2D nano-materials beyond graphene: from synthesis to tribological studies

Recent Developments and Future Perspectives of Molybdenum Borides and MBenes

Metal borides have received a lot of attention recently as a potentially useful material for a wide range of applications. In particular, molybdenum-based borides and MBenes are of great significance, due to their remarkable properties like good electronic conductivity, considerable stability, high surface area, and environmental harmlessness. Therefore, in this article, the progress made in molybdenum-based borides and MBenes in recent years is reviewed. The first step in understanding these materials is to begin with an overview of their structural and electronic properties. Then synthetic technologies for the production of molybdenum borides, such as high-temperature/pressure methods, physical vapor deposition (PVD), chemical vapor deposition (CVD), element reaction route, molten salt-assisted, and selective etching methods are surveyed. Then, the critical performance of these materials in numerous applications like energy storage, catalysis, biosensors, biomedical devices, surface-enhanced Raman spectroscopy (SERS), and tribology and lubrication are summarized. The review concludes with an analysis of the current progress of these materials and provides perspectives for future research. Overall, this review will offer an insightful reference for the understanding molybdenum-based borides and their development in the future.

Review of two-dimensional nanomaterials in tribology: Recent developments, challenges and prospects

From our ordinary lives to various mechanical systems, friction and wear are often unavoidable phenomena that are heavily responsible for excessive expenditures of nonrenewable energy, the damages and failures of system movement components, as well as immense economic losses. Thus, achieving low friction and high anti-wear performance is critical for minimization of these adverse factors. Two-dimensional (2D) nanomaterials, including transition metal dichalcogenides, single elements, transition metal carbides, nitrides and carbonitrides, hexagonal boron nitride, and metal-organic frameworks have attracted remarkable interests in friction and wear reduction of various applications, owing to their atomic-thin planar morphologies and tribological potential. In this paper, we systematically review the current tribological progress on 2D nanomaterials when used as lubricant additives, reinforcement phases in the coatings and bulk materials, or a major component of superlubricity system. Additionally, the conclusions and prospects on 2D nanomaterials with the existing drawbacks, challenges and future direction in such tribological fields are briefly provided. Finally, we sincerely hope such a review will offer valuable lights for 2D nanomaterial-related researches dedicated on tribology in the future.

Properties and tribological evaluation of graphene and fullerene nanoparticles as additives in oil lubrication

In this study, SAE-0W20 engine oil was mixed with graphene and fullerene nanoparticles. The goal of this study was to evaluate and compare the effects of different carbon nanoparticles on the thermal, rheological, and tribological properties of engine oil, such as thermal degradation, viscosity, friction, and wear. Using a two-step process, graphene and fullerene nanostructures were dispersed in low-viscosity SAE-0W20 engine oil at a concentration of 0.05 wt.%. The friction and wear characteristics were evaluated in a customized cylindrical block-on-ring tribology test according to the ASTM G77 standard. Graphene and fullerene nanoparticles protect contact surfaces by forming a very thin protective film between moving mechanical parts thus resulting in wear and friction reduction. The results showed graphene nanoparticles have improved significantly the tribological performance of SAE-0W20 engine oil.

Tribology of Copper Metal Matrix Composites Reinforced with Fluorinated Graphene Oxide Nanosheets: Implications for Solid Lubricants in Mechanical Switches

The potential for the use of copper coatings on steel switching mechanisms is abundant owing to the high conductivities and corrosion resistance that they impart on the engineered assemblies. However, applications of these coatings on such moving parts are limited due to their poor tribological properties; tendencies to generate high friction and susceptibility to degradative wear. In this study, we have fabricated a fluorinated graphene oxide-copper metal matrix composite (FGO-CMMC) on an AISI 52100 bearing steel substrate by a simple electrodeposition process in water. The FGO-CMMC coatings exhibited excellent lubrication performance under pin-on-disk (PoD) tribological sliding at 1N load, which reduced CoF by 63 and 69%, compared to the GO-CMMC and pure copper coatings that were also prepared. Furthermore, FGO-CMMC achieved low friction and low wear at higher sliding loads. The lubrication enhancement of the FGO-CMMCs is attributed to the tribochemical reaction of FGO with the AISI 52100 steel counterface initiated by the sliding load. The formation of an asymmetric tribofilm structure on the sliding track is critical; the performance of the FGO/Cu tribofilm formed in the track is boosted by the continued fluorination of the counterface surface during PoD sliding, passivating the tribosystem from adhesion-driven breakdown. The FGO-CMMC and GO-CMMC coatings also provide increased corrosion protection reaching 94.2 and 91.6% compared to the bare steel substrate, allowing for the preservation of the long-term low-friction performance of the coating. Other influences include the improved interlaminar shear strength of the FGO-containing composite. The excellent lubrication performance of the copper matrix composite coatings facilitated by FGO incorporation makes it a promising solid lubricant candidate for use in mechanical engineering applications.

