TRANSITION WEAR BEHAVIOR OF SIC-PARTICULATE REINFORCED AND SIC-WHISKER-REINFORCED 7091 AL METAL MATRIX COMPOSITES

The wear behavior of unreinforced and reinforced 7091 Al, the latter containing either 20 vol.% SiC particulates (SiC(p)) or 20 vol.% SiC whiskers (SiC(w)), was studied as a fuction of sliding distance and sliding velocity under unlubricated conditions. At sliding velocities below 1.2 m s-1, SiC reinforcement does not affect wear resistance. Wear debris produced from both the unreinforced and reinforced materials was predominantly metallic and was small in dimension and dark in color. The mechanism of wear under these conditions was surface-fatigue-related surface cracking. At sliding velocities greater than 1.2 m s-1, the wear rates of the reinforced materials were lower than for the unreinforced matrix. Both the unreinforced alloy and the SiC-reinforced composites exhibited elevated wear rates during the initial period of sliding, the mechanism of wear under these conditions, i.e. high velocity and short sliding distance, being controlled by subsurface-cracking-assisted adhesive transfer and by abrasion. During steady state sliding, these elevated wear rates were maintained by the unreinforced alloy, reduced wear rates being observed in the reinforced composites. The initial wear rates of the composites depend strongly upon reinforcement orientation, the highest wear rates being observed on the perpendicularly oriented SiC(w) composite. However, the steady state wear rates of the composites were generally independent of reinforcement geometry (particular vs. whiskers) and orientation (perpendicular vs. parallel) with the exception of wear at 3.6 m s-1 where the parallel-oriented SiC(w) composite was superior.