Topological Rearrangement-Induced Mesoscale Phase Redistribution to Enhance the Fatigue Resistance of Polymer Blends.

Maintaining a high modulus to simultaneously withstand deformation and increase fatigue resistance to restrict crack propagation in a material presents a significant challenge. In this work, a straightforward strategy was developed to address this issue in polymers. A dynamic network was incorporated into a permanent one prior to the formation of the latter, and two incompatible polymer networks were created to prevent common phase separation. The mechanical and fatigue resistance properties were substantially enhanced by the exact modulation of the soft and hard phase distribution by precise control over the densities of dynamic and permanent networks as well as the number of reprocessing steps. The experimental results demonstrated a nearly 9-fold increase in the fatigue life of polyurethane compared with traditional design methods and a 2.5 times increase in modulus. This strategy shows potential for the design of fatigue-resistant thermosetting and thermoplastic materials. The results offer new insight into the development of durable, high-performance materials that are reprocessable and compatible.

Effects of the Mesoporous Filler and Oleophilic Property on the Tribological Behavior of Polyetheretherketone Composites.

Porous composites, such as polyimide and polyetheretherketone (PEEK) polymer composites, became more attractive as a result of excellent specific strength, lightweightness, and smart lubrication. However, revealing the influence of a porous filler on the friction behaviors of polymer composites remains a challenge. The current work examines the effects of the pore structure and wettability on the mechanical and tribological performances of polymer composites reinforced with fillers. Three kinds of particles (non-porous, porous, and oleophobic porous) act as fillers within PEEK, respectively. It was identified that adding porous zeolite particles into PEEK slightly decreased the mechanical property. The zeolite particles significantly increased the oil adsorption of the composite, leading to a significant increase in the friction coefficient at 10 wt % content. However, the friction coefficient of the PEEK composite filled with 20 wt % modified zeolite was relatively low as a result of the weak adsorption. The interface analyses indicated that the thickness of the oil film was controlled via porous structures, changing the lubrication regime of the tribopair. The reason is the appropriate adsorption of oil and the release of zeolite from the composites during the friction process. These results can offer technical guidance to control the friction behaviors of composites under oil-lubricating conditions by a porous particle and pore modification.

Self-Reinforced Piezoelectric Response of an Electroluminescent Film for the Dual-Channel Signal Monitoring of Damaged Areas.

Organic piezoelectric nanogenerators (PENGs) show promise for monitoring damage in mechanical equipment. However, weak interfacial bonding between the reinforcing phase and the fluorinated material limits the feedback signal from the damaged area. In this study, we developed a PENG film capable of real-time identification of the damage location and extent. By incorporating core-shell barium titanate (BTO@PVDF-HFP) nanoparticles, we achieved enhanced piezoelectric characteristics, flexibility, and processability. The composite film exhibited an expanded output voltage range, reaching 41.8 V with an increase in frequency, load, and damage depth. Additionally, the film demonstrated self-powered electroluminescence (EL) during the wear process, thanks to its inherent ferroelectric properties and the presence of luminescent ZnS:Cu particles. Unlike conventional PENG electroluminescent devices, the PENG film exhibited luminescence at the damage location over a wide temperature range. Our findings offer a novel approach for realizing modular and miniaturized real-time damage mapping systems in the field of safety engineering.

Partially Oxidized Violet Phosphorus as an Excellent Lubricant Additive for Tribological Applications.

As a novel two-dimensional material, violet phosphorus (VP) has attracted a considerable amount of attention due to its high carrier mobility, anisotropy, wide band gap, stability, and easy stripping properties. In this work, the microtribological properties of partially oxidized VP (oVP) and the mechanism of reducing friction and wear as additives in oleic acid (OA) oil were studied systematically. When adding oVP to OA, the coefficient of friction (COF) decreased from 0.084 to 0.014 with the steel-to-steel pair, and the ultralow shearing strength tribofilm consisting of amorphous carbon and phosphorus oxides that formed resulted in the reductions of COF and wear rate individually by 83.3% and 53.9%, respectively, compared with those of pure OA. The results extended the application scenarios for VP in the design of lubricant additives.

