NEAR-SURFACE DEFORMATION IN AN ALUMINA SILICON-CARBIDE WHISKER COMPOSITE DUE TO SURFACE MACHINING

Deformation due to two different surface-machining conditions-grinding (126 mu m diamond) and polishing (3 mu m diamond)-in an uniaxial hot- pressed Al2O3-30%-SiC-whisker composite has been investigated. A Warre n-Averbach analysis of grazing incidence X-ray diffractometry data sho ws that the deformation is localized to the very top surface zone. The cell size and the root mean square of the strain show a gradient in t he deformed layer. Transmission electron microscopy studies, in cross- sectional view, also show a near-surface deformation zone containing d islocations, twins, and cracks. This is seen for both machining proced ures, but the depth of the zone and the degree of deformation, in term s of dislocation density and number of cracks, is much higher in the r oughly ground specimen than in the polished one. For comparison, a mon olithic Al2O3 sample also has been studied after grinding. The deforma tion zone is very similar to the Al2O3-SiC sample with the same grindi ng condition, but cracks and dislocations are present at a slightly la rger depth. The deformation depth for the polished Al2O3-SiC sample is similar to 50 nm. In the ground Al2O3-SiC sample, the deformation dep th is 1-1.5 mu m and corresponds to the grain size. The deformation zo ne in the ground monolithic Al2O3 sample is 1.5-2 mu m deep. The obser ved grain-boundary cracks are almost parallel to the surface and may o riginate from nonaccommodated plastic flow between grains.