Correlation between roughness, film thickness, and friction coefficient with pitting wear resistance of spur gears

This research aims to examine the relationship between roughness, film thickness, and friction coefficient in relation to pitting wear resistance during the rolling-sliding contact fatigue of forged and plasma-nitrided gears. In the present work, forged gears in continuous cooling bainitic steel were manufactured and plasma nitrided with three sets of N2-H2 gas mixtures, containing 5 vol%, 24 vol%, and 76 vol% N2. The present investigation evaluated the contact analysis between gears from the macroscopic point of view and the microscopic. At the same time, the evolution of roughness and pitting damage area after each test stage was monitored. The study determined the stress field after each loading cycle and correlated the regions of higher stresses with surface and sub-surface fatigue. Weibull's statistical approach showed that nitrided gears with 24 vol% N2 exhibit the most reduced pitting wear rates among the gears tested. This outcome is attributed to the optimal balance of surface hardness, fracture toughness, compound layer depth, and surface phases. The most pitting damage occurred in the dedendum and pitch line regions, being the regions that reported the greatest Hertz contact pressure. This is due to the rolling direction being opposed to the friction force in the dedendum region. This paper shows that pitting wear intensifies with increasing roughness, but this same behavior was not observed between the wear evolution and the maximum shear stress field in the sub-surface. Another interesting fact is that nitrided gears with 24 vol% N2 (best condition) have a greater film thickness at the end of the rolling-sliding contact fatigue, which gives it greater protection, less friction, and pitting wear. In general, the cracks were observed in areas very close to the surface and, with the course of the propagation stage, reached the surface, causing the formation of damage by pitting or spalling.