Oxidation Control with Chromate Pretreatment of MCrAlY Unmelted Particle and Bond Coat in Thermal Barrier Systems
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Hideaki Yamano, Kazumi Tani, Yoshio Harada, and Takema Teratani (Submitted August 31, 2007; in revised form December 12, 2007) 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 topcoat can occur due to a thermally grown oxide (TGO) that grows at the interface between the bond coat and the topcoat. In this study, the effect of chromate treatment was investigated. Prior to topcoat deposition, a thin film of Cr2O3 was formed on the bond coat surface. High-temperature oxidation tests were carried out, 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. As a result, the chromate-treated bond coat showed outstanding oxidation resistance. Calculations that take into account the oxidation of particles in the topcoat indicated the generation of internal stress to cause local fracture of the topcoat.
Keywords
chromate treatment, gas turbine, plasma spraying, thermal barrier coating, thermally grown oxide, unmelted particle
1. Introduction As the operating temperature of a gas turbine has increased, development of heat-resistant alloy materials and heat-resistant coating has become an important theme in addition to demand for high efficiency of a cooling system, especially for hot-section parts such as burners, turbine blades, and vanes (Ref 1-4). Thermal barrier coating (TBC) systems, consisting of yttria partially stabilized zirconia (YSZ) topcoat and MCrAlX (M: Ni, Co, or their combinations; X: Y, Ce, La, and Hf) bond coat, have been widely used as thermal protective coating (Ref 5) since 1950s when their basic concept was built. However, delamination at the bond coat/topcoat interface occurs because of thermal stresses (Ref 6) or local stresses developed by the growth of thermally grown oxide (TGO) consisting of Co, Ni, Cr, and Al (Ref 7). Therefore, the development of a countermeasure against oxidation has become an important issue. Several studies have proposed growth-inhibiting methods for TGO, such as improvement of chemical compositions of MCrAlX alloy as a bond coat (Ref 8, 9), control of ambient during heat treatment of a bond coat (Ref 10), electron beam irradiation to a bond coat surface (Ref 11), and laser irradiation to a bond coat surface (Ref 12). Hideaki Yamano, Kazumi Tani, Yoshio Harada, and Takema Teratani, Thermal Spraying Technology R&D Laboratory, Tocalo Co., Ltd., Akashi 674-0093, Japan. Contact e-mail: [email protected].
Journal of Thermal Spray Technology
In our previous study, attention was focused on the wart-like oxide that grew as complex oxide near interface, and it was found that wart-like oxide generates cracks in the topcoat (Ref 10). In addition, it was reported that complex oxides may originate during the rapid oxidation of unmelted MCrAlY particles attached to the bond coat surface during spraying. Furthermore, to reduce the gro
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