Scanning probe microscopy of automotive anti-wear films

Abstract

Atomic force microscopy (AFM) and lateral force microscopy (LFM) have been used to examine the local topography and frictional properties of anti-wear films formed during tribological testing using a reciprocating Amsler test — a laboratory mechanical test to simulate engine valve train conditions — and a real engine component. Measurements were made in ambient conditions, under a model hydrocarbon fluid (dodecane), and under a lubricating oil. AFM gave high resolution images together with an estimate of the distribution of film thicknesses and coverages in good agreement with previous electron microscopy results. Force-distance curves were sensitive to soft liquid-like surface overlayers found to be present in some cases. Using LFM, relative friction coefficients were determined on microscopic smooth areas of Amsler specimens prepared using different lubricant additives, and the values compared with macroscopic values measured at the end of the Amsler test. Absolute values for these micro-regions were also measured, with a lower degree of confidence. The possibility of using the AFM tip to simulate a single asperity tribological contact was investigated.