Influence of abrasives and graphite on processing and properties of sintered metallic friction materials

In this study, the influence of abrasives (size and morphology) and graphite on the processing and properties of friction materials were investigated. Friction materials based on bronze matrix, graphite as solid lubricant and different abrasives (silica, mullite and zircon) were prepared following two routes. On the one hand, following the traditional P/M technology (pressing-sintering) and on the other hand, using an alternative P/M route, which consists on sintering a powder blend free deposited in a mold and subsequently cold pressing. Sinterability, microstructure and physical-mechanical properties of the processed friction materials have been studied. Tribological and wear tests were carried out on a pin-on-disc system at different loads and sliding speeds using samples of 20 mm in diameter. Results show that the influence of abrasives size is especially relevant in the alternative P/M route, where materials including fine abrasives present unsuitable properties. Graphite also plays an important role on tribological behavior, in this work it has been found that friction materials with 4 wt.% graphite have better tribological properties than those with 2 wt.% graphite, despite having lower density and mechanical resistance.