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As a promising lubricate additive, the excellent lubrication performance of graphene oxide (GO) is attributed to the formation of GO tribofilm. However, the poor adsorption of GO on engineering materials surfaces limits the formation of tribofilm, resulting in losing the expected lubricating performance as a lubricant additive. In this study, to improve the adsorption of GO on bearing steel, the boronized functionalization layer was prepared on a bearing steel surface by an electrochemical boronizing process. Our experiment results indicate that an ultralow friction state with a friction coefficient (COF) of 0.03 was achieved on the boronized surface lubricated with 1 wt% GO nanosheets as water-based additives. Together with TEM, EDS and Raman analysis, we confirmed the formation of GO tribofilm with a thickness of 280 nm on the sliding surface due to the synergistic effect of the boronized surface and GO. Molecular dynamics (MD) simulation revealed that the high adhesion energy between the boronized surface and GO endows the preferential adsorption of GO on the sliding surface to form the tribofilm. The ultralow interlayer shear strength of GO tribofilm further contributed to the impressive synergistic lubrication performance. Our study expands the application scope of the ultralow friction behavior of GO by enhancing its adhesion energy on the engineering alloy surface. [Display omitted] • Synergistic lubrication effect of GO nanosheets and boronizing surface was proved. • The adhesion energy of GO on steel surface was enhanced by boronizing treatment. • The preferential adsorption of GO on the substrate was ensured during sliding. • A GO tribofilm with a thickness of 280 nm was formed on the sliding surface. • An ultralow friction state was achieved on the steel lubricated by GO solution. [ABSTRACT FROM AUTHOR]

The goal of the work is to investigate the flow pattern of MoS 2 nanoparticles, which influences the lubrication properties of the lubricant. The tribological behavior of MoS 2 nanoparticles was studied in unidirectional or reciprocating ball-on-disk sliding tribometer. Friction reduction of 0.25 wt% MoS 2 nanoparticles in reciprocating mode reached 53.1% compared with that of base oil; whereas the friction coefficient of 0.25 wt% MoS 2 nanoparticles in unidirectional mode was similar with that of base lubricant (about 0.18). Flow pattern results of MoS 2 nanoparticles around contact area show that particles piled up heavily in the inlet of contact zone in unidirectional mode, leading to starved lubrication. However, loose net-like aggregations were formed in reciprocating mode, benefiting to reduce friction and wear. [ABSTRACT FROM AUTHOR]

The purpose of this work was to expand the applications of molybdenum disulfide (MoS 2 ) nanosheet additives by investigating their effectiveness and advantages for lubricating line contacts between bearing steel and medium carbon steel surfaces. The friction and wear effects of MoS 2 nanosheet additives were investigated using block-on-ring tests, and compared with those from zinc dialkyldithiphosphates (ZDDP). Both additives were combined with ISO VG 32 white oil as base fluid. Results suggest 0.25 wt% MoS 2 nanosheets exhibited similar friction coefficient, oil temperature and wear scar width with those of ZDDP at low loads; while the reductions of friction coefficient and wear scar width were 28.6% and 34.3%, respectively, compared to the behavior of ZDDP at high load (459 N). Furthermore, the wear scar widths of the ZDDP containing oil significantly increased in 2 h tests, while they remained constant with a smaller size for 0.25 wt% additives of MoS 2 nanosheets at a load of 326 N. Scanning electron microscope observations showed some pits on the wear scar lubricated with ZDDP, compared with spalls for 0.06 wt% MoS 2 nanosheets. The wear scar was smooth and had a patched film for tests using 0.25 wt% MoS 2 nanosheets. Energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy analysis revealed that the patched surface film was composed of MoS 2 and phosphate, and that film reduced friction coefficient and wear. The results highlight how high concentration MoS 2 nanosheets exhibit significant advantages over ZDDP in reducing friction and wear in high load in line contact. [ABSTRACT FROM AUTHOR]

