Fabrication and Characterization of HVOF Sprayed CoNiCrAlY-6%Al₂O₃ Coating to Improve High-Temperature Oxidation Resistance

Alumina nanoparticles were incorporated into CoNiCrAlY powders to fabricate an overlay coating of improved oxidation resistance for gas turbine blades via thermal spraying. In this regard, 6 wt.% alumina nanoparticles were agglomerated with CoNiCrAlY powders by modified suspension route and applied to samples of CMSX-4 nickel-based superalloy by high velocity oxygen fuel (HVOF) process. The coatings were characterized by X-ray diffraction, scanning electron microscopy and field emission scanning electron microscopy, EDS and elemental mapping, Vickers hardness and roughness measurement. Cyclic oxidation tests were performed to study the high-temperature oxidation behavior at 1100 °C. The results showed an increase in hardness, roughness and porosity with the addition of alumina nanoparticles to the coating. Furthermore, the oxidation resistance of CoNiCrAlY + 6 wt.% Al₂O₃ was improved as compared to conventional CoNiCrAlY after 100 cycles of oxidation; a reduction in the thickness of oxide layer and β depletion zone was observed. Formation of a dense and protective α-Al₂O₃ phase, instead of θ-Al₂O₃, was confirmed during the oxidation process in the coatings containing nanoparticles. It was concluded that nanoparticles prevent the penetration of elements to the surface and reduce the formation of non-protective oxide layer.