•Zr/CrN multilayer coatings with different modulation ratios were fabricated.•The textures and properties of multilayer coatings depended on modulation ratios.•Coatings with thick CrN layer possess superior wear and corrosion resistances.•Multilayer coatings presented delamination layer by layer.•The main wear mechanism for Zr/CrN multilayer coatings was oxidation wear.Zr/CrN multilayer coatings with different modulation ratios were fabricated by multi-arc ion plating. By virtue of X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM), the influences of modulation ratios on coatings microstructures were investigated. The corrosion and wear behaviors of coatings in seawater solutions were evaluated. Results showed that the textures and properties of Zr/CrN multilayer coatings depended on the modulation ratios. The hardness, corrosion resistances and tribological performances of Zr/CrN multilayer coatings increased as the increase of individual CrN layers thickness. Multilayer coating with thick CrN layer exhibited superior anti-corrosion and tribological performances. The high charge transfer resistance maybe contribute to the better corrosion resistances, while the wear mechanisms were identified as oxidation wear.

•Ultrathin period thickness results in superior mechanical and tribological properties.•Columnar structure originates from the surface irregularities of intermediate layer.•The more heterointerfaces, the lower internal stress.•Interfacial sliding between WO3 rich tribofilm and C rich surface generates low friction.Nanomultilayered WC/a-C coatings with modulation period raning from 1.3 to 11.5 nm were successfully fabricated using unbalanced magnetron sputtering process and the evolution of their microstructure, mechanical and tribological properties with the modulation period were systemically investigated. It has been demonstrated that the columnar structure in WC/a-C nanomultilayers originates from the surface irregularities of intermediate Cr/WC/C layer and the column diameter is correlated with the initial Cr layer thickness in this intermediate stage. Hardening and toughening through the nanomultilayer structure design have been achieved and particularly pronounced when the modulation period ranges from 5.8 to 10 nm. The enhanced hardness and fracture toughness are mainly attributed to dislocation/crack pinning effect from the thin individual layer and the heterointerfaces between WC and a-C layers. It is also found that the more heterointerfaces, the lower internal stress. What's more, the WC/a-C nanomultilayers with small modulation period (≤5.8 nm) exhibit low COF about 0.05 and good wear resistance due to the interfacial sliding between a WO3-rich tribofilm and a carbon-rich wear surface; once the modulation period above 5.8 nm, it fails to build up such a low friction interface between the tribopairs, which combining with the high intrinsic friction caused by its own structure characteristics result in high COF about 0.18–0.2 and poor wear resistance.Download high-res image (281KB)Download full-size image

Friction on a nanoscale revealed rich load-dependent behavior, which departs strongly from the long-standing Amonton's law. Whilst electrostatic repulsion-induced friction collapse for rare gas sliding over metallic surfaces in a high-load regime was reported by Righi et al. (Phys. Rev. Lett., 2007, 99, 176101), the significant role of attraction on frictional properties has not been reported to date. In this study, the frictional motion of Xe/Cu(111), Xe/Pd(111) and Ar/Cu(111) was studied using van der Waals corrected density functional calculations. An attraction-induced zero friction, which is a signal of superlubricity, was found for the sliding systems. The superlubric state results from the disappearance of the potential corrugation along the favored sliding path as a consequence of the potential crossing in the attractive regime when the interfacial pressure approaches a critical-value. The finding of an attraction-driven friction drop, together with the repulsion-induced collapse in the high-load regime, which breaks down the classic Amonton's law, provides a distinct approach for the realization of inherent superlubricity in some adsorbate/substrate interfaces.

Fluorine and sulfur co-doped amorphous carbon (a-C:S:F) films were synthesized from C2H2 and SF6 plasma mixture, and the bonding states of S and F in films were scrutinized. The a-C:S:F films exhibited a more ordered carbon structure than hydrogenated films. The friction behavior of films was tested against GCr15 bearing steel balls under high vacuum (HV) conditions. Ultra-low steady-state friction coefficient (0.01–0.02) was achieved for a-C:S:F films containing about 2.0 at.% H, depending on the concentrations of S and F in films. Graphite-like transfer layer on the ball surface provided a low-adhesive sliding surface terminated by –CF2- and –CF3 groups. The electrostatic repulsive interaction between the F-terminated transfer layer and the S- and F-terminated carbon structure was probably responsible for the ultra-low friction behavior. Intriguingly, the sp2-C clusters containing ‘thiophene-S’ (–C–S–C–) structure in films might contribute to the low friction when sliding against an highly fluorinated surface. These results demonstrate that the lubricating properties of a-C films in vacuum can be achieved through the S and F co-doping, which is of great significance for developing a-C films as vacuum lubricants.

Electrochemical measurements, salt spray tests and immersion tests were employed to investigate the influence of deposition periods and corrosive medium (NaCl, H2SO4, HCl, NaOH) on the corrosion behaviors of silicon doped multilayer diamond-like carbon (DLC) coatings. The results showed that the corrosion resistance of the multilayer DLC coatings was significantly improved with the increase of deposition periods. Interestingly, the coating with the highest deposition periods provided good corrosion protection in neutral and acidic solutions while poor corrosion protection in alkaline and acidic chloride solutions.

•Low F content in a-C:H:F films reduces the friction coefficient.•High F content in a-C:H:F films increases the friction coefficient.•A composite-like tribo-layer on Al surface changes with the F content.•Adhesion, shear and abrasion competitively control the friction.Fluorinated amorphous carbon (a-C:H:F) films synthesized from C2H2 and CF4 were examined by Raman spectra and X-ray photoelectron spectroscopy (XPS). Their tribological properties were tested against aluminum balls in dry sliding. The a-C:H:F films with low F content showed lower friction coefficient than hydrogenated amorphous carbon (a-C:H) film, whereas high F content in films resulted in a significant friction increase. Contact surfaces were analyzed in detail to elucidate the possible sliding mechanism. Results indicated that the friction behavior was closely related to the nature of a composite-like tribo-layer consisting of Al compounds and carbon components formed on Al ball, relying on the tribochemical processes of contact interface. The accumulated F atoms on tribo-layer reduced the adhesion across sliding interface because of strong repulsion between F atoms, and thereby lowered the friction of a-C:H:F films. However, with increasing F contents in films, the enhanced tribochemical reaction between Al and F caused crack, delamination and fragmentation of the tribo-layer, and then a marked abrasive process at the sliding interface. Consequently, the increased shearing and abrasive actions strongly opposed the contribution of the reduced adhesion to friction, and result in a rather high friction of highly fluorinated a-C:H:F films.

Complicated tribochemical reactions with the surrounding media often occur at the prosthesis material, which is a dominant factor causing the premature failure in revision surgery. Graphite-like carbon (GLC) film has been proven to be an excellent tribological adaption to water-based media, and this work focused on the friction and wear behavior of Mo-doped GLC (Mo-GLC)-coated poly(aryl ether ether ketone) sliding against Al2O3 counterpart in physiological saline, simulated body fluid, and fetal bovine serum (FBS), which mainly emphasized the interface interactions of the prosthetic materials/lubricant. Results showed different tribological responses of Mo-GLC/Al2O3 pairs strongly correlated with the interfacial reactions of the contacting area. Particularly, a transfer layer was believed to be responsible for the excellent wear reduction of Mo-GLC/Al2O3 pair in FBS medium, in which graphitic carbon and protein species were contained. The wear mechanisms are tentatively discussed according to the morphologies and chemical compositions of the worn surfaces examined by scanning electron microscope as well as X-ray photoelectron spectroscopy.Keywords: biological medium; friction and wear; graphite-like carbon; interfacial interactions; PEEK

A remarkable synergetic effect between the graphene oxide (GO) layers and multiwalled carbon nanotubes (MWCNTs) in improving friction and wear on sliding diamond-like carbon (DLC) surfaces under high vacuum condition (10–5 Pa) and low or high applied load is demonstrated. In tests with sliding DLC surfaces, ionic liquid solution that contains small amounts of GO and MWCNTs exhibited the lowest specific friction coefficient and wear rate under all of the sliding conditions. Optical microscope images of the wear scar of a steel ball showed that GO/MWCNT composites exhibited higher antiwear capability than individual MWCNTs and GO did. Transmission electron microscopy images of nanoadditives after friction testing showed that MWCNTs support the GO layers like pillars and prevent assembly between the GO layers. Their synergistic effect considerably enhances IL-GO/MWCNT composites.Keywords: graphene oxide; ionic liquids; multiwalled carbon nanotubes; nanoadditive; synergistic effect; tribology; vacuum;

