Promising Solid Lubricant Research Articles

Graphene-induced reconstruction of the sliding interface assisting the improved lubricity of various tribo-couples

Graphene has emerged as one of the most promising solid lubricants owing to their exceptional lubricity. The atomically thin nature and its ability to conformally adsorb on the sliding surfaces provide unprecedented pathways for modifying the friction and wear behaviors of the mechanical moving parts. Here, the tribological responses of several representative tribo-couples including bare steel, diamond-like carbon and ceramic materials are investigated to explore the potentials of graphene as surface modifier. Specific emphasis is devoted to the graphene-induced reconstruction of the sliding interface and the growth mechanism of nanostructured tribofilms formed on the contact surface using high-resolution microscopic technique. The results reveal the unique properties of graphene regarding the friction reduction and wear protection irrespective of the types of counterpart materials, graphene-processed methods, dry or humid tests environments. Nevertheless, the interfacial features and the bonding characteristics of the tribofilms are diversified in each specific rubbing case, demonstrating the distinguished adaption capacity of graphene to the tribo-testing conditions. The present findings shed light on the lubrication phenomenon of graphene at the microscale and may provide useful design criterion for 2D-based solid lubricants.

Elucidating the atomic mechanism of the lubricity of graphene on the diamond substrate

Graphene is a promising solid lubricant for diamond-based MEMS/NEMS devices to implement reliable and effective lubrication. While, few available studies focus on nanoscale friction behaviors of graphene on the diamond substrate and the underlying mechanism behind such lubricity remains unclear. Here, we demonstrated in a set of AFM tests that applying a layer of single layer graphene (SLG) onto the polycrystalline diamond substrate results in a reduction of the coefficient of friction by an order of magnitude and elimination of the anisotropy of friction coefficient. We elucidated the atomic mechanism behind such lubricity performance in systematical classical molecular dynamics simulations. It is found the friction of the SLG coated diamond surface is determined by the distribution of atomic forces acted on the atoms of the Si tip surface and positively correlated with the maximum value of these atomic forces. The atomic mechanism of the lubricity of graphene on the diamond substrate can be explained by the balanced atomic force distribution and reduced maximum atomic force on the mated tip with graphene on the sliding interface. We believe that the findings achieved in the present study would promote the application of graphene as a solid lubricant for diamond-based MEMS/NEMS devices.

Fluorinated Graphene: A Promising Macroscale Solid Lubricant under Various Environments.

Graphene-related materials are promising solid lubricants owing to their easy shear between lattice layers. However, the coefficient of friction (COF) of graphene is not sufficiently low at the macroscale, and the lubrication performance is largely restricted by the external environment. In this study, we fabricated a fluorinated graphene (FG) coating on a stainless-steel substrate by a simple electrophoretic deposition in ethanol. The FG coating exhibited an excellent lubrication performance, which reduced the COF by 54.0 and 66.2% compared to those of pristine graphene and graphene oxide coatings, respectively. The lubrication enhancement of FG coating is attributed to its extremely low surface energy and interlaminar shear strength. The formation of ionic metal-fluorine chemical bonds provided a robust solid tribofilm and transfer layer on the friction pairs, which further increased the lubrication performance of the FG coating. The limited influence of the humidity on the lubrication performance of the FG coating is attributed to the hydrophobicity of the FG nanoflakes, which could prevent the influence of water molecules on the sliding interface. The excellent lubrication performance and better environmental adaptability of the FG make it a promising solid lubricant for mechanical engineering applications.

Prediction of Macroscopic Properties of Diamond-like Carbon from Atomic-Scale Structure

The macroscopic mechanical properties of diamond-like carbon (DLC), which is a promising solid lubricant and protective coating, are directly determined by the atomic-scale structure. Understanding the relation between the atomic-scale structure and the macroscopic properties is necessary and helpful for the improvement of DLC; however, experimental investigations of the atomic-scale structure and macroscopic properties require huge time and economic costs, limiting the progress of elucidating their correlation. In this work, the atomistic simulation approach was used to study the atomic-scale structure of DLC and its effect on the Young’s modulus of DLC. Structural analysis of DLC showed that the sp and sp2 hybridizations decrease while the sp3 hybridization increases with increasing density and hydrogen concentration of DLC. The degree of graphitization of DLC was further evaluated and found to be linearly proportional to the sixth power of the sp2 ratio. Then, it was demonstrated that Young’s modulus of DLC can be singly predicted by the effective coordination number (CNeff), which is defined as the average coordination number of carbon atoms without the contributions from hydrogen atoms. Furthermore, this work successfully proposed quantitative relations among CNeff, density, and Young’s modulus, which were verified by comparison to experimental results.

