Development of thermal barrier coating systems from Al microparticles. Part I: Influence of processing conditions on the mechanisms of formation.

This work presents the mechanisms of formation on pure nickel of full thermal barrier coating systems (aluminide coating, thermally grown oxide and thermal barrier top coating) from micro-sized Al particles dispersed in a slurry annealed in different atmospheres (Ar, synthetic air, water vapour and mixtures thereof). The simultaneous formation of nickel aluminides and of a thermal barrier made of sintered hollow alumina spheres involved self-propagating high-temperature synthesis in all conditions. However, the microstructures and adherence of the top coatings changed markedly with either a diffusion step (700°C-2 h) or a complete heat treatment (700°C-2 h + 1100°C-2 h) depending on the atmosphere. Whereas fast consumption of Al occurred in Ar to form the nickel aluminides, synthetic air and water vapour fostered the peripheral oxidation of Al micro-sized particles that impeded the release of Al and its diffusion towards the substrate. This resulted in heterogeneous diffusion layers but thicker top coatings with better sintering and thicker alumina shells. • Hollow alumina spheres coatings were obtained from Al microspheres slurries using different oxidizing atmospheres. • Under inert gas atmosphere (Ar), the oxide shell of the microspheres was too thin and result in collapsed coatings. • Using oxidizing atmospheres (synthetic air or Ar + H 2 O vap), the coatings showed poor adherence due to excessive oxidation. • However, a good compromise between adherence and oxide shell thickness was achieved using hybrid atmosphere heat treatment. [ABSTRACT FROM AUTHOR]