The role of combustion synthesis in the formation of slurry aluminization

Slurry processes have been investigated for several years as an alternative technique to conventional CVD-derived aluminizing to achieve similar diffusion coatings. This study investigates the coating formation mechanisms during heat treatment processes on pure nickel using slurries, which contain high amounts of micro-sized aluminium particles. At temperatures in the range of 550 °C–1000 °C, aluminium diffuses into the nickel substrate, promoting the formation of intermetallic nickel–aluminide layers. In order to control this process, it is important to understand the mechanisms that occur in the initial stages, when the metallic aluminium powder melts and reacts in contact with nickel. While a conversion of closely pressed nickel–aluminium into aluminide by combustion synthesis is well known, DTA measurements were undertaken to investigate if and when such processes occur in loosely packed powders. Two compositions of nickel with aluminium or eutectic aluminium–silicon alloyed particles were used in order to reveal a potential influence of the melting point of the aluminium alloy particles. The influence of the atmosphere was studied by comparing results during exposure in argon and air. Subsequently, for comparison with the more complex mechanism of slurry aluminizing, both powders were applied to pure nickel substrate and the coating formation during heat treatment at 600 °C, 650 °C, and 700 °C was investigated. The results clearly show the importance of combustion synthesis on the formation of slurry coatings on nickel. Based on the observations, four steps were identified, which lead to the formation of aluminides and the subsequent growth of the aluminide layer: After melting of the aluminium powder, a network of molten aluminium forms within and between the particles, followed by dissolution of nickel in the aluminium melt. If enough Al is available, combustion synthesis between Ni and Al takes place. After this highly exothermic reaction, solid state diffusion controls the further formation of slurry coatings on nickel. Finally, the mechanism was verified by coating industrially used superalloys with the Al-based slurry in air and argon.