Rolling-contact-fatigue wear characteristics of diamond-like hydrocarbon coatings on steels

558

Wear, 162-164 (1993) 558-568

Rolling-contact-fatigue wear characteristics hydrocarbon coatings on steels

of diamond-like

Ronghua Wei and Paul J. Wilbur* Colorado State Universiy, Fort Collins, CO 80523 (USA)

Mary-Jo Liston NTN Technical Center, Ann Arbor, MI 48108 (USA)

Gayle Lux Charles Evans and Associates, Redwood City, CA 94063 (USA)

Abstract Adherent diamond-like hydrocarbon (DLHC) coatings were applied on AISI M-50, 52100, 4118 and 440C steel rods by using an approximately 0.2 pm amorphous-silicon-hydrocarbon (a-SiHC) bonding layer between the DLHC and the steel. Both the a-SiHC and DLHC coatings were applied using a single, broad-beam ion source. The rods were subjected to rolling-contact-fatigue (RCF) testing under high, cyclic Hertzian stress (5.5 GPa), low lubricant-film-thickness parameter (lambda = 0.7) conditions. Order-of-magnitude increase in the fatigue lives of all four rod materials were observed. Systematic RCF tests coupled with microscopic examination after various test intervals show that micro-polishing by hard DLHC coating fragments may play an important role in prolonging fatigue lives. Raman spectroscopic measurements suggest that cyclic stressing of the DLHC layer causes it to transform from what was initially amorphous carbon into the more lubricous and stable graphite phase.

1. Introduction Because rolling-contact-fatigue (RCF) is a major cause of failure in rolling-element bearing materials, tribological research has addressed various means by which the time-to-failure for these materials might be extended. The development of improved materials using vacuum-induction melting and vacuum-arc remelting techniques to reduce oxide, carbide and other nonmetallic inclusions which facilitate subsurface failures [l, 21 and the enhancement of the near-surface properties of conventional materials using such techniques as ion implantation [3-51, Cu and TiN coating [6] and diamond-like hydrocarbon (DLHC) coating [7] are typical of approaches that have been investigated. A recent review of near-surface treatment techniques by Erdemir [8] summarizes their beneficial effects and deficiencies. Recognition that DLHC coatings have desirable tribological properties has led to widespread research into their suitability for tribological applications [g-12]. For example, hardness as great as 13 000 kgf mm-* [13], friction coefficients as low as 0.02 (in dry argon *Author

to whom correspondence

0043-1648/93/$6.00

should be addressed.

or nitrogen environments) [14] and extreme resistance to sliding wear [14] make them attractive. Recently, we reported that DLHC coatings alter the RCF lives of M-50 steel significantly and suggested this could be because the coatings, which are amorphous, were inhibiting surface crack initiation and propagation [7]. This paper extends that work by demonstrating similar RCF life improvements in other bearing steels and presenting test results that address questions regarding the mechanisms by which DLHC coatings improve the lives of these materials.

2. Apparatus and procedures 2.1. Test materials AISI M-50 (0.85 wt.%C, 4 wt.%Mo, 4 wt.%Cr, 1 wt.%V}, 52100 (1 wt.%C, 0.35 wt.%Mn, 1.45 wt.%Cr}, carburized 4118 (0.2 wt.%C, 0.8 wt.%Mn, 0.5 wt.%Cr, 0.1 wt.%Mo} and 440C (1.1 wt.%C, 17 wt.%Cr, 0.75 wt.%Mo, 1 wt.%Si, 1 wt.%Mn, 0.04 wt.%P, 0.03 wt.%S} steels were selected for this study because they are commonly used, microstructurally distinct bearing materials. Testing involved establishing basic trends using

0 1993 - Elsevier

Sequoia.

