Reduction In COF Research Articles

Multiscale study on the solid-liquid synergistic lubrication mechanism of graphite and liquid lubricant in polymer composites

Solid-liquid synergistic lubrication, in combination with the advantages of solid and liquid lubrication, has gained the great popularity in recent years. However, the mechanism of solid-liquid synergetic lubrication is still unclear. In this paper, graphite and poly α-olefin base oil (PAO) were used as self-lubricating additives to evaluate their synergistic effect. The results show that the composites possess good mechanical and excellent tribological properties with a nanoindentation modulus of 3.8 GPa, a coefficient of friction of 0.0998, as well as a wear rate of 1.3 × 10−14 m3/N·m. Both transfer film and lubricating oil film exist on the friction interface, and the lubricating oil film protects the counter ball from excessive oxidation. At the same time, with combination of molecular dynamics (MD) simulation and first-principles calculations, the synergistic lubrication mechanism of graphite and PAO was manifested. PAO destroyed the interlayer structure of graphite, making it easier to form a transfer film on the counter ball surface. The weak chemical bond between graphite and PAO facilitates more PAO molecules to gather at the interface while forming the transfer film, which benefits the formation of lubricating oil film. The synergistic effect of graphite and PAO molecules is critical in promoting the further reduction of COF.

Temperature-adaptive lubrication of Ag doped Cr2AlC nanocomposite coatings

Self-lubricating nanocomposite carbides and nitrides coatings that enable to exhibit both low friction coefficient (CoF) and high wear resistance over a wide temperature range, are in high demand for mechanical components used in aerospace and marine systems. In this work, we developed high-purity Ag-doped Cr2AlC (Ag–Cr2AlC) nanocomposite coatings using a facile arc/sputter deposition technique followed by annealing. Compared to typical nanocrystalline Cr2AlC MAX phase coatings, the incorporation of Ag resulted in a unique nanocomposite structure, where Ag encapsulates elongated Cr2AlC nanograins. The dry sliding wear of this material was evaluated using the ball-on-disc method, with an Al2O3 ball as the counter body, at temperature ranging from 25 °C to 700 °C. Interestingly, Ag dopants contributed to a significant reduction in COF and wear rate at temperatures below 500 °C, while the synergistic effect of the Ag distributed at grain boundaries and the outermost oxides of Cr2O3 and Al2O3 generated during friction dominated the excellent self-lubrication at 500–700 °C. Notably, the Ag–Cr2AlC coatings promoted the in-situ formation of gradient bilayer oxides, consisting of a loosely coarse-grained inner layer and a densely refine-grained outmost layer. The combination of unique gradient oxides and a combined nanocomposite matrix allows the coating to accommodate plastic strains without extended cracks or localized brittle fracture.

Casting light on the tribological properties of paraffin-based HDPE enriched with graphene nano-additives: an experimental investigation

The impressive mechanical properties and robust resistance to wear recorded by nano-polymeric composites have positioned them as a viable alternative in many applications. When it comes to frictional materials, high-density polyethylene (HDPE) emerges as one of the best candidate materials that can be used. However, it tribological properties need more enhancement to suite with wide variety of applications. The objective of the current study is to identify the optimal loading ratio using a comprise of paraffin oil and nano-graphene with varying loading compositions. Different experiments were carried out to assess the modulus of elasticity, hardness, and strength. Additionally, the friction coefficient and wear resistance of the proposed nanocomposite have been estimated. Surfaces topographies were analyzed to recognize the wear mechanism. The results pointed that samples containing 5% paraffin oil and 0.5 wt% have relatively better mechanical and tribological behavior compared to further compositions; where, a 38% decrease in wear and a 34% reduction in COF compared to other composite samples.