Interaction of Water and Oxygen Molecules with Phosphorene: An Ab Initio Study

Phosphorene, the 2D form of black phosphorus, has recently attracted interest for optoelectronic and tribological applications. However, its promising properties are affected by the strong tendency of the layers to oxidize in ambient conditions. A significant effort has been made to identify the role of oxygen and water in the oxidation process. In this work, we introduce a first-principles study of the phosphorene phase diagram and provide a quantitative estimate of the interaction of pristine and fully oxidized phosphorene layers with oxygen and water molecules. Specifically, we study oxidized layers with oxygen coverages of 25% and 50% that keep the typical anisotropic structure of the layers. We found that hydroxilated and hydrogenated phosphorene layers are both energetically unfavorable, leading to structural distortions. We also studied the water physisorption on both pristine and oxidized layers, finding that the adsorption energy gain doubled on the oxidized layers, whereas dissociative chemisorption was always energetically unfavorable. At the same time, further oxidation (i.e., the dissociative chemisorption of O2) was always favorable, even on oxidized layers. Ab initio molecular dynamics simulations of water intercalated between sliding phosphorene layers showed that even under harsh tribological conditions water dissociation was not activated, thus further strengthening the results obtained from our static calculations. Overall, our results provide a quantitative description of the interaction of phosphorene with chemical species that are commonly found in ambient conditions at different concentrations. The phase diagram that we introduced confirms the tendency of phosphorene layers to fully oxidize due to the presence of O2, resulting in a material with improved hydrophilicity, a piece of information that is relevant for the application of phosphorene, e.g., as a solid lubricant. At the same time, the structural deformations found for the H- and OH- terminated layers undermine their electrical, mechanical, and tribological anisotropic properties and, therefore, the usage of phosphorene.

Perspectives of 2D MXene Tribology

The large and rapidly growing family of 2D early transition metal carbides, nitrides, and carbonitrides (MXenes) raises significant interest in the materials science and chemistry of materials communities. Discovered a little more than a decade ago, MXenes have already demonstrated outstanding potential in various applications ranging from energy storage to biology and medicine. The past two years have witnessed increased experimental and theoretical efforts toward studying MXenes' mechanical and tribological properties when used as lubricant additives, reinforcement phases in composites, or solid lubricant coatings. Although research on the understanding of the friction and wear performance of MXenes under dry and lubricated conditions is still in its early stages, it has experienced rapid growth due to the excellent mechanical properties and chemical reactivities offered by MXenes that make them adaptable to being combined with other materials, thus boosting their tribological performance. In this perspective, the most promising results in the area of MXene tribology are summarized, future important problems to be pursued further are outlined, and methodological recommendations that could be useful for experts as well as newcomers to MXenes research, in particular, to the emerging area of MXene tribology, are provided.

Effect of copper oxide@boron nitride nanosheet hybrid nanocomposite on tribological properties of paraffin liquid

Abstract

Due to the significant friction-reduction and anti-wear properties of nano particle, nano copper oxide (CuO) particle decorating on boron nitride nanosheets (BNNS) nanocomposites are fabricated with polydopamine (PDA) as the linking agent. The structural and morphological characteristics of the CuO@BNNS composite are characterized by scanning electron microscopy (SEM), X-ray diffraction patterns, fourier transform infrared spectroscopy and thermogravimetric analysis. The synergistic effect of CuO and BNNS on extreme pressure of lubrication oil are investigated. A four-ball tribometer is adopted to investigate the tribological behaviors of the as-prepared oil with different additives. Optical microscope and SEM are used to analyze the topography of worn surface. Energy dispersive spectroscopy is adopted to investigate the element distribution on worn surface. The results demonstrate that the CuO@BNNS nanoparticle could effectively improve the friction-reduction and anti-wear properties of the paraffin liquid, compared to paraffin liquid containing modified CuO (f-CuO) or PDA-BNNS respectively. The worn surface of steel ball presents smooth morphology for oil containing CuO@BNNS, contrast to oil containing f-CuO or PDA-BNNS respectively. With the tribo-film formation on the worn surface, the elements distribute uniformly on the worn surface for oil containing CuO@BNNS. Compared to lower load, the effect of load-carrying capacity and easy-shear property of CuO@BNNS outstand under 392 N, which results in the smooth worn surface compared to oil containing other additives.

