2016 Volume 11 Issue 2 Pages 308-319
In this study, an experimental apparatus was developed where the relative humidity (RH) and oxygen concentration could be controlled between 0.01-10%RH and 1-100000 ppm, respectively. This apparatus was used to study the roles of water and oxygen molecules on the friction of carbon nitride (CNX) coatings. The friction properties of Si3N4 balls sliding against CNX (Si3N4/CNX) under a nitrogen atmosphere were classified into the following three modes according to their average friction coefficients (μa) and the stability of the friction: Mode I, μa < 0.05 (stable); Mode II, 0.05 < μa < 0.3 (stable); and Mode III, 0.3 < μa (unstable). To achieve a low friction coefficient (e. g. < 0.05), the optimum RH and oxygen concentration were 0.1-1.0%RH and 100-10000 ppm, respectively. Time-of-flight secondary ion mass spectroscopy (TOF-SIMS) showed that hydrogen and hydroxyl radicals derived from water molecules chemisorbed onto the worn surfaces of the CNX coatings. Raman spectroscopy suggested that the structure of carbon on the worn surfaces was different from that of the deposited CNX coating. We concluded that low friction (Mode I) arose from the water and oxygen molecules terminating carbon dangling bonds and structural changes in CNX.