Wc-17% Co Thermal Spray Powders, And Its Wear-resistant Coatings Prepared Study

Due to the advantage in combination of hardness and toughness, tungsten carbide/cobalt cermet coatings deposited by thermal spraying, have been widely used in various areas, particularly in aerospace, automotive, metallurgy and machinery industries, to enhance the wear resistance of many types of engineering components or to repair the worn parts. However, during thermal spraying of WC-Co powder, decarburization giving rise to the formation of ditungsten carbide(W₂C),η (Co₃W₃C, Co₆W₆C), even to metallic tungsten occurs, which are undesirable for resistance to fracture toughness and wear resistance of the coatings. Feedstock powder characteristics, such as carbide grain size, initial phase content, morphology size of powders, which are usually determined by the techniques for the synthesis of feedstock powders, have great influence on the phase distribution and the wear performance of the coatings.Studies on WC-17%Co feedstock powders for thermal spraying prepared by means of sintered-crushed method, especially on ultra-fine tungsten carbide powder which contain WC grains in the approximate size 0.68 μ m and cobalt powder as the main raw materials and with free carbon as an additive, are limited. In this study, ultra-fine WC-17%Co feedstock powders for thermal spraying were prepared by means of sintered-crushed method, and a suitable amount of free carbon was introduced into the powder to prevent the decomposition of the WC phase. The influences of sintering temperature, organic binder and free carbon on the feedstock powder characteristics and the differences between ultra-fine and coarse feedstock powder were investigated mainly by X-ray diffraction(XRD), scanning electron microscopy(SEM) and energy-dispersive spectroscopy(EDS). Then, wear-resistant, WC-17%Co coatings were thermally sprayed by an air plasma spraying (ASP) system from four sintered powders prepared with various carbide size distributions. The mechanism for microstructural development was analyzed and the effect of feedstock powder characteristics on the phase distribution and the microstructure of coatings were studied. The experimental results are as follows. 1250℃ is the better sintering temperature for preparing ultra-fine grain WC-17%Co feedstock powder forthermal spraying, at which feedstock powders with a particle size of 45⁷5 u m can be prepared using the mixed powders with ultra-fine tungsten carbide powder which contains WC grains in the approximate size 0.68 U m and cobalt powder with a mean grain size of 2.39 u m as raw materials. Superior WC-17%Co feedstock powders can be prepared when mixing raw powders with a suitable amount of organic binder and free carbon. In the prepared powders, carbon exists mainly in a free state and can retard the decomposition of WC. Preparing ultra-fine grain WC-17%Co feedstock powders has greater sintering degree than coarse ones under identical process conditions. A substance, unclearly seen by SEM, is Y solid solution with Co as matrix. Comparing the four coatings, the coatings with ultra-fine WC grains display superior ability to fill the porosity, the surface of which is smoother than the coatings with coarse and medium WC grains, in which particles melting and deformation are not homogeneous. The finer WC grains in feedstock powders are, the higher superficial Rockwell hardness(HRA). The fine carbon additive in feedstock powders not only increases the hardness of the coatings but also decreases the degree of decarburization. Research results are very important for the optimization of preparation technique of feedstock powders for thermal spraying and eventually for improving the performance of the coatings.