Open Access Research Article Issue
Superlubricity induced by partially oxidized black phosphorus on engineering steel
Friction 2023, 11 (9): 1592-1605
Published: 23 March 2023

Macroscale superlubricity has attracted increasing attention owing to its high significance in engineering and economics. We report the superlubricity of engineering materials by the addition of partially oxidized black phosphorus (oBP) in an oleic acid (OA) oil environment. The phosphorus oxides produced by active oxidation exhibit lower friction and quick deposition performance compared to BP particles. The H-bond (–COOH···O–P, or –COOH···O=P) formed between P–O bond (or P=O) and OA molecule could benefit the lubricating state and decrease the possibility of direct contact between rough peaks. The analysis of the worn surface indicates that a three-layer tribofilm consisting of amorphous carbon, BP crystal, and phosphorus oxide forms during the friction, which replaces the shear interface from the steel/steel to carbon–oBP/carbon–oBP layer and enables macroscale superlubricity.

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 Editorial Issue
Editorial for the Special Issue on 10th Anniversary of Friction
Friction 2022, 10 (10): 1439-1442
Published: 11 October 2022

Open Access Research Article Issue
Extremely low friction on gold surface with surfactant molecules induced by surface potential
Friction 2023, 11 (4): 513-523
Published: 31 May 2022

An extremely low friction state was observed on the gold surface induced by applying a specific negative potential in cationic surfactant solution. The friction force showed a remarkable reduction from 8.3 to 3.5 × 10−2 nN (reduced by 99.6%) with increasing the period of negative applied potential, and the final friction coefficient could reduce down to 3 × 10−4. The extremely low friction state was robust, and it also exhibited an excellent load bearing capacity, which cannot be damaged by a high load. Moreover, the extremely low friction state achieved under negative applied potential could keep stable even after the removal of potential, but failed in a short time, once a specific positive potential was applied. It was demonstrated that there was a stable electro-adsorption of surfactant molecules on the gold surface induced by applying a negative potential, leading to the formation of a bilayer structure on the gold surface. The hydration layers of the bilayer on the gold surface and micelles on the silica probe provided a shear plane with an extremely low shear strength, leading to the extremely low friction state on the gold surface. This study provides a method to achieve extremely low friction state by applied potential.

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 Editorial Issue
Guest editorial: Special Issue on Biomedical Tribology
Friction 2022, 10 (1): 2-3
Published: 07 December 2021

Open Access Editorial Issue
Friction will be published monthly from 2022
Friction 2022, 10 (1): 1
Published: 07 December 2021

Open Access Research Article Issue
Dynamic friction energy dissipation and enhanced contrast in high frequency bimodal atomic force microscopy
Friction 2022, 10 (5): 748-761
Published: 12 March 2021

Dynamic friction occurs not only between two contact objects sliding against each other, but also between two relative sliding surfaces several nanometres apart. Many emerging micro- and nano-mechanical systems that promise new applications in sensors or information technology may suffer or benefit from noncontact friction. Herein we demonstrate the distance-dependent friction energy dissipation between the tip and the heterogeneous polymers by the bimodal atomic force microscopy (AFM) method driving the second order flexural and the first order torsional vibration simultaneously. The pull-in problem caused by the attractive force is avoided, and the friction dissipation can be imaged near the surface. The friction dissipation coefficient concept is proposed and three different contact states are determined from phase and energy dissipation curves. Image contrast is enhanced in the intermediate setpoint region. The work offers an effective method for directly detecting the friction dissipation and high resolution images, which overcomes the disadvantages of existing methods such as contact mode AFM or other contact friction and wear measuring instruments.

Open Access Review Article Issue
A review on tribology of polymer composite coatings
Friction 2021, 9 (3): 429-470
Published: 28 November 2020

Self-lubricating polymer composite coatings, with tailorable tribological and mechanical properties, have been widely employed on mechanical parts to reduce friction and wear, which saves energy and improves the overall performance for applications such as aerospace satellite parts, shafts, gears, and bushings. The addition of functional fillers can overcome the limitations of single-polymer coatings and extend the service life of the coatings by providing a combination of low friction, high wear resistance, high load bearing, high temperature resistance, and high adhesion. This paper compares the heat resistance, and the tribological and mechanical properties of common polymer matrices, as well as the categories of functional fillers that improve the coating performance. Applicable scopes, process parameters, advantages, and limitations of the preparation methods of polymer coatings are discussed in detail. The tribological properties of the composite coatings with different matrices and fillers are compared, and the lubrication mechanisms are analyzed. Fillers reduce friction by promoting the formation of transfer films or liquid shear films. Improvement of the mechanical properties of the composite coatings with fillers of different morphologies is described in terms of strengthening and toughening mechanisms, including a stress transfer mechanism, shear yielding, crack bridging, and interfacial debonding. The test and enhancement methods for the adhesion properties between the coating and substrate are discussed. The coating adhesion can be enhanced through mechanical treatment, chemical treatment, and energy treatment of the substrate. Finally, we propose the design strategies for high-performance polymer composite coating systems adapted to specific operating conditions, and the limitations of current polymer composite coating research are identified.

Open Access Research Article Issue
Micro/atomic-scale vibration induced superlubricity
Friction 2021, 9 (5): 1163-1174
Published: 08 July 2020

With the rapid development of industry, the inconsistency between the rapid increase in energy consumption and the shortage of resources is becoming significant. Friction is one of the main causes of energy consumption; thus, the emergence of superlubricity technology can substantially improve the energy efficiency in motion systems. In this study, an efficient method to control superlubricity at the atomic-scale is proposed. The method employs vibrational excitation, which is called vibration induced superlubricity (VIS). The VIS can be easily and steadily achieved by employing external vibration in three directions. The simple method does not depend on the type of sample and conductivity. Dependence of oscillation amplitude, frequency, scanning speed, and normal force (FN) on friction were investigated. Experimental and simulated explorations verified the practical approach for reducing energy dissipation and achieving superlubricity at the atomic-scale.

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