All rights reserved

R Wei et al. I Fatigue wear characteristics of diamond-like hydrocarbon coatings

M-50 steel and then investigating the other materials, at a standard processing condition, to identify differences and similarities introduced by material changes. A group of rods were produced from each steel which were identical to each other with respect to material heat, heat treatment and final grind. The rods were 0.95 cm in diameter and 8 cm long and met the requirements for RCF testing [15]. All rods were cleaned ultrasonically in chlorothene, acetone and methyl alcohol before they were either subjected directly to RCF testing or placed in a vacuum chamber for ion beam processing before RCF testing. 2.2. Ion beam processing procedures A rod was given the complete DLHC coating procedure using a sequential three-step process accomplished without interrupting the vacuum. It was first sputter-cleaned in a 1 keV, approximately 2 mA cm-’ argon ion beam for 30 min to remove surface contaminants. Next, a 0.2 pm amorphous-silicon-hydrocarbon (a-SiHC) interface layer was reactively sputter-coated onto the rod. This bonding layer is required because the DLHC films will not adhere directly to steels. Finally, a layer of DLHC was applied by bombarding it with 450 eV carbonaceous molecular ions derived from the ion source while it was being operated on methane. Some rods received only the first or first two processing steps, but in all cases they were rotated continuously to assure processing uniformity. Generally, active cooling was used throughout the processing to maintain rod temperatures near 100 “C although they rose to approximately 160 “C during the sputter-cleaning phase of the processing on a few occasions. Additional detail on the procedures followed in the coating of rods utilizing a single, broad-beam ion source [16] is given in ref. 7. 2.3. Rolling contact fatigue testing All of the test rods in this study were subjected to rolling-contact-fatigue testing according to specifications given in ref. 15. These tests involve the rotation of a 9.5 mm diameter rod while it is loaded by three 12.7 mm diameter balls that are equally spaced around a common rod circumference on which a wear track develops as the balls are rolled around its surface. Uncoated and roughened AISI 52100, grade-24 load balls, alternately stress the test rod to a calculated Hertzian contact stress of 5.5 GPa. This stress was sufficient to cause plastic deformation in the rods used in these tests. The resultant stress was, therefore, less. The contact region was drip lubricated with Mil-L23699C synthetic turbine oil. The lubricant-film-thickness parameter (lambda) was calculated to be 0.7 for the uncoated rods under the prescribed conditions of

559

contact stress, initial rod and ball rot&messes and lubricant properties (test temperature was estimated at room temperature). Protilometric measurements on the coated rods, of the type and thickness used in these tests, did not induce any significant changes in rod roughness. So lambda was approximated at 0.7 for coated rods at the beginning of an RCF test. The output data for each RCF test was the number of stress cycles required to induce a rod failure (i.e. a spall). In order to assure statistically significant results, at least 10 tests were run per steel/coating condition evaluated. The resulting failure data were analyzed using Weibull statistics [17] so the median rank OS. stress cycles to failure could be plotted and BlO and B50 lives and Weibull slopes could be determined. New balls were used for each test and the test cups were replaced at the first opportunity following 300 h of testing (155 X lo6 cycles). 2.4. Rod sugace characterization

Surface finishes on as-received and ion-beam-processed rods were measured using a stylus profilometer capable of detecting 0.02 pmvariations in surface height. Measurements were also made on RCF test balls before and at the conclusion of RCF testing. Surface finish values could not be obtained in the test rod wear track after testing because the material deformation interfered with the measurements. Raman spectroscopy which is widely used to characterize the microstructure of DLHC coatings [lS] was employed along with secondary ion mass spectroscopy (SIMS) to study the comparative microstructures and compositions of DLHC coatings both within typical RCF wear tracks and on adjacent unworn regions of the test rods. A 514.5 nm, 300 mW laser was used to excite a 2 mm* spot on a rod for Raman measurements. A nominal 0.5 PA Cs’ primary ion beam rastered over a (400 pm)’ area with negative secondary ion extraction from the central 25 pm diameter region was used to conduct the SIMS analysis. Hydrogen concentrations were quantified using relative sensitivity factors developed through the analyses of ion implants into diamonds. Sputtering depth calibration was accomplished using stylus profilometric measurements of analytical craters and sputtering times. Because the sputtering rates for Sic and DLHC by Cs’ are similar, the uncertainty of these measurements is quite good (f7%). 2.5. Wear-track development histories The surface morphological and depth histories of wear tracks from coated and uncoated tracks were determined in a series of RCF tests run at different axial locations on a rod for various times (1 min (9 x 103 cycles), 10 min (9 X 104 cycles), 30 min (2.6 X 105 cycles),

560

R. Wei et al. I Fatigue wear characteristics of diamond-like

1 h (5.2 X 105 cycles) and 6 h (3.1 X lo6 cycles)). The wear tracks were characterized using scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) capability to record surface morphological differences. A profilometer. was used to determine differences in wear-track profiles induced during each of the RCF test intervals.

