SILICON-CARBIDE PLATELET ALUMINA COMPOSITES .3. TOUGHENING MECHANISMS

This is the last part of a series of papers on the processing and fracture behavior of SiC-platelet/Al2O3 composites. The objective of this paper was to identify the mechanisms involved in the toughening process. A hot-pressed composite with a SiC volume fraction of 0.3 was chosen as the model system for study. Based on microstructural observations, crack deflection and grain bridging were both identified as possible toughening mechanisms and were further investigated. A modified two-dimensional crack deflection model is presented to account for the anisotropic microstructure in hot-pressed platelet composites, in which preferred platelet orientation was present. Relative toughness values were predicted for two crack propagation directions assuming crack deflection is the toughening mechanism. Fracture toughness measurements for specific crack directions were made using a bridge indentation technique. The correlation of experimental results with theoretical predictions is discussed. To distinguish the effect of grain bridging from crack deflection, an in situ observation of crack growth was conducted. The results showed no distinct rising T-curve behavior for cracks in the size range 80 to 500 mum. Measurement of fracture surface roughness was also made and the implications are discussed. The results indicate that crack deflection is the dominant toughening mechanism in the SiC-platelet/Al2O3 composites studied herein.