SILICON-CARBIDE PLATELET ALUMINA COMPOSITES .3. TOUGHENING MECHANISMS

This is the last part of a series of papers on the processing and frac ture behavior of SiC-platelet/Al2O3 composites. The objective of this paper was to identify the mechanisms involved in the toughening proces s. A hot-pressed composite with a SiC volume fraction of 0.3 was chose n 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-d imensional crack deflection model is presented to account for the anis otropic microstructure in hot-pressed platelet composites, in which pr eferred platelet orientation was present. Relative toughness values we re predicted for two crack propagation directions assuming crack defle ction is the toughening mechanism. Fracture toughness measurements for specific crack directions were made using a bridge indentation techni que. The correlation of experimental results with theoretical predicti ons is discussed. To distinguish the effect of grain bridging from cra ck deflection, an in situ observation of crack growth was conducted. T he results showed no distinct rising T-curve behavior for cracks in th e 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.