Oxidation Behavior of Overlay NiCoCrAlY and Diffusion Aluminide Coatings Deposited on a Directionally Solidified Nickel-Based Superalloy: A Comparative Study

The high-temperature oxidation resistance of an overlay NiCoCrAlY coating deposited by HVOF process on directionally solidified CMSX-4 nickel-based superalloy was compared with a conventional diffusion aluminide coating. The coatings, produced by industrial processes, were characterized by optical and scanning electron microscopy, EDS micro-analysis and x-ray diffraction. High-temperature oxidation tests were carried out at 1100 degrees C for 300 cycles. It was shown that both coatings improved the oxidation resistance of the superalloy to different levels. After 300 cycles of oxidation, the oxide layers formed on the surface of both coatings included cracks and spallations but the diffusion aluminide coating with a dense structure and a rich aluminide reservoir experienced lower weight changes and therefore higher oxidation resistance than the HVOF overlay NiCoCrAlY coating.