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Evolution of real contact area during stick-slip movement observed by total reflection method

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Chinese Physics B 2019年第10期28卷 401-407页

作者： Zhijun Luo Baojiang Song Jingyu Han Shaoze Yan State Key Laboratory of tribology Department of Mechanical Engineering Tsinghua University

We build an experiment system based on total reflection(TR) method to observe the evolution of real contact area of polymethyl methacrylate(PMMA) in the continual stick-slip movement. The bilateral friction is adopted to overcome the bending moment in the lateral friction movement. Besides some classical phenomena of stick-slip movement such as periodical slow increase of frictional force in sticking phase and a sudden drop when slipping, a special phenomenon that the contact area increases with the tangential force is observed, which was called junction growth by Tabor in *** processing methods are developed to observe the variation of the junction area. The results show that the center of the strongest contact region will keep sticking under the tangential force until the whole slipping, the strongest point undergoes three stages in one cycle, which are named as sticking stage, fretting stage, and cracking stage, respectively. The combined analysis reveals a physical process of stick-slip movement: the tangential force causes the increase of the real contact area, which reduces the pressure between the contact spots and finally leads to the slipping. Once slipping occurs,the real contact area drops to the original level resulting in the pressure increase to the original level, which makes the sticking happen again.

关键词： total reflection stick-slip real contact area detachment junction growth

Stress-augmented thermal activation:tribology feels the force

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Friction 2018年第1期6卷 1-31页

作者： Hugh SPIKES tribology Group Department of Mechanical EngineeringImperial College

In stress-augmented thermal activation, the activation energy barrier that controls the rate of atomic and molecular processes is reduced by the application of stress, with the result that the rate of these processes increases exponentially with applied stress. This concept has particular relevance to tribology, and since its development in the early twentieth century, it has been applied to develop important models of plastic flow, sliding friction, rheology, wear, and tribochemistry. This paper reviews the development of stress-augmented thermal activation and its application to all of these areas of tribology. The strengths and limitations of the approach are then discussed and future directions considered. From the scientific point of view, the concept of stress-augmented thermal activation is important since it enables the development of models that describe macroscale tribological performance, such as friction coefficient or tribofilm formation, in terms of the structure and behaviour of individual atoms and molecules. This both helps us understand these processes at a fundamental level and also provides tools for the informed design of lubricants and surfaces.

关键词： stress activation stress augmented thermal activation mechanochemistry friction EHD friction wear Eyring

Relationship between the real contact area and contact force in pre-sliding regime

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Chinese Physics B 2017年第7期26卷 212-217页

作者：宋保江阎绍泽 State Key Laboratory of tribology Department of Mechanical Engineering Tsinghua University

The pre-sliding regime is typically neglected in the dynamic modelling of mechanical systems. However, the change in contact state caused by static friction may decrease positional accuracy and control precision. To investigate the relationship between contact status and contact force in pre-sliding friction, an optical experimental method is presented in this *** this method, the real contact state at the interface of a transparent material can be observed based on the total reflection principle of light by using an image processing technique. A novel setup, which includes a pair of rectangular trapezoidal blocks, is proposed to solve the challenging issue of accurately applying different tangential and normal forces to the contact interface. The improved Otsu＇s method is used for measurement. Through an experimental study performed on polymethyl methacrylate（PMMA）, the quantity of contact asperities is proven to be the dominant factor that affects the real contact area. The relationship between the real contact area and the contact force in the pre-sliding regime is studied, and the distribution of static friction at the contact interface is qualitatively discussed. New phenomena in which the real contact area expands along with increasing static friction are identified. The aforementioned relationship is approximately linear at the contact interface under a constant normal pressure, and the distribution of friction stress decreases from the leading edge to the trailing edge.

关键词： real contact area contact interface pre-sliding regime contact force

Simulation of Dual-Electrode Capacitively Coupled Plasma Discharges

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Plasma Science and Technology 2016年第12期18卷 1175-1180页

作者：路益嘉季林红程嘉 State Key Laboratory of tribology Department of Mechanical EngineeringTsinghua University

Dual-electrode capacitively coupled plasma discharges are investigated here to lower the non-uniformity of plasma density. The dual-electrode structure proposed by Jung splits the electrode region and increases the flexibility of fine tuning non-uniformity. Different RF voltages,frequencies, phase-shifts and electrode areas are simulated and the influences are discussed. RF voltage and electrode area have a non-monotonic effect on non-uniformity, while frequency has a monotonic effect. Phase-shift has a cyclical influence on non-uniformity. A special combination of 224 V voltage and 11% area ratio with 10 MHz lowers the non-uniformity of the original set（200 V voltage and 0% area ratio with 10 MHz） by 46.5%. The position of the plasma density peak at the probe line has been tracked and properly tuning the phase-shift can obtain the same trace as tuning frequency or voltage.

