Effects of ZrSiO4 in non-metallic brake friction materials on friction performance

The effects of ZrSiO4 (zirconium silicate or zircon) as an abrasive on brake friction performance and friction layers of non-metallic brake friction materials were evaluated. The experimental results indicated that ZrSiO4 enhances friction coefficient, but depresses wear rate. However, ZrSiO4 can improve the negative wear rate of the friction materials. The formation and development of friction layers are complex so that the friction layers formed during friction process were carefully characterized using scanning electron microscopy (SEM), light microscopy (LM), and X-ray diffraction (XRD) methods. Following characteristics of friction layers were identified-(1) dynamic behavior: the structure of friction layers changes at the different surfacial positions and across sample's thickness; (2) friction condition dependence: formation of friction layers depends upon temperatures, time, and thermal history such as fade and recovery; and (3) compositional dependence: the compositions of friction surface and bulk differ, nevertheless the bulk's composition determine the friction layers. The phenomena as baryte films, altered layers, iron patches, and zircon loose areas formed on the friction surfaces were observed. Baryte films were detected on the friction surfaces of Zr-0 (sample without zircon). Baryte films have positive effect on wear property, but the films disappear in the presence of ZrSiO4. The amount of carbonaceous materials decreases with the increase in ZrSiO4. Only negligible thickness of altered layers was found on the friction surfaces of Zr-0 sample, while samples containing zircon show out relatively thick altered layers. Both iron-patches and zircon loose areas increase with the ZrSiO4 contents. The relationships among formulation, friction performance, and friction surfaces were summarized.