Abstract
A MCrAlY alloy bond coat is widely used in thermal barrier coating (TBC) systems to protect substrates from high-temperature oxidizing environments. However, failure of the ceramic top coat can occur due to a thermally grown oxide (TGO) layer that grows at the interface between the bond coat and the top coat. Local stresses are produced by multiple oxides that grow separately at the interface. It seems that the multiple oxides may originate in the rapid oxidation of unmelted MCrAlY particles attached to the bond coat during spraying. Thus, it has become an important issue to control the growth behavior of TGOs.
In the present study, the effect of chromate treatment was investigated. Prior to top coat deposition, a thin film of Cr2O3 was formed on the bond coat surface. High-temperature oxidation tests were carried out using samples coated both with and without chromate pretreatment, and the oxidation rates were determined by inspection of cross sections. Similar oxidation tests were carried out using MCrAlY powder material assumed to be unmelted particles. Chromate-treated bond coat showed outstanding oxidation resistance in comparison with bond coat without chromate pretreatment. In the case of pretreated MCrAlY powder, only the surfaces of particles were oxidized, even under conditions in which most particles without pretreatment were fully oxidized. This outcome is thought to result from the preferential formation of an oxidation-resistant Al2O3 layer in the early stage of oxidation. Calculations that take into account the oxidation of particles in the top coat indicate the generation of sufficient internal tensile stress to cause local fracture of the top coat. Because of its simplicity and applicability to complicated shapes, the chromate treatment is quite useful as a countermeasure against bond-coat oxidation and local fracture of the top coat near the interface.