Proposal of a new formation mechanism for hydrogenated diamond-like carbon transfer films: Hydrocarbon-emission-induced transfer

Diamond-like carbon (DLC) is one of the most promising solid lubricants for sliding against other materials, such as steel, alumina, and silicon carbide (SiC). During sliding, a DLC transfer film is usually formed on the counterpart surface, affording a low friction coefficient. It is well known that hydrogen in DLC strongly promotes the formation of the DLC transfer film. To further improve the lubricity of DLC, we investigate the formation mechanisms of the DLC transfer film on amorphous SiC and the influence of hydrogen on transfer film formation using reactive molecular dynamics simulations. In addition to the conventional transfer mechanism induced by surface adhesion, we herein propose the new transfer mechanism of “hydrocarbon-emission-induced transfer”. In the proposed transfer mechanism, hydrocarbon molecules are emitted from the DLC surface and subsequently adsorb on the counterpart surface during the continuous grinding of the sliding interface, ultimately generating the DLC transfer film. Furthermore, the addition of hydrogen atoms to DLC slightly increases the adhesion-induced transfer and greatly accelerates the “hydrocarbon-emission-induced transfer”, collaboratively contributing to substantial DLC transfer film formation. Thus, we suggest that the experimentally observed promotion of DLC transfer film formation by hydrogen is largely attributable to our proposed mechanism of “hydrocarbon-emission-induced transfer”.

Study on the effectiveness of WS2 and CaF2 on the performances of self-lubricating micro impregnated diamond bits

Self-lubricating micro impregnated diamond bits (IDBs) were manufactured for the first time via hot-pressing powder metallurgy to investigate the effectiveness of two promising solid lubricant additives, WS2 and CaF2, in improving IDBs' self-lubricating and drilling performances through laboratory drilling test and micro analyses. It was found that, with increase in lubricant concentration, the rate of penetration (ROP) of WS2 IDBs increased exponentially, while that of the CaF2 IDBs decreased monotonically from its maximum value at 6 vol% CaF2 concentration. For WS2 IDBs, the increase in ROP was at the sacrifice of substantial deterioration of IDBs' service life, while the service life of CaF2 IDBs gradually increased to surpass that of lubricant-free IDBs at 8 vol% CaF2 addition and continued to grow. Micro analyses, including measurement of diamond protrusion height and micro morphological analysis of IDBs' worn surface, were conducted using a confocal laser scanning microscopy (CLSM). The 10 vol% CaF2 IDBs were found under good protection from either matrix or diamond wear. By contrast, the surface of the 6 vol% WS2 IDBs suffered severe wear. Based on CLSM images, it was inferred that the high ROP and low service life of WS2 IDBs were a result of enhanced three-body abrasive wear by breakaway diamond grits. In general, self-lubricating IDBs with 10 vol% CaF2 addition acquired best overall performances in that they achieved longest service life while having higher ROP than lubricant-free IDBs.

Impacts of the substrate stiffness on the anti-wear performance of graphene

Owing to its excellent mechanical and tribological properties, graphene has been proposed to be a promising atomically-thin solid lubricant for engineering applications. However, as a typical two-dimensional (2D) material, graphene has an exceptionally high surface-to-volume ratio and is very susceptible to the surrounding environments. By performing nanoscale scratch tests on graphene deposited on four different substrates, we have shown that the anti-wear performance of graphene, characterized by the maximum load carrying capacity, is not an intrinsic material property. Instead, its value is significantly affected by the stiffness the substrates: Stiffer substrate typically results in a higher load carrying capacity. As revealed by finite element simulations, stiffer substrate can effectively share the normal load and reduce the in-plane stress of graphene by limiting graphene deformation, which enhances the overall load carrying capacity. In addition to the load sharing mechanism, the experimental results also suggest that the frictional shear stress during scratch tests may facilitate wear of graphene by lowering its equivalent strength. The deformation mechanism of graphene/substrate systems revealed in this work provides guidelines for optimizing the mechanical performance of 2D materials for a wide range of tribological applications.

Origin of low friction for amorphous carbon films with different hydrogen content in nitrogen atmosphere

Abstract To date, hydrogenated amorphous carbon(a-C:H) film is a promising solid lubricant to meet growing needs for energy-saving and environmentally friendly. Here, friction behaviors of four a-C:H films with different hydrogen content in high purity N2 atmosphere are compared and they all behave low friction and wear rates. Detailed experimental studies reveal that N2 atmosphere-induced graphited transfer films and hydrogen passivation have synergic effects on low friction for different a-C:H films. Besides, N2 molecule adsorbed on the sliding interfaces due to π-π∗, -C-H···π and O···π interactions also accounts for low friction in N2 atmosphere. This work will provide a new pathway for reducing friction and wear by applying gas lubrication to engineering fields.

