Effect of oxygen plasma treatment on the tribological properties of Si-DLC coatings

In the present study, the effects of silicon oxide layer of Si-DLC coatings on the tribological properties were investigated. The Si-DLC coatings were deposited on Si substrates using a bi-polar type plasma-based ion implantation and deposition technique (bi-polar type PBII&D), and the surface of the Si-DLC coating was treated by oxygen plasma for 180 seconds. The friction coefficient of the Si-DLC coatings is effectively reduced due to the treatment of oxygen plasma whereas the wear depth of oxidized Si-DLC coatings increases due to the wear of soft oxide layer. A transferred layer composed of silicon oxide and graphitic carbon was observed on the counter steel ball surface, and the amount of the transferred layer increases by the treatment of oxygen plasma. The transferred layer prevents contact between the steel and DLC surfaces when sliding, resulting in a low friction coefficient. It seems that the silicon oxide plays a roll to bind the lubricative graphitized carbon debris on the steel ball surface. The treatment of the oxygen plasma on the top surface of the Si-DLC coatings can effectively reduce the friction coefficient without hardness reduction in the entire Si-DLC coatings. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.