Tribological Properties Of Graphene Research Articles

Improving Surface Wear Resistance of Polyimide by Inserting KH550 Grafted GO.

To improve the wear resistance of polyimide (PI), surface modification was developed. In this study, the tribological properties of graphene (GN), graphene oxide (GO), and KH550-grafted graphene oxide (K5-GO) modified PI were evaluated by molecular dynamics (MD) at the atomic level. The findings indicated that the addition of nanomaterials can significantly enhance the friction performance of PI. The friction coefficient of PI composites decreased from 0.253 to 0.232, 0.136, and 0.079 after coating GN, GO and K5-GO, respectively. Among them, the K5-GO/PI exhibited the best surface wear resistance. Importantly, the mechanism behind the modification of PI was thoroughly revealed by observing the wear state, analyzing the changes of interfacial interactions, interfacial temperature, and relative concentration.

Tribological Behavior and Wear Protection Ability of Graphene Additives in Synthetic Hydrocarbon Base Stocks

Graphene carbon materials show good tribological properties due to their unique layered structures. In this work, the tribological properties of graphene (GN) and fluorinated graphene (FGN) were studied in two kinds of synthetic hydrocarbon base stocks at different working conditions. Firstly, the structures of GN and FGN were characterized comparatively using FT-IR, Raman, XRD, and TGA. Secondly, the tribological properties of GN and FGN as the lubrication additives both in PAO6 and CTL6 were studied on a four-ball tester. Finally, the surfaces of friction counterparts, before and after tribological tests, were analyzed to disclose the lubrication mechanism using UV, micro-Raman, and EDS. The results show that GN and FGN can be stably dispersed in the selected synthetic hydrocarbon base stocks with 1 wt.% T161 as the dispersant, and the optimal addition of graphene additive is 100 ppm, which shows better friction reducing and anti-wear properties. GN and FGN also show better tribological performance at a higher load (not less than 392 N), and their compatibility with PAO6 is better. The worn surface analysis shows that the graphene additive participates in the lubrication film formation during friction by frictional chemical reaction with friction counterparts, which could improve the stability and tribological performance, resulting in an increased application temperature of synthetic hydrocarbon base stock by at least 10 °C.

Molecular Dynamics Simulation of the Thermomechanical and Tribological Properties of Graphene-Reinforced Natural Rubber Nanocomposites

The thermomechanical and tribological properties of graphene (GNS)-reinforced NR were investigated using molecular dynamics (MD) simulations. The amorphous molecular dynamics models of two nanocomposites, i.e., natural rubber (pure NR) and graphene/natural rubber (GNS/NR), were established. In addition, the thermodynamic properties of the two materials, before and after the incorporation of graphene into the natural rubber matrix, were investigated through analytical comparison. The results showed that after the graphene was added to the rubber matrix as a reinforcing material, the elastic modulus and shear modulus were increased by 110% and 94.8%, respectively, the tensile property was increased by 178%, the overall thermal conductivity of the composite system was increased by 59%, the glass transition temperature increased from 223 K to 236 K, and the rigidity of the material matrix was significantly improved. The inherent interactions and wear mechanisms of the polymer nanocomposites were discussed at the atomic scale by analyzing the changes in temperature, atomic velocity, relative atomic concentration, and radial distribution functions at the friction interface in the thickness direction.

Effect of N-doping on tribological properties of graphene by molecular dynamics simulation

