Synergistic Lubricating Effect Research Articles

Friction and wear properties of GLC and DLC coatings under ionic liquid lubrication

Abstract Cr-doped graphite-like carbon (Cr-GLC) coatings and Cr-doped diamond-like carbon (Cr-DLC) coatings were prepared by PVD and PECVD, respectively. The lubrication behaviour of the solid-liquid composite lubrication systems was investigated using two ionic liquids (ILs) as lubricants. The results show that the friction coefficient was reduced by approximately 40% compared with that under the dry condition, and the composite system exhibited a good synergistic lubrication effect. The Cr-DLC coating showed better tribological behaviour than the Cr-GLC coating, which can be ascribed to a better physicochemical film formation under friction and the compact microstructure of the Cr-DLC coating. The synergistic effect of the composite systems was affected by the viscosity and corrosiveness of the ILs and the microstructure of the coatings.

Synthesis of poly(ionic liquid)s brush-grafted carbon dots for high-performance lubricant additives of polyethylene glycol

Poly(ionic liquid)s brush-grafted carbon dots (CDs-PILs-X) were synthesized for the first time by surface-initiated atom transfer radical polymerization, where X representing polymerization time was 1, 3, 6, 12, and 18 h. Their tribological properties as lubricant additives of polyethylene glycol (PEG200) were evaluated under the steel/steel contact and linear reciprocating mode. The CDs-PILs-1 exhibited the best friction-reducing and antiwear performance at a concentration (c) of 1.5 wt%. Specifically, the friction coefficient and wear volume of PEG200 were reduced by 61.9% and 82.5%, respectively. At a concentration (c) of 1.5 wt%, the performance of CDs-PILs-1 apparently outperformed that of PILs-1, reflecting the particularly important role of carbon cores in boundary lubrication. In addition, the CDs-PILs-1 was comparable to CDs-PILs-3 and much superior to CDs-PILs-6-18, suggesting that the carbon cores and PILs shells of CDs-PILs played a synergistic lubrication effect and an optimal degree of polymerization of 3.0 (close to that of CDs-PILs-1) should exist. Wear scar surface analyses revealed that robust boundary tribofilms with a thickness of approximately 190 nm, that is, the tribochemical film embedded with carbon cores, were formed on the sliding surfaces lubricated by CDs-PILs-1 dispersion, directly accounting for the excellent tribological properties of CDs-PILs-1.

In Situ Green Synthesis of the New Sandwichlike Nanostructure of Mn3O4/Graphene as Lubricant Additives.

Nanoparticles and two-dimensional (2D) nanosheets are well-investigated as lubricant additives, which can significantly reduce frictional energy consumption. However, the tribological properties of the additives will deteriorate because of the occurrence of aggregation in the lubricant and the difficulty in entering the frictional contact area. In the present work, the new sandwichlike nanostructure of Mn3O4 nanoparticles and graphene nanosheets (Mn3O4@G) has been developed by an in situ green synthesis method; i.e., the impurities of Mn2+ ions in crude graphite oxide as the precursor are directly transferred into Mn3O4 precipitate between the graphene sheets. The graphene has a lamellar structure without folds and wrinkles, and the Mn3O4 nanoparticles are not only uniformly anchored on the graphene surfaces but also intercalated in the layers of the graphene nanosheets. The Mn3O4@G exhibits excellent tribological properties and high stability because of a synergistic lubrication effect between the graphene nanosheets and the Mn3O4 nanoparticles. Even at an ultralow concentration (0.075 wt %) and a high temperature of 125 °C, the friction coefficient and the wear depth have been reduced by 75% and 97% compared with base oil, respectively. The synthesis method and the Mn3O4@G nanocomposite have significant potential in various tribological applications for saving energy.