Space mechanisms require multialkylated cyclopentanes (MACs) more lubricious, more reliable, more durable, and better adaptive to harsh space environments. In this study, two kinds of additives were added into MACs for improving the tribological properties under simulated space environments: (a) solid nanoparticles (tungsten disulfide (WS2), tungsten trioxide (WO3), lanthanum oxide (La2O3), and lanthanum trifluoride (LaF3)) for steel/steel contacts; (b) liquid additives like zinc dialkyldithiophosphate (ZDDP) and molybdenum dialkyldithiocarbamate (MoDTC) for steel/steel and steel/diamond-like carbon (DLC) contacts. The results show that, under harsh simulated space environments, addition of the solid nanoparticles into MACs allows the wear to be reduced by up to one order magnitude, while liquid additives simultaneously reduce friction and wear by 80% and 93%, respectively. Friction mechanisms were proposed according to surface/interface analysis techniques, such as X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectroscopy (TOF-SIMS). The role of solid nanoparticles in reducing friction and wear mainly depends on their surface enhancement effect, and the liquid additives are attributed to the formation of tribochemical reaction film derived from ZDDP and MoDTC on the sliding surfaces.Keywords: boundary lubrication; diamond-like carbon; liquid lubricants; solid nanoparticles; solid−liquid composite coatings;

Graphene oxide (GO) is a layered material bearing a variety of oxygen-containing functional groups on its basal planes and edges, which allow it as a substrate to conduct a variety of chemical transformations. Here modified graphene oxide (MGO) was prepared using alkyl imidazolium ionic liquids (ILs) (1-butyl-3-methylimidazolium tetrafluoroborate (LB104), 1-butyl-3-methyl imidazolium hexafluorophosphate (LP104) and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl) amide (LF106)) via epoxide ring-opening reaction, cation-π stacking or van der Waals interactions, with LB104 modified graphene (MG) exfoliated from graphite rod by a moderate electrochemical method as a comparison. The stability and tribological properties of MGO and MG as multialkylated cyclopentanes (MACs) additives were investigated in detail. The results show that GO is converted into graphene through the chemical modification using ILs, and MGO with good dispersion and stability in MACs significantly improves the tribological performance (friction and wear were reduced about 27% and 74% with pure MACs as a comparison, respectively). The excellent tribological properties are attributed to the formation of an ILs-containing graphene-rich tribofilm on the sliding surfaces, which as the third body can prevent the sliding surfaces from straight asperity contact and improve friction reducing and anti-wear behaviors.

The anticorrosion and tribocorrosion properties of a multilayer diamond-like carbon (DLC) film were systematically investigated in NaCl solution. Electrochemical measurements suggest that the corrosion performance of the multilayer DLC film is superior to those of the substrate and single layer DLC film in NaCl solution, which is attributed to the successively multilayered structure with a well-bonded interface and the formation of Si oxides. An extremely high Warburg impedance value, higher than 107 Ω cm2, of the multilayer DLC film has been observed. Tribocorrosion tests show that the multilayer DLC film presents lower wear rate in NaCl solution, with the substrate and single layer DLC film as comparisons. We demonstrate that the multilayer DLC film is an excellent protective material for improving both corrosion and wear performance of the substrate.

Recent research on carbon films has introduced interesting low friction properties of self-mated fluorinated carbon films. In particular, the low friction mechanism of self-mated fluorinated carbon films was consistently attributed to anti-bonding repulsive forces. However, no experimental data reported to date for the limitations of this low friction mechanism comply with the results obtained using first-principles calculations. In this investigation, we attempt to clarify the limitations of the low friction mechanism of self-mated fluorinated carbon films.

Run-in behavior of amorphous carbon films significantly affected their tribological properties. Generally, for amorphous carbon films, a low friction coefficient was obtained after a run-in period during which the friction coefficient was high and gradually decreased to a low value under vacuum. However, in this paper, the friction coefficient was initially low and gradually increased to a high value during run-in period under vacuum. According to experimental results, first principles calculations were selected to probe the possible mechanism of the anomalous run-in behavior. The results showed that the weak and strong atomic interactions should be attributed to low and high friction.

●Effects of atmosphere and load on the friction behavior of a DLC film are studied.●The DLC film is a low-hydrogenated one with H content about 7.86 at.%.●The film can show a superlow friction coefficient (0.003) in a specific condition.●Friction mechanisms underlying the environment- and load-dependence are discussed.We examine the influence of gaseous atmospheres and applied loads on the friction behaviors of a low-hydrogenated diamond-like carbon (DLC) film. The results show that friction behaviors of the DLC film can be manipulated by adjusting the contact stress in conjunction with the test atmosphere, particularly the humidity. By this way, surprisingly, the low-hydrogenated DLC film is enabled to display a superlow friction coefficient (~0.003) with a long wear life (>20000 sliding cycles). The mechanical and mechanochemical effects of the contact stress and the effect of humidity on the friction behaviors of the DLC film are elaborately discussed.

•The PHC-PECVD method allows deposition of superthick DLC-based films.•Superthick DLC film exhibit excellent tribological property at 500 °C.•A layer that consists of SiC was formed on the top of wear track at 500 °C•Thin film is more likely to fail compared with thick film at high temperature.A plasma-enhanced chemical vapor deposition system was used to deposit super thick diamond-like carbon (DLC)-based films ((Six-DLC/Siy-DLC)n). The aim of this work is to investigate the properties of super thick films to verify that increasing the thickness of DLC films offers the possibility of improving their properties at high temperatures. The investigation revealed that superthick (Six-DLC/Siy-DLC)n film exhibited excellent tribological property up to 500 °C. One reason is that a thin layer that consists of nanocrystals SiC is formed on the top of wear track. Another is that the stress mostly concentrates near the top surface.

•Ionic liquids (ILs) gels were readily prepared by grinding multiwall carbon nanotubes (MWCNTs) in ILs.•MWCNTs were effeciently modified by ILs during formation process of ILs gels.•ILs gels possess high stability, high conductivity and excellent tribological performance.•Friction mechanisms of ILs gels are discussed according to the synergy of ILs and MWCNTs with their respective outstanding characteristics.Two ionic liquids (ILs) gels were prepared by grinding multiwall carbon nanotubes (MWCNTs) in two kinds of ILs, and their physical and tribological properties were investigated in detail. Results demonstrate that ILs gels possess high conductivity and excellent tribological performance which mainly depends on the synergy of ILs and MWCNTs with their respective outstanding characteristics. ILs modified MWCNTs through van der Waals and π–π stacting interactions significantly improve the dispersibility and compatibility with lubricants, which greatly enhances the conductivity and tribological properties of the lubricants. The friction mechanism for the ILs gels is attributed to the synergetic lubrication of ILs and MWCNTs.

•A multilayer carbon film was developed through PECVD system.•This film exhibits superior corrosion-wear properties in 0.1 M HCl solution.•Solid-like film and microstructure of the film assure the superior tribological properties in HCl solution.In this paper, an amorphous carbon film containing F–Si-doped multilayer structure was developed through a simple deposition technique. The deposited film was able to attenuate stress concentrations, and improve the adhesion between film and substrate. The film had strong adhesion to substrate and could well withstand high scratch loads. Results showed superior wear resistance of film/GCr 15 pairs in 0.1 M HCl strongly correlated with the interfacial reactions of the contacting area. A solid-like film was believed to be responsible for the excellent wear reduction of film/GCr 15 pairs in 0.1 M HCl solution, where SiC nanocrystallines were contained. The wear mechanism was tentatively discussed according to the morphologies and chemical compositions of the debris examined by transmission electron microscope and X-ray photoelectron spectroscopy.