Upcycling of Spent Lithium Cobalt Oxide Cathodes from Discarded Lithium-Ion Batteries as Solid Lubricant Additive.

This work provides an alternative solution to the challenge of battery recycling via the upcycling of spent lithium cobalt oxide (LCO) as a new promising solid lubricant additive. An advanced solid lubricant mixture of graphene, Aremco binder, and recycled LCO was formulated into a spray with the use of excess volatile organic solvent. Numerous flat steel disks were spray-coated with the new lubricant formulation and naturally dried followed by curing at 180 °C. When tested on a ball-on-disk up to 230 m in distance, the composite new solid lubricant reduced the coefficient of friction (COF) by 85% between two steel surfaces compared to unlubricated surfaces under a constant 1 GPa Hertzian pressure in an ambient environment. The tribofilm composition, particle size, and type of contact are identified as important parameters in the improvement of the COF. Scanning electron microscopy was used to study its morphology, and energy dispersive X-ray spectroscopy was used to analyze the composition of pristine and tested tribofilms. Upcycled spent low value LCO powder was used as a lubricant additive in tribology for the first time with exceptional lubricious properties.

Nickel Aluminum Matrix Solid-Lubricating Composite Lubricated by Silver and Silver Vanadate Formed by Tribochemistry at Elevated Temperature

The synergistic effect of solid lubricants plays a significant role in wide-temperature-range lubrication, where the combination of lubricious oxide and Ag is the promising solid lubricants. In this paper, the friction and wear performances of Ni3Al with the addition of Ag and V2O5 solid-lubricating composites were evaluated from room temperature to 1000 °C. It was found that Ni3Al matrix composite with the addition of V2O5 has high friction coefficient of 0.3–0.7, while Ni3Al matrix composite with simultaneous addition of Ag and 2 wt % V2O5 has a relatively low friction coefficient of 0.25–0.4 between room temperature and 1000 °C and wear rate with the magnitude of 10−5 mm3/N m at high temperatures. The results revealed that nickel aluminum matrix solid-lubricating composite lubricated by silver and in situ formed silver vanadate at elevated temperature achieves a wide-temperature-range lubrication, which is attributed to the synergistic action of silver and silver vanadate formed at high temperatures.

Three-dimensional graphene nanosheet films towards high performance solid lubricants

Graphene nanosheet films (GNSF) show great potential as solid lubricants. They, however, usually suffer from short lifetime and low load capability, limiting their further engineering application. In this work, we try to address these crucial challenges, for the first time, by inducing a unique three-dimensional microstructure into the GNSF. The as-prepared three-dimensional graphene nanosheet films (3D-GNSF) are composed of many crumpled and partially standing graphene nanosheets (GNS) that overlap with each other and form a three-dimensional interconnected network. The morphology, composition and structure of the 3D-GNSF are investigated in detail using scanning electron microscope, X-ray diffractometer, transmission electron microscope and X-ray photoelectron spectroscopy to deduce their possible formation mechanism. Tribological tests are conducted in air as a function of contact pressure that is up to about 1GPa. The results suggest it is the unique architecture of the 3D-GNSF that quickly produces a compact and stable sliding interface consisting of GNS against GNS, enabling 3D-GNSF as promising solid lubricants with low friction, excellent anti-wear ability, good durability and high load capability.

Probing the difference in friction performance between graphene and MoS2 by manipulating the silver nanowires

Graphene and molybdenum disulfide (MoS2) are promising solid lubricants to deal with the interfacial friction and adhesion concerns in silver nanowires (Ag NWs)-based nanodevices, but their difference in friction performance is seldom considered. Here, the difference in friction performance between graphene and MoS2 has been comparatively studied by manipulating the Ag NWs on graphene and MoS2 surfaces through atomic force microscopy (AFM) tip-on-side and tip-on-top manipulations. The tip-on-side and tip-on-top manipulations demonstrate the atomically thin MoS2 has better friction performance than graphene. The tip-on-top manipulation further shows the shear strength in NW–MoS2 interface is smaller than in NW–graphene interface. The underlying mechanism for the friction difference between the two interfaces is attributed to the different interfacial adhesion interactions. The relatively small adhesion interaction in NW–MoS2 interface leads to a small interfacial shear strength, resulting in a small friction when the NW slides on MoS2 surface. The measured water contact angles, calculated work of adhesion and the adhesion force directly measured by AFM tip confirm the relatively small adhesion interaction in NW–MoS2 interface. These findings suggest that MoS2 may be more appropriate as lubricants for application in the NW-based nanodevices.

