Comparative investigation of microstructure and high-temperature oxidation resistance of high-velocity oxy-fuel sprayed CoNiCrAlY/nano-Al₂O₃ composite coatings using satellited powders

Satellited CoNiCrAlY–Al₂O₃ feedstocks with 2wt%, 4wt%, and 6wt% oxide nanoparticles and pure CoNiCrAlY powder were deposited by the high-velocity oxy fuel process on an Inconel738 superalloy substrate. The oxidation test was performed at 1050°C for 5, 50, 100, 150, 200, and 400 h. The microstructure and phase composition of powders and coatings were characterized by scanning electron microscopy and X-ray diffraction, respectively. The bonding strength of the coatings was also evaluated. The results proved that with the increase in the percentage of nanoparticles (from 2wt% to 6wt%), the amount of porosity (from 1vol% to 4.7vol%), unmelted particles, and roughness of the coatings (from 4.8 to 8.8 µm) increased, and the bonding strength decreased from 71 to 48 MPa. The thicknesses of the thermally grown oxide layer of pure and composite coatings (2wt%, 4wt%, and 6wt%) after 400 h oxidation were measured as 6.5, 5.5, 7.6, and 8.1 µm, respectively. The CoNiCrAlY–2wt% Al₂O₃ coating showed the highest oxidation resistance due to the diffusion barrier effect of well-dispersed nanoparticles. The CoNiCrAlY–6wt% Al₂O₃ coating had the lowest oxidation resistance due to its rough surface morphology and porous microstructure.