Testing to Reveal Tribology Mechanisms for Advancing Bearing Steels

摘要

The risk of introducing advanced bearing steels can be reduced by Technology Readiness Level (TRL) testing methods that reveal surface and subsurface tribology mechanisms. Abrasive wear screening tests (TRL 3) show a correlation with hardness for AMS 6491 (M50), AMS 6278 (M50 NiL), and three versions of a high-nitrogen stainless steel AMS 6898 (N360). A TRL 3 load capacity test method for oil qualification reveals lubrication and failure mechanisms for both oil formulations and bearing steels. Load capacity tests with legacy bearing steels AMS 6491 (M50) and AMS 6278 (M50 NiL) show superior performance over newer bearing and gear steels AMS 6509 (Ferrium C64), AMS 5930 (Pyrowear 675), and AMS 5898 (N360). Surface analysis and traction behavior are used to reveal the accommodation of shear phenomenon on an asperity scale. M50 NiL shows high traction and good antiwear (AW) behavior where shear is accommodated at the surface film, without much asperity shear or wear. The high-nitrogen stainless steels, or any material with a stainless oxide layer or a soft phase with limited shear resistance, tend to have low traction and poor AW attributes. AW and extreme pressure (EP) attributes are clearly revealed from tests with various bearing steels, as well as oil formulations. A third attribute, run-in polishing (RIP), is introduced to characterize interactions resulting in polishing of roughness features. Asperity polishing extends micro-elastohydrodynamic mechanisms and lowers the traction coefficient. Bearing steel and oil formulation attributes for AW, EP, and RIP can easily be at odds with surface and subsurface initiated fatigue, particularly for long-term operation. This requires extended TRL 3 testing for these attributes, along with TRL 4 simulation testing for component tribology design for service. TRL testing and analysis provide the methodology to design and apply component contact interfaces with greatly enhanced innovation and reduced risk.