Synthesis of self-regenerating NiAl-Al2O3 composite coatings

The aluminothermic reactions involved either in the mixtures or the stack of NiO and Al powders were elucidated in a previous work (Troncy et al., 2020) [1]. The present work now investigates the formation of potential self-regenerating coatings using the aluminothermic reaction between nickel oxide and aluminum. The use of this exothermic reaction allows the simultaneous formation of both alumina and nickel aluminides. The coatings were obtained by first pre-oxidizing the nickel substrate, then spraying an aluminum slurry before applying an adequate heat treatment. The most promising self-regenerating coatings display a first composite layer, made of an intermetallic compound (Al3Ni or NiAl) and alumina, and a second layer with a higher concentration of oxide. The first layer is expected to play the role of an Al reservoir coating and the second one that of a diffusion barrier to slow down the Al flux to maintain the Al reservoir in the coating. The effects of preoxidation time and of temperature on the resulting coating microstructures are discussed as a function of time.