Nano-Tribological Analysis by Molecular Dynamics Simulation—A Review

Silicon carbide (SiC) is widely employed as a friction-pair material in high-load and extreme-service environments due to its superior wear resistance and high-temperature stability. However, continuum-based tribological models have limited capability in capturing interfacial deformation and material removal mechanisms at the atomic scale. In this work, molecular dynamics simulations were performed using LAMMPS and analysed/visualised with OVITO to investigate how temperature (300, 700, and 1100 K), penetration depth (10, 12, and 14 Å), and sliding velocity (0.2, 0.5, and 0.8 Å/ps) influence the tribological response of SiC. The results indicate that increasing temperature generally reduces both friction and normal forces, consistent with thermal softening and enhanced atomic mobility, while the friction coefficient shows only a weak and slightly non-monotonic dependence across the studied range. Penetration depth plays a dominant role: deeper indentation markedly increases tangential resistance and the friction coefficient, which is associated with an expanded plastic deformation zone and a thicker third-body debris layer. By contrast, under a fixed penetration depth of 10 Å, the average friction coefficient is largely insensitive to sliding velocity within 0.2–0.8 Å/ps, although higher velocity intensifies force fluctuations. These findings provide atomic-scale insights into the governing mechanisms of SiC friction and wear and offer guidance for performance optimisation under extreme operating conditions.

Self-assembled monolayer (SAM) films have attracted immense attention for both fundamental and applied research. A SAM is composed of a large number of molecules with a head group that chemisorbs onto a substrate, a tail group that interacts with the outer surface of the film, and a spacer (backbone) chain group that connects the head and tail groups resulting in a coating. Interactions between spacer groups of different molecules, such as van der Waals forces and/or hydrogen bonding, hasten SAM film formation and contribute to its stability. In this dissertation, SAM and thin films have been formed onto copper and aluminum oxide surfaces by reaction with 1H,1H,2H,2H-perfluorodecyldimethylchlorosilane (PFMS), 1H,1H,2H,2H-perfluorodecyltrichlorosilane (PFTS), 1H,1H,2H,2H-perfluorodecylphosphonic acid (PFDP), octylphosphonic acid (OP), decylphosphonic acid (DP), and octadecylphosphonic acid (ODP). The properties and stability of the films were investigated employing complementary surface analysis techniques: X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), friction force microscopy (FFM), a derivative of AFM, contact angle measurements (CAMs), and Fourier transform infrared reflection/absorption spectroscopy (FT-IRRAS). The perfluoroalkylsilane SAM on Cu is found to be extremely hydrophobic typically having sessile drop static contact angles of more than 130° for pure water and a surface energy of 14 mJ/m2 (mN/m). FFM showed a significant reduction in the adhesive force and friction coefficient of PFMS modified Cu (PFMS/Cu) compared to unmodified Cu. Treatment by exposure to harsh conditions showed that PFMS/Cu SAM can withstand boiling nitric acid (pH=1.8), boiling water, and warm sodium hydroxide (pH=12, 60 °C) solutions for at least 30 minutes. Furthermore, no SAM degradation was observed when PFMS/Cu was exposed to warm nitric acid solution for up to 70 min at 60 °C or 50 min at 80 °C. XPS and FT-IRRAS data reveal a coordination of the PFMS silicon (Si) atom with a cuprate (CuO) molecule present on the oxidized copper substrate. The data give good evidence that the stability of the SAM film on the PFMS modified oxidized Cu surface is largely due to the formation of a siloxy-copper (-Si-O-Cu-) bond via a condensation reaction between silanol (-Si-OH) and copper hydroxide (CuOH). For a PFTS modified Cu surface (PFTS/Cu), the sessile drop static contact angle of pure water has been measured to be more than 125° and the surface energy to be typically less than 16 mJ/m2. Stability tests show that the PFTS/Cu film can survive in boiling pure water for one hour, boiling nitric acid (pH 1.5 or 1.8) for 30 minutes, sodium hydroxide solution (pH 12, 70 °C) for 30 minutes, and autoclave conditions (steam at 134 °C and 3 atmospheres) for 15 minutes. The more commonly used self-assembled monolayer (SAM) modifications of Cu surfaces, e.g. thiol compounds, are significantly less stable than PFTS/Cu. Extremely hydrophobic (low surface energy) and stable PFMS/Cu SAMs and PFTS/Cu films could be useful as corrosion inhibitors in micro/nanoelectronic devices and/or as promoters for anti-wetting, low adhesion surfaces or drop-wise condensation on heat exchange surfaces. XPS analysis confirmed the presence of perfluorinated and non-perfluorinated alkylphosphonate molecules on the PFDP, DP, and ODP SAMs deposited at the aluminum oxide coated silicon (Al/Si) surfaces. The sessile drop static contact angle of pure water on PFDP SAMs was typically more than 130° and on DP and ODP typically more than 125° indicating that the phosphonic acid SAMs reacted with Al samples were very hydrophobic. The surface roughness for PFDP/Al, DP/Al, ODP/Al, and bare Al was approximately 35 nm, as determined by AFM. The surface energy for PFDP/Al was determined to be approximately 11 mJ/m2 by the Zisman plot method compared to 21 mJ/m2 and 20 mJ/m2 for DP/Al and ODP/Al, respectively. PFDP/Al gave the lowest adhesion and friction force while bare Al gave the highest. The adhesion and friction forces for ODP/Al and DP/Al SAMs were in between. ODP, DP, and OP SAMs have been studied in detail on relatively flat aluminum oxide surfaces. The rms surface roughness for ODP/Al, DP/Al, OP/Al, and bare Al was less than 15 nm, as determined by AFM. The sessile drop static contact angle of pure water on ODP/Al and DP/Al was typically more than 115° and on OP/Al typically less than 105°. The surface energy for ODP/Al and DP/Al was determined to be approximately 21 mJ/m2 and 22 mJ/m2, respectively, compared to 26 mJ/m2 for OP/Al. ODP/Al and OP/Al were studied by FFM to better understand their micro-/nano-tribological properties. ODP/Al gave the lowest coefficient of friction values while bare Al gave the highest. The adhesion forces for ODP/Al and OP/Al were comparable. The chemical stability of ODP/Al, PFDP/Al, DP/Al, OP/Al, and PFMS/Al SAMs has been inspected by exposure to warm nitric acid (pH 1.8, 30 min, 60-95 °C). The XPS data and stability against harsh chemical conditions indicate that a type of bond forms between a phosphonic acid (PA) or silane molecule and the oxidized Al surface. Stability tests using warm nitric acid (pH 1.8, 30 min, 60-95 °C) show ODP/Al SAMs to be most stable followed by PFDP/Al, DP/Al, PFMS/Al, and OP/Al SAMs. For PFTS/Al, stability tests demonstrate that modified aluminum is able to survive exposure to warm nitric acid (pH = 1.8, 60 °C, 30 min) indicating some degree of robustness. Hydrophobic, low adhesion, and robust aluminum surfaces have useful applications for micro/nano-electromechanical systems (MEMS/NEMS), such as digital micro-mirror devices (DMDs). These studies are expected to aid in the design and selection of proper lubricants for MEMS/NEMS.

