The effects of graphite, coke and ZnS on the tribological and surface characteristics of automotive brake friction materials

Purpose - The purpose of this paper is to investigate the role of solid lubricants (graphite, coke, ZnS) on brake performance. Design/methodology/approach - In this study, the tribological and surface characteristic of non-asbestos organic type brake friction materials containing three different solid lubricants (graphite, coke, and ZnS) in different proportions were examined and evaluated experimentally. The coefficient of friction (COF) and wear behavior of the samples were tested on a chase-type friction tester, and particular emphases were given to the effect of temperature and number of braking cycles on the COF. Each of the lubricants was added to the mixtures in different amounts and seven different brake linings were manufactured, provided that the total amount of solid lubricants and other ingredients were not changed. The worn surfaces of the specimens were analyzed using a scanning electron microscope with energy-dispersive X-ray microanalysis. Findings - The experimental results indicate that graphite has a positive effect on the tribological properties of brake linings. However, brake linings containing higher concentrations of ZnS and coke showed an unstable friction coefficient relationship with the temperature and number of braking cycles. The formation of friction layers was detected on the friction surface of these samples, which indicates that an increase in coke and ZnS content increases the discontinuous and unstable friction film areas. Originality/value - This paper fulfils the effects of solid lubricants (graphite, coke, ZnS) in brake friction materials with detailed tests and analysis.