive wear component is also increased. Although the intrinsic adhesion of titanium nitride on steel is low and adhesive wear is not the dominant wear mechanism for denser coatings with the zone T microstructure, for these low bias coatings adhesive wear is the dominant mechanism as material is pulled out from the coating due to the adhesion of the sphere to the transferred iron that is keyed-in to the microstructure. 4. CONCLUSIONS If the behaviour of a coated component in a particular application is to be fully understood it is necessary to know the microstructure of the coating and how this influences its properties. Fundamental physical property measurements are not enough to select a component for a wear application as these do not allow predictions of which wear mechanisms will be dominant in a given situation. However, the coating microstructure does play and important role in determining these mechanisms since as well as controlling the inter-relationship between the various physical properties of the coating layer, it can have a direct influence on what occurs during wear. If a coated component is to be produced for optimum performance it is necessary for those properties which affect the dominant wear mechanisms in its application be optimised. This may be achieved by producing a coating with the microstructure optimised for the application. However, the best properties may only be achieved by considering the coating as an integral part of the component to which it is to be applied, and this may lead to changes in the design of the coated component from the uncoated version. 5. ACKNOWLEDGEMENTS The authors wish to acknowledge the

Department of Trade and Industry for the provision of funding.

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[2] RICKERBY, D.S., and BURNETT, P.J., Thin Solid Films, 157 (1988) 195. [3] MOCHVAN, B.A., and DEMCHISHIN, A.V., Fiz. Met. Metalloved., 28 (1969) 653. [4] THORNTON, J.A., Ann. Rev. Mat. Sci., 7 (1977) 239. [5] MESSIER, R., GIRI, A.P., and ROY, R.A., J. Vac. Sci. Technol., A2 (1984) 500. [6] MATTHEWS, A., (1985) 93.

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[7] MATTOX, D.M. and KOMINIAK, G.J., J. Vac. Sci. Technol., 9 (1972) 528.

181 MATTOX, D.M., J. Vac. Sci. Technol., 1 0 (1973) 47. [9] BLAND, R.D., KOMINIAK, G.J., and MATTOX, D.M., J. Vac. Sci. Technol., 11 (1974) 671.

1101 KNOLL, R.W., and BRADLEY, E.R., Thin Solid Films, 117 (1984) 201. [lll RICKERBY, D.S., J. Vac. Sci. Technol., A4 (1986) 2809. [12] RICKERBY, D.S., JONES, A.M., and BELLAMY, B.A., Surf. Coat. Technol., 37 (1989) 111. [131 BURNETT, P.J. and RICKERBY, D.S., Surface Engineering, 3 (1987) 195. [141 BULL, S.J., RICKERBY, D.S., MATTHEWS, A., LEYLAND, A., and PACE, A.R., Surf. Coat. Technol., 36 (1988) 503. 1151 BULL, S.J., and RICKERBY, D.S., Proc. 16bh Int. Conf. Metallurgical Coatings, San Diego, California, April 17-21, 1989, in press. [16] HELMERSSON, U., SUNDGREN, J.-E., and GREENE, J.E., J.Vac. Sci. Technol., 4 (1986) 500. [17] RICKERBY, D.S., ECKOLD, G., SCOTT, K.T., and BUCKLEY-GOLDER, LM., Thin Solid Films, 154 (1987) 125. [181 BURNETT, P.J., and RICKERBY, D.S, Thin Solid Films, 148 (1987) 41.

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[191 RANDHAWA, H., Surf. Coat. Technol., 3 3 (1987) 53. [20] RICKERBY, D.S., and BULL, S.J., submitted to Surf. Coat. Technol. [21] RICKERBY, D.S., and BURNETT, P.J., Surf. Coat. Technol., 33 (1987) 191. [22] BULL, S.J., RICKERBY, D.S., ROBERTSON, T. and HENDRY, A., Surf. Coat. Technol., 36 (1988) 743.

[B] BURNETT, P.J., and RICKERBY, D.S., J. Mat. Sci., 23 (1988) 2429. [24] BUNSHAH, R.F., and RAGHURAM, A.C., J. Vac. Sci. Technol., 9 (1972) 1389.

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[N] BULL, S.J.,, RICKERBY, D.S., and JAIN, A., Proc. 16th Int. Conf. Metallurgical Coatings, San Diego, California, April 17-21,1989, in press. [311 BOWDEN, F.P., and TABOR, D., "The Friction and Lubrication of Solids," Clarendon Press, Oxford, 1954.

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