Phase formation in a Ni-50Cr HVOF coating

A complex, fine scale microstructure of non-equilibrium phases is obtained by HVOF deposition of Ni-50Cr alloy due to the rapid cooling experienced by the splats which make up the as-deposited coatings. XRD analyses indicated that the as-deposited coatings consisted predominantly of a single fcc γ-Ni phase. Two small peaks suggested the presence of NiO and/or NiCr₂O₄ at the limit of detection (∼ 5%). Shoulders on the main γ-Ni peaks were interpreted as evidence of a second, lower Cr content γ-Ni phase. Characterization of the oxide content of the as-deposited coatings by X-ray diffraction, image analysis of backscattered electron images, and electron probe microanalysis yielded conflicting results due to the size of the microstructural features present relative to the spatial resolution of these techniques. Due to the nature and feature size of the non-equilibrium oxide phase(s), direct measurement of the oxygen content by EPMA was found to be the most accurate technique. Heat treatment of an as-deposited coating at 650 °C in vacuum resulted in coarsening of the microstructural features, and an approach towards a mixture of equilibrium phases consisting of γ-Ni, α-Cr, and Cr₂O₃. Evidence was also seen in the XRD pattern of an intermetallic σ phase that has previously only been reported in thin films of Ni-Cr alloys.