Promising Solid Lubricant Research Articles

Spectroscopic Study of Strain, Oxidation, and Formation of Few-Layer Graphene at Steel Surfaces Tribologically Tested against MoS2 Films.

MoS2 not only has unique optoelectronic properties realizing photonic and semiconductor applications but also serves as a promising solid lubricant in tribological three-body contacts due to its advantageous friction and wear behavior. Its functionality is defined by elementary processes including strain, oxidation processes, and material mixing. However, these mechanisms were not elucidated for MoS2 having transferred from the MoS2 film synthesized at the main body to a steel counter body during tribological ball-on-disk tests. Using spatially and spectrally high-resolved Raman spectroscopy, we study the compressive and tensile strain within the MoS2 transfer material and analyze the oxidation of molybdenum, sulfur, and iron. In addition, we elaborate on the impact of transition metals modifying the MoS2 films on the strain distribution and oxidation processes. Decreasing intensities of the MoS2 Raman lines are accompanied with enhanced intensities of sulphur and molybdenum oxide Raman signatures which are particularly agglomerated at the edges of the tribological track. The formation of tribochemical oxides, including Mo4O11 in the Magnéli-phase, depends weakly on the type of modifying element, and an oxidation of a modification element itself is not detected. We also identified a tribologically induced formation of disordered few-layer graphene at counter-body surface areas which experienced weak thermo-mechanical tribological load. Our results characterize structural and chemical features of the MoS2 transfer material, thus predicting material failure at the microscopic level.

Maintaining the 2D Structure of MXene via Self-Assembled Monolayers for Efficient Lubrication in High Humidity.

MXene is considered as a promising solid lubricant due to facile shearing ability and tuneable surface chemistry. However, it faces challenges in high-humidity environments where excessive water molecules can significantly impact its 2D structure, thus deteriorating its lubricating properties. In this work, the self-assembled monolayers are formed on MXene by surface chlorination (MXene-Cl) and fluorination (MXene-F), and their friction behaviors in high/low humidity are investigated. The results indicate that MXene-F and MXene-Cl can maintain a relatively constant friction coefficient (CoF) (MXene-F ∼0.76, MXene-Cl ∼0.48) under both high (75%) and low (25%)-relative humidity (RH) environments. Meanwhile, the MXene-F and MXene-Cl display a lower CoF than the pristine MXene (MXene CoF∼1.18) in high humidity. The above phenomena are mainly attributed to the preservation of its 2D layered structure, the increased layer spacing, and superficial partial oxidation for SAMs-functionalized MXene under high humidity during friction. Interestingly, MXene-Cl with moderate water resistance has a lower CoF than that of MXene-F with complete water resistance. The nanostructured water adsorption capacity and larger interlayer spacing of MXene-Cl make it exhibit a lower CoF compared to MXene-F. The findings of this study offer valuable guidance for tailoring MXene by surface chemical functionalization as an efficient solid lubricant in high humidity.

Why is Superlubricity of Diamond-Like Carbon Rare at Nanoscale?

Hydrogenated diamond-like carbon (HDLC) is a promising solid lubricant for its superlubricity which can benefit various industrial applications. While HDLC exhibits notable friction reduction in macroscale tests in inert or reducing environmental conditions, ultralow friction is rarely observed at the nanoscale. This study investigates this rather peculiar dependence of HDLC superlubricity on the contact scale. To attain superlubricity, HDLC requires i) removal of ≈2nm-thick air-oxidized surface layer and ii) shear-induced transformation of amorphous carbon to highly graphitic and hydrogenated structure. The nanoscale wear depth exceeds the typical thickness of the air-oxidized layer, ruling out the possibility of incomplete removal of the air-oxidized layer. Raman analysis of transfer films indicates that shear-induced graphitization readily occurs at shear stresses lower than or comparable to those in the nanoscale test. Thus, the same is expected to occur at the nanoscale test. However, the graphitic transfer films are not detected in ex-situ analyses after nanoscale friction tests, indicating that the graphitic transfer films are pushed out of the nanoscale contact area due to the instability of transfer films within a small contact area. Combining all these observations, this study concludes the retention of highly graphitic transfer films is crucial to achieving HDLC superlubricity.

Structural superlubricity at the interface of penta-BN2.

