Structure characterization and tribological study of magnetron sputtered nanocomposite nc-TiAlV(N,C)/a-C coatings

Grown by reactive unbalanced magnetron sputtering in a mixed N2 and CH4 gaseous medium, heterogeneous nanocomposite coatings in the Ti-Al-V-N-C system show extraordinarily excellent tribological performance of coated machining tools. Using analytical high resolution TEM, EELS, FEG-SEM, XRD, and Raman spectroscopy, this paper reports detailed structural and chemical characterization of the coatings grown at various CH4: N2 ratios. Meanwhile, the mechanical and tribological properties were also measured, including hardness, Young’s modulus, residual stress and the dry-sliding friction and wear at varying environmental humidity. When CH4 gas was introduced in the deposition, the structure of the coatings has been found to experience a change from nano-scale TiAlN-VN multilayer architecture to a complex mixture of columnar grains of nc-TiAlV(N,C)/a-C nanocomposites and inter-column network of sp2-type amorphous carbon. Carbon incorporation and segregation also shows remarkable influence on the columnar growth model by leading to finer grain size. As compared to the carbon-free nitride coating, the nanocomposite coatings showed substantially reduced residual stress owing to the free-carbon precipitation, whereas the coatings maintained comparable hardness to the carbon-free TiAlN/VN. Their tribological properties were found to be strongly dependent on the environment. In humid air at RH > 30%, the coatings showed low friction coefficient less than 0.4 and extremely low wear rate at a scale of ~10-17 m3N-1m-1.