Graphene has been recognized as an excellent lubrication material owing to its two-dimensional structure and weak interlayer interactions. However, most extant works concerning superlubricity ...involving graphene oxide have been limited to nanoscale or microscale dimensions (of the order of 1–10 μm). In present work, realization of a robust macroscale superlubricity state (μ = 0.0037), by taking advantage of the synergy effect of graphene-oxide nanoflakes (GONFs) and ethanediol (EDO) at Si3N4–SiO2 interfaces is reported. GONFs have been observed as being adsorbed on friction surfaces, thereby preventing direct contact between surface asperities. The extremely low shear stresses developed between these asperities contribute toward enhanced superlubricity and the resulting super-low wear. Meanwhile, the formation of partial-slip hydrodynamic boundary condition at the GONFs–EDO interface along with the formation of hydrated GONFs–EDO networks through hydrogen-bond interactions contribute to the generation of extremely low shear stresses of the liquid lubricating film. Such macroscale superlubricity provides a new approach toward realization of extremely low friction in GONFs through the synergy effect with liquids.
Graphene-related materials and molybdenum disulfide (MoS2) are good candidates as solid lubricants owing to their easy-shear property between lattice layers. However, the coefficients of friction ...(COFs) of graphene or graphene oxide (GO) are not sufficiently low at the macroscale and the friction and wear performance of MoS2 is significantly deteriorated by humidity. Herein, GO–MoS2 nanocomposite film is fabricated on a stainless steel (SS) substrate by a simple electrophoretic deposition (EPD) method in water. The deposited nanocomposite film can reduce the COF to 10% of that for a SS substrate and 50% of that for a pristine GO film and can extend the lifetime in comparison to a MoS2 film under ambient conditions. A tribofilm forms in the wear track where GO nanoflakes wrap MoS2 nanoflakes in the tribofilm; furthermore, a MoS2 transfer layer forms on the steel ball surface during the friction test. This excellent lubrication performance arises from the synergetic effect between GO and MoS2, where the layered MoS2 dominates the lubrication performance owing to its easy-shear property and the surrounding GO protects MoS2 from humidity and oxidation. This has implications for developing nanocomposite films with excellent lubrication and wear-resistance performance for industrial applications.
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Due to complex semantics, a sample may be associated with multiple labels in various classification and recognition tasks. Multilabel learning generates training models to map feature vectors to ...multiple labels. There are several significant challenges in multilabel learning. Samples in multilabel learning are usually described with high-dimensional features and some features may be sequentially extracted. Thus, we do not know the full feature set at the beginning of learning, referred to as streaming features. In this paper, we introduce fuzzy mutual information to evaluate the quality of features in multilabel learning, and design efficient algorithms to conduct multilabel feature selection when the feature space is completely known or partially known in advance. These algorithms are called multilabel feature selection with label correlation (MUCO) and multilabel streaming feature selection (MSFS), respectively. MSFS consists of two key steps: online relevance analysis and online redundancy analysis. In addition, we design a metric to measure the correlation between the label sets, and both MUCO and MSFS take label correlation to consideration. The proposed algorithms are not only able to select features from streaming features, but also able to select features for ordinal multilabel learning. However streaming feature selection is more efficient. The proposed algorithms are tested with a collection of multilabel learning tasks. The experimental results illustrate the effectiveness of the proposed algorithms.
Various microRNAs have been demonstrated to play roles in a number of human diseases. Several microRNA-disease network reconstruction methods have been used to describe the association from a systems ...biology perspective. The key problem for the network is the similarity computation model. In this article, we reviewed the main similarity computation methods and discussed these methods and future works. This survey may prompt and guide systems biology and bioinformatics researchers to build more perfect microRNA-disease associations and may make the network relationship clear for medical researchers.
Several ionic liquids (ILs) are formed in situ with monovalent metal salts and ethylene glycol (EG). The macroscale superlubricity and antiwear properties of the ILs were studied between ceramic ...materials. Superlow coefficients of friction of less than 0.01 could be obtained for all ILs at silicon nitride (Si3N4) interfaces induced by tribochemical reactions. Notably, the IL (Li(EG)PF6) formed with LiPF6 and EG exhibited the greatest superlubricity and antiwear properties. The results of film thickness calculations and surface analysis showed that the lubrication regime during the superlubricity period was the mixed lubrication, and a composite tribochemical layer (composed of phosphates, fluorides, silica (SiO2), and ammonia-containing compounds), hydration layer, and fluid film contributed to superlubricity and wear protection. It was found that the small size of metal cations was beneficial for alleviating wear, and PF6 – anions exhibited the smallest friction and best antiwear performance at Si3N4 interfaces. This work studied the lubricity and antiwear properties of ILs with different cations and anions, enriching the range of alternative ILs for macroscale superlubricity and low wear, and is of importance to engineering applications.
Recently, a multigranulation rough set (MGRS) has become a new direction in rough set theory, which is based on multiple binary relations on the universe. However, it is worth noticing that the ...original MGRS can not be used to discover knowledge from information systems with various domains of attributes. In order to extend the theory of MGRS, the objective of this study is to develop a so-called neighborhood-based multigranulation rough set (NMGRS) in the framework of multigranulation rough sets. Furthermore, by using two different approximating strategies, i.e., seeking common reserving difference and seeking common rejecting difference, we first present optimistic and pessimistic 1-type neighborhood-based multigranulation rough sets and optimistic and pessimistic 2-type neighborhood-based multigranulation rough sets, respectively. Through analyzing several important properties of neighborhood-based multigranulation rough sets, we find that the new rough sets degenerate to the original MGRS when the size of neighborhood equals zero. To obtain covering reducts under neighborhood-based multigranulation rough sets, we then propose a new definition of covering reduct to describe the smallest attribute subset that preserves the consistency of the neighborhood decision system, which can be calculated by Chen’s discernibility matrix approach. These results show that the proposed NMGRS largely extends the theory and application of classical MGRS in the context of multiple granulations.