Two-dimensional (2D) materials have been widely used as lubricants due to their weak interlayer interaction and low shear resistance for interlayer sliding. Composed entirely of five-membered rings, penta-BN2 monolayer has excellent thermal and mechanical stability, higher hardness and a negative Poisson's ratio. In this work, we investigate the frictional properties at both the commensurate and incommensurate contacting interfaces of penta-BN2 by adopting the molecular dynamics (MD) simulation method. Our calculations demonstrate robust superlubricity at the incommensurate contacting interface of penta-BN2. The ultra-low friction is explained by the potential energy surface (PES) fluctuations, interlayer binding energy and out-of-plane motion amplitude of the sliding layer. In addition, our calculations show that the anisotropy of friction at the commensurate contacting interface is more obvious compared with that at the incommensurate contacting interface. Finally, the influences of the size of the Moiré pattern, normal force, temperature and sliding velocity on the friction are examined. Our results show that 2D penta-BN2 is a promising solid lubricant, enriching the family of 2D lubrication materials.

Enhancing tribological performance of graphene sheet-embedded carbon films on steel substrates via texture engineering and chromium doping

Amorphous carbon (a-C) film is a promising solid lubricant coating due to its unique structure and excellent properties. However, its poor adhesion and long running-in period hinder the application in metal components. Graphene sheet-embedded carbon (GSEC) film has a significantly shorter running-in period but suffers from inferior tribological properties due to its rough surface and weak mechanical characteristics. In this study, we utilized an electron cyclotron resonance (ECR) plasma nano-surface manufacturing device to deposit GSEC films onto steel substrate surfaces. By manipulating the surface/interface texture and incorporating chromium (Cr) doping, the tribological properties of the GSEC films were improved both chemically and physically. The results demonstrated that the textured carbon film exhibited superior adhesion to the steel substrate and a wear life that was three orders of magnitude longer than the untextured GSEC film, under low normal load conditions while sliding against the GCr15-bearing steel ball. After doping the Cr into textured carbon films, there was a significant reduction in the friction coefficients, and the wear type of carbon film transformed from adhesive wear to abrasive wear. Moreover, the doped carbon films demonstrated prolonged wear lives under high normal loads. This improvement can be attributed to the formation of hard CrC nanocrystalline, which effectively reduced surface roughness and enhanced the mechanical properties of the carbon films. Notably, even under a contact pressure of 0.83 GPa, the doped carbon film maintained favorable tribological performances. These findings have extended the potential applications of carbon films on metal substrates.

Tribological behavior of few-nanometer-thick MoS2 prepared by low-temperature conversion of atomic layer deposited MoOx films

Molybdenum disulfide (MoS2) is a popular lamellar material with desirable properties who's form, and function can vary widely, from particles to monolayer/μm thick films and applications in semiconductors to aerospace lubricants and many more. Physical vapor deposition (PVD) is commonly used to produce dense, conformal micron thick MoS2 coatings, but lacks the ability to coat more complex geometries due to line-of-sight constraints and typically exhibit amorphous or nanocrystalline microstructures. Atomic layer deposition (ALD) has also been employed to deposit monolayer to 10's of nanometers thick lamellar solids like MoS2 & WS2 in transistor, sensors or electrocatalyst applications but not commonly for applications as solid lubricants in aerospace like their PVD counterparts. While recent work has shown that ALD MoS2 can exhibit favorable microstructures for solid lubricant applications and allows for non-line of sight deposition on complex geometries, it has not been widely adopted due to high deposition temperatures (>500 °C) that can lead to softening of steel substrates. In this work, we show one of the first applications highlighting the use of ultra-thin (5–10 nm) low temperature (∼250 °C) MoS2 coatings as a promising solid lubricant for macroscale mechanical interfaces. Post-deposition chemical conversion of ALD MoOx to MoS2 by annealing in H2S at temperatures ranging from 200 to 550 °C resulted in highly-ordered basally-oriented surface microstructures. The results from this work suggest that converted-ALD MoS2 coatings can serve as viable solid lubricants for aerospace applications requiring low-temperature processing, including those with complex geometries.

Investigation of the Hardness Development of Molybdenum Coatings under Thermal and Tribological Loading

The increasing global demand for innovative and environmentally friendly lubricants can be met through the use of solid lubricants. By switching from conventional lubricants such as various oils or grease to solid lubricants, new scopes of application can also be opened up. The main requirements for solid lubricants are a reduction in the coefficient of friction (CoF) and an increase in wear resistance. Due to the favourable material properties, molybdenum (Mo) coatings fulfil the tribological requirements and are therefore promising solid lubricants which can be applied via physical vapour deposition (PVD). In this work, the impact of substrate temperature on the hot hardness of deposited Mo coatings was determined. The specimen with the highest hot hardness was then tribologically examined both at the micro and nano level. Through an analysis of the wear tracks by means of nanoindentation and scanning electron microscopy (SEM), it was possible to detect the influence of the tribological load separately from that of the thermal loads. The results showed that the tribological load influenced the Mo coating by significantly increasing its hardness. This was achieved due to the work hardening of the Mo layer leading to an increase in the wear resistance of the coating.

