Research Article Issue
Modulation mechanism of electron energy dissipation on superlubricity based on fluorinated 2D ZIFs
Nano Research 2024, 17 (4): 3198-3209
Published: 02 February 2024

Electron energy dissipation is an important energy dissipation pathway that cannot be ignored in friction process. Two-dimensional zeolite imidazole frameworks (2D ZIFs) and fluorine doping strategies give 2D Zn-ZIF and 2D Co-ZIF unique electrical properties, making them ideal materials for studying electron energy dissipation mechanism. In this paper, based on the superlubricity modulation of 2D fluoridated ZIFs, the optimal tribological properties are obtained on the 2D F-Co-ZIF surface, with the friction coefficient as low as 0.0010. Electrical experiments, density functional theory (DFT) simulation, and fluorescence detection are used to explain the mechanism of fluorine doping regulation of tribological properties from the two stages, namely energy transfer and energy release. Specifically, the energy will transfer into the friction system through the generation of electron–hole pairs under an external excitation, and release by radiation and non-radiation energy dissipation channels. Fluorination reduces energy transfer by altering the electronic properties and band structures of ZIFs, and slows down the charge transfer by enhancing the shielding efficiency, thus slowing the non-radiative energy dissipation rate during the energy release stage. Our insights not only help us better understand the role of fluorine doping in improving tribological properties, but also provide a new way to further explore the electron energy dissipation pathway during friction.

Research Article Issue
Achieving ultrafast superlubricity with layered double hydroxides
Nano Research 2023, 16 (5): 6940-6950
Published: 07 January 2023

Layered double hydroxides (LDHs) have the potential to be superlubricated materials due to their strong adsorption effect and weak internal interaction. However, obtaining stable superlubricity during the ultrafast time (< 10 s) is still a challenge. Here, we demonstrated macroscale superlubricity based on LDHs of multiple metal ions at high surface roughness, achieving superlow friction coefficients (0.006) and ultrafast wearing-in time (< 7 s), which mainly originated from tribochemical reactions and the formation of nanostructured adsorption layers. Through cross-sectional analysis and density functional theory, we revealed the properties of the protective tribofilm to achieve ultrafast superlubricity. LDHs strongly adsorbed on the surface of the bearing steel, and the sliding interface transformed into a heterogeneous interface between the polytetrafluoroethylene and LDH, leading to macroscale superlubricity. These findings demonstrate that tribochemical treatment of surfaces produces tribofilm that effectively reduces wearing-in time and promotes ultralow friction.

Open Access Research Article Issue
Adjustable superlubricity system using polyalkylene glycol with various acid aqueous solutions
Friction 2023, 11 (7): 1138-1149
Published: 16 July 2022

Polyalkylene glycol (PAG) aqueous solutions have recently been demonstrated to exhibit an ultralow friction coefficient (COF, μ < 0.01). However, the prolonged running-in period and low bearing capacity have limited its widespread application. In this study, we determined that the running-in period can be decreased by more than 75% when the pH value of the lubricant is controlled at 3 by introducing various acid solutions. Additionally, less time was required to realize stable superlubricity with inorganic acid at lower pH values. This was mainly attributed to the acceleration effect of hydrogen ions around the contact region. In case of PAG aqueous solution with organic acid, the wear loss between sliding solid surfaces was reduced, and thus the bearing pressure during the superlubricity period was significantly improved from approximately 30 to 160 MPa. Furthermore, the organic acid molecules were considered to form strong hydrogen bonds with PAG macromolecules and solid surfaces. This in turn strengthened the structure of the adsorption layers. The unique effect of different acids in aqueous polymer lubrication can potentially significantly aid in advancing the study of polymer tribology and broadening industrial applications.

