Effect of the particle size distribution of spinel on the mechanical properties and thermal shock performance of MgO-spinel composites

The influence of varying the amounts of spinel with a similar median particle size, but with different distribution, on the mechanical properties and thermal shock performance of MgO-spinel composites was investigated. Mechanical properties of composites decreased significantly with increasing spinel content due to the thermal expansion mismatch. However, gamma(WOF) values of composites increased markedly, because of a significant change in the fracture mode from transgranular to intergranular fracture. A narrow distributed spinel A (Alcoa MR66) particles resulted in shorter initial crack propagation distances from the spinel particles, but spinel B (Britmag 67) particles with a significantly broader distribution were the origins of longer interlinked cracks. The improved resistance to thermal shock in MgO-spinel composites can therefore be attributed to the microcrack networks developed around the spinel particles, associated with the high values of gamma(WOF), and not to an increased K-1c. On the basis of theoretically calculated R''' values and experimentally found gamma(WOF)/gamma(i) ratios, resistance to thermal shock damage would be more strongly favoured with materials containing spinel B particles, rather than spinel A, for which a much larger volume% was required to achieve a similar improvement. (C) 2003 Elsevier Ltd. All rights reserved.