The mechanical properties of the polymer matrix are crucial for forming a stable and dense transfer film between the friction pairs, facilitating friction reduction and wear resistance. This work introduces a novel polyurethane composite (PUC) system, prepared by incorporating the MXene (Ti3C2Tx) into a polyurethane elastomer, which exhibits a robust strength of similar to 64 MPa and an excellent toughness of similar to 155.8 MJ/m(3). The PUC containing 0.5 wt % MXene (PUC0.5) exhibited excellent tribological properties, with the friction coefficient and wear rate (66 N, 0.54 m/s) reduced by 45.1 % and 206 % in comparison to pure PU, respectively. The excellent tribological properties of PUC0.5 are attributed to exceptional mechanical properties and low shear of MXene.

"Life" represents a distinctive attribute inherent to organisms in nature, evident in their capacity to actively adapt to changes in their environment. In contrast to the static and intricate constructs of additive manufacturing (AM), the dynamic structure of 4D printing (4DP) adeptly integrates AM technology, responsive mechanisms, and external stimuli, imbuing it with a semblance of "life." This fusion significantly broadens its functional applications across biomedicine, actuators, and metamaterials. The escalating demand across diverse fields necessitates heightened criteria for 4DP, encompassing rapid response, multi-stimulus response, large shape change, and specific mechanical properties (e.g., high strength, high modulus) capable of accommodating varying environmental conditions. In recent years, shape memory polymers (SMPs) have garnered increasing attention among 4DP researchers due to their ease of design and preprogramming at the molecular level, facilitating controlled transformations along predictable pathways. However, 4DP of high-strength SMPs, as an indispensable part of the high-performance field, is full of challenges because the intrinsic properties of the raw materials are not well compatible with the printing principle and the printed configuration is not flexible enough. Consequently, this paper provides a concise overview of the response mechanisms and applications of five prominent high-strength SMPs utilized in 4DP: epoxy resin, poly(ether-ether-ketone), polyimide, polylactic acid, and polyurethane. Additionally, it delves into the associated challenges and prospects, offering researchers valuable insights into the potential value of high-strength SMPs within the domain of 4DP.

4D printing (4DP) of high-performance shape memory polymers (SMPs), particularly using digital light processing (DLP), has garnered intense global attention due to its capability for rapid and high-precision fabrication of complex configurations, meeting diverse application requirements. However, the development of high-performance dynamic shape memory polymers (DSMPs) for DLP printing remains a significant challenge due to the inherent incompatibilities between the photopolymerization process and the curing/polymerization of high-strength polymers. Here, a mechanically robust DSMP compatible is developed with DLP printing, which incorporates dynamic covalent bonds of imine linking polyimide rigid segments, exhibiting remarkable mechanical performance (tensile strength ≈41.7 MPa, modulus ≈1.63 GPa) and thermal stability (Tg ∼ 113 °C, Td ∼ 208 °C). More importantly, benefiting from the solid-state plasticity conferred by dynamic covalent bonds, 4D printed structures demonstrate rapid network adaptiveness, enabling effortless realization of reconfiguration, self-healing, and recycling. Meanwhile, the extensive π-π conjugated structures bestow DSMP with an intrinsic photothermal effect, allowing controllable morphing of the 4D configuration through dual-mode triggering. This work not only greatly enriches the application scope of high-performance personalized configurations but also provides a reliable approach to addressing environmental pollution and energy crises.

In this work, a macroporous oil-containing metal-organic framework material (CuBTCO) was prepared by a solvothermal method and vacuum impregnation. The tribological behaviors of PTFE/PEEK fabric composites (PFC) filled with CuBTCO were evaluated by the pin-on-disk tribometer under different loads. The coefficient of friction and mass wear rate of PFC containing 6 wt% CuBTCO (PFC-CuBTCO-6 %) were 0.017 and 1.1 x 10-9 g/ (Nm), respectively, which were reduced by 63.8 % and 76.3 % in comparison to PFC containing 6 wt% CuBTC under the same conditions. The excellent tribological properties of PFC-CuBTCO-6 % are attributed to the solid-liquid synergistic lubrication between the adsorption film formed by the oleylamine released from CuBTCO and the PTFE-based transfer film. This work provides a novel approach for the structural design of solid-liquid synergistic lubrication of fabric liner composites.