Self-Adaptive Macroscale Superlubricity Based on the Tribocatalytic Properties of Partially Oxidized Black Phosphorus.

The high-flash heat generated by direct contact at asperity tips under high contact stress and shear significantly promotes the tribocatalytic reaction between a lubricating medium and a friction interface. Macroscale superlubricity can be achieved by using additives with good lubrication properties to promote the decomposition and transformation of a lubricating medium to form an ultralow shear interface during the friction process. This paper proposed a way to achieve self-adaptive oil-based macroscale superlubricity on different tribopairs, including steel-steel and steel-DLC (diamond-like carbon), which is based on the excellent lubricating performance of black phosphorus with active oxidation and the catalytic cleavage behavior of oil molecules on the surface of oBP. This work potentially expands the industrial application of superlubricity.

Tribological and Antibacterial Properties of Polyetheretherketone Composites with Black Phosphorus Nanosheets.

Over the past few decades, polyetheretherketone (PEEK) artificial bone joint materials faced problems of poor wear resistance and easy infection, which are not suitable for the growing demand of bone joints. The tribological behavior and wear mechanism of polyetheretherketone (PEEK)/polytetrafluoroethylene (PTFE) with black phosphorus (BP) nanosheets have been investigated under dry sliding friction. Compared with pure PEEK, the COF of PEEK/10 wt% PTFE/0.5 wt% BP was reduced by about 73% (from 0.369 to 0.097) and the wear rate decreased by approximately 95% (from 1.0 × 10-4 mm3/(N m) to 5.1 × 10-6 mm3/(N m)) owing to the lubrication of the BP transfer film. Moreover, BP can endow the PEEK composites with excellent biological wettability and antibacterial properties. The antibacterial rate of PEEK/PTFE/BP was assessed to be over 99.9%, which might help to solve the problem of PEEK implant inflammation. After comprehensive evaluation in this research, 0.5 wt% BP nanosheet-filled PEEK/PTFE material displayed the optimum lubrication and antibacterial properties, and thus could be considered as a potential candidate for its application in biomedical materials.

Load-dependent energy dissipation induced by the tip-membrane friction on suspended 2D materials.

The tip-membrane interface plays a critical role in characterizing the mechanical properties of ultrathin 2D materials by commonly employed nanoindentation based on atomic force microscopy (AFM). However, the reliability of the assumption that the tip-membrane interface remains pinned during nanoindentation remains unclear, which may introduce unignorable uncertainty in evaluating their true mechanical properties. In this work, it is reported that load-dependent frictional behavior would occur on the tip-membrane interface during nanoindentation tests on monolayer and multilayer suspended WS2 and graphene, and the curve hysteresis could be well explained by the stick-slip behavior. Further analyses and finite element simulations demonstrated that the frictional energy dissipation should be mainly attributed to the frictional behavior along the direction parallel to the cantilever beam. Meanwhile, the in-plane membrane stiffness was mainly responsible for the different frictional behavior on monolayer and multilayer 2D materials. Based on these analyses, some suggestions were proposed to help reduce the uncertainty when extracting the mechanical properties of 2D materials. These findings not only facilitate the deep understanding of the origin of the curve hysteresis during nanoindentation, but also help to evaluate the mechanical properties of 2D materials in a more reliable way.

Reduced Fracture Strength of 2D Materials Induced by Interlayer Friction.

The potential applications of 2D layered materials (2DLMs) as the functional membranes in flexible electronics and nano-electromechanical systems emphasize the role of the mechanical properties of these materials. Interlayer interactions play critical roles in affecting the mechanical properties of 2DLMs, and nevertheless the understanding of their relationship remains incomplete. In the present work, it is reported that the fracture strength of few-layer (FL) WS2 can be weakened by the interlayer friction among individual layers with the assistance of finite element simulations and density functional theory (DFT) calculations. The reduced fracture strength can be also observed in FL WSe2 but with a lesser extent, which is attributed to the difference in the interlayer sliding energies of WS2 and WSe2 as confirmed by DFT calculations. Moreover, the tip-membrane friction can give rise to the underestimation of the Young's modulus except for the membrane nonlinearity. These results give deep insights into the influence of interfacial interactions on the mechanical properties of 2DLMs, and suggest that importance should be also attached to the interlayer interactions during the design of nanodevices with 2DLMs as the functional materials.

