Restoring WC in plasma sprayed WC–Co coatings through spark plasma sintering (SPS)

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Decarburization of WC–Co is a common phenomenon during atmospheric plasma spray (APS) deposition, resulting in a large amount of W 2C and other carbon-deficient phases in the coating. Consequently, the properties and performance of the coating may be inadequate and unreliable. In this study, spark plasma sintering (SPS) is applied as a post-spray heat treatment to reinstate the WC phase in the APS WC/Co coating. Three sets of sample configurations have been employed, namely: direct contact of graphite punch with APS WC–Co coating, packing the coating in WC–Co powder, and finally, packing APS WC–Co coatings in yttria-stabilized zirconia (YSZ) powder. The treatment was carried out at 800 °C for 5 min. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) are applied to investigate the microstructure and phase composition of APS WC–Co coatings before and after SPS treatment. Rietveld refinement is utilized to quantitatively determine the coatings’ phase composition. Results showed that SPS treatment with the graphite punch directly in contact with the coating surface could restore the WC phase in the coating to approximately 52.3 wt.% of the overall phases in the coating, which is close to the amount of WC found in the original WC–Co feedstock used in APS coating. Comparatively, the other sample configurations employing the packing of WC–Co or YSZ powders yielded a maximum of 17.6 wt.% WC. It is believed that the direct carbon diffusion from the graphite punch during SPS played a significant role in the enhanced WC content in the coating. A comparison with the effect brought about by conventional inert atmospheric heat treatment was made by the Rietveld-refining of the XRD data from available literature, and it showed that the conventional inert atmospheric heat treatment could only cause the recrystalization of the Co 3W 3C phase and the transformation of W 2C into Co 3W 3C phase. The present study showed that the SPS working as the post-spray process, which has promising effect on reverting W 2C or other related metastable phases to the original WC in APS WC–Co coatings. Furthermore, the microhardness of the WC coating improved significantly after the SPS post-spray heat treatment.