Fabric composites are widely employed in self-lubricating bearing liners as solid lubrication materials. Although the tribological behaviors of fabric composites have been extensively studied, the cryogenic tribological properties and mechanisms have been scarcely reported and are largely unclear to instruct material design for aerospace and other high-tech applications. Herein, the tribological properties of polytetrafluoroethylene (PTFE)-based hybrid-fabric composites were investigated at cryogenic and ambient temperatures in the form of pin-on-disk friction under heavy loads. The results suggest that the friction coefficients of the hybrid-fabric composites obviously increase with a decrease in wear when the temperature drops from 25 to −150 °C. Moreover, thermoplastic polyetherimide (PEI), as an adhesive for fabric composites, has better cryogenic lubrication performance than thermosetting phenol formaldehyde (PF) resin, which can be attributed to the flexible chemical structure of PEI. The excellent lubrication performance of hybrid-fabric composites is attributed to the transfer film formed by PTFE fibers on the surface of fabrics.

The time evolution and temperature dependence of friction along with transfer film morphology and composition for the polytetrafluoroethylene (PTFE) fabric composite lubricated spherical plain bearing was unraveled through a home-made cryogenic tribotester integrating in-situ Fourier Transform Infrared (FTIR) microscopy. Specifically, the temperature dependence of which shows two regions divided by 190 K, and PTFE plays a crucial role on the lubrication of the fabric composite. Thin and uniform transfer film generally forms at 323 K, however, with decreasing temperature, friction increases and transfer film formation is suppressed, which is attributed to the freezing molecular chain movement and inhibition of molecular reorientation of PTFE. After reaching the highest value around 190 K, friction coefficient slightly decreases with further decreasing temperature due to the brittle delamination of PTFE fragments and formation of less-oriented and patch-like uneven transfer film. The transition temperature around 190 K is closely related to the gamma relaxation of PTFE owing to totally restricted mobility of PTFE molecules. Furthermore, inner ring with a surface roughness of 200-300 nm is found to be beneficial to materials transfer and lower friction by increasing plastic flow of PTFE lubricating material at cryogenic temperatures.

Hybrid fabric composites with self-lubricating properties have been applied to various joint bearing liners, especially under cryogenic temperature conditions where liquid lubricant fails. Herein, we report a novel Nomex/polytetrafluoroethylene (PTFE) fabric composite with a hierarchical distribution of PTFE by sintering a layer of PTFE/MoS2 coating. The tribological properties of the PMS composite were investigated over a temperature range from - 150-25 celcius on a pin-on-disk tribometer. The results show that the friction coefficient of the PMS composite has a nonmonotonic relationship with temperature, which is related to the molecular conformation and mobility of PTFE at different temperatures. The wear of the PMS composite decreased with decreasing temperature, exhibiting an unmeasurable 'zero wear' at - 150 degrees C.

