Wear behavior of triode-sputtered MoS 2 coatings in dry sliding contact with steel and ceramics

WEAR ELSEVIER

Wear 195 (1996)

7-20

Wear behavior of triode-sputtered MoS, coatings in dry sliding contact with steel and ceramics I.L. Singer, S. Fayeulle ‘, P.D. Ehni 2 USNaval Research Laboratory, Code 6176, Washington. DC 20375, Received 20 September

USA

1994; accepted 6 April 1995

Abstract The endurance of MoS, sputter-coated steel balls was measured in continuous and stop-start sliding tests in a four-ball wear tester. Test variables were counterface materials, uncoated steel and two ceramics (Co-bonded tungsten carbide and sapphire), and gaseous atmospheres, dry Ar and dry air. In continuous tests, coating endurance increased from 14 to 80 min; combinations were ranked as follows: steel in air = ceramics in air < steel in Ar < ceramics in Ar. Coating endurance in stop-start tests was the same as in continuous test for all combinations except steel in Ar, which failed sooner in stop-start tests. MO& wear tracks were analyzed at intervals from 1 min to failure by optical microscopy, energy-dispersive X-ray spectroscopy (EDX) and Auger electron spectroscopy ( AES) . Tracks run in Ar were smoother and showed ductile flow; tracks in air were rougher and developed ragged scratches. Blisters formed and contributed to failure in both atmospheres: in Ar they led to a slower ductile failure whereas in air they led to more rapid brittle failure. Abrasion by the worn steel ball accelerated coating wear in Ar. EDX of wear tracks showed that 2/3 of the coating was removed within the first 10 min in both atmospheres; AES of the same tracks found higher levels of oxygen in MO& run in air. Mechanisms of wear, transfer and lubrication are discussed, and a quantitative model for blister formation in ductile MoS, coatings is presented. It is recommended that hard ceramics replace abrasion-prone steel to enhance the wear life of MoS,-coated steel in oxygen-free environments. Keywords: MoS, coatings;

Solid lubrication;

Steel bearings; Ceramic bearings; Wear mechanisms;

1. Introduction The sliding wear behavior of burnished [ l-61 and bonded MoS, coatings [ 7,8] has been studied extensively over the past 30 years [ 9, lo]. It has been shown that, during run-in, a coating compacts to a fraction of its original thickness, at which time some of it is lost as wear debris. Thereafter, coating endurance is very dependent on the atmosphere, always drastically reduced when water vapor is present [l31. When sliding tests are performed in the presence of oxygen, coating loss is due mainly to the oxidation of MoS, to MOO, [ 4,5]. In argon, depletion is caused by mechanical flow of MO& platelets [ 2,4]. Blistering, a surface feature of MoS, coatings run in air and in argon, leads to failure when the blisters open and expose the underlying substrate [ 1,2,57]. In air, blisters result from oxidation, and they form more rapidly in dry air than in humid air [ 21. Failure is sometimes achieved so quickly that blisters are not even seen [4]. In ’Present address: Departement Materiaux-Mecanique Centrale de Lyon, BP163,69131 Ecully Cedex France. 2 ONT/NRL postdoctoral fellow: present address: Wheeling Jesuit College, Wheeling WV 26003, USA.

Physique,

Ecole

Dept. of Physics,

0043-1648/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSDIOO43-1648(95)06661-6

Auger spectroscopy

inert gas, where blisters have also been reported, mechanical causes such as descaling may be responsible for this phenomenon [ 4,8]. Although blister formation is understood qualitatively, no quantitative models for nucleation and growth have been proposed. Sputter deposition techniques have been used recently to develop thin coatings of MO& with longer life and lower friction than the earlier burnished and bonded coatings [ 111. These techniques offer great potential for modifying certain coating properties known to control friction and wear behavior; such properties include morphology, thickness, crystal orientation, grain size, chemicalcomposition [ 12-151 as well as crystal structure and intercrystallite slip [ 13,16,17]. They also offer the processing and (potential) contamination control associated with vacuum plasma technologies. One objective of this study is to assess the wear behavior of a commercially available MoS, coating [ 141, produced by DC triode sputtering, in light of what is known about the wear behavior of coatings produced by earlier (burnished and bonded) MoS, coating technologies. The wear behavior of the latter can be described by the classical wear modes for detaching material from a surface: abrasive, adhesive and

8

I.L. Singeretd/Wear

fatigue wear. However, it is often useful to invoke more specific wear modes that account for behavior peculiar to solid lubricating films, like deformation without material loss and film transfer to the counterface. These modes will be reviewed in Section 2. A second objective of this study is to investigate the endurance of MoS2 coatings against non-ferrous counterfaces. The early investigations of burnished and bonded coatings were made exclusively against steel substrates. Recently, there has been considerable interest in lubricating ceramic-coated triboelements [ 18-2 I]. Ceramic coatings and advanced ceramics offer numerous advantages over bearing steels, not the least being their high hardness and abrasion resistance [22,23]. However, the performance of ceramics against MoS, coatings has received very little attention [ 241. In this study, sliding wear tests were performed at a contact stresses between 1 and 1.5 GPa against steel and two ceramic counterface materials (Co-bonded tungsten carbide and sapphire) in dry air or argon. Optical microscopy, energy-dispersive X-ray spectroscopy (EDX) and Auger electron spectroscopy (AES) were performed with the objective of characterizing mechanical and chemical aspects of the wear behavior. The optical studies are similar to the ones performed by Fusaro on burnished coatings [ 4,6]. In addition, both EDX and AES were used to characterize compositions of the coating and transfer films and the loss of coating in wear tracks. Experimental details are given in Section 3. In the results Section 4, it is shown that the wear behavior of sputtered MO& coatings is different in air and argon and for different counterface materials. In the discussion Section 5, the wear behavior of MoS, coatings is summarized along with recent studies of transfer films generated by MoS, coatings [ 25,261. The summaries are followed by analysis of the observed wear behavior in terms of the wear modes reviewed in Section 2. In addition, a quantitative model for blister formation and a mechanism for blister wear is presented. The role of transfer films in wear resistance is then treated, followed by answers to the following questions: ( 1) How is the endurance affected by atmosphere (air or argon) and rider material (steel, WC-Co or sapphire) ? (2) What are the wear modes under these sliding conditions? (3) Do sputter-deposited coatings have different failure modes from those of burnished or bonded coatings?

