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 Warren-Averbach analysis of grazing incidence X-ray diffractometry data shows 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 the deformed layer. Transmission electron microscopy studies, in cross-sectional view, also show a near-surface deformation zone containing dislocations, twins, and cracks. This is seen for both machining procedures, but the depth of the zone and the degree of deformation, in terms of dislocation density and number of cracks, is much higher in the roughly ground specimen than in the polished one. For comparison, a monolithic Al2O3 sample also has been studied after grinding. The deformation zone is very similar to the Al2O3-SiC sample with the same grinding condition, but cracks and dislocations are present at a slightly larger depth. The deformation depth for the polished Al2O3-SiC sample is similar to 50 nm. In the ground Al2O3-SiC sample, the deformation depth is 1-1.5 mu m and corresponds to the grain size. The deformation zone in the ground monolithic Al2O3 sample is 1.5-2 mu m deep. The observed grain-boundary cracks are almost parallel to the surface and may originate from nonaccommodated plastic flow between grains.