MODEL OF ANOMALOUS CARBON DIFFUSION DURING SPARK PLASMA SINTERING OF TUNGSTEN CARBIDE

A model to explain the causes of anomalous carbon diffusion in tungsten carbide during spark plasma sintering (SPS) of α-WC + W₂C powders has been proposed. The phenomenon of anomalously deep carbon penetration into tungsten carbide ceramics during SPS of a powder billet in graphite tooling, as discovered in previous studies by the authors, is characterized by the formation of an anomalously deep layer (∼ 50 mm in thickness) of pure tungsten monocarbide (a-WC) on the ceramic surface. The formation of this layer is due to the reduction of tungsten semicarbide (W₂C) during its interaction with carbon diffusing from the tooling surface. The thickness of the reduced layer (∼ 50 mm) is several orders of magnitude greater than the values that follow from calculations of the depth of carbon diffusion, based on tabulated values of the activation energy of carbon diffusion in tungsten carbide. The model is based on the assumption that to facilitate the phase transformation of W₂C → α-WC, during which, due to the restructuring of the atomic crystal structure, a change in the volume of the unit cell occurs, it is necessary to ensure not only the flow of additional carbon to the surface of the W₂C particles, but also the formation of additional volume, the value of which is proportional to the change in density during this phase transformation (∼ 10%). The creation of additional volume is provided by the flow of nonequilibrium vacancies from the sample surface. The concentration of such vacancies is proportional to the value of the change in density during the phase
transformation of W₂C→α-WC and the volume fraction of the W₂C particles, and in this case exceeds the equilibrium concentration of vacancies in tungsten carbide by several orders of magnitude. The presence of these vacancies facilitates the accelerated diffusion of carbon in tungsten carbide. The model allows for the estimation of changes in the diffusion coefficient and enables the calculation of the depth of carbon penetration into the sintered material.