Despite continuous improvements in machine elements over the past few decades, lubrication issues have impeded human exploration of the universe because single solid or liquid lubrication systems have been unable to satisfy the ever-increasing performance requirements of space tribology. In this study, we present an overview of the development of carbon-based films as protective coatings, with reference to their high hardness, low friction, and chemical inertness, and with a particular focus on diamond-like carbon (DLC) films. We also discuss the design of carbon-based solid-liquid synergy lubricating coatings with regards to their physicochemical properties and tribological performance. Solid-liquid composite coatings are fabricated via spinning liquid lubricants on solid lubricating films. Such duplex lubricating coatings are considered the most ideal lubrication choice for moving mechanical systems in space as they can overcome the drawback of adhesion and cold-welding associated with solid films under harsh space conditions and can minimize the crosslinking or chain scission of liquid lubricants under space irradiation. State of the art carbon-based solid-liquid synergy lubricating systems therefore holds great promise for space applications due to solid/liquid synergies resulting in superior qualities including excellent friction reduction and anti-wear properties as well as strong anti-irradiation capacities, thereby meeting the requirements of high reliability, high precision, high efficiency, and long lifetime for space drive mechanisms.

The present investigation has been conducted in order to evaluate the friction and wear behaviors of polyaryl-ether-ether-ketone coated with a Si/GLC film sliding against 100Cr6 steel, Al2O3 and Si3N4 balls in a biological medium of simulated body fluid, using the ball-on-disk tribological tests. The test results show that the wear volume loss of the rubbing pairs is two orders of magnitude greater than that determined for the tribo-pairs in SBF lubrication, coefficient of friction decreases by 50 % at least, as compared to that in dry conditions. The friction coefficient showed the same varied trend with Hertzian contact radius. The wear rate showed the inverse varied trend with the contact pressure. The interfacial tribochemically reacted with moisture available in SBF medium plays an effect role of the difference in the wear-resistant and lubricating behavior. Furthermore, some indexes including hardness ratio of pair and film were employed to predict the wear behavior of GLC composite films sliding against different counterparts. The Si/GLC nanocomposite films would be a new kind of promising materials applied to artificial heart valves and stents.

Compared to the reference coatings, the coating nc-(Ti, W)C/a-C(Al) exhibited a “metal-reservoir” behavior, which was essential for its low-friction behavior within a wide load range. This “metal-reservoir” phenomenon was based on the stability tendency of the transition metal carbides. Additionally, this tendency could also explain the reacting activity of Me-DLC in previous reports, and specially, in this system, the coating replacing the functional additives controlled the boundary films. This result opens a new route for us to design nanocomposite coating operated in lower viscosity oils with reduced sulfur and phosphorous.

•Cr/Cr2N nano-multilayer coatings were fabricated by multi-arc ion plating.•Coating with proper thickness ratio of Cr and Cr2N layer possessed high toughness.•Corrosion behavior of multilayer coating depended on porosity and interface effect.•High toughness and tribolayer contributed to good tribological behavior of coating.Cr/Cr2N nano-multilayer coatings with varied individual thickness of Cr2N layer were fabricated on 316L stainless steel substrates by multi-arc ion plating system. The evolutions of microstructures and mechanical properties were investigated by scanning electron microscopy, X-ray diffraction, nanoindentation and scratch testing. The corrosion behaviors and tribological properties in seawater conditions were evaluated using polarization resistance and reciprocating-sliding friction tests, respectively. Results showed that the multilayer coatings with thicker individual Cr2N layer presented higher hardness at the expense of toughness, and the coating with thickness ratio of Cr and Cr2N layers at 0.45 possessed both high hardness and toughness. The dominant feature influencing the corrosion resistance of deposited multilayer coating was the porosity and interface effect. While the excellent tribological performances of coatings with high load bearing capacity were attributed to the optimization of thickness ratio presenting high mechanical properties. Furthermore, the tribolayer formed during wear process, which small debris particles of chromium nitride embedded into the lubricative matrix and well adhered in the wear track, played a significant role in the anti-wear property. The superior anti-corrosion and anti-wear performances of nano-multilayer coatings made them good candidate protective materials in engineering applications in seawater environments.

Although ionic liquids (ILs) as a class of promising materials have a wide range of applications due to the excellent properties, their potential as space lubricants has been not systematically explored. Here two kinds of conductive alkyl imidazolium ILs greases were prepared using 1-hexyl-3-methylimidazolium tetrafluoroborate (LB106) and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl) amide (L-F106) as base oil and the polytetrafluoroethylene (PTFE) as thickener, with multiple-alkylated cyclopentane grease (MACs) as a comparison. Their chemical composition and tribological properties were investigated in detail under simulated space environment which is composed of high vacuum, high temperature and irradiation. Results show that the high conductive ILs greases not only possess good adaptive abilities to space environment and thermal stability but also provide excellent friction reducing and antiwear behaviors as well as high load carrying capacities. The unique physicochemical properties are attributed to a combination of special anions and cations, the excellent tribological properties are strongly dependent on a boundary protective film on the rubbing surfaces.Keywords: conductivity; ionic liquids; space irradiation; tribochemistry; vacuum

The novel graphene–C60 hybrid films have been fabricated successfully on silicon surfaces by a multistep self-assembly process, and showed synergistic effects beyond individual performance in micro/nano-tribological behaviors. It is expected that the graphene–C60 hybrid films may find wide applications as high performance lubricating films in MEMS.

The lubrication of diamond-like carbon (DLC) films has commonly been ascribed to two hypotheses, i.e., ‘friction-induced graphitization’ mechanism and passivation mechanism. To clarify the primary low-friction mechanism of DLC, we investigate the effects of sliding velocity and vacuum pressure on the friction behavior of DLC film. Counterintuitively, examination of wear tracks by Raman spectroscopy reveals that a higher friction coefficient is accompanying with a higher degree of graphitization. We therefore claim that it is the higher friction force that results in the higher degree of graphitization, which subverts the concept of a higher degree of graphitization leading to a lower friction coefficient. Besides, the friction coefficient is found to depend on the ratio of ambient pressure to rotating speed, indicating that the passivation mechanism is at play. Besides, the additional slide–hold–slide test in room air also gives evidence that cannot be understood in terms of the ‘friction-induced graphitization’ mechanism.

A continuing desire exists to explore graphene as a lubricant additive and increase the performance of oil/grease products in efforts to acquire a fundamental knowledge of its tribology. As compared to graphite and ionic liquid, multilayer graphene (MLG) as a bentone lubricating grease additive not only provides lower friction and better wear resistance, but also greatly improves the load-bearing capacities and thermal stability of bentone lubricating grease. These benefits are strongly dependent on the formation of a versatile boundary lubricating film, which is provided by the laminated structure and good adsorption action of MLG on the rubbing surfaces, as well as good dispersion of MLG in grease.

It is currently a challenge for space tribology to develop a long lifetime and high bearing capacity lubricant meeting the requirements of space applications. Herein, we dispersed graphene into ionic liquid, prepared novel composite coatings of diamond-like carbon (DLC)/ionic liquid (IL)/graphene with different graphene concentrations, and investigated its space performance under high vacuum and space radiation conditions. IL/graphene nanofluids with different concentrations were examined by Fourier transform infrared spectroscopy (FTIR). Furthermore, IL/graphene nanofluids after friction tests were investigated by X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HRTEM). The results showed that the graphene concentration would obviously affect the spatial tribology performance of the composite coatings. Because the excess graphene in the IL would tend to form irreversible agglomerates, leading to reduction of the effective graphene dose, an optimum graphene concentration (0.075 mg ml−1) in IL for the composite coatings was required to exhibit the lowest friction coefficient, the highest bearing capacity and the strongest anti-irradiation in a simulated space environment. In addition, XPS spectra further confirmed that the formation of a fluorinated oil-containing carbon-rich tribofilm between the friction pairs further ensured the good antifriction and wear resistance performance of DLC/IL/graphene.

Based on the microbumps of graphene nanosheets and the nanostructure of carbon nanotubes (CNTs), novel graphene/CNTs composite films with hierarchical micro- and nanoscale surface roughness were successfully fabricated by simply spraying the mixed acetone dispersion of graphene nanosheets and CNTs onto stainless steel substrates. The as-prepared composite films exhibited controlled surface hydrophobic, adhesive and electrowetting properties via altering the film surface structure and surface energy. Among them the composite film with a 1:5 mass ratio of graphene to CNTs showed high hydrophobicity and conductivity, low water adhesion and contact angle sensitivity to the external electric field, which would help to resolve the surface electrostatic problems and unstable hydrophobicity under applied potential that exist in many conventional insulating hydrophobic materials, and could be useful in some application fields.

