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

• Failure mode maps identifying conditions causing micropitting vs macropitting. • Higher Hertz pressure favours macropitting, higher roughness favours micropitting. • Λ ratio has a comparatively weaker influence on type of failure mode. • Higher R q and lower Λ increase severity of micropitting and shorten its incubation. • Transition of initial micropitting to eventual macropitting promoted by higher pressure but prevented by severe micropitting rate. 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 only 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 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. Transition of initial micropitting to a more damaging macropitting mode is promoted by higher Hertz pressures but can be prevented by more severe rates of micropitting wear as caused by higher roughness and lower Λ-ratio. 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 relevant machines.