In present work, a superlubricity phenomenon of phosphoric acid (H3PO4) was found under ambient conditions. An ultralow friction coefficient of about 0.004 between glass/Si3N4 and sapphire/sapphire ...tribopairs was obtained under the lubrication of a phosphoric acid aqueous solution (pH 1.5) at high contact pressure (the maximum pressure can reach about 1.65 GPa) after a running-in period of about 600 s. The experimental results indicate that the superlow friction state was very stable for more than 3 h. In such a state, solidlike films formed on the two sliding surfaces, which are hydrates of phosphoric acid with a hydrogen-bonded network according to the Raman spectrum. The superlubricity mechanism is mainly attributed to the hydrogen bond effect that forms a hydrated water layer with low shearing strength, and the dipole–dipole effects that form an interfacial Coulomb repulsion force also make some contributions to low friction. This work may help us to introduce a new approach to superlubricity and may lead to the wide application of superlubricity in future technological and biomedical areas.
2D or 3D layered materials, such as graphene, graphite, and molybdenum disulfide, usually exhibit superlubricity properties when sliding occurs between the incommensurate interface lattices. This ...study reports the superlubricity between graphite and silica under ambient conditions, induced by the formation of multiple transferred graphene nanoflakes on the asperities of silica surfaces after the initial frictional sliding. The friction coefficient can be reduced to as low as 0.0003 with excellent robustness and is independent of the surface roughness, sliding velocities, and rotation angles. The superlubricity mechanism can be attributed to the extremely weak interaction and easy sliding between the transferred graphene nanoflakes and graphite in their incommensurate contact. This finding has important implications for developing approaches to achieve superlubricity of layered materials at the nanoscale by tribointeractions.
Superlubricity between graphite and silica is achieved under ambient conditions, induced by the formation of multiple transferred graphene nanoflakes (GNFs) on the silica surfaces. The friction coefficient can reduce to 0.0003 at a maximal local contact pressure of 700 MPa, which is attributed to the extremely weak interaction and easy sliding between the transferred GNFs and graphite in incommensurate contact.
Achievement of macroscale superlubricity from the hydration effect could provide a potential application for artificial articular cartilage. In this work, two zwitterionic polymers, ...2-methacryloyloxyethyl phosphorylcholine (MPC) and sulfobetaine methacrylate (SBMA), were used to synthesize a P(MPC-co-SBMA) copolymer hydrogel through the additional polymerization of the alkane carbon-carbon double bond. The incorporation of MPC promoted the water-binding property of copolymer hydrogels as well as improved their anti-compression capability. A superlubricity state with a friction coefficient of approximately 0.002 was achieved when a P(MPC-co-SBMA) copolymer hydrogel hemisphere slid on a sapphire in water, which demonstrated its dependence on sliding velocity, load, and aqueous lubricants. The superlubricity mechanism was mainly attributed to the hydration effect from zwitterionic MPC and SBMA polymer chains, which led to the formation of a uniform hydration layer on the hydrogel surface and the strong adsorption of water molecules on the sapphire surface, which provided additional stabilized hydration layers. These findings might provide insight into the superlubricity mechanism of zwitterionic hydrogels based on the hydration effect, thereby broadening their extensive applications.
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•A novel macro-scale superlubricity system was established between copolymer hydrogel and sapphire.•MPC provided an improvement in compressive capacity for the copolymer hydrogel's network.•MPC promoted the formation of hydration layer on the hydrogel surfaces and enhanced the lubrication properties of hydrogel.•This work provides an idea for the study of macro-scale superlubricity mechanism of zwitterionic hydrogels.
Superlubricity of graphite sliding against graphene can be easily attained at the nanoscale when it forms the incommensurate contact under a low contact pressure. However, the achievement of ...superlubricity under an ultrahigh contact pressure (>1 GPa), which has more applications in the lubrication of micromachine and nanomachine, remains unclear. Here, this problem is addressed and the robust superlubricity of graphite is obtained under ultrahigh contact pressures of up to 2.52 GPa, by the formation of transferred graphene nanoflakes on a silicon tip. The friction coefficient becomes as low as 0.0003, a state that is attributed to the extremely low shear strength of the graphene/graphite interface in the incommensurate contact. When the pressure exceeds some threshold, the superlubricity state collapses suddenly with the friction coefficient increasing ≈10 times. The failure of superlubricity originates from the delamination of the topmost graphene layers on graphite under ultrahigh contact pressures, which requires the tip to provide additional exfoliation energies during the sliding process. The results demonstrate that the superlubricity of graphite sliding against graphene can exist stably under ultrahigh contact pressure, which would appear to accelerate its application in nanoscale lubrication.
Robust superlubricity of graphite sliding against graphene nanoflake under ultrahigh contact pressures of 2.52 GPa is obtained. The friction coefficient is reduced to 0.0003, a state that is attributed to the extremely low shear strength of the graphene/graphite interface. When the pressure exceeds a certain threshold, the superlubricity state collapses, which originates from the delamination of the topmost graphene layers on graphite under ultrahigh contact pressures.
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