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•Periodically multilayered and superlubricious MoS2-Ag film is designed based on ion beam assisted deposition (IBAD).•MoS2-Ag film demonstrates a superior lubricating durability with ...a superlow friction coefficient of 0.0065 triggered by DLC film.•Superlubricity highly depends on the synergistic effect from both physicochemical properties of shear-induced tribofilm and transformation of the MoS2-Ag surface.
Robust superlubricity of the disulphide-based film or diamond-like carbon (DLC) coating has been systematically investigated in relation to interfacial nanostructures and environmental atmospheres. Nevertheless, the frictional performances of the tribo-couple between them, especially the underlying mechanisms governing superlubricity are scarcely touched. In this study, we designed the periodically multilayered MoS2-Ag film based on ion beam assisted deposition (IBAD) and found a superior lubricating durability of the film with a superlow friction coefficient of 0.0065 triggered by DLC film under a load of 10 N in N2. The in-depth structural analysis on the contact areas of tribopairs were performed through Raman and high-resolution transmission electron microscopy (HRTEM). The results demonstrate that sliding-affected regions of the film generally undergo a deep amorphization transformation and the materials are transferred to counterfacing ball to form a multiphase tribofilm. The prevailing mechanisms emphasize that the superlubricious status highly depends on the synergistic effect from both the physicochemical properties of shear-induced tribofilm and the structural transformation of the MoS2-Ag surface by different contact mechanics and reconfiguration pathways. The findings shed light on the nature of tribochemical mechanisms of MoS2-Ag film and provide a new strategy to achieve macroscale superlubricity of the disulphide-related lubricants.
PTFE matrix composites reinforced with α-Al2O3 fillers are considered a gold standard polymeric solid lubricant. α-Al2O3 PTFE exhibits ultra-low wear rates (k ∼10−8 mm3/Nm) when sliding against steel ...in the presence of water vapor. Mechanochemical interactions involving the breaking of PTFE chains, and subsequent oxidation in the presence of water vapor helps promote growth of interfacial films that are strongly bound to the sliding interface. In the absence of moisture, wear rates of α-Al2O3 PTFE can increase by as much as three orders of magnitude, underscoring its importance to ultra-low wear behavior. A similar increase in wear rates of α-Al2O3 PTFE in humid environments has recently been observed when sliding against brass counter surfaces, though reasons for this increase remain unclear. These findings suggest that mechanisms of ultra-low wear not only rely on ambient humidity but are also strongly dependent on the counter surface chemistry. In the present work, we test the hypothesis that counter surface chemistry drives alumina PTFE wear by systematically investigating the wear of an α-Al2O3 PTFE composite against brass and aluminum substrates within the context of the morphological and mechanochemical evolution of the sliding interface. Wear tests are conducted under reciprocating, dry sliding conditions. Results presented in this work offer insights on mechanochemical mechanisms that drive wear of α-Al2O3 PTFE against brass and aluminum, and provide a framework for developing strategies for controlling wear.
•Dry sliding wear of α-Al2O3 PTFE increases when sliding against brass and aluminum, relative to steel by nearly three orders of magnitude.•Spectroscopic analysis suggests increase in wear is due to partial oxidation of PTFE, resulting in less strongly bound interfacial films.•Preconditioning of polymer pins is explored as a viable route for recovering ultra-low wear against non-ferrous substrates.
Recent literature showed that Bulk Metallic Glasses can demonstrate repeatable tribological response, and that it might be controlled through a careful selection of the sliding counterpart’s ...composition. The present study focuses on the effect of Cr content of a steel ball sliding over a Ni62Nb33Zr5 BMG. Three different steels were chosen (C90, 100Cr6, and X105CrMo17) because they demonstrate similar constitutive elements, similar mechanical properties, but different Cr contents. Increasing Cr content resulted in an increase of friction coefficient from 0.13 to 0.85 while maintaining extremely low wear. In depth chemical analysis showed that increasing Cr content favors the loss of the ductile Nb2O5 of protective FeNbO4, and NiNb-oxide to the benefit of NiCrO, CrNbO4, NiFe-oxides, and Cr(VI) oxide.
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•Ni62Nb33Zr5 BMG shows very low wear in dry friction against three different steel.•Cr in steel (from 0 % to 17 %) however drives friction coefficient from 0.15 to 0.85.•Cr favors the loss of lubricious Nb2O5, FeNb- and NiNb- oxides.•Cr favors the creation of protective CrNbO4, NiCr- and NiFe- oxides.•Ni62Nb33Zr5 demonstrates applicability in non-lubricated application.
