Friction and wear property of a-CNx coatings sliding against Si3N4 balls in water

Amorphous carbon nitride coatings (a-CNx) were deposited on Si(1 0 0) wafers and 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 behavior of a-CNx coating sliding against a Si3N4 ball in water was investigated. The results indicated that the a-CNx coatings contained 12 at.% nitrogen and the major chemical bonding was sp2 CN and sp3 C-N. The nano-hardness of the a-CNx coatings was 29 GPa. At a sliding velocity of 0.16 m/s and after running-in, the mean steady-state friction coefficient varied around 0.02 when the normal load was lower than 3.5 N, and then decreased abruptly from 0.018 to 0.007 at 5 N. For self-mated Si3N4, the specific wear rate of a Si3N4 ball was a little higher than that of a Si3N4 disk, while for a-CNx/Si3N4, the specific wear rate of a Si3N4 ball was slightly smaller than that of a-CNx coating. Furthermore, the specific wear rate of Si3N4 ball sliding against a-CNx coating was reduced by a factor up to 35 in comparison to that against Si3N4 in water. This indicated that the wear mechanism of a-CNx coating/Si3N4 ball was the formation of a carbonaceous transfer film on the a-CNx coatings via a tribochemical reaction between a-CNx coatings and water induced by friction, while that of self-mated Si3N4 ceramics was the formation of silica gel on the contact zone via the reaction of silicon nitride and water.