Synthesis of Core-Shell Micro/Nanoparticles and Their Tribological Application: A Review.

Owing to the diverse composition, adjustable performance, and synergistic effect among components, core-shell micro/nanoparticles have been widely applied in the field of tribology in recent years. The strong combination with the matrix and the good dispersion of reinforcing fillers in the composites could be achieved through the design of core-shell structural particles based on the reinforcing fillers. In addition, the performance of chemical mechanical polishing could be improved by optimizing the shell material coated on the abrasive surface. The physical and chemical state of the core-shell micro/nanoparticles played important effects on the friction and wear properties of materials. In this paper, the synthesis methods, the tribological applications (acted as solid/liquid lubricant additive, chemical mechanical polishing abrasives and basic units of lubricant matrix), and the functionary mechanisms of core-shell micro/nanoparticles were systematically reviewed, and the future development of core-shell micro/nanoparticles in tribology was also prospected.

Thickness-dependent Young's modulus of polycrystalline α-PbO nanosheets.

Litharge, in two dimensional (2D) nanostructure form, has recently ignited considerable theoretical interest due to its excellent photoelectric and magnetic properties. However, the lack of an efficient synthesis method hinders its development. Here, we provide an interfacial solvothermal strategy for controllably synthesizing ultrathin hexagonal polycrystalline α-PbO nanosheets in micrometer scale. This strategy can also be utilized for the synthesis of other 2D materials. Experimental atomic force microscope nanoindentation measurements reveal the relationship between the thickness of polycrystalline α-PbO nanosheets and the corresponding Young's modulus, expressed as E = E0 + Kt -1. First-principles calculation supports the result and ascribes the cause to interlayer sliding from particular weak interlayer interactions. Additionally, the enhanced mechanical strength of the polycrystalline structure compared to its single-crystal counterpart is attributed to the alternate arrangement of grain-boundaries effects. The summative formula may be extended to other 2D materials with weak interlayer interactions, which has the potential to provide guidance for constructing flexible devices.

Super-Slippery Degraded Black Phosphorus/Silicon Dioxide Interface.

The interfaces between two-dimensional (2D) materials and the silicon dioxide (SiO2)/silicon (Si) substrate, generally considered as a solid-solid mechanical contact, have been especially emphasized for the structure design and the property optimization in microsystems and nanoengineering. The basic understanding of the interfacial structure and dynamics for 2D material-based systems still remains one of the inevitable challenges ahead. Here, an interfacial mobile water layer is indicated to insert into the interface of the degraded black phosphorus (BP) flake and the SiO2/Si substrate owing to the induced hydroxyl groups during the ambient degradation. A super-slippery degraded BP/SiO2 interface was observed with the interfacial shear stress (ISS) experimentally evaluated as low as 0.029 ± 0.004 MPa, being comparable to the ISS values of incommensurate rigid crystalline contacts. In-depth investigation of the interfacial structure through nuclear magnetic resonance spectroscopy and in situ X-ray photoelectron spectroscopy depth profiling revealed that the interfacial liquid water was responsible for the super-slippery BP/SiO2 interface with extremely low shear stress. This finding clarifies the strong interactions between degraded BP and water molecules, which supports the potential wider applications of the few-layer BP nanomaterial in biological lubrication.

In-Plane Potential Gradient Induces Low Frictional Energy Dissipation during the Stick-Slip Sliding on the Surfaces of 2D Materials.

Understanding the nanoscale friction properties of 2D materials and further manipulating their friction behaviors is of great significance for the development of various micro/nanodevices. Recent studies, taking advantage of the close relationship between friction and surface charges, use an external out-of-plane electric field to control the interfacial friction. Nevertheless, friction increases with the application of the out-of-plane electric field in most cases. Here, an in-plane potential gradient is applied for the investigation of the contribution of electric charges to friction on the surfaces of 2D materials. Experimental results show that the friction between an atomic force microscope tip and the flakes of 2D materials decreases with the application of the in-plane potential gradient, and the higher the potential gradient, the greater the friction decrease. By comparing the in situ atomic-level stick-slip maps before and after the application of the in-plane potential gradient, it is proposed that the promotion of low friction dissipative motion during the stick-slip process owing to the presence of the potential gradient gives rise to the friction reduction. These results not only help to reveal the origin of friction, but also provide a novel way to manipulate friction through an electrically-controlled sliding process.

