Thermal conductivity and phase evolution of plasma-sprayed multilayer coatings

Multilayer coatings were prepared using small-particle plasma spray to inve stigate the effect of interfaces on thermal conductivity and phase stabilit y. Monolithic and multilayer alumina and yttria partially-stabilized zircon ia coatings, with 0, 3, 20, and 40 interfaces in 200-380 mum thick coatings were studied. Thermal conductivity was determined for the temperature rang e 25 degreesC to 1200 degreesC using the laser flash method and differentia l scanning calorimetry. Thermal conductivity of the multilayer coatings was accurately modeled by a series heat transfer equation, indicating that int erfacial resistance plays a negligible role in heat transfer in the directi on perpendicular to the coating plane. Powder X-ray diffraction results ind icate that identical phase transitions occur in all the coatings. Independe nt of coating microstructure (i.e. layer thickness), as-sprayed gamma -Al2O 3 transforms to alpha -Al2O3 after 100 hours at 1200 degreesC; as-sprayed m etastable t'-ZrO2 converts to a mixture of t-ZrO2 and c-ZrO2 after 100 hour s at 1300 degreesC. Thus, the results indicate that the interfaces do not a id in stabilizing the as-sprayed phases after prolonged severe heat treatme nts. (C) 2001 Kluwer Academic Publishers.