2. Wear modes-summary

of the literature

Three classical forms of wear (i.e. wear modes) have been identified [ 271: ?? abrasive wear, in which a hard, rough counterface slides against a softer surface, such as a soft coating, and plows grooves in the softer surface, removing material from the groove; ?? adhesive wear, in which two smooth surfaces slide over each other, the two surfaces adhere and a particle is pulled out of one surface;

195(1996)

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0 fatigue or delamination wear, in which repeated sliding contact generates loading and unloading cycles, leading to subsurface cracks then loss of a particle by fracture. In the latter two wear modes, when the particle is removed from the oxide layer, the mode is called oxidational wear. We note that particles removed by these wear modes may be retained within the sliding contact by transferring to the counterface surface and, possibly, back to the first surface, or may be lost from the contact as loose wear debris. In the MoS, wear literature, several wear modes specific to MoS, have been proposed. ?? Platelet “wear”. The platelet “wear” mode describes the deformation, compaction and fracture of the MoS, platelets that make up sputter-deposited coatings (platelets will be confirmed later in Section 4 and in Fig. 2). An extension of a model proposed by Spalvins [ 121, it postulates that during the initial stage of sliding, upright platelets become reoriented parallel to the surface. The platelets may simply deform, causing compaction of the porous coating, or fracture, leading to loose particles. Reorientation of sputtered MO& coatings has been verified by Fleischauer et al. [ 13,28 ] using XRD and the deformation/ compaction without wear by Ehni and Singer [ 291 using Michelson interferometry and EDX analysis. Recently, Moser and Levy [ 30 ] have presented cross-section TFM evidence that demonstrates intercrysallite flow and fracture within this compacted layer. The loss of material from the track that accompanies platelet wear usually occurs by one of four processes: Film transfer to a (stationary) counte$ace. An initially bare surface can acquire a transfer film after as little as one sliding pass [ 291. The film, composed of MO& and its reaction products with the atmosphere [25,31], bonds to the counterface by chemical reaction [ 3 1,321. Transfer films can grow up to a micron thick without removing much of the coating because of volume conservation, e.g. a 2.5 nm thick layer from a track 40 mm long can produce a transfer film 1 km thick by 100 p,rn long. Transfer films become basally oriented along the sliding direction, regardless of initial coating orientation [ 25,331. Abrasion by transferfilm. Transfer films themselves have been shown to cause wear; for example, hard Fe304 particles, embedded in a MoS, transfer film, behaved as abrasives and scored the coating [ 261. In contrast, abrasives can also be beneficial if abrasive roughening of a counterface enhances lubricant retention/entrapment [ 341. Transverseflow. This mode, associated with wear of burnished MoS, coatings in dry argon, describes the gradual depletion of MoS, particles/platelets from the center to the edges of a wear track by lateral flow [ 41. Oxidation and ejection of oxide debris. Three oxidation modes have been proposed to account for the wear of MoS, coatings run in air. First, surface oxides can be “wiped off” the coating and transferred to the ball [ 25,311; this is an oxidational wear mode. Second, oxygen can permeate the coating, causing it to lose cohesion. As suggested by Gardos

I.L.

Singer et al. /Wear 195 (1996) 7-20

[5], mixtures of MoS, and MOO, are less cohesive than MoS,, leading to more rapid breakup of the coating with subsequent loss of material by all of the loss mechanisms discussed here. This wear mode is dependent on the rate of oxygen penetration and the rate of oxide formation and, therefore, should be independent of the counterface composition, structure or hardness. A third mode, associated with blister formation, is described in the next paragraph. 0 Blister wear. Like platelet “wear,” blister “wear” identifies a mode of particle detachment, not material loss. Two blister mechanisms are known to affect MoS, coatings. The first is due to oxidation [ 1,2,5,6]. It is prevalent in thick MoS, coatings run in air, as detailed by DeGee et al. [ 1,2] for burnished coatings. As described by Gardos [ 51, blisters are thought to develop around voids that form when MoS, oxidizes to Moo3 and SO*. The voids nucleate because of volume mismatch-Moo3 has a 10% lower molar volume than MO&---then blisters grow as the SOP gas fills the voids. During sliding, blisters disintegrate owing to embrittlement, depending on a critical combi nation of oxide content, wall thickness and size. Fracture of the blisters finally leads to particle detachment. The second mode involves mechanical formation and failure of blisters. This mode accounts for blisters observed on coatings run in argon where oxidation does not play a major role [ 4,6,35-371; it is also applicable to other solid lubricant coatings like graphite [38]. Formation and breakup of the blisters have been related to localized stress concentration [ 351, spalling [ 4,6], fatigue [ 361 and internal stresses [ 371. We present a model for the formation of blisters by mechanical stress in Section 5.2.3.

3. Experimental 3.1. Materials and wear test procedures Three ball materials were used: type 52100 steel, Cobonded tungsten carbide (WC-Co) and sapphire. The 52100 steel and the WC-Co balls were Grade 25, with a 0.04 km ( 1.5 pin) finish; the sapphire balls were optical grade, single crystal sapphire, surface finish unknown. The balls, 12.7 mm in diameter, were degreased with benzene in a Soxhlet extractor and stored in toluene. Prior to coating or testing, the balls were ultrasonically cleaned in acetone then rinsed with 2-propanol. One set of 52100 steel balls was coated with MO& (codeposited with nickel) [ 141 by DC triode sputtering to a nominal thickness of 1 km. Selected coatings were analyzed by Rutherford backscattering spectroscopy (RBS) (2 and 6.2 MeV (Y particles) and X-ray diffraction (XRD) taken with Cu radiation at a fixed incident angle of 2”. Wear tests were performed at room temperature in a fourball wear tester. One coated 52100 steel ball was run against three uncoated balls of the same material. The load was fixed at 49 N, giving mean Hertzian stresses of 1.15 GPa for steel,

1.5 GPa for WC-Co,

9

and 1.4 GPa for sapphire; these stresses

are below the yield stress of the three counterface materials. The speed was set at 0.2 m s- ’ (600 rev min-I), which translates to 1800 contacts (passes) min- ’ against the coating. Tests were performed under a steady flow of air or argon (99.995% pure), both dried by passing the gas through a dryice trap and a desiccant, anhydrous CaSO,; this procedure is expected to reduce the relative humidity to below 2%, based on measurements carried out in a nearby plexiglass chamber. Only after testing was completed was it realized that the argon flow did not completely eliminate oxidation reactions [25,26]. Two types of sliding tests were run. Continuous tests were run for a fixed duration (e.g. 1 and 10 min) or until failure, defined as when the friction coefficient exceeded 0.1. Stopstart tests were run for 5 min intervals, then the test was stopped and the uncoated balls rotated; the rotation caused the worn coated surface to be placed in contact with an fresh (unworn) area of the uncoated surface. Stop-start tests were continued at 5 min intervals until failure. 3.2. Methods of analysis The wear behavior of MoS,-coated steel was investigated by a combination of optical interference (Nomarsky) and scanning electron microscopy (SEM) , energy-dispersive Xray spectroscopy (EDX) , and Auger electron spectroscopy (AES) in conjunction with argon ion sputtering. SEM micrographs were taken in a dedicated SE microscope and in the scanning Auger microprobe. Auger spectra were acquired with a 5 keV electron beam; sputtering was performed using a 3 keV argon ion beam. Auger sputter depth profile data were normalized using elemental sensitivity factors appropriate to the cylindrical mirror analyzer: 0.28 for Mo,a6,_v, 0.75 for S,,, ._v,0.40 for 0s03 ,_vand 0.30 for CZT2eV, the latter value determined for carbidic carbon. No attempt was made to account for preferential sputtering effects (e.g. of S with respect to MO). EDX analysis was performed in the same UHV chamber as the Auger analyzer. Spectra were acquired with a thin-window detector, with a beam energy of 20 keV, a beam current of 5.9 nA and a detector take-off angle of 25”. Spectra of Fe KY and of MO La + S Kcx, whose X-ray lines overlap around 2.3 keV, were recorded. AES and EDX were used to examine selected wear tracks on MoS, coatings. Eight sets of wear tracks were produced according to the following combinations: against steel and WC-Co balls in air and in argon, at one and ten min respectively. With AES sputter depth profiles, wear was inferred from changes in composition and sputtering dose (sputter time X current density) required to reach the coating-steel interface. With EDX, coating losses were determined by comparing compositions in and out of the track. EDX linescans were taken across the track at several sectors along a track in order to assess both the contour (e.g. plowed or uniform) of wear across the track and the variation in wear around the track.

