A Study of the Relationship Between Transfer Film Chemistry and Friction Performance in Automotive Braking Systems

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, has been examined in detail. The chemistry of the TBL has been studied using energy dispersive X-ray (EDX) and X-ray photo electron spectroscopic (XPS) analytical techniques. Optical and scanning electron optical methods have been used to examine the friction surfaces of the two elements of the couple.

Evidence is presented which relates the friction characteristics of the couple to transfer film chemistry. It is also demonstrated that transfer film chemistry is dependent upon asperity temperature and the composition of the asbestos free composite, especially lubricant additions.

The effectiveness of molybdenum sulphide as a layer lubricant at low asperity temperatures is confirmed by imaging (XPS). More significantly, the technique provides evidence which is used to explain the temperature dependence of friction coefficient in some asbestos free friction materials.