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 sp(2)/sp(3) 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 H-3/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 H-3/E*(2) exhibit low coefficient of friction of 0.07 and low wear rate of 5.4 x 10(-9) mm(3) N-1 m(-1), which is ascribed to the high sp(2)/sp(3) and (Ti- C)/(C-C) ratios, dense microstructure and smooth surface morphology.