Compositional Dependence of Phase Formation Mechanisms at the Interface Between Titanium and Calcia-Stabilized Zirconia at 1550 degrees C

Abstract

ZrO(2) samples with various CaO contents were fabricated by hot pressing, whereby CaO was dissolved by and/or reacted with ZrO(2) to form a solid solution and/or CaZr(4)O(9), respectively. After a reaction with Ti at 1550 degrees 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 Y(2)O(3)-ZrO(2) system. The 5 mol% CaO-ZrO(2) sample was relatively stable due to the formation of a thin TiO layer acting as a diffusion barrier phase. However, alpha-Ti(O), beta'-Ti (Zr, O), and/or Ti(2)ZrO were found in 9 or 17 mol% CaO-ZrO(2) due to extensive interdiffusion of Ti, O, and Zr with a much thinner (beta'-Ti+alpha-Ti) layer in 17 mol% CaO-ZrO(2) than in 9 mol% CaO-ZrO(2). Because CaO was hardly dissolved into Ti, it fully remained in the residual ZrO(2), leading to the formation of spherical CaZrO(3) in 9 mol% CaO-ZrO(2) and columnar CaZrO(3) in 17 mol% CaO-ZrO(2). In the region far from the original interface, abundant intergranular alpha-Zr was formed in 5 or 9 mol% CaO-ZrO(2). Scattered alpha-Zr and CaZrO(3) were found in 17 mol% CaO-ZrO(2) 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.