Definition of Atomic-Scale Contact: What Dominates the Atomic-Scale Friction Behaviors?

On the Friction Behavior of SiO2 Tip Sliding on the Au(111) Surface: How Does an Amorphous SiO2 Tip Produce Regular Stick-Slip Friction and Friction Duality?

Friction behaviors of an amorphous SiO₂ tip sliding on the Au(111) surface in atomic force microscopy (AFM) are investigated through molecular dynamics (MD) simulations. We observed a regime of extremely low, close-to-zero friction at low normal loads with clear stick-slip friction signals. The friction is almost independent of the applied normal load below a threshold value. However, above this loading threshold, friction can remain low or increase sharply. Such an unexpected friction duality is attributed to the high probability of defect formation at the sliding interface that can induce plowing friction in a high-friction state. The energy difference between the low-friction state and the high-friction state is surprisingly low, which is comparable to kT (∼25 meV) at room temperature. These findings are consistent with previous AFM friction measurements using silicon AFM tips. Further MD simulations show that one can always use an amorphous SiO₂ tip to image the crystalline surface with regular stick-slip friction signals. This is largely due to the fact that there is always a small fraction of contacting Si and O atoms at the sliding interface that are sitting on the relatively stable, close-to-hollow sites of the crystalline Au(111) surface during the stick stage; thus, they are capable of sampling local energy minima. We anticipate that regular stick-slip friction can be achieved even in the intermediate loading range, so long as the low-friction state is maintained when friction duality occurs.

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.

Oxidation and hydrogenation of monolayer MoS2 with compositing agent under environmental exposure: The ReaxFF Mo/Ti/Au/O/S/H force field development and applications

An atomistic modeling tool is essential to an in-depth understanding upon surface reactions of transition metal dichalcogenides (TMDs), such as molybdenum disulfide (MoS2), with the presence of compositing agents, including Ti and Au, under different environmental exposures. We report a new ReaxFF reactive force field parameter set for Mo, Ti, Au, O, S, and H interactions. We apply the force field in a series of molecular dynamics (MD) simulations to unravel the impact of the Ti dopant on the oxidation/hydrogenation behaviors of MoS2 surface. The simulation results reveal that, in the absence of Ti clusters, the MoS2 surface is ruptured and oxidized at elevated temperatures through a process of adsorption followed by dissociation of the O2 molecules on the MoS2 surface during the temperature ramp. When the MoS2 surface is exposed to H2O molecules, surface hydrogenation is most favored, followed by oxidation, then hydroxylation. The introduction of Ti clusters to the systems mitigates the oxidation/hydrogenation of MoS2 at a low or intermediate temperature by capturing the O2/H2O molecules and locking the O/H-related radicals inside the clusters. However, OH− and H3O+ are emitted from the Ti clusters in the H2O environment as temperature rises, and the accelerating hydrogenation of MoS2 is consequently observed at an ultra-high temperature. These findings indicate an important but complex role of Ti dopants in mitigating the oxidation and hydrogenation of MoS2 under different environmental exposures. The possible mechanisms of oxidation and hydrogenation revealed by MD simulations can give an insight to the design of oxidation resistant TMDs and can be useful to the optical, electronic, magnetic, catalytic, and energy harvesting industries.