Ultra-low Friction Coefficient Research Articles

A facile method for fabricating super-slippery surface with long term and high-efficiency sustained release performance

Slippery liquid infused porous surfaces (SLIPS) process potential applications in surface liquid repellency, self-cleaning and anti-adhesion, while exhibiting long-term and stable release of lubricants is remaining a great challenge. Herein, an efficient liquid injection method was proposed to construct SLIPS by altering the surface energy and adjusting the inner porous structure using the facile method of direct UV irradiation. The infusion efficiency and release behavior were significantly improved and verified with various techniques such as wetting measurement, confocal fluorescence microscopy and optical microscopy. It was found that the infusion and release behavior of lubricants were varied for different specimens with different UV irradiation time. Compared to the prime substrate, PDMS treated with 90 min UV irradiation showed the highest volume of lubricant penetration which was increased almost by 833 %. Additionally, the amount of infusion consisted with the surface energy and specimens with moderate wetting exhibited the most effective lubricating performance. For the sample with 90 min UV irradiation, an ultra-low friction coefficient of 0.018 was observed under water lubrication and its releasing could be detected even after a long period of 768 h. Our results provided a facile method for the future development of long-term lubricating surfaces which may broaden their applications in fields of anti-corrosion, anti-icing and anti-fog, anti-fouling, micro-flow control and oil-water ultra-slippery occasions.

Toward ultralow friction and wear of ultrananocrystalline diamond film by formation of graphene nanosheets with a Cu catalyst

The present work has realized ultralow friction and wear of ultrananocrystalline diamond (UNCD) film through in-situ formation of graphene nanosheets by introducing Cu catalyst. UNCD film was first coated with a Cu layer by magnetron sputtering method and then annealed with the protection of Ar. After that, some thin and wrinkled graphene-like nanosheets were in-suit formed in UNCD film, which avoided the contamination of graphene during transfer process and enabled better adhesion between graphene and UNCD film. A comparative study of tribological behavior of UNCD, Cu/UNCD and annealed Cu/UNCD films were performed against Si3N4 ball on a ball-on-disk tribometer. The results showed that graphene nanosheets over UNCD film enhanced its tribological performance evidently. The annealed Cu/UNCD film exhibited ultralow friction coefficient of 0.025 and a kind of zero wear as the wear track is higher than the film surface. The findings in this study present a novel and effective method to enhance the lubricity of any engineering UNCD film.

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.

Synergistic Lubrication Effect between Oxidized Black Phosphorus and Oil Molecules Triggers Superlubricity.

Oxidized black phosphorus (BP) has been demonstrated as a promising oil-based nanoadditive because of its superior friction-reducing capability. However, the synergistic lubrication effect between oxidized BP and oil at the molecular level dominating the friction properties remains unclear. In this Letter, the synergistic lubrication effect between oxidized BP and two typical oil molecules (nonane and nonanoic acid) was explored with an atomic force microscope. The superlubricity of oxidized BP with an ultralow friction coefficient of 0.006 was achieved in the nonanoic acid environment, exhibiting a 96% reduction compared with that in the nonane environment. There was a confined nonanoic acid layer in the contact zone with a tilt angle of 35° because of the hydrogen bonding interaction, contributing to the superlubricity. This observation sheds light on the exploration of the lubrication mechanism of oxidized BP as a nanoadditive in oil, which reveals the considerable implications for the design of high-performance lubrication system.

Adjustable superlubricity system using polyalkylene glycol with various acid aqueous solutions

Polyalkylene glycol (PAG) aqueous solutions have recently been demonstrated to exhibit an ultralow friction coefficient (COF, μ < 0.01). However, the prolonged running-in period and low bearing capacity have limited its widespread application. In this study, we determined that the running-in period can be decreased by more than 75% when the pH value of the lubricant is controlled at 3 by introducing various acid solutions. Additionally, less time was required to realize stable superlubricity with inorganic acid at lower pH values. This was mainly attributed to the acceleration effect of hydrogen ions around the contact region. In case of PAG aqueous solution with organic acid, the wear loss between sliding solid surfaces was reduced, and thus the bearing pressure during the superlubricity period was significantly improved from approximately 30 to 160 MPa. Furthermore, the organic acid molecules were considered to form strong hydrogen bonds with PAG macromolecules and solid surfaces. This in turn strengthened the structure of the adsorption layers. The unique effect of different acids in aqueous polymer lubrication can potentially significantly aid in advancing the study of polymer tribology and broadening industrial applications.

