PHASE-TRANSFORMATION OF YTTRIA-STABILIZED ZIRCONIA PLASMA-SPRAYED COATINGS IN A HUMID ATMOSPHERE

The tetragonal to monoclinic phase transformation resulting from the h ydrothermal ageing of 4-20 mass % yttria-stabilized zirconia plasma-sp rayed coatings formed from commercial powders has been studied with re spect to the Y2O3 distribution inside each coating. The phase transfor mation was prevented when the nominal Y2O3 concentration was greater t han 8 mass % with its point to point distribution ranging between 6.82 to 9.90 mass %. It is suggested that a close correlation exists betwe en the durability of the tetragonal phase in a humid atmosphere and th e Y2O3 distribution in the tetragonal zirconia coatings. A content of 6.3-6.8 mass % Y2O3 was calculated to be necessary to prevent the tran sformation.