A New Approach to the Effect of EP Additive and Surface Roughness on the Pitting Fatigue of a Line-Contact System

Abstract

A gear-cam adaptor was used to investigate the effect of an extreme pressure (EP) additive in the base oil and roller’s surface roughness on the pitting life of a disk specimen under oil lubrication with rolling-sliding line contacts. Incorporating the mean friction coefficient, increases in the Vickers microhardnesses within the layer of strain hardening can explain the influence of the EP additive on the material toughness of the ratchetting layer. The pull-off forces obtained from the tip/sample’s force-distance curve of an atomic force microscopy (AFM) can further reveal the ever-growing characteristic of the boundary layer, which is benefit to retard the growth of cracks. Weibull plots for disk’s pitting life are provided for three lubrication cases. The use of the EP additive in the base oil can expand the range of pitting-life distribution; on the other hand, increasing the roller’s surface roughness can also elevate the pitting life of the specimen lubricated by the same oil. From the study of contact subsurface, the distributions of four material responses: boundary layer, ratchetting layer, shakedown layer, and elastic layer, can be found. The EP additive in the base oil affects the behavior of the first two layers and roller’s surface roughness, the dominant factor, even does the third one.