Over the past two decades, covalent organic frameworks (COFs) have become the most widely studied porous crystalline materials. Their specific physical and chemical properties are determined by the arrangement of atoms (crystal structure). Therefore, the determination of their structure is arguably the most important characterization step for COFs. Although single-crystal X-ray diffraction is the most widely used method for structure determination, confirmation of the structure of COFs is limited to lattice fringes in transmission electron microscopy (TEM) because of their small crystal size (nanocrystals) or poor crystal quality. At present, many two-dimensional COFs (2D-COFs) have clear powder X-ray diffraction (PXRD) patterns, but specific lattice fringes are not available for all 2D-COFs. This severely hinders the development of the COF field. Here, we discovered the lattice shrinkage behavior of COFs under electron beam irradiation by comparing the lattice fringes of 2D-COFs under different conditions. By comparing the lattice fringes of a 1,3,5-tris-(4-aminophenyl)triazine-1,3,5-tris-(4-formylphenyl)triazine covalent organic framework (TAPT-TFPT COF) at room temperature and under liquid nitrogen freezing conditions, we found that the lattice fringes are in good agreement with the PXRD and the theoretical values of the COF (2.213 nm) under freezing conditions. However, the lattice fringe spacing is only 1.656 nm at room temperature. The discovery not only provides new insights into the TEM characterization of COFs, but also further expands the range of crystalline COF materials.

The development of smart lubricating materials with lasting lubrication and outstanding wear resistance while achieving speed response is of great significance in the field of bearing cage applications but has proven to be challenging. Herein, polyimide (PI) molding powder containing two kinds of PI with different rigidities is synthesized in situ to fabricate a two‐scale porous polyimide (PPI) by (cold and hot) isostatic pressing. The unique two‐scale porous structure originates from the bimodal particle size distribution of the PI molding powder. The oil stored in the two‐scale PPI is gradually thrown out as the rotation speed increases, which makes the two‐scale PPI suitable for bearing cages with a wider rotation speed range and realize persistent and stable oil supply under different speeds. The porosity of PPI decreases with increasing pressure during hot isostatic pressing, but the effect of temperature on the porosity is not obvious. Moreover, the two‐scale PPI maintains good mechanical properties, and its ring tension is as high as 22.3 MPa. Due to the high oil content (>15%) and excellent oil retention rate (93.3%, at 3000 rpm for 180 min), the two‐scale oil‐containing PPI exhibits a stable low friction coefficient (≈0.096) and ultralow wear rate (≈0.6810−6mm3 (Nm)−1).

High-performance polymer friction materials with tunable tribological behavior to fit varied work conditions remain a challenge of widespread interest for a variety of applications. Shape memory polymer exhibits morphing and modulus changing over temperature changing provides a promising material to adjust the friction process. Herein, we investigated the tribological properties of shape memory cyanate ester (SMCE) under different conditions. The SMCE exhibits the tribological behavior of good friction material with stable high coefficient of friction (COF) and a low wear rate. Besides, the COF increases and wear rate decreases with the temperature increasing show the tunable friction property of the SMCE. We propose a new model of wear-compensation through shape recovery to explain the adjustable friction behavior of thermal-responsive polymer from the aspect of shape recovery and energy conversion. This study provides a high-performance friction material and paves the route for the application of shape memory polymer (SMP) in tribology field with tunable property.

The mechanism of enhancing the mechanical and tribological properties of polytetrafluoroethylene (PTFE) via the addition of nano-ZrO2 was investigated by virtue of molecular dynamic (MD) simulation from an atomic level. The MD was used to explore stretch and break of molecular chains for understanding the inherent mechanism of nanocomposites. The results found that elastic modulus of PTFE and PTFE/ZrO2 were 1.42 GPa and 6.43 GPa, respectively. The average friction coefficients of PTFE and PTFE/ZrO2 were 0.168 and 0.113, respectively. Compared to PTFE, the friction coefficient of PTFE/ZrO2 decreased by 32.7%. To explore the microscopic friction mechanism, the radial distribution function, atomic concentration, and atomic velocity were simulated and interpreted accordingly during the tribological process.