Simultaneous low friction and ultra-low wear enabled by a 2,5-Furandicarboxylic acid derived bio-based ionic liquid

Bio-based protic ionic liquids (bio-PILs) based on 2,5-Furandicarboxylic acid and polyetheramine was synthesized, which was explored as a neat lubricant and lubricant additive for different alcohols that has different hydroxyl numbers under varied test conditions. Long-term service performance testing showed that the bio-PILs could achieve ultra-low coefficient of friction and wear simultaneously, which decreased coefficient of friction of 95.94% and wear rate of 99.87% compared to dry friction under Si3N4-to-steel tribo-pairs. The distinguished tribological performance is contributed to the bio-PILs can absorb on the rubbing surface and form a lubricating protective film to avoid direct contact of super hard upper Si3N4 ball against lower steel plate. This work presents a facile approach to synthesize high-performance bio-based protic ionic liquids using biomass as bridging agent and provides new insight into the underlying lubricating mechanism, which might help guide the development of novel bio-based lubricants in the future.

Macro-superlubric triboelectric nanogenerator based on tribovoltaic effect

<h2>Summary</h2> Reducing the coefficient of friction (COF) and wear rate is significant for triboelectric nanogenerators (TENGs). Here, we devised a new macroscale superlubric triboelectric nanogenerator (SL-TENG) that can withstand high-contact stress with ultralow COF (<0.01) and ultralow wear rate based on a hydrogenated diamond-like carbon (DLC) film from the tribological perspective. Specifically, the SL-TENG produced an output with the peak short-circuit current of approximately 60 nA and a power density of up to 5.815 W/m² during the friction of <i>DLC</i> versus <i>steel ball</i> with a Hertz contact stress of 1.37 GPa. Moreover, first-principle calculations demonstrate that the ultralow friction on DLC is mainly due to the hydrogen-terminated surface. The SL-TENG can generate direct current (DC) output caused by tribovoltaic effect and can light up several light-emitting diodes (LEDs) without rectification. Online superlubric friction monitoring and early warning can be realized, owing to the close correspondence between tribology and triboelectricity.

Boundary Slip of Oil Molecules at MoS2 Homojunctions Governing Superlubricity.

Molybdenum disulfide (MoS2) nanoflakes are widely used as nano-additives in oil for the excellent lubrication performance. However, the molecular mechanism of MoS2 nanoflakes in oil governing the friction properties remains elusive. In this study, MoS2 homojunctions were constructed by combining the fabricated MoS2 probe and MoS2 crystal with an atomic force microscope (AFM), and the superlubricity with an ultralow friction coefficient of approximately 0.003 at MoS2 homojunctions was attained after the formation of a confined oil layer, exhibiting a 67% reduction of the friction coefficient in comparison to that under a nitrogen atmosphere. The boundary slip of oil molecules on the MoS2 crystal with a small energy barrier was observed, causing the shear to occur at the interface of oil/MoS2 crystal with an extremely low shear strength, which contributes to the achievement of superlubricity. This boundary slip of oil molecules at MoS2 homojunctions can be extended to the macroscale for friction reduction, supplying a fundamental insight into the lubrication mechanism of MoS2 nanoflakes in oil, which has potential applications for designing an efficient lubrication system with nano-additives.

Ultralow friction of graphene-coated silica nanoparticle film

Reductions in friction and wear are widely sought to promote energy-saving and reliability in ubiquitous moving machine components. In this work, we demonstrate ultralow friction of graphene-coated silica nanoparticle films on diamond (111) surfaces by performing molecular dynamics simulations. Coated nanoparticles are prepared by enclosing 2.7 nm sized silica nanoparticles in C720 fullerene cages, which are used to construct a monolayer film with the face-centered-cubic (111) structure. Results indicate an ultralow friction coefficient, μ ∼ 0.07, for loads up to 18 GPa. The ultralow friction and high compressive strength are attributed to the coated structure that synergizes the outstanding low friction coefficient of C720 fullerenes with the nanobearing capability of silica nanoparticles. The realization of coated nanoparticles film-based ultralow friction has the potential to enhance the reliability and extend the working life of machines and devices while reducing their operating energy requirements.