3. Results 3.1. Rolling contact fatigue tests 3.1.1. M-50 steel The Weibull plots in Fig. 1 are RCF failure data from the uncoated rods and rods subjected to partial and complete coating processes. Forty tests were performed on as-received rods. The data points lie close to their median-rank line and the slope of this line (Weibull slope) is steep. Collectively, these results imply narrow confidence bands around the mean-rank line and indicate good uniformity of the M-50 rod material and reliable equipment and test procedures. The Weibull slope and BlO and B50 lives for the data shown in Fig. 1 are listed in Table 1. The 3.5 Weibull slope is the same as that measured in standard RCF tests of AISI M-50 steel [15]. The results in Table 1 suggest sputter-cleaning alone caused the BlO and B50 lives to increase and the Weibull slope to decrease relative to the as-received material, but these changes are not always observed. The addition of the sputtered a-SiHC caused the BlO life and the Weibull slope to increase while the B50 life remained near the sputter-cleaned value. Adding a 0.5 pm DLHC coating, like sputter-cleaning, appeared to cause the BlO and B50 lives to increase and the Weibull slope to increase. Taken together, the data of Fig. 1 suggest that the cleaning and DLHC coating

LIFE(MiUionrofStmrCJclu)

Fig. 1. AISI M-50 steel rolling-contact-fatigue

test results.

hydrocarbon

coatings

processes which involve direct ion impact on the rods, degrade Weibull slopes and increase lives. The a-SiHC sputter-coating process, which involved surface attachment of low energy atoms, on the other hand, appeared to induce an increase in the Weibull slope. The observations that cleaning changed the life and that the application of the a-SiHC layer changed the Weibull slope suggest that these processing steps do more than simply clean the steel surface and effect bonding between the steel and the DLHC coating. 3.1.2. 52100, 4118 and 440C steels When DLHC coatings were applied to rods made from other bearing steels and the rods were RCF tested along with control rods from the same material lots, the results shown in Figs. 2-4 and the bottom of Table 1 were obtained. As these data show, dramatic increases in the BlO and B50 lives and modest degradations in the Weibull slopes appear to be induced by DLHC coating of all of the steels. 3.2. Characterization of the coatings A typical Raman spectrum of an untested DLHC film is shown in Fig. 5 and a corresponding de-convolution showing the Gaussian components associated with its microstructure is given in Fig. 6. The deconvolution reveals (i) a G-band at 1538 cm-l, which is probably composed of broadened amorphous-carbon peaks; (ii) a D-band at 1352 cm-’ that appears to be due to disordered graphite; (iii) a small peak at 1116 cm-’ which indicates significant disorder. Because of its sensitivity to confinement, the D-band reveals information about grain size. For these data, models of this sensitivity [19,20] suggest a small graphite crystallite size (about 9 nm). Although no strong evidence of four-fold (diamond) bonding is indicated in the spectrum, it does suggest a durable DLHC film because edge pinning of the hexagonal planes of graphite through inter-layer, three-fold bonds is indicated. These bonds are much smaller in length and stronger than the normal inter-layer pi-bonds of crystal graphite. The bond-angle disorder and small crystallite size promote a structure with a very rigid lattice which exploits the strength of the three-fold bonds. A small spot centered within a region of a test track that retained substantial DLHC coverage after a 100 million cycle RCF test was examined using Raman spectroscopy. The resulting spectrum is shown in Fig. 7. Compared with the unworn DLHC coating of Fig. 5, it shows dramatic growth in the D-band around 1360 -I. Growth of this peak, which corresponds to a i:order mode in the small graphite crystallite [19], implies that the effect of fatigue wear of the coating is to break the inter-layer bonds which characterize hard-carbon films, thereby allowing the DLHC material