Using molecular dynamics simulations and a coarse-grained model of ionic liquids (ILs), we study mechanisms of electrotunable friction measured in friction force microscopy experiments, where only one layer of IL is present between the tip and the electrode (substrate). We show that the variation of the friction force with the electrode surface charge density is determined by the regime of motion of the confined IL relative to the substrate and tip. The latter depends on the strengths of the ion–substrate and ion–tip interactions and on the commensurability between the characteristic ion dimensions and lattice spacings of the substrate and tip surfaces. Related with those factors, our simulations predict two strictly different scenarios for the variation of the friction force with the electrode surface charge. Revealing mechanisms of frictional energy dissipation in nanoscale IL films offers a way for controlling friction by tuning ion–substrate interactions and electrical polarization of sliding surfaces.

Molecular dynamics simulation with embedded atom method/Finnis–Sinclair potential function was utilised to study the friction properties of the friction-triggering process at the mesoscopic scale. The sliding friction process of a spherical probe with different diameters on the surface of a single-crystal aluminum matrix was simulated and the influences of sliding friction on the atomic behaviour of the surface under different contact conditions were analysed. The relationship between contact force and friction coefficient with spherical diameter was studied. The research shows that the plastic groove, normal force and friction force increase with the increase in probe diameter in the sliding friction process. The contribution of increasing spherical diameter to normal force is greater than that of friction force. The coefficient of friction varies with the ratio of the actual contact area to the normal force. After the running-in process is stable, the friction coefficient fluctuates around a certain value as theoretically verified by the two-term friction law. The increase in the diameter of the spherical probe leads to the increase in the number of dislocation atoms in the workpiece and the formation of dislocation rings, which result in the difference in the sliding friction process under indenters with different diameters. The results provide valuable reference for friction-triggering measurement based on 3D nanometre probes such as friction force microscopy.

