Dissolution mechanism of tungsten carbide in cobalt-based liquids
Olivier Lavergne, Colette H Allibert
The cemented carbides, tungsten carbide – cobalt, are processed by liquid phase sintering at 1400 – 1500 °C. The understanding of the densification and microstructural evolution during liquid phase sintering requires a good understanding of the tungsten carbide dissolution in liquid cobalt. The dissolution mechanism is studied by experimental determination of the liquid composition evolution, versus the dissolution time, with controlled stirring of the liquid. The effects of the initial carbon to tungsten ratio, and of the addition of small chromium or vanadium contents in the liquid, are examined at 1450 °C. The tungsten carbide/liquid interface morphology is observed. The significant dissolution acceleration produced by increasing the stirring velocity indicates a process limited by diffusion. The liquid composition evolution is correctly represented by the Berthoud equation. This leads to deduced dissolution rate constants of the order of 4.9 x 10-5 and 1.6 x 10-5 m s-1, respectively, for the turbulent and laminar flows investigated. Such values do not significantly change for initial liquid contents with various carbon/tungsten ratios or chromium and vanadium additions. From the flow computation analysis, carried out in a joint work, the diffusion coefficient D, in the liquid, is calculated to be about 2 x 10-9 m2 s-1. This leads to the conclusion that tungsten carbide dissolution is controlled by the diffusion of tungsten in the cobalt-based liquids.