SOLID-SOLUTION EFFECTS ON THE FRACTURE-TOUGHNESS OF MULLITE-ZRO2 COMPOSITES

The toughening observed in the mullite-ZrO//2 composites by the dispersion of ZrO//2 particles into a mullite matrix cannot be adequately explained by the stress-induced phase transformation t-ZrO//2. The existence of a solid solution far off the equilibrium at the mullite-ZrO//2 grain boundary is assumed to be a dissipation-energy region which avoids the crack propagation. Taking into account this effect together with the martensitic transformation of ZrO//2 in the expression of the total energy of the crack, a toughness expression which explains correctly the experimental K//I//C values is obtained. Consequently, this new mechanism should also be considered in designing new tough materials in many ceramic systems where metastable solid solution occurs.