针对空间高精密轴承用多孔聚酰亚胺保持架的等静压成型要求,开展了聚酰亚胺模塑粉的等静压法制备多孔材料的成型工艺研究.从聚酰亚胺模塑粉自身的结构特性、包套软硬质地、冷、热等静压成型工艺调控(温度、压强、时间)等角度,系统研究了结构因素和冷、热等静压工艺因素对多孔聚酰亚胺材料成孔性能的影响,并通过其微观形貌演变提出了聚酰亚胺模塑粉在冷等静压过程中的主要致密化机制为前期的粒子靠近及重排和中后期的剪切变形,在热等静压过程中主要致密化机制为高温高压下的剪切变形和塑性变形,同时利用封存其中的气体受热膨胀实现扩孔,结果表明,冷等静压工艺对最终多孔聚酰亚胺材料的成孔性能的影响更为根本,热等静压的最高温度提高和保压时间延长有利于获得更大的孔径,为后续采用等静压法制备多孔含油保持架材料并根据应用条件进行成孔性能的精确调控提供了科学依据.

The tribological models of polyimide (PI) reinforced by carbon nanotubes (CNTs) with different orientations were constructed via a molecular dynamic simulation. The variation of PI molecular chains and frictional interface properties were explored for understanding microscopic tribological mechanism of different oriented CNTs. The average friction coefficients of PI composites reinforced by X-, Y-, and Z-oriented CNTs were 0.242, 0.270, and 0.243. Meanwhile, the abrasion rates of XCNT, YCNT, and ZCNT were 11.5% 34.8%, and 28.0%, respectively. The atomic concentration, interfacial temperature and shear strength of XCNT and ZCNT were higher than those of YCNT. The radial distribution function was simulated and interpreted during the friction process to determine the inherent interaction between PI molecular chains and CNTs with specific orientations.

Viologen-based covalent triazine frameworks (Vio-CTFs) were constructed via ionic thermal polymerization in melt ZnCl2 at 400-550 degrees C. The as-prepared Vio-CTFs showed excellent NO2 sensing ability with high response values, fast response and recovery, unique selectivity and stability. The fragment of viologen was recognized as the active site for NO2 adsorption via in-situ UV-Vis spectra, and the weak interaction between NO2 and viologen induced the NO2 sensing process. The DFT calculation was also conducted to explore the NO2 sensing mechanism. Some electrons transferred from NO2 molecule to viologen group when NO2 adsorbed on a viologen-contained model molecule. The negative response during NO2 exposure indicated n-type semiconductor property of Vio-CTFs and which was further confirmed through Mott-Schottky measurement.

In the present study, graphene nanosheets (GNS) supporting monodispersed Ag nanoparticles (GNS/Ag) hybrid was constructed via an in-situ synthesis. The enhancement mechanism of friction and wear performance of Kevlar/PTFE (Poly-p-phenylene terephthamide)/(Polytetrafluoroethylene) phenolic composites reinforced by GNS/Ag hybrid was investigated by virtue of macroscopic experiments and microscopic molecular (MD) simulation. The results found that Ag nanoparticles were homogeneously anchored on the GNS surfaces. When 9 wt% GNS/Ag hybrid was incorporated into fabric composites, the friction coefficient and wear rate of Fabric-9GNS/ Ag decreased by 40 % and 72 % because of synergistic enhancement of GNS and Ag nanoparticles. The MD simulation demonstrated that GNS/Ag hybrid had a strong interaction with molecular chains of PTFE, Kevlar and phenolic resin.

With the development of energy-saving industries, the tribology and thermal management of composites were increasingly urgent. Here, we explored the dual-network graphene/epoxy composites with enhanced tribological and thermal management. The 3D graphene was prepared using calcium ions as the crosslinker and melamine foam as template. Subsequently, the reinforced concrete-like composites were obtained by dipping in silica-filled epoxy. The research results show that the thermal conductivity of the composites was enhanced by 1015.52% compared to the silica-filled epoxy. Simultaneously, the friction coefficient of the composites was reduced to 0.29 with the wear rate be reduced to 0.19 x 10(-5) mm(3)/N.m under the condition of 5 N and 2 cm/s. This dual-network model provides a new strategy for the development of solid lubricating materials with excellent thermal management.