An investigation into frictional surface interactions and their effect on brake judder.

The chemical nature of the Transfer Film (T.F.) or Third-Body Layer (T.B.L.) formed at the friction interface of an automotive friction brake during off-brake motoring has been studied using Energy Dispersive X-ray (E.D.X.) analysis and Scanning Electron Microscopy (S.E.M.). Although these third-body layers are deposited on both mating surfaces of the friction couple, special attention has been paid to those formed on the disc brake rotor surface. Concurrently, detailed investigations have been undertaken examining the temperature-dependent, physico-chemical interactions of friction material constituents with each other, atmospheric oxygen and countermember materials using X-Ray Diffraction (X.R.D.).Evidence is presented relating the tribological performance of the friction pair to both the transfer film thermochemistry and the friction material composition. Among those characteristics describing the tribological performance of the friction couple, particular attention has been applied to the generation of Disc Thickness Variations (D.T.V.) induced by Off-Brake or Non-Braking Wear (O.B.W. or N.B.W.). The critical role of solid lubricants and abrasive friction modifiers and their effectiveness over a range of contact pressures / temperatures has received particular attention. Information obtained using various surface analytical techniques combined with detailed dimensional assessments of the affected triboelements has been used to show the considerable significance of abrasive particle size in determining the overall tribological behaviour of the friction pair, especially with respect to the wear regime and extent encountered at the surface of the countermember during O.B.W.Wear mechanisms are described for the generation of off-brake wear, these varying with friction material formulation. Dynamic and temperature-dependent influences on the level of in-service disc brake rotor runout are named as causes for particular forms of disc thickness variation generated by aggressive friction materials. Keywords: Third-body layer; Transfer film; Tribochemistry; Automotive Friction Braking; Cold Judder; Disc Thickness Variation; Disc Brake; Friction Material.