I.L. Singer et al. /Wear 195 (1996) 7-20

10

An example of Fe and MO + S EDX linescans superimposed on the wear scar where they were taken is shown in Fig. 1. In this paper, only the MO Lo + S Ka line spectra will be reported and interpreted [ 391. Although some variations were seen, e.g. some tracks showed wear only along a 3060” sector, it was usually possible to assess a “typical” wear depth for each track. The depth, in this paper, is taken as the difference between the MO + S intensities in and out of the wear track. This approximation can be justified in the present study for the following reason. The X-ray intensity is linear with depth so long as the thickness of the coating is no more than the peak depth in the X-ray depth production curve for the “thin film” under consideration [40]. With the 20 keV electron beam used in this study, the peak depth in MO& is 0.20 mg cm-*, which also happens to be the mass-equivalent thickness of the porous, 1 pm thick sputtered MoS, coatings. A more quantitative wear track analysis by EDX is presented elsewhere [ 391.

Fig. 2. SEM micrograph

of a DC triode sputtered MoS, coating.

4. Results 4.1. As-deposited

MoS, coatings

MoS, coatings were examined by SEM, RBS and XRD. A high-magnification SEM micrograph of the coating, shown in Fig. 2, indicates that the coating was a porous network of thin (5-15 nm) MoS, platelets, similar to findings in Refs. [ 13,411. Although nominally 1 p.m thick, an earlier study showed that the thickness of these coatings compacted to 0.5 p,rn after one pass of (wearless) sliding, i.e. the thickness was reduced by l/2 [29]. XRD indicated that the coating was oriented, with the basal plane of the MoS, crystallites perpendicular to the substrate surface, and the c/a ratio of the lattice ( =4.35) was larger than that of the bulk MoS, (c/a = 3.89) [ 251. RBS established that the batch of coatings used in this investigation were nearly stoichiometric (S/MO ratio = 2.0 + 0.1) but highly contaminated with carbon and oxygen - the composition in atomic percent was Mo2,S41C210,5Ni2; other batches of MoS, coatings from the same supplier had S/MO ratios varying from 1.3 to 2.4, and

0

100 SPUTTERING

200 300 DOSE @A-minkm?

Fig. 3. Auger sputter depth profile of an as-deposited

MO.!&coating

all exhibited high levels (25-40%) of contamination [ 29 ] commonly observe in sputtered coatings [ 161. Fig. 3 shows a typical Auger sputter depth profile of the as-deposited MO& coating. Besides the expected amounts of S and MO (the S/ MO ratio is decreased by preferential sputtering), the coating contained non-uniform concentrations of oxygen and carbon from the surface to the Fe interface; the non-uniform contamination levels, we suspect, were caused by heating which released gaseous carbon and oxygen species in the deposition chamber. 4.2. Endurance tests

Fig. 1. EDX linescans of MO La + S Ka and Fe Ka superimposed on the SEM image of a wear track produced by a 10 min 4-ball test in dry air. ,Linescans were taken at a beam energy of 20 keV across the area bracketed by the white lines.

Endurance tests of MoS,-coated steel against WC-Co, steel and sapphire balls are presented in Table 1. MO& coatings had 2 to 4 times greater endurance in argon than in air. In argon, the endurance against steel was half that of the two harder materials, while in air the endurance was the same for steel and for WC-Co balls (sapphire was not tested in air). Endurance during stop-start tests was comparable to that of continuous tests in all cases except for the steel ball in argon, where stop-start tests failed in less than 2/3 the time of continuous tests. In all cases friction coefficients settled at a low (0.02 to 0.04) steady-state value, then rose rapidly when the coating failed. Finally, we note that the average wear rates of these nominally 1 p,rn thick coatings were very

I.L. Singer et al. /Wear

Table 1 Endurance Atmosphere

of MoS,-coated

steel balls in continuous

Ball

Test type

and stop-start

tests

Number of tests

Endurance (mm)

Dry argon

WC-CO WC-CO sapphire sapphire 52100 steel 52100 steel

continuous stop-start continuous stop-start continuous stop-start

4 2 2 1 8 2

65*8 55.60 60.80 65 32*7 20.20

Dry air

WC-CO WC-CO 52100 steel 52100 steel

continuous stop-start continuous stop-start

5 2 6 2

18f3 16,20 18*3 14,20

low: in terms of average coating life per contact pass, from 0.003 nm pass - ’ in Ar to 0.01 nm pass-’ in air; in more traditionalterms,2X10-8mm3N-‘m-1and8X10-8mm3 N-’ m-r, respectively, based on Table 1 and scar widths taken from Fig. 6 and Fig. 9. 4.3. Analysis of wear tracks run in argon Optical. Fig. 4, parts (a)-(d) are series of optical micrographs of coatings obtained after 1, 10 and 62 min and after failure against WC-Co balls in argon. After 1 min (Fig. 4(a) ) the wear track appeared specularly reflective (burnished smooth), with few scratches and almost no debris.

Fig. 4. Optical micrographs

195 (1996) 7-20

11

The burnished appearance of the track results from plastic deformation of the MO& platelets, deformation which includes a significant amount of compaction, as deduced from a previous study on similar coatings [ 291. After 10 min (Fig. 4(b) ) , a few small blisters (2-6 p,m) are seen, mainly along scratches. On longer runs, including stop-start tests from 10 to 60 min, the number of blisters remained the same (not shown) but some became larger ( = 10 pm). Shortly before failure, some of these blisters began to peel open (Fig. 4(c)), exposing what appears to be the steel surface. After failure, a narrow strip of coating was removed along the entire length of the wear track, exposing the steel surface, which can be seen as a rough vertical feature 20-30 pm wide in Fig. 4(d) . Presumably the coating peeled along open blisters, which led to metal-metal contact and the rise in friction. Fig. 5 is the equivalent series of micrographs from sliding tests against steel balls in argon ( 1, 10 and 30 min and after failure). These tracks, from 1 min on (Fig. 5(a) ) , showed more scratches than those against WC-Co. The scratches were formed by grooves in the contact area of the counterface steel ball [ 261, which suffered from abrasion during run-in. After 10 min (Fig. 5 (b) ) , small blisters (2-6 pm) formed mainly along scratches; after 30 min (Fig. 5 (c) ) , their numbers increased and some became larger ( 10-15 pm). Failure against steel occurred in the same way as against WC-Co: the coating was removed along a strip 2mO p.m wide (Fig. 5(d)) that ran the full circumference of the track. In

of MoS, wear tracks obtained after (a) 1, (b) 10, (c) 62 min and (d) after failure against WC-Co balls in argon.

