The effect of friction on micropitting

Abstract

Micropitting is a type of surface fatigue damage that occurs in rolling-sliding contacts operating under thin oil film conditions. It is caused by stress fluctuations, brought about by surface asperity interactions, which lead to initiation and propagation of numerous surface fatigue cracks and subsequent loss of material. Despite its increasing importance to gear and bearing reliability, the mechanisms of micropitting are poorly understood. This is particularly the case concerning the effects of friction on micropitting which are difficult to study under controlled conditions. This is because it is difficult to isolate the friction effects from other influential factors, in particular from the build-up of any anti-wear tribofilm and its subsequent effect on the running-in of counterface roughness that is known to strongly affect micropitting through its influence on severity of asperity stresses. This paper presents new data on the impact of friction on micropitting obtained using a new test methodology. Micropitting tests were conducted using a ball-on-disc MTM rig with the additional functionality to continuously monitor the growth of tribofilm during the test. Friction was varied by using custom-made oils containing different concentrations of MoDTC. Crucially, the effect of friction was isolated from the effect of counterface roughness running-in by introducing the MoDTC blend only after the running-in period was completed with a ZDDP solution alone. This approach eliminates the influence of MoDTC on ZDDP anti-wear tribofilm growth in early stages and hence ensures the same running-in takes place in each test. This gives similar asperity pressure history, regardless of the amount of MoDTC present.Results show that friction has a very significant impact on micropitting; for example, the extent of micropitting was reduced by a factor of 10 when friction coefficient was reduced from about 0.1 to 0.04. Lower friction results in fewer surface cracks which grow at a shallower angle to the surface than those at higher friction. Numerical analysis of contact stresses present under tested conditions indicates that the primary mechanism by which friction affects micropitting is through its effects on near surface stress fields: reducing friction reduces the local tensile and shear stresses in the near surface, asperity-influenced region, which may in turn be expected to mitigate crack initiation and propagation. The results presented may help in designing oil formulations that can extend component lifetimes with respect to both wear and micropitting damage through controlling tribofilm growth and friction.