Article Highlights

CuO@BNNS hybrid nanoparticle was prepared with a simple and mild method.

CuO@BNNS presents superior anti-friction and anti-wear properties due to the synergistic effect of nanoparticle.

The result obtained in the paper promotes the application of h-BN based nanocomposites in tribology.

Impact of Graphene Nano-Additives to Lithium Grease on the Dynamic and Tribological Behavior of Rolling Bearings

In recent years, reduced graphene oxide (rGO) received considerable interest as a lubricant nano-additive for enhancing sliding and rolling contacts. This paper investigates the tribological and dynamic behavior of ball bearings lubricated by lithium grease at different weight percentages of rGO. Full bearing tests were conducted for experimental modal analysis, vibration analysis, ultrasonic analysis, and infrared thermography. Modal analysis indicated considerable improvements of the damping ratio values up to 50% for the bearings with rGO nano-additives. These findings were confirmed by the corresponding reductions in vibrations and ultrasound levels. The steady-state temperatures of bearings running with lithium grease reached 64 °C, whereas the temperature of bearings lubricated by grease with 2 wt.% rGO measured only 27 °C. A Timken Load test was conducted on grease samples with and without rGO additives. Grease samples having 2, 3.5, and 5 wt.% rGO showed the highest OK load with an increase of 25%, 50%, and 100% as compared to values of lithium grease. For comparison, all tests were conducted on samples of the same grease blended with graphite and MWCNTs’ nano-additives. The results proved the superiority of graphene in enhancing the load-carrying capacity and damping of grease in rolling bearings.

A triphenylamine-based aggregation-enhanced emission probe for viscosity and polarity analysis of lubricating oils

Lubricating oils offers abundant information about the operating state of machines. In this work, a donor-acceptor aggregation-enhanced emission fluorescent molecule, (Z)-4-(1-cyano-2-(4-(diphenylamino)phenyl)vinyl)benzonitrile (CPA-TPA), was synthesized to investigate its function to sense the viscosity and polarity of lubricant base oils. The results indicated that the molecule showed steadily-increased fluorescence emission with the base oil viscosity. In a mixture of poly-α-olefin/ester base oils, its optimal emission gradually red-shifted with the ester oil fraction, i.e., the polarity of the lubricant. A good correlation could be established between the emission intensity of the probe and the oil viscosity or between the optimal emission wavelength and the oil polarity. This demonstrates a dual-response fluorescent probe used for lubricating oil analysis and opens a new opportunity to develop a molecule-based method for the evaluation of lubricating oil quality.

Structural superlubricity in 2D van der Waals heterojunctions

Structural superlubricity is a fundamentally important research topic in the area of tribology. Van der Waals heterojunctions of 2D materials are an ideal system for achieving structural superlubricity and possessing potentially a wide range of applications in the future due to their ultra-flat and incommensurate crystal interfaces. Here we briefly introduce the origin and mechanism of structural superlubricity and summarize the representative experimental results, in which the coefficient of friction has achieved the order of 10^-5. Furthermore, we analyze the factors affecting structural superlubricity of 2D materials, including dynamic reconstruction of interfaces, edge effects, interfacial adsorption, etc, and give a perspective on how to realize the macroscopic expansion and where it can be applied in practice.

Environmental Molecular Effect on the Macroscale Friction Behaviors of Graphene

This study investigated the friction behavior of graphene in air and nitrogen atmosphere environments. The microstructural evolution caused by the variation of atmosphere environments and its effect on the friction coefficient of the graphene is explored. It is demonstrated that graphene can exhibit excellent lubricating properties both in air and nitrogen atmosphere environments. In air, a highly ordered layer-by-layer slip structure can be formed at the sliding interface. Oxygen and H₂O molecules can make edge dangling bonds and defects passive. Thus the interaction between the nanosheets and the layers of nanosheets is weak and the friction coefficient is low (0.06–0.07). While the friction coefficient increases to 0.14–0.15 in a nitrogen atmosphere due to the interaction of defects generated in the sliding process, the nitrogen molecules with lone pair electrons can only make the nanosheets passive to a certain degree, thus the ordered slip structure is destroyed and friction is higher. This work reveals the influence of environmental molecules on the macroscale tribological performances of graphene and its effect on the microstructure at the sliding interface, which could shed light on the lubricating performance of graphene in environmental atmospheres and help us to understand the tribological behaviors of graphite at the macroscale.

