**2.3. Failure criterion**

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

Rolling Contact Fatigue in Ultra High Vacuum 101

**Figure 3.** Auger electron spectroscopy depth profile of a silver film deposited on a 7.94 mm diameter

**Figure 4.** Auger electron spectroscopy depth profile of a nickel-copper-silver film deposited on a 7.94

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

for bearing and gear type RCF failures. For comparison, a shape factor less than 1 would

is to be expected

configurations. Historically, a Weibull shape factor in the range 1 4

ANSI T5 steel ball.

mm diameter Si3N4 ball.

**3. Experimental results**
