Effect of powder characteristics on shrinkage kinetics of WC-Co-Cr Zn-reclaimed submicron powders

Zn-reclaimed WC-Co-Cr powders are strongly agglomerated, requiring additional milling to achieve homogeneous particle size distribution and adequate compaction behavior. As milling time (i.e. milling energy) increases, both powder specific surface area and oxygen content increase. A limiting milling time has been identified, beyond which the finest and more oxidized powders tend to re-agglomerate. Solid state shrinkage during sintering, fully inhibited in un-milled Zn-reclaimed materials, is strongly activated as milling time increases up to the point at which powder re-agglomeration occurs. Activation energies, derived from shrinkage rate values assuming viscous flow-like behavior, confirm a transition between at least two diffusion-controlled regimes around 980-1000 °C. At lower temperatures, the “effective” viscosity of compacts during shrinkage decreases with milling time, except for the case of re-agglomeration. Nevertheless, at elevated temperatures, all materials show similar activation energies, suggesting that the lattice defects induced by milling-which promote solid state diffusion- are progressively annihilated through annealing.