Monodisperse Cu nanoparticles @ MoS2 nanosheets as a lubricant additive for improved tribological properties

Monodisperse Cu nanoparticles @ MoS2 nanosheets were prepared via a facile in-situ reduction method. The anchored Cu nanoparticles with average diameters of 8–13 nm were homogeneously decorated on the polydopamine (PDA) functionalized MoS2 nanosheet surface (Cu/PDA/MoS2). The resulting Cu/PDA/MoS2 nanosheets were highly dispersed in sunflower oil because the PDA film provided a large number of active anchoring sites. The tribological performances of the Cu/PDA/MoS2 nanosheets as a sunflower oil lubrication additive were investigated. The Cu/PDA/MoS2 nanosheets demonstrated optimal lubrication properties in all the sliding tests. For 0.5 wt% Cu/PDA/MoS2 nanosheets mixed with sunflower oil, the average friction coefficient and wear scar width were decreased by 37.8% and 28.5% compared with those for the neat sunflower oil, respectively. The excellent lubrication performance can be ascribed not only to the synergistic lubricating effects of the monodisperse Cu nanoparticles and MoS2 nanosheets but also the formation of stable transfer films on the rubbing surfaces. Furthermore, the lubricating mechanism of the Cu/PDA/MoS2 nanosheets was investigated by studying the worn steel disk surfaces via energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS).

Boundary and Elastohydrodynamic Lubrication Behaviors of Nano-CuO/Reduced Graphene Oxide Nanocomposite as an Efficient Oil-Based Additive.

Copper oxide/reduced graphene oxide (CuO/rGO) nanocomposite was synthesized by an effective approach combining supercritical hydrolysis and facile thermolysis. The as-synthesized CuO/rGO exhibited a unique microstructure in which ultrafine CuO nanoparticles were encapsulated by several stacked rGO nanosheets. The highly dispersed CuO nanoparticles on the rGO nanosheets were in spherical shape with diameters around 6 nm and had a base-centered monoclinic crystal structure. Tribological performances of the CuO/rGO nanocomposite as a lubricant additive of 10w40 engine oil were evaluated by a ball-on-disc tribotester under boundary lubrication. As compared to the bare 10w40 engine oil, the engine oil with 0.06-0.18 wt % CuO/rGO exhibits the reductions of 46.62 and 77.05% for friction coefficient and wear rate, respectively. In addition, CuO/rGO as a nanoadditive displays superior lubricating abilities than rGO, CuO, and their mechanical mixture. This synergistic lubricating effect of the CuO/rGO nanocomposite was mainly contributed to its special nanostructure rather than only the coexistence of CuO and rGO. The antiwear and friction-reducing mechanism of the CuO/rGO nanoadditive mainly results from its synergistic lubricating effect and the formed tribofilms on the wear surface due to the spontaneous deposition behavior of the nanocomposite. In addition, influences of the nanoadditives including rGO, CuO, the mixture, and CuO/rGO on the rheological behaviors and the elastohydrodynamic lubrication (EHL) oil film of the modified engine oil were also investigated. Results show that these nanoadditives can increase the viscosity of the engine oil to a certain extent throughout the test temperature range of 0-60 °C. The remarkable increase in viscosity by addition of the CuO/rGO nanoadditive corresponds to the thickening of the EHL film and the strengthening of the EHL state for the engine oil with CuO/rGO as compared to the bare one.

Structural and tribological properties of CrMoCN coatings with various Mo contents in artificial seawater

CrMoCN coatings with various Mo contents were prepared on Ti6Al4V disks and Si wafers using unbalanced magnetron sputtering at various Mo target currents, and their crystallite phases and bonding structures were characterized using XRD and TEM as well as XPS, respectively. The mechanical properties of coatings were measured by nano-indenter and the tribological properties of CrMoCN/SiC tribopairs in artificial seawater were tested by ball-to-disk tribo-tester. The results showed that Cr, Mo and N elements mainly existed in the form of (Cr, Mo)N solid solution crystals, while C element mainly existed as amorphous a-C and amorphous a-CNx. The hardness and elastic modulus of CrMoCN coatings increased first, and then decreased as Mo target current further increased to 3.0 A. The friction coefficients and wear rates of CrMoCN/SiC tribopairs decreased overall with the increase in the Mo target current due to the synergistic lubrication and wear-reducing effect of tribolayers (MoO3 and SiO2·2H2O). The abrasive and tribo-chemical wear were the wear mechanisms of CrMoCN/SiC tribopairs. CrMoCN-3.0 coating showed the best tribological properties in the artificial seawater.

