This is the third part of a three-part series that investigates the rolling contact fatigue initiation and spall propagation characteristics of three bearing materials, namely, AISI 52100, VIM-VAR M50, and VIM-VAR M50 NiL steels. Though there is substantial prior work published on the rolling contact fatigue initiation of these materials, little is known about their spall propagation characteristics after spall initiation. In Part III, 208-size, 40-mm-bore bearings are examined for changes in appearance of the microstructure as well as residual stress as a function of depth in the circumferential direction. The correlations between the experimental results from Part I and computer modeling in Part II are made.

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Difficult tribology problems with wear, scuffing and contact fatigue in aeropropulsion are being addressed with a systematic approach. Solutions to these problems have been found using a systematic tribology approach involving detailed failure analysis and simulation testing. The risks in advanced development have been substantially reduced by the inclusion of tribology attributes in mechanical system design. These attributes control lubrication and failure mechanisms (wear, scuffing and contact fatigue). The developmental process involves five key tribology parameters. Simulation of wear, scuffing and micro-pitting can be accomplished with the control of the five key tribology parameters: entraining velocity (U e ), sliding velocity (U s ), film thickness-tosurface roughness ratio (h/), contact stress (both global and asperity scale) and contact temperature. Specialized test machines, test specimens and test protocols that control these parameters are able to replicate the failure mechanisms experienced in service. This approach has been used to develop next generation jet engine oil formulations that are compatible with stainless steel bearing materials. Simulation testing involving oil formulations, along with bearing steel composition and heat treatments, shows what technologies have the greatest impact on performance and where future design and development efforts should be focused.