A comprehensive study on electrophoretic deposition of a novel type of collagen and hexagonal boron nitride reinforced hydroxyapatite/chitosan biocomposite coating

A novel family of Hydroxyapatite/Chitosan/Collagen/h-BN biocomposite coatings were successfully produced by electrophoretic deposition (EPD). A new type of polyelectrolyte consisting of water, ethanol and isopropyl alcohol was used for EPD process. Collagen (main ligament of human bone) and h-BN (a promising solid lubricant) has been used as supporting materials for biocomposite coatings. The effect of h-BN concentration in the EPD suspension has been investigated. Crystallographic, morphological, spectroscopical, corrosion protection performance, thermal behaviour, mechanical and tribological, topographical and in-vitro biocompatibility investigations have been carried out by XRD, FE-SEM, FTIR, Tafel extrapolation and EIS, TGA/DSC, nanoindentation and nanoscratch, SPM and 12-weeks of immersion into r-SBF, respectively. Corrosion protection performance and mechanical and tribological properties of the coatings have been improved with increase in h-BN concentration in deposition suspension up to 5 g·L−1 and worsen with further increase. The results are encouraging for in-vivo applications of HA/CTS/COL/h-BN biocomposite coatings.

Tribochemical reactions and graphitization of diamond-like carbon against alumina give volcano-type temperature dependence of friction coefficients: A tight-binding quantum chemical molecular dynamics simulation

Diamond-like carbon (DLC) is a promising solid lubricant used as a protective coating to reduce friction against alumina. Friction properties of DLC/alumina are strongly affected by temperature. To improve the friction performance of DLC, we investigate the friction behaviors of DLC/alumina at various temperatures and reveal the mechanisms by using our tight-binding quantum chemical molecular dynamics method. We observe an interesting volcano-type temperature dependence of friction coefficients in our friction simulations. Friction coefficients of DLC/alumina are low and show little change at 300–600 K because no tribochemical reactions occur at the interface. However, as the temperature increases, friction coefficients increase at 600–800 K and subsequently decrease at 800–1000 K. At 600–800 K, interfacial C-O and C-Al bonds between two substrates are formed during friction, leading to a high friction coefficient. Interestingly, further increment of temperature to 800–1000 K induces the graphitization of DLC. The graphite-like surface suppresses the interfacial bond formation, reducing the friction coefficient. We reveal that the volcano-type temperature dependence of friction coefficients is due to the tribochemical reactions generating interfacial bonds at 600–800 K and the graphitization of DLC reducing the number of interfacial bonds at 800–1000 K.

Characterization and tribologic study in high vacuum of hydrogenated DLC films deposited using pulsed DC PECVD system for space applications

This paper focuses on diamond-like carbon (DLC) films for space applications. Specifically, it reports the structure, morphology, adhesion, and the high-vacuum tribological performance of several DLC films with different hydrogen content. DLC films have been studied as a promising solid lubricant since liquid lubricants are ineffective and undesirable for many space applications. The films were deposited by pulsed Direct Current Plasma Enhanced Chemical Vapor Deposition (DC PECVD) technique with an additional cathode. An amorphous silicon interlayer was deposited in order to guarantee the adhesion between DLC coating and substrate. For the films characterization, Raman spectroscopy, Scanning Electron Microscopy (SEM), Rockwell C indentation test, and Ion Beam analysis (IBA) were performed. Additionally, the friction coefficient was measured in high vacuum and atmospheric conditions. Results showed that the increase of the deposition voltage led to the decrease of hydrogen content of the films and the increase of DLC films hardness. Furthermore, films deposited at −200V, −300V, and −400V showed a decrease in their friction coefficients under high vacuum conditions when compared to their friction coefficients at atmospheric ambient conditions. Moreover, DLC films produced with the highest hydrogen content presented lower friction coefficients under high vacuum rather than in ambient conditions. These results demonstrated that DLC films deposited by the DC PECVD technique with additional cathode have tribological properties that enable their use in space applications. This work also validated that the use of an additional cathode represents an improvement of the DC PECVD technique. Consequently, films with higher adhesion, hardness, and hydrogen content can be produced.

