Effects of oxide stringers on the β-phase depletion behaviour in thermally sprayed CoNiCrAlY coatings during isothermal oxidation

This paper details an investigation of the effects of oxide stringers on the β-phase depletion behaviour in thermally sprayed CoNiCrAlY coatings. Vacuum Plasma Sprayed (VPS) CoNiCrAlY coatings, which are free of oxide stringers, are used as the reference materials in comparison with High-Velocity Oxy-Fuel (HVOF) sprayed CoNiCrAlY coatings during isothermal oxidation at 1100 °C. An outer layer of spinel oxides and an inner layer of alumina are formed in the as-sprayed coatings, while only a single alumina scale is found in the heat-treated coatings. Less β-phase depletion occurred in the HVOF coatings than in the VPS coatings. It was found that the β phases tend to coalesce at the oxide stringers in the HVOF coatings, which is likely due to the internal oxide particles and stringers acting as short diffusion barriers to tie up the β phase and inhibit the β-phase depletion.