Graphene has attracted a lot of attention due to its excellent electrical properties, however, the gapless nature of graphene limits its further applications. Doping is an effective way to open the bandgap, in which nitrogen-doped (N-doped) graphene has potential applications, but the study of its tribological properties is still lacking. In this work, the effects of nitrogen doping on the tribological properties of graphene under different interfacial structures are investigated by molecular dynamics simulation. The simulation models include a hexagonal graphene sheet, graphene or N-doped graphene substrate. The results show that the nitrogen doping has different effects on friction when interface structure is in a commensurate state and an incommensurate state. In a commensurate state, N-doping reduces the friction between interfaces in all cases, but the friction first goes up and then decreases with the increase of doping ratio of nitrogen. The local maximum value of friction occurs at a doping ratio of 7.5%. This phenomenon results from the interface structure and the change of van de Waals force between interfaces. The introduction of nitrogen atoms causes the lattice of graphene to distort, which results in the formation of local incommensurate state, thus reducing the interfacial potential barrier and friction. However, the van der Waals force between nitrogen atom and carbon atom between layers is stronger than that between carbon atoms and carbon atoms, which causes the friction to increase. When the doping ratio is low or high, lattice distortion plays more important role. The friction of N-doped graphene shows much smaller increase with load than that of ideal graphene, which indicates that the N-doped graphene possesses a better performance under high load. When the interface structure is in an incommensurate state, the introduction of nitrogen atoms shows slight influence on lattice mismatch, therefore, the change of atomic type plays a dominant role in determining the friction between interfaces, which goes up with the increase of N-doping ratio. When the substrate is graphene with vacancy defects, the value of friction between interfaces is larger than the ideal graphene substrate or N-doped graphene substrate, which indicates that the doping of nitrogen atoms has positive effect on reducing the friction of graphene with defects.

Molecular dynamics simulation to enhance the mechanical and tribological properties of polyimide composites by graphene reinforcement

Background: Polyimide is one of the organic polymer materials with the best comprehensive performance. It has outstanding mechanical properties, excellent thermal stability and excellent corrosion resistance, but pure polyimide has high coefficient of friction and wear rate. By combining graphene with polyimide, the mechanical properties of the composite are significantly reformatived, and the friction coefficient and wear rate can be reduced. Objective: The molecular models were developed to study the mechanical and tribological properties of graphene as a reinforced material. Methods: In this paper, the mechanical properties and friction and wear mechanism of materials are studied by molecular dynamics method from the microscopic point of view. The Young’s modulus and hardness of composites were calculated using the strain constant method. Results: Molecular dynamics simulation results expressed that the Young’s modulus and hardness of polymer composites benefited by approximately 115% and 42%, respectively, after the addition of the graphene-reinforced material. The average friction coefficient and wear rate of polymer composites fall by 35% and 48%, respectively. Through the calculation and statistics of the micro-information in the process of friction simulation, the internal mechanism of various situations is revealed in the atomic dimension. Conclusions: Graphene can adsorb on the surface of polymer chain segment, a strong polymer matrix, through van der Waals and electrostatic forces and can effectively resist external loading.

An investigation on the tribological properties of graphene and ZDDP as additives in PAO4 oil

The prime objective of this study was to extend the application of graphene additive in oil solution via investigating the frictional interaction of graphene and ZDDP composite additives in point contact lubrication. Accordingly, 0.05 wt% graphene, 1 wt% ZDDP and their mixtures as additives were added into PAO4 base oil. The ball-on-disk reciprocating experiments were carried out under the load of 30 N, 80 N, 155 N and 310 N. The tribological performance of the single system containing graphene and ZDDP were evaluated separately to compare the tribological property of the composite system containing graphene + ZDDP. The result revealed that the friction coefficient of the graphene/ZDDP composite system was slightly higher compared to PAO4 oil under high load. Whereas the composite system exhibited excellent wear resistance under all loads, reducing the wear rate by 77% in comparison to that of PAO4 oil under a load of 30 N. The reduction in wear is attributed to the formation of the graphene tribofilm and the protective polyphosphate film, which avoided the direct contact with the sliding surface and effectively inhibited the wear of steel materials.

Preparation and Tribological Properties of Graphene Oxide/Polydopamine-Derived Carbon Films on Silicon Substrate

In this study, dopamine and graphene oxide (GO) solutions were spin-coated on the silicon wafers to obtain the GO/polydopamine (PDA) films. The carbon films were subsequently fabricated by annealing the GO/PDA films in argon atmosphere. The atomic force microscopy analysis was performed to characterize the morphology and tribological properties of the composite films. The developed carbon films exhibited lower friction coefficient and high adhesion between composite films and substrate as compared to the untreated films, which was attributed to the enhanced hydrophobicity, improved hardness and elastic modulus and chemical bonding of the films with substrate. The findings obtained in this study would lead to an efficient design of the anti-wear nanofilms for the micro-/nanoelectronic mechanical systems.

