Thermal conductivity and phase evolution of plasma-sprayed multilayer coatings
HSIN WANG, WALLY D. PORTER, Y. JENNIFER SU
JOURNAL OF MATERIALS SCIENCE
Abstract
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 m thick coatings were studied. Thermal conductivity was determined for the temperature range 25 C to 1200 C using the laser ash 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 -Al2O3 transforms to -Al2O3 after 100 hours at 1200C; 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.
Key Words：layer thickness; multilayer coatings; plasma spray; series heat transfer
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