Low temperature phase equilibria and ordering in the ZrO₂-rich region of the system ZrO₂-CaO

Abstract

From co-precipitated powder samples, the solid state reactions occurring between room temperature and 1500° C in the ZrO₂-CaO system have been studied. At low temperatures, compositions containing < 25 mol% CaO show a complex picture of phase transformation and ordering in the system. From the obtained results the following singular reactions have been established. (i) Tetragonal zirconia solid solution decomposes eutectoidally at 7 mol% CaO and 1048 ± 4° C into monoclinic zirconia solid solution and calcium zirconate (CZ). (ii) Cubic zirconia solid solution undergoes a eutectoidal decomposition at 17.5 mol% Cao and 1080 ±20° C into tetragonal solid solution + calcium zirconate. (iii) The monoclinic ordered phase, CaZr₄O₉ (Φ₁), ), undergoes an order-disorder transformation into cubic zirconia solid solution at 1232 ± 5° C. (iv) Cubic zirconia solid solution undergoes a eutectoidal decomposition into two ordered phases, Φ₁ + Φ₂ at 21 mol% CaO and 1200 ± 10°C. (v) Hexagonal ordered phase Ca₆Zr₁₉O₄₄ (Φ₂) decomposes peritectoidally into cubic zirconia solid solution + calcium zirconate at 1360 ± 10° C. The two ordered phases Φ₁ and Φ₂ seem to be unstable below ≈ 1100° C. By using DTA, X-ray diffraction and SEM techniques, the extent of the tetragonal and cubic zirconia solid solution fields have been established. From the above experimental results a new tentative phase diagram is given for the ZrO₂-rich region of the system, ZrO₂-CaO.