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Thermal conductivity and sintering characteristics of plasma sprayed dysprosia-yttria-zirconia thermal barrier coatings Wang, Steven Yuan Jun


Yttria-stabilized zirconia has long been the favoured refractory material for demanding applications such as thermal barrier coatings on turbine components. Its low thermal conductivity, relatively high thermal expansion coefficient, and good fracture toughness are most useful parameters when acute thermal cycles are considered. In recent years however, the demand for higher turbine operating temperatures has led to novel and innovative research in improving the thermal conductivity and sintering resistance of thermal barrier coatings. Rareearth doped zirconia, rare-earth zirconates, and lanthanum hexa-aluminate have all been proposed as candidate materials for the next generation of thermal barrier coatings. Drawn from research conducted during 2003-2005, this study focuses on dysprosia as a ternary dopant to yttria-stabilized zirconia, examining the relationship of dopant content with overall thermal conductivity and sintering behaviour under cyclic thermal loading between room temperature and 1100°C. Air plasma spray deposition technique was employed for coatings deposition. Based on existing published works, this study is prefaced with four hypotheses: 1. increasing levels of dysprosia would likely result in lower overall thermal conductivity; 2. best improvement occurs at about 10 mol% total dopant (Dy + Y); 3. addition of dysprosia is also likely to increase sintering resistance during thermal cycling, since Dy cation radius is larger than Zr; 4. higher dopant concentrations, between 10 mol% and 50 mol%, should increasingly lead to shorter coating life under thermal cycling. As-sprayed coating heat capacity, thermal diffusivity, and porosity were measured by differential scanning calorimetry, laser flash method, and image analyses, respectively. Post-cycle coating porosity levels were compared against data for as-sprayed coatings. A theoretical model for estimating the thermal conductivity of plasma sprayed zirconia coatings was derived and constructed from previous works by other researchers. Experimental data and theoretical model presented in this study offer positive confirmations for the hypotheses, with the exceptions that the greatest reduction in thermal conductivity was seen at 15 mol% total dopant and that increased levels of dysprosia did not result in continued reductions in thermal conductivity. Literature data suggests long range ordering of oxygen vacancies could be a contributing factor in this trend.

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