Tungsten carbide coatings deposited by high-velocity oxy-fuel spraying on a metallized polymeric substrate

A tripler system consisting of WC(Co) hard coating, metallic bond layers (Ni,Tu/Ni), and acrylonitrile-butadiene-styrene as substrate, was investigated. The WC (88 wt.%)-Co (12 wt.%) coating was deposited by the high-velocity oxy-fuel spraying technique modified by the addition of CO2 gas hows. The microstructure, the composition, and the phases present in the coating were studied by means of transmission and scanning electron microscopies both combined with energy dispersive analysis, X-ray diffraction. and Auger electron spectroscopy. It was found that during the spraying a fraction of the WC phase partially decomposes into alpha- and beta-W2C and beta-WC1-x and reacts with cobalt to form the ternary carbide, Co3W3C. The decomposition occurred on the surface of the WC particle to a thickness of several tenths of a nanometer. The microstructure of the coating consisted of tungsten carbide particles bonded by a binder matrix composed mainly of Co. The microstructure of the binder phase was found to be dependent on the cooling rate. As a result of the variation in the particles' velocities and temperatures, different interface morphologies as well as different fractions of binder phase and pore densities were observed in the coating. A theoretical model describing the dynamic and the thermal performance of the powder particles in high-velocity oxy-fuel spraying was developed, correlating between the particles' properties (size, density, emissivity) and their velocity and temperature. The factors that influence the powder velocity and temperature and thereby the coating microstructure and composition are discussed.