Evidence for bulk residual stress strengthening in Al2O3/SiC nanocomposites

The fracture behavior of Al2O3/SiC nanocomposites has been studied as a function of the SiC volume fraction and compared to that of the pure Al2O3 matrix. A pronounced strengthening effect was only observed for materials with low SiC content (i.e., less than or equal to 10 vol%) although no evidence of concurrent toughening was found. Assessment of near-tip crack opening displacement (COD) could not experimentally substantiate significant occurrence of an elastic crack-bridging mechanism, in contrast with a recently proposed literature model. Quantitative fractography analysis indicated that transgranular crack propagation in Al2O3/SiC nanocomposites depends on the location of the SiC dispersoids within the matrix texture; the higher the fraction of transgranularly located dispersoids, the more transgranular the fracture mode. Experimental evidence of remarkably high residual stresses arising from thermal dilatation mismatch (upon cooling) between Al2O3 and SiC phases were obtained by fluorescence and Raman spectroscopy. A strengthening mechanism is invoked which merely arises from residual stress through strengthening of Al2O3 grain boundaries.