Two-dimensional layered materials: from mechanical and coupling properties towards applications in electronics.

With the increasing interest in nanodevices based on two-dimensional layered materials (2DLMs) after the birth of graphene, the mechanical and coupling properties of these materials, which play an important role in determining the performance and life of nanodevices, have drawn increasingly more attention. In this review, both experimental and simulation methods investigating the mechanical properties and behaviour of 2DLMs have been summarized, which is followed by the discussion of their elastic properties and failure mechanisms. For further understanding and tuning of their mechanical properties and behaviour, the influence factors on the mechanical properties and behaviour have been taken into consideration. In addition, the coupling properties between mechanical properties and other physical properties are summarized to help set up the theoretical blocks for their novel applications. Thus, the understanding of the mechanical and coupling properties paves the way to their applications in flexible electronics and novel electronics, which is demonstrated in the last part. This review is expected to provide in-depth and comprehensive understanding of mechanical and coupling properties of 2DLMs as well as direct guidance for obtaining satisfactory nanodevices from the aspects of material selection, fabrication processes and device design.

Core-shell nanospheres to achieve ultralow friction polymer nanocomposites with superior mechanical properties.

Core-shell nanospheres have been widely used in catalysis, batteries, medicine, etc. owing to their unique structural characteristics, which exhibit optimal performance and integrated functions of both the core and shell materials. To simultaneously achieve outstanding mechanical properties and remarkable lubrication properties in desirable polymer composites, core-shell nanospheres with polytetrafluoroethylene (PTFE) as the core and poly methyl methacrylate (PMMA) as the shell have been adopted as structural units to form bulk nanocomposites. We demonstrated that the mechanical and lubrication properties of the nanocomposites prepared using core-shell nanospheres as the continuous matrix were dramatically improved. Specifically, when compared with that of pure PTFE, the compressive strength of the PTFE@PMMA nanocomposite obviously increased up to one order of magnitude (from ∼9 to ∼90 MPa), the friction coefficient reduced to 25% (the lowest value was 0.03), and the wear rate decreased up to two orders of magnitude. Moreover, the mechanical and lubrication properties of the nanocomposites could be adjusted by changing the core-shell ratio, and an appropriate core-shell ratio was beneficial for achieving the desired comprehensive properties. It has been proposed that the properties, such as the confinement effect, improved dispersion capacity, etc., imparted by the core-shell structure effectively lead to high dispersion of the reinforcement phase, improvement of the binding force of the transfer film to the friction surface, and interruption of the wear process of the polymer composite.

Superlubricity of Black Phosphorus as Lubricant Additive.

Superlubricity is defined as a sliding regime in which friction, or the resistance to sliding of two relatively moving surfaces, almost vanishes. From a practical point of view, the development and use of new materials that can enable superlubricity (coefficient of friction, COF < 0.01) in moving mechanical systems will have huge positive impact on energy-saving and emission reduction. In this work, the use of a new two-dimensional material, black phosphorus (BP) as a high-performance water-based lubricant additive that can significantly reduce friction and achieve superlubricity has been explored. A lowest COF value of 0.0006 ever measured by an application-orientated ball-on-plate tribometer has been found. Robust superlubricity in the aqueous solution with ultrafine BP nanosheets modified by NaOH (BP-OH) has been observed for a wide range of additive concentrations, contact pressures, and sliding velocities owing to the very low shear resistance of the water layer retained by BP-OH nanosheets. This finding has the potential of opening up a new approach to dramatically reduce or even eliminate friction by using BP nanomaterials as lubricant additives.

Black Phosphorus: Degradation Favors Lubrication.

