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 sprayi ng 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 th e phases present and in the overall degree of decarburization. A model desc ribing the evolution of microstructure has been developed. As a WC-Co parti cle 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 f rom 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, W2 C is observed in both coatings, whereas W is found only in the nanocomposit e deposit. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Lt d. Ail rights reserved.