Research Article Issue
Electric field controlled superlubricity of fullerene-based host–guest assembly
Nano Research 2023, 16 (1): 583-588
Published: 04 July 2022

Controlling friction by the electric field is a promising way to improve the tribological performance of a variety of movable mechanical systems. In this work, the assembly structure and microscale superlubricity of a host–guest assembly are effectively controlled by the electric field. With the help of the scanning tunneling microscopy (STM) technique, the host–guest assembly structures constructed by the co-assembly of fullerene derivative (Fluorene-C60) with macrocycles (4B2A and 3B2A) are explicitly characterized. Combined with density functional theory (DFT), the distinct different assembly behaviors of fullerene derivatives are revealed at different probe biases, which is attributed to the molecular polarity of the fullerene derivative. Through the control on the adsorption behavior, the friction coefficient of host–guest assembly is demonstrated to be controllable in the electric field by using atomic force microscopy (AFM). At positive probe bias, the friction coefficient of the host–guest assembly is significantly reduced and achieves superlubricity (μmin = 0.0049). The efforts not only help us gain insight into the host–guest assembly mechanism controlled by the electric field, but also promote the further application of fullerene in micro-electro-mechanical systems (MEMS).

Open Access Research Article Issue
Hierarchical self-assembled structure and frictional response of phthalocyanine molecules
Friction 2023, 11 (3): 354-368
Published: 12 April 2022

Solid evidence is needed to demonstrate the effect of molecular orientation and structure on the frictional property of boundary lubricants. In this work, the frictional properties of phthalocyanine self-assembled monolayers (SAMs) with face-on (aromatic cores parallel to the substrate) and edge-on (aromatic cores stand on the substrate) orientations have been compared and the in situ structural variation of edge-on SAMs under frictional shear has been revealed by atomic force microscope (AFM). Face-on oriented SAMs show lower adhesion, lower friction, and stronger wear resistance, compared with edge-on oriented SAMs. Hierarchical structures of edge-on oriented SAMs have been revealed by frictional topography, which are consisted of nanoscale columns, micron-scale stripes, and centimeter-scale monolayer. The column structure deforms under increasing load force, leading to a stepwise friction force curve and a transition among three friction states (ordered friction, collapsed friction, and worn friction). The structural deformation depends on both the order degree and anisotropic stiffness of columns. Columns in phthalocyanine SAMs show a larger stiffness when shearing against molecular plane than shearing along the molecular plane. The presented study on the interfacial structure and frictional mechanism promisingly supports the designing of novel boundary lubricants and their application in engineering.

Open Access Research Article Issue
Morphological residual convolutional neural network (M-RCNN) for intelligent recognition of wear particles from artificial joints
Friction 2022, 10 (4): 560-572
Published: 05 April 2022

Finding the correct category of wear particles is important to understand the tribological behavior. However, manual identification is tedious and time-consuming. We here propose an automatic morphological residual convolutional neural network (M-RCNN), exploiting the residual knowledge and morphological priors between various particle types. We also employ data augmentation to prevent performance deterioration caused by the extremely imbalanced problem of class distribution. Experimental results indicate that our morphological priors are distinguishable and beneficial to largely boosting overall performance. M-RCNN demonstrates a much higher accuracy (0.940) than the deep residual network (0.845) and support vector machine (0.821). This work provides an effective solution for automatically identifying wear particles and can be a powerful tool to further analyze the failure mechanisms of artificial joints.

Research Article Issue
Macroscale superlubricity under ultrahigh contact pressure in the presence of layered double hydroxide nanosheets
Nano Research 2022, 15 (5): 4700-4709
Published: 02 February 2022

It is difficult to achieve macroscale superlubricity under high contact pressures and high normal loads. Layered double hydroxide (LDH) nanoadditives were introduced into an ionic liquid alcohol solution (IL(as)) with contact pressures up to 1.044 GPa, which resulted in a friction coefficient (COF) of 0.004 and a robust superlubricity state lasting for 2 h. Compared with the LDH particles (LDH-Ps) with ca. 90-nm widths and 18-nm thickness, micron-scale LDH nanosheet (LDH-N) additives with ca. 1.5-µm width and 6-nm thickness increased the load-bearing capacity by approximately three times during superlubricity. The lubricant film thickness and the ultrathin longitudinal dimension of the LDH-N additives did not influence the continuity of the fluid film on the contact surface. These improvements resulted from the protective adsorption layer and ion distribution formed on the contact interface, as revealed by detailed surface analyses and simulation studies. In particular, the sliding energy barrier and Bader charge calculation revealed that weak shear sliding between the nanosheet and the solid surface formed easily and the anions in the liquid adsorbed on the solid surface exhibited electrostatic repulsion forces, which generated stable tribological properties synergistically. This research provides a novel method for obtaining macroscale superlubricity for practical industrial applications.