Hot temperature tribological behavior of Mg-Zn-Gd alloy under graphite surfactant functionalized SN500

The effect of SN500 and SN500 +G lubricants on the wear behavior of Mg-Zn-Gd alloy was studied. For this purpose, Mg-Zn-Gd alloy was produced by the stir casting method. The wear test was performed under 20, 40, and 80 N with a velocity of 0.5 m/s for 1000 m at different temperatures (25, 100, 150, 200, and 250 °C) under SN500 and SN500 +G lubricants. The results showed that in the presence of precipitates weight loss and wear rate were reduced. With the increasing load in all samples, weight loss and wear rate increased. The lowest wear rate was obtained at 250 under 20 N and was equal to 0.036 and 0.030 m3/m, and with an increase in the load to 80 N, it reached 0.044 and 0.042 m3/m for samples under SN500 and SN500 +G, respectively. It was also observed that lubrication of SN500 +G in all samples created better properties than SN500. The addition of graphite to SN500 also reduced the coefficient of friction. So that the friction coefficient of the tested sample under 80 N at 250 °C under SN500 has decreased by 15% compared to SN500 +G. The worn surfaces SEM micrographs of samples under SN500 +G lubricant show the presence of the mechanically mixed layer (MML) with the adhesive wear mechanism. With increasing load, the abrasion, delamination, and MML mechanisms were dominant, and a significant reduction of COF and wear rate were observed. At the high temperature and high loads, the crushing of precipitates and their orientation in the wear sliding was caused to dwindle applied load and prevented the activation of other wear mechanisms.

Investigation of tribological properties of polypropylene (PP)—Acrylonitrile butadiene styrene (ABS) blends reinforced with graphene nano-platelets (GNPs)

The tribological response of PP-ABS/GNPs nanocomposites fabricated via extrusion and injection molding with varying wt% GNPs fractions were examined under varying normal load and sliding speed wear test parameters. The specimens with 3% GNPs and 11% GNPs respectively exhibited the lowest and highest specific wear rates and COF under all test conditions. Increased load led to significantly reduced COF values, whereas increased sliding speed led to a slight reduction in COF by contributing to the formation of a continuous transfer layer. Addition of GNPs helped to retain the worn material on the contact surface and led to improved wear resistance up to the weight fraction of 3% after which non-homogeneous dispersion of GNPs occurred.

Ultrafine MoS2 crumpled submicrosphere grown by clean laser irradiation for excellent anti-friction performance

2 H-MoS2 nanoparticles with excellent structural characteristics has become one of the main research objects of lubricating additives. Herein, one crumpled MoS2 submicrosphere staggered by ultrafine nanosheet was successfully synthesized by clean and fast laser irradiation in liquids at ambient conditions. Such crumpled sphere exhibits good wettability on steel surface in water. As water additives, the crumpled MoS2 submicrosphere can significantly improve the lubrication performance. Notably, the crumpled MoS2 submicrosphere have remarkable anti-friction with 80% reduction of COF compared with the pure water. The enhanced lubrication performance benefits from the lamella structure, microbearing and protective tribofilm of crumpled submicrosphere.

Friction and Wear Characteristics of Aqueous ZrO2/GO Hybrid Nanolubricants

Aqueous nanolubricants containing ZrO2 nanoparticles, graphene oxide (GO) nanosheets, or hybrid nanoparticles of ZrO2 and GO were formulated using a cost-effective ultrasonication de-agglomeration method. The friction and wear characteristics of these water-based nanolubricants were systematically investigated using a block-on-ring testing configuration with a stainless- and alloy steel contact pair. The concentrations and mass ratios of nanoadditives were varied from 0.02 to 0.10 wt.% and 1:5 to 5:1, respectively, to obtain optimal lubrication performance. The application of a 0.06 wt.% 1:1 ZrO2/GO hybrid nanolubricant resulted in a 57% reduction in COF and a 77% decrease in wear volume compared to water. The optimised ZrO2/GO hybrid nanolubricant was found to perform better than pure ZrO2 and GO nanolubricant in terms of tribological performance due to its synergistic lubrication effect, which showed up to 54% and 41% reductions in friction as well as 42% and 20% decreases in wear compared with 0.06 wt.% ZrO2 and 0.06 wt.% GO nanolubricants. The analysis of wear scars revealed that using such a ZrO2/GO hybrid nanolubricant yielded a smooth worn surface, with 87%, 45%, and 33% reductions in Sa compared to water and 0.06 wt.% ZrO2 and 0.06 wt.% GO nanolubricants. The superior tribological performance can be ascribed to the combination of the rolling effect of ZrO2 nanoparticles and the slipping effect of GO nanosheets.

