Comparative Study of Thermal Shock Behavior of the Arc Ion Plating NiCrAlY and the Enamel Based Composite Coatings

From the view of material point, high-temperature protective coatings are divided into the following two categories: ceramic coating and metallic coating. Metallic coating possesses higher toughness and bond strength to the alloy substrate than ceramic coating does. Its protectiveness relies on the formation of a slow-growing and adherent oxide scale at high temperatures. However, with increasing the oxidation time, the oxide scale will experience cracking and spalling as it has grown to the critical thick- ness. Ceramic coating due to its chemical inertness has been used in many corrosive environments for protection. But the weak interfacial bond and big mismatch of coefficient of thermal expansion with the alloy substrate limit its application in thermal shock environments. Since glass-ceramics combine the generally superior properties of crystallite ceramics with the easy processing of glasses, it is expected that glass-ceramic coating should show a higher spallation resistance than ceramic one under thermal shock. Cast K444 superalloy is widely used in advanced aircraft engine and gas turbine. Its protection from hightemperature oxidation under thermal shock becomes a critic issue. In this work, NiCrAlY and enamel based composite coatings on the K444 superalloy substrate by arc ion plating and spray-firing methods were prepared, respectively. Thermal shock behavior from 900℃ to room temperature of these two coatings was studied comparatively. One cycle of thermal shock contained the holding of samples at 900℃ for 1.5 h and the following cooling down in air or water. Results indicated that thermal shock resistance of the NiCrAlY coating was low. As the NiCrAlY coating was thermal shocked by water, its oxide scale cracked severely after 30 cyc, and certain crack had already transported the scale and penetrated into the interior of the underlying metallic coating; for the enamel based composite coating, however, its thermal shock resistance was high. No cracks were detected at the coating surface or interior after thermal shock test. Besides, the enamel coating still adhered well with the alloy substrate. The high resistance to thermal shock of the enamel based composite coating originated from: (1) the coefficient of thermal expansion of the enamel based composite coating matched well with that of the alloy substrate; (2) the addition of nano-sized Ni and NiCrAlY metallic particles improved the toughness of the enamel coating, in addition to enhancing its coefficient of thermal expansion.