Tribological properties of a synthetic mica-organic intercalation compound used as a solid lubricant

Solid lubricants have long been widely used to enhance lubricity and to achieve a longer die life in the field of tribological applications. Molybdenum disulfide (MoS2) is the most desirable solid lubricant because of its low friction coefficient, good anti-seizure ability stemming from easy cleavage and excellent adhesion to the metal surface. Although MoS2 performs well, it has a disadvantage in that its black appearance significantly stains the working environment. Therefore, non-black solid lubricants are strongly desired. Herein, we investigated the properties of a clay-organic intercalation compound as a solid lubricant. It is well known that some types of layered clay minerals have a cation exchange capacity and are capable of being intercalated with an organic cation between the silicate layers. This reaction is based on the ion exchange of exchangeable inorganic ions (Na+, K+, etc.) in the layered clay minerals. In this study, we examined the properties of synthetic mica, whose silicate layers were intercalated with a cationic organic compound (dioctadecyl dimethyl ammonium chloride). We evaluated the lubrication performance of the mica using a newly devised upsetting-ironing type tribometer. X-ray diffraction (XRD) results show that the distance between the silicate layers of the intercalated synthetic mica increased to 36.6Å from 12.6Å for a non-intercalated one. Owing to the increase in the interlayer distance, the lubrication coating containing the synthetic mica-organic intercalation compound showed a much improved lubrication performance than that of the non-intercalated one. These findings indicate that the lubricity of the clay minerals is highly dependent on the friction between the different silicate layers, and that the cationic organic compound incorporated into the interlayers can improve the lubricity. This result means that the clay-organic intercalation compound is a promising solid lubricant as an alternative to MoS2.

Tribological Properties of TiAl Matrix Self-Lubricating Composites Containing Multilayer Graphene and Ti3SiC2 at High Temperatures

An investigation is conducted on the unexplored synergistic effects of multilayer graphene (MLG) and Ti3SiC2 in self-lubricating composites for use in high-temperature friction and wear applications. The tribological properties of TiAl matrix self-lubricating composites with different solid lubricant additions (Ti3SiC2-MLG, MLG) are investigated from room temperature to 800°C using a rotating ball-on-disk configuration. Tribological results suggest the evolution of lubrication properties of MLG and the excellent synergistic lubricating effect of MLG and Ti3SiC2 as the testing temperature changes. It can be deduced that MLG has great potential applications as a promising high-temperature solid lubricant within 400°C, and a combination of MLG and Ti3SiC2 is an effective way to achieve and maintain desired tribological properties over a wide temperature range.

Chemical Basis of the Tribological Properties of AgTaO3 Crystal Surfaces

The chemical properties of a surface determine the friction and wear behavior of a material during sliding. In this article, we study the mechanisms underlying the sliding behavior of the AgTaO3 perovskite material, a promising high-temperature solid lubricant that presents excellent friction properties and is chemically inert. In particular, by employing a combination of molecular dynamics simulations and density-functional theory calculations, we show that the low friction of AgTaO3 at high temperature is explained by silver aggregation on the surface, which is enabled by the low energy barriers associated with silver migration. Two different surface terminations (AgO and TaO2) are studied, and we show that the migration barrier on the AgO surface is smaller, favoring silver aggregation, which affects both friction and wear. Regardless of the termination, the formation of soft silver clusters dominates the sliding behavior when enough energy (mechanical or thermal) is imparted to the surface.

Relationship between the structure of C60 and its lubricity: A review

AbstractBased on analyses of the calculations of quantum theories, such as electron charge density, the energy level distribution of orbitals, and the forces between C atoms and between the molecules in C60 crystals, the author concludes that C60 crystal is a promising solid lubricant. Furthermore, owing to the unique spherical structure of its molecules, high chemical stability, and the phase transition of the crystal fullerene C60 may have some particular tribological properties that are superior to traditional graphite and MoS2, especially under crucial conditions such as high vacuum, high pressure or in a humid environment. The tribological tests that have been reported show the lubricity of fullerene C60, and the effects of the lubricating properties of C60 on some metals are also predicted.

On the Lubrication Mechanism of SbSbS4

Recent investigations have shown that SbSbS4 is a promising solid lubricant. It exhibits outstanding extreme pressure (EP) performance, and good antiwear behavior under laboratory test conditions. An investigation was undertaken to identify the mechanism by which SbSbS4 functioned when used as a lubricant in the dry powder form and as an additive to oils and greases. Friction and wear behavior of SbSbS^ was investigated using several different wear tests carried out in air at temperatures ranging from 20°C to 500°C.