Paper TF-ThM6
Effect of Oxygen on the Self-formation of Carbonaceous Tribo-layer with Carbon Nitride Coatings under a Nitrogen Atmosphere

Thursday, December 15, 2016, 9:40 am, Room Makai

Carbon nitride (CN_X) is an expected coating material which shows high hardness and relatively low friction under a nitrogen atmosphere. These properties make it a good candidate for small precision machines in order to reduce energy consumption and material losses. We have already reported that a carbonaceous tribo-layer, whose structure is altered from that of the deposited coating, has been found on the worn surface of counterpart. Thus, a structural change of carbon is necessary condition for super-low friction. However, the outstanding properties cannot be achieved in the presence of oxygen or humidity, which has meant these coatings has not been used in practical applications under ambient air. In contrast, CN_X coatings can give super-low friction under ambient air when they are heated to 100 ˚C. Besides, we have reported that hydrogen atoms derived from water molecules in a nitrogen atmosphere chemisorb the worn surfaces, which provides super-low friction. These results indicate that CN_X coating has possibility to reduce friction and wear regardless of whether the nitrogen gas is mixed with oxygen and humidity. In this study, the effect of oxygen and humidity on friction and wear of CN_X coatings are clarified in detail. The possibility of super-low friction under an oxygen-contained humid nitrogen atmosphere is discussed from the viewpoint of formatting carbonaceous tribo-layer.

The CN_X coating is produced on the surfaces of SiC ball and disk using an ion beam assisted deposition system. We conduct a ball-on-disk friction and wear test in a glove box, which allows for control of the relative humidity (RH) and oxygen concentration within the range of 0.01-40 %RH and 0.01-25 vol.%, respectively.

When friction and wear tests of CN_X coatings are conducted, friction coefficients and wear volumes increase with increasing RH (3-40 %RH) and oxygen concentration (0.01-25 vol.%). However, friction coefficients and wear volumes remain low values regardless of RH and oxygen concentration when CN_X slides against hydrogenated-CN_X (CN_X:H) coating. Besides, we found that CN_X coatings gave specific wear rate below 3*10^-8 mm³/Nm when they achieved low friction coefficients below 0.05. These results indicate that there is a critical value of specific wear rate for formatting carbonaceous tribo-layer on the worn surfaces. TOF-SIMS depth analysis provides evidences that carbon derived from CN_X:H transfer to the surface of CN_X. This means that the carbonaceous tribo-layer is self-formed during friction. CN_X slid against CN_X:H achieved low friction coefficient below 0.05 regardless of RH and oxygen on the condition that the carbonaceous tribo-layer is self-formed.