Micropitting has been studied using a disc machine in which a central carburised steel test roller contacts
three, harder, counter-rollers (“rings”) with closely controlled roughness. We varied the roughness using
different finishing techniques and investigated the effects of different oil base- stocks and additives, whilst
keeping the viscosity approximately constant. We also developed a predictive model for the approximate
analysis of rough-surface elastohydrodynamic lubrication based on the FFT approach of Hooke.
Damage on the test rollers included dense micropitting and“micropitting erosion” in which tens of microns of
the test surface were completely removed. This phenomenon is particularly damaging in gear teeth where it
has the potential to destroy profile accuracy. It was found that anti-wear additives led to a high rate of
micropitting erosion and that the effect correlated more or less inversely with simple sliding wear results.
There were also appreciable effects from base- stock chemistry.
The key parameter affecting the severity of damage seemed to be the near- surface shear stress amplitude
arising from the evolved roughness; different chemistries led to the evolution of different roughness during
initial running and thence to different contact stresses and different levels of damage.

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