Residual Stresses and Apparent Strengthening in Ceramic-Matrix Nanocomposites

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Abstract
Residual stresses in Al2O3 and Si3N4-matrix nanocomposites (added with beta-SiC or WC phase) have been measured by piezo-spectroscopy. The highest residual stress value was found in Al2O3-matrix materials. Machining procedures have been found to play a minor role in the formation of residual stresses. Analyses of both spectroscopic and strength data suggest that, when the coefficient of thermal expansion of the matrix material is much higher than that of the added dispersoid, an apparent strengthening effect may arise in nanocomposite materials. However, this effect does not arise from an actual enhancement of the inherent material toughness, since it is proved that any strength enhancement vanishes in presence of externally introduced flaws greater than or equal to 30 mu m.

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