Abstract
For the purpose of clarifying the water vapor effect on brake pad wear, two experiments were performed. In the first set of experiments, wear tests were conducted using four pads under three relative humidity conditions (0.8%RH, 35%RH and 65%RH). The tested pads were simplified formulation of commercial brake pad, each of which contains either CeO2, CuO, graphite as additive or no additives. Wear test results showed that wear amount of the three pads containing CeO2, CuO and non-additive at 0.8%RH was larger than those of pads at 35%RH and 65%RH. Analyses of wear debris and of the worn surfaces of both pad and disk showed that adhesion between pad and disk was relatively high at 0.8%RH. In the second set of experiments, adsorption tests of pad specimens left under 35%RH and 65%RH conditions were conducted. Test results indicated that water vapor in the environment physically adsorbed the pad materials. From the two sets of experiments, it is concluded that water vapor in ambient air adsorbed on the worn surfaces of pad and disk contributed to reduce the adhesion between them. On the other hand, adding graphite to the pad material decreased pad wear even at 0.8%RH. In order to elucidate this mechanism, the specimen of pure graphite was rubbed against disk material at 0.8%RH, 2.5%RH, 3.5%RH, and 65%RH. Wear of graphite at 0.8%RH and 2.5%RH was by some orders of magnitude higher than that at the other humidity conditions. On the disk specimen, graphite was clearly transferred at 0.8%RH. These results mean that the adhesion of graphite is very high at 0.8%RH. Therefore, the wear reduction mechanism of the pad with the addition of graphite at 0.8%RH was concluded as follows. Degraded gases from phenolic resin adsorbed the graphite surface at frictional interface, which caused to reduce its high adhesion to the disk material under very low humidity condition.
