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 gamma-Ni phase. Two small peaks suggested the presence of NiO and/or NiCl2O4 at the limit of detection (similar to 5%). Shoulders on the main gamma-Ni peaks were interpreted as evidence of a second, lower Cr content gamma-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 degrees C in vacuum resulted in coarsening of the microstructural features, and an approach towards a mixture of equilibrium phases consisting of gamma-Ni, alpha-Cr, and Cr2O3- Evidence was also seen in the XRD pattern of an intermetallic sigma phase that has previously only been reported in thin films of Ni-Cr alloys. (C) 2008 Elsevier B.V. All rights reserved.