Surface fatigue in lubricated contacts: Mapping the failure modes of micropitting versus macropitting

Abstract

This paper presents a surface-fatigue failure mode map to identify contact conditions which preferentially lead to (i) micropitting, (ii) initial micropitting transitioning to surface-initiated macropitting, and (iii) surface-initiated macropitting failure modes in lubricated rolling-sliding contacts representative of rolling bearings and gears. The study used a triple disc fatigue rig to systematically investigate the effect of contact pressure, surface roughness and Λ-ratio on the type of surface fatigue damage mode. Specimens made of case-carburised 16MnCr5 steel and a custom-blended PAO + ZDDP oil were used. Results show that higher Hertz contact pressures strongly favour the occurrence of macropitting over micropitting; In present tests no macropitting was detected in any tests at Hertz pressures less than 1.5 GPa. Conversely, lower pressures favour micropitting. This is likely due to higher macro-pressure being able to drive the surface-initiated cracks deeper into the subsurface material, which was shown to be a pre-requisite for formation of macropits. Higher roughness favours micropitting due to higher asperity stresses, while the influence of Λ-ratio on the type of failure mode is relatively weak as long as contact is within mixed/boundary lubrication regime. Higher roughness and lower Λ increased the severity of micropitting. Micropitting incubation time was between 100,000 and 1 million cycles depending on contact conditions, it is shorter for higher R_q  and/or lower Λ. Transition of initial micropitting to a more damaging macropitting mode is promoted by higher Hertz pressures but impeded by more severe rates of micropitting wear which occur at higher roughnesses and lower Λ-ratios. Given the different implications of micropitting versus macropitting, the presented failure mode map can be used during the design process to help improve the reliability of machines.