Microstructural evolution in thermally sprayed WC-Co coatings: Comparison between nanocomposite and conventional starting powders

WC-Co coatings have been deposited by high velocity oxy-fuel thermal spraying of conventional and nanocomposite powders which contain WC grains in the size range 2-5 mu m and 70-250 nm, respectively. The coatings differed not only in microstructural scale, bur also in the nature and proportion of the phases present and in the overall degree of decarburization. A model describing the evolution of microstructure has been developed. As a WC-Co particle is heated in the hot gas jet, the cobalt phase melts and the WC grains begin to dissolve in it. The periphery of the semi-molten particle becomes decarburized by oxidation, promoting further WC dissolution in this region. Particle quenching on impact with the substrate results in precipitation from the melt of W2C and possibly W depending on the local melt composition. The larger surface to volume ratio of the WC in the nanocomposite material promotes more rapid dissolution and thus decarburization. Consequently, W2C is observed in both coatings, whereas W is found only in the nanocomposite deposit. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. Ail rights reserved.