Effects of grafting oxygen atoms on the tribological properties of graphene: Molecular dynamics simulation and experimental analysis

In this work, we investigated the effect of oxidation treatment on tribological properties of multilayered graphene using a combination of different characterization techniques and molecular dynamics (MD) simulation. The sliding friction of graphene and oxidized graphene (GO-1 h, GO-3 h and GO-6 h) were examined by conducting ball-on-disk tests. Tribological studies show that the friction coefficient (COF) and run-in period of graphene increase with the degree of oxidation, but its wear life tends to increase first and then decrease, and GO-1 h has the best value. The MD simulation results corroborate these observations and further reveal the reasons for these changes by studying the morphology, diffusion rate, interlayer distance, carbon skeleton and sliding distance of graphene and GO sheets during friction. The research mainly includes three aspects: 1. the effect mechanism of grafted oxygen atoms on the structure, COF and wear life of graphene; 2. the microstructure changes of graphene and GO flakes under different loads; 3. the influence of each oxygen-containing functional group on the tribological properties of graphene. Thus, our study provides a better understanding of the lubrication and failure mechanisms of graphene and GO and proposes optimized graphene structure with better tribological performances.

Effect of graphene concentration on tribological properties of graphene aerogel/TiO2 composite through controllable cellular-structure

The microstructure design of the packing is a crucial factor in determining the performances of sealing capacity. Graphene aerogel/TiO2 (GA/TiO2) composites were fabricated by an infiltration method using porous GA as a framework. GA with different pore sizes and morphologies was tuned by changing concentration of graphene. The structural morphology of the graphene layer structure transformed into porous structure as the graphene concentrations increased. In addition, mechanical properties and tribological behaviors of GA/TiO2 composites with different graphene concentrations were comparatively investigated. The results show that the TiO2 particles are more uniformly filled into the GA with higher graphene concentration due to the porous separation effect. Therefore, the GA/TiO2 composites of higher concentrations have a better elongation and mechanical properties. Simultaneously, the tribological behavior is excellent owing to the self-lubricating properties of graphene and tiny TiO2, and the TiO2 cluster can effectively prevent the crack propagation. Moreover, the tribological performance of the GA/TiO2 samples under wet friction conditions is better than that under dry friction conditions due to the self-transport operation of lubricants assisted by the through capillary action, besides the self-lubricating effect of graphene and tiny TiO2 particles.

Graphene Oxide/Silver Nanoparticle Coating Produced by Electrophoretic Deposition Improved the Mechanical and Tribological Properties of NiTi Alloy for Biomedical Applications.

The aim of this study was to evaluate the mechanical and tribological properties of graphene oxide/silver nanoparticle (GO/AgNP) coated medical grade nickel-titanium (NiTi) alloy. The alloy substrates were coated using electrophoretic deposition at 30 V for 1, 5, and 10 min and were characterized by SEM, Raman spectroscopy, EDS, and surface profilometer. Mechanical and tribological tests were performed for hardness, Young's modulus, and friction coefficient. The data were analyzed using the Kruskal-Wallis test at a significance level of 0.05 to compare the coatings' roughness, thickness, friction coefficient, and hardness at the different coating times. The GO/AgNP coatings were confirmed with Raman spectroscopy, which demonstrated the presence of D bands and G bands at ∼1300 cm-1 and ∼1600 cm-1. The intensity ratios of the D and G bands (ID/IG) were 0.838, 0.836, and 0.837 in the 1, 5, and 10 min coated groups, respectively. The coating thickness ranged from 0.46-1.34 μm and the mean surface roughness (Ra) ranged from 50.72-69.93 nm. Increasing the coating time from 1-10 min increased the roughness, thickness, and Young's modulus of surface coating. The friction coefficients of the coated NiTi alloy were significantly lower compared with that of the uncoated NiTi allloy (p < 0.001). The GO/AgNP nanocomposite coated NiTi alloy demonstrated improved mechanical strength and a reduced friction coefficient that would be more favorable for biomedical applications.

