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

Three different alumina (90 vol.%)-calcium hexaluminate (10 vol.%) composit es have been prepared by colloidal processing of high purity starting powde rs in aqueous media and conventional sintering at 1500, 1550 and 1600 degre esC. Calcium hexaluminate has been synthesised by high temperature reaction of alumina and calcium carbonate powders. The optimum dispersion condition s for the mixture have been selected from data for alumina and calcium hexa luminate slips studied independently. The microstructure of the obtained ma terials has been characterised by scanning electron microscopy. All of them showed a highly dispersed distribution of small CA(6) grains inside the al umina matrix. Grain size and shape of the alumina grains was highly depende nt on the sintering temperature whereas the grain size of calcium hexalumin ate remained almost constant. The fracture behaviour of the composites was analysed using Vickers indentation and optical and scanning microscopies. H igher loads could be applied to the composites without additional lateral c racking 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 alum ina material. Results have been discussed in terms of residual thermal stre sses between the grains due to thermal expansion anisotropy. (C) 2000 Elsev ier Science Ltd. All rights reserved.