INTRODUCTION Background History of Microscopy SCANNING PROBE MICROSCOPES (SPMS) Scanning Tunneling Microscope Electron Tunneling Atomic Force Microscope (AFM) Modes of Operation of AFM Simultaneous AFM and Scanning Near-Field Fluorescence (SNOM and SNOM-AFM) Friction Force Microscopy (FFM) STM and AFM Studies under Fluids Sample Preparation Procedures for STM and AFM Calibration and Image Analysis of STM and AFM Comparative Studies of Diverse Molecules by STM and AFM LIPID-LIKE MOLECULES ON SOLIDS AND SAMS Collapsed Lipid Monolayers (Self-Assembly) Domain Patterns in Monomolecular Film Assemblies Mixed Lipid Molecule Assemblies Holes in LB Films of Self-Assembly Monolayers Visualization of Vesicles by AFM LB Films of Liquid Crystals STM and AFM Studies of Diverse Molecules on Solids STM Studies on the Effect of Functional Group BIOPOLYMERS AND SYNTHETIC POLYMERS STRUCTURES BY STM AND AFM DNA Structures by STM and AFM SPM Studies of Three-Dimensional Protein Structures Protein Adsorption Studies by AFM Biological Macromolecular Structures Synthetic Polymers Studies by SPMs Mixed Monolayers of Macromolecules and Lipids Diverse Macromolecular Properties as Studied by SPMs Monolayers of Synthetic Polyamino Acids Biopolymer SAM Structures at Interfaces by STM and AFM CRYSTAL STRUCTURES BY STM AND AFM Crystal Structures of Small Molecules Surface Adsorption Studies by SPMs Macromolecule Crystals by STM and AFM STUDIES OF SOLID SURFACES BY SPMS Wetting Properties of Solid Surfaces AFM Analyses of Surface Acid-Base Properties Measurement of Attractive and Repulsive Forces by Atomic Force Microscope (AFM) DIVERSE APPLICATIONS OF SPMS (STM AND AFM, ETC.) AND NANOTECHNOLOGY STM and AFM in Organic Chemistry Semiconductor Study by SPM STM and AFM in Inorganic Chemistry Nanolithography and Nanomachining Qualitative and Quantitative Analyses at Nanoscale Application of SPMs under Dynamic Conditions Application of AFM to Immunodiagnostic Systems Applications of STM and AFM in Industry SPM Studies of Nanoscale Reactors Nanoscale Evaluation of Surface Roughness by SPMs Application of STM and AFM in Pollution Control Friction Force Microscope (FFM) Time-Resolved Analyses by STM REFERENCES

In order to dynamically clarify the friction mechanism between specimen surface and probe tip in the surface observation by atomic force microscope (AFM) or friction force microscope (FFM), several molecular dynamics simulations have been performed. In the simulation, a 3-dimensional simulation model is proposed where the specimen and the probe are assumed to consist of mono-crystalline copper and rigid diamond or carbon atom, respectively. The effect of cantilever stiffness of AFM/FFM is also taken into considered. The surface observation process on a well-defined Cu {100} is simulated. From the simulation results, it is verified that the surface images and the 2-dimensional atomic-scale stick-slip phenomenon just as observed through the real AFM/FFM surface observation can be detected from spring force acting on the cantilever. From the evaluation, it is clarified that the double slip and the dispersion in the stick-slip period occurs when the stiffness in the scan direction of the cantilever is low.

Grafted polymers inside cylindrical tubes in good and Θ solvents are systematically studied by numerical self-consistent mean-field theory (SCFT) and molecular dynamics (MD) simulations. We demonstrate that within the SCFT and at low enough concentrations the properties of these systems are fully determined by the overall polymer concentration plus two reduced parameters which are derived from the SCFT. Via a suitable change of variables, the results for very different systems can thus map onto universal curves. We show that MD and SCFT results at finite concentrations nearly obey these mappings as well. These findings allow for generalized predictions of the polymer properties such as concentration profiles, chain end distributions, and when there are polymer-free "open" channels at the center of a cylinder. This provides useful guidance for the development of micro- and nanofluidic devices.

Fretting wear is one of the most important failure forms of alloy 690 heat exchanger tubes in nuclear power plants. The key to understanding the fretting wear of alloys lies in the friction process, especially at the atomic scale. In this study, molecular dynamics simulations were performed on alloy 690 to investigate the nanoscale friction behavior and its influencing factors, laying a foundation for further understanding the fretting wear mechanism of alloy 690. The friction processes of a single-asperity (probe) on a smooth polycrystalline surface (matrix) were investigated by molecular dynamics simulations at the atomic scale, and the variation law of friction force during the friction process was calculated. The factors that affected the friction force were discussed, including the pressing depth, temperature, and sliding speed of the probe, and the friction force was positively correlated with the pressing depth and sliding speed of the probe, while the temperature had little effect on the friction force. Observations of the generation and evolution of dislocations during the friction process and related factors such as grinding grooves and wear debris were also reported.

