THERMOMECHANICAL BEHAVIOR AND PHASE-RELATIONSHIPS OF PLASMA-SPRAYED ZIRCONIA COATINGS

The performance of plasma-sprayed zirconia coatings, used as thermal barriers in gas turbine applications, is influenced by residual stresses. Coating stresses are induced by the plasma spray process and are affected by thermophysical properties of the system. Stress alteration as a function of temperature and time was evaluated using bimaterial dilatometry. Reversible changes in stress associated with DELTAalphaDELTAT relationships and phase transformations were observed along with irreversible changes owing to oxidation, metastable phase transformations and sintering of the coating materials. Dilatometric studies of yttria (8 wt.%)-stabilized zirconia coatings which had been chemically removed from the substrate indicated that a stable phase transformation occurs at around 1200-degrees-C. The presence of this phase transformation suggests that changes are required in the generally accepted phase diagram by Scott. The changes and supporting evidence will be presented,