Temperature-variant hardness behavior of plasma sprayed ceramic coatings

The temperature-variant hardness response of a few plasma-sprayed duplex ceramic coatings is reported. The coating materials investigated are 99% alumina and partially stabilized zirconias with yttria and MgO. The Vickers microhardness of all four coatings studied decreases as the temperature increases from room temperature to 800°C. However, while the hardness of ZrO 2-8%Y 2O 3 coatings does not recover to the room temperature hardness on cooling, the hardness of ZrO 2-24%MgO and alumina coatings is almost completely recovered to the room temperature hardness on cooling. This different hardness response, which undergoes reversible or irreversible change during the temperature-variant indentation experiments, can be explained in terms of relaxation of the residual compressive stress during the first heating cycle and thus the coating microstructure. An activation energy approach for the temperature dependence of hardness is also discussed.