A FUNDAMENTAL TRIBOCHEMICAL STUDY OF THE 3RD BODY LAYER FORMED DURING AUTOMOTIVE FRICTION BRAKING

The chemical nature of the transfer film (TF) or third body layer (TBL) formed at the friction interface during automotive friction braking has been studied in detail using energy dispersive X-ray (EDX) and X-ray photoelectron spectroscopic (XPS) analytical techniques. Optical and scanning electron optical methods have been used to examine the friction surfaces of the two elements of the friction couple, grey cast iron and asbestos free friction materials. Evidence is presented which relates the friction characteristics of the couple to transfer film chemistry and friction material composition. The role of lubricants and abrasive additions to the friction material and their effectiveness at a range of contact pressures/temperatures, is examined in detail. Information obtained from imaging X-ray photo-electron spectroscopy studies will be used to show that transfer film chemistry is dependent upon asperity temperature/contact pressure and the composition of the friction couple components. These data are used to explain changes in friction coefficient which may occur during automotive braking. The influence of transfer film chemistry upon wear rate of the friction couple components is also discussed. Friction performance was found to be independent of transfer film thickness or topography but sensitive to transfer film composition. The chemistry of the transferred layer was influenced by the morphology and mechanical properties of the friction material abrasive. Friction material wear rates and friction coefficient were not interrelated.