A study of transfer films formed on the surface of automotive friction couples.

During recent years both ride comfort expectations and vehicle refinement have increased such that any brake vibration problems are quickly highlighted. Much emphasis has been placed on the elimination of uneven disc wear and the reduction of un-sprung mass. Problems generally associated with the use of cast iron as the brake rotor, including uneven wear, corrosion and distortion, have been overcome using a thermally conducting coating. The thermally sprayed coating (HIPAC) is a composite of dispersed silicon carbide abrasives in a copper matrix. The coating has excellent thermal conductivity and wear resistance with high energy absorption and heat dissipation. With careful selection of friction material adequate levels of friction can be generated over a wide range of operating temperatures. When applied to aluminium discs dramatic reductions in rotor mass can be achieved without the interface melting problems generally associated with aluminium metal matrix discs. However, problems may still be encountered with excessive bulk temperatures. As a result the coating may find initial success when applied to cast iron discs.It is widely accepted that the performance of automotive friction couples is influenced by the transfer film or third body layer formed between the friction surfaces. Optical, scanning electron optical and energy dispersive X-ray techniques have been used to examine the frictional surfaces of the two elements of the couple. Films on both cast iron and HIPAC coating have been examined extensively. Evidence is presented which relates the friction characteristics to the distribution and chemistry of the transfer film.