Hardness and Friction Coefficient Analysis of Electrochemically Anodized Ti-6Al-4V

Electrochemical anodization was performed for surface modification of Ti-6Al-4V, using an HCl-based electrolyte. The effect of increasing the electrode spacing of the electrochemical cell by 1, 2, and 2.5 cm on the hardness and COF was investigated. Using a 0.5 M solution of HCl and glycerol in different percentages (85, 90, and 95 %), a voltage of 30 V and a time of 5 minutes. The results revealed the growth of nanotubular structures, which contribute to the increase in hardness, obtaining a maximum of 7.1 GPa for the anodized samples and the reduction of COF to a minimum of 0.5. While the heat-treated samples increased their microhardness up to 10.25 GPa.

Advanced Tribological Characterization of DLC Coatings Produced by Ne-HiPIMS for the Application on the Piston Rings of Internal Combustion Engines

Piston rings (PR) are known for almost a quarter of the friction losses in internal combustion engines. This research work aims to improve the tribological performance of PR by a recently developed variant of Diamond-like Carbon (DLC) coatings deposited in a mixture of Ar and Ne plasma atmosphere (Ne-DLC) by high-power impulse magnetron sputtering (HiPIMS). For the benchmark, the widely used Chromium Nitride (CrN) and DLCs deposited in pure Ar plasma atmosphere (Ar-DLC) were used. The tribological tests were performed on a block-on-ring configuration under different lubrication regimes by varying temperatures and sliding speeds. The analysis of the results was performed by Stribeck curves corresponding to each sample. An improvement of the tribological performance was observed for Ne-DLC films by up to 22.8% reduction in COF compared to CrN in the boundary lubrication regime, whereas, for the Ar-DLC film, this reduction was only 9.5%. Moreover, the Ne-DLC films achieved ultralow friction of less than 0.001 during the transition to a hydrodynamic lubrication regime due to better wettability (lower contact angle) and higher surface free energy. Increasing the Ne up to 50% in the discharge gas also leads to an increase of hardness of DLC films from 19 to 24 GPa.

Ultra-low friction of a-C:H films enabled by lubrication of nanodiamond and graphene in ambient air

Hydrogenated amorphous carbon (a-C:H) is subjected to abnormal high friction in ambient air, and the possibility to retain an ultra-low friction state remains as a great challenge. Here, nanodiamond and graphene were used as solid lubricants to improve the tribological properties of two representative types of a-C:H films with 20 at.% and 40 at.% hydrogen contents, respectively. The results emphasize the exceptionally synergetic lubrication effect of nanodiamond + graphene composite with a mass ratio of 1:1 and a solution-processed concentration of 0.1 mg/mL. An ultra-low friction coefficient of ∼0.02 was achieved for a-C:H (20 at.% H) film, and more strikingly, a dramatic reduction in COF from 0.52 to 0.07 was realized in a-C:H (40 at.% H) film. Meanwhile, the wear rates of the counterparts in both cases are significantly reduced in the presence of nano-lubricants. The lubricity mechanisms are mainly based on the in-situ growth of nanostructured tribolayers. The roles of a-C:H film bonding characteristic and the tribo-induced structural evolution of nano-lubricants in the build-up of anti-friction and wear-resistant tribolayers are discussed. These findings can enrich the understanding of surface modification pathways to a-C:H films via low-dimensional nano-lubricants and help to develop more adaptive and robust solid carbon films.