12

I.L. Singer et al. /Wear 195 (1996) 7-20

Fig. 5. Optical micrographs of MoS, wear tracks obtained after (a) 1, (b) 10, (c) 30 min and (d) after failure against steel balls in argon (dashed line shows edge where coating is removed).

both cases, the coating was removed “cleanly,” i.e. without ragged edges. Figs. 6(a)-6(c) are micrographs of wear tracks against sapphire balls after 10, 15 and 25 min of sliding in dry argon. The track features were nearly identical to those obtained with WC-Co balls. This sequence of micrographs, taken at the same area of the wear track, illustrates two properties of MoS, coatings conducive to wear resistance in argon. First, the large scratch on the right side “filled in” over time, indicating the coating’s ductility (ease of flow). Second, the virtual disappearance of blisters, visible on the left side of the track after a 10 min test (Fig. 6(a) ) but barely visible upon further running of the coating (Figs. 6(b) and 6(c) ) , shows that blistering can be suppressed. In argon, therefore, the coating behaved like a very ductile film, which allowed it to “self-heal’ ’from surface defects (blisters and scratches). A final observation is that relatively small amounts of debris accumulated in the tracks or at the edges of coatings run in argon. EDX. Fig. 7 shows several MO Lo +S Ko linescans acquired from wear tracks produced after 1 and 10 min of sliding against WC-Co and steel balls in argon. After 1 min of sliding against a WC-Co ball, very little MoS, was lost from the track. The track had a few scratches, in some sectors as deep as l/3 of the way through the coating (not shown in Fig. 7(a) ) , and some debris at the edge of the track. After

10 min, about 213 of the coating was removed, but not much debris piled up at the edge of the track. The removal was uniform along much of the track except in one sector (shown in Fig. 7 (b) ) , where several narrow scratches nearly penetrate the coating. After 1 min against a steel ball, the track had a “plowed” contour, with about 20% of the coating removed from the track and, apparently, deposited at the edge of the track (see Fig. 7 (c) ) . After 10 min, the coating was removed uniformly across the track (see Fig. 7 (d) ) in some sectors but removed unevenly in others; the greatest material loss occurred where removal was uneven. On average, at least 213 of the coating was removed; moreover, very little debris was found beside the track. Thus after 10 min of continuous sliding in argon, WC-Co and steel produced quite similar material loss (true “wear” depth) and loss contours. AES. Auger sputter depth profiles of the wear tracks were consistent with EDX observations of wear. For example, a depth profile taken in a scratch worn l/3 the way through the track against a WC-Co ball after 1 min is shown in Fig. 8. The profile, aside from subtle features discussed below, resembles the as-deposited coating (Fig. 3), but with the outer layer removed. Depth profiles for all four tracks run in argon indicated no major changes in composition, other than loss of material. Subtle differences in the depth profiles between worn (Fig. 8) and as-deposited coating (Fig. 3) are that the S/MO ratio was up to 20% higher and the C concen-

I.L. Singer et al. / Wear 195 (I 996) 7-20

13

WC IN ARGON

STEEL IN ARGON

a

TRACK POSITION Fig. 7. EDX Ii :scans of MO Lo + S Ko from wear tracks produced in argon against WC-Co balls after (a) 1 mitt and (b) 10 min and against steel balls after (c) 1 min and (d) 10 min (A.U. means arbitrary units; zero along lower horizontal lines).

.-

SPUTTERING

Fe --

l

DOSE (lIA-min/cm2)

Fig. 8. Auger sputter depth profiles taken in a scratch worn l/3 the way through the track by a WC-Co ball after 1 min in argon.

4.4. Analysis of wear tracks run in air

Fig. 6. Optical micrographs of MO& wear tracks obtained after (a) 10, (b) 15 and (c) 25 min of sliding against sapphire balls in argon.

tration from 20% to 80% lower in the worn areas. These changes, however, may be due to a sputtering artifact associated with the textural differences between the as-deposited coating, a porous coating of edge-oriented flakes (see Fig. 2)) and the wear track, a more compact (as in Figs. 4-6) film of basal-oriented flakes [ 131. Although only a few debris were found beside a track after 1 min against WC-Co, sputter depth profiles showed them to have the same composition as the as-deposited coating. These debris were probably agglomerations of fractured platelets that were pushed out of the track, as has been reported by others including Fusaro [ 41.

Optical. Optical micrographs obtained after 1 and 10 min sliding tests against a WC-Co ball are shown in Fig. 9. Tracks from 1 min tests (Fig. 9, top) differed in three ways from those in argon: many more scratches were seen in air, small blisters had already formed along the scratches and much more debris collectedat the edge of wear tracks. After 10 min (Fig. 9, bottom), the wear tracks had a rough, lumpy appearance and still more debris collected at the edge of tracks. The tracks obtained after 1 and 10 min sliding tests against a steel ball, shown in Fig. 10, are remarkably similar to those against WC-Co. In neither case can we explain the origin of the rough texture that developed over time; it could be due to the flattening of the blisters or to the debris retransferred from the ball to the track. Fig. 11 is an optical micrograph of a failed wear track, typical of tests with both steel and WC-Co run in air. The coating wore most heavily along scratches, as it did in argon (Figs. 4(d) and 5(d) ) . The scratches in air, however, had more ragged edges, which suggests that the coating delaminated by brittle rather than ductile fracture. EDX. Fig. 12 shows MO La + S Kol linescans of wear tracks after 1 and 10 min of sliding in air against WC-Co and

14

I.L. Singer et al. /Wear

Fig. 9. Optical micrographs of MO& wear tracks obtained after (top) (bottom) 10 min against WC-Co balls in air.

1and

steel balls. After 1 min against a WC-Co ball, the coating developed a plowed contour, with about l/3 of the coating removed about the center of the track. Excess MoS, can also be seen piled up at the sides of the track, perhaps pushed from the center to the edge of the track (see Fig. 12(a) ) . After 10 min (Fig. 12(b) ) , at least 2/3 of the coating was removed and debris was piled up beside the track. After 1 min of sliding against a steel ball, MO& loss was uneven along the track, with sectors having little loss next to sectors showing up to 50% loss locally (see Fig. 12(c)). Debris were piled up at the edge of the track. After 10 min (Fig. 12(d) ), MoS, was removed at least 2/3 the way through the coating and debris were piled up beside the track. Thus, after 1 min runs, more MoS2 was lost in air than in argon. But, after 10 min runs, approximately the same amount of MoS, was lost in air and in argon! The only significant difference seen by EDX in the air and argon tracks was the lesser amount of debris at the edge of tracks run in argon. AES. After I min sliding tests in air against both WC-CO or steel balls, Auger sputter depth profiles revealed the same composition found after sliding in argon and no excess oxygen was detected. After 10 min, however, there was a noticeable enhancement of subsurface oxygen in worn areas of tracks. This can be seen in Fig. 13, a profile taken at a spot in the track shown by EDX to be worn 213 the way through;

I95 (1996) 7-20

Fig. 10. Optical micrographs of MoS, wear tracks obtained and (bottom) 10 min against steel balls in air.

Fig. 11. Optical micrograph of a MoS, wear track obtained ( 18 min) against a steel ball in air.

after (top

after failure

this profile is representative of depth profiles taken after 10 min of sliding against both steel or WC-Co balls. Compared with the track run in argon (Fig. 8), the oxygen is higher and the S is lower. The increase in the oxygen level correlates with a decrease in S in the near surface region, suggesting the conversion of MO& to molybdenum oxide. Again the S/MO ratio in the wear track was slightly higher than in the as-deposited coating. Fig. 14 shows an Auger sputter depth profile of debris at the side of a track after a 10 min test against a WC-Co ball.

I.L. Singer et al. /Wear

WC IN AIR

STEEL IN AIR

1

TRACK POSITION Fig. 12. EDX linescans of MO Lo + S Ka from wear tracks produced in air against WC-Co balls after (a) 1 min and (b) 10 min and against steel balls after (c) 1 min and (d) 10 min (A.U. means arbitrary units; zero along lower horizontal lines).

