Preparation And Gradient Of The Gradient Structure Of Carbide Formation Mechanism Analysis

Cemented carbides with gradient structure have attracted great attention, as they possess high surface hardness, wear-resistance, strength and excellent impact toughness at the same time which is a contradictory problem in conventional hard alloy with homogenous microstructure. In this kind of materials, there is a gradient cobalt distribution across the section of the alloy, e.g, the cobalt content of surface layer is much lower than its nominal content which provides high surface hardness and good wear resistance, the cobalt content of the middle layer is much higher than its nominal content to permit high transverse rupture strength and toughness, while, the microstructure of the core consists of WC+Co+η three phases.In this paper, by using optical microscopy analysis(OP), X-raydiffraction analysis(XRD), scanning electron microscopy analysis(SEM),transmission electron microscopy analysis(TEM), mechanical andphysical properties tests, the influence of carbon content, sinteringtemperature, holding time and carbusintering methods on themicrostructure, mechanical and physical properties of the alloy wereextensively investigated. The equation used to calculate the diffusioncoefficient of carbon atoms in liquid cobalt phase was established afterinvestigating impurity diffusion in liquid metal. An effective equationwhich can be used to describe the relationship between thickness ofgradient structure and carbusintering time, carbon content was alsoestablished, and the calculated diffusion coefficient of carbon atoms wasused to predict the thickness of gradient structure. The formationmechanism of gradient structure was analyzed in this paper. The mainresults are listed as follows:1) Carbon atoms diffuse into alloy through cobalt phase during carbusintering, thus, the diffusion procedure is mainly controlled by diffusion coefficient of carbon atoms and effective diffusion area of surface layer. Generally, the thickness of gradient structure increases with carbusintering time and carbon content. And high hardnesssurface as well as high transverse rupture strength and good impact toughness can be obtained through careful control of carbusintering procedure.2) Diffusion coefficient of impurity atoms in liquid metal was calculated under different temperature, the results showed that the calculated values accord well with experimental values and calculated results of other authors, this equation can also be used to calculate the diffusion coefficient of carbon atoms in liquid cobalt phase. Thickness of gradient structure at different carbusintering time was also calculated by using the dynamic equation established in this paper, and the calculated values shows good consistency with experimental results.3) The formation mechanism of gradient structure is mainly ascribed to solution and precipitation of W and C atoms on WC grains as well as harmonious rotation and coalition of WC grains during carbusintering above eutectic temperature.