KINETICS OF CHEMICAL VAPOR-DEPOSITION OF TUNGSTEN CARBIDE

Hard tungsten carbide based coatings were deposited by chemical vapor deposition (CVD) at low temperatures onto metal substrates by hydrogen reduction of tungsten hexafluoride (WF6) in the presence of dimethyl ether (DME). These coatings contain a mixture of tungsten and tungsten carbide in the form Of W3C, W2C, or a combination of W3C and W2C depending on reaction conditions. They exhibit layered morphology, which is primarily responsible for their excellent erosion and wear-resistance properties. The deposition of these coatings with layered morphology is believed to involve two simultaneous reactions, leading to the deposition of tungsten and tungsten carbide phases with crystallite size varying between 50 and 150 angstrom. Since the deposition of these coatings in a large-scale commercial reactor requires knowledge about the rate and order of simultaneous reactions, a pilot-scale CVD reactor was designed and operated to investigate reaction kinetics. This paper describes an experimental approach to determine the apparent reaction order and kinetic rate constant for depositing the coating with optimal properties. It also discusses the significance of these kinetic parameters on scaling up reactor design.