Microstructure, mechanical properties and friction behavior of magnetron-sputtered V-C coatings

This study describes reciprocating sliding friction experiments of three types of magnetron-sputtered vanadium carbide/amorphous carbon nanocomposite coatings, VC1 − x/a-C, against alumina counterparts under unlubricated conditions. It links micro- to macroscale tribology in order to provide a better understanding of the sliding mechanisms of these nanocomposite coatings with varying constitution and microstructure (i.e. by varying the C/V concentration ratio between 1.2 and 2.9). Correspondingly, the volume fraction of the amorphous carbon phase (a-C) increases with increasing the carbon content as shown by electron microprobe studies, X-ray diffraction and Raman spectroscopy; simultaneously, the volume fraction of nanocrystalline vanadium carbide (VC1 − x) decreases, and the crystallite size and crystallinity of the thin films decrease with increasing carbon content. These constitution and microstructure changes have an impact both on the mechanical and the tribological properties of the coatings. Mechanical properties characterization shows that the indentation hardness (H) and the reduced modulus (Er) of the coatings decrease with increasing the carbon concentration (i.e. H ranges between 15 and 33 GPa and Er ranges between 239 and 391 GPa for the low and high vanadium concentration respectively). A similar behavior was also observed with the results on the critical load of failure obtained from scratch tests, where the coatings with the highest vanadium concentration exhibits higher critical load of failure and thus, better coating adhesion. However, reciprocating micro- and macroscale tribological tests performed at a relative humidity level of 45 ± 5% (i.e. normal load in the microtribological experiments is between 100 mN and 1 N and for the macroscale tribometry the normal load is 20 N) reveal higher friction values and increased wear with the high vanadium content coatings. This sliding behavior is attributed to differences in the third body formation (i.e. carbon based transfer- and tribo-film) and velocity accommodation modes, which are analyzed ex situ by means of X-ray photoelectron spectroscopy (XPS), micro-Raman spectroscopy and atomic force microscopy.