Growth And Friction Mechanisms Of Hydrogenated Diamond-like Carbon Films

Diamond-like carbon(DLC) film has high hardness, low friction and good wear resistance, giving rise to its widespread application in the field of tribology. In this thesis, hydrogenated DLC film is chosen as the research object. The growth process, as well as the friction mechanism of hydrogenated DLC film, is studied by molecular dynamic simulation, which is based on a second-generation reactive empirical bond order(REBO) potential.During the growth process of hydrogenated DLC films, the structural properties such as film density, hydrogen content and sp3 fraction are defined and calculated. And the influence of incident energy and incident species(such as hydrogen content and radical characters) on film properties is the focus of this thesis. At low energies, molecular adsorption dominates the process of the film growth, so the incident molecules tend to preserve their original molecular structures. As the incident energy rises, film density increases firstly and then becomes stabilized, while hydrogen content decreases.The growth mechanism of hydrogenated DLC film can be explained as follows: at low energies, the formation of sp3 structures mainly attributes to the hydrogen adsorption; while at high energies, the subplantation of carbon atoms is responsible for the formation of sp3 structures. Hence, the growth mechanism of hydrogenated DLC film differs from that of hydrogen-free DLC film. An increase of hydrogen content in source gas could lead to lower film density, higher hydrogen content in film and a general increase of sp3 fraction. The existence of dangling bonds in incident radicals facilitates easier adsorption than neutral molecules at low energies, resulting in higher deposition yield and sp3 fraction. This trend, however, diminishes at high energies when extensive molecular fragmentation occurs.By using a single asperity model, the frictional behavior during the sliding process between the diamond tip and as-deposited hydrogenated DLC films with different hydrogen concentration is analyzed, under different normal loads. Hydrogen concentration and normal load are found to play essential roles in the tribological behavior of hydrogenated DLC film. With low hydrogen content, the hydrogenated DLC film shows high adhesion and friction even at very low normal loads. At high normal loads, formation of nanocrystalline graphene-like lamellar structures is observed locally, acting as a lubricating agent, resulting in the reduction of friction coefficient. With high hydrogen concentration, the friction shows increasingly linear dependence on the normal load and higher load bearing capability. Hydrogen passivation effect becomes more obvious with higher hydrogen content especially at low loads, where a hydrogen-rich monolayer is found to attach onto the asperity. The hydrogen-rich transfer layer, along with the high hydrogen concentration on the surface of the a-C:H film, forms apparent hydrogen passivated sliding interface, which results in the low friction of film. By means of the comprehensive analysis of microstructure and tribological properties of hydrogenated DLC film, it can be deduced that, in order to achieve superlow friction, the optimal incident energy ranges from 20-45 e V, and the incident species can be chosen as CH2 radical.