Influence of carbon chemical bonding on the tribological behavior of sputtered nanocomposite TiBC/a-C coatings

The tribological performance of nanocomposite coatings containing Ti-B-C phases and amorphous carbon (a-C) are studied. The coatings are deposited by a sputtering process from a sintered TiB2:TiC target and graphite, using pulsed direct current and radio frequency sources. By varying the sputtering power ratio, the amorphous carbon content of the coatings can be tuned, as observed by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The crystalline component consists of very disordered crystals with a mixture of TiB2/TiC or TiBxCy phases. A slight increase in crystalline order is detected with the incorporation of carbon in the coatings that is attributed to the formation of a ternary TiBxCy phase. An estimation of the carbon present in the form of carbide (TiBxCy or TiC) and amorphous (a-C) is performed using fitting analysis of the C 1s XPS peak. The film hardness (22 to 31 GPa) correlates with the fraction of the TiBxCy phase that exists in the coatings. The tribological properties were measured by a pin-on-disk tribometer in ambient conditions, using 6 mm tungsten carbide balls at 1 N. The friction coefficients and the wear rates show similar behavior, exhibiting an optimum when the fraction of C atoms in the amorphous phase is near 50%. This composition enables significant improvement of the friction coefficients and wear rates (mu similar to 0.1; k < 1 x 10(-6) mm(3)/Nm), while maintaining a good value of hardness (24.6 GPa). Establishing the correlation between the lubricant properties and the fraction of a-C is very useful for purposes of tailoring the protective character of these nanocomposite coatings to engineering applications. (C) 2010 Elsevier B.V. All rights reserved.