**5. Conclusions**

A ball-rod RCF test platform has been successfully demonstrated for testing thin solid film lubricants in ultra-high vacuum and at high rotational speeds. The film systems tested included silver and nickel-copper-silver and results indicate that film thickness initially present on the balls has the most influence over test life based on 118 tests and a thickness range of 180 to 200 nm. The Weibull shape factor for all test data was in the range of 2.5 to 2.8, indicating typical bearing-type failure modes and test-times. A shape factor in this range suggests predictability of the test method, which enables repeatable conclusions concerning tribology testing of rolling bearing elements.

For the two ball sizes investigated, the 7.94 mm ANSI T5 balls with nickel-copper-silver gave longer RCF test life than the 12.7 mm M50 steel balls for similar Hertzian contact stress loading. Surface finish of the balls and races also influenced test longevity, with longer life and higher load capability for the 7.94 mm T5 balls with a surface Ra of 0.04 compared to the 12.7 mm M50 balls with a surface Ra of 0.32. The surface roughness of the races directly affects the volume of silver that may be stored within the wear track over the course of the test. Rod material and surface finish did not influence test time for the materials Rex 20 steel and Si3N4.

Two types of failure modes were observed for all tests: i) early spall failure and, ii) silver film depletion. In the former, a film-spall on at least one ball resulted in increased friction and plastic yielding within the contact area leading to the onset of spall at the ball-film interface. In the latter failure mode, as the lubricant film became depleted the friction and vibration increased and the test was stopped.

Surface finish of the rod and race elements influences the rate of solid film transfer from the ball surface to the rod and races. Inspection of the wear surfaces of suspended test components suggested a third-body transfer mechanism of the lubricating film could be used to predict test longevity. A third-body self-replenishing model approach was applied and there is good agreement with measured steady state film thickness and predicted film height.

A Lundberg-Palmgren empirical model fit of the RCF test data for *L10* life was used to calculate the load-capacity parameter for three test configurations. Stable prediction of a load capacity parameter will enable better planning for expanded used of the UHV-RCF test platform. The load capacity parameters for configurations involving 12.7 mm M50 balls with M50 races varied with applied load, suggesting that further work is needed to establish load-reduction correction factors for these configurations. Load-reduction factors for configurations involving 7.94 mm T5 balls with nickel-copper-silver film are not required. Future work with the UHV-RCF platform will involve continued testing for influence of process parameters and deposition methods, such as magnetron, on RCF test life similar to Danyluk and Dhingra (2011, 2012a). Expansion of the platform for study of surface interaction with heat and fatigue under high vacuum will also be explored.