195 (1996) 7-20

15

5. Discussion Section 5.1.1 summarizes the observed wear behavior of sputter-deposited MoS2 coatings. Coating wear alone, however, cannot give enough of the picture to fully interpret wear behavior; just as it takes two hands clapping to make a sound, so it takes two materials rubbing to produce wear. The other half of the picture, summarized in Section 5.1.2, is the wear behavior of the counterface ball, which was reported earlier [25,26]. It addressed issues of counterface wear, transfer films and their chemistry. With the counterface properties accounted for, Section 5.2 analyzes the wear behavior and identifies wear modes that can account for the observed mechanical and chemical changes in worn coatings. Section 5.3 then discusses the role of transfer films in wear resistance and Section 5.4 answers questions posed in the introduction. 5.1. Summary of wear behavior

0

100 SPUTTERING

200 300 DOSE bA-min/cm*)

Fig. 13. Auger sputter depth profile after 10 mitt of sliding against a steel ball in air.

SPUTTERING

DOSE @A-min/cm2)

Fig. 14. Auger sputter depth profile of debris at the side of a track, from a 10 mm test against a WC-Co ball in air.

The composition was very uniform: oxygen remained at 2025% throughout the debris while C was less than 5%. The debris has the identical composition of that on the worn surface of the track, but is much thicker; it also has the same composition as the transfer film found on the ball [ 251. Thus, it was not possible from these studies to decide whether the debris at the edge of the track originated from surface layers pushed transversely out of the track or debris redeposited from the ball.

5.1.1. MoS,-coated steel The tribological behavior of the four combinations of ball material and environment is summarized in Table 2. MO& coating endurance in air was the same for ceramic and steel balls. In argon, endurance against ceramic balls (65-70 min) was nearly four times that in air; against steel balls, endurance was twice that in air. Tracks formed in argon were smoother than in air, although steel produced more severe scratches and blisters than did ceramics. All combinations failed along blister-laden scratches where a narrow strip of coating peeled away from the track, but the peeling was more ductile in argon and more brittle in air. AES added that oxygen was incorporated in tracks run in air, but not in argon. Nonetheless, despite the differences, EDX analysis showed that all combinations lost about 213 of the coating after 10 min. 5.1.2. Uncoated ball contacts and transferfilms Contact areas of uncoated steel and ceramic balls slid against MoS,-coated steel were examined by the same set of tools used in the present study. Three features of the contact area after 1 and 10 min of sliding were identified: (1) the texture of the contact area; (2) the texture, composition and phase of the transfer film; and (3) the interface chemistry between the transfer film and the ball. Only the first two will be reported here. In dry argon [ 25,261, WC-Co and sapphire balls remained smooth and were covered by uniformly thin transfer films. Steel balls, by contrast, had abrasion grooves and transfer films that were thicker and patchy. It was calculated that, at the pressures applied ( 1.1 GPa) , the steel matrix would be elastically depressed about 1 p,rn during contact, allowing the harder and less deformable carbides to protrude and plow the uncoated surface. These grooves, in turn, produced numerous scratches in the coatings. TEM (see Table 3) and AES analysis of the transfer films can be summarized as follows. Transfer films after 1 and 10 min runs were mainly basally oriented

I. L. Singer et al. / Wear 195 (1996) 7-20

16

Table 2 Summary

of endurance,

wear behavior and wear scar analysis of MoS,-coated Dry argon WC-Co;

Uncoated counterface ball material Endurance relative to air Appearance (optical) lmin

3V4

up to 30% 60-70% none

a Endurance

is same for continuous

Table 3 Transmission

electron diffraction

sapphire

WC-CO

steel

1.5-2 (cant) l-l.4 (s/s)

1

1

scratches scratches scratches fine more numerous more numerous scratches are filled in small blisters; debris small blisters; debris . . blisters (2-6 km) form along scratches . .. . . . t compressed or opened more and larger lumpy texture; debris pileup at edges narrow strip of coating peeled away along blisters ductile tearing at edges ductile tearing at edges brittle fracture at edges brittle fracture at edges

Material removed from track (at.%) (EDX) lmin 10 min Increased oxygen in coating (at.%) (AES)

WC-CO

steel

burnished fine scratches are tilled in ......... .............

at failure

52100 steel

Dry air

of wear track

10 min

Ball

sapphire

steel

(cant)

up to 20% 60-70% none

and stop/start

(s/s)

up to 30% 60-70% = 10%

up to 50% 60-70% = 10%

tests, except for steel in dry argon.

analysis of transfer layers after 1 and 10 min tests in dry argon and dry air (from Ref. [25] )

Time (min)

1 10 1 10

I 10

Phases observed In dry argon

In dry air

MO&, MoS, MoS, MO&, MO&, MO&,

MoS,, MoS,, MO&, MO&, _ _

Fe,O,

CoMoO,, WO,, Co MOO, MOO,, A1203

MO&. In addition, against a steel ball, iron oxide (Fe,O,) was detected at the beginning of the test; the Fe,O, likely formed from particles plowed from the steel ball. Against a WC-Co ball, a Co-based ternary oxide, CoMoO,, was also found. In dry air [25], WC-Co and sapphire balls remained smooth and were covered by uniform, but somewhat thicker transfer films than found in argon. Steel balls had fewer scratches than those run in argon, and transfer films were thicker. TEM (see Table 3) and AES analysis of the transfer films can be summarized as follows: Although MO& was present in all films, oxides were the major constituents. Moo3 was the main compound formed in sliding tests longer than 1 min. Mixed iron-molybdenum oxide or cobalt-molybdenum oxide were also detected in tests run against steel or WC-Co respectively. The phases detected in both argon and air were those predicted by thermochemical equilibrium calculations [ 251. These oxidation products are evidence that a slow-wearing interface behaves like a miniature “tribo-reactar,” generating equilibrium phase compounds by gas-solid reactions.

MOO,, FeMoO,Fe,MoO,, MOO,, FeMoO, MOO,, CoMoO, MOO,

steel inclusions

In summary, transfer films formed within the first minute on all of the (initially uncoated) counterfaces, and the films exposed to oxygen formed equilibrium reaction products. 5.2. Analysis of coating wear Wear of MO& coatings occurred by a mixture of the competing mechanical and tribochemical processes discussed above. Experimental results suggest that wear can be divided into two stages: the first stage, including run-in (within the first 10 min) and a second stage (after 10 min) ; these are discussed in Section 5.2.1. Contributions of oxygen to wear in air are presented in Section 5.2.2. Wear processes not directly due to oxygen are presented in Section 5.2.3. 5.2. I. Stages of wear The first stage. During the first 10 min, 2/3 the original coating was worn away for all combinations. Since wear was independent of atmosphere, it must have been caused by mechanical processes. Platelet “wear” was clearly the major contributor to the run-in of MO& coatings in both dry argon

I.L. Singer et al. / Wear I95 (1996) 7-20

and dry air. During the first minute, deformation and compaction gave tracks their specular appearance. At the same time, some of the coating transferred to each ball, even in the case of WC-Co in argon where no significant wear loss was measured. The small “plowing” wear observed against steel in argon was likely due to abrasive wear by the transfer film that contained Fe304 particles. The large loss, 2/3 of the coating, was probably due to gross plowing and ejection of the compacted platelets. In general, the amount of MoS2 lost by these processes will depend on the morphology of the coating, as demonstrated by Hilton et al. [ 281. The second stage (after 10 min). The wear behavior of the remaining l/3 coating, by contrast, was very dependent on atmosphere and ball material (in argon). In air, the MO& coatings failed in less than 20 min against both balls. This wear behavior can be accounted for by a series of oxidationdriven wear modes presented in the next section. In argon, the remaining l/3 coating survived an additional 60 min against WC-Co and sapphire but only 15 min against steel. Wear tracks on the three were similar, except that steel balls produced more scratches. Slow oxidational wear could have taken place in dry argon, owing to oxygen contamination; the presence of an oxide layer and oxide phases in transfer films on MoS, run in argon for 10 min attests to the presence of oxygen [ 25,261. However, the ductility of the coating, demonstrated by the self-healing of scratches and blisters, suggests that different mechanisms led to failure. We speculate that the dominant second stage wear mode in argon is the mechanical formation and failure of blisters; this will be discussed along with a quantitative model for blister failure two sections hence.

