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Introduction

Whisker-reinforced ceramic matrix composites have recently received a great deal of attention for applications as high temperature structural materials in, for example, advanced heat engines and high temperature energy conversion systems. For applications requiring mechanical reliability, the improvements that can be realized in fracture strength and fracture toughness are of great interest. Of particular importance for optimizing the mechanical reliability of these composites is the effect of the whisker/matrix interfacial characteristics on the strengthening and toughening mechanisms. Whisker reinforcements are primarily utilized to prevent catastrophic brittle failure by providing processes that dissipate energy during crack propagation. The degree of energy dissipation depends on the nature of the whisker/matrix interface, which can be controlled largely by the matrix chemistry, the whisker surface chemistry, and the processing parameters. It is generally believed that a strong interfacial bond results in a composite exhibiting brittle behavior. These composites usually have good fracture

strengths but low fracture toughnesses. If the interfacial bond is weak, the composite will not fail in a catastrophic manner due to the activation of various energy dissipation processes. These latter composites tend to have high fracture toughnesses and low fracture strengths. Generally, the interface should be strong enough to transfer the load from the matrix to the whiskers, but weak enough to fail preferentially prior to failure. Thus, local damage occurs without catastrophic failure. It is therefore necessary to control the interfacial chemistry and bonding in order to optimize the overall mechanical performance of the composites. Numerous investigators have demonstrated that the fracture toughness and/or fracture strength of polycrystalline ceramics can be significantly improved by reinforcing them with single-crystal SiC whiskers. The whiskers typically have Young's elastic moduli of approximately 700 GPa and fracture strengths in excess of 7.0 GPa. The SiC whiskers have been successfully incorporated in AI2O3,1"9 mullite,10 spinel,11 Si3N4,1215 MoSi2,1617 Zr0 2 , 18 and glass-ceramic19 matrices. Table I summa-

rizes typical values of the fracture strengths and fracture toughnesses reported in these systems. As shown in Table I, fracture strength and fracture toughness improvements are not consistently realized upon SiC whisker addition to the appropriate matrix. The variability in the expected property improvements is due partly to the nature of the whisker/ matrix interface. This article reviews the role of interfacial characteristics in the potential strengthening and toughening mechanisms in whisker-reinforced ceramic matrix composites and illustrates the concepts with an example in the SiC whisker/ A1203 matrix system. Strengthening and Toughening Mechanisms

The nature of the whisker/matrix interface is affected by several factors, including chemical bond formation, mechanical interlocking, and t