What does Coercivity tell us?

Coercivity is determined by fully magnetizing a sample in a strong magnetic field created by an electrical force running in a coil. An opposite electrical force applied through the same coil then demagnetizes the sample; the force necessary to demagnetize the sample is measured in oersteds and is given as the Coercivity.

Coercivity is a measure of the residual magnetism of a cemented carbide sample. The measurement is independent of sample size or weight. The sample must meet the criteria of the coils maximum dimensions and position within the test coil. Pure cobalt in cemented carbides has normal magnetic properties. During sintering some of the cobalt in the cemented carbides will alloy with either carbon or tungsten carbide. Therefore the carbon imbalance of the sample will affect the Coercivity. For Coercivity it is important to know the metallurgical condition of the sample in terms of free carbon, normal or Eta-phase, as these will affect the Coercivity measurement.

During sintering all the individual cobalt powder agglomerates melt and fuse together forming a solid matrix of WC grains (non-magnetic) and cobalt (magnetic); The finer the matrix the higher the Coercivity. A fine cobalt matrix will have a lot more residual magnetism, as it will require more force to neutralize all the magnetic nodes (cobalt pockets) in the matrix. The size of the cobalt pockets is a direct measurement of the size of the WC grains, i.e. if the WC grains are small the cobalt pockets are small. Therefore, in carbon-balanced samples, Coercivity is a measure of the grain size of the sintered samples; The higher the Coercivity the finer the grain size. Since grain size is a direct measure of proper sintering, Coercivity is a measure of how well cemented carbides have been sintered. The following table shows the relationship.

Coercivity of a 10% cobalt cemented carbide grade as grain size varies.