Shrinkage, liquid phase formation, phase transformation and gas reactions during the sintering of WC-Co hard metals

The outgassing and sintering processes occurring during the heating period are very decisive for the quality of the sintered hard metal.The largest proportion of the compaction takes place in the solid state, i.e.before the formation of the liquid phase.In the intensive stage of shrinkage the reduction of porosity is determined by particle rearrangements.On the other hand the solution of tungsten and carbon in the cobalt binder is determined by diffusion-controlled processes.The formation of the eutectic and the solution of tungsten and carbon in the cobalt binder are characterized by calorimetric studies (DSC) and determination of the Curie temperature (magnetic transformation) by means of a special thermobalance.The effects are shown to be closely correlated with sintering conditions, cobalt content and WC grain size.Additionally in-situ measurement of volatile components during outgassing and sintering (evolved gas analysis by means of thermobalance-mass spectrometer coupling) allow s the characterization of relevant mechanisms (e.g.reduction of oxidic impurities of the binder and the hard material).The results yield new possibilities of optimization of temperature-time-atmosphere-profiles for the production of hard metals.