PHASE-STABILITY OF ZIRCONIA-BASED THERMAL BARRIER COATINGS .1. ZIRCONIA YTTRIA ALLOYS

Coatings of zirconia alloyed with 4-12 wt.% yttria were prepared by plasma spraying. A comprehensive sequence of annealing experiments comprising times from 1 to 100 h at temperatures between 1200 and 1600-degrees-C was carried out on the alloy containing zirconia-8wt.% yttria. It was shown that the T' phase decomposes into high and low yttria phases following high temperature annealing. It was further shown in the course of thermal expansion tests that the low yttria phase will transform to monoclinic during slow heating and cooling. These findings were substantiated for coatings of alloys containing 4, 6, 10 and 12 wt.% yttria.