Effect of lubricants on bearing damage in rolling-sliding conditions: Evolution of white etching cracks

To understand the role of lubricants in generating white etching cracks (WECs), cylindrical roller thrust bearings were life-tested in different oils in rolling-sliding conditions. The bearings were damaged prematurely when the tests were performed in an oil with additives (so-called "WEC critical oil"). Bearing life in the WEC critical oil was less than 5% of that of the mineral oil without additives. In the bearings tested in the WEC critical oil, post-test investigations revealed a non-uniform -100 nm thick tribo-film and micro-scale line cracks parallel to the contact line in the slide zones of the tribo-surface. In contrast, typical "point surface origin" (PSO)-type damages were observed when the bearings were tested in the mineral oil. Electron microscopy investigations showed that the line cracks in the case of the WEC critical oil tests propagated deeper into the subsurface. Only in the negative slide zone, the cracks then became WECs. The PSO damages generated in the mineral oil testing did not show subsurface cracking or white etching features. The results support the conclusion that, in the case of the WEC critical oil, a tribo-film formed and caused an increase in surface shear forces, which consequently resulted in the line cracks on tribo-surface that subsequently propagated deep into the subsurface.