Nanostructure and properties of TiC/a-C:H composite coatings

TiC/a-C:H nanocomposite coatings, deposited with closed-field unbalanced magnetron sputtering, have been scrutinized with atomic force, scanning and high-resolution transmission electron microscopy, nanoindentation and tribo-tests. These coatings consist of 2–5 nm TiC nanocrystallites embedded in an amorphous hydrocarbon (a-C:H) matrix. A transition from a columnar to a glassy microstructure has been observed in the nanocomposite coatings with increasing substrate bias or carbon content. Microcracks induced by nanoindentation or sliding wear readily propagate through the column boundaries whereas the coatings without a columnar microstructure exhibit substantial toughness. The toughening of the nanocomposite coatings has been achieved effectively on two different scales, namely by restraining the formation of columns on a microscale and by manipulating the nanostructure on a nanoscale. The hardness (H) and elastic modulus (E) of the coatings are found to increase monotonically with increasing substrate bias, whereas the ratio of the hardness to the elastic modulus (H/E) remains approximately constant. In contrast, H/E increases with C content. Ball-on-disc tribo-tests confirm that the nanocomposite coatings possess superior wear resistance and strong self-lubrication effects with a coefficient of friction as low as 0.05 in ambient air and below 0.02 in dry air, under dry sliding against uncoated bearing steel balls. Physical arguments are presented to explain the toughening mechanism and the ultra-low friction.