Friction and wear property of a-CNx coatings sliding against ceramic and steel balls in water

The amorphous carbon nitride coatings (a-CNx) were deposited on Si3N4 disks using ion beam assisted deposition (IBAD), and their composition and chemical bonding were determined by X-ray photoelectron spectroscopy (XPS). The a-CNx coatings' hardness was measured by nano-indentation and the friction and wear property of the a-CNx coatings sliding against Si3N4, SiC, Al2O3, SUS440C and SUJ2 balls in water were investigated by using ball-on-disk tribo-meter. The worn surfaces were observed using optical microscopy and analyzed by XPS. The results of XPS analysis showed that the a-CNx coatings contained 12 at.% nitrogen and the major chemical bonding was sp2 C = N and sp3C-N. The nano-hardness of the a-CNx coatings was 29 GPa, higher than those of balls. Among five kinds of tribo-systems, the lowest friction coefficient was obtained in the range of 0.01 to 0.02 for the tribo-systems with SiC and Si3N4 balls, the largest wear rate of the a-CNx coating of 1.77 × 10− 7 mm3/Nm was obtained as sliding against Al2O3 ball, while the smallest wear rate of a-CNx coating of 1.44 × 10− 8 mm3/Nm was gotten as sliding against Si3N4 ball. However, SUJ2 ball showed the highest wear rate of 7.0 × 10− 7 mm3/Nm, whereas Al2O3 ball exhibited the lowest wear rate of ball of 3.55 × 10− 9 mm3/Nm. The XPS analysis on the worn surface for the a-CNx coatings displayed that the nitrogen concentration decreased and the sp2-bonding-rich structure was formed after sliding tests in water.