Fracture behaviour of alumina–calcium hexaluminate composites obtained by colloidal processing

Three different alumina (90 vol.%)–calcium hexaluminate (10 vol.%) composites have been prepared by colloidal processing of high purity starting powders in aqueous media and conventional sintering at 1500, 1550 and 1600°C. Calcium hexaluminate has been synthesised by high temperature reaction of alumina and calcium carbonate powders. The optimum dispersion conditions for the mixture have been selected from data for alumina and calcium hexaluminate slips studied independently. The microstructure of the obtained materials has been characterised by scanning electron microscopy. All of them showed a highly dispersed distribution of small CA6 grains inside the alumina matrix. Grain size and shape of the alumina grains was highly dependent on the sintering temperature whereas the grain size of calcium hexaluminate remained almost constant. The fracture behaviour of the composites was analysed using Vickers indentation and optical and scanning microscopies. Higher loads could be applied to the composites without additional lateral cracking than to a monophase alumina material of the same average grain size. The fracture toughness values of the composites were highly dependent on the microstructure and for two of them values were larger than for the alumina material. Results have been discussed in terms of residual thermal stresses between the grains due to thermal expansion anisotropy.