Influence of run-in procedures and thermal surface treatment on the anti-wear performance of additive-free lubricant oils in rolling bearings

When rolling bearings are operated in the boundary and mixed friction regime, there is a risk of wear which can be minimized by the application of suitable lubricants with anti-wear additives. These lubricants form surface-bound additive-based tribolayers under tribological strain. The trend towards a reduction of additive concentration requires new methods of effective wear protection. In this study, two additive-free lubricants were investigated using highly wear-critical cylinder roller thrust bearings. The results show that both oils exhibit insufficient wear protection. This protection can be increased for one of the oils by an iron oxide-based tribolayer which is built up during a suitable run-in procedure. Furthermore, it is shown how the run-in tribolayer can be reproduced by a thermal treatment of the bearings. The increase in wear protection can be explained by a better lubricant-surface interaction due to the tribolayers, which leads to a better separation of the steel surfaces. The tribolayers are characterized using EPMA, FIB-TEM, and nano-indentation analyses.