Lubricating Properties of Water-Based Drilling Fluid Improvement Using Lignite NPs as well as Their Effect on Rheological and Filtration Properties

In the process of drilling directional, extended-reach, and horizontal wells, the frictional forces between the drill string and the wellbore or casing can cause severe problems including excessive torque which is one of the most important problems during drilling oil and gas well. Drilling fluid plays an important role by reducing these frictional forces. In this research, an enhancement of lubricating properties of drilling fluids was fundamentally examined by adding Lignite NPs into the water-based drilling fluid. Lubricity, Rheology and filtration properties of water-based drilling fluid were measured at room temperature using OFITE EP and Lubricity Tester, OFITE Model 900 Viscometer, and OFITE Low-Pressure Filter Press, respectively. Lignite NPs were added at different concentrations (0.05 %, 0.1 %, 0.2 %, 0.5 %, and 1 %) by weight into water-based drilling fluid. Lignite NPs showed good reduction in COF of water-based drilling fluid. The enhancement was increased with increasing Lignite NPs concentrations; 23.68%, 35.52%, and 45.3 % reduction in COF were obtained by adding 0.2%, 0.5%, and 1% by weight Lignite NPs concentration, respectively.

Lubrication effectiveness investigation on the friendly capped MoS2 nanoparticles

AbstractThe compatibility and effectiveness of nanoparticles with the existing additives in formulated oil are still unclear. In the present study, some lubricant additives were selected to modify nanoparticles to obtain friendly capped nano‐MoS2. Various polyisobutyleneamine succinimide (PIBS) concentrations were applied to investigate the lubrication effectiveness of capped nano‐MoS2. The results showed that the reduction in COF and wear volume of friendly capped nanoparticles without PIBS reached about 35% and 75% in comparison with those of the base oil respectively. However, the average coefficient of friction and wear volume loss of nano oil increased with PIBS concentration in the range of 0.05%–1%. By scanning electron microscope, energy‐dispersive X‐ray spectroscopy and X‐ray photoelectron spectroscopy analysis, it is identified that (i) MoS2 tribofilm was formed on the wear track for the oil with nano‐MoS2 and (ii) wear scar was smooth for nano‐MoS2 with low PIBS concentration and without PIBS, while it showed plowing wear when containing high PIBS concentration. Copyright © 2016 John Wiley & Sons, Ltd.

Low friction behaviour of boron carbide coatings (B4C) sliding against Ti–6Al–4V

Boron carbide (B4C) is a potential tool coating for machining of titanium alloys due to its high hardness and high temperature stability. In addition, B4C coatings annealed at 600°C, form a layer that acts as solid lubricant leading to a low coefficient of friction, COF, against titanium. This paper shows that a transfer layer consisting of graphite could be established on B4C surface when sliding against Ti–6Al–4V and passivation of graphitized carbon would reduce COF. Ball-on-disk (B4C) type sliding tests were performed in dry air, argon and nitrogen atmospheres (<0.5% RH), and in air with humidity levels varying between 25% and 85% RH. B4C samples were also tested while they were immersed in water, ethanol (C2H5OH) and iso-propyl alcohol (C₃H₇OH). The B4C coatings exhibited high COF values of 0.59–0.65 in dry atmospheres. Sliding under an air atmosphere containing 70% RH reduced the steady state COF to 0.25 and a further reduction of COF to 0.20 was recorded in air with 85% RH. A COF of 0.15 was measured when the tests were carried out in ethanol and 0.07 in iso-propyl alcohol. Low COF values of B4C were attributed to H and OH passivation of the graphitized transfer layers observed by the Raman, Fourier transform and X-ray photoelectron spectroscopy techniques.

