Evaluation of microstructure and growth kinetics of tungsten carbide ceramics at the interface of iron and tungsten

Abstract

The microstructure evolution and growth kinetics of tungsten carbide (WC or W2C) ceramics at the interface of iron and tungsten during isothermal annealing process from 1100 °C to 1150 °C were investigated by X-ray diffraction (XRD), electron backscattered diffraction (EBSD) and scanning electron microscopy (SEM), respectively. The results show that carbide formation that involves W2C, WC and a small quantity of Fe2W2C, is strongly dependent on a diffusion-controlled reaction. During the evolution process of tungsten carbides, W2C emerges as an initial phase by the reaction of W and C, and WC is formed as the second phase, because the value of Gibbs free energy of formation of W2C is more negative than that of WC at the annealing temperature. The initial W2C transforms into WC until complete consumption of the W2C. Additionally, Fe2W2C between W2C and WC was formed at a relatively low temperature (1100 °C and 1125 °C) with a long annealing time (60 min). After the W2C is consumed completely, WC grows by C diffusion, where the growth kinetics of a dominant WC ceramic exhibits a parabolic growth law before metal tungsten is completely consumed. The growth activation energy of the WC layer is calculated to be achieved with a value of 208.68 kJ/mol. When the metal tungsten is completely consumed after annealing at 1150 °C for 60 min, the W2C and Fe2W2C disappear, and only the WC phase exists in the final product.