Structural investigations of detonation-deposited tungsten carbide-cobalt coatings

The phase composition of detonation-deposited VK type coatings differs from that of the starting mixtures. A deposited layer contains about 40–50% of the tungsten carbide WC present in the starting mixture, metallic tungsten and cobalt, the intermetallic compounds Co7W6 and Co3W, traces of W2C, and a small amount of complex carbides. The amount and particle size of the tungsten carbide in a coating are determined by the particle size distribution of the powder being deposited and detonation process parameters. Our investigations have shown that the optimum powder particle size range for the process of detonation deposition of hard-metal mixtures may be taken to be 5–40 Μm. During detonation deposition under the conditions investigated the usual hard-metal structure and composition are not obtained. Most of the starting tungsten carbide decomposes with the formation of tungsten and cobalt intermetallic compounds. Attempts should now be made to improve deposition conditions so as to increase the phase and structural homogeneity of layers being deposited. In coatings deposited by an optimum method the ductile Co binder will be present in an amount close to that in a sintered VK type hard metal. The fracture of coatings and basis metal on the antivibration shelves of compressor blades has a fatigue character. Crack initiation takes place in the surface and inner layers of detonation-deposited coatings as a result of their considerable brittleness, which is due to the presence of metastable intermetallic phases as well as of defects in the form of pores and blowholes.