Tungsten Carbide Additives (2/2)

Tungsten carbide additives play an important role in inhibiting the WC grain growth, and there are several major additive inhibitory effects: VC> Cr3C2> NbC> TiC> Mo2C> TiC> ZrC> HfC. Their inhibition principle includes:

1. Growth inhibitor changed the energy and characteristics of the solid / liquid interface, hindering the process of tungsten carbide precipitation and dissolution;

2. According to two-dimensional nucleation theory, the border energy leads to subtle changes in a significant change in the control of grain growth of two-D nucleation. During sintering grain growth inhibitors can be added by increasing the boundary can improve dimensional nucleation energy obstacle, making it the nucleation rate dropped significantly, WC grain coarsening has been suppressed;

3. Grain growth inhibitor can change the interface energy or interfere the surface dissolution- precipitation process; and it has higher solubility and diffusion coefficient in the binder phase Co, mainly through migration or diffusion bonding phase in the WC / Co interface, so it is likely to grow crystal grains in the activation WC lattice deposition occurs, thereby preventing the further growth of grains;

4. Add grain growth inhibitors will reduce the rate of diffusion of the activation energy is increased, and it will migrate along the WC / Co interface segregation occurs thereby restricting grain interface.

As an anti-corrosive component, Cr3C2 can be used as additives to improve the corrosion resistance of cemented carbide. It usually added in an amount to be controlled within the solid solubility of Cr in the γ phase, so it can improve its corrosion resistance without compromising the strength of the alloy. The basic principle is dissolved in Cr and Co binder phase in which surface to form a layer of passive film, can significantly reduce the current density, and the higher the ratio of Cr / Co and blunt alloy is greater. The addition of rare earth elements, such as molybdenum (Mo), niobium (Nb), tantalum (Ta), rhenium (Re), ruthenium (Ru) can significantly improve the high temperature properties of the cemented carbide. Ru can keep Co hexagonal structure stable at high temperature and improve the toughness and wear resistance of tungsten carbide; Re can form solid solution with Co, and Co-Re binder may significantly increase the softening temperature and reduce tackiness during processing.

These metal elements are usually added in the wet milling process, except some added in the form of master alloy. In addition, there are some non-metal can be used as additives, such as P, B and some carbide. It forms low-melting eutectic by reacting with other metals, and WC grain will rapidly dissolute in the liquid at relatively low sintering temperature so the alloy can be densified rapidly; while carbide can be synthesized at high temperature and pressure diamond diffuse carbide, which has a hardness close to diamond as well as close to conventional carbide toughness.

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