Effect of pre-treatment temperatures on the oxidation behaviors and surface roughening mechanisms of NiCoCrAlYHf coating

Abstract

The NiCoCrAlYHf coating has been widely used as a bond layer in thermal barrier coating system, of which the oxidation behaviors and surface roughening mechanisms in service are greatly influenced by pre-treatment temperature. To clarify the effects of pre-treatment temperatures, the characteristics of the pre-treated coatings and isothermally exposed coatings were characterized and analyzed, including microstructure, weight change, TGO thickness and surface roughness. When the pre-treatment temperature was relatively low, the coating surface did not form a TGO with integrity. When the pre-treatment temperature was relatively high, mixed oxides were generated on the coating surface during the pre-treatment process, which damaged the integrity of TGO. The overall performance of the coatings pretreated at too high or low temperatures was insufficient. Moreover, the phase transformation of coating is strongly correlated with diffusion of Al element. The pre-treatment process could change the as-deposited coating from a single-phase to a duplex-phase structure due to the diffusion of Al element. In the oxidation process, the duplex-phase structure changed to a single-phase structure due to the consumption of Al element. In addition, it was found that there was a competitive relationship between grain sliding caused by high-temperature stress and Al2O3 growth in the coating depression, which was one of the main surface roughening mechanisms of NiCoCrAlYHf coating.