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Why Cobalt is the Prefered Binder
Although nickel and various alloys are used in modern grades, cobalt is still most common.
There are several criteria which govern the performance of a binder for Tungsten carbides:
a) Cobalt has a high melting point 1493°C (2719F)
b) Cobalt has excellent strength at high temperature
c) It forms a liquid phase with WC at a suitable temperature of 1275°C. This pulls the sintered part together by surface tension and eliminates voids.
d) Cobalt dissolves WC. Cobalt forms a eutectic with WC at 1275°C/1350°C and at that temperature dissolves 10% WC.
e) On cooling, WC should reprecipitate in the Cobalt bond giving hardness combined with toughness.
f) Cobalt can be produced as a very fine powder well under 1 micron. The binding agent should be capable of being ground very finely to mix with the hard carbide particles. Cobalt can be produced very finely and grinds down to << 1µ. On grinding, it reverts to the close packed form which is brittle although in the carbide product, it retains the more ductile cubic form at room temperature.
Cobalt fulfills all the needs of a binder while others, like Ni, Fe, etc., only fulfill some. It is this fact that has kept it irreplaceable in carbides. However other binders such a Nickel and Chrome can add corrosion resistance and toughness. They are harder to use and thus more expensive but the increased performance can be well worth it.