Superthick diamond-like carbon (DLC) films [(Six-DLC/Siy-DLC)n/DLC] were deposited on 304 stainless steel substrates by using a plane hollow cathode plasma-enhanced chemical vapor deposition method. The structure was investigated by scanning electron microscopy and transmission electron microscopy. Chemical bonding was examined by Raman, Auger electron, and X-ray photoelectron spectroscopy techniques. Mechanical and tribological properties were evaluated using nanoindentation, scratch, interferometry, and reciprocating-sliding friction testing. The results showed that implantation of a silicon ion into the substrate and the architecture of the tensile stress/compressive stress structure decreased the residual stress to almost 0, resulting in deposition of (Six-DLC/Siy-DLC)n/DLC films with a thickness of more than 50 μm. The hardness of the film ranged from 9 to 23 GPa, and the adhesion strength ranged from 4.6 to 57 N depending on the thickness of the film. Friction coefficients were determined in three tested environments, namely, air, water, and oil. Friction coefficients were typically below 0.24 and as low as 0.02 in a water environment. The as-prepared superthick films also showed an ultrahigh load-bearing capacity, and no failure was detected in the reciprocating wear test with contact pressure higher than 3.2 GPa. Reasons for the ultrahigh load-bearing capacity are proposed in combination with the finite-element method.Keywords: finite-element method; load-bearing capacity; superthick DLC film;

The high friction of diamondlike carbon (DLC) films in vacuum impedes achieving their application in space environment. Here we show that the vacuum friction coefficients can be lowered below 0.1 by avoiding formation of carbonaceous transfer layers on the counterfaces. First-principles calculations reveal that the low friction coefficients stem from intrinsically weak-interacting sliding interfaces. Conversely, formation of transfer layers and carbonaceous sliding interfaces thus established are invalid for friction reduction for DLC in vacuum. The mating materials are thus highlighted for their crucial roles in transfer-layer formation.Keywords: adhesion; diamondlike carbon; first-principles calculation; friction; high vacuum; mating material;

We have addressed the friction and wear behaviors of multilayer Cr/CrN/GLC (graphite-like carbon) coatings sliding against different rubbers in water environment. Stribeck curves and start–stop tests were presented to analyze the water-lubrication mechanism and tribological properties of coating/rubber tribopairs. Results exhibited that applied loads had a minor influence on the tribological performances of coating/rubber tribopairs compared to sliding speeds. The friction coefficients fluctuated during the test process, including in the start–stop process, which related to the viscoelasticity of rubbers. Furthermore, the different tribological performances of tribopairs were attributed to the hardness, surface wettability and tensile strength as well as viscoelasticity.Highlights► The tribological performances of coatings sliding against rubbers were investigated. ► The water-lubrication mechanism of three coating/rubber tribopairs were presented. ► 500 times repeated start–stop tests were conducted in succession for three tribopairs.

A model based on energy dissipation was developed to describe the tribological behavior of solid lubricant films in sand–dust environment. A relationship between wear rate and coefficient of friction was obtained. Theoretical results kept well consistent with the experimental data in reported publications (Qi et al., Tribol Lett 38:195–205, 2010; Surf Interface Anal 43:836–846, 2011; Wear 271:899–910, 2011). It was pointed out that the absolute value of slope of the simulated straight line is closely related with mechanical properties of solid lubricant films. The results increased our understanding about the individual friction and wear mechanism for solid lubricant films in sand–dust environment.

•A rang of Si doped DLC films were prepared by HCPIII method.•Si-DLC film with tensile stress and compressive stress can be tailored.•Both the H and E suffer degradation when the Si concentration is low.•Tribological behavior of Si-DLC film is improved when Si reaches to a certain value.Diamond-like carbon (DLC) films have been extensively studied over the past decades due to their unique combination of properties; in particular, silicon-doped DLC (Si-DLC) films are of significant interest for tribological effects. But there are contradictory reports in the literature with regard to the effect of silicon content on the properties of DLC films. In this study, Si-DLC films were deposited by hollow cathode plasma immersion ion implantation (HCPIII) method, using mixtures of C2H2, Ar and diluted SiH4 (SiH4/Ar 10:90). The influences of Si addition on the surface morphology, structure, mechanical and tribological properties were investigated by a combination of surface analysis methods, nanomechanical and friction measurements. It was observed that addition of Si into DLC films lead to a decrease in the Raman band intensity ratio ID/IG. The root mean square values of Si-DLC films were increased along with the increase of Si concentration. Both the hardness and elastic modulus suffered degradation when the silicon concentration was low, but these properties recovered when Si concentration increased. The Si-DLC film with tensile stress and compressive stress can be obtained by choosing distinct contents of Si in the film. The coefficient of friction (COF) of Si-DLC films against GCr 15 steel ball under atmosphere firstly increased as the Si concentration increased up to 8.41 at.%, then COF of Si-DLC films decreased with a further increase of Si concentration. The mismatch in the bond length, the difference of the mechanical property and the alteration of the colliding particles' energy were determined to be the basis for the changes in these properties.

•Ionic liquid DLC-based solid–liquid lubricating coatings were prepared.•High and low temperature alternating tribological behavior was investigated in high vacuum.•The friction coefficient at 100 °C was the lowest and that was the largest at −100 °C.•The disc wear rates were lowest at the room temperature and that was the highest at 150 °C.In this paper, ionic liquid (IL) diamond-like carbon (DLC)-based solid–liquid lubricating coatings were used to conduct friction and wear experiments under a high-vacuum condition with alternating temperatures between −100 and 150 °C. The results showed that the friction coefficients at 100 and −100 °C were the lowest and the largest, respectively. Considering that the mobility, activity, spreadability, and self-repairing capacity of IL were significantly better at high temperatures compared with those at low temperatures, the DLC film surface slightly graphitized at a relatively high temperature. The disc wear rates were the lowest at room temperature and the highest at 150 °C because the surface graphitization of the DLC film and the large friction coefficient resulted in an increase in wear rate.

Combination of solid and liquid lubricants to meet emission or environmental requirements of future tribological systems while providing the levels of desired friction and wear performance have received considerable research attention in the near term. The aim of the present work was to investigate the tribological behavior of oil-lubricated (PAO, PFPE, SO, IL and MAC) DLC coated surfaces under the conditions without and with sand-dust particles. The effects of applied load, frequency, and sand-dust particles on the tribological performance of DLC coating were systemically studied. The analysis results showed that solid–liquid lubricating coatings including SO and IL exhibited excellent anti-friction (∼0.026) but relative poor wear-resistance performances under the conditions without and with sand-dust environments. But for PFPE and PAO, they demonstrated the worst tribological behaviors with high friction coefficient and wear rates. The added sand-dust particles lead to the wear rates to the one order of magnitude large than that without sand-dust conditions for all the selected liquid lubricants. The viscosity, contact angle and work of adhesion played an important part in affecting the tribological performances. The lubrication regimes in Stribeck curve for the five kinds of liquid lubricants were affected obviously by the sand-dust particles in different way. The formed transfer films on the coating surface and pin have much influence on the tribological behavior and the transition between lubrication regimes.Highlights► Sand-dust increases friction coefficient and wear rate under oil-lubricated conditions. ► Viscosity, contact angle and adhesional work affect tribological performance. ► Stribeck curve clarified the lubrication regime. ► XPS presents the probable chemical reaction on the worn surface. ► Tribofilm influences tribological behavior and transition between lubrication regimes.

Friction has a direct relation with the energy efficiency and environmental cleanliness in all moving mechanical systems. To develop low friction coatings is extremely beneficial for preserving not only our limited energy resources but also the earth’s environment. This study proposes a new design for low friction carbon-based nanocomposite coatings by tailoring the microstructure and phase segregation, and thereby it contributes to better controlling the mechanical and tribological properties. Experimental findings and theoretical calculations reveal that high-hardness (18.2 GPa), high-adhesion strength (28 N) as well as low-internal stress (−0.8 GPa) can be achieved by a nanocrystallite/amorphous microstructure architecture for the nc-WC/a-C(Al) carbon-based nanocomposite coating; in particular low friction (∼0.05) can be acquired by creating a strong thermodynamic driving force to promote phase segregation of graphitic carbon from the a-C structure so as to form a low shear strength graphitic tribo-layer on the friction contact surfaces. This design concept is general and has been successfully employed to fabricate a wide class of low friction carbon-based nanocomposite coatings.

