Thermal conductivity and phase evolution of plasma-sprayed multilayer coatings

Multilayer coatings were prepared using small-particle plasma spray to investigate the effect of interfaces on thermal conductivity and phase stability. Monolithic and multilayer alumina and yttria partially-stabilized zirconia coatings, with 0, 3, 20, and 40 interfaces in 200-380 mum thick coatings were studied. Thermal conductivity was determined for the temperature range 25 degreesC to 1200 degreesC using the laser flash method and differential scanning calorimetry. Thermal conductivity of the multilayer coatings was accurately modeled by a series heat transfer equation, indicating that interfacial resistance plays a negligible role in heat transfer in the direction perpendicular to the coating plane. Powder X-ray diffraction results indicate that identical phase transitions occur in all the coatings. Independent of coating microstructure (i.e. layer thickness), as-sprayed gamma -Al2O3 transforms to alpha -Al2O3 after 100 hours at 1200 degreesC; as-sprayed metastable t'-ZrO2 converts to a mixture of t-ZrO2 and c-ZrO2 after 100 hours at 1300 degreesC. Thus, the results indicate that the interfaces do not aid in stabilizing the as-sprayed phases after prolonged severe heat treatments. (C) 2001 Kluwer Academic Publishers.