关键词： dual-electrode non-uniformity capacitively coupled plasma fluid model

A method for detection and quantification of meshing characteristics of harmonic drive gears using computer vision

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Science China(Technological Sciences) 2016年第9期59卷 1305-1319页

作者： MA DongHui WU JiaNing YAN ShaoZe State Key Laboratory of tribology Department of Mechanical Engineering Tsinghua University

A lack of accurate description of the meshing characteristics and the corresponding frictional mechanism of the harmonic drive gear has limited progress toward modeling the hysteresis stiffness. This paper presents a method for detection and quantification of the meshing characteristics of the harmonic drive gear based on computer vision. First, an experimental set-up that integrates a high speed camera system with a lighting system is developed, and the image processing is adopted to extract and polish the tooth profiles of the meshed teeth pairs in each acquired video sequence. Next, a physical-mathematical model is established to determine the relative positions of the selected tooth pair in the process of the gear engagement, and the combined standard uncertainty is utilized to evaluate the accuracy of the calculated kinematics parameters. Last, the kinematics analysis of the gear engagement under the ultra-low speed condition is performed with our method and previous method, and the influence of the input rotational speed on the results is examined. The results validate the effectiveness of our method, and indicate that the conventional method is not available in the future friction analysis. It is also shown that the engaging-in phase is approximately a uniform motion process, the engaging-out phase is a variable motion process, and these characteristics remain unchanged with the variation of the input rotational speed. Our method affords the ability to understand the frictional mechanism on the meshed contact surfaces of the harmonic drive gear, and also allows for the dynamic monitoring of the meshing properties.

关键词： harmonic drive gear gear engagement computer vision image processing kinematics analysis uncertainty analysis

A measurement method for distinguishing the real contact area of rough surfaces of transparent solids using improved Otsu technique

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Chinese Physics B 2015年第1期24卷 354-360页

作者：宋保江阎绍泽向吴维凯 State Key Laboratory of tribology Department of Mechanical Engineering Tsinghua University

An experimental method of measuring the real contact area of transparent blocks based on the principle of total internal reflection is presented, intending to support the investigation of friction characteristics, heat conduction, and energy dissipation at the contact interface. A laser sheet illuminates the contact interface, and the transmitted laser sheet is projected onto a screen. Then the contact information is acquired from the screen by a camera. An improved Otsu method is proposed to process the data of experimental images. It can compute the threshold of the overall image and filter out all the pixels one by one. Through analyzing the experimental results, we describe the relationship between the real contact area and the positive pressure during a continuous loading process, at different loading rates, with the polymethyl methacrylate（PMMA）material. A hysteresis phenomenon in the relationship between the real contact area and the positive pressure is found and explained.

关键词： real contact area total internal reflection improved Otsu method hysteresis phenomenon

A comprehensive method for joint wear prediction in planar mechanical systems with clearances considering complex contact conditions

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Science China(Technological Sciences) 2015年第1期58卷 86-96页

作者： XIANG WuWeiKai YAN ShaoZe WU JiaNing State Key Laboratory of tribology Department of Mechanical Engineering Tsinghua University

A comprehensive method to predict wear in planar mechanical systems with clearance joints is presented and discussed in this *** method consists of a system dynamic analysis and a joint wear *** the size and shape of the clearance are dictated by wear and evolve with the dynamic response of the system,the contact between the journal and bearing could be conformal or non-conformal,which makes the contact conditions in clearance joints quite *** a modified contact force model is employed to evaluate the joint reaction force in this *** the nonlinear stiffness coefficient is related to the physical and geometrical properties of contact bodies and varies with the deformation,this contact force model is applicable to different contact conditions between the journal and ***,based on the Archard’s wear model,the amount of wear can be quantified in the *** the geometry is updated to reflect the evolving contact ***,the wear process and the contact force model are integrated into the motion equations of the system to perform coupled iterative analyses between system dynamic response and joint wear *** addition,a slider-crank mechanism is simulated as an example to demonstrate efficiency of the proposed method and to carry out a parametric study on mechanical systems considering joint *** influence of clearance size and driving power are discussed and compared *** index of concordance is introduced to quantify contributions of contact pressure and sliding distance to wear rate under different types of journal *** study could help to predict joint wear in mechanical systems with clearances and optimize mechanisms in design.

关键词： wear joint clearance dynamics

An experimental method for quantitative analysis of real contact area based on the total reflection optical principle

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Chinese Physics B 2019年第5期28卷 158-165页

作者： Zhijun Luo Baojiang Song Jingyu Han Shaoze Yan State Key Laboratory of tribology Department of Mechanical EngineeringTsinghua University

The