Due to its innate instability, the degradation of black phosphorus (BP) with oxygen and moisture was considered the obstacle for its application in ambient conditions. Here, a friction force reduced by about 50% at the degraded area of the BP nanosheets was expressly observed using atomic force microscopy due to the produced phosphorus oxides during degradation. Energy-dispersive spectrometer mapping analyses corroborated the localized concentration of oxygen on the degraded BP flake surface where friction reduction was observed. Water absorption was discovered to be essential for the degraded characteristic as well as the friction reduction behavior of BP sheets. The combination of water molecules as well as the resulting chemical groups (P-OH bonds) that are formed on the oxidized surface may account for the friction reduction of degraded BP flakes. It is indicated that, besides its layered structure, the ambient degradation of BP significantly favors its lubrication behavior.

Electromechanical failure of MoS2 nanosheets.

Molybdenum disulfide (MoS2) has attracted particular attention as a promising electronics and optoelectronics material due to its significant physical properties. In this research, the electromechanical properties of MoS2 nanosheets are systematically investigated with the conductive AFM nanoindentation method. The suspended MoS2 nanosheets with the thickness of tens of nanometers can sustain external applied load (∼4.3 µN) until the bias increases to a critical value (+4 V). Small external load (∼400 nN) may also lead to a failure when the bias (+7 V) overcomes the contact Schottky barrier and generates high electric current. The extent of destruction of MoS2 nanosheets is related to the external applied load and the bias. Besides, the MoS2 nanosheets suspended on the holes are more likely to be damaged than those supported on the substrate under the same conditions. The volcanic volume expansion profiles of the damaged area after the electromechanical failure are caused by electric current-induced local heating anodic oxidation and buckling-induced structural instability. The emergence of gas bubbles in the damaged area proves the strong oxidation process. These experiments have proved that the electric current can promote the mechanical failure of MoS2 nanosheets. The findings can also provide beneficial guidance for the electromechanical applications of MoS2 nano-devices.

Nanotribological Properties of Graphite Intercalation Compounds: AFM Studies.

Tetraalkylammonium salts have larger ions than metal ions, which can greatly change the interlayer space and energy, and then potentially tune the properties of graphite. In this work, various graphite intercalation compounds (GICs) have been synthesized by intercalating tetraoctylammonium bromide (TOAB) ions into graphite through electrochemical interactions under different reduction potentials. Different degrees of expansion between graphite layers as well as their corresponding structures and topographies have been characterized by different analytical techniques. The nanoscale friction and wear properties of these GICs have been investigated by AFM-based nanofrictional and scratch tests. The results show that electrochemical intercalation using tetraalkylammonium salts with different interaction potentials can tune the friction and wear properties of graphite. Under relatively large applied loads of AFM tips, friction increase and wear can be easier to occur with the increase of the intercalation potential. It is inferred that the increases of both the interlayer space of graphite and the number of ions on the surface give rise to puckered effect and formation of rougher surfaces. This work gives us deep insight into the friction and wear properties of GICs as composite lubrication materials, which would be of great help for material design and preparation.

Ultralow Friction Self-Lubricating Nanocomposites with Mesoporous Metal-Organic Frameworks as Smart Nanocontainers for Lubricants.

Smart nanocontainers with stimuli-responsive property can be used to fabricate a new kind of self-lubricating nanocomposite, while the practical potential of the metal-organic frameworks (MOFs) as nanocontainers for lubricants has not been realized. In this work, mesoporous Cu-BTC MOFs storing oleylamine nanocomposites were explored from synthesis and microstructure to self-lubricating characterization. The stress stimuli-responsiveness behavior of the Cu-BTC storing oleylamine (Cu-BTCO) for lubrication has been investigated by subjecting it to macroscopic ball-on-disc friction tests. The steady-state coefficients of friction (COFs) of the Cu-BTC nanocomposites without lubricants were ca. 0.5. In contrast, after oleylamine as the lubricant was incorporated into the Cu-BTC container in the nanocomposite, ultralow friction (COF, ca. 0.03) was achieved. It has been demonstrated that the improved lubricating performance was associated with the lubricating film which was in situ produced by the chemical reaction between the oleylamine released from the nanocontainer and the friction pairs. Therefore, the nanocomposite with smart Cu-BTC container holds the promise of realizing extraordinary self-lubricating properties under stress stimuli.