Thermal shock behaviour of magnesia–spinel composites

The incorporation of up to 30% of spinel powder of median particle sizes 3, 11 and 22 μm into dense magnesia gives improved resistance to thermal shock as indicated both by the R‴ thermal shock resistance parameters calculated from measured mechanical properties, and by a standard quench test. The loss of strength of the composite materials in the quench test matched satisfactorily that predicted by the R‴ parameter. The maximum benefit from spinel additions was obtained with spinel of the largest particle size, at a loading of 20%. It is postulated that the pre-formed cracks in the composites, resulting from the thermal expansion mismatch between magnesia and spinel, inhibit the accumulation of strain energy during thermal shock.