Characterization of the W2C phase formed during the high velocity oxygen fuel spraying of a WC+12 pct Co powder

A variety of experimental techniques have been used to study a WC-12 pet Co powder and the coatings produced by high velocity oxygen fuel (HVOF) spraying of the powder onto a steel substrate, Many of the structural characteristics of the powder were also found in the coating. However, when the metallic matrix of the powder was melted during thermal spraying, the carbides were partially dissolved and a very heterogeneous liquid phase was produced in which the W/C ratio varied from about 1 to 4. These variations have been Linked with oxidation of the liquid phase during spraying. The factors influencing the formation of W2C in the coating have been identified as (1) an in situ transformation of WC into W2C maintaining the original WC faceted morphology and (2) the precipitation of W2C from the W-rich liquid phase matrix as the coating cools. A cobalt containing carbide of the M6C-M12C type has also precipitated from the liquid phase when the W/C and W/Co ratios were high.