Recent Progress on Wear-Resistant Materials: Designs, Properties, and Applications

There has been tremendous interest in the development of different innovative wear-resistant materials, which can help to reduce energy losses resulted from friction and wear by ≈40% over the next 10-15 years. This paper provides a comprehensive review of the recent progress on designs, properties, and applications of wear-resistant materials, starting with an introduction of various advanced technologies for the fabrication of wear-resistant materials and anti-wear structures with their wear mechanisms. Typical strategies of surface engineering and matrix strengthening for the development of wear-resistant materials are then analyzed, focusing on the development of coatings, surface texturing, surface hardening, architecture, and the exploration of matrix compositions, microstructures, and reinforcements. Afterward, the relationship between the wear resistance of a material and its intrinsic properties including hardness, stiffness, strength, and cyclic plasticity is discussed with underlying mechanisms, such as the lattice distortion effect, bonding strength effect, grain size effect, precipitation effect, grain boundary effect, dislocation or twinning effect. A wide range of fundamental applications, specifically in aerospace components, automobile parts, wind turbines, micro-/nano-electromechanical systems, atomic force microscopes, and biomedical devices are highlighted. This review is concluded with prospects on challenges and future directions in this critical field.

Tribochemistry, Mechanical Alloying, Mechanochemistry: What is in a Name?

Over the decades, the application of mechanical force to influence chemical reactions has been called by various names: mechanochemistry, tribochemistry, mechanical alloying, to name but a few. The evolution of these terms has largely mirrored the understanding of the field. But what is meant by these terms, why have they evolved, and does it really matter how a process is called? Which parameters should be defined to describe unambiguously the experimental conditions such that others can reproduce the results, or to allow a meaningful comparison between processes explored under different conditions? Can the information on the process be encoded in a clear, concise, and self-explanatory way? We address these questions in this Opinion contribution, which we hope will spark timely and constructive discussion across the international mechanochemical community.

Recent development in friction of 2D materials: from mechanisms to applications

Two-dimensional (2D) materials with a layered structure are excellent candidates in the field of lubrication due to their unique physical and chemical properties, including weak interlayer interaction and large specific surface area. For the last few decades, graphene has received lots of attention due to its excellent properties. Besides graphene, various new 2D materials (including MoS₂, WS₂, WSe₂, NbSe₂, NbTe₂, ReS₂, TaS₂and h-BN etc.) are found to exhibit a low coefficient of friction at the macro- and even micro-scales, which may lead to widespread application in the field of lubrication and anti-wear. This article focuses on the latest development trend in 2D materials in the field of tribology. The review begins with a summary of widely accepted nano-scale friction mechanisms contain surface friction mechanism and interlayer friction mechanism. The following sections report the applications of 2D materials in lubrication and anti-wear as lubricant additives, solid lubricants, and composite lubricating materials. Finally, the research prospects of 2D materials in tribology are presented.

Investigation of the Tribological Behavior of Mineral Lubricant Using Copper Oxide Nano Additives

In this study, tribological properties of custom formulated and stabilized nano lubricant are investigated. Spherical CuO nanoparticles are suspended in 20W-50 mineral base lubricant using Oleic Acid (OA) as a surfactant. Three different nano lubricant concentrations with 0.2, 0.5, and 1 wt.% were analyzed through ASTM G-99 pin-on-disc tribometer standardized test under boundary/mixed lubrication regimes. The generated friction and wear analyses confirm a consolidation of tribological properties with a reduction in friction coefficient in the range of 14.59–42.92%, compared with the base lubricant. Analysis of worn surfaces (SEM/EDX) as well as (AFM) was conducted. Combined hypotheses were proposed from the analysis of worn surfaces; these hypotheses suggested that CuO nanoparticles exhibit an integrated effect of two phenomenal lubrication mechanisms. Additionally, dispersion stability evaluation of the suspended nanoparticles was performed through Zeta potential, (FTIR), and sedimentation analyses. Stability results showed that steric stabilization is the dominating effect of the repulsive forces between nanoparticles, surpassing the electrostatic repulsive forces.

Fast MoS$$_2$$ thickness identification by transmission imaging

Determining the thickness of a few-layer 2D material is a tough task that often involves complex and time consuming measurements. Here we discuss a rapid method for determining the number of layers of molybdenum disulfide, MoS$$_2$$ 2 , flakes based on microscopic transmission imaging. By analyzing the contrast of the red, blue and green channels of the flake image against the background, we show that it is possible to unequivocally determine the number of layers. The presented method is based on the light absorption properties of MoS$$_2$$ 2 and its validity is confirmed by micro-Raman measurements. The main advantage of this method against traditional methods is to quickly determine the thickness of the material in the early stages of the experimental process with low cost apparatus.