Nanocomposite TiC/a-C and TiC/a-C:Al carbon-based coatings were fabricated on stainless steel and silicon wafer substrates. X-ray diffraction (XRD), high-resolution transmission electron microscope (HRTEM), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) were used to investigate the composition and structure of as-fabricated coatings. Nanoindenter, interferometer, scratch tester and ball-on-disc tribometer were used to evaluate the mechanical and tribological properties. Results showed that the TiC nanocrystallites were formed and uniformly dispersed in the amorphous carbon matrix by incorporating strong-carbide-forming Ti metal while the size of TiC grain and surface roughness were decreased by co-incorporating weak-carbide-forming Al metal. Particularly, the co-incorporation of Al could drastically diminish the magnitude of internal stress, improve adhesion strength and maintain relatively high hardness for as-fabricated coating. As a result, the duplex doped nc-TiC/a-C:Al coating achieved lower friction coefficient and specific wear rate compared to simplex Ti-doped nc-TiC/a-C coating. Mechanism analysis revealed that the improved self-lubricating and anti-wear performances of this nc-TiC/a-C:Al coating were mainly attributed to the good combining mechanical properties and easily formed continuous and compacted graphitized interlayer on the contact area.Highlights► nc-TiC/a-C:Al coating presents typical nanocrystallite/amorphous structure. ► nc-TiC/a-C:Al coating has superior combining mechanical properties. ► nc-TiC/a-C:Al coating easily forms continuous and compacted graphitized tribofilm. ► nc-TiC/a-C:Al coating exhibits improved self-lubricating and anti-wear behaviors.

Providing a low, predictable and stable friction as well as low wear for mechanical components across a wide range from dry to wet atmosphere is of great importance in many engineering applications, the urgent challenge is to control tribological moisture sensitivity of their protective coatings. In this paper, the a-C:Si:Al carbon-based coating was fabricated successfully using multifunctional magnetron sputtering system. The microstructure and mechanical properties of as-fabricated coating were investigated; especially the friction and wear of coating sliding against Si3N4, SiO2, GCr15, brass and aluminum counterparts were evaluated under different relative humidity atmospheres. Results showed that the as-fabricated a-C:Si:Al carbon-based coating presented typical amorphous microstructure and good combined mechanical properties. Under low relative humidity conditions, the graphitized carbon tribo-layers on the contact surfaces were responsible for low friction; under high relative humidity conditions, the low shear strength colloidal silica on the contact surfaces induced by tribo-chemical reaction were mainly responsible for low friction. In particular, the lowest tribological moisture sensitivity can be achieved by the a-C:Si:Al/SiO2 pair, which was mainly attributed to the formation of tribo-layers on the contact surfaces induced by the graphitization and tribo-chemical reaction under different relative humidity conditions. These indicate that the a-C:Si:Al carbon-based coating might be a good candidate as low tribological moisture sensitivity material in engineering applications.Highlights► The a-C:Si:Al coating presented amorphous microstructure and good mechanical properties. ► Compacted carbonaceous tribo-film was formed in low relative humidity conditions. ► Colloidal silica tribo-film was formed in high relative humidity conditions. ► The a-C:Si:Al/SiO2 pair provided lowest tribological moisture sensitivity.

The existing challenges due to the extremely wear condition and variable serving environments require urgent improvements in the performance of solid lubrication coatings in helium atmosphere applications. In this paper, TiC/a-C:H and Ti–MoS2 coatings were deposited using magnetron sputtering process. The tribological performance of TiC/a-C:H and MoS2-based coatings were comparatively investigated under helium gas atmospheres. Results show that TiC/a-C:H coatings exhibit a super-low friction coefficient of around 0.02, and the wear rate of the TiC/a-C:H coatings was nearly one order of magnitude lower than Ti-MoS2 coatings. While the Ti–MoS2 coatings were characterized by severe adhesion wear and plastic deformation. The significant improvement in the tribological performance of TiC/a-C:H coatings can be attributed to the higher hardness and the large extent coverage of transferred carbon films on counterfaces. The good balance between hardness and toughness, super-low friction and excellent anti-wear properties of TiC/a-C:H coatings make them good substitution as solid lubricating coating for helium applications.Highlights► TiC/a-C:H coatings with good balance between hardness and toughness were deposited. ► TiC/a-C:H coatings exhibit super-low friction and lower wear rate than MoS2. ► TiC/a-C:H coatings are good solid lubricating coating for helium applications.

The friction and wear behaviors of DLC-based solid–liquid lubricating coatings for three liquid lubricants sliding against different counterface materials were examined under high vacuum conditions. Seven kinds of balls with the diameter of 3 mm were chosen as counterparts, which were GCr15, bronze, ZrO2, Al2O3, SiC, WC, and Si3N4. Under high vacuum condition, the friction coefficient (COF) and wear rate of carbon-based solid–liquid lubricating coatings sliding against different counterparts were diverse, due to different liquid lubrications and counterface materials. In analyzing the friction and wear mechanism, the contact radius and the contact pressure were introduced. The COF showed the inverse varied trend with Hertzian contact radius for the three liquid lubricants. COF decreased with the increase of contact radius, which was different from the dry sliding of DLC film. The possible reason was that the synergetic lubrication between the DLC films and the liquid lubricants formed a new solid–liquid synergistic system (DLC-oil-DLC), which improved the ability of plasticity-resistant deformation, reduced the real contact area and friction coefficient. The contact pressure was consistent with the wear rate varied trend for the three liquid lubricants, and this varied trend was in good agreement with each other for MACs lubricant.Graphical abstractHighlights► The tribological behavior of DLC solid–liquid lubricating coatings was studied in high vacuum. ► Seven kinds of balls of 3 mm diameter were chosen as counterparts. ► The friction coefficient showed the inverse varied trend with Hertzian contact radius. ► The contact pressure was consistent with the wear rate varied trend.

Graphite-like carbon (GLC) coatings are being increasingly used in mechanical seals, especially under the water-lubricated condition, to improve component durability by providing a low friction coefficient and high wear resistance. We have addressed the friction and wear performances of multilayer Cr/CrN/GLC coatings sliding against polyether–ether–ketone (PEEK), polyimide (PI), and polytetrafluoroethylene (PTFE) in distilled water under various applied loads and sliding speeds using a ring-on-block test rig. Stribeck curves were developed to analyze the water lubrication mechanisms. Start–stop tests were also carried out to evaluate the tribological performances of the coatings. Our results showed that the friction coefficients and wear rates of the polymers decreased with increasing sliding speeds. Compared to the applied load, the sliding speed had a major effect on the friction and wear performance of the polymer. Three coating/polymer tribopairs demonstrated different tribological behaviors, which were attributed to their different molecular structures, the different mechanical properties of the polymers, and the different lubrication mechanisms. The transfer film determined the lubrication mechanism of the three tribopairs and also resulted in the low friction coefficient and wear rate. The initial and steady-state friction coefficient also decreased with increases in the repeated start–stop times. Based on our results, we concluded that the coating/PEEK tribopair presented an excellent tribological performance that we ascribed to the chemical properties of PEEK and the hydrodynamic lubrication regime of the coating/PEEK tribopair at high sliding speeds.

In this paper, a kind of textured amorphous carbon film with the pattern of micro dots matrix was developed by irradiating amorphous carbon film with Nd–yttrium aluminum garnet laser system. Confirmed by the characterizations is that the produced micro dots are protuberant and in nanocrystalline graphite phase with a porous structure and reduced hardness. The micro tribological behavior of textured film was studied experimentally using steel balls and Si3N4 balls as the counter body. It turns out that the influences of laser treatment on the tribological performance of amorphous carbon film are strongly dependent on the friction pairs. By specially probing into the effects of localized micro graphite bulges, possible friction reduction mechanisms are discussed.Highlights► Textured a-C film is prepared by irradiating a-C with pulse laser. ► Irradiation results in nc-graphite bulges with a porous structure and reduced hardness. ► Laser treatment is negative to lower the friction when paired with Si3N4 ball. ► Laser treatment is very effective to lower the friction when paired with steel ball. ► The formed lubrication film enriched with nc-G is important to friction reduction.