The effect of surface conditions on the elevated temperature sliding contact deformation of AA5083 alloy

Elevated temperature sliding contact experiments were carried out on an Al–Mg alloy AA5083 to study the formation of oxide fibres on their surface. Deformation and friction tests were conducted at a constant strain rate of 4×10−2s−1 at temperatures 350°C, 450°C and 545°C. The samples were tested in two surface conditions, namely in the as-received condition, with surfaces covered with a tribolayer generated during previous rolling processes and after being polished to remove the tribolayer. The coefficient of friction, COF, and the wear of the AA5083 surfaces as well as the amount of material transferred to the counterface increased with the temperature. The presence of tribolayers on the surface was beneficial, as the polished surfaces showed generally higher COF and wear, with a tendency to form localised adhesive joints with the counterface. Oxide fibres formed during plastic deformation (with diameters of about 100nm) were generated within the cracks on the tribolayers and inside the cavities at the Al–Mg grain boundaries. High resolution scanning and transmission electron microscopy showed that the fibres were composed of an outer shell of MgO, Al2MgO4 and Al2O3 and an inner core of Al nanograins. The visco-plastic deformation behaviour of oxide fibres observed within the sliding track could have contributed to the reduction of COF.

Continuously extruded micro‐textured polypropylene films

The continuous extrusion of micro‐textured isotactic polypropylene (i‐PP) film and its tribological properties are reported. As analyzed by scanning electron microscopy, a rectangular‐semicircular micro‐patterned die successfully produced extruded cast‐films with hill‐like micro‐texture. Such films displayed static and kinetic coefficients of friction (COFs and COFk) values of 0.247 ± 0.028 and 0.245 ± 0.003, respectively, for a steel fixture sliding along the micro‐texture of the film. The equivalent COFs and COFk values for non‐textured (NT) films were 0.369 ± 0.036 and 0.340 ± 0.024, respectively, confirming that textured films displayed a reduction in COF of about 30%. For textured and NT films sliding on textured films, the COFk values were even lower at 0.161 ± 0.013 and 0.113 ± 0.004, which represents about 20–40% reduction as compared with that for the NT‐NT counterparts. Films coated with a silicone lubricant displayed COFs and COFk values of 0.334 ± 0.042 and 0.099 ± 0.012, respectively, for NT films sliding over lubricated NT films, and 0.426 ± 0.031 and 0.063 ± 0.006, respectively, for textured films sliding over lubricated textured films. The COFk values for textured/non‐lubricated films approach those of NT/lubricated films, indicating that micro‐textured polypropylene films may be used in environmentally sensitive applications where lubricants and fluorinated additives may not be used for reducing friction. POLYM. ENG. SCI., 54:2147–2154, 2014. © 2013 Society of Plastics Engineers

Scratch resistance analysis of plasma-nitrided Ti–6Al–4V alloy

A modified plasma nitriding treatment developed for Ti–6Al–4V alloy at a low temperature (600°C) and in a dilute nitriding atmosphere (3% N2) has shown to improve the alloy tribological properties and resistance to surface crack initiation and propagation. This modification formed a thin compound layer (CL) (1.9μm) on the surface of Ti–6Al–4V followed by an α-Case (1.8μm) and a deep nitrogen diffusion zone (DZ) (44.4μm). Transmission electron microscopy (TEM) and x-ray diffraction (XRD) analyses revealed that the compound layer consisted of Ti2N, TiN, and TiN0.3 nitrides. The failure mechanisms of the nitrided samples subjected to micro-scratch testing under both constant and progressive loading were investigated using scanning and transmission electron microscopy, focused ion beam milling (FIB), and optical surface profilometry. The onset of failure events was observed as the applied load reached 0.75N (mean Hertzian pressure of 4.4GPa) with the appearance of angular cracks on scratch path rims and subsurface microcracks within the CL likely at grain boundaries. Normal loads higher than 8N (9.7GPa) indicated the advent of tensile surface cracks perpendicular to the scratch direction and subsurface cracks which propagated beyond the CL. As the normal load exceeded 15.7N (12.1GPa), angular and tensile cracks joined and formed semi-circular cracks. FIB observations indicated that these cracks penetrated into the diffusion zone but were effectively stopped by ductile β particles at α grain boundaries. The plasma-nitriding also led to a significant reduction in COF without any signs of compound layer spallation and delamination during micro-scratch testing.