Molybdenum disulfide (MoS) nanosheets are a promising lubricant additive for enhanced engine efficiency in cars. However, high-cost production methods and poor dispersion have limited their application in industry. In this study, the ball milling process is demonstrated as a low-cost and high-efficient method for fabrication of oil-dispersible MoS nanosheet, and the ball milling parameters are optimized. Moreover, the lubrication effectiveness of ball-milled MoS nanosheet was also evaluated. Results indicated that well-dispersed MoS nanosheets with a size of 250 nm can be manufactured with optimized surfactants of zinc dialkyldithiphosphates (ZDDP) and polyisobutylene succinimide (PIBS) after being ball milled for 36 h. Tribological results revealed that a friction coefficient of white oil with 0.25% MoS nanosheets reached 0.075, much lower than that of lubricant without nanosheets (0.16). The wear scar radius of 0.015% MoS nanosheets was similar with that of Hertz contact, and the wear scar radius reduction reached 20% compared with that of 1% ZDDP. In addition, EDS and XPS results indicated the formation of a MoS and FeS tribofilm on the wear surface. [ABSTRACT FROM AUTHOR]

The goal of the work is to investigate the lubrication mechanism of MoS 2 nano sheet. Polyisobutyleneamine Succinimide (PIBS) provides the steric repulsion to disperse nanoparticles in oil due to its long carbon chain. In the present study, the tribological behavior of nano-MoS 2 sheet mixed with different percentages of PIBS was explored. The dispersed state and the mode of nano-MoS 2 sheet entering the contact point were studied. Ball-on-disk results illustrated that the COF of nano-MoS 2 in paraffin oil with free of PIBS was 26.2% compared with those containing 3 wt% PIBS, and the reduction of wear depth reached 41.9%. The nano-MoS 2 sheets with 3 wt% PIBS were in discrete state in paraffin oil, which moved with side flow and could not enter the contact area to cause poor lubrication. Aggregation of particles took place in the case of low PIBS concentration and without PIBS, it entered the contact area and formed well-distributed MoS 2 tribofilm to reduce friction and wear. [ABSTRACT FROM AUTHOR]

The as-cast AlCoCrFeNi 2.1 eutectic high-entropy alloy (EHEA) has high fracture strength and ductility, which displays an excellent application prospect in key structural materials. However, its low surface hardness and poor wear resistance at room temperature limit the actual applications in tribology. Here we showed the enhancement of surface strengthening and self-lubrication properties of AlCoCrFeNi 2.1 EHEA by a powder-pack boronizing treatment. SEM and XRD analysis indicate that a double affected layer with a thickness of about 42 μm was formed on the surface at 900 °C for 4 h, which were mainly consisted of Ni 2 Si and FeB. The surface hardness of the alloy was significantly increased from 396 HV to 1108 HV after the treatment. Tribological results show a remarkable self-lubrication property of the boronized EHEA. The friction coefficient of boronized EHEA under the water condition was reduced by 21%, and the corresponding wear volume was decreased by 65%. EDS and XPS analysis demonstrate the formation of silicon/boron-containing tribofilm on the wear scar, which benefits in the achievement of self-lubrication properties with friction and wear reduction under the water condition. Our research provides a general strategy with the powder-pack boronizing method to strengthen AlCoCrFeNi 2.1 EHEA and enhance self-lubrication properties, which would certainly boost its industry application. • An impressive enhancement of surface hardness was achieved after boronizing. • The affected layer showed a double-layer structure, which was mainly Ni 2 Si and FeB. • Boronized EHEA showed the remarkable self-lubrication property in water. • The formed silicon/boron lubrication film benefited in self-lubrication property. [ABSTRACT FROM AUTHOR]

The CoCrNi medium-entropy alloy (MEA) exhibits an impressive ductility, but its low hardness (∼352 HV) and poor wear resistance limit its engineering applications. In this study, a significant increase of surface hardness and enhancement of self-lubrication were achieved for the CoCrNi MEA using powder-pack boronizing. Results indicate that dual reaction layers with a total thickness of about 50 μm were formed on the surface. These two strengthened layers are mainly consisted of silicide (Ni 2 Si) and borides (CoB, Cr 5 B 3). The hardness of the affected region reaches to 11–21 GPa dependence on the cross-sectional depth, being about 6 times higher than that of untreated MEA. In the deionized water condition, the friction and wear rate reductions of the boronizing treated specimen reach to about 25% and 63%, respectively. Energy Dispersive Spectrometer (EDS) and X-ray Photoelectron Spectroscopy (XPS) analysis on the wear scars showed that a silicon/boron-containing tribofilm was formed under the water environment, which reduces the friction coefficient and enhances the wear resistance. Our study demonstrates that powder-pack boronizing is a powerful way to enhance surface hardness and self-lubrication properties of CoCrNi MEA. • Double-strengthened layers were observed on the boronized CoCrNi alloy surface. • An obvious improvement of surface hardness was obtained. • The self-lubrication performance of boronized CoCrNi MEAs in the water is excellent. • The self-lubrication enhancement was due to the formation of silicon-boron based tribofilm. [ABSTRACT FROM AUTHOR]