Insights on the Formation Mechanism of Ultra-Low Friction of Phenolic Resin Graphite at High Temperature

In the present paper, the influences of high temperature on the tribological properties of phenolic resin graphite (PRG) sliding against tungsten carbide-nickel (WC-Ni) alloy in ambient air were investigated systematically. Results demonstrated that the antifriction behaviors of PRG was sensitive to high temperature and PRG exhibits ultra-low coefficient of friction (CoF) of about 0.01–0.015. The low CoF is attributed to the formation of graphite tribofilms, which shows different formation processes on the contact interface at different temperatures (room temperature, 200, 300 and 400 °C). These findings provide insight into the formation mechanism of graphite tribofilms, and provide an important basis for improving the tribological properties of graphite-based friction materials and manufacturing new graphite for seal applications.

OPTIMIZATION OF TRIBOLOGICAL PROPERTIES OF AN EPOXY HYBRID POLYMER COMPOSITE REINFORCED WITH ZrB2 AND PTFE PARTICLES USING RESPONSE SURFACE METHODOLOGY FOR HIGH-TEMPERATURE TRIBOLOGICAL APPLICATIONS

Hybrid PTFE/epoxy composites are widely used as materials for self-lubricating spherical bearing which are used in a high-temperature environment. In the present work, zirconium diboride (ZrB2) particles are incorporated to enhance high-temperature tribological properties of PTFE/epoxy composites. Pin on disc experiment is conducted with the aid of design of experiments (DOE) using central composite-response surface methodology (RSM). Under a load of 40 N and 1.25 m/s sliding speed, the optimum content 5.95 vol% of PTFE and 5.05 vol% of ZrB2, yields an ultralow coefficient of friction (COF) in conjunction with a low wear rate of the composite. The addition of ultra-high-temperature ceramic ZrB2 particles and solid lubricant PTFE is found to enhance the thermal conductivity and improve the heat transfer thereby reducing contact temperature. The use of optimum composition of the composite is capable of reducing the wear rate and high local temperature due to friction, implying its potential use as a self-lubricating spherical bearing liner material.

Material transfer mechanism for fabrication of superlubricity interface by reciprocating rubbing on graphite under high contact stress

Superlubricity is attractive in many scientific and technological fields due to energy-saving and wear reduction. Two incommensurate contacting surfaces (superlubricity interface) slide against each other exhibiting ultralow dynamic friction coefficient below 10−3. Fabrication of the superlubricity interface is the essence to obtain superlubricity. Here we developed a plasma-assisted mechanical exfoliation and tribo-transfer method to fabricate graphene nanoflake wrapped atomic force microscopy (AFM) tips. Superlubricity between graphene-wrapped tips with different diameters and graphite was achieved with a friction coefficient as low as 10−4. Systematic investigations were conducted on material transfer mechanism mainly including delamination, deformation and lateral displacement, and fracture and transfer of graphene nanoflake from thick graphene. Negative friction-load phenomenon and abnormal stick-slip motion on graphene were observed, demonstrating top layers of graphene were deformed and delaminated during rubbing process. Weakened strengthening effect on monolayer graphene and bubble on thick graphene after reciprocating rubbing may prove interlayer sliding and lateral displacement. The general nature of the results may apply to other 2D materials. These results not only provide a technical route to acquire superlubricity interface but also intensify the understanding of superlubricity which is the basis of its application in moving mechanical systems.

Dual-Phase Nanocomposite TiB2/MoS1.7B0.3: An Excellent Ultralow Friction and Ultralow Wear Self-Lubricating Material.

