Tribofilm characterization and residual stress changes in rolling/sliding contacts under low lambda conditions

Abstract

Low lambda applications, such as gears and bearings in off-road vehicle drivetrains, are an area of interest regarding tribological performance and failure mode investigations. In previous work [1], tests were conducted with variable rolling/sliding contact conditions under low lambda (λ < 0.5), demonstrating that a transition point exists where the primary failure mode changes from pitting/abrasive wear to micropitting, driven primarily by the lambda ratio. That work has been extended here to λ ≈ 1.2, showing another transition in the failure mode from micropitting to mild abrasive wear/no failure. These transitions tend to occur rather abruptly, suggesting that the amount of asperity contact (lambda ratio) alone doesn't explain them. The goal of the present work is to investigate the reason these failure modes transition with respect to increasing lambda ratio by evaluating the tribofilm, residual stress, and retained austenite at three different lambda ratios (0.05, 0.4, and 1.2) that represent the conditions under which the different failure modes occur. These investigations reveal that there are differences in the tribofilm composition and coverage, while the thickness is similar for all conditions. The presence of ZDDP elements in the tribofilm promotes micropitting, and increased tribofilm coverage results in longer life. Additionally, it is demonstrated that higher levels of retained austenite and compressive residual stress contribute to longer life.