X-ray photoelectron spectroscopy characterisation of third body layers formed during automotive friction braking

Abstract

X-ray photo electron spectroscopy (XPS) has been used to determine the nature of the “TRANSFER FILM” (TF) or “THIRD BODY LAYER” (TBL) formed at the friction interface of some simple friction couples using materials commonly employed in automotive braking systems. Imaging X-ray photo electron spectroscopy (IXPS) has been used with good effect to illustrate both the interaction and relative distribution of the various chemical species present on the grey cast iron surface after rubbing against a number of different asbestos free friction materials. Evidence is presented which highlights the relationship between transfer film chemistry, friction material composition and work done during braking. The role of lubricant additions to the friction material, in particular their effectiveness at a range of asperity temperatures, is examined in detail. The influence of chemical changes at the interface is also discussed. The results obtained in this study allows conclusions to be drawn about the dependence of transfer film chemistry on the asperity temperature and the composition of the friction material. This information is used to explain some of the differences in friction performance which may occur during automotive braking.