Proper lubrication is essential to the reliable and efficient operation of mechanical systems ranging from the industrial to the nanoscale. Self-lubricating materials that can self-generate and sustain concurrent ultralow friction and ultralow wear in harsh environments open up a unique realm that is unattainable by traditional external lubrication mechanisms, but developing such extraordinary materials has been a long-standing grand challenge. Here, we devise an unconventional strategy to construct a dual-phase nanocomposite (DPNC) that comprises a wear-resistant phase (TiB2) and an internal lubricant source (MoS1.7B0.3). Tribological tests demonstrate simultaneous ultralow friction coefficient (∼0.03) and ultralow wear rate (∼10-10 mm3·N-1·m-1) of the synthesized DPNC specimen in ambient environments; these superb properties remain intact after the specimen has been annealed at 400 °C in air. First-principles energetic and stress-strain calculations elucidate atomistic mechanisms underpinning DPNC TiB2/MoS1.7B0.3 as an ultimate self-lubricating material. This accomplishment solves the classic lubricity-durability tradeoff dilemma, enabling advances to meet the most challenging lubrication needs.

Improved biotribological performance of Ti6Al4V alloy through the synergetic effects of porous TiO2 layer and zwitterionic hydrogel coating

To improve the biotribological properties of Ti6Al4V alloy for artificial joints, motivated by the composite structure of ‘natural articular cartilage - subchondral bone’ and the hydration lubrication mechanism of articular cartilage, a soft/hard bilayer coating composed of a porous TiO 2 layer and a zwitterionic hydrogel coating was designed and fabricated on Ti6Al4V alloy through micro-arc oxidation (MAO) combined with ultraviolet (UV) radiation technology. Characterization results revealed that the bio-inspired bilayer coating could be successfully constructed on Ti6Al4V alloy, and such a coating displayed good hydrophilicity (WCA: 32°). Notably, by ball - on - disk friction test, this biomimetic bilayer coating exhibited an ultra-low friction coefficient (∼0.038) in PBS solution, which was reduced by an order of magnitude compared with that of the Ti6Al4V alloy substrate. This was due to the synergistic effects of soft/hard composite structure and the hydration lubrication of zwitterionic MPC polymer. This work provides a novel and effective strategy for surface modification of Ti6Al4V alloy for artificial joints.

Polyethylene glycol derived carbon quantum dots nanofluids: An excellent lubricant for diamond-like carbon film/bearing steel contact

Polyethylene glycol derived carbon quantum dots nanofluids were synthesized via a slow thermal oxidation process. The size of carbon quantum dots was ca. 2 nm and had a decreasing trend with the increase of oxidation time. When used as lubricant in a diamond-like carbon film/bearing steel interface, the nanofluids achieved an ultra-low friction coefficient (μ ≈ 0.02), much lower than that of original polyethylene glycol (μ = 0.12). The worn surface analyses revealed that the nanofluids could effectively inhibit the tribo-oxidation of steel counterpart that occurred under original polyethylene glycol lubrication, and hence reduced the abrasion component of friction. Especially, the poly-hydroxyl carbon dots and oxidized polyethylene glycol species in nanofluids induced a hydroxyl-rich sliding interface via their tribochemical reactions with friction surfaces, which promoted the adsorption of polyethylene glycol molecules on sliding surfaces. Along with the mild corrosion wear of steel counterface, this shifted the boundary lubrication to a mixed/film lubrication regime, thereby achieving an ultra-low friction coefficient. The above results suggest that the polyethylene glycol derived carbon quantum dots nanofluids should be a quite excellent candidate lubricant for solid-liquid synergy lubrication based on diamond-like carbon films.

Regulating Vacuum Tribological Behavior of a-C:H Film by Interfacial Activity.

A hydrogenated amorphous carbon (a-C:H) film shows an ultralow friction coefficient (COF, lower than 0.01); however, its wear life is short in vacuum, and the mechanisms are still not well-understood. This study demonstrates the vacuum tribological behaviors of the a-C:H film can be regulated by interfacial activity. The strong interfacial activity induced continuous transfer of carbon from the film to counterface, causing the formation of a porous transfer film and severe wear of the a-C:H film. Interestingly, weak interfacial activity is beneficial to form spherical-like carbon at the sliding interface, which shields the interaction of dangling bonds and contributes to lower COF and wear of film. Notably, the catalytic nature of Au induced perfect graphene nanoscrolls around Au nanoparticles at the sliding interface, achieving ultralong vacuum wear life. This Letter unifies the understanding of vacuum tribological properties of a-C:H film and provides new insight for prolonging the life of carbon films in vacuum.

