Atomic-Scale Insights into the Dynamic Friction Regulation Mechanisms of Nanolubricant Molecules at the Fe/PTFE Interface

Abstract

Surface and interface science play an important role in the tribological properties of materials. Recently, research in this field has extended from the macroscopic scale to the molecular level to elucidate energy dissipation and structural evolution mechanisms at sliding interfaces. In this work, we propose a nanolubricant strategy based on carbon nanocages (CNCs). Three types of lubricating molecules—oleylamine (amine), oleic acid (carboxyl), and stearyl alcohol (hydroxyl)—were encapsulated into a polytetrafluoroethylene (PTFE) matrix to construct a composite tribological interface model. Molecular dynamics simulations were employed to investigate the interfacial enrichment, diffusion, and interaction mechanisms of these molecules with PTFE chains and the Fe counterface. Particular emphasis was placed on how different functional groups regulate energy transfer and dissipation pathways. This study deepens the molecular–level understanding of structure–lubrication relationships and provides theoretical guidance for designing high–performance polymer–based tribological materials.

1. Introduction

Friction, as a ubiquitous phenomenon in both natural and engineering systems, is a critical factor governing energy consumption, operational efficiency, and service lifetime of mechanical components [1,2,3]. It is widely acknowledged that friction– and wear–induced energy losses and material failures impose a substantial economic burden on the global industrial system each year [4]. Under high–speed, heavy–load, or extreme operating conditions, interfacial behaviors within friction pairs become particularly complex, leading not only to severe energy dissipation and material degradation but also to cascading failures of mechanical equipment [5,6]. Conventional material systems have gradually reached their performance limits in terms of lubrication stability, wear resistance, and adaptability to harsh working environments [7,8]. Consequently, regulating the structure and dynamic behavior of friction interfaces from the perspective of materials design, and developing advanced tribological materials that simultaneously exhibit low friction, high wear resistance, and robust environmental tolerance, has emerged as a frontier topic at the intersection of tribology and materials science.

Polytetrafluoroethylene (PTFE), owing to its extremely low surface energy, excellent chemical inertness, and intrinsic self–lubricating characteristics, has been widely employed in seals, coatings, and sliding components [9,10,11]. Nevertheless, PTFE still suffers from inherent drawbacks under high loads or complex service conditions, including poor wear resistance, limited load–bearing capacity, and inadequate compatibility with lubricating additives, which often lead to accelerated wear and premature failure during long–term operation [12,13]. In recent years, constructing polymer–based composite materials by incorporating functional fillers (e.g., nanocontainers) has emerged as an effective strategy to overcome the inherent limitations of PTFE [14,15,16,17,18]. Among these, nanocarriers like carbon nanocages (CNCs) have attracted considerable attention due to their advantages, including relatively low cost, high specific surface area, good chemical stability, and strong interfacial compatibility [19,20]. Specifically, the strategy of employing lubricant–encapsulated nanocapsules is particularly promising. Loading liquid lubricant molecules into nanocarriers and dispersing them within the polymer matrix not only improves the uniform dispersion of the additives but also enables the controlled release of lubricant under shear and pressure during friction. This process facilitates the formation of a continuous lubricating protective layer at the interface, significantly enhancing the anti–friction and anti–wear properties of the composite. This lubrication mechanism is commonly attributed to the synergistic effects of various factors, including the formation of a surface protective film, damage repair, a rolling effect, and a polishing effect [21,22].

In particular, the chemical structure of lubricating molecules—especially the type and polarity of terminal functional groups—plays a decisive role in determining their adsorption behavior on metal surfaces, interfacial bonding strength, and stability under shear. Long–chain organic molecules such as oleylamine (OAm), oleic acid (OA), and stearyl alcohol (SA), bearing amine, carboxyl, and hydroxyl functional groups, respectively, are capable of inducing distinct physical adsorption and chemical interactions at friction interfaces. These differences significantly influence interfacial shear responses, molecular alignment, and the evolution pathways of lubricating films. However, most existing studies primarily focus on macroscopic performance enhancement, while the dynamic interfacial behavior and synergistic mechanisms of lubricating molecules during friction remain insufficiently understood.

Molecular dynamics (MD) simulation provides a powerful tool for elucidating the microscopic structural evolution and multiscale lubrication mechanisms at friction interfaces [23,24,25,26,27]. Compared with macroscopic experiments, MD simulations enable direct tracking of adsorption configurations, orientational rearrangements, interfacial energy dissipation, and the coupled responses between polymer chains and metal surfaces under combined shear and load. This capability is particularly valuable for multiphase composite systems, where complex many–body interactions among nanofillers, lubricating molecules, and polymer matrices are difficult to decouple experimentally. MD simulations therefore offer a theoretical bridge linking molecular–scale interfacial processes to macroscopic tribological responses.

