Influence of graphite on the features of phase transformations during vacuum annealing of α-WC + 10% W2C ceramics

The effect of vacuum annealing (1750℃, 3 h) on the phase composition, structure, and properties of the surface layer of fine-grained α-WC + W2C ceramics has been studied. Ceramics of the α-WC + 10% W2C composition with a high relative density and an average grain size of 0,1–0,2 μm were produced using the Spark Plasma Sintering (SPS) method. Vacuum annealing of the ceramic samples was carried out in two modes: (i) annealing of samples in contact with graphite; (ii) annealing of samples without contact with graphite. Changes in the phase composition and microstructure of the surface layers of ceramics after annealing have been studied using X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). Microhardness, nanohardness, and Young's modulus of the ceramics were measured after annealing. It has been shown that contact between the ceramic sample and graphite leads to carbon diffusion and transformation of W2C into α-WC. Vacuum annealing of the samples not in contact with graphite results in the transformation of α-WC into W2C and tungsten. Microstructure areas containing only tungsten are characterized by abnormally large grains up to ~80 µm in size, elongated perpendicular to the surface. After annealing, areas with increased microhardness up to ~40 GPa were found on the surface of ceramics.