Complex carbide powders for plasma spraying

Abstract

Various carbide-containing powders are used for plasma spraying. Most of these consist of tungsten carbide. Very frequently the tungsten carbide is mixed with cobalt to produce coatings similar to cemented carbides. The powders can be made by agglomeration of the carbides and the metal matrix powders or by coating the carbides with the matrix metal. As in cemented carbides, cobalt and nickel form the metal binder of the coatings. The coating metals can be increased to 20% of the total weight. Further metal matrix powders can be added also. The W 2C, WC, W 2C-WC eutectic phases and mixtures of them are used as tungsten carbide. The carbon content is very important and can be controlled better in coated particles than in an agglomerated powder. In the case of WCCo the carbon content has to correspond to MC to avoid the embrittling η phase and to achieve a strength of the coating comparable with that of cemented carbides. In addition to the composition, shape and grain size distribution of the powders, the spraying conditions are very important for the properties of the coating. Coated carbide powders are less sensitive to spraying conditions. When it is possible to control the carbon content in carbides better during spraying it will be feasible to use complex carbides also. Titanium carbide forms a solid solution with WC over a wide range of composition and forms mixed crystals with tantalum carbide and niobium carbide. These binary and ternary mixed carbide crystals, sometimes containing additional tungsten carbide, are used in cemented carbides to increase the wear resistance. By spraying these cobalt- or nickel-coated complex carbide powders similar properties of the coatings can be achieved. Spraying conditions and the shape, grain size and grain size distribution of the powders are important. Results will be given.