Effect of substrate temperatures on the properties of PLD Mo–V–Ag–O nanocomposite thin films

Nanocomposite Mo–V–Ag–O films were deposited on Inconel 718 by pulsed laser deposition (PLD) technique with self-prepared composite target. The effects of substrate temperatures on the growth characteristics, mechanical and tribological properties of nanocomposite films were investigated. The phase transformation, micro-structure and element content of target and films were investigated by XRD and FESEM equipped with EDS, respectively. The chemical and phase changes in wear tracks were measured by Raman spectrometer. The results showed that the high ingredient uniformity of target and onset of crystallization of film occurred at substrate temperature of 400 °C. Higher substrate temperature could reduce defects and improve the crystallization and micro-hardness of the composite films. Furthermore, the composite film showed the lowest friction coefficient about 0.24 at 500 °C which could be ascribed to the synergistic lubricating effect of vanadium oxide (VnO2n±1), molybdenum oxide (MoO3) and silver vanadate (AgVO3) and the formation of lubrication glaze on the worn surfaces.

Branched polyelectrolyte grafted carbon dots as the high-performance friction-reducing and antiwear additives of polyethylene glycol

Branched polyethyleneimine (PEI) grafted carbon dots (CDs-PEI0.6-10k) were facile synthesized by hydrothermal treatments of citric acid and PEI with various molecular weights of 0.6, 1.8 and 10 k. With the aid of protonation and anion exchange (between Cl− and bistrifluoromethanesulfonimide abbreviated as NTf2-) processes in aqueous phase, the branched polyelectrolyte grafted CDs (CDs-PEI0.6-10k-NTf2) derived from CDs-PEI0.6-10k were gained as the precipitates. The tribological properties of CDs-PEI0.6-10k-NTf2 as the lubricant additives of polyethylene glycol (PEG200) were evaluated. The CDs-PEI0.6-10k-NTf2 exhibited superb friction-reducing and antiwear performances followed the sequence: CDs-PEI10k-NTf2 > CDs-PEI1.8k-NTf2 > CDs-PEI0.6k-NTf2. Typically, the friction coefficient and wear volume of PEG200 reduced by 53.8% and 79.9%, respectively, when only 0.07 wt% of CDs-PEI10k-NTf2 was introduced. The surface analysis results revealed that a tribochemical reaction film composed of iron compounds (including iron fluorides, sulfates, nitrides, oxides and carbides) and organic compounds containing various elements (such as C, O, N, S and F) should form on the rubbing surfaces lubricated by CDs-PEI10k-NTf2 (0.07 wt%)/PEG200 dispersion. The distinguished friction-reducing and antiwear properties of CDs-PEI10k-NTf2 could be attributed to their nanolubrication effects and the above tribochemical film, i.e. the synergistic lubrication effect of their carbon cores and surface groups.

Solvent-free graphene liquids: Promising candidates for lubricants without the base oil

Reduced graphene oxide modified by hyperbranched polyamine-ester (RGO-HBPE) liquids was successfully fabricated through the surface chemical engineering and tested for serving as a solvent-free novel lubricant. Structural characterization, dispersibility and rheology behavior of the lubricant and the related frication performance on steel plate were investigated thoroughly. The results manifest that the RGO-HBPE exhibited good dispersity in distilled water and liquid behavior without any solvent at ambient temperature. And this RGO-HBPE liquids could be directly introduced onto the surface of steel plate as lubricants without any additional base oil. Tribological results and the proposed lubricating mechanism of RGO-HBPE imply that the fluidity of RGO-HBPE is favorable for lubrication and is crucial to reduce the friction coefficient. The spontaneous flow of RGO-HBPE provide a spreading effect to form the lubricating film. The specific spreading effect of RGO-HBPE and the synergistic lubricating effect between HBPE and graphene demonstrate that RGO-HBPE could be directly used as promising candidates for lubricants in nowadays moving machines.