5.2.2. Wear behavior in air Oxygen contributes to the observed wear behavior in air in four ways: First, oxidized surfaces are more susceptible to adhesive wear and thereby accelerate transfer film formation. Secondly, surface layers weakened by oxygen permeation and oxide formation lose cohesion and are vulnerable to transverse flow and further adhesive wear. Thirdly, oxygen permeation nucleates blisters and continued oxidation embrittles them, accelerating fracture and detachment of blister tops. Finally, the mixed MO& and molybdenum oxide debris, found on the track surface and in transfer films, do not attach well to the track because of the low chemical affinity of MoS, and MOO,; they are thermochemically stable compounds (co-existing phases) in contact [ 2.51. Instead of replenishing the lost lubricant, mixed debris are ejected from the track onto and around the contact zone. Since the wear rate depends on the cohesion and retention of the solid lubricant coating inside the track, it is controlled by the rate of oxidation, not by the counterface composition, structure or hardness. Therefore, oxidation processes produce the same endurance against both steel and WC-Co balls.

17

5.2.3. Wear behavior in argon We speculate that the dominant second stage wear mode in argon is the formation and failure of blisters in a ductile coating. We first present a quantitative model for the formation of blisters, based on the mechanism of buckling of compressed thin coatings described by Evans et al. [ 421. Modeled after a clamped, circular plate, buckling occurs when the compressive edge stress on the coating exceeds a critical stress, crc, according to: a,=[kE/12(1-~*)](tla)*

(1)

where k = 14.68 is a geometrical constant, E = elastic modulus, t = thickness of the coating, a = radius of a blister and 17= Poisson’s ratio. For MoS,, 7 = 0.13 [ 431. If the coating has been reoriented by friction to the basal direction, i.e. (001) planes parallel to the surface, the value of the elastic modulus is given by E = C, 1= 238 GPa [ 431. The size of blisters expected on MoS, during sliding contact can be calculated with Eq. ( 1) . From the results section, the actual thickness of the coating is105 pm. Setting t = 0.5 km in Eq. ( 1) gives a2=74/u,

(2)

where a is the radius of the blister in microns and crc is in gigapascals. The critical stress needed to produce blisters of radius a can be estimated from the Tresca criterion, I cl - u2 I 5 co, where (+i and g2 are the normal and edge stresses, and g0 is the yield strength of the coating. The normal stress is the mean contact pressure, which for steel couples is o, = 1.15 GPa; the yield strength is estimated to be about u 0 = 50 MPa, twice the value of the shear strength of MoS, obtained from friction measurements of the same coating [ 451. Since (me< (T,, u2 = (+i, then (T*= 1 GPa. At this value of edge stress, according to Eq. (2), blisters of radius 2 8.6 p,m can be formed. In fact, blisters of radii lO15 pm were observed after 10 min tests on all coatings in argon (see Figs. 4-6). Of course, as the wear scar widens, the contact pressure decreases, allowing even larger blisters. According to the model [ 421, microvoids must be present in the coating in order for buckling to occur. In the compacted platelet film, many sources of microvoids can be found: at misoriented grains and interfacial cracks within the worn coating [ 301 or at spots where MoS, is oxidized to MOO,, as described by Gardos [ 51. Oxygen-induced microvoids are probable, considering the rather high oxygen concentration (15%) in these coatings and the likelihood that traces of oxygen were present during testing with argon. It has also been shown that buckling is made easier by an increase in the density of microvoids [ 441. Although buckling produces blisters, blisters themselves, as shown earlier, do not necessarily cause immediate coating failure. Failure requires that blister tops be torn out, leading to contact between the steel substrate and the counterface. Tearing could occur if the blisters become very brittle, e.g. due to oxidation. However, tracks worn in argon consistently showed ductile behavior and no significant oxygen permea-

18

I.L. Singer et al. /Wear

tion was observed. Alternatively, fatigue-resulting from the numerous deformations of the coating due to pressing-down of the blisters ( 1800 times each minute of sliding) -is more likely responsible for blister failure. This is our speculation. The accelerated wear of MO& run against steel in argon is mainly due to enhancement of fatigue stresses produced by the abraded ball contact. The contact area of the ball was grooved during run-in, generating the iron oxides found in the transfer layer at the beginning of the test [26]. The grooves and the harder oxide particles in the debris plowed the wear track to a greater extent than the smoother surface of WC-Co [26], as evidenced by the larger number of scratches seen in the wear tracks. Plowing increases the local contact stresses on the coating, thereby increasing the fatigue stress. The buckling model can be used to estimate the size of blisters on grooved surfaces. At an edge of a scratch in steel, the contact stresses can reach the yield strength (elastic limit) of steel, cri = 3 GPa. If the coating thickness is t = 0.5 ym, Eq. (2) predicts blisters of radius a 2 5 km; in l/3 thinner areas (t’ = 0.33 pm), a’ 2 2.2 p,m, Blisters observed after 1 min tests against steel were indeed smaller, from 2-6 p.m, suggesting that they too arose from locally high stresses along scratches. Moreover, these smaller blisters at the edge of scratches are subjected to higher fatigue stresses. As blister wear proceeds the coating gets thinner, which, according to Eq. ( l), further lowers the critical stress or the radius of blisters. This “run-away’ ’mode of blister formation and wear may account for the shorter endurance against the abraded surface of steel. It also provides a rationale (hard and abrasion resistance) for choosing a ceramic as a counterface to run against a MoS,-coated substrate. Finally, a second factor-loss of transfer film-is invoked to account for the anomalously low endurance of steel balls in stop-start tests in dry argon. Abrasion of the contact area of steel prevented the buildup of well-adherent transfer films that were found on WC-Co balls [ 261. In continuous tests, the abraded area was eventually covered by a transfer film. However, in stop-start tests, the ball was rotated to a new area which became abraded each time the test was resumed. Hence, before a transfer film had time to form, the abraded area exposed the track to additional cycles of high stress sliding, which clearly accelerated the wear of the remaining coating. The fact that continuous and stop-start endurance values were the same for WC-Co and sapphire in argon provides evidence that: 1) film transfers to the uncoated ball with very little wear of the coating, as described earlier; and 2) transfer films are responsible for the high endurance of MoS, coatings. 5.3. Role of transfer-films

in wear resistance

Although transfer films contributed to several modes of wear, overall they protected MoS, coatings from wear. Transfer films shielded the coating from the defects (machining marks, pits, protruding second phases, . ..) found on normal