A novel ultrathin dual-layer film, which contained both bonded and mobile phases in ionic liquids (ILs) layer, was fabricated successfully on a silicon substrate modified by a self-assembled monolayer (SAM). The formation and surface properties of the films were analyzed using ellipsometer, water contact angle meter, attenuated total reflectance Fourier transform infrared spectroscopy, multi-functional X-ray photoelectron spectroscopy, and atomic force microscope. Meanwhile, the adhesive and nanotribological behaviors of the films were evaluated by a homemade colloidal probe. A ball-on-plate tribometer was used to evaluate the microtribological performances of the films. Compared with the single-layer ILs film deposited directly on the silicon surface, the as-prepared dual-layer film shows the improved tribological properties, which is attributed to the special chemical structure and outstanding physical properties of the dual-layer film, i.e., the strong adhesion between bonded phase of ILs and silicon substrate via the chemical bonding with SAM, the interlinked hydrogen bonds among the molecules, and two-phase structure composed of steady bonded phase with load-carrying capacity and flowable mobile phase with self-replenishment property.Graphical abstractUltrathin dual-layer ionic liquid lubrication film, which contained both bonded and mobile phases in ionic liquids layer was assembled on silica surface.Research highlights► Dual-layer ultrathin ionic liquid lubrication film which contained both bonded and mobile phases was formed on a silicon substrate by a two-step self-assembly process. ► Both anchor layer (self-assembled monolayer) and bonded ionic liquid phase acts very important effects in enhancing load-carrying capacity of the ionic liquid ultrathin lubrication film. ► The synergic effect between flowable mobile phase and steady bonded phase plays a significant role in improving tribological properties of the ionic liquid lubrication film.

Tungsten trioxide nanoparticles doped amorphous hydrogenated carbon (WO3/a-C:H) films were successfully fabricated on silicon substrates by electrochemical deposition technique under atmospheric pressure. The as-deposited films were characterized by X-ray photoelectron spectroscopy, Transmission electron microscopy, Fourier transform infrared spectroscopy and Raman spectroscopy, respectively. Results showed that nanocrystalline tungsten trioxide particles with a grain size in the range of 8–12 nm were homogeneously embedded in the amorphous carbon matrix, and the sp3-hybridized carbon content in the a-C:H films increased. Compared with pure amorphous carbon film, the hardness and elastic recovery were significantly improved due to the doping of WO3. At the end, an understandable model was proposed to interpret the growth mechanism of the WO3/a-C:H composite films.

Nanocomposite materials based on graphene and ionic liquids (ILs) with unique and highly attractive properties have received considerable interest in various research fields, including biosensors, electrochemical sensors, and so on. Given the excellent mechanical properties and frictional properties of graphene nanosheets, nanocomposite ultrathin films composed of graphene nanosheets and ionic liquids (ILs) with excellent lubricating property are expected to possess improved comprehensive tribological performance. In the current paper, various functionalized graphene–IL nanocomposite ultrathin lubrication films on Si substrates, on the basis of the good dispersion of graphene nanosheets that were noncovalently functionalized by imidazolium-based ILs in acetone, were successfully prepared by an electrostatic adsorption method and were confirmed by several characterization techniques. Appropriate amounts of functionalized graphene nanosheets uniformly distributed on the substrate surface without overlapping greatly enhanced the load-carrying capacity of the ultrathin lubrication films, and the new nanocomposite films gave excellent micro/nanotribological properties. The novel nanocomposite films are hoped to find promising applications in the lubrication of micro/nanoelectromechanical systems (MEMS/NEMS).

Owing to the requirements of the stable operation for mechanical components, the urgent challenges are to control tribological moisture sensitivity of protective coatings. In this letter, a-C:Si and a-C:Si:Al carbon-based coatings were successfully fabricated via magnetron sputtering Si, Al, and C. The microstructure, mechanical properties, and tribological moisture sensitivity of as-fabricated carbon-based coatings were comparatively investigated. Results showed that the as-fabricated a-C:Si and a-C:Si:Al coatings were dominated by typical amorphous structure. The co-introduction of Al could effectively relax internal stress and improve adhesive strength as well as maintain the moderately high hardness for the as-fabricated coating. The striking improvement in tribological moisture sensitivity of a-C:Si:Al carbon-based coating was mainly attributed to the superior mechanical properties and the formation of continuously compacted graphitized tribofilm under low relative humidity condition as well as low shear strength colloidal silica tribofilm under high relative humidity condition. The good balance between the hardness and toughness, low internal stress, and superior low tribological moisture sensitivity of a-C:Si:Al coating make it a good candidate for solid lubricating coating in engineering applications.

Solid lubricant films have received considerable research attention in the last decades owing to their remarkable improved tribological characteristics. In this paper, the abrasive wear behaviour of five types of solid lubricant films (magnetron-sputtered diamond-like carbon, magnetron-sputtered molybdenum disulfide, bonded molybdenum disulfide, bonded polytetrafluoroethylene and bonded graphite) in sand-dust environment has been investigated using a reciprocating pin-on-disc test rig. The effects of applied load, amount of sand and particle size on the tribological performance of these films were systemically studied. Experimental results show that magnetron-sputtered films give excellent anti-friction and wear-resistance performances under sand-dust environments compared to bonded solid lubricant films. The significant differences of surface roughness, hardness, microstructure and intrinsic lubricating property directly lead to the different tribological performances and worn morphology. The formed composite transfer layer plays a vital role in reducing friction and wear due to its anti-friction and shielding action of the film surface from the hard metal asperities. Two main abrasive wear mechanisms (three-body rolling wear and two-body grooving wear) occur simultaneously in the tribological process under sand-dust environments. A transfer layer-hardening composite wear modeling was established to further explain the anti-wear mechanisms and friction-reducing capacity of these solid lubricant films under sand-dust environments.Highlights► The sand-dust markedly affects the tribological performance of solid lubricant film. ► The sand particles reduce the COF of film but not increase the wear rate. ► The transferred composite layer plays the vital role in reducing friction and wear. ► The films possess a certain extent sand-dust environmental adaptability. ► A transfer layer-hardening composite model to clarify the wear mechanisms.

In the present study, five kinds of liquid lubricants, including MACs, IL, Zdol, PAO and silicon oil, were homogeneously spun on DLC coatings and consequently the DLC-based solid–liquid lubricating dual-layer coatings were successfully fabricated. The tribological behaviors of the DLC/steel combinations and steel/steel combinations lubricated with above five liquid lubricants were comparatively investigated using a high-vacuum tribometer simulated for space environments. The SEM, 3D surface profiler, Raman spectrum and EDS were employed to analyze the worn surfaces of the friction pairs. The analysis results showed that three kinds of solid–liquid lubricating coatings including MACs, IL, Zdol exhibited excellent tribological properties, in which DLC/steel combinations lubricated with these three liquid lubricants exhibited smoother running-in and much lower steady-state friction than steel/steel combinations, and the wear rates of DLC/steel combinations were in the range of 1–3 orders of magnitude lower than that of steel/steel combinations. But for PAO and silicon oil, they demonstrated the poorest tribological behaviors with high friction coefficients and wear rates. Such significant improvement in tribological performance of DLC-based solid–liquid lubricating dual-layer coatings under high vacuum can be attributed to the synergy lubrication mechanism by the combination of the solid lubrication effect of DLC film and the boundary lubrication of the liquid lubricants. Moreover, the differences among different liquid lubricants in tribological properties of DLC-based solid–liquid lubricating dual-layer coatings under high vacuum were possible due to the differences in viscosity and components of the five kinds of liquid lubricants.Highlights► Five DLC-based solid–liquid lubricating dual-layer coatings were successfully fabricated. ► Solid–liquid lubricating coatings of MACs, IL, Zdol exhibited excellent tribological properties. ► Solid–liquid lubricating coatings of PAO and silicon oil showed the poor tribological behaviors. ► Such difference depends on nature of liquid lubricant, and synergy lubricating mechanism with DLC.