Layered boric acid (BA) nanoparticle was considered as a promising 'green' lubricant additive due to its bio-degradable. However, its unclear lubrication mechanism and poor dispersion stability cause serious concerns for practical applications. Here, highly oil-dispersed alkyl-functionalized BA nanoparticles with size of about 100 nm were fabricated using a scalable strategy. The friction coefficient reached to around 0.11 with the addition of BA nanoparticles, about 30% lower than that of base oil (0.16). Tribological tests show that the wear reduction reached 40%, and the scuffing load capacity was improved by about 3 times. Lubrication mechanism investigation by molecular dynamics (MD) simulation shows that BA molecules preferentially adsorbed on the friction interface through hydrogen bond. A compact net-like adsorption layer from the decomposed nanoparticles, as well as boron-containing tribofilm, was formed on the rubbing surface to enhance lubrication performance. These solid evidences make BA nanoparticle to be a very promising candidate in practical applications as high performance green additive. Image 1 • MD simulation demonstrates the key role of hydrogen bond at friction interface. • Boric acid (BA) molecule preferentially adsorbed on the sliding surface. • Alkyl-functionalized BA nanoparticles were fabricated using a simple strategy. • The oil suspension with BA nanoparticles remains stable for more than 2 months. • Scuffing load capacity was improved by 3 times with the addition of BA nanoparticles. [ABSTRACT FROM AUTHOR]

The commonly accepted interlayer shear mechanism for layered materials as oil additives remains a challenge with the emergence of two-dimensional materials. In this work, the lubrication effectiveness of MoS 2 and BN nanosheets with a similar size was investigated as oil additives using block-on-ring tests. BN nanosheets reveal remarkable anti-wear performance and a reduction of wear depth of about 50%, whereas, the expected friction and wear reduction were not found for MoS 2 nanosheets. X-ray photoelectron spectroscopy (XPS) investigation indicates the generation of boron-based tribofilm for BN nanosheets and MoS 2 -complex tribofilm for MoS 2 nanosheets. Together with the thermal stability analysis, we found that it is the high strength tribofilm with high oxidation resistance, but not the layer structure, that plays a critical role in lubrication enhancement. This study highlights the advance of 'depositing tribofilm' mechanism in novel additive design. Image 1 • Layered materials with similar size and shape were selected. • Expected friction and wear reduction was not found for MoS 2 nanosheets in this study. • BN nanosheets reveal the remarkable anti-wear property with strong tribofilm strength. • It is a challenge to explain the results by using interlayer shear mechanism. • Depositing tribofilm mechanism would be preferable for novel particle additive design. [ABSTRACT FROM AUTHOR]

The influence of anti-oxidation property of nanoparticles on the tribological properties is rarely reported. In this study, the alkyl-functionalized graphene oxide (FGO) and alkyl-functionalized boric acid (FBA) were fabricated as oil additives to explore their lubrication effectiveness. FBA exhibited remarkable lubricating properties which reduced friction coefficient by approximately 24 % and the wear depth by about 94 %. However, the friction coefficient and wear depth of FGO were reduced by only 9.5 % and 44 %, respectively. Worn surface analysis shows a bunch of deep-plowing on rubbing surface in the case of FGO, while the wear scar was smooth and almost wear-free for FBA. Lubrication mechanism analysis by X-ray Photoelectron Spectroscopy revealed the formation of carbon-based tribofilm for FGO and boron-based tribofilm for FBA during rubbing. In the thermal behavior analysis, graphene oxide (GO) nanoparticles were completely decomposed at 650 °C, whereas, B 2 O 3 nanoparticles remained stable. It indicated that the excellent thermal stability and anti-oxidation property of boron-based tribofilm benefited to the lubrication enhancement. This study highlights the importance of oxidation resistance in designing novel nanoparticle additives, and the enhancement of oxidation resistance of GO will be a promising way to further application as oil additives. [Display omitted] [ABSTRACT FROM AUTHOR]