195 (1996) 7-20

counterfaces. In addition, the films, made of basal-oriented MoS, flakes, altered sliding at the interface. This interface had a much lower shear strength ( = 20 MPa) [45] than either hard counterface material ( > 1000 MPa) and therefore became the preferred plane of sliding (or “velocity accommodation plane”). Since low-friction ( = 0.02) minimizes the sliding stresses transmitted across an interface [ 461, surface “damage” was kept to a minimum. We attribute the overall low wear rate of MoS, coatings to the localization of sliding along this interface. 5.4. Summary The overall high endurance of MO& coatings is attributed to the localization of sliding along a well-defined plane consisting of a transfer film containing basal-oriented MoS, against a track of mainly reoriented and compacted MoS, platelets. This interface is the preferred “velocity accommodation plane” because of its low shear strength. Wear, however, is due to the complex mixture of competing mechanical and tribochemical processes discussed above. Ceramic counterfaces avoided two of the wear modes that accelerated coating wear against steel in argon. We are now able to answer the questions posed in the introduction: ?? In air, the lifetime of MoS, coatings is limited by oxidation and not the counterface ball material. An oxidizing atmosphere leads to continuous loss of material from the track for three reasons: ( 1) oxygen can permeate MO& making it more brittle; (2) oxidation produces a non-cohesive transfer film which is easily ejected from the contact; (3) oxidation accelerates blister wear. These phenomena are independent of the ball material. 0 In argon, the lifetime is longer and determined mainly by the mechanical blister wear mode. Blisters form by stressinduced buckling at microvoids in the coating. The coating remains ductile, and failure is controlled by fatigue of blisters along stress raisers like scratches. Grooves in the counterface contact area also produce higher local contact stresses, increasing blister formation and fatigue stresses. The lifetime against steel balls is reduced because they abrade more easily than the harder ball materials WC-Co and sapphire. Stop-start test against steel in dry argon shorten the wear life even further owing to the repeatedly abraded, fresh steel surfaces and the concurrent absence of a protective transfer layer. 0 This study has demonstrated that DC triode sputtered MoS, coatings have the same wear and failure modes as burnished or bonded coatings in dry air and dry argon. In all cases, run-in leads to a thinning and compaction of MO& particles, and wear is controlled by the chemical and timedependent (fatigue) mechanical processes outlined above. There is more work to be done in determining the chemical aspects of adhesive wear (both adhesion of transfer films and detachment of surface layers), decohesion of oxygen-penetrated MO& and interfacial vs. intercrystalline slip processes.

I.L. Singer et al. /Wear

Further study is also needed to clarify nucleation, oxygen embrittlement and detachment ‘of blisters. It is also recommended that polished ceramic triboelements replace steel for applications in which MoS, coatings are subjected to sliding in oxygen-free atmospheres or vacuum.

6. Conclusions 1. The endurance of sputter-deposited MoS, coatings under concentrated contact, dry sliding conditions is decreased by the presence of oxygen. The endurance in air is controlled by oxygen and, thereby, is independent of the counterface ball material. The endurance in argon, however, is greater against harder materials, WC-Co and sapphire, than against steel, because of the latter’s susceptibility to abrasion. Two distinct stages of wear were detected with DC triode sputtered films: 4.1. Initially, about two-thirds of the MoS, coating was worn away regardless of ball material or atmosphere. The wear mode appears to be platelet wear. 4.2. Thereafter, in air, wear was controlled by oxidational wear and oxygen embrittlement of blisters; in argon, wear was controlled by fatigue of blisters.

Acknowledgements We thank Bob Bolster for assistance with the wear testing, Bob Gossett for RBS analysis, Bernie Stupp at Hohman Plating and Mfg. for supplying the coatings, and SD10 Tribomaterials TIWG for financial support. ILS would like to thank Kathy Wahl, Larry Seitzman and Dave Venezky for comments on the manuscript.

References [I] G. Salomon, A.W. DeGee and J.H. Zaat, Mechano-chemical factors in MoS, lubrication, Wear, 7 ( 1964) 87-101. [2] A.W. DeGee, G. Salomon and JH. Zaat, On the mechanisms of MoS, film failure in sliding friction, ASLE Trans., 8 (1965) 156-163. [ 31 C. Pritchard and J.W. Midgley, The effect of humidity on the friction and life of unbonded molybdenum disulphide films, Wear, 13 (1969) 39-50. [4] R.L. Fusaro, Effects of substrate surface finish on the lubrication and failure mechanisms of molybdenum disulfide films, ASLE Trans., 25 (1981) 141-156. [5] M.N. Gardos, Synergistic effects of graphite on friction and wear of MoS, in air, Tribol. Trans., 31 (1987) 214227. [6] R.L. Fusaro, A comparison of the lubricating mechanisms of graphite fluoride and molybdenum disulfide films, in Proc. 2nd Int. Conf on Solid Lubrication, ASLE, Park Ridge, IL, Vol. SP-6, 1978, pp. 59-78. [ 71 W.J. Bartz and J. Xu, Wear behavior and failure mechanism of bonded solid lubricants, Lubr. Eng., 43 (1987) 514-521. [ 81 G.D. Gamulya, G.V. Dobrovol’skaya, IL. Lebedeva and T.P. Yukhno, General characteristics of wear in vacuum for solid film lubricants formulated with lamellar materials, Wear, 93 (1984) 319-332.

I95 (1996) 7-20

19

[9] For comprehensive review of early studies see W.O. Winer, Molybdenum disulfide as a lubricant: A review of the fundamental knowledge, Wear, IO (1967) 422452. [ 101 B. Bhushan and B.K. Gupta, Handbook of Tribology, McGraw-Hill, New York, 1991, Chapters 5 and 13. 1111 M.J. Todd, Solid lubrication of ball bearings for spacecraft mechanisms, Tribol. bat., I5 ( 1982) 33 l-337; also R.A. Rowntree and M.J. Todd, in L.E. Pope, L. Fehrenbacher and W.O. Winer (eds.), New Materials Approaches to Tribology: Theory and Applications, MRS. Pittsburgh, PA, Vol. 140, 1989, pp. 21-34. [ 121 T. Spalvins, A review of recent advances in solid film lubrication, Thin Solid Films, 73 (1980) 291-297; J. Vat. Sci. Technol., A5 (1987) 212-219. [ 131 P.D. Fleischauer and R. Bauer, Chemical and structural effects on the lubrication properties of sputtered MO& films, Tribal. Trans., 31 (1988) 239-250. [ 141 B. Stupp, Synergistic effects of metals co-sputtered with molybdenum disulfide, Thin Solid Films, 84 (1981) 257-266; Performance of conventionally sputtered MoS, versus co-sputtered MoS, and nickel, Proc. 3rd Int. Conf on Solid Lubrication, Denver, CO, 7-10 August 1984, ASLE SP-14, p. 217. [ 151 E.W. Roberts and M.J. Todd, Space and vacuum tribology, Wear, 136 (1990) 157-167. [ 161 P.D. Fleischauer, Effects of crystallite orientation on environmental stability and lubrication properties of sputtered molybdenum disulfide thin films, ASLE Trans., 27 (1983) 82-88. [ 171 W.E. Jamison, Structure and bonding effects on the lubricating performance of crystalline solids, Trans. ASLE, 1.5 (1972) 296-305. [ 181 S.V. Didziulis, P.D. Fleischauer, B.L. Soriano and M.N. Gardos, Chemical and tribological studies of MoS, films on Sic substrates, Surf. Coat. Technol., 4344 (1990) 652662. [ 191 M.R. Hilton and P.D. Fleischauer, Applications of solid lubricant films in spacecraft, Surf: Coat. Technol., 54-55 (1992) 435441. [20] H.J. Boving and H.E. Hintermann, Wear-resistant hard titanium carbide coatings for space applications, Tribal. Int., 23 (1990) 129133. [21] A. Erdemir, A review of the lubrication of ceramics with thin solid films, in S. Jahanmir (ed.), Friction and Wear of Ceramics, Marcel Dekker, New York, 1994, pp. 119-162. [22] L. Pope, L. Fehrenbacher and W. Winer (eds.), New Materials Approaches to Tribology: Theory andApplications, MRS, Pittsburgh, PA, Vol. 140, 1989; see articles in Part V on ceramics and ceramic lubrication. [23] S. Jahanmir (ed.), Friction and Wear of Ceramics, Marcel Dekker, New York, 1994. [24] E.W. Roberts and W.B. Price, In vacua, tribological properties of bighrate sputtered MO& applied to metal and ceramic substrates, in L. Pope, L. Fehrenbacher and W. Winer, eds., New MaterialsApproaches to Tribology: Theory and Applications, MRS. Pinsburgh, PA, Vol. 140,1989, pp. 251-264. [25] S. Fayeulle, P.D. Ehni and I.L. Singer, Role of transfer films in wear of MoS, coatings, in D. Dowson, C.M. Taylor and M. Godet (eds.), Mechanics of Coatings, Tribology Series 17, Elsevier, Amsterdam, 1990 pp. 129-138. [ 261 S. Fayeulle, P.D. Ebni and IL. Singer, Analysis of transfer films formed on steel and Co/WC during sliding against MO&-coated steel in argon, Sue Coat. Technol., 41 (1990) 93-101. [27] See, for example, E. Rabinowicz, Friction and Wear of Materials, Wiley, New York, 1965, Chapters 5-7. 1281 M.R. Hilton, R. Bauer and P.D. Fleischauer, Tribological performance and deformation of sputter-deposited MoS, solid lubricating films during sliding wear and indentation contact, Thin Solid Films, I88 (1990) 219-236. [29] P.D. Ehni and I.L. Singer, Composition of sputter deposited MoS, films during run-in in vacuum, in L.E. Pope, L. Fehrenbacher, and W.O. Winer, eds., New Materials Approaches to Tribology: Theory andApplications, MRS, Pittsburgh, PA, Vol. 140, 1989, pp. 245-250.

