Kinetics of densification and grain growth in ultrafine WC-Co composites

Abstract

Dilatometer densification measurements were performed on WC-Co composites containing 2, 5 and 10 wt% Co. The runs were carried out under a flow of 10% H2-Ar up to 800 °C followed by vacuum up to 1400 °C. Analysis of diffusional densification kinetics suggest an initial solid state densification from 800 to about 1190 °C attributed mainly to grain growth-densification indicated by acceptable fittings to a modified Coble intermediate stage model. This behavior was confirmed by grain growth analysis (range 820 to 1400 °C) measured in fractures surfaces of partially densified pellets. It was detected either a softening solid state stage or eventually liquid formation at ~ 1150-1200 °C and a kind of viscous flow densification behavior operating prior to apparently the solution-precipitation liquid phase sintering. The densification model for diffusional liquid phase sintering applied well in the 1260-1400 °C range with 105-150 kcal mole- 1 activation energy depending on Co content. The standard rearrangement stage was also valid partially within the temperature range 1320 to 1400 °C. Certain fittings of present densification data using phenomenological continuous mechanics approach were carried out, obtaining activation energies ranging between 6 and 35 kcal mole- 1. Although no clear assignments of such values to probable matter diffusion processes could be made, it is noted the latter value (35 kcal mole- 1) is close to the 25-36 kcal mole- 1 values obtained for the classical liquid rearrangement stage.