Ni/Si3N4 composite coatings and their water lubrication behaviors

In view of the excellent water lubrication performance of Si3N4, the composite coatings of Ni/Si3N4 with different Si3N4 particle contents were fabricated and the ultra-low friction coefficient of the coatings sliding against Si3N4 ball in water was expected. Experimental results manifested that only the Ni/Si3N4 composite coatings with a high particle content can achieve the ultra-low friction coefficient (0.008). The SEM and 3Dprofiler measurements indicated that the formation of consecutive and smooth contact interface is one of the essential conditions to achieve the ultra-low friction. Besides, the tribo-chemical product, silicon hydroxide, is another requirement. The major wear mechanism changes from the mechanically dominated wear during the running-in process to the tribo-chemical wear at the ultra-low frictional stage. Such result indicates that low friction is not a specific feature of Si3N4 and SiC self-mated pairs. This behavior of ceramic composite coating may develop a new approach for the low energy loss water lubrication systems.

Ultra low friction of conductive carbon nanotube films and their structural evolution during sliding

Solid lubricating coatings with high conductivity are widely used in aerospace, tram and micro mechanical systems. In this study, a solid lubricating film with ultralow friction coefficient and good electrical conductivity was prepared using hydroxylated CNTs through electrophoretic deposition, and the structural evolutions when the CNT films act as lubricants are investigated in-depth. It was found that the electric resistance reduced significantly with the increase of the deposited CNT film thickness, and then it reached a saturation value of approximately 40 Ω at the film thickness of 0.7 ± 0.10 μm deposited at 50 V. Moreover, the friction coefficient was reduced by 71% at the deposition voltage of 40 V compare to the single crystal silicon. It is worth noting that the original carbon nanotubes transformed into nano-scrolls under the action of friction shearing and heating. At the same time, this work provided new insights for the structure evolution of carbon nanotube films during the rubbing process.

MoS2@ZIF-8 doped waterborne polyurethane membranes with water vapor permeable, lubricating, and antibacterial properties

In this research, a multi-functional waterborne polyurethane (WPU) membrane was developed through the in-situ synthesis of molybdenum disulfide/zeolitic imidazolate framework-8 (MoS2@ZIF-8) nanoparticles and then the introduction of these nanoparticles into a waterborne polyurethane emulsion. The photocatalysis of MoS2@ZIF-8 nanoparticles were improved due to the rapid transfer of photo-inspired electrons from MoS2 to ZIF-8, resulting in higher yields of reactive oxygen species (ROS) to kill bacteria. Hence, inactivating 93% and 99.3% of E. coli and S. aureus after 60 min, respectively. Moreover, the inactivation using the MoS2@ZIF-8 nanoparticles was dramatically superior to the pure MoS2 and ZIF-8. Additionally, compared with a pure WPU membrane, the mechanical properties of the composite membranes increased significantly, where the maximum tensile strength and elongation at break reached 51 MPa and 237%, respectively. Meanwhile, the introduction of the MoS2@ZIF-8 nanoparticles into the WPU membrane at a concentration of 0.5% improved the water vapor permeability to as high as 215.55 g/(m2·24 h) and enhanced the tribological properties with an ultralow friction coefficient (0.05). Therefore, the MoS2@ZIF-8/WPU composite membranes show promise as multifunctional coating materials due to their high water vapor permeability, good lubricating performance, and photo-responsive disinfection properties.

Research on Ti-GLC/TiCN/TiN composite multilayer coating with ultra-low friction coefficient in various environments

The Ti-GLC (graphite-like carbon)/TiCN/TiN composite multilayer coatings with ultra-low friction coefficient were prepared by High Power Impulse Magnetron Sputtering technique (HiPIMS). The friction and wear properties of the coatings in multiple environments including artificial seawater, 0.5 mol/L H2SO4 (pH = 0) and 0.1 mol/L NaOH (pH = 13) solution were studied. During the sliding process, the friction coefficients of the coatings in the environments mentioned above are 0.059, 0.109 and 0.037, respectively. Meanwhile, the wear rates of the coatings all remain at 10−7 orders of magnitude mm3•N−1•m−1 in aqueous environments. The results of the tribo-corrosion test show that the coating has a significant passivation, which protects the coating effectively. The friction and wear properties of the coatings are maintained at a very considerable level in various environments.