Influence of glycerol dispersions of graphene oxide on the friction of rough steel surfaces

The tribological properties of graphene have become an object of great scientific interest. However, the graphene oxide friction performance on rough surfaces remains comparatively unexplored. In this work, we focused on the effect of the multilayer graphene oxide (GO) dispersions in glycerol on the steel-steel tribosystem. Macroscale tribological tests were performed using a tribometer having a pin-on-disc configuration (normal loads 98 and 196 N, rotating speed 200 rpm). The wear samples were made of 40Cr steel with a roughness of about 110 nm. The study of the morphology of worn surfaces showed the dependence of lubricant efficiency on GO concentration. The use of 0.025 wt% GO led to the formation of a movable lubricating layer without deposition of tribofilm. The lowest friction coefficient was found at a concentration of 0.05 wt%. The decrease correlated with the formation of tribofilm on the steel surface. At high GO concentration (0.10 wt%), the agglomerated GO particles scratch the rubbing surfaces, and the friction coefficient increases significantly.

Tribological characteristics of graphene as grease additive under different contact forms

Abstract The tribological properties of graphene as grease additive were investigated under different contact forms and the worn surfaces were examined by X-ray photoelectron spectroscope (XPS) and Raman Spectrometer to reveal tribological mechanism. By the addition of graphene, the antiwear ability and friction reduction performance of grease were significantly improved. It is concluded that graphene can not only act as a deposition film to protect the substrate, but also stimulate formation of Fe2O3 and Li2O tribofilm. And the synergism of deposition film and tribo-chemical film significantly improve tribological properties of grease. In addition, the easy shear capability of graphene is prone to interlayer slip under the action of friction, thus decreasing the friction coefficient and promoting friction reduction performance of grease.

Self-Assembled Graphene Film as Low Friction Solid Lubricant in Macroscale Contact.

Promoted by the demand for solid lubricants, graphene has been proved to be a promising material for potential applications in reducing friction and wear. Here, a novel lubricating system where graphene sliding against graphene is developed to achieve low friction in macroscale contact. And the large area graphene film used here were prepared by a unique self-assembly technique based on Marangoni effect. Low friction coefficient of about 0.05 is obtained, and it is demonstrated that the film thickness, applied normal load and annealing process all have important influences on the tribological properties of graphene. The expedient fabrication procedure of large-area graphene film with excellent transferability and high-performance friction-reducing behaviors of the developed lubricating system both have a promising perspective in engineering applications.

Preparation and tribological properties of graphene oxide/nano-MoS2 hybrid as multidimensional assembly used in the polyimide nanocomposites

A three-step strategy was employed to prepare a self-lubricating and anti-wear graphene oxide/nano-MoS2 (GO/nano-MoS2, abbreviated GMS) hybrid by chemical compounding as a novel multidimensional assembly.

Cross-Linking-Induced Frictional Behavior of Multilayer Graphene: Origin of Friction

The tribological properties of graphene have attracted intensive attentions over the past few years. It was found that the frictional behavior of multilayer graphene was dependent on the multilayer thickness. However, the origin for such phenomenon is still under discussion. In this study, the mechanism of the thickness-dependent friction was explored based on molecular dynamics simulations of the scratching process of multilayer graphene. We found that the friction coefficient dropped dramatically as the number of layers increased under the same scratch depth. Further analysis of the graphene structure variation during the scratching process showed that the amount of the cross-linking decreased when the number of layers increased, which accounts for the dependence of the friction coefficient on the thickness. Finally, a novel mechanism was proposed that the thickness-dependent friction of multilayer graphene was caused by the formation of cross-linking between graphene layers. This study provides basic understanding of the origin of friction in multilayer graphene.

Graphene with outstanding anti-irradiation capacity as multialkylated cyclopentanes additive toward space application.

Multialkylated cyclopentanes (MACs), a class of synthetic hydrocarbon fluid have attracted intensive interest as possible space lubricants due to a series of unique physical and chemical properties. Here, we used graphene with high mechanical strength and chemical inertness as lubricant additive to explore its potential for space application. The effects of space irradiation on graphene and the tribological properties of graphene as lubricant additive were firstly investigated in detail under simulated space environment composed of high vacuum, high/low temperature and irradiation. Results demonstrate that graphene not only possesses outstanding anti–irradiation capacity but also significantly improves the space performance and tribological properties of MACs, which depends on the excellent physicochemical properties and high load-carrying ability of graphene as well as more effective separation of the sliding surfaces.