Based on this background, the present work employs CNCs as nanocontainers to encapsulate representative lubricating molecules—OAm, OA, and SA—and incorporates them into a PTFE matrix to construct composite tribological materials. Through MD simulations, the dynamic behaviors and evolution mechanisms of these functional–group–dependent lubricating molecules at the Fe–PTFE friction interface are systematically investigated. Particular emphasis is placed on elucidating, from an interfacial dynamics perspective, the adsorption stability, orientational rearrangement, and shear–load–coupled responses of lubricating molecules with different polarities, as well as their regulatory effects on PTFE chain conformations and Fe surface adhesion. This work not only provides new insights into the design of high–performance polymer–based tribological materials but also reveals, at the atomic scale, the distinct roles of functional–group–engineered lubricating molecules in interfacial lubrication, thereby offering a solid theoretical foundation for the development of next–generation intelligent lubrication systems.

2. Simulation Methods

All MD simulations in this study were performed using the LAMMPS package, version 22 Jul 2025 [28]. The initial atomic configurations were constructed with the PACKMOL tool [29], while the all–atom force–field parameters for PTFE and the lubricating molecules were generated using Moltemplate [30]. Atomic–scale structural visualization and data analysis were carried out with OVITO version 3.15.1 [31].

The atomic–scale friction model is illustrated in Figure 1. The simulation box dimensions were set to 50 Å × 50 Å × 190 Å. Periodic boundary conditions were applied along the X and Y directions, while a non–periodic boundary condition was used along the Z direction. The CNCs structures were approximated by partially removing atoms from a C720 fullerene, which preserves a curved carbon framework and provides a simplified yet computationally efficient representation of nanoscale carbon structures. The upper counterface consisted of α–Fe with a body–centered cubic (BCC) structure (50 Å × 50 Å × 70 Å), while the lower counterface comprised a PTFE substrate decorated with CNCs of 12 Å in diameter, resulting in a total height of 120 Å. To represent the state of the lubricant after its release into the tribological interface, 35 lubricant molecules were introduced into the confined region between the Fe layer and the PTFE substrate. This number was selected to approximate the target mass density of the lubricant within the interfacial volume while ensuring sufficient statistical resolution of intermolecular interactions during sliding. The PTFE was rendered transparent to facilitate clear observation of its different components.

A hybrid force–field strategy was employed in the simulations. The OPLS–AA force field [32] was used to describe PTFE and the lubricating molecules, the Fe layer was modeled using the MEAM potential [33], and the CNCs were described by the AIREBO potential [34]. Interactions between different components were represented by the Lennard–Jones (LJ) potential [35,36]. All simulations were conducted under the canonical (NVT) ensemble at a constant temperature of 298.15 K. The cutoff distance for non–bonded interactions was set to 10 Å, and long–range electrostatic interactions were calculated using the particle–particle–particle–mesh (PPPM) method with a relative accuracy of 10⁻⁴. The equations of motion were integrated using the Verlet algorithm with a time step of 1 fs.

The simulation system was constructed in a stepwise manner to ensure a well–defined and reproducible initial configuration. First, the PTFE–CNCs–lubricant system was generated and equilibrated. The center of the CNC was fixed at (25 Å, 25 Å, 85 Å) within the simulation box. Subsequently, 160 PTFE chains with a degree of polymerization of 30 were randomly packed into the region spanning 0–120 Å along the Z direction. A total of 35 lubricant molecules were then introduced into the interfacial region between 100 Å and 120 Å along the Z direction, corresponding to the expected release zone of the lubricant. The constructed system was equilibrated for 500 ps under the canonical (NVT) ensemble to eliminate unfavorable contacts and achieve a stable initial configuration. Equilibration was considered complete when key thermodynamic properties (e.g., potential energy, temperature, and pressure) exhibited stable fluctuations. Following equilibration, the Fe layer was brought into contact with the top surface of the PTFE–CNCs–lubricant system. A normal pressure of 400 MPa was then applied along the Z direction to establish intimate interfacial contact. This pressure corresponds to the local stress level that can be achieved within real contact regions at the nanoscale and is intended to emulate the high–pressure conditions associated with asperity contacts [37]. The applied pressure was maintained constant throughout the subsequent simulations. The system was then further equilibrated for 500 ps to ensure mechanical equilibrium prior to tribological testing. Finally, to rapidly overcome the initial running–in stage and achieve a steady frictional state, a constant sliding velocity of 0.5 Å/ps was imposed on the Fe layer along the X direction, with a total sliding duration of 1 ns. This velocity is commonly adopted in MD simulations of polymer tribology and facilitates a more realistic representation of the interfacial sliding behavior observed in experiments [38].