Vacuum Tribological Performance of WS2–MoS2 Composite Film Against Oil-Impregnated Porous Polyimide: Influence of Oil Viscosity

To achieve the desired levels of performance and durability for bearings applied in space, a novel solid–liquid dual lubricating system was established via the WS2–MoS2 composite films sliding against oil-impregnated porous polyimide (PPI), in which polyalphaolefin (PAO) oils with different viscosities were used, respectively. The contact angle measurement indicated that the PAO oil had good wettability on the surfaces of the composite film and the PPI. The vacuum tribological behaviors of single WS2–MoS2 composite film and the dual lubricating system were mainly evaluated under different loads and speeds. In comparison with single WS2–MoS2 composite film, the dual lubricating system exhibited the low friction coefficient and synergistic lubricating effect for the low-viscosity PAO oil under severe sliding conditions. It was concluded that the cleavages of the WS2 and MoS2 crystals along the basal plane were more sufficient and formed small and thin WS2 and MoS2 platelets in the dual lubricating system. Meanwhile, it was found that oil-impregnated PPI readily released PAO lubricating oil with low viscosity, which further decreased friction on the contact interface. The lubricating mechanism of the dual lubricating system was also revealed after correlating the tribological behaviors of the different lubricating systems.

Composite Nanoparticles Based on Hydrotalcite as High Performance Lubricant Additives

Composite nanoparticle additives of hybrid layered double-hydroxide/graphene nanosheets (LDH/GO) and layered double-hydroxide/molybdenum disulfide (LDH/MoS2) have been fabricated by electrostatic self-assembly. The morphologies and compositions are confirmed by FESEM, EDS, XRD, Raman, and FT-IR. Zeta potential testing confirms that the composite nanoparticles have good dispersion stability in poly-α-olefins-4 (PAO-4). Meanwhile, the nanocomposites as additives can dramatically enhance the lubricating properties of PAO-4. Research results indicate that the good friction and antiwear properties of LDH/GO and LDH/MoS2 composites can be attributed to the synergistic lubricating effects between LDH and GO or MoS2.

Preparation and tribological properties of WS2 and WS2/TiO2 nanoparticles

Abstract A method was developed to prepare WS2 and WS2/TiO2 nanoparticles. A blue tungsten oxide precursor was synthesized from Na2WO4, CH3CSNH2, and HCl. WS2 nanoplatelets was produced after sulfurizing the precursor at 300 °C. Nano-TiO2 could be activated by HCl and provide active sites for the deposition of the precursor to produce WS2/TiO2 nanoparticles after sulfurization. The as-prepared two nano-additives slightly affected the friction reduction of diisooctyl sebacate (DOS), but they, especially WS2/TiO2, remarkably improved the wear resistance of DOS. Compared with the pure DOS, the 0.75% WS2/TiO2-containing DOS decreased the wear volume by ∼81%. The anti-wear performance of WS2/TiO2 is ascribed to the formation of a lubricating film and the synergistic lubrication effect of WS2 and TiO2.

Ga-based liquid metal: Lubrication and corrosion behaviors at a wide temperature range

Novel Ga-based liquid metal (GLM) may be a promising coolant and lubricant in the advanced liquid-metal nuclear reactor, due to its excellent physicochemical properties. But the lubrication and corrosion behaviors of GLM over a wide temperature range are unclear, which are vital to the service safety of the nuclear reactor. In this work, a high-temperature T91 steel was chosen to slide against Al2O3 ball in GLM medium and vacuum atmosphere from room temperature (RT) to 800 °C, to clarify this issue. It was found that liquid-metal will decompose and form a Ga-rich film on the contact interface and thus can provide good boundary lubrication at RT and 200 °C. At 400 °C, a Ga-rich film also exists and the steel begins to react with GLM to form nano-FeGa3 crystal, providing the synergistic lubricating effect. At 600 and 800 °C, GLM corrodes severely the T91 steel and various polymetallic intermetallics are formed, counteracting the lubricity of GLM. In addition, the distinct corrosion mechanisms of GLM on the T91 steel at 600 and 800 °C were preliminary studied and proposed, namely, inside-out growth at 600 °C and outside-in dissolution corrosion at 800 °C.