Electron-transfer reactions in non-aqueous media. VII. Reduction of CoF(NH3)52+ and Co(HCOO)(NH3)52+ by copper(I) in dimethyl sulfoxide-water mixtures

Previous work on the reduction of a series of cobalt(III) complexes by iron(II) in dipolar aprotic solvents and in aqueous mixtures has been extended to reduction by copper(I). The greater stability of copper(I) to disproportionation in these media has permitted the study of the reduction of CoF(NH3)52+ and Co(HCOO)(NH3)52+ in range of solvents over a number of temperatures with a precision not possible in previous studies in water. The results are consistent with an inner-sphere mechanism in which the copper(I) reductant is preferentially solvated by dimethyl sulfoxide to the exclusion of water in mixed solvents.

Electron-Transfer Reactions in Non-Aqueous Media. V. Solvent Effects in the Reduction of CoN3(NH3)52+ by Iron(II)

The reduction of CoN3(NH3)52+ by iron(II) is rate-determined by a two-stage process involving the reversible formation of an azide-bridged precursor complex prior to electron transfer in each of the solvents water, Me2SO, aqueous Me2SO and HCONMe2. The activation parameters in H2O and Me2SO, and the trends shown with increasing Me2SO concentrations in aqueous Me2SO, are similar to the properties of the previously studied CoCl(NH3)52+ and CoBr(NH3)52+ systems and contrast with the reduction of COF(NH3)52+. The results are consistent with a bridged precursor complex octahedral at both the iron and cobalt atoms in water but with tetrahedral coordination about the iron in Me2SO. In HCONMe2, as in the reduction of COF(NH3)52+, COCl(NH3)52+ and COBr(NH3)52+, the precursor complex is a significant part of the reacting solutions, and as a result the experimental pseudo-first-order rate constants for the loss of CoIII are not linearly dependent on the concentration of FeII. The initial spectra of the reacting solutions in this system also indicate significant concentrations of the precursor complex.

Electron-transfer reactions in non-aqueous media. II. Reduction of CoF(NH3)52+, CoCl(NH3)52+ and CoBr(NH3)52+ by iron(II) in dimethyl sulphoxide-water mixtures

The kinetics of the reduction of the cobalt(111) octahedral complexes, CoF(NH3)52+, CoCl(NH3)52+ and CoBr(NH3)52+, by iron((11) in various Me2SO-H2O mixtures have been studied over a range of temperatures. The activation parameters obtained for the chloro and bromo systems are consistent with a change in the stereochemistry of the iron (11) atom in the bridged intermediate from octahedral in water to tetrahedral with increasing Me2SO concentration. The fluoro system, however, has activation parameters which are less sensitive to solvent composition and consistent with the iron(11) atom maintaining octahedral coordination.

Electron-transfer reactions in non-aqueous media. III. Reduction of CoF(NH3)52+, CoCl(NH3)52+ and CoBr(NH3)52+ by iron(II) in N,N-dimethylformamide

The reduction of the cobalt(111) octahedral complexes CoF(NH3)52+, CoCl(NH3)52+ and CoBr(NH3)52+ by iron(11) in HCONMe2 proceeds through a mechanism involving reversible formation of a halide bridged binuclear intermediate prior to electron transfer and decomposition of the intermediate to products. Values of the activation parameters for reduction and the equilibrium constants for formation of the bridged intermediate are consistent with a tetrahedral stereochemistry for the iron(11) atom in the bridged intermediate for the chloro and bromo systems. However, for the fluoro system these values are consistent with the iron(11) atom maintaining octahedral coordination. These results are compared to those obtained in Me2S0.1,2