Nanofluids containing nanocomposite materials have broad application prospect in lubrication. In the present study, MoS2-Al2O3 nanocomposite was synthesized by solvothermal method. Then water-based ...nanofluid containing MoS2-Al2O3 nanoparticles was prepared and it exhibited optimal stability and wettability. The tribological properties of MoS2-Al2O3 nanofluid under steel/steel contact were investigated. Through chemical analysis of worn surface, tribochemical reactions occurred and tribofilm generated at friction interface were studied to reveal the lubrication mechanism. The results showed that MoS2-Al2O3 nanofluid exhibited outstanding effect on reducing the friction force and wear rate of friction pairs. The superior lubrication performance was partly attributed to the entry and movement of nanoparticles at friction interface. More importantly, due to interfacial tribochemistry, a tribofilm with double layer structure composed of adsorption film and reaction layer was formed on the metal surface, preventing the direct contact of steel surfaces. The adsorption film with a thickness of about 16 nm was predominated by amorphous substrates and crystal phases (ultrafine Al2O3 and MoS2 debris) embedded in the top. And the reaction layer beneath adsorption film contained high mechanical properties iron oxides (mainly Fe3O4 and Fe2O3) and self-lubricating Fe2(SO4)3, which further alleviated friction and enhanced the wear-resistance of materials.
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•MoS2-Al2O3 nanocomposite was synthesized and applied as lubricating additive.•MoS2-Al2O3 nanofluid showed excellent performance under steel/steel contact.•A double-layered tribofilm due to interfacial tribochemistry was formed on surface.•Iron oxides and Fe2(SO4)3 in reaction layer further alleviated friction and wear.
A novel nanomaterial reduced graphene oxide-Al2O3 nanocomposite (rGO-Al2O3) was synthesized through the hydrothermal method. The nanofluid containing rGO-Al2O3 nanocomposite was prepared as lubricant ...and exhibited superior dispersion stability. To clarify the lubrication mechanism of rGO-Al2O3 nanofluid, tribological tests and cold rolling lubrication experiments were conducted using the four-ball tribometer and two-high rolling mill. The results indicated that excellent anti-wear and friction-reducing properties as well as desired strip surface topography could be obtained under the lubrication of 0.20 wt% rGO-Al2O3 nanofluid. Induced by the interfacial tribochemical reaction, a bilayer lubrication film composed of adsorption film and reaction layer was formed at the friction interface. Through theoretical calculation and experimental characterizations, the thickness of lubrication film was about 25 nm. The adsorption film, dominated by nano-Al2O3 and graphene oxide fragments, played a vital role in synergistic lubrication. Meanwhile the reaction layer contained iron oxides (primarily Fe2O3 and FeO) with high mechanical properties. This bilayer structure ensured the denseness and continuity of the lubrication film, thus achieving significant lubrication performance.
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Over the decades, the application of mechanical force to influence chemical reactions has been called by various names: mechanochemistry, tribochemistry, mechanical alloying, to name but a few. The ...evolution of these terms has largely mirrored the understanding of the field. But what is meant by these terms, why have they evolved, and does it really matter how a process is called? Which parameters should be defined to describe unambiguously the experimental conditions such that others can reproduce the results, or to allow a meaningful comparison between processes explored under different conditions? Can the information on the process be encoded in a clear, concise, and self-explanatory way? We address these questions in this Opinion contribution, which we hope will spark timely and constructive discussion across the international mechanochemical community.
The functional group in the molecular chain of lubricants is a key factor for a boundary lubricating film, which can further enhance the friction performance by interfacial tribochemistry. In this ...paper, the tribological behavior of epoxy/graphene (EP/Gra) composites was investigated through varying lubrication conditions, including dry friction, or different lubricants (such as water, hexadecane, oleylamine, and oleic acid). The porosity and Shore hardness (SHD) of the EP composites were significantly improved by the addition of graphene. Results showed that the porosity of the EP/0.75 wt.% Gra composites was reduced by 75.6% and their SHD increased by 5.4 compared with the EP/0.05 wt.% Gra composites. The average coefficient of friction (COF) of the composites was reduced by at least 27.3% under dry friction conditions. For water as lubricant, the EP/0.5 wt.% Gra composite exhibited the lowest COF (0.0455). The EP/0.75 wt.% Gra composite displayed the lowest COF (0.0466) when hexadecane acted as lubricant. By contrast, EP/0.05 wt.% Gra showed the lowest COF (0.0389) with oleylamine as lubricant, and the EP/0.25 wt.% Gra composites gave the lowest COF (0.0433) under oleic acid. The amine group of oleylamine reacted with the EP group to form a hydroxyl group, resulting in a polymer with low friction adhered to the friction interface. Simultaneously, the EP group reacted with the carboxyl group, forming a network of hydrogen bonds with graphene, which reduced the shear stress on the friction interface. Therefore, the functional groups of lubricants have a vital influence on the tribological properties of frictional composites, which may provide a new approach to solving the problem of wear.
Anti‐friction mechanism diagram of the composites under different lubrication conditions.
This review summarizes recent advances in the area of tribology based on the outcome of a Lorentz Center workshop surveying various physical, chemical and mechanical phenomena across scales. Among ...the main themes discussed were those of rough surface representations, the breakdown of continuum theories at the nano- and microscales, as well as multiscale and multiphysics aspects for analytical and computational models relevant to applications spanning a variety of sectors, from automotive to biotribology and nanotechnology. Significant effort is still required to account for complementary nonlinear effects of plasticity, adhesion, friction, wear, lubrication and surface chemistry in tribological models. For each topic, we propose some research directions.
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