Why Cemented Carbide Also Called Cermet

The main use of tungsten (in the form of tungsten carbide) is now in the manufacture of cemented carbide. Cemented carbide, or hard metal as it is often called, is a material made by "cementing" very hard tungsten mono-carbide (WC) grains in a binder matrix of tough cobalt metal by liquid phase sintering. This material is classified technically as a “brittle” material since it exhibits little or no plastic deformation preceding the initiation of a crack. All materials contain some amount of defects in the form of voids, pores or micro-cracks. These defects lead to reduced material strength. In the case of ductile materials such as aluminum, mild steels or copper, the frequency of defects is less critical than in brittle materials.

Sintered tungsten carbide exhibits a broader range of scatter-of-fracture stresses, due primarily to the existence of micro-voids, when compared to ductile materials. The value of the stress at fracture can also vary widely with size, stress state (tensile, bending, torsion), shape, and type of loading. Despite the significant variability of the stress at failure, cemented carbide has considerably high strength for what some consider to be a “ceramic” or cermet material.

The definition of a ceramic material is the marriage of a metal to a nonmetal, for example silicon (metal) carbide (carbon, non-metal), aluminum oxide, silicon nitride or tungsten carbide. A cermet is a composite material composed of ceramic (cer) and metallic (met) materials. A cermet is ideally designed to have the optimal properties of both a ceramic, such as high temperature resistance and hardness, and those of a metal, such as the ability to undergo plastic deformation. The metal is used as a binder with oxide, boride, carbide, or alumina. Generally, the metallic elements used are cobalt, nickel, and molybdenum. It is the addition of the metallic binder, i.e. cobalt or nickel that makes the cemented carbide (WC - Co) a cermet and differentiates it from truly brittle materials, that is, the ceramic family of materials.

The combination of WC and metallic cobalt as a binder is a well-adjusted system not only with regard to its properties, but also to its sintering behavior. The high solubility of WC in cobalt at high temperatures and a very good wetting of WC by the liquid cobalt binder result in an excellent densification during liquid phase sintering and in a pore-free structure. As a result of this, a material is obtained which combines high strength, toughness and high hardness.

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