Microstructural characterization, mechanical properties, and corrosion resistance of the CaO–TiO₂system

Ceramic materials with chemical compositions of CaO–Xmole % TiO₂, whereXranges from 3 to 40, have been prepared for the evaluation of microstructures, mechanical properties, thermal shock resistance, resistance to hydration in air, and resistance to corrosion in molten salt saturated with calcium. When the TiO₂content is less than 40.3 mole %, the CaO forms two-phase microstructures consisting of CaO and Ca₃Ti₂O₇, and the composition CaO−40.3 mole % TiO₂corresponds to the (single-phase) compound Ca₃Ti₂O₇. The data on mechanical properties suggest that the single-phase material has better flexural strength and fracture toughness than do the two-phase materials. It is also demonstrated that the stabilizing effect of TiO₂on CaO imparts resistance to atmospheric hydration. Both scanning electron microscopy and energy-dispersive x-ray analysis of the specimens exposed to molten CaCl₂−20 mole % CaF₂saturated with calcium at 900 °C show that corrosion resistance increases with an increase in TiO₂content, but that corrosion does occur in all materials. On the basis of experimental observations and thermodynamic considerations, the degradation of Ca₃Ti₂O₇is shown to be associated with the reduction of Ca₃Ti₂O₇by calcium to form CaO and metallic titanium followed by the dissolution of the reaction product CaO in the molten salt.