Sliding friction and wear property of a-C and a-CN_x coatings against SiC balls in water

The amorphous carbon coatings with and without nitrogen ion incorporation (a-CN_x and a-C) were deposited on Si (100) wafers and SiC disks by using ion beam assisted deposition, and their composition, structure and chemical bonding were characterized by using Auger electronic spectroscopy, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS), respectively. The friction and wear properties of the a-CN_x coatings sliding against SiC ball in water were investigated and compared with those of the a-C coatings. The worn surfaces of the a-CN_x coatings were observed by scanning electron microscopy and analyzed by XPS. The micro-analysis results showed that the a-CN_x coatings contained 12 at.% nitrogen and the sp³ fraction in the a-CN_x coatings was lower than that in the a-C coatings. There were two kinds of N 1s bonding states such as sp²C{double bond, short}N and sp³C-N in the a-CN_x coatings. The nanohardness of the a-CN_x coatings was 29 GPa, lower than that of the a-C coatings (34 GPa). The tribological tests in water showed the friction coefficients of the a-CN_x/SiC tribo-pair were in the range of 0.02-0.05, smaller than those of the a-C/SiC tribopair (0.03-0.07), and the specific wear rates of the a-C coating and SiC ball in the a-C/SiC tribo-pair were slight larger than those in the a-CN_x/SiC tribopair. The XPS analysis on the a-CN_x coatings' wear track showed that the surface nitrogen concentration of the a-CN_x on the wear track decreased. This indicated that the nitrogen removal would change the wear track surface structure, and then improve the tribological properties of the a-CN_x coatings in water.