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.

Tribological behaviour of Ti3C2Tx nano-sheets: Substrate-dependent tribo-chemical reactions

MXenes, a newly emerging class of layered two dimensional (2D) materials, are promising solid lubricants due to their 2D structure consisting of weakly-bonded layers with a low shear strength and ability to form beneficial tribo-layers. This work aims at evaluating for the first time MXenes lubrication performance and tribofilm formation ability on different metallic substrates (mirror-lapped Fe and Cu discs). After depositing MXenes via ethanol (1 wt%) on the substrates, pronounced differences in the resulting substrate-dependent frictional evolution are observed. While MXenes are capable to reduce friction for both substrates after the full evaporation of ethanol, MXenes lubricating effect on Cu is long-lasting, with a 35-fold increased lifetime compared to Fe. Raman spectra acquired in the wear-tracks of the substrates and counter-bodies reveal notable differences in the friction-induced chemical changes depending on the substrate material. In case of Fe, the progressive failure of MXenes lubrication generates different Fe oxides on both the substrate and the ball, resulting in continuously increasing friction and a poor lubrication effect. For Cu, sliding induces the formation of a Ti3C2-based tribofilm on both rubbing surfaces, enabling a long-lasting lubricating effect. This work boosts further experimental and theoretical work on MXenes involved tribo-chemical processes.

Copper ions cross-linking graphene oxide nanosheet coatings towards robust solid lubricants

Graphene oxide (GO) nanosheet coatings are considered as promising solid lubricants. They, however, still suffer short lifetime and low load capability, limiting their further mechanical engineering industrial application. In this work, copper ions are in-situ incorporated into GO nanosheet coatings by applying a low potential towards a copper substrate in a GO suspension. The generated copper ions not only induce chemical cross-linking of neighboring GO nanosheets via bounding to carboxylic acid groups at the edges of individual GO nanosheets but also reduces the C O functional groups on their basal planes. These factors facilitate retaining GO nanosheets with few structural deteriorations in the contact interface when copper covered with these coatings slides against steel ball. Consequently, the COF is as low as 0.17 even under a maximum Hertz contact pressure of 1.1 GPa, the durability is 10 times higher than the air-sprayed GO coating and the wear rate of the copper substrate is reduced more than 20 times compared with the bare copper substrate. • A low-cost and green method is reported to deposit graphene oxide (GO) coatings. • GO coatings show low friction coefficient, long durability and high-load capability. • Mechanism of improved tribological performance is explored via various techniques.

Repetitive nanoindenter scratch testing of polydopamine/polytetrafluoroethylene-based thin coatings

Repetitive nanoindenter scratch testing and scanning electron microscopy were used to evaluate the scratch resistance, deformation, and mechanisms of failure of polydopamine (PDA)/polytetrafluoroethylene (PTFE)-based coatings deposited on both polished and roughened 60NiTi substrates. PDA was used as an adhesive layer between the substrate and the PTFE top layer with and without 0.25 wt% graphite particles (GPs). The roughened substrate enabled mechanical interlocking with the coatings that prevented global delamination which occurred with the coatings on the polished substrate due to weak adhesion. The inclusion of GPs improved the coating cohesion and enabled the PDA/PTFE coating with GPs on the roughened substrate to compact into a more plowing-resistant layer with stronger interlocking with the substrate roughness. Therefore, the GP-containing coating deposited on a roughened substrate showed far superior performance, and it is a promising solid lubricant for tribological applications that involve repetitious sliding at high contact pressures, such as bearings and gears.

