Silicon Carbide Platelet/Alumina Composites: II, Mechanical Properties

The mechanical properties, i.e., Young's modulus, fracture toughness, and flexural strength, of SiC-platelet/Al₂O₃ composites with two different platelet sizes were studied. Both Young's modulus and the fracture toughness of composites using small platelets (12 μm) increased with increasing SiC volume fraction. Maximum values for toughness and Young's modulus of 7.1 MPa·m^1/2 and 421 GPa were obtained for composites containing 30 vol% platelets. Composites fabricated using larger platelets (24 μm), however, showed spontaneous microcracking at SiC volume fractions of ≤0.15. The presence of microcracks decreased Young's modulus and the fracture toughness substantially. Two types of radial microcracks were identified by optical microscopy and found to be consistent with a residual stress analysis. Anisotropy in fracture toughness was identified with a crack length indentation technique. Cracks propagating in a plane parallel to platelet faces experienced the least resistance, which was the the lowest toughness plane in platelet composites with preferred orientation. Enhanced fracture toughness was found in the plane parallel to the hot-pressing direction, but no anisotropy in toughness was observed in this plane. The flexural strength of alumina showed a decrease from 610 to 480 MPa for a 30 vol% composite and was attributed to the presence of the platelets.