DCB Methods for Examining Toughening in Alumina Reinforced with Carbon Coated SiC Whiskers

Abstract

The mechanical properties of Al_2O_3 have been substantially improved by the addition of SiC whiskers. Room temperature strength and fracture toughness have been reported for a variety of whisker sources and whisker fractions. The composite microstructure and the properties of the whisker/matrix interface allow various toughening mechanisms to be active in the SiC/Al_2O_3 system including crack branching and deflection, whisker and grain bridging, and whisker debonding and pullout. Modifications of the whisker/matrix interface are one way of tailoring the composite microstructure. Tiegs et al. have made composites from heat-treated and acid-leached whiskers to modify the whisker surface chemistry and the whisker/matrix interfacial chemistry. A second means of modifying the whisker/matrix interface has been suggested by Hsueh et al. in which a low modulus film is used to modify the residual stress. Using this theory, a 50 Å carbon film on a 0.7 μm diameter whisker would result in a nearly ten percent reduction of residual stress at the whisker/matrix interface in SiC whisker-reinforced alumina. SiC whiskers coated with carbon are used in this study as a way of modifying the chemical and mechanical bonding at the whisker/matrix interface by directly changing the interfacial chemistry and by modifying the residual thermal stresses found in these materials due to the thermal expansion mismatch of the composite constituents. In this study, two DCB methods of analysis are used to examine the effects of modified interfaces on fracture toughness in these materials.

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