Patterned mixture component surfaces, offering a means for controlling the adhesion and the wetting behavior of materials, have attracted great interest. In this letter, a patterned dual-component lubricant film consisting of multiply-alkylated cyclopentanes (MACs) mobile lubricant trapped and maintained among the patterned octadecyltrichlorosilane self-assembled monolayer (OTS-SAM) network was fabricated on surfaces of silicon using an elastomeric stamp. Tribological behavior of the patterned MACs–OTS dual-component film was investigated, comparing with MAC film and patterned OTS-SAM. The patterned MACs–OTS dual-component film has shown to have improved load-bearing capacity, anti-wear, and self-lubricating ability. It can remain as an effective lubricant layer for more than 3,600 s as the load increased to 0.4 N, and the average friction coefficient is about 0.1. Compared with patterned OTS-SAM and MACs film, patterned MACs–OTS dual-component film showed best load-carrying capacity and durability at applied loads of 0.1–0.5 N.

Textured diamond-like carbon (DLC) films with the pattern of parallel grooves were developed by depositing DLC on textured stainless substrates in a PVD system. The texturing effects on tribological performance of DLC in water-lubricated condition were investigated. Results show that introducing specific patterns into DLC film not only retains the low friction coefficients, but also dramatically extends coating lifetime through affecting the coating delamination behavior and graphitization process during friction. Besides the adherence difference induced by surface texturing which could influence the delamination, another possible mechanism, “buffer stripes”, which is characteristic of the lateral soft/hard periodical structure, was proposed by us based on the Micro-Raman spectroscopy and nanoindentation analysis. Additionally, a much lower graphitization for textured DLC during friction may also be responsible for the improved wear resistance.

A duplex treatment involving nitrogen ion pre-implantation and gradient interfacial transition was performed to obtain a high-performance graphite-like carbon (GLC) coating on a Ti6Al4V alloy. Characteristics of the as-deposited coating systems were systemically investigated by Raman spectrometry, scanning electron microscopy, atomic force microscopy, nano-indentation, and scratch tests. The friction and wear behaviors in distilled water and sea water environments were evaluated by a ball-on-disk tribometer. The results showed that the GLC multilayer coating on nitrogen ion-implanted Ti6Al4V possessed a greater hardness and adhesion strength than to that on un-implanted Ti6Al4V. The tribological performances of these duplex process systems showed a great improvement in both the distilled water and sea water environments. In particular, the Cr/CrN/GLC coatings on nitrogen ion-implanted substrates demonstrated the best friction and wear behaviors. These striking improvements were attributed to the greatly enhanced interface strength between substrate and coating by the nitrogen ion implantation process and improved adhesion strength between gradient layers by the appropriate gradient interlayers with a similar thermal expansion coefficient.

Fast development of micro/nanoelectromechanical systems (MEMS/NEMS) and high-density storage technology (HDT) have stimulated the development of new materials that require hydrophobic surfaces with low adhesion and friction. Micro/nanohierarchical structures and chemical modification are two useful methods for improving nanotribological properties of mechanical components. In this study, Au surfaces with micro/nanohierarchical structures were prepared by replication of micropatterened silicon surfaces using PDMS and self-assembly of alkanethiol [CH3(CH2)9SH] to create hydrophobic micro/nanohierarchical structures and to improve nanotribological properties of MEMS/NEMS. The effects of nanoscaled roughness (including pillar height and pillar fractional surface coverage) and chemical modification on the wetting and nanotribological properties of surfaces were systemically investigated. Results show that with the increasing of nanoscale roughness and lowering of surface energy, the surface becomes more hydrophobic, and the adhesive force and friction force are reduced greatly.Keywords: height and fractional surface coverage; MEMS/NEMS; nanotribological performance; pillar; SAMs

Inspired by plants and animals in nature that show obvious superhydrophobic performance with high or low adhesion, developing artificial ways to mimic these surfaces is meaningful and practical for mankind. In this work, we used a simple, efficient, and highly reproducible method for producing large-area positive and negative lotus and rice leaf topography on Au surfaces based on PDMS, and then the as-prepared surfaces were chemically modified with alkanethiol to enhance hydrophobicity. Surface morphologies of Au surfaces with biomimetic micro/nanobinary textures were examined by SEM and 3D noncontact optical profilometry. Hydrophobic properties of surfaces were characterized by the contact angle and sliding angle between a water droplet and the as-prepared Au surfaces, and the smooth Au surface was provided as a comparison. Results show that both positive and negative biomimetic textures (lotus leaf and rice leaf) were successfully generated on Au surfaces, and Au surfaces with biomimetic textures exhibited improved hydrophobic ability after chemical modification. Adhesion properties between water droplet and Au surfaces with positive and negative biomimetic micro/nanotextures showed nearly opposite performance. An understandable model is proposed to interpret the mechanism which causes the different adhesion performance between Au surfaces with positive and negative biomimetic structures. The strong adhesion is attributed to van der Waals and the capillary force interactions between the biomimetic Au surfaces with negative plant topographies and water droplet.

Surface micro/nanohierarchical structure designing and thin film lubrication are two main methods to alleviate adhesive and frictional problems encountered by micro/nanoelectromechanical systems (MEMS/NEMS). In this study, silicon surfaces with micro/nano grooves were prepared by photolithography and further chemically modified by multiply-alkylated cyclopentane (MAC) thin films to improve the tribological behaviors of MEMS/NEMS. The effects of nanoscaled roughness (including groove pitch and groove fractional surface coverage) and chemical modification on the wetting and tribological properties of surfaces were systemically investigated. The results of contact angle measurement indicated that the surface hydrophilicity decreases with increasing of surface roughness and lowering of the surface energy (chemical treatment with MACs). Tribological study showed that with the increasing of nanoscale roughness and combined with chemical modification, the tribological properties are greatly improved, which may be affected by real area of contact and the surface chemistry.Graphical abstractResearch highlights▶ MACs thin film modified silicon surfaces with micro/nano grooves were prepared. ▶ The surface hydrophilicity of MACs film modified silicon with grooves decreases. ▶ The micro/nanotribological properties are improved greatly. ▶ The improvement could be due to the increasing area density and chemical treatment.

Two kinds of ultrathin dual-layer films, which contain both bonded and mobile phases in ionic liquids layer, were fabricated on silicon substrates by a two-step process. As an anchor layer, (3-aminopropyl)triethoxylsilane (APS) and N-[3-(trimethoxylsilyl)propyl]ethylenediamine (DA) separately self-assembled on silicon surfaces, then a few ionic liquids molecules were chemically bonded to the silicon substrates modified by the self-assembled monolayers (SAMs) to form two-phase structure. The formation and surface properties of the films were analyzed by means of ellipsometric thickness measurement, water contact angle measurement, attenuated total reflection Fourier transform infrared (ATR-FTIR) spectrometry, multi-functional X-ray photoelectron spectra (XPS), and atomic force microscope (AFM), It is shown that DA form more densely packed films than APS, and the density of the amino-terminated underlayer largely affects the density of the subsequent bonded ionic liquids layer. The adhesive and nanotribological behaviors of the films were evaluated by a homemade colloidal probe. A ball-on-plate tribometer was used to test the microtribological performances of the films. As a result, the two kinds of dual-layer films show the improved tribological properties than the single-layer ionic liquids film deposited directly on the silicon surface, which is ascribed to synergic effect between flowable mobile phase and steady bonded phase. In particular, the dual-layer film containing DA-SAM possesses excellent micro/nanotribological properties characterized by lower friction and higher antiwear ability due to more densely packed bonded phase and a few mobile phases. By studying the influences of self-assembled underlayer on tribological properties of ultrathin dual-layer film, we might find the way to further improve tribological properties and acquire insights into their potential in resolving the tribological problems of micro-electromechanical systems (MEMS).Graphical abstractUltrathin dual-layer ionic liquid lubrication film with bonded and mobile phases on SAM modified Si substrate.Research highlights▶ Two kinds of dual-layer ultrathin ionic liquid lubrication films were formed on a silicon substrate by a two-step self-assembly process. The synergic effect between flowable mobile phase and steady bonded phase plays a significant role in improving tribological properties of the ionic liquid lubrication film. ▶ More densely packed SAMs produce more densely packed bonded phase and reduce meniscus effect resulting from excessive mobile molecules because more ionic liquid molecules are chemically bonded thereon, which would lead to low micro/nano-friction coefficient of dual-layer films. ▶ The improved durability of dual-layer films is closely related to high load-carrying capacity of more densely packed and ordered bonded phase.

