Martensitic toughening of ceramics

Abstract

Current knowledge of displacive phase transformations in non-metals (in particular, in ceramics) is reviewed. Using the tetragonal-to-monoclinic transformation in zirconia ( ΔV = 4.9% at room temperature) as a model system, the factors controlling the metastability of confined particles are discussed, i.e. matrix constraint, chemical composition position and nucleation barrier to the transformation. The model for martensitic nucleation in ceramics is based on a localized soft mode mechanism at strain singularities or concentrations in faceted particles. Other potential transformation togerther alternative to zirconia have been identified. For the first time, preliminary mechanical toughness data using the new tougheners is briefly presented. Composites of 20wt.% BTb 2O 3 in 80 vol.% MgO causes a thresfold increase at 1400°C. Monoclinic (β) dicalcium silicate dispersed in calcium zirconate exhibits a fivefold increase in toughness. Other new materials including refractory sulfides are examined from the point of view of transformation toughening. The application of high pressures during processing makes it feasible to alter thermodynamic phase fields and to develop yet other new transformation-toughened composites.