Durability of Super-Low Friction of Hydrogenated Carbon Nitride Coatings in High-Vacuum Environment

Abstract

The durability of super-low friction of hydrogenated carbon nitride (CN x :H) coatings sliding against Si 3 N 4 balls in high-vacuum (HV) environments and the phenomena that occur at the interface were clarified. In an HV, CN x :H achieved super-low friction (µ<0.01), which increased after a certain number of cycles even without coating delamination, indicating the existence of "the lifetime of super-low friction." During super-low friction, a carbonaceous tribolayer with a transformed structure from the initial CN x :H was formed on Si 3 N 4 and transformed to a similar structure to the initial CN x :H after friction increased. Scanning transmission electron microscopy and time-of-flight secondary ion mass spectroscopy (TOF-SIMS) revealed that the carbonaceous tribolayer during super-low friction had 10-15 nm thickness and chemically bonding with the Si 3 N 4 bulk. This changed to the interface with patch-like carbon adhesions after friction increased, indicating that the carbon transfer states from CN x :H transited during the lifetime. TOF-SIMS depth analysis revealed that desorption of the hydrogen content of CN x :H occurred on the friction surface at a depth of less than 10 nm. We concluded that the lifetime of super-low friction is due to the transformation of the tribolayer formed on Si 3 N 4 owing to the desorption of the hydrogen content of CN x :H.