Study of the growth kinetics of in situ WC grains in tungsten carbide layers produced by a diffusion-controlled reaction

Abstract

To refine the microstructure, the growth behaviour of ceramic grains in tungsten carbide cemented layers produced by diffusion-controlled in situ reactions at 1000 °C, 1050 °C and 1100 °C for 1–5 h was investigated in the solid-state. The cemented layers were etched in a 25% hydrochloric acid solution for 36 h until the substrates were dissolved. Hence, ceramic grains adhered to the thin tungsten plate were obtained. The phase composition, microstructure and three-dimensional morphology of tungsten carbide ceramic were characterized by utilizing X-ray diffraction (XRD) and scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS). The indexed phase structure for the ceramic grains consists mainly of W2C and WC, and a transformation of W2C into WC occurred with the elevated temperature. The presence morphology of platelet and triangular prism and hexagonal prism plays a dominant role in tungsten carbide grains. The average value of basal plane radii (r) and prismatic plane length (L) were counted by means of the three-dimensional shape and size of grains. The growth kinetics and growth mechanism of ceramic grains were evaluated based on the classical power law. The in situ tungsten carbide grains grown exhibit a grain growth exponent equal to 3, and the growth rates increase with temperature. The calculated growth activation energy for the basal plane radii has a value of 554.64 kJ/mol, which is greater than the values obtained under liquid phase sintering because the inhibition effect of the precipitated phase may reduce the grain growth driving force, indicating that the growth of WC grains has difficulty overcoming the large energy barrier. The grain growth mechanism was determined to be grain boundary migration controlled by the volume diffusion of W and C elements.