Acoustic emission monitoring of lubricant additive behavior in sliding contacts

Acoustic emission (AE) signals were recorded during tribological tests on 52100 steel specimens, sliding under boundary lubrication conditions in the presence of different chemical additives (an anti-wear, 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. The measured friction, ECR, surface roughness and wear varied between tests as expected due to the action of whichever additive was present. Within each test, the variation over time of certain AE frequencies correlated strongly with the variation in friction and while other frequency components correlated with the ECR signal. AE and friction, surface roughness and wear parameters were also averaged over each test, and the results plotted against each other while varying the lubricant additive. This showed that the average of certain AE frequencies correlated well with the average friction, certain AE frequencies correlated with the surface roughness and while other average frequency components correlated with the wear volume per test. Together this demonstrates that the AE signal is rich in tribological information and is sensitive to asperity interactions and surface film composition. It is therefore a powerful tool to monitor tribological behavior and lubricant condition.