Influence of the carbon-overcoat on CNx coating for the friction and wear properties

Carbon Nitride (CNx) coating showed superlow friction when it slid against a Si3N4 ball in dry N2, and this phenomena took place nitrogen atoms desorption from topmost layer of CNx and structural change from CNx to graphitic layer. However, it was taken place after several thousand friction cycles, so-called running-in period. In this study, we proposed Carbon-overcoat deposition on CNx due to shorten the running-in period, and investigated the effects of its thickness on runningin period, minimum friction coefficient and specific wear rate. 5 different thickness specimens as 0 meant an as-deposit CNx, 3, 5, 10, and 15 nm as Carbon-overcoat which were deposited on CNx coating by IBAD (Ion Beam Assisted Deposition) method. Tribological measurements of these coatings sliding against Si3N4 balls were carried out with a pin-on-disk apparatus and 5 different thickness disks in matched pairs under a 0.1 N normal load, 0.13 m/s sliding speed, in N 2. The minimum friction coefficient was within a range of 0.01-0.015 when 3 nm thickness Carbon-overcoat on CNx coating slid against a Si 3N4 ball after short running-in period (around 400 cycles) rather than other specimens, and 1.8×10-8 mmVNm as minimum specific wear rate was obtained. Furthermore, Raman spectroscopy indicated that Carbon-overcoat specimens changed CNx to graphitic layer easier than only CNx coating.