In this investigation, commercial Al2O3 and Y2O3 nanopowders were used to produce a composite powder feedstock that was plasma sprayed on graphite substrates. An Al2O3/Y3Al5O12 (YAG) eutectic crystalline structure was expected in the coating due to the high enthalpy, large temperature gradient, and rapid solidification of the plasma spraying process, but microstructure and phase analysis of as-sprayed deposits revealed only a small amount of crystalline grains in an amorphous coating matrix. After heat treating, the expected structure was obtained and found to have excellent microstructure and microhardness stability at high temperatures.

In the present study, Al2O3 and Al2O3-Y2O3 composite coatings were deposited by atmospheric plasma spraying and compared based on microstructure, hardness, and sliding wear behavior. The mass ratio of α-Al2O3/γ-Al2O3 was found to be 0.53 in the composite, compared to 0.08 in the alumina layer, which explains why the Al2O3-Y2O3 coatings are harder and better conductors of heat. Tribological tests show that the friction coefficients of the composite coating samples are more stable and the wear rates lower, which may be related to the formation of a graphite transferred film on abraded surfaces.