Open Access Research Article Issue
Effect of contact geometry on the friction of acrylamide hydrogels with different surface structures
Friction 2022, 10 (3): 360-373
Published: 23 November 2020

Polyacrylamide (PAAm) hydrogels with brush-covered or crosslinked surfaces were produced and their tribological behavior was studied over a wide range of sliding speeds for two different contact geometries: sphere-on-flat and flat-pin-on-flat. Irrespective of the contact geometry, the brushy hydrogel surfaces displayed up to an order of magnitude lower coefficients of friction µ (COF) compared to the crosslinked surfaces, even achieving superlubricity (µ < 0.01). In general, a hydrogel sphere showed a lower coefficient of friction than a flat hydrogel pin at a similar contact pressure over the entire range of sliding speeds. However, after normalizing the friction force by the contact area, the shear stress of hydrogels with either crosslinked or brushy surfaces was found to be similar for both contact geometries at low speeds, indicating that hydrogel friction is unaffected by the contact geometry at these speeds. At high sliding speeds, the shear stress was found to be lower for a sphere-on-flat configuration compared to a flat-pin-on-flat configuration. This can be attributed to the larger equivalent hydrodynamic thickness due to the convergent inlet zone ahead of the sphere-on-flat contact, which presumably enhances the water supply in the contact, promotes rehydration, and thus reduces the friction at high sliding speeds compared to that measured for the flat-pin-on-flat contact.

Open Access Review Article Issue
Superlubricity achieved with two-dimensional nano-additives to liquid lubricants
Friction 2020, 8 (6): 1007-1024
Published: 23 July 2020

The topic of superlubricity is attracting considerable interest around the world while humanity is facing an energy crisis. Since various liquid superlubricity systems can be commonly achieved on the macroscale in ambient conditions, it is considered an effective solution to reduce unnecessary energy and material losses. However, certain practical problems such as low load-bearing pressure, dependence on hydrogen ions, and relatively long running-in processes still limit its widespread application. Two- dimensional (2D) nano-additives with ultrathin longitudinal dimensions can lower the shear resistance between sliding solid surfaces, and thus further optimize the applied conditions. In this review, the latest studies on 2D nano-additives with a combination of various water-based lubricants in the state of superlubricity are reported, typically including black phosphorus (BP), graphene oxide (GO), and layered double hydroxide. During the sliding process, composite lubricants effectively improved the load capacity (up to 600 MPa), reduced wear, and accelerated the running-in period (within 1,000 s) of the liquid superlubricity system. Both macromechanical experiments and microscopic tests are conducted to precisely analyze various interactions at the interfaces of the nano-additives and solid surfaces. These interactions can be described as tribochemical reactions, physical protection, and adsorption enhancement, and improved wear resistance. This review provides better guidance for applying 2D nanomaterials in liquid superlubricity systems.

Open Access Research Article Issue
Tribological behavior of layered double hydroxides with various chemical compositions and morphologies as grease additives
Friction 2021, 9 (5): 952-962
Published: 20 May 2020

The layered double hydroxide (LDH) is a kind of natural mineral, which can also be manually prepared. It has been practically applied in various fields due to its unique crystal structure and diversity of composition, size, and morphology. In this work, LDHs with different chemical compositions (Co2+, Mg2+, Zn2+, and Ni2+) and topographical features (flower-like, spherical, and plate-like) were successfully prepared by controlling the reaction conditions. Then, they were mechanically dispersed into base grease and their tribological properties were evaluated by a ball-on-disk tester under a contact pressure of 2.47 GPa. It was found that the variation of morphology, instead of chemical composition, had great influence on the tribological performance. The "flower-like" LDH sample with high specific surface area (139 m2/g) was demonstrated to show the best performance. With 1 wt% additive, the wear volume was only about 0.2% of that lubricated by base grease. The tribofilm with unique microscopic structure and uniform composition was derived from tribochemical reaction between LDH additives and sliding solid surfaces, effectively improving tribological properties of the lubrication system. This work provided the guidance for optimizing lubricant additives and held great potential in future applications.

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