Compositional dependence of phase formation mechanisms at the interface between titanium and calcia-stabilized zirconia at 1550°C

ZrO₂ samples with various CaO contents were fabricated by hot pressing, whereby CaO was dissolved by and/or reacted with ZrO₂ to form a solid solution and/or CaZr₄O₉, respectively. After a reaction with Ti at 1550°C for 6 h in argon, the interfacial microstructures were characterized using X-ray diffraction and analytical electron microscopy. Experimental results were very different from those found previously in the Y₂O₃-ZrO₂ system. The 5 mol% CaO-ZrO₂ sample was relatively stable due to the formation of a thin TiO layer acting as a diffusion barrier phase. However, α-Ti(O), β′-Ti (Zr, O), and/or Ti₂ZrO were found in 9 or 17 mol% CaO-ZrO₂ due to extensive interdiffusion of Ti, O, and Zr with a much thinner (β′-Ti+α-Ti) layer in 17 mol% CaO-ZrO₂ than in 9 mol% CaO-ZrO₂. Because CaO was hardly dissolved into Ti, it fully remained in the residual ZrO₂, leading to the formation of spherical CaZrO₃ in 9 mol% CaO-ZrO₂ and columnar CaZrO₃ in 17 mol% CaO-ZrO₂. In the region far from the original interface, abundant intergranular α-Zr was formed in 5 or 9 mol% CaO-ZrO₂. Scattered α-Zr and CaZrO₃ were found in 17 mol% CaO-ZrO₂ because a high concentration of extrinsic oxygen vacancies, which were created by the substitution of Ca⁺² for Zr ⁺⁴, effectively retarded the reduction of zirconia.