What Determines the Low-Friction Mechanism of the Silicon-Doped Diamond-like Carbon Film in a Water Environment: An Atomic-Level Understanding.

It is widely acknowledged that doping silicon can significantly enhance the friction performance of diamond-like carbon (DLC) films in a water environment. However, the mechanism of low friction caused by doped silicon is still highly controversial. Therefore, this article compares the interface interaction between DLC and Si-DLC films in a water environment through first-principles calculations of physisorption and chemisorption effects. The results indicate that water molecules are predominantly chemically adsorbed rather than physically adsorbed on the Si-DLC surface. Further study reveals that when OH-termination is formed on the Si-DLC surface, water molecules are predominantly physically adsorbed rather than chemically adsorbed on the Si-DLC hydroxylation surface. Consequently, a more stable hydration layer is formed on the surface through the hydrogen bond network formed by Si-OH groups, ultimately leading to lower friction. Moreover, molecular dynamics simulations further suggest that the lower friction coefficient of Si-DLC films in a water environment may be due to more water molecules at the friction interface and fewer interface covalent bonds. In short, the low-friction coefficient of the Si-DLC film in a water environment may be caused not only by the chemisorption of water molecules on its surface but also by the physisorption of water molecules on the Si-DLC film after surface hydroxylation.

Probing the Effect of Cuttings Particle Size on the Friction and Wear Mechanism at the Casing Friction Interface: A Molecular Dynamics Study.

Cuttings particles of different sizes in the drilling fluid are the leading cause of wear at the casing and drill pipe joints, and diamond-like carbon (DLC) films have excellent research potential in reducing tool wear due to their ultra-low friction coefficient and high wear resistance. In this paper, a corresponding molecular dynamics model was developed using LAMMPS to investigate the effect of silica particles of different particle sizes on the friction and wear mechanisms of Fe/DLC friction pairs at the microscale. The results show that small cuttings particles in a dry environment are more likely to cause interface wear between the casing and drill pipe joint, while in a water environment, the opposite is true. The main reason is that small particles in a dry environment have smaller contact areas and greater indentation depth, leading to greater wear at the friction interface. The movement of water molecules in the water environment will promote the composite movement of large particles, thereby exacerbating the wear of the interface. Moreover, the relevant research results at the micro-scale indicate that DLC films can effectively reduce wear, which provides theoretical support for its application in drill pipe joints.

Understanding the Effect of Oil-Based Lubricants on the Tribological Behavior of Fe-Cr Alloys from Reactive Molecular Dynamics.

In this paper, the frictional behaviors of Fe-Cr alloys in the lubricating effect of oil-based lubricant are investigated through reactive molecular dynamics. It is shown that the oil-based lubricant achieves ultralow friction through hydrodynamic lubrication by linear alpha olefin (C8H16) and passivation of the friction pairs by hydrogen gas (H2) and free H atoms generated by the friction chemistry. Moreover, there is a critical value for the transition of the crystal structure of Fe-Cr alloy from body-centered cubic (Bcc) to amorphous structure (Other), leading to a dramatic change in friction. Meanwhile, a sliding interface consisting of a large number of amorphous structures is formed near the rigid layer, which keeps the friction force stable.

Effect of the Graphitization Mechanism on the Friction and Wear Behavior of DLC Films Based on Molecular Dynamics Simulations.

Whether a graphitization mechanism can control the low-friction behavior of DLC films is still controversial. In this paper, we establish the molecular dynamics model of the DLC film with graphene (DLC-GR-DLC) by LAMMPS and study the influence of the graphitization mechanism on the friction and wear behavior of the DLC film. The friction force of the DLC-GR-DLC model in the running-in stage is significantly smaller than that of the DLC film and then gradually increases to the same size as that of the DLC film. Further analysis indicates that the graphitization mechanism could indeed reduce the shear stress of the friction interface when graphene remains intact. However, the curling and breaking of the graphene structure will lead to an increase in shear force at the friction interface.

Probing Friction Properties of Hydrogen-Free DLC Films in a Nitrogen Environment Based on ReaxFF Molecular Dynamics.

In this paper, ReaxFF molecular dynamics simulations were used to look at how load and the number of nitrogen molecules affect how friction behavior in hydrogen-free diamond-like carbon (DLC). The presence of nitrogen molecules will inhibit the formation of C-C covalent bonds between the contact surfaces of the upper and lower DLC, thereby effectively suppressing the increase in friction during the initial friction phase. After the initial friction stage, the mechanical mixing of the contact surfaces brought on by the diffusion of nitrogen molecules results in considerable shear stress, which has significant impacts on the friction force. In addition, due to the existence of nitrogen molecules, the effect of graphitization of hydrogen-free DLC on friction is almost negligible.