Influence of graphene coating on the adsorption and tribology of Xe on Au(1 1 1) substrate

The adsorption and tribological properties of graphene have received increasing attention for the further development of graphene-based coatings in applications. In this work, we performed first principles calculations with the inclusion of the nonlocal van der Waals correction to study the effect of graphene coating on the adsorption geometries, sliding frictions and electronic properties of Xe monolayer on the Au(1 1 1) substrate. The calculated activation energies indicate that Xe becomes movable on pure Au(1 1 1) surface at a temperature of around 30 K, whereas its motion can be activated only at a high temperature of ~50 K on graphene and on graphene-coated Au(1 1 1) substrates, in good agreement with recent experimental measurements by quartz crystal microbalance technique.

Frictional dependence of graphene and carbon nanotube in diamond-like carbon/ionic liquids hybrid films in vacuum

The use of graphene and multi-walled carbon nanotubes (MWCNTs) additives as lubricants has received considerable interest because of their excellent mechanical and frictional properties. Given their structural differences, both carbon nano-additives are expected to have different synergistic effects at various conditions (from boundary to mixed lubrication). For applications in space, the tribological properties of graphene and MWCNTs additives in diamond-like carbon/ionic liquids hybrid films in different lubricating states at high vacuum were compared. The wear surfaces, transfer films, wear debris, and microstructures of the hybrid films were analyzed via Raman spectroscopy, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy. The results showed that MWCNTs and graphene present the different nano-scale tribological mechanisms and produce different lubricating effect on the hybrid films at different lubricating states.

Nanotribological Properties of Fluorinated, Hydrogenated, and Oxidized Graphenes

Recently, the tribological properties of graphene have been intensively examined for potential applications in micro- and nano-mechanical graphene-based devices. Here, we report that the tribological properties can be easily altered via simple chemical modifications of the graphene surface. Friction force microscopy measurements show that hydrogenated, fluorinated, and oxidized graphenes exhibit, 2-, 6-, and 7-fold enhanced nanoscale friction on their surfaces, respectively, compared to pristine graphene. The measured nanoscale friction should be associated with the adhesive and elastic properties of the chemically modified graphenes. Density-functional theory calculations suggest that, while the adhesive properties of chemically modified graphenes are marginally reduced down to ~30 %, the out-of-plane elastic properties are drastically increased up to 800 %. Based on these findings, we propose that nanoscale friction on graphene surfaces is characteristically different from that on conventional solid surfaces; stiffer graphene exhibits higher friction, whereas a stiffer three-dimensional solid generally exhibits lower friction. The unusual friction mechanics of graphene is attributed to the intrinsic mechanical anisotropy of graphene, which is inherently stiff in plane, but remarkably flexible out of plane. The out-of-plane flexibility can be modulated up to an order of magnitude by chemical treatment of the graphene surface. The correlation between the measured nanoscale friction and the calculated out-of-plane flexibility suggests that the frictional energy in graphene is mainly dissipated through the out-of-plane vibrations, or the flexural phonons of graphene.

Tribological Properties of Graphene and Boron-Nitride Layers: A Fully Atomistic Molecular Dynamics Study

ABSTRACTGraphene has been one of the most important subjects in materials science in the last years. Recently, the frictional characteristics of atomically thin sheets were experimentally investigated using atomic force microscopy (AFM). A new mechanism to explain the enhanced friction for these materials, based on elastic compliance has been proposed. Here, we have investigated the tribological properties of graphene and boron-nitride (single and multi-layers) membranes using fully atomistic molecular dynamics simulations. These simulations were carried out using classical force fields, as implemented in the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) code. The used structural models contain typically hundreds of thousands of atoms. In order to mimic the experimental conditions, an artificial AFM tip was moved over the membranes and the tribological characteristics determined in terms of forces and energies. Our results are in good agreement with the available experimental data. They show that the observed enhanced tribological properties can be explained in terms of out-of-plane geometrical distortions and elastic waves propagation. They validate the general features of the model proposed by Lee et al. (Science 328, 76 (2010).