The Zirconia-Yttria System

The phase relationships in the zirconia‐yttria system have been established up to 2000°C. The addition of 0 to 5 mole per cent yttria lowers the temperature at which zirconia transforms from the tetragonal to the monoclinic form. From 7 to 55 mole per cent yttria is soluble in zirconia and this solid solution is cubic (stabilized zirconia). A two‐phase region appears between 55 and 76 mole per cent yttria. With more than 76 per cent yttria, zirconia is soluble in yttria. These phase boundaries refer to the specimens rapidly cooled from 2000°C. The effect of lowering the temperature to 1375°C is to increase the width of the two‐phase region between cubic zirconia and yttria solid solution. A tentative phase diagram of the zirconia‐yttria system is presented. The mechanism of stabilization of zirconia by an oxide of the yttria type is discussed in relation to the crystal structure of these two oxides. It is concluded that scandia, as well as other oxides of the rare earth group from element 62 to element 71, should stabilize zirconia by the same mechanism. Experimental results obtained on scandia, gadolinia, and samaria indicate that, as for yttria, the minimum amount of these oxides necessaryto produce stabilization is about 6 mole per cent.