On the deformation mechanism of SiC under nano-scratching: An experimental investigation

Several molecular dynamics simulations have been performed to clarify dynamically the contact mechanism between specimen surface and probe tip in the surface observation by an atomic force microscope (AFM) or friction force microscope (FFM). In the simulation, a three-dimensional simulation model is proposed where the specimen and the probe are assumed to consist of monocrystalline copper and rigid diamond or a carbon atom, respectively. The effect of cantilever stiffness of the AFM/FFM is also taken into consideration. The surface observation process is simulated on a well-defined Cu{100} surface. As a result, the influence of the probe tip shape on the force images was evaluated, and it was proven that the surface atomic images could be observed even when a flat tip probe was used due to the periodicity of the specimen surface.

We investigate the microscopic mechanism of energy dissipation in the friction force microscope (FFM), which is a modification of the atomic-force microscope for application to friction. Based on ab initio results for the interaction between Pd and graphite, we determine the atomic-scale modulation of the friction force and the corresponding stick-slip motion at the interface during the relative motion between these solids. We propose two idealized versions of the FFM and show that the friction force depends not only on the Pd-graphite interaction potential, but even more critically on the construction parameters of such a microscope.

In recent years, short peptide self-assembled materials, prepared under the control of the thermolysin catalyst, have been investigated extensively and shown to acquire various morphologies and fun...

Observation of the phase-separated Langmuir-Blodgett films on vapor-deposited silver films with atomic force microscopy and friction force microscopy

The surface microstructures of monolayers of polyamic acid alkylamine salts having long alkyl chains were analyzed by atomic force microscopy (AFM) and friction force microscopy (FFM). Two different aromatic structures of polyamic acids and four different single chain alkylamine structures were used for the preparation of polyamic acid alkylamine salt monolayers. Although single chain alkylamines tend to form an island-like domain structure, the homogeneous island-free surface structure of monolayers were observed for the long alkyl chain dimethylamine derivative. The island-like structure was formed by the coagulation of alkylamine during the film preparation process. It was found that the coagulation of alkylamine chains could be eliminated by the selection of the starting single chain alkylamine structure and by the conditions of monolayer deposition, such as surface pressure.

Friction plays a crucial role in both natural phenomena, from the flow of blood cells to earthquakes, and technological applications, from car engines to wind turbines. One of the most fundamental aspects of tribology is friction anisotropy, that is, the dependence of the friction force vector on the direction of sliding. Even when two identical surfaces slide against each other, differences in sliding direction can lead to significantly different friction forces. While atomistic and semi-empirical models provide a good understanding of friction and friction anisotropy at the molecular and macroscale levels, a comprehensive understanding at the nano- and microscale remains elusive. Unravelling the mechanisms of friction anisotropy at these intermediate length scales is crucial for bridging the gap between the current atomistic and macroscale models, as well as for advancing technological applications, such as nano-/micro-electromechanical systems (NEMS/MEMS). One experimental technique which has significantly contributed to the study of nano- and micro-scale friction anisotropy is scanning probe microscopy, particularly the form of atomic force microscopy (AFM). AFM enables investigations at the nanometre-scale resolution, providing at equilibrium topographical, physical, and chemical information of interfaces with atomic precision. AFM can also measure friction forces with pN-level sensitivity while operating in vacuum, air or fluid environments. The versatility of AFM and its applicability to both soft and hard materials have made it an indispensable tool for understanding the nano- and microscale mechanisms of friction. In this review, we examine the contributions of AFM-based techniques to the study of friction anisotropy. First, we summarise key AFM findings on friction anisotropy arising from the periodic corrugation of atomically flat crystals, quasicrystals, 2D materials and organic materials. In the second part, we explore friction anisotropy mechanisms influenced by the presence of solid or fluid adsorbates, such as polymers or liquid lubricants. By synthesising insights from the last 30 years of research, our review contributes to a deeper understanding of friction anisotropy, identifying common mechanisms across the diverse systems. This knowledge will not only refine theoretical models but also drive technological advancements, in a wide range of applications from biomedical devices and 3D printing to engines and advanced lubricant formulations.