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[ 301 J. Moser and F. Levy, MoS2_, lubricating films: structure and wear mechanisms investigated by cross-sectional transmission electron microscopy, Thin Solid Films, 228 (1993) 257-260. [31] IL. Singer, A thermochemical model for analyzing low wear-rate materials, Sur$ Coat. Technol., 49 (1991) 474-481. [ 321 P.D. Fleischauer and R. Batter, The influence of surface chemistry on molybdenum sulfide transfer film formation, ASLE Trans., 30 ( 1987) 160-166. [33] J.R. Lince and P.D. Fleischauer, Crystallinity films, J. Mater. Res, 2 (1987) 827-838.

of rf sputtered

MO&

[ 341 J.K. Lancaster, Stabilization of the friction and wear of non-graphitic carbons

by additives,

in Proc. 2nd fnt. Conf on Solid Lubrication,

ASLE, Vol. SP-6, 1978, pp. 176188. [35] A.W.J. DeGee, A. Begelinger and G. Salomon, Influence of the atmosphere on the endurance of some solid lubricants compared at constant layer thickness, Proc. Inst. h4ech. Eng., 183 ( 1968-69) 1827. [36] W.J. Bartz, R. Holinsky and J. Xu, Wear life and frictional behavior of bonded solid lubricants, Lubr. Eng., 42 (1986) 762-769.

[ 371 J.W. MacCain, A theory and tester measurement

correlation MoS, dry film lubricant wear, SAMPE J., 6 ( 1970) 17-26.

about

[38] B.R.G. Swinnerton and M.J.B. Turner, Blistering of graphite films in sliding contacts, Wear, 9 ( 1966) 142-159.

[ 391 P.D. Ehni and I.L. Singer, Electron-beam wear behavior of sputter-deposited (1992) 45-53.

microprobe

analysis of the

MO& coatings, Appl. Surj: Sci., 59

[40] J.1. Goldstein, D.E. Newbury, P. Echlin, D.C. Joy, C. Fiori and E. Lifshin, Scanning Electron Microscopy and X-ray Microanalysis, Plenum, New York, 1981, pp. 355-358. [41] M.R. Hilton and P.D. Fleischauer, Structural studies of sputterdeposited MoS, solid lubricant films, in L.E. Pope, L. Fehrenbacher and W.O. Winer (eds.), New Materials Approaches to Tribology: Theory and Applications, MRS, Pittsburgh, PA, Vol. 140,1989,227235. [42] A.G. Evans and J.W. Hutchinson, On the mechanics of delamination and spalling in compressed films, Int. J. Solids Structures, 20 (1984) 455466. [43] M.N. Gardos, On the elastic constants of thin solid lubricant films, in Coatings, D. Dowson, C.M. TaylorandM. Godet (eds.),Mechanicsof Tribology Series 27, Elsevier, Amsterdam, 1990, pp. 3-13. [44] L.M. Keer, S. Nemat-Nasser

and A. Oranratnachai,

and splitting in compressed brittle elastic arrays, J. Appl. Mech., 49 (1982) 761-767.

Surface instability

solids containing

crack

[45] IL. Singer, R.N. Bolster, J. Wegand, S. Fayeulle and B.C. Stupp, Hertzian stress contribution to low friction behavior of thin MO& coatings, Appl. Phys. Lett., 57 (1990) 995-997.

[46] G.M. Hamilton and L.E. Goodman, The stress field created by a circular sliding contact, Trans. ASME, J. Appl. Mech., 33 ( 1966) 371378; see also, D.A. Hills and D.W. Ashelby, The influence of residual stresses on contact-load-bearing capacity, Wear, 7.5 ( 1982) 221-240.

Biographies Peter D. Ehni: received a B.S. in physics from Wheeling Jesuit College in 1981, then an M.Sc. in 1984 and Ph.D. in physics in 1986 from the University of Maine, Orono. His graduate work was in ion implantation modification of steel and Ni. He did postdoctoral work at Naval Research Laboratory from 1987 to 1990 on the chemistry and wear behavior of MoS, coatings. Since 1990 he has been teaching physics at Wheeling Jesuit College and is currently the chair of the Physics Department. Serge Fayeulle: is a research chemist from the CNRS in the Material Department of Ecole Centrale de Lyon, France, specializing on surface characterization and tribology. His Ph.D. studies were on microstructural and tribological characterization of ion-implanted steels. He spent two years as a visiting scientist at the Naval Research Lab, Washington DC, from 1987 to 1989. Irwin L. Singer is a Supervisory Research Physicist in the Surface Chemistry Branch of the Chemistry Division at the Naval Research Laboratory (NRL) in Washington DC. After receiving a Ph.D. in physics from Indiana University in 1976, he did postdoctoral research at NRL in the Surface Chemistry Branch. In 1978, he joined NRL’s Tribology Section, and in 1983 was appointed Tribology Section Leader. During his tenure at NRL, Dr. Singer has also been a visiting scientist at Instituto de Fisica, UNAM, Mexico ( 1981-82)) Cambridge University, England ( 1985-86) and Ecole Centrale de Lyon ( 1992-93). His research has focused on analyzing and modifying tribological properties of engineering surface. He has over 80 publications, 2 patents on ion implantation processing and is co-editor of a recently published book Fundamentals of Friction: Macroscopic and Microscopic Processes (Kluwer Academic Publishers, Dordrecht, 1992).