Atomistic Insights into the Running-in, Lubrication, and Failure of Hydrogenated Diamond-Like Carbon Coatings

Abstract

The tribological performance of hydrogenated diamond-like carbon (DLC) coatings is studied by molecular dynamics simulations employing a screened reactive bond-order potential that has been adjusted to reliably describe bond-breaking under shear. Two types of DLC films are grown by CH2 deposition on an amorphous substrate with 45 and 60 eV impact energy resulting in 45 and 30% H content as well as 50 and 30% sp(3) hybridization of the final films, respectively. By combining two equivalent realizations for both impact energies, a hydrogen-depleted and a hydrogen-rich tribo-contact is formed and studied for a realistic sliding speed of 20 m s(-1) and loads of 1 and 5 GPa. While the hydrogen-rich system shows a pronounced drop of the friction coefficient for both loads, the hydrogen-depleted system exhibits such kind of running-in for 1 GPa, only. Chemical passivation of the DLC/DLC interface explains this running-in behavior. Fluctuations in the friction coefficient occurring at the higher load can be traced back to a cold welding of the DLC/DLC tribo-surfaces, leading to the formation of a transfer film (transferred from one DLC partner to the other) and the establishment of a new tribo-interface with a low friction coefficient. The presence of a hexadecane lubricant leads to low friction coefficients without any running-in for low loads. At 10 GPa load, the lubricant starts to degenerate resulting in enhanced friction

Non-equilibrium phase behavior and friction of confined molecular films under shear: A non-equilibrium molecular dynamics study

“…This must not influence the calculated physical properties to any great extent. With such, 'global' barostats, 44,[48][49][50] the extra τ term is added to improve the ability to control the imposed load and eliminate possible instabilities caused by the barostat. Our equations to enforce an applied normal pressure overcome some of the above mentioned difficulties, as no instantaneous pressure, P inst , needs to be calculated to drive the system.…”

Low Friction Property of Diamond-Like Carbon Coating Films and Oxygen Transmission Ratio of Amorphous Carbon Films Deposited by Advanced Coating Processes

“…This ultra-low friction is thought to be generated by forming very thin and weak tribofilm at the mixed lubrication condition. [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] Martin et al proposed the possible mechanism of the super low friction was the forming water layer by the tribochemical reaction between ta-C and glycerol. [28][29][30][31][32][33] In this paper, the formation of water and acid was indicated as one possible chemical reaction for glycerol, as shown in (a) (a) (b) (b) For the oleic acid test, the water can be supplied from the ambient air.…”

MD Sliding Simulations of Amorphous Tribofilms Consisting of either SiO2 or Carbon

“…Although the beneficial effects on friction and wear reduction are beyond dispute, the reasons are not so obvious. Pastewka et al performed MD simulations on hydrogenated amorphous carbon (a-C:H) [8]. They observed two effects which might be responsible for the observed friction reduction.…”