Nanotribological properties play an important role in many applications, such as micro/nanoelectromechanical systems (M/NEMS) and high-density storage technologies (hard drivers). Therefore, it is important to study the nanotribological properties of these surfaces which are separated by only a couple of nanometers. Patterning feature or surface texture with micro/nano-scale dimension is of great importance for solving these problems. In this work, micro/nano-patterned Au surfaces with different topography features were fabricated via a convenient method and then chemically modified by alkanethiol SAMs for optimization of their nanotribological performance. Surface composition and morphologies of Au surfaces with different micro/nano-textures and chemical modification were evaluated by XPS, contact angle measurements, 3D non-contact optical microscopy and AFM. AFM/FFM was used to investigate the nanotribological behaviors of Au surfaces with different micro/nano-textures and corresponding chemical modification. Results obtained in this work demonstrated the feasibility of fabricating surface textures with micro/nano-scale cylindrical holes and the possibility of controlling chemical composition on surface to improve the nanotribologcal performance of Au surface. Nanotribological properties of textured Au surfaces were greatly determined by the fractional surface coverage of cylindrical holes and consequent chemical modification. Au textured surface with dense cylindrical holes and further chemically modified with self-assembly monolayer showed significantly enhanced hydrophobicity and nanotribological performance compared with Au textured surface with sparse cylindrical holes without chemical modification.

The performance of micro- and nano-electromechanical systems (M/NEMS) depends on the surface and interface properties of the substrate, such as chemical composition, roughness, friction, adhesion, and wear. In order to solve these problems and improve the performance of M/NEMS, molecularly thin films of room temperature ionic liquid (RTIL)-1,3-di(2-hydroxyethyl)imidazolium hexafluorophosphate which has two terminal hydroxyl groups were prepared on silicon substrate. Thermal stability of the RTIL was evaluated using thermogravimetric analysis in a nitrogen atmosphere. A multi-functional X-ray photoelectron spectrometer was used to investigate the chemical compositions of the films. The morphology, nano-friction and nano-adhesion properties of RTIL films with different heat treatment were experimentally investigated at nanoscale using atomic force microscopy/friction force microscopy. The wear-resistant property was tested on a ball-on-plate microtribometer. The results revealed that the micro/nano-friction and adhesion properties of RTIL films were significantly improved with appropriate heat treatment. The corresponding friction reduction and anti-adhesion mechanisms of the tested ultra-thin RTIL films under tested condition were proposed based on the experimental observations. For the micro/nano-friction, bonding ratio of the lubricant film had great effect on the RTIL's performance.

Graphite-like carbon (GLC) nanocomposite films were fabricated by DC magnetron sputtering using high pure graphite target at ambient temperature. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) investigation showed that the as-deposited GLC films have high concentration of sp2-hybridized carbon. High-resolution transmission electron microscopy (HRTEM) images and selected area diffraction patterns (SADP) indicated a complex nanocomposite microstructure of the GLC films. As well as nanocrystalline graphite, a face-center cubic (fcc) diamond with a grain size in the range of 3–8 nm were dispersed in the amorphous carbon matrix inhomogenously and integrally. The nanocomposite GLC film had high hardness of 23 GPa, which was attributed to the mutual strengthening effect of nanoparticles and amorphous matrix. More importantly, the as-deposited nanocomposite GLC film exhibited excellent self-adapted tribological properties in different environments of ambient air, different relative humidity and water. The friction coefficients were 0.053 in ambient air and 0.046 in distilled water, while specific wear rates were 4.5 × 10−16 m3 N−1 m−1 and 1.6 × 10−16 m3 N−1 m−1, respectively. The friction regimes and mechanisms in different environments were elaborated. This film is foreseen to high potential in protecting and solid lubricating material in humidity or water environment.

Friction has a direct relation with the energy efficiency and environmental cleanliness in all moving mechanical systems. To develop low friction coatings is extremely beneficial for preserving not only our limited energy resources but also the earth’s environment. This study proposes a new design for low friction carbon-based nanocomposite coatings by tailoring the microstructure and phase segregation, and thereby it contributes to better controlling the mechanical and tribological properties. Experimental findings and theoretical calculations reveal that high-hardness (18.2 GPa), high-adhesion strength (28 N) as well as low-internal stress (−0.8 GPa) can be achieved by a nanocrystallite/amorphous microstructure architecture for the nc-WC/a-C(Al) carbon-based nanocomposite coating; in particular low friction (∼0.05) can be acquired by creating a strong thermodynamic driving force to promote phase segregation of graphitic carbon from the a-C structure so as to form a low shear strength graphitic tribo-layer on the friction contact surfaces. This design concept is general and has been successfully employed to fabricate a wide class of low friction carbon-based nanocomposite coatings.

It is currently a challenge for space tribology to develop a long lifetime and high bearing capacity lubricant meeting the requirements of space applications. Herein, we dispersed graphene into ionic liquid, prepared novel composite coatings of diamond-like carbon (DLC)/ionic liquid (IL)/graphene with different graphene concentrations, and investigated its space performance under high vacuum and space radiation conditions. IL/graphene nanofluids with different concentrations were examined by Fourier transform infrared spectroscopy (FTIR). Furthermore, IL/graphene nanofluids after friction tests were investigated by X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HRTEM). The results showed that the graphene concentration would obviously affect the spatial tribology performance of the composite coatings. Because the excess graphene in the IL would tend to form irreversible agglomerates, leading to reduction of the effective graphene dose, an optimum graphene concentration (0.075 mg ml−1) in IL for the composite coatings was required to exhibit the lowest friction coefficient, the highest bearing capacity and the strongest anti-irradiation in a simulated space environment. In addition, XPS spectra further confirmed that the formation of a fluorinated oil-containing carbon-rich tribofilm between the friction pairs further ensured the good antifriction and wear resistance performance of DLC/IL/graphene.

Based on the microbumps of graphene nanosheets and the nanostructure of carbon nanotubes (CNTs), novel graphene/CNTs composite films with hierarchical micro- and nanoscale surface roughness were successfully fabricated by simply spraying the mixed acetone dispersion of graphene nanosheets and CNTs onto stainless steel substrates. The as-prepared composite films exhibited controlled surface hydrophobic, adhesive and electrowetting properties via altering the film surface structure and surface energy. Among them the composite film with a 1:5 mass ratio of graphene to CNTs showed high hydrophobicity and conductivity, low water adhesion and contact angle sensitivity to the external electric field, which would help to resolve the surface electrostatic problems and unstable hydrophobicity under applied potential that exist in many conventional insulating hydrophobic materials, and could be useful in some application fields.

The novel graphene–C60 hybrid films have been fabricated successfully on silicon surfaces by a multistep self-assembly process, and showed synergistic effects beyond individual performance in micro/nano-tribological behaviors. It is expected that the graphene–C60 hybrid films may find wide applications as high performance lubricating films in MEMS.

Friction on a nanoscale revealed rich load-dependent behavior, which departs strongly from the long-standing Amonton's law. Whilst electrostatic repulsion-induced friction collapse for rare gas sliding over metallic surfaces in a high-load regime was reported by Righi et al. (Phys. Rev. Lett., 2007, 99, 176101), the significant role of attraction on frictional properties has not been reported to date. In this study, the frictional motion of Xe/Cu(111), Xe/Pd(111) and Ar/Cu(111) was studied using van der Waals corrected density functional calculations. An attraction-induced zero friction, which is a signal of superlubricity, was found for the sliding systems. The superlubric state results from the disappearance of the potential corrugation along the favored sliding path as a consequence of the potential crossing in the attractive regime when the interfacial pressure approaches a critical-value. The finding of an attraction-driven friction drop, together with the repulsion-induced collapse in the high-load regime, which breaks down the classic Amonton's law, provides a distinct approach for the realization of inherent superlubricity in some adsorbate/substrate interfaces.