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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 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]

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]