Binder-phase enrichment by dissolution of cubic carbides

Sintering of a cemented carbide body containing nitrogen in a vacuum or in a nitrogen-free atmosphere results in the formation of a cubic-carbide-free gradient zone adjacent to the insert surface. The mechanism for the formation of the gradient zone is, despite its having been thoroughly investigated by others, not fully understood. In the present paper, a kinetic model based on thermodynamic calculations and some kinetic considerations will be presented. According to this model, gradient formation is the result of nitrogen diffusion out of the insert and the coupled diffusion of cubic-carbide-forming elements in the opposite direction, from the surface region into the interior of the insert. Based on the model, a comprehensible explanation of the influence of sharp edges on the gradient growth is given. The gradient is usually thinner close to sharp edges. The present model has been successfully used to rationalize experimental data and to explain and predict the influence of variations in composition, sintering time, temperature, and atmosphere.