Thermal cyclic resistance and long term inter-diffusion properties of slurry aluminide coatings modified with Si

High temperature oxidation resistant aluminide coatings fail due to their inability to maintain a protective scale. The corresponding mechanism of protection is based on of forming a thin Al2O3 layer, which depends on the availability of Al at the interface with the intermetallic coating. However, the Al content in the coating’s surface decreases with time at high temperatures due to interdiffusion with the substrate and/or frequent regeneration of the protective Al2O3 scale when it spalls. Si addition is known to enhance the life of Ni aluminide coatings under certain corrosion environments. In an effort to develop a stable, long lasting coating for industrial components exposed to metal dusting, diffusion aluminide coating has been produced by applying Al-Si slurries of various compositions on alloy 601. Aluminide coatings were prepared by applying a water base, Al slurry to which 1, 10 and 20 wt. % of Si were added, followed by a diffusion heat treatment. The microstructures of the coatings were similar, exhibiting an outer layer consisting of an Al-rich β-NiAl matrix, an almost stoichiometric β-NiAl inner layer and an interdiffusion zone between the coating and the alloy. Thermal cyclic tests in air at 1100 °C have demonstrated good adhesion and stability of all coatings. In addition, long term isothermal tests at 850 °C for up to 3 months in air showed high oxidation resistance of the coating but different degree of coating degradation depending on the composition. The coating with the lowest Si addition was the most stable.