Characteristics and morphology of wear particles from laboratory testing of disk brake materials

The geometrical characteristics and morphology of wear particles generated from brake materials are important for environmental and tribological reasons. Low- and high-speed, pin-on-disk friction and wear testing of a commercial truck brake pad material against cast iron was conducted in which wear debris was collected. The sliding speed was held constant either at 0.275 or at 5 m/s, and the nominal contact pressure was varied between 0.125 and 1.25 MPa in room temperature air. In low-speed experiments, some tests were conducted with the pin in continuous sliding contact and others in which the pin specimen was raised and lowered periodically. Laser scattering examination of wear debris revealed two distinct peaks in the plot of frequency versus the mean particle size. The first peak occurs around 350 nm and does not vary with respect to the pressure and the sliding speed. The location of the second peak varies between 2 and 15 μm, depending on the pressure and the sliding speed. Energy dispersive X-ray (EDX) analysis of wear particles revealed particles having a high concentration of carbon, silicon, aluminum, iron, oxygen, molybdenum, and sulfur. It was also found that the continuity or discontinuity of sliding contact affects the size distribution of wear particles. In general, when the motion was discontinuous, as is the case in a repeated braking action, smaller wear particles are generated.