561

R Wei et al. / Fatigue wear characteristics of diamond-like hydrocarbon coatings TABLE 1. RCF Weibull statistics B10 life x 106 (cycles)

Treatmenta

Material

B50 life x lo6

Weibull slope

B50 B50,

(cycles)

M-50 M-50 M-50 M-50b

AR C a-SiHC DLHC

4.4 5.8 9.8 28.6

7.5 15.6 14.1 76.4

3.5 2.0 5.1 2.0

1 2 1.9 10.2

52100 52100b

AR DLHC

6 75.5

16.5 232

1.9 1.7

1 14.1

4118 4118b

AR DLHC

11.5 87

32 327

1.8 1.4

1 10.2

44oc 44oc

AR DLHC

3.2 7.1

2.9 1.61

1 3.7

6.1 22.7

mt’,

‘AR: as-received (uncoated); C: sputter-cleaned only; a-SiHC: sputter-cleaned then coated with a-SiHC, DLHC: sputter-cleaned then coated with a-SiHC and DLHC. bDLHC coatings frequently resulted in lives that exceeded 100 million cycles. These tests were generally suspended at 100 million cycles because of the time required to test these rods and the increased likelihood of ball or race failures, at the high test times. These suspended data are considered in the computation of the median-rank lines, but are not shown on Weibull plots. In one case, where a rod was tested significantly beyond this suspension point (to 300 million cycles) no evidence of a rod spa11 was found.

I 2 i

I11llll 3 11.3719

I 2

I 3

I

,,I111 4 16789

I‘n&m&m

a

I 3

I ,,I,, 4 36789

100

02 StrcsI cycled

Fig. 2. AISI 52100 steel rolling-contact-fatigue test results (the 199 data points collected on as-received rods are not plotted because they are too closely positioned).

to return to the most stable carbon form - graphite. It is considered possible that such a transformation would provide a solid lubricant and facilitate reduced friction at the points of asperity contact that are expected at low lubricant-film-thickness conditions. Secondary ion mass spectrometry was used to determine composition profiles for selected elements through the DLHC and a-SiHC films in an RCF track of an M-50 rod and on the unworn region adjacent to it. The typical secondary ion count profiles for the unworn material shown in Fig. 8(a) indicate that the coating thicknesses were approximately 0.2 wrn and approximately 0.5 pm for a-W-E and DLHC, respectively. Unfortunately, only the elemental concentrations

Fig. 3. AISI 4118 steel rolling-contact-fatigue

test results.

of H could be quantified (approximately 4.5 X 1022atoms cmm3 in both films). Assuming a carbon density close to that of diamond for the DLHC film, a C density of 1.5 X 1O23atoms crne3 is computed and an H-to-C ratio near 0.3 is obtained. In the a-SiHC layer an Hto-Si ratio near unity is computed over a reasonable range of C concentrations. Raman and X-ray diffraction measurements of the a-SiHC films indicate that these films are amorphous, but their carbon content is uncertain. The compositional profile measured on the bottom of a typical wear track after 100 million cycles of RCF testing is shown in Fig. 8(b). It indicates the a-SiHC layer thickness is about the same as it was in the unworn test grove, but the DLHC layer is almost gone. Comparison of the profiles for the worn and unworn films shows the relative concentration of C in the a-

562

R. Wei et al. I Fatigue wear characteristics of diamond-like

hydrocarbon

coatings

- - - - - extradisorder

LIFE (hmtolu

of strewCycles)

Fig. 4. AISI 440C steel rolling-contact-fatigue

1200

1000

test results.

1400

Fig. 6. De-convolution of Raman spectrum a-SiHC coated rod (before RCF testing).

1000

1200

1400

1600

a-SiHC/DLHC

for sputter-cleaned,

1800

RAMAN WAVENUMBER SHIFT (cm-‘)

Fig. 5. Raman spectrum of sputter-cleaned, rod (before RCF testing).