3. Results and Discussion

Based on the interfacial structural evolution of the four tribological systems shown in Figure 2 at 0 ps, 500 ps, and 1000 ps, the pronounced role of functionalized lubricating molecules during shear can be clearly identified. In the lubricant–free Fe–PTFE system, direct contact between the two surfaces occurs from the initial stage. As sliding proceeds, PTFE chains exhibit evident adhesion and plastic deformation, and by 1000 ps the interface becomes highly disordered, indicating that the neat polymer lacks an effective interfacial protection mechanism under shear. In contrast, the introduction of lubricating molecules encapsulated in CNCs leads to a pronounced regulation of interfacial behavior. The released lubricants rapidly spread at the interface during the early stage and form a stable and continuous adsorbed layer at approximately 500 ps, effectively preventing direct contact between the Fe surface and PTFE chains. Even at 1000 ps, this adsorbed layer largely retains its structural integrity and molecular ordering, demonstrating the formation of a robust, shear–resistant protective film on the metal surface.

Figure 3 illustrates the evolution of the coefficient of friction (COF) for different friction systems during 1 ns of sliding. Compared with the pristine PTFE system, which remains in a high–friction regime with pronounced fluctuations and an average COF of 0.431, all three composite materials containing carbon nanocage–encapsulated lubricant molecules exhibit a significant and stable reduction in COF. Among them, the CNCs/OA/PTFE system shows the lowest average COF (0.212), indicating the most effective friction–reducing performance. The CNCs/SA/PTFE system displays a slightly higher average COF of approximately 0.248, while the CNCs/OAm/PTFE system exhibits the highest COF among the composites (0.269). It is noteworthy that all composite systems exhibit moderate COF fluctuations during the intermediate stage of sliding. These fluctuations are closely associated with the dynamic interfacial behaviors of the lubricant molecules under shear, including adsorption, interfacial reorganization, and partial desorption. In the following section, the differences in tribological behavior are further elucidated by correlating the functional group structures of the lubricant molecules with their distinct interfacial interaction mechanisms.

Figure 4a,b illustrates the effect of frictional shear on the spatial distribution of lubricating molecules bearing different terminal functional groups within the Fe/PTFE interfacial region (Z = 105–125 Å). In the simulation model, the PTFE substrate occupies the region from Z = 0 to 120 Å, whereas the Fe layer is located between Z = 120 and 190 Å. The pre–sliding distributions (Figure 4a) reveal pronounced interfacial enrichment for all lubricating molecules. Among them, SA, bearing a hydroxyl (-OH) group, exhibits the highest and sharpest density peak at Z ≈ 118 Å, reflecting its strong initial interfacial adsorption affinity driven by the polar terminal group. After frictional shear (Figure 4b), the spatial distributions undergo substantial reorganization. The density peaks of OA and SA shift toward Z = 120 Å, i.e., closer to the Fe side of the interface. This redistribution indicates that molecules with polar terminal groups are more prone to migrate into the transition zone between Fe and PTFE under shear, thereby participating in the construction of a coupled metal–polymer interfacial layer. The dynamically formed interfacial structure plays a critical role in maintaining interfacial stability during sliding. Furthermore, based on the number density distribution of lubricant molecules after friction, the thickness of the interfacial adsorption layer was quantitatively evaluated using the full width at half maximum (FWHM) method [39]. The results show that the interfacial film thicknesses for OA, OAm, and SA are 6.2 Å, 5.2 Å, and 4.8 Å, respectively. This clearly indicates that, although SA exhibits a sharper adsorption peak under initial static conditions, OA—bearing the -COOH functional group—can form a thicker and denser interfacial film during dynamic shear.

Figure 5 illustrates the regulatory role of functional groups in tribological interfacial behavior from an MD perspective. The mean square displacement (MSD) analysis (Figure 5a) shows that SA molecules (-OH group) exhibit the lowest diffusivity, indicating strong confinement within the Fe/PTFE interface. Lubricant introduction markedly alters PTFE chain mobility (Figure 5b). In pristine PTFE, segments display the highest MSD due to pronounced shear-induced migration. Conversely, lubricants suppress PTFE mobility in all composite systems. Notably, while OA molecules themselves have relatively high diffusivity (Figure 5a), the PTFE segments in the CNCs/OA/PTFE system show the lowest MSD. This counterintuitive behavior suggests that OA acts as more than an interfacial lubricant; it forms strong interactions with the polymer matrix, creating an “anchoring effect” that restricts chain motion. The evolution of van der Waals (vdW) interactions corroborates this (Figure 5c). Initially, OA–PTFE interactions are strongest, while SA–PTFE interactions are weakest. As sliding proceeds, these interaction energies converge, with a significant enhancement in SA–PTFE interactions. This trend suggests that SA molecules progressively infiltrate the polymer interface under coupled shear and normal loads.

