Thermal evolution of WC/C nanostructured coatings by raman and in situ XRD analysis

In this work, a series of WC/C nanostructured films were deposited on silicon substrates by changing the ratio of sputtering power applied to graphite and WC magnetron sources (P_c/ P_wc: 0, 0.1, 0.5, 1). The thermal stability of WC/C coatings was followed in situ by means of X-ray diffraction measurements up to 1100°C in vacuum (10^-1 Pa). Initially, the film microstructure is composed of nanocrystalline WC _1-x and W₂C phases. As the P_c/P_wc ratio increases the crystallinity decreases, and WC_1-x becomes the predominant phase from P_c/ P_wc = 0.1. The results show that the structural evolution with temperature of all studied layers depends essentially on their initial phase and chemical composition (determined by the synthesis conditions: ratio P_c/P_wc)- The coating deposited at P_c/P_wc = 0 reveals a transformation of W₂C phase into W and W₃C phases at 400°C. However, the samples with P_c/P_wc greater than 0 exhibits an improved thermal stability up to 600-700°C where the WC_1-x begins to transform into W₂C and WC phases. At 900°C, WC is the predominant phase, especially for those coatings prepared with higher ratios. Further annealing above 1000°C yields W as the foremost phase. The thermal behaviour was later studied by means of Raman spectroscopy measurements at certain temperatures where the main changes in phase composition were observed. Particularly, a fitting analysis was carried out on the D and G bands typical of disordered and amorphous carbon. The changes induced during heating are discussed in terms of the positions of D and G lines, and full width at half maximum (FWHM).