Synthesis of Si-C-N coatings by thermal Plasmajet chemical vapour deposition applying liquid precursors

Materials in the system Si-C-N feature excellent properties for wear protec tion applications, even at elevated temperatures, and an excellent thermal shock resistance. As these materials have no melting point, coatings have t o be manufactured via a synthesis. Conventional chemical vapour deposition (CVD) processes have the disadvantage of low deposition rates. Thermal Plas majet CVD processes with liquid feedstock feature the highest deposition ra tes among the gas-phase synthesis processes. Single and triple DC torches a nd HF torches with supersonic nozzles have successfully been applied to pro duce Si-C(-N) coatings on different steel, aluminium, titanium and copper a lloys, as well as on graphite. Besides chlorosilanes, hexamethyldisiloxane, tetramethyldisiloxane and hexamethyldisilazane have been used as liquid si ngle precursors. Deposition rates up to 1500 mum h(-1) have been achieved. The coatings show cauliflower, columnar or dense morphology and an amorphou s or nanocrystalline structure. The formation of both alpha- and beta -Si3N 4 has been verified by X-ray diffraction. The application of chlorosilanes always results in chlorine-containing coatings. The chlorine causes severe corrosion in the interface to mild carbon steel substrates. The processes a re compared taking into account their characteristics concerning the inject ion modes, gas temperature and velocity profiles determined by enthalpy pro be measurements. The process conditions are correlated to the coating micro -guidelines for the optimum production of Si-C-N coatings by Plasmajet CVD are deduced. Emission spectroscopy is used to determine the mechanisms of t he coating formation. Full dissociation of the liquid feedstock in the plas ma jet has been verified. Copyright, (C) 2001 John Wiley & Sons, Ltd.