Mechanical and tribological properties of plasma sprayed NiAl composite coatings with addition of nanostructured TiO2/Bi2O3

Spray-dried nanostructured TiO2/Bi2O3 feedstock powders were successfully incorporated into plasma sprayed NiAl composite coatings. The tribological properties of the composite coatings from RT to 800 °C were investigated. The results show that the nanostructured TiO2/Bi2O3 phases performed the typical bimodal microstructure resulting in a significant enhancement of mechanical and tribological properties of the composite coatings. The lowered friction and wear at 800 °C is attributed to the synergistic lubricating effect of Bi4Ti3O12 and NiTiO3 formed on worn surfaces by tribo-chemical reactions together with TiO2 and NiO. A continuous lubricating layer including NiTiO3 and Al2TiO5 was formed on counterfaces, which further improved the tribological behavior of composite coating at high temperatures.

Understanding the synergistic lubrication effect of 2-mercaptobenzothiazolate based ionic liquids and Mo nanoparticles as hybrid additives

Abstract Ionic liquids (ILs) are increasingly expected to be used as lubricants or additives in industry as they feature many attractive properties. The interaction effects between ILs and traditional additives become significant for future application. This paper investigates the synergistic effects of 2-Mercaptobenzothiazolate based ILs and Mo nanoparticles in polyethylene glycol (PEG) base oil. The hybrid additives outperform neat PEG oil and PEG with single additives. Surface analyses (SEM-EDS, Raman, XPS, cross-sectional TEM-EDS) demonstrate that MoS 2 formed in the tribofilm, which is a critical factor to the observed excellent tribological properties. The outcomes of this work can not only provide fundamental insights into the formation of the tribofilm, but also promote the industrial application of ILs as lubricating additives.

Tribological Properties of New Cu-Al/MoS2 Solid Lubricant Coatings Using Magnetron Sputter Deposition

The increasing demands of environmental protection have led to solid lubricant coatings becoming more and more important. A new type of MoS2-based coating co-doped with Cu and Al prepared by magnetron sputtering, including Cu/MoS2 and Cu-Al/MoS2 coatings, for lubrication applications is reported. To this end, the coatings were annealed in an argon atmosphere furnace. The microstructure and the tribological properties of the coatings prior to and following annealing were analyzed using scanning electron microscopy, energy dispersive spectrometry, X-ray diffractometry (XRD) and with a multi-functional tester for material surface properties. The results demonstrated that the friction coefficient of the Cu/MoS2 coating was able to reach as low as 0.07, due to the synergistic lubrication effect of the soft metal Cu with MoS2. However, the wear resistance of the coating was not satisfied. Although the lowest friction coefficient of the Cu-Al/MoS2 coatings was 0.083, the wear resistance was enhanced, which was attributed to the improved the toughness of the coatings due to the introduction of aluminum. The XRD results revealed that the γ2-Cu9Al4 phase was formed in the specimen of Cu-Al/MoS2 coatings. The comprehensive performance of the Cu-Al/MoS2 coatings after annealing was improved in comparison to substrate heating, since the heat-treatment was beneficial for the strengthening of the solid solution of the coatings.

Fabrication of novel ionic liquids-doped polyaniline as lubricant additive for anti-corrosion and tribological properties

Abstract Two types of ionic liquids-doped polyaniline (ILs-doped PANI) were synthesized based on an improved interfacial polymerization and evaluated as anti-wear and anti-corrosion additives in poly alpha olefin (PAO) and polyurea grease. Their nanostructures and functionalization were analyzed in detail. The anti-corrosion performance of ILs-doped PANI was assessed via salt spray test, which reveals the excellent anti-corrosion performance. Tribological tests demonstrated that ILs-doped PANI as additives can remarkably reduce friction coefficients and wear volumes in PAO and also in polyurea grease. The scanning electron microscope (SEM) micrographs and X-ray photoelectron spectroscopy (XPS) spectra of the worn surfaces suggested the excellent friction reduction and anti-wear abilities of ILs-doped PANI were strongly dependent on the synergistic lubricating effect during the friction process.

