Effects of atomic structure on the frictional properties of amorphous carbon coatings

• Unsaturated carbon causes formation of CC bonds and stick-slip at the interface.
• The stick-slip behavior in DLC/DLC is more irregular than that in diamond/DLC.
• Increase of sp3 content and doped amorphous carbon decreases friction force.
• Structures of sp3 and sp2 hybridization dominate the friction force behavior.

Diamond-like carbon (DLC) films are widely used as protective coatings against wear and corrosion and have attracted a lot of attention in the last few decades. However, it is very difficult to determine the hybridization and structure of amorphous carbon experimentally during sliding. Therefore, in this work, molecular dynamics (MD) simulations are employed to investigate the nano-tribological properties of amorphous carbon with and without hydrogen atoms doped. The simulation models are built with DLC/DLC interface and diamond/DLC interface for different densities of carbon. As we found, both the carbon bonding network and the carbon hybridization have significant effect on the frictional properties of DLC films. The unsaturated carbon atoms, which are in sp2 or sp1 hybridization, can lead to the formation of covalent bonds between the two contacting surfaces resulting in stick-slip type of friction. With an increase of sp3 bonding content in tetrahedral carbon and doping of amorphous carbon with hydrogen atoms decrease the steady-state value of friction force and reduce the amount of the stick-slip motion.