The Effect of Heat Treatment on the Oxidation Behavior of HVOF and VPS CoNiCrAlY Coatings

Free-standing VPS and HVOF CoNiCrAlY coatings were produced. The as-sprayed HVOF coating retained the gamma/beta microstructure of the feedstock powder, and the VPS coating consisted of a single (gamma) phase. A 3-h, 1100 A degrees C heat treatment in vacuum converted the single-phase VPS coating to a two-phase gamma/beta microstructure and coarsened the gamma/beta microstructure of the HVOF coating. Oxidation of free-standing as-sprayed and heat-treated coatings of each type was carried out in air at 1100 A degrees C for a duration of 100 h. Parabolic rate constant(s), K (p), were determined for free-standing, as-sprayed VPS and HVOF coatings as well as for free-standing coatings that were heat treated prior to oxidation. The observed increase in K (p) following heat treatment is attributed to a sintering effect eliminating porosity from the coating during heat treatment. The lower K (p) values determined for both HVOF coatings compared to the VPS coatings is attributed to the presence of oxides in the HVOF coatings, which act as the barrier to diffusion. Oxidation of the as-sprayed coatings produced a dual-layer oxide consisting of an inner alpha-Al2O3 layer and outer spinel layer. Oxidation of the heat-treated samples resulted in a single-layer oxide, alpha-Al2O3. The formation of a thin alpha-Al2O3 layer during heat treatment appeared to prevent nucleation and growth of spinel oxides during subsequent oxidation.