1800

1600

RAMAh’ WAVENUMBER SHIFI’ (an-‘)

coated

0-l 1100

I

I

1300 RAMAN WAVJBUhiBER

SiHC after wear is about double that in the unworn layer. This suggests carbon from the DLHC film either difIirses or is forced mechanically into the a-SiHC layer. 3.3. SE&f and EDS examination Figure 9(a) is a scanning electron micrograph showing the appearance of a typical spalled test track and adjacent rod surface on the M-50 rod that was cleaned and coated with a-SiHC. While some a-SiHC has been removed from the test-track region (the small, white specks represent regions where a-SiHC has gone), the figure shows most of the a-SiHC still intact after over 8 million stress cycles. Most of the removal occurred at the edges of the test track where micro-slip is expected. The spa11shown in Fig. 9(a) was typical of spalls observed on all rods regardless of their surface state (i.e. whether they were sputter-cleaned, coated or not processed). The test track in Fig. 9(b) is typical of the tracks

Fig. 7. Raman spectrum of sputter-cleaned, rod (after RCF testing).

f

IMO SHIFT

I 1700

(cm-‘)

a-SiHC/DLHC

coated

observed on DLHC-coated M-SO rods and looks similar to those on the 4118, 440C and 52100 rods. Beyond the edge of the test track, DLHC coverage is complete, but in the track itself, it is scant. The lighter greycolored regions are areas of a-SiHC as verified by EDS, but there are still lighter spots where all of the coatings have been removed. Examination of the M-50 rods that had not spalled after long intervals of RCF testing (over 100 million stress cycles) frequently revealed the development of small pits near the middle of the test tracks apparently produced when carbide particles dislodged from the M-50 matrix material. This suggests that surface defects induced by particle removal are insufficient to initiate the rapid development of a spall. It is noteworthy that

563

R. Wei et al. I Fatigue wear characteristics of diamond-like hydrocarbon coatings

(4

DEPTH @m)

(b)

DEPTH (am)

Fig. 8. Secondary ion mass spectrometric profiles for sputtercleaned, a-SiHC/DLHC coated rod: (a) unworn coating; (b) coating in wear track.

a-SiHC and in some cases significant DLHC were still present in the wear tracks associated with these long duration tests, but no correlation was found between the amount of coating remaining in the wear track and the test life. 3.4. Profilometric measurements The surface roughnesses of the bodies in contact in an RCF test are important because they influence the oil-film parameter (lambda) and therefore the degree to which the oil film separates the asperities at the ball/rod contact zone. The effects of ion-beam processing (sputtering, a-SiHC and DLHC coatings) on the roughnesses of typical M-50 rod surfaces are given in Table 2. These data show the uncoated and coated M-50 steel rods had roughness values that were essentially the same (0.063-0.075 pm). This suggests that all RCF tests were initiated under the same film-parameter test conditions. The surface roughness (R,) of the pre-test balls was approximately 0.1 pm. 3.5. The evolution of a typical wear track To understand the mechanism involved in extending the life of the DLHC coated rods, a comparison of

(a)

0.5mm

(b) Fig. 9. Typical wear-track appearance at failure: (a) cleaned and a-SiHC coated rod (spa11 at center); (b) cleaned and a-SiHC/ DLHC coated rod (spa11 at upper right).

TABLE 2. Typical roughness 50 rods

(R,) measurements

Rod treatmenta

R, (pm)

AR C a-SiHC DLHC

0.075 0.071 0.069 0.063

of untested

M-

“AR: as-received (uncoated); C: sputter-cleaned only; a-SiHC: sputter-cleaned then coated with a-SiHC, DLHC: sputter-cleaned then coated with a-SiHC and DLHC.

the histories of wear-track development for uncoated and DLHC-coated rods was performed. The data are shown in Figs. 10-12. These figures show the development sequences for the rod-wear-track depth profile and the surface morphologies of the contacting wear track and ball surfaces, respectively. The wear-track depth profiles in Fig. 10 show that wear tracks extend to their full depth within the first minute of testing for both the as-received and DLHC-coated rods. Substantial plastic deformation resulting in a track 1.5-2 pm deep occurred during this short interval and then the track cross-sections appeared to stabilize in both

