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 μm thick coatings were studied. Thermal conductivity was determined for the temperature range 25°C to 1200°C 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 γ-Al₂O₃ transforms to α-Al₂O₃ af 100 hours at 1200°C; as-sprayed metastable t′-ZrO₂ converts to a mixture of t-ZrO₂ and c-ZrO₂ after 100 hours at 1300°C. Thus, the results indicate that the interfaces do not aid in stabilizing the as-sprayed phases after prolonged severe heat treatments.