Research Article Issue
Interlayer friction behavior of molybdenum ditelluride with different structures
Nano Research 2023, 16 (8): 11375-11382
Published: 17 July 2023

The interlayer friction behavior of two-dimensional transition metal dichalcogenides (TMDCs) as crucial solid lubricants has attracted extensive attention in the field of tribology. In this study, the interlayer friction is measured by laterally pushing the MoTe2 powder on the MoTe2 substrate with the atomic force microscope (AFM) tip, and density functional theory (DFT) simulations are used to rationalize the experimental results. The experimental results indicate that the friction coefficient of the 1T'-MoTe2/1T'-MoTe2 interface is 2.025 × 10−4, which is lower than that of the 2H-MoTe2/2H-MoTe2 interface (3.086 × 10−4), while the friction coefficient of the 1T'-MoTe2/2H-MoTe2 interface is the lowest at 6.875 × 10−5. The lower interfacial friction of 1T'-MoTe2/1T'-MoTe2 compared to 2H-MoTe2/2H-MoTe2 interface can be explained by considering the relative magnitudes of the ideal average shear strengths and maximum shear strengths based on the interlayer potential energy. Additionally, the smallest interlayer friction observed at the 1T'-MoTe2/2H-MoTe2 heterojunction is attributed to the weak interlayer electrostatic interaction and reduction in potential energy corrugation caused by the incommensurate contact. This work suggests that MoTe2 has comparable interlayer friction properties to MoS2 and is expected to reduce interlayer friction in the future by inducing the 2H-1T' phase transition.

Open Access Research Article Issue
Active control of friction realized by vibrational excitation: Numerical simulation based on the Prandtl–Tomlinson model and molecular dynamics
Friction 2023, 11 (7): 1225-1238
Published: 19 November 2022

Superlubricity and active friction control have been extensively researched in order to reduce the consumption of fossil energy, the failure of moving parts, and the waste of materials. The vibration-induced superlubricity (VIS) presents a promising solution for friction reduction since it does not require high-standard environment. However, the mechanism underlying the VIS remains unclear since the atomic-scale information in a buried interface is unavailable to experimental methods. In this paper, the mechanism of VIS was examined via numerical calculation based on the Prandtl–Tomlinson (PT) model and molecular dynamics (MD) simulations. The results revealed that the pushing effect of stick–slip is one of the direct sources of friction reduction ability under vibrational excitation, which was affected by the response amplitude, frequency, and the trace of the tip. Moreover, the proportion of this pushing effect could be modulated by changing the phase difference when applying coupled vibrational excitation in x- and z-axis. This results in a significant change in friction reduction ability with phase. By this way, active friction control from the stick–slip to superlubricity can be achieved conveniently.

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 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.

Open Access Research Article Issue
Film forming behavior in thin film lubrication at high speeds
Friction 2018, 6 (2): 156-163
Published: 21 July 2017

The film forming condition may transit into thin film lubrication (TFL) at high speeds when it is under severe starvation. Central film thicknesses and film thickness profiles are obtained via a technique of relative optical interference intensity. These profiles show a critical film thickness lower than which the absolute values of the film thickness gradient against speed or time decrease. It is possible to be in the thin film lubrication mode under such conditions. The high speed flow drives the lubricant molecules to rearrange in TFL and critical film thickness higher than 100 nm is achieved. The viscosity is one of the main factors controlling the decreasing rate and the critical film thickness. This paper is designed to investigate the thin film lubrication behavior at high speeds.

Open Access Research Article Issue
Effects of nano thickener deposited film on the behaviour of starvation and replenishment of lubricating greases
Friction 2016, 4 (4): 313-323
Published: 23 December 2016

The mechanism of grease replenishment in and around a starved point contact was studied in this work. Greases made of different thickeners and same base oil were tested and compared. Disappearing and re-formation of a dynamic grease reservoir during operation revealed that grease bled oil to replenish contact. However, the replenishment process was slow because of the presence of grease fingers along the track and thickener-deposited film inside the track. The contact angles of base oil on the chromium-coated surface and thickener-deposited surfaces were measured. Results proved that the contact angle on the deposited film remarkably increased compared with that on the chromium-coated surface from 25° to more than 40°. However, the deposited film could be consumed with continuous rolling, and replenishment was then enhanced.

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