Strengthening mechanisms in Al2O3/SiC nanocomposites

A strengthening mechanism merely arising from internal (residual) microstre sses due to thermal expansion mismatch is proposed for explaining the high experimental strength data measured in Al2O3/SiC nanocomposites. Upon cooli ng, transgranular SiC particles undergo lower shrinkage as compared to the surrounding matrix and provide a hydrostatic "expansion" effect in the core of each Al2O3 grain. Such a grain expansion tightens the internal Al2O3 gr ain boundaries, thus shielding both weakly bonded and unbonded (cracked) gr ain boundaries. It is shown that the shielding effect by intragranular SiC particles is more pronounced than the grain-boundary opening effect eventua lly associated with thermal expansion anisotropy of the Al2O3 gains, even i n the "worst" Al2O3-grain cluster configuration. Therefore, an improvement of the material strength can be found. However, a large stress intensificat ion at the grain boundary is found when intergranular SiC particles are pre sent, which can produce a noticeable wedge-Eke opening effect and trigger g rain-boundary fracture. The present model enables us to explain the experim ental strength data reported for Al2O3/SiC nanocomposites and confirms that the high strength of these materials can be explained without invoking any toughening contribution by the SiC dispersion. (C) 2001 Elsevier Science B .V. All rights reserved.