The effect of ultraviolet light irradiation on tribological properties of hydrogenated DLC

The carbonaceous coating such as Diamond-Like Carbon is one of the promising materials as well tribological properties, especially low friction coefficient. The friction coefficient of hydrogenated DLC was reported not lower than 0.1 when it slid in lubricants. However, the hydrogenated DLC is easy to coat to the three dimensional mechanical parts, so it is required to decrease the friction coefficient of hydrogenated DLC in lubricants. The low friction property is important for hydrogenated DLC, however, it is worse to get the low friction property by the running-in process because of the complex procedure, long period to get low friction. So, it is required to obtain the frictional property by a substitute process. From the view point of reformation of the topmost surface of hydrogenated DLC, we focused on ultraviolet light. It is assumed that ultraviolet light will change the DLC topmost surface to graphite-like layer because the light intensity exceeds C-C bonding energy, and then generated dangling bonds will combine. We hypothesized that ultraviolet light irradiation could make the DLC coating topmost surface graphitic. We prepared the DLC coating irradiated ultraviolet light of 254, 312, and 365 nm wave length for 30 min. The C-C bond is 348 kJ/mol and the other ultraviolet lights are 469, 382, and 327 kJ/mol. Then these specimens and as-deposited DLC coating were tested in air to compare frictional property. DLC coating was synthesized with PECVD as 1.8 μm thickness on Si(100) substrate. Friction tests were carried out by a ball-on-disk type frictional tester in air, under lubricant, sliding against Si₃N₄ ball and SUJ2 ball which diameters were 8 mm.