Microstructure and high temperature oxidation behavior of Pt-modified aluminide bond coats on Ni-base superalloys

Over the years, diffusion aluminide bond coats have been widely used for providing oxidation resistance to Ni-base superalloy components of advanced gas turbine engines. Modification of these coatings by Pt addition improves their high temperature oxidation resistance considerably. This effect of Pt has prompted intense research on the microstructure and oxidation behavior of Pt-modified aluminide coatings over past several decades. The present review seeks to comprehensively collate all the available information on the subject and critically assess the status based on most recent findings. The review includes traditional β-(Ni,Pt)Al bond coats as well as the Pt-modified γ–γ′ type of bond coats that have gained prominence in more recent times. A brief description of typical process steps involved in the formation of Pt–aluminide coatings is provided followed by a detailed assessment of coating microstructures reported under various processing conditions. The influence of process parameters such as the time–temperature schedule used for prior diffusion treatment, on the coating microstructure is highlighted. Further, the importance of the contents of two key elements, namely Pt and Al, on the microstructure and oxidation performance of Pt–aluminide bond coats is discussed. The various mechanisms pertaining to the role of Pt in enhancing the oxidation resistance of aluminide coatings, as suggested in the literature, are discussed. The gradual microstructural degradation of the bond coats during high temperature oxidation exposure in terms of thickness increase, phase transformations, surface roughening, void formation and cracking is described. The current trends in the aluminide bond development are also provided.