PHASE TRANSFORMATION OF ZrO_2-8%Y_2O_3 DURING ANNEALING AT HIGH TEMPERATURE

In this study,coatings of zirconia alloyed with8wt% yttria were prepared by plasma spraying.A comprehensivesequence of annealing experiments comprising time from 1 to 100hours at temperatures between 1100 and 1500℃ were carried out.It is shown that the T′phase in coatings decomposes into low yttriatetragonal phase(T_1′)and high yttria tetragonal phase(T_2′)following high temperature annealing.The content of YO_(1.5)in T_1′decreases with the increasement of annealing time.Both T_1′andT_2′phases are also “non-transformable”tetragonal phases.Andfurther annealing causes a cubic phase (C) to precipitate fromT_2′.The increasement of C phase is accompanied by the decrease-ment of T_2′phase.The probable mechanism of T′→T_1′ T_2′phase transformation is of spinodal decomposition and that of T_2′→C is of precipitation.