Figure 5d,e demonstrates that both SA and OA establish stronger simultaneous interactions with Fe and PTFE than OAm, constructing a stable coupling layer. Hydrogen bond analysis (Figure 5f) provides further mechanistic support. OA forms the most hydrogen bonds with PTFE, as its carboxyl (-COOH) groups act as effective donors that form dynamic networks with fluorine atoms. This mechanism drives the “locking” effect on polymer segments, underpinning the superior confinement of PTFE in OA–containing systems. Additionally, Figure 6 uses box plots to show the temporal evolution of the radius of gyration (Rg) of PTFE chains. The pristine system maintains small, narrowly distributed Rg values, reflecting coiled conformations. In contrast, the CNCs/SA/PTFE system shows a progressive increase in Rg. This reflects the continuous insertion of SA molecules during friction, where resulting steric hindrance drives PTFE chains from coiled to extended configurations.

Figure 7 illustrates the energy dissipation characteristics of the friction systems by tracking the evolution of angle (Figure 7a), bond (Figure 7b), and dihedral (Figure 7c) energies within the slip layer (Z = 20–120 Å). Pristine PTFE consistently exhibits the highest internal energy, confirming its role as the primary phase for bearing and dissipating mechanical energy. In this system, PTFE chains absorb and dissipate external energy through continuous bond–angle vibrations, chain bending, and conformational transitions. Conversely, adding lubricant molecules reduces these internal energy contributions across all composite systems, effectively alleviating energy accumulation. The CNCs/OA/PTFE system shows the lowest internal energy levels. This suggests that the -COOH functional group suppresses excessive excitation and energy localization of polymer chains through strong interfacial interactions.

Regarding nonbonded interactions, this system also exhibits the highest van der Waals and Coulombic energies (Figure 7d,e). This highlights the advantage of -COOH groups in enhancing metal–polymer interfacial coupling and promoting uniform stress transfer. Meanwhile, the potential and kinetic energy profiles (Figure 7f,g) show that pristine PTFE maintains high energy levels with pronounced fluctuations, reflecting intense internal energy accumulation. In contrast, all lubricant–containing systems display substantially lower potential and kinetic energies with dampened fluctuations. This comparison demonstrates that lubricants effectively redistribute localized shear stress, allowing mechanical energy to dissipate within the slip layer in a more homogeneous and stable manner, thereby enhancing tribological stability.

Based on the above simulation results, the correlation between molecular–scale interactions and macroscopic friction and wear behavior can be rationalized in terms of interfacial shear stability. Molecular dynamics simulations reveal that the −COOH functional group exhibits high interaction energy and strong hydrogen–bonding capability, thereby acting as a “molecular anchor” that effectively bridges the polymer matrix and the metallic counterpart. From an engineering perspective, this anchoring effect enhances the critical shear strength of the interfacial boundary film. Under high load conditions, such a mechanism suppresses the extrusion of lubricant molecules from the contact region, thereby maintaining a continuous and stable protective film and preventing direct metal–polymer contact. This provides an atomistic explanation for the markedly improved tribological performance observed in the system.

For practical composite systems, these findings offer clear engineering implications. In the design of lubricant additives for polymers such as PTFE, it is essential not only to consider the intrinsic low–shear characteristics of the molecules but also to emphasize their chemical affinity with both fillers and the polymer matrix. In this context, the −COOH functional group can be regarded as an effective interfacial “coupling agent” that enhances the interfacial bonding strength across multiphase systems, thereby providing a molecular–level design strategy for improving the durability and reliability of composites under harsh operating conditions

4. Conclusions

This study employs MD simulations to elucidate the atomic–scale lubrication mechanisms of molecules with different terminal functional groups at the Fe/PTFE sliding interface. The results demonstrate that the chemical nature of the terminal groups fundamentally governs interfacial adsorption, molecular mobility, and energy dissipation behavior.

Specifically, SA with a hydroxyl group exhibits strong initial interfacial enrichment; however, its limited mobility under shear leads to molecular confinement, restricting its ability to continuously accommodate interfacial deformation. In contrast, OA with a carboxyl group demonstrates a distinct “anchoring–buffering” mechanism. Although OA molecules retain relatively higher mobility, they simultaneously form abundant hydrogen bonds and strong van der Waals interactions with PTFE chains. This dual effect enables dynamic interfacial adaptation while effectively constraining polymer segment motion. As a result, the OA system suppresses local stress concentration and prevents excessive energy accumulation during sliding. Energy analysis further confirms that the CNCs/OA/PTFE system significantly reduces fluctuations in bond, angle, and kinetic energies within the slip layer, indicating a more homogeneous redistribution and efficient dissipation of shear–induced energy.

Overall, these findings reveal that the -COOH functional group provides a superior balance between interfacial anchoring and molecular mobility, leading to enhanced tribological stability. This work not only clarifies the microscopic origin of functional–group–dependent lubrication behavior but also offers theoretical guidance for the rational design of high–performance polymer–based lubricating systems.