Iron ions induced self-assembly of graphene oxide lubricating coating with self-adapting low friction characteristics

Graphene oxide (GO) is a promising solid lubricant for moving mechanical systems to implement reliable and effective lubrication due to its easy modification and excellent tribological properties. However, obtaining highly dense GO coatings simultaneously with high hardness and extensibility is still a challenge for available self-assembly techniques, and these characteristics are beneficial to tribological properties. Inspired by the advanced abrasion resistance system of mussel byssus threads, a surprisingly simple and versatile metal ions (namely Fe2+ and Fe3+ ions) directed self-assembly strategy has been developed to rapidly fabricate GO coatings. GO as a solid lubricant is highly sensitive to the interface structure and test environment. Consequently, the influence of the valence state of iron ions on the macroscopic tribological properties of GO-based coatings in air, nitrogen gas and vacuum environments are comparatively investigated. The results show that the GO-Fe3+ coating with a highly compact characteristic and unique granular microarchitecture resulting in higher hardness and strain capacity displays a lower friction coefficient and remarkable wear resistance than the GO-Fe2+ coating. Additionally, because water molecules generate strong hydrogen bonding gradually disrupting the ordered layered structure, both GO-Fe2+ and GO-Fe3+ coatings exhibit a higher friction coefficient in air than in nitrogen gas and vacuum.

Tribo-chemical wear of various 3d-transition metals against DLC: Influence of tribo-oxidation and low-shear transferred layer

Diamond-like carbon (DLC) is a promising solid lubricant owing to its excellent anti-friction and wear-resistance characteristics. However, the high hardness of DLC causes two-body abrasion to its counterpart, which generally are ferrous and brass metals having a relatively low hardness, resulting in severe wear of counterpart. Therefore, it is required to solve the wear-imbalance issue, leading to noise and vibration in tribo-system. In this study, we aimed to investigate the wear behavior of counterparts sliding against DLC. We used four-3d-transition metals (Ti, Fe, Ni, and Cu) as the counterpart of DLC, based on the chemical properties for carbon and oxygen, i.e., the activation energy of oxidation and work of separation with carbon. Our finding is suggested that the wear of transition metals was governed by oxidative wear rather than mechanical wear (e.g., adhesive wear and abrasive wear). Moreover, their oxidative wear was prevented by carbon-containing tribofilm, which is transferred from DLC continuously. It revealed that severe oxidational wear was caused in copper and iron, because of their chemical properties.

Structural superlubricity in graphene/GaSe van der Waals heterostructure

Two-dimensional layered materials arise enormous research enthusiasm for achieving superlubricity. We theoretically study the atomic friction behaviors of the heterogeneous graphene/GaSe bilayer system under normal load and defect/functionalization effects through first-principles methods. The heterogeneous graphene/GaSe exhibits much lower friction and shear strength than the homogeneous graphene/graphene and GaSe/GaSe, and the introduced OH-functionalized graphene can further reduce the friction. The ultralow friction coefficients of 0.0033∼0.080 indicate the heterogeneous graphene/GaSe is a promising solid lubricant with superlubricity. The larger electrostatic repulsion caused by the incommensurate interface of the heterogeneous bilayer system and the less variation of the charge density differences along the whole sliding is responsible for the lower friction.

Potential application of graphene nanoplatelets as a high temperature lubricant for hot rolling

Graphene has been shown to be a promising solid lubricant to reduce friction and wear of the sliding counterparts, and currently is reported to only function below 600 °C. In this study, its potential as a lubricant above 600 °C was studied using a ball-on-disc tribo-meter and a rolling mill. Friction results suggest that a reduction up to 50% can be obtained with graphene nanoplatelets (GnP) under lubricated conditions between 600–700 °C when compared with dry tests. and this friction reduction can last more than 3 min. At 800 and 900 °C, the friction reduction is stable for 70 and 40 s, respectively, which indicates that GnP can potentially provide an effective lubrication for hot metal forming processes. Hot rolling experiments on steel strips indicate that GnP reduces the rolling force by 11%, 7.4%, and 6.9% at 795, 890, and 960 °C, respectively. These friction reductions are attributed to the easily sheared GnP between the rubbing interfaces. A temperature higher than 600 °C will lead to the gasification of the residual graphene on the strip surface, which is believed to reduce the black contamination from traditional graphite lubricant.

Tribological properties of defect-rich molybdenum disulfide / graphene oxide composite coating under various environments

It is said that molybdenum disulfide (MoS2)/graphene oxide (GO) composite coating is a promising solid lubricant due to their synergetic effect. However, there is few reports on how the content of MoS2 affects the tribological performance of the composite coating. In addition, considering the requirements of production and environmental protection, more types of MoS2 nanomaterials, except the commonly used two dimensional MoS2, are needed to be developed. In this work, defect-rich MoS2 nanosheets obtained by a facile hydrothermal method are proposed to combine with GO nanosheets to form a robust solid lubricant. The influence of defect-rich MoS2 content on the tribological properties of GO/MoS2 composite coating is studied via sliding tests under dry nitrogen and ambient conditions, and its mechanism is revealed by analyzing the composition and microstructure evolution of the contacting interface. The results have implications for developing GO based nanocomposite coatings with excellent lubrication and wear-resistance performance for industrial applications.

