抄録
Carbon Nitride (CNx) coating has been attracted materials as well tribological property, especially low friction coefficient. Recently, the authors have reported the mechanism of superlow friction as running-in process. During the friction between CNx coating and a Si_3N_4 ball in N2 gas, nitrogen atoms desorbed from the CNx coating surface by cyclic friction in nitrogen gas, structural reformation of topmost surface to low shearing strength was taken place, simultaneously. From the view point of nitrogen desorption from CNx coating, it was assumed that ultraviolet ray could break C-N single bond when the ray intensity exceeded C-N single bonding energy. In this study, we hypothesized that ultraviolet ray irradiation could make the CNx coating topmost surface graphitic without friction in N_2 gas. We carried out preparing the CNx coating irradiated ultraviolet ray as 254, 312, and 365 nm wave length, because the C-N single bond is 305 kJ/mol and the other ultraviolet rays are 469, 382, and 327 kJ/mol. Then these specimens were tested in N_2 gas to compare frictional property of with and without ultraviolet ray irradiation. CNx coating was synthesized with Ion beam assisted deposition method as 100 nm thickness on Si(100) substrate. Ar ion beam irradiated to carbon target, sputtered carbon was deposited on the substrate and nitrogen ion beam was irradiated to the specimen, simultaneously. Friction tests were done with ball-on-disk type frictional tester which was set in vacuum chamber and the test conditions were that normal load was 0.1 N, sliding speed was approx. 0.04 m/s, and counter material was Si_3N_4 ball with 8 mm diameter. Before the friction test, the vacuum chamber was evacuated to approx. 0.1 Pa, then nitrogen gas was filled into the chamber approx. atmospheric pressure. All specimens did not exhibit superlow friction coefficient at the initial of friction test, however, the coatings which were irradiated exhibited superlow friction phenomena after several friction cycles. We defined that the running-in period was friction coefficient which became lower than 0.05. All irradiated specimens exhibited shorter running-in period than as-deposit CNx. The average of initial several cycle frictions of as-deposit CNx exhibited 0.04, on the other hand, in the case of 312 nm ultraviolet ray irradiated CNx exhibited 0.01 or lower average friction coefficient. These results indicated that ultraviolet ray could break the C-N single bond, these broken bonds were restructured to C=C double bond, and nitrogen atoms desorbed from the coating as the cause of restructuring to N_2 molecule. The effect of ultraviolet ray irradiation on destruction of CNx was assumed as ultraviolet ray irradiation period. Some early studies of ultraviolet ray irradiation to DLC were reported that carbonaceous network included in DLC was completely destructed with long period ultraviolet ray irradiation. According to the destruction of these networks, we investigated the effect of long period ultraviolet ray irradiation on. Thus, the friction coefficient had not been superlow (such as 0.01 or lower), and it often exhibited high value such as 0.2 or more. The structural changing of CNx coating's topmost surface was important dominant to decide friction coefficient, and it was clear that ultraviolet ray irradiation period should be controlled to obtain superlow friction. The ultraviolet ray irradiation was conventional method compare with changing the coating condition, and it could exhibit shorter running-in period than as-deposit CNx coating.
