Effect of Natural Graphite Particle size in friction materials on thermal localisation phenomenon during stop-braking

Thermal localisation (TL) phenomenon is defined as localisation of high temperature as consequences of high strain gradients developed in the bulk of materials of a tribo-pair. High extent of localisation of thermal gradient is rated as a non-desirable parameter for a good friction material because it affects both the surfaces leading to thermal crazing, cracks and fatigue, especially of the disc apart from affecting wear. In the present work, four composites with identical parent composition (containing ingredients of all the four categories) but differing in the average particle size of natural graphite (21, 41, 71 and 137 μm) were developed and evaluated on specially designed tribometer for enabling the studies of TL phenomenon. Two series of testing modes viz. inertia mode and rpm-mode were selected to bring out the correlation of selected graphite and their tendency to produce hot bands, hot spots and related phenomena on the counter-face. Overall, it was observed that the composites with lower particle size of graphite rendered better results than that with higher particle size. The study of hot-spot appearance and the TL behavior of tribo-pair are discussed in details in the paper.