STUDY OF FRACTURE SURFACE BRIDGING EFFECT IN CERAMICS

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Some toughness reinforced ceramic-based composites )polycrystalline aluimina containing a few vol% stainless-steel particles, β-spodumene glass ceramics containing 30.vol% Ni or Co metal, and silicon nitride ceramics reinforced with 20 vol% silicon carbide whisker , etc.) were prepared. We tried to clarify the toughening mechanism by 1) Spectroscopic techniques at micro scale, 2) R-curve evaluation, and 3) Crack profile observation. The R-curve behavior and crack opening displacement (COD) profiles for those composites were evaluated and mechanism for fracture resistance was examined. Those composites showed remarkable R-curve behavior within short crack extension of 1 mm. The results were discussed, associated with microstructural features and fracture surface geometry of those composites. For COD profile observation in glass or ceramic single crystals, the interaction between fracture surfaces was not observed. In the COD profiles of toughness reinforced ceramic-based composites on the other hand, the experimental value was lower than the theoretical one (Irwin parabola) near the crack tip, resulting in the enhancement of the fracture surface interaction. Especially, it was obvious that external stress shielding effects acted more strongly near the crack tip in highly toughened composite. The increase in matrix grain size and addition of metal powder or whisker were found to enhance the interaction between fracture surfaces. The relationship between the applied stress intensity and the stress intensity at the tip of crack as a function of K(appl)/K(tip) indicate that the agents for reinforcement acted as elastic bridging elements in the toughness reinforced ceramic-based composites. The size of bridging zone in COD profile for each specimen was clarified to correspond to that of rising extension in R-curve characteristics.