Low friction coefficient of superhard nc-TiC/a-C:H nanocomposite coatings deposited by filtered cathodic vacuum arc

Nanocomposite coatings formed by metal (Ti, Al, Cr, etc)-doped diamond like carbon coatings can exhibit excellent tribological properties due to the combined improvement in hardness and toughness in optimized deposition conditions. In this work, Ti-doped a-C:H coatings are successfully deposited at C₂H₂ flow rate (f _C2H2) of 50–250 sccm on Si (100) and AISI 304 L stainless steel substrates using filtered cathodic vacuum arc. Chemical bonding state, microstructure, and mechanical and tribological properties of Ti-doped a-C:H coatings were investigated using XPS, FESEM, nanoindentation and ball-on-disc tribometer, respectively. Nc-TiC/a-C:H nanocomposite coating with dense microstructure and smooth surface morphology are obtained at f _C2H2 of 50–250 sccm thanks to high-efficiency magnetic filtered depositions. The sp²/sp³ ratio gradually decrease from 1.6 to 1.1, while (Ti–C)/(C–C) ratio shows a sharp decrease from 0.361 to 0.047. Hardness and H/E* and H³/E*² ratios decrease from 41.2 GPa, 0.098 and 0.41 at 50 sccm to 22.9 GPa, 0.071 and 0.117 at 250 sccm, respectively. Residual stress ranges from −0.39 to −1.72 GPa. Superhard nc-TiC/a-C:H nanocomposite coating with the low residual stress, high H/E* and H³/E*² exhibit low coefficient of friction of 0.07 and low wear rate of 5.4 × 10⁻⁹ mm³ N⁻¹ m⁻¹, which is ascribed to the high sp²/sp³ and (Ti–C)/(C–C) ratios, dense microstructure and smooth surface morphology. (paper)