Effects of water films and sliding speed on the frictional behavior of truck disc brake materials

Friction materials for car and truck disc brake pads are typically complex composites containing metals, ceramics, and polymers that are designed to provide stable frictional performance over a range of vehicle operating conditions, exhibit acceptable wear life, and produce minimal noise and vibration. Despite the fact that brakes must operate under a variety of environmental conditions, most standardized and developmental laboratory tests for brake materials are conducted under dry sliding conditions. Studies of wet braking behavior are rare in the tribology literature. The present work investigated the frictional behavior of commercial truck brake pad materials sliding on gray cast iron simulated brake discs in the presence of water films. An instrumented sub-scale testing machine equipped with a water spray was used. There was a significant effect of water films on reducing pad/disc friction compared with dry tests on the same materials and at the same sliding speeds. The friction coefficient under water spray conditions was inversely proportional to the square of the sliding speed. The reduction of friction with increasing speed was principally due to hydrodynamic effects, although the water spray also reduced frictional heating and changed the nature of the transfer film formation on the cast iron contact surface. Frictional recovery occurred within seconds after the water spray was removed. (C) 2003 Elsevier Science Ltd. All rights reserved.