Synthesis of ionic liquid decorated muti-walled carbon nanotubes as the favorable water-based lubricant additives

Ionic liquid decorated muti-walled carbon nanotubes (MWCNTs–IL) were prepared by amidation, and their tribological property as a kind of water-based additives was investigated under four-ball mode. The MWCNTs–IL exhibited the best friction-reducing and antiwear performance at optimal additive concentration of 0.015 wt% and the load of 40 N. Under these conditions, the friction coefficient and wear volume of base liquid reduced by 42 and 72%, respectively. The obvious friction-reducing and outstanding antiwear properties of MWCNTs–IL could be attributed to the synergistic lubrication effect of ionic liquid groups and carbon nanotubes. The ionic liquid groups not only endowed the MWCNTs–IL with good dispersion stability in base liquid, but also resulted in the preferable embedded stability of MWCNTs–IL between the rubbing surfaces by electrostatic interactions. Then the friction and wear of the rubbing surfaces were greatly reduced owing to the nanolubrication effects of MWCNTs (such as polishing, mending and rolling effects).

Abrasion properties of self-suspended hairy titanium dioxide nanomaterials

Considering the excellent solubility of pyrrolidone ring organic compounds, the synthesized N-(trimethoxysilyl) propyl-N-methyl-2-pyrrolidone chlorides was tethered onto titanium dioxide (TiO2) nanoparticles to improve dispersion of TiO2, and then polyethylene oxide (PEO) oligomer through ion exchange embraced the tethered TiO2 to obtain a novel self-suspended hairy TiO2 nanomaterials without any solvent. A variety of techniques were carried out to illustrate the structure and properties of the self-suspended hairy TiO2 nanomaterials. It was found that TiO2 nanoparticles embody monodispersity in the hybrid system though the “false reunion” phenomenon occurring due to nonpermanent weak physical cross-linking. Remarkably, self-suspended hairy TiO2 nanomaterials exhibit lower viscosity, facilitating maneuverable and outstanding antifriction and wear resistance properties, due to the synergistic lubricating effect between spontaneously forming lubricating film and nano-lubrication of TiO2 cores, overcoming the deficiency of both solid and liquid lubricants. This make them promising candidates for the micro-electromechanic/nano-electromechanic systems (MEMS/NEMS).

Laser processing parameter optimization and tribological characteristics of different surface treatment

The laser surface processing parameters and tribological properties of a textured surface filled with Gr-MoS2-PI-carbon nanotube nanocomposite solid lubricant under oil lubrication were investigated. Micro-dimples were fabricated on GCr15-bearing steel material using Gaussian-shaped beam profile nanosecond semiconductor sound and light-pumped Nd:YAG laser machining equipment. Friction and wear tests were performed using MMW-1A testing machine with the flat-on-flat configuration under different conditions in air. The results show that micro-dimple morphology is affected strongly by the laser head to sample surface distance, pulse repetition frequency, and single pulse energy. Additionally, the textured surface filled with solid lubricant (TSL) exhibited an enhanced tribological performance as compared to the textured (T) and smooth (S) surfaces under dry friction condition. In the oil medium, the efficiency of the textured surface (TO) was 18%, while the textured surface filled with solid lubricant (TSO) was 22% as compared to the plateau smooth surface (SO). Furthermore, under different conditions of load and rotational speed, the solid-oil composite lubricating film effectively reduces the friction and wear on the surface of the sample. The research results identified that the good lubrication and anti-wear properties of the TSO sample were mainly attributed to the double-action lubrication system and synergistic lubricating effect generated by the composite solid lubricant and oil.