High-temperature oxidation of HVOF thermally sprayed NiCr–Cr₃C₂ coatings: microstructure and kinetics

In this study, 25 wt% Ni(Cr)–75 wt% Cr₃C₂ powder was thermally sprayed onto 304 stainless steel substrate to produce coatings of ~400-µm thick and the oxidation behaviour of the coatings was investigated. The kinetic behaviour of oxidation of freestanding coatings was investigated over short times at 650, 700 and 800 °C using the thermogravimetric analysis technique. The oxidation rate for all samples was parabolic in nature and the parabolic rate constant (k) increased with increasing temperature. The activation energy (Q) for oxidation was found to be around 226 kJ mol⁻¹. Furnace oxidation tests were also carried out on the as-sprayed samples at 700 and 800 °C in laboratory air for times ranging from 1 h up to 16 days (384 h). The oxide scale growth, morphology and phase formation were investigated using scanning electron microscopy, energy dispersive X-ray analysis and X-ray diffraction. An oxide scale covered the surface of the cermet coating following oxidation at both 700 and 800 °C; the scale was significantly thicker at the higher temperature. It was found to be Cr₂O₃ and it covered both the metal matrix and the Cr₃C₂ particles present in the coating. The oxide grains had an angular shape after 16 days of exposure at both temperatures but the grain size was significantly larger after exposure at 800 °C.