R. Wei et al. I Fatigue wear characteristics of diamond-like

TEST DURATION

-0.127

mm

*

,,

:,,‘,I#

60 min

Fig. 10. Histories of typical rod-wear-track-contour developments: (a) as-received; (b) cleaned, a-SiHC/DLHC coated rod.

cases. The wear track on the uncoated rod appeared smoother than the original surface (compare the roughness at the edge of the track with the roughness within it). The DLHC-coated rod, in contrast, retained a substantial roughness in the track and this roughness appeared to remain essentially the same to the end of the test. The grinding marks that were still evident on the DLHC-coated rod at the end of the 6 h test suggest that the rod itself did not wear significantly over this time interval. The SEM images in Fig. 11 show the rod-surfacemorphology changes over time. Evidence of plastic deformation and smoothing in the wear-track surface for the as-received rod was observed. Some of the original features of the rod surface (deformed grinding marks), though, can still be seen. The smoothing of the as-received rod appears to stabilize after about 30 min. In comparison, the DLHC-coated rod shows evidence of continuing DLHC coating removal with time. Although there was some evidence of a-SiHC removal, most of this layer seemed to remain attached to the steel beneath it. The axes of ball rotation generally remained parallel to the rod axis during testing so circumferential tracks

hydrocarbon

coatings

also developed on the balls where they contacted the rod wear tracks. Comparison of the ball-track surface morphologies for the two cases (Fig. 12) suggests that the ball in contact with the uncoated rod retained its roughened morphology. In contrast, the surface of the ball in contact with the DLHC-coated wear track showed substantial polishing as the test proceeded. After 30 min, Fig. 12 shows that most of the initial surface features of the ball in contact with the DLHC surface had been removed. The surface finish of the balls run for 60 min against as-received and DLHC-coated rods were measured at 0.027 and 0.023 pm, respectively. These measured R, values suggest the surface finish has not changed as much as the micrographs suggest. Still, these roughnesses are between l/3 and l/4 of the initial ball roughness and they correspond to a lubricantfilm-thickness parameter increase from approximately 0.7 to approximately 1.2. Longer tests, particularly those conducted on 4118 steel, suggested a more significant difference in the R, values for balls run against asreceived and DLHC-coated rods (0.05 and 0.023 pm, respectively). These roughnesses suggest still greater film-thickness parameters (1.06 and 1.18, respectively). 3.6. Cross-sectional studies of the test tracks To understand if the coating process affected the macro-stress distribution on the M-50 test rods, selected test tracks from the coated and uncoated test rods were sectioned axially and circumferentially, polished and examined metallurgically. Long and short test lives were examined. All of the microstructures revealed a subsurface stress band from approximately 0.05 mm to 0.17 mm below the bar surface. This band indicated the region in which the material experienced maximum shear stress during the test. The region of stress was characterized by a “white-etched band”, which is typical for rolling contact fatigue, Fig. 13. The amount of whiteetched area observed was dependent on the number of stress cycles the test track saw (the higher the number of cycles, the denser the white etched area), rather than the uncoated or coated condition of the test bar. The depth of the subsurface stress area was not significantly different for coated and as-received rods and this indicates the a-SiHC and DLHC coatings did not affect the bulk stress state the material experienced. The extent to which they may have affected the nearsurface (residual) stress was not measured.

4. Discussion Some insight into the mechanism coatings induce increased RCF lives by considering test results (including that many wear tracks appeared to

by which DLHC can be obtained the observation have no DLHC

R. Wei et al. / Fatigue wear charactetitics of diamond-like hydrocarbon coatings 0.5 mm

565

TEST DURATION

lb)

Fig. 11. Histories of typical rod-wear-track surface morphologies: (a) as-received; (b) cleaned, a-SiHCDLHC designate the edge of wear track and direction of ball motion.

coated rod. Arrows

R. Wei et al. / Fatigue wear characteristics of diamond-like

hydrocarbon

coatings

10 min

30 min

60 min

(a) Fig. 12. Histories of typical load-ball surface morphoiogies: with cleaned, a-SiHC/DLHC coated rod.

