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

Abstract

Carbon-containing transition metal nitride coatings promise excellent mechanical and tribological properties owing to nanocomposite structure. This paper reports the fundamental structural characteristics of nanocomposite nc-TiAlV(N,C)/a-C coatings which govern the mechanical and tribological properties. The coatings were grown on hardened steel substrate by reactive unbalanced magnetron sputtering in a mixed N2 and CH4 gases. After measurements of the hardness, Young’s modulus, residual stresses, and ball-on-disk friction and wear properties in dry sliding against an alumina ball, comprehensive chemical and chemical characterisations have been carried out by analytical high resolution TEM, EELS, FEG-SEM, EDX, XRD, and Raman spectroscopy. The structure of the coatings has been characterised to be a complex architecture of columnar grains of nc-TiAlV(N,C)/a-C nanocomposites and inter-column network of amorphous carbon. For the phase constituents, the majority of carbon was condensed as sp2-type amorphous carbon, whereas some carbon atoms/ions were dissolved in the TiAlV(N,C) carbonite. The segregation of carbon has played a decisive role in the nanocomposite formation. As compared to the carbon-free nitride coating, the major difference of the carbon-containing composite coatings are the substantially reduced residual stress of -1.14 GPa and Young’s modulus of 298 GPa, whereas the coatings maintained high hardness of 29 GPa. Their tribological properties were found to be strongly dependent on the environment by showing high friction and wear in dry atmosphere and significantly lower friction and wear when the relative humidity was higher than 30 %. Moreover, the coating grown at the CH4 to N2 ratio of 1:1 contained large amount of amorphous carbon, resulting in much lower hardness and Young’s modulus and substantially high wear rate. 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

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