Microstructural characterisation of high velocity oxyfuel thermally sprayed Stellite 6

Abstract

High velocity oxyfuel (HVOF) thermal spraying with an oxygen-propylene combustion flame was used to deposit coatings of a Co–Cr–W–C alloy (similar in composition to Stellite 6) from gas atomised feedstock powder. The microstructural characteristics of the deposits were investigated using the combined techniques of X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). In particular, the tripod polishing technique was used to prepare large areas of electron transparent membrane for TEM analysis; these areas were typically of the order of several hundred square micrometres. Accordingly, SEM and TEM analysis could be directly correlated, providing a clear understanding of the overall nature of the coatings. The coatings exhibited characteristic splatlike, layered morphologies due to the deposition and resolidification ofmolten or semimolten powder particles. The as sprayed microstructure consisted of a face centred cubic Co rich metallic matrix with a small fraction of CoCr₂O₄ spinel oxide. The latter formed either as intersplat lamellae or intrasplat globules. Under the HVOF spray conditions employed, a high proportion of the feedstock powder appeared to have been fully melted prior to impact on the substrate, with partially or unmelted powder accounting for < 20% of the microstructure. Formation of the M₇C₃ phase, normally present in Stellite 6, appeared to have been suppressed by the high cooling rate during spray deposition.