Tribo‐Induced Structural Transformation and Lubricant Dissociation at Amorphous Carbon–Alpha Olefin Interface

Atomistic Insights into Interfacial Optimization Mechanism for Achieving Ultralow-Friction Amorphous Carbon Films under Solid-Liquid Composite Conditions

The combination of fluid lubricants and textured amorphous carbon (a-C) can provide an ultralow friction state, which can improve the reliability and service life of dynamic machinery. However, the coupling effects of the contact pressure and oil content on the friction-reducing efficiency is still lack of study, and the corresponding friction mechanism is also not fully understood, which cannot be achieved by experiment due to the limitation of in situ characterization. In this study, using the reactive molecular dynamics simulation, the insight into the evolution of interfacial structures induced by both contact pressures and oil contents on a-C surface was systematically investigated to explore the fundamental mechanism. In particular, the friction difference between textured and untextured a-C films was evaluated comparatively. Results indicate that the tribological performance strongly depends on the interfacial lubrication state, which is jointly determined by the oil content and contact pressure; the best operating condition to achieve ultralow friction coefficient (0.002) is obtained, and the evolution of friction coefficient with oil content and contact pressure is highly dominated by the lubricant mobility, cross-linking between mating a-C surfaces, or competition/synergy of the H stress state from the lubricant with interfacial passivation. Furthermore, the difference in friction reduction between textured and untextured systems is unveiled; with the increase of contact pressure, the role of texturing a-C surface in antifriction changes from positive to negative effect, which is related to the transformation of interfacial hybridized structure and anomalous flow of lubricant. These results can significantly enhance the understanding of composite lubrication systems through computation and also provide a roadmap for the R&D of the advanced lubrication system according to the working conditions.

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.

Insights into Superlow Friction and Instability of Hydrogenated Amorphous Carbon/Fluid Nanocomposite Interface

Hydrogenated amorphous carbon (a-C:H) film exhibits the superlubricity phenomena as rubbed against dry sliding contacts. However, its antifriction stability strongly depends on the working environment. By composting with the fluid lubricant, the friction response and fundamental mechanisms governing the low-friction performance and instability of a-C:H remain unclear, while they are not accessible by experiment due to the complicated interfacial structure and the lack of advanced characterization technique in situ. Here, we addressed this puzzle with respect to the physicochemical interactions of a-C:H/oil/graphene nanocomposite interface at atomic scale. Results reveal that although the friction capacity and stability of system are highly sensitive to the hydrogenated degrees of mated a-C:H surfaces, the optimized H contents of mated a-C:H surfaces are suggested in order to reach the superlow friction or even superlubricity. Interfacial structure analysis indicates that the fundamental friction mechanism attributes to the hydrogenation-induced passivation of friction interface and squeezing effect to fluid lubricant. Most importantly, the opposite diffusion of fluid oil molecules to the sliding direction is observed, resulting in the transformation of the real friction interface from a-C:H/oil interface to oil/oil interface. These outcomes enable an effective manipulation of the superlow friction of carbon-based films and the development of customized solid-fluid lubrication systems for applications.

High-Temperature Sliding Friction Behavior of Amorphous Carbon Films: Molecular Dynamics Simulation

With the development of the aerospace industry, the requirement for mechanical parts, which are serviced under extreme conditions such as high temperature, is more and more severe. Amorphous carbon (a-C) films are widely used in the aviation field as a protective coating because of their excellent antiwear and friction-reduction properties. However, a-C films are vulnerable to failure in a high-temperature environment, and a series of complex changes in the friction process make it a challenge to put forward the friction mechanism. Here, the sliding friction behaviors of amorphous carbon (a-C) films at different simulated temperatures (STs) (300-1300 K) were analyzed by molecular dynamics. The density, average coordination number, and local residual stress as well as the hybridization of sp, sp², and sp³ of a-C films were analyzed to reveal the high-temperature sliding friction mechanism of a-C films. The results show that the friction coefficient (μ) of a-C films increased with increase in ST. Meanwhile, the friction mechanisms of a-C films are different at an ST lower than 800 K and higher than 1100 K. Compared with those before sliding, the local residual stress of all a-C films is relaxed, which causes transformation of sp³ into sp². Moreover, when ST is lower than 800 K, the μ increased with increase in sp³%. When ST is higher than 1100 K, the stability of a-C films is broken, which results in the rapid increase in μ.

Tailoring the Nanostructure of Graphene as an Oil-Based Additive: toward Synergistic Lubrication with an Amorphous Carbon Film

Graphene exhibits great potential as a lubricant additive to enhance the antifriction capacity of moving mechanical components in synergism with amorphous carbon (a-C) as a solid lubricant. However, it is particularly challenging for experiments to accurately examine the friction dependence on the physical nanostructure of the graphene additive and the corresponding interfacial reactions because of the inevitable complexity of the graphene structure fabricated in experiments. Here, we address this puzzle regarding the coeffect of the size and content of the graphene additive at the a-C interface using reactive molecular dynamics simulations. Results reveal that the friction-reducing behavior is more sensitive to graphene size than content. For each graphene structure, with increasing content, the friction coefficient always decreases first and then increases, while the friction behavior exhibits significant dependence on the graphene size when the graphene content is fixed. In particular, the optimized size and content of the graphene additive are suggested, in which an excellent antifriction behavior or even superlubricity can be achieved. Analysis of the friction interface indicates that with increasing graphene size, the dominated low-friction mechanism transforms from the high mobilities of the base oil and graphene additive in synergism to the passivation and graphene-induced smoothing of the friction interface. These outcomes disclose the roadmap for developing a robust solid-liquid synergy lubricating system.

In situ Synthesizing Carbon-Based Film by Tribo-Induced Catalytic Degradation of Poly-α-Olefin Oil for Reducing Friction and Wear

Reducing friction and wear in a convenient and economical way has always been desired for industrial production. Here, a carbon-based film with excellent friction-reducing and antiwear abilities was formed in situ from the degradation of poly-α-olefin oil (PAO10) on the friction interfaces of the MoN/Pt coating sliding against the Si₃N₄ ceramic ball during the rubbing process. The MoN/Pt coating was prepared on stainless steel by direct current magnetron sputtering, in which an active 10 nm Pt layer grew well on the MoN layer. The MoN/Pt coating, lubricated by trace amounts of 5 mL PAO10 oil, exhibited a super low friction coefficient of 0.042 and an extremely low wear rate of 1.08 × 10^-8 mm³ (N m)^-1 after a long duration of applied friction under a high Hertz contact stress of 1.7 GPa. Raman spectra and transmission electron microscopy images revealed that the carbon-based film was composed of amorphous carbon phase dotted with sporadic Pt, MoO₃, and SiO₂ crystal phases. Molecular dynamics simulations illustrated that the MoN/Pt coating had catalytic action and resulted in the degradation of PAO10 during the rubbing process, which corresponded to the formation of the amorphous carbon-based film on the wear surfaces.