Acoustic emission monitoring of lubricant additive behavior in sliding contacts

Acoustic emission (AE) signals were recorded during tribological tests on 52,100 steel samples sliding under boundary lubrication conditions in the presence of various chemical additives (an anti-wear agent, two friction modifiers, an extreme pressure additive, and a dispersant). Various parameters of the AE signal were analyzed and compared with the coefficient of friction (CoF), electrical contact resistance (ECR), surface roughness, and wear volume. As expected, the measured CoF, ECR, surface roughness, and wear volume varied between tests due to the action of the additive. Within each test, the variation over time in certain AE frequencies correlated strongly with changes in CoF, while other frequency components correlated with the ECR signal. AE and CoF, surface roughness, and wear volume parameters were also averaged over each test, and the results were plotted against each other while the lubricant additive was varied. These findings revealed that the average certain AE frequencies correlated well with the average CoF, that certain AE frequencies correlated with the surface roughness, and that other average frequency components correlated with the wear volume per test. Together, these results demonstrate that the AE signal contains rich tribological information and is sensitive to asperity interactions and surface film composition. Therefore, AE is a powerful tool for monitoring tribological behavior and lubricant conditions.