**3. Experimental results**

100 Performance Evaluation of Bearings

**2.4. Thin film diagnostics**

Based on previous testing using the RCF rig in Figure 1, the onset of a spall on at least one ball is detectable over a vibration range of 0.22 g to 0.35 g with the accelerometer mounted on the top section of the test chamber. To be clear, each test chamber may have different vibration transmissibility based on its assembly. Therefore a new test rig should be characterized for the first detectable vibration of the onset of coating spall, and verified by post-test autopsy. The failure threshold for the rig presented in Figure 1 is 0.35 g, as measured by an accelerometer mounted on the top section. For the first 15 minutes of the test, a run-in period occurs and the vibration can reach 0.30 g, then settles to a steady state range of 0.06 g to 0.15 g for length of the test until failure. The test fixture temperature measurement shown in Figure 2c tends to increases with vibration and in general tended to increase over the length of the test. This is to be expected since as the test proceeds the solid lubrication is depleted and there is an increase in surface friction and ultimately increased vibration. Temperature at failure may vary based on coating system and contact stress loading. For the test rig shown in Figure 2c a thermocouple has been placed in contact with the top race to monitor temperature during the test. An optical temperature measurement

Confirmation of thin film composition and thickness prior to RCF testing can give insight and predictability to the test results. RCF test results may be confounded due to unknown and unwanted constituents within the coating. Auger Electron Spectroscopy (AES) may be used to measure and sample atoms within the coating. The AES process uses a high energy electron beam to bore a small diameter hole, on the order of 1 – 2 nm, into the coating and ball surface. The material that is removed during the process is analyzed using an in-situ mass spectrometer to determine its species. Figure 3 presents constituent information related to a thin film of silver that was deposited onto a 7.94 mm diameter ANSI T5 steel ball. Starting from the left, which correlates to the surface of the coated-ball, carbon, oxygen, and silver are present in the coating. Moving to the far right in Figure 3, there is a strong transition from silver to iron and chromium, which are two constituents of T5. Based on the known composition of T5 and the AES results of Figure 3 the coating thickness may be approximated as 190 nm. More interesting however is the composition of the coating through its thickness, specifically, the iron, oxygen, and nickel present throughout the silver layer. The results of Figure 4 illustrate element composition of a nickel-copper-silver coating deposited on to a Si3N4 ball using a physical vapor deposition ion plating process. Referring to Figure 4, there is a high concentration of nickel and iron near the ball-coating interface at about 120 nm. Since the ball itself does not contain Ni or Fe, the presence of those elements and their concentrations suggest that contamination occurred during the deposition process. The results of Figures 3 and 4 suggest interlayer mixing and coating contamination during the deposition process which is likely to influence the lubrication properties of the film, and ultimately the RCF life. For more information concerning thin film diagnostics and

deposition plasma diagnostics related to RCF life, see (Danyluk and Dhingra, 2012b).

may be taken as well from the large port in the top section.

**2.3. Failure criterion**

Statistical tools may be used to extract necessary information from RCF data. For example, Weibull analysis and high-cycle fatigue software tools such as ReliasoftTM may be used to correlate and compare test results independent of the coating and test elements in the test. An inverse power law life model with Weibull distribution may be used to check the experimental setup by comparing the Weibull parameters against known test configurations. Historically, a Weibull shape factor in the range 1 4 is to be expected for bearing and gear type RCF failures. For comparison, a shape factor less than 1 would indicate a flawed test method or infant failure. It is good practice to fit the RCF data to a Weibull distribution model starting after the fifth test so that one may confirm right away that the test results reflect coating and material performance and not a flawed assembly process or inadequate test preparation.

Rolling Contact Fatigue in Ultra High Vacuum 103

tested. When choosing the stress-use parameter, make sure the data is collected over a sufficient range of contact stresses that includes the stress-use value. The data presented in Figures 6 and 7 was taken over a stress-use range of 1.4 to 3.5 GPa. The results in Figures 5 through 7 strongly suggest rolling contact fatigue failure since the contact stresses for each of these tests are 1/3 less than the calculated tensile yield strength of each component as

calculated from the hardness measurements presented in Table 2.

**Figure 6.** Reliability data for RCF elements in configuration 1 of Table 1.

**Figure 7.** Reliability data for RCF elements in configuration 2 of Table 1.
