Friction Behavior Of Diamond-like Carbon Films Research Articles

Probing the friction mechanism of diamond-like carbon films in aqueous environments based on molecular dynamics simulations

Abstract The frictional behavior of diamond-like carbon (DLC) films in an aqueous environment is of tremendous interest. To compare the impacts of the quantity of water molecules, sliding velocity, and loading on the friction behavior of DLC films, a molecular model of DLC films in an aqueous environment was built. The results show that the DLC film under high load and high sliding velocity leads to severe friction behavior, but the water molecules can prevent the occurrence of this severe friction behavior. Also increasing the number of water molecules can effectively reduce the increase in friction during the running-in stage and keep it at a low value. The primary explanation is that if there are enough water molecules, they will create a stable water film at the friction interface and prevent large-scale contact and distortion between the upper and lower DLC films, significantly reducing friction.

The synergistic mechanism between graphitized tribofilm and graphitized surface of diamond-like carbon film under different temperature environments

The tribology characteristics of diamond-like carbon (DLC) film are studied under different experiment temperatures. The outcomes reveal the average COF of DLC film will gradually decrease with the increase of temperature from 25 °C to 225 °C, but its wear rate will gradually increase. Especially at 225 °C, the DLC film is worn through in a very short time. The Raman characterizations imply that the low friction behavior of DLC film may be mainly caused by the synergistic effect of graphitized transfer layer and graphitized worn surface of DLC film as the temperature increases from 25 °C to 175 °C. Further analysis shows that the high wear of DLC film at high temperature may be mainly due to the reduction of wear resistance of worn DLC surface and the formation of soft oxide layer at friction interface.

Rehybridization analysis of C atoms of Cu/Diamond and Ni/Diamond interfaces under vertical pressure

Diamond-like carbon (DLC) film enormous interest in both scientific and engineering communities because of its excellent tribological properties. And graphitization has been widely used to explain the low friction behavior of DLC film. However, the microscopic understanding of phase transition and graphitization mechanisms has never been conducted due to the limitations of existing characterization techniques. Herein, using first-principles calculations, we comparatively investigated the phase transition behavior of interface C atoms and the electronic structure information of the interface in the two interface models (Cu/Diamond and Ni/Diamond). The phase transition behavior is mainly due to the change in the hybridization mode of some C atoms at the interface under the vertical pressure, namely sp3 to sp2. We further used the transfer of the interface charge density to express the difference in tribological properties. The calculation results show that the re-hybridization behavior of interface C atoms has different effects on the charge transfer between the two interfaces. That is, there are different effects on its tribological properties. This work helps to understand the graphitization phenomenon under vertical pressure from the microscopic electronic structure.

Formation of rod-shaped wear debris and the graphitization tendency of Cu-doped hydrogenated diamond-like carbon films

Diamond-like carbon (DLC) films have good lubrication properties, and the structure of wear debris or tribolayer at the friction interface affect the friction behavior of DLC films. In the present work, Cu-doped hydrogenated DLC (Cu-DLC:H) films with Cu contents of 3.19 at.%, 8.21 at.%, and 11.28 at.% were fabricated by a reactive high-power impulse magnetron sputtering. The formation of wear debris in the friction interfaces of Cu-DLC:H and the effects of Cu content on the tribological properties of the Cu-DLC:H films were investigated. The results show that Cu has uniform distribution in the Cu-DLC:H films with Cu contents of 3.19 at.% and 8.21 at.%. By contrast, for the Cu-DLC:H film with 11.28 at.% Cu, nanocrystalline Cu is surrounded by an amorphous carbon matrix. The wear resistance of the Cu-DLC:H films is better than that of the DLC:H film without Cu. Moreover, the wear debris of the Cu-DLC:H films on the friction interface is rod-shaped, and Cu accumulates in the debris, which can improve the graphitization of the wear debris on counterpart and further improve the wear resistance of the Cu-DLC:H films.

Probing the low-friction mechanism of diamond-like carbon by varying of sliding velocity and vacuum pressure

The lubrication of diamond-like carbon (DLC) films has commonly been ascribed to two hypotheses, i.e., ‘friction-induced graphitization’ mechanism and passivation mechanism. To clarify the primary low-friction mechanism of DLC, we investigate the effects of sliding velocity and vacuum pressure on the friction behavior of DLC film. Counterintuitively, examination of wear tracks by Raman spectroscopy reveals that a higher friction coefficient is accompanying with a higher degree of graphitization. We therefore claim that it is the higher friction force that results in the higher degree of graphitization, which subverts the concept of a higher degree of graphitization leading to a lower friction coefficient. Besides, the friction coefficient is found to depend on the ratio of ambient pressure to rotating speed, indicating that the passivation mechanism is at play. Besides, the additional slide–hold–slide test in room air also gives evidence that cannot be understood in terms of the ‘friction-induced graphitization’ mechanism.