Fracture mechanics study of a transformation toughened zirconia alloy in the CaOZrO2 system

The flexural strength, elastic modulus, fracture toughness (K_tc) and grain size were determined for a partially stabilized calcia-zirconia alloy (Ca-PSZ) which was progressively aged at 1300°C. Data for the same properties were obtained also for a fully stabilized cubic magnesia-zirconia alloy (Mg-CSZ) which was used as a reference material. The growth of the zirconia precipitate phase in the Ca-PSZ material was monitored. The flexural strength and fracture toughness increased smoothly to peak values of 645 MPa and 9.6 Mpa $\text{m}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, respectively, at a critical value of the ageing time and thereafter declined rapidly. The precipitate phase coarsened during ageing. Its structure was tetragonal up until the critical ageing time and thereafter the majority of the particles transformed to monoclinic. The peak strength increased three times relative to the cubic stabilized material. The grain size and elastic modulus showed only a slight dependence on ageing time. The study confirmed the hypothesis that the enhanced strength of transformation toughened zirconia alloys arises from an increase in the fracture energy. This increase is brought about by the presence of tetragonal particles, metastable at room temperature, which can be transformed by stress.