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 degrees C in vacuum (10(-1) Pa). Initially, the film microstructure is composed of nanocrystalline WC1-x and W2C phases. As the P-C/P-WC ratio increases the crystallinity decreases, and WC1-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 degrees C. However, the samples with P-C/P-WC greater than 0 exhibits an improved thermal stability up to 600-700 degrees C where the WC1-x begins to transform into W2C and WC phases. At 900 degrees C, WC is the predominant phase, especially for those coatings prepared with higher ratios. Further annealing above 1000 degrees 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).