Effect of pressure
The effects of pressure on the wear rate for MMC and matrix material are presented in Fig. 4. For MMC, the wear rate is negligible until the pressure is 0.7 MPa. The wear rate increases rapidly with the further increase of pressure. This results in larger wear debris and a rough MMC surface. A second transition from severe wear to sliding wear occurs with the further increase of pressure (0.9 MPa) though the rate of increase is lower than that at a pressure of 0.7 MPa. This is most likely due to rapid oxidation of the surface material and work hardening. The friction coefficients are lower compared with those of the wear versus distance experiments where lower pressure was used. A possible reason for this is that the higher pressure increases the surface temperature which causes more oxidation on the specimen surface. The evidence of surface oxidation is supported by the progressive increase of hardness and darker regions in the surface with the increase of pressure. The presence of a metal oxide film proved to be a valuable factor in improving the wear resistance of MMCs