Bi2S3 and MoS2 Soft Coatings: A Comparative Study of Their Frictional Behavior Under Different Humidity Levels, Normal Loads, and Sliding Speeds

Bi2S3 is a promising solid lubricant for demanding applications, such as oil extraction, heavy mining, and aerospace components. However, its tribological behavior has not been studied in depth. Burnished Bi2S3 soft coatings were evaluated utilizing reciprocating sliding tests under different levels of relative humidity level, normal load, and sliding speed and the results were benchmarked against MoS2 coatings. In addition, ab initio calculations using density functional theory modeling were employed for studying the interaction between the sulfides and the steel substrates and the effect of water intercalation at an atomic level. It was found that Bi2S3 coatings present excellent environmental adaptability and substrate protection over a wide range of testing conditions, thanks to their characteristic crystal structure.

Atomistic insights into friction and wear mechanisms of graphene oxide

Graphene oxide (GO) is a promising solid lubricant used as a protective coating to reduce friction, while a detailed atomic scale understanding about its tribological behavior is required. In this study, ReaxFF reactive molecular dynamics simulations are used to explore the friction and wear mechanisms of GO. At low load, the structure of GO can be tailored via the tribochemical induced conversion from graphene oxide to graphene (GO-to-Gr), achieving low friction during the sliding process. The effects of multiple sliding and system temperature on the GO-to-Gr conversion are considered. At high load, wear of GO at the nanoscale is characterized by the emergence of the CC bond and CO bond breaking, both of which are related to the transformation from the epoxide groups to the ether groups. Furthermore, we show that decreasing the concentration of epoxide groups is a feasible method to enhance wear resistance of GO.

Electric resistance as a sensitive measure for detecting graphene wear during macroscale tribological tests

Super-high mechanical strength and excellent lubrication property at the nanoscale render graphene a promising ultra-thin solid lubricant. However, macroscale friction tests have shown that wear of graphene often occurs resulting in failure of interface lubrication. As graphene coatings are atomically thin, a sensitive and reliable detection method for monitoring wear has been a great challenge to date. In this work, we performed tribological experiments on monolayer graphene on SiO2/Si while simultaneously measuring the in-plane electric resistance of graphene during the sliding tests. Our experiments showed that the electric resistance exhibited a sudden increase signifying the onset of graphene wear well before the macroscale friction increased rapidly. The variation of in-plane electric resistance was found to depend on the wear track length and relative orientation. Longer wear track and wear track perpendicular to the current would typically lead to higher increase in graphene electric resistance. Our work demonstrates that monitoring in-plane electric resistance can be a simple, sensitive and predictive method for detecting graphene wear, which makes it a useful inspecting technique for a wide range of mechanical applications.

A comparison study between the Lennard-Jones and DRIP potentials for friction of graphene layers

Graphene is a promising solid lubricant, in particular for small-length scale devices such as nano/micro-electro-mechanical systems. Atomistic simulations such as molecular dynamics is a popular tool to study the frictional behaviors of graphene layers and it is of critical importance to accurately describe the interlayer interactions in order to give a reliable prediction on the friction of graphene. In this study, the interlayer interactions between graphene layers are examined by using two interatomic potentials, Lennard-Jones (LJ) potential and dihedral-angle-corrected registry-dependent interlayer (DRIP) potential, in the molecular dynamics simulations of friction sliding of multilayer graphene structures. While both potentials have the identical attractive interactions, DRIP models the repulsive interaction by registry-dependent modifiers considering transverse distance and dihedral angle. The sliding simulations mimic the atomic force microscope experiment and are carried out in the zigzag (ZZ) and armchair (AC) directions. The simulation results reveal that the friction forces of the DRIP models are about one order of magnitude larger than those of the LJ models in both sliding directions. It turns out that the modification of the repulsive term in DRIP introduces additional energy corrugations which increase the friction force. Moreover, it is found that the sliding direction is another important factor on friction force of graphene layers so that in both LJ and DRIP models the friction force in the AC sliding direction is about two to three times larger than that of the ZZ sliding.