Why Cobalt Is the Preferred Binder

Cobalt was the first material successfully used as a binder for tungsten carbide.  It is still the most widely used but other materials, such as Nickel and Chrome, can add significant advantages in areas such as corrosion resistance, lubricity, temperature resistance and similar.  

There are several criteria which govern the performance of a binder for 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 fulfils all the needs of a binder whilst 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.