(b) (a) ball surface in contact with as-received

rod; (b) ball surface in contact

R. Wei et al. / Fatigue wear charactetitics

Fig. 13. SEM image ofwear-track a-SiHC/DLHC-coated rod.

cross-section

for sputter-cleaned,

coating on them at the time of spalling or suspension). The experimental conditions associated with the RCF tests (e.g. surface finish and lambda values) should favor surface or near-surface failures. This type of failure can initiate with a surface crack followed by spalling, as discussed by Bamberger et al. [21]. Since crack initiation requires more time than other events in the failure sequence, prolonging initiation should increase the RCF life significantly. It is argued that crack initiation can be delayed by reducing the size and/or loading on asperities that tend to stimulate crack formation as they are flexed during the rolling process. Hence, evidence of reductions in asperity size or the forces on asperities could explain observed increases in RCF life. This study has shown that the softer of the two surfaces in rolling contact with each other tends to be polished during the early phase (approximately 30 min) of the testing process. As-received rods are softer and they are polished by the balls. As the DLHC is flexed on the coated rods, tiny abrasive particles are released into the oil film. The loading balls are soft in comparison with these particles and any coating that remains on the rod and the balls are, therefore, polished. The aSiHC layer which appears to adhere to the rod to varying degrees is hard and may protect the rod from abrasion. The smoothing of the loading balls has an effect on the lambda condition during the test. Since the smoothing of the balls occurs very quickly (approximately 30 min), a higher lambda condition is experienced for the majority of the DLHC-coated rod tests. Lambda has a significant effect on the RCF life of bearings; an increase reduces asperity interaction. Where long RCF lives are observed with the DLHC-coated rods, it appears that the combined rod and ball wear is more severe. It is suggested that the abrasive wearing process that occurs in this case results in improved conformity

of diamond-like hydrocarbon coatings

567

between the balls and rods. This in turn increases the lubricant-film-thickness parameter, reduces the contact stress in the rod and retards crack initiation and spalling failure by reducing asperity flexure. Sputter cleaning and a-SiHC coating, which also improved RCF lives, may also induce modest reductions in surface roughness that could not be detected in these tests. Further evidence of the relative importance of the wearing process compared with the protective properties of the DLHC film is found in the fact that the RCF life of a particular wear track does not seem to depend on relative DLHC coverage at the end of the test. For example, many tests that lasted beyond the test suspension period had essentially no DLHC fihn at the time of suspension. The smoothing effect, though, would not explain the increase in life seen with the sputtercleaned only and the sputter-cleaned and a-SiHC-coated test rods. Finally, it is noted that the gradual graphitization of the DLHC film during RCF testing could be expected to provide boundary lubrication which would tend to reduce the asperity coefficient of friction and loading and as a result impede crack initiation,

5. Conclusions Diamond-like hydrocarbon coatings induce order-ofmagnitude increases in rolling contact fatigue lives of typical bearing steels. An amorphous silicon-hydrocarbon layer sputter-coated between the steel and the DLHC coating facilitates sufficiently good adherence that significant fractions of the film will remain in the RCF wear track for substantial periods of time under what are typically very severe test conditions. When no DLHC coating is applied, the RCF balls that contact the rod tend to polish it. When a DLHC coating is applied, the rod is not worn, but fragments of this very hard coating polish the balls. It is argued that the ball polishing facilitates improved conformity between the ball and the wear track and more favorable lambda conditions, that these reduce asperity contact and loading and retard surface-crack initiation in the wear track. Repeated stressing of the DLHC coating causes it to revert from a hard, amorphous-carbon state to graphite.

Acknowledgments The financial support of the Argonne National Laboratory (DOE Contract 0372401) for part of this work and Raman spectroscopic analysis and interpretation by Tom Furtak of Colorado School of Mines are gratefully acknowledged.

568

R. Wei et al. 1 Fatigue wear charactetitics

References 1 K. Kakuta, The state of the art of rolling bearing system technology, JSME Int. J., 32 (2) (1984) 179-186. 2 E. V. Zaretsky, Selection of rolling-element bearing steels for long-life applications, in J. J. C. Hoo (ed.), Effect of Steel Manufacturing Process on the Quality of Bearing Steels, ASTM, Philadelphia, PA, 1988, pp. S-43. 3 R. G. Vardiman and J. E. Cox, A study of the mechanisms of fatigue life improvement in an ion implanted nickel-chromium alloy,Acta. Metall., 33 (11) (1985) 2033-2039. 4 J. Mendez, P. Violan and M. F. Denanot, Influence of nitrogen implantation on the fatigue properties of metals related to the nature of crack initiation mechanisms, Nucl. Instrum. Methods Phys. Res., B19J20 (1987) 232-235. 5 F. M. Kustas, M. S. Misra and P. J. Wilbur, Ultrahigh current density N implantation of 400C steel for improved RCF lifetimes, Surf. Coat. Technol., 39140 (1989) 573-585. 6 R. F. Hochman, A. Erdemir, F. J. Dolan and R. L. Thorn, Rolling contact fatigue behavior of Cu and TiN coatings on bearing steel substrates, I: Vat. Sci. Technol., A3 (6) (1985) 2348-2352. 7 R. Wei, P. J. Wilbur and F. M. Kustas, A rolling contact fatigue study of hard carbon coated M-50 steel, ASME J. Trib., II4 (2) (1992) 298-303. 8 A. Erdemir, Rolling-contact fatigue and wear resistance of hard coating on bearing-steel substrates, Surf: Coat. Technol., in press. 9 W. Scharff, K. Hammer, 0. Stenzel, J. Ullman, M. Vogel, T. Frauenheim, B. Eibish, R. Roth, S. Schulze and I. Miihling, Preparation of amorphous I-C films by ion-assisted methods, Thin Solid Films, 171 (1989) 157-169. 10 C. Weissmantel, K. Breuer and B. Winde, Hard films of unusual microstructure, Thin Solid Films, IO0 (1983) 383-389.

of diamond-like hydrocarbon coatings 11 K. Miyoshi, J. J. Pouch and S. A. Alterovitz, Plasma-Deposited Amorphous Hydrogenated Carbon Films and Their Tribological Properties, NASA Technical Memorandum 102379, 1989. 12 R. Wei, P. J. Wilbur, A. Erdemir and F. M. Kustas, The effects of beam energy and substrate temperature on the tribological properties of hard-carbon films on aluminum, Surf: Coat. Technol., 51 (1992) 139-145. 13 P. J. Martin, S. W. Filipczuk, R. P. Netterfield, J. S. Field, D. F. Whitnall and D. R. McKenzie, J. Mater. Sci. Lett., 7 (1988) 410. 14 A. Erdemir, M. Switala, R. Wei and P. J. Wilbur, A tribological investigation of the graphite-to-diamond-like behavior of amorphous carbon films ion beam deposited on ceramic substrates, Surj: Coat. Technol., 50 (1991) 17-23. 15 D. Glover, A ball-rod rolling contact fatigue tester, in J. J. C. Hoo (ed.), Rolling Contact Fatigue Testing of Bearing Steels, ASTM, Philadelphia, PA, 1982, pp. 107-124. 16 J. S. Sovey, Improved ion containment using a ring-cusp ion thruster, J. Spacecraft Rockets, 21 (5) (1984) 488-495. 17 L. G. Johnson, The Statistical Treatment of Fatigue Experiments, Elsevier, New York, 1974. 18 R. J. Nemanich, J. T. Glass, G. Lucovsky and R. E. Shroden, Raman scattering characterization of carbon bonding in diamondlike thin films, J. Vat. Sci. Technol. A, 6 (3) (1988) 1783-1787. 19 R. E. Shroder, R. J. Nemanich and J. T. Glass, Analysis of the composite structure in diamond thin films by Raman spectroscopy, Phys. Rev., 41 (6) (1990) 3738-3745. 20 P. Lespade, R. Al-Jishi and M. S. Dresselhaus, Model for Raman scattering from incompletely graphitized carbons, Carbon, 20 (5) (1982) 427-431. 21 E. N. Bamberger, B. L. Averbach and P. K. Peason, Improved Fatigue Life Bearing Development, Interim Report AFWALTR-87-2059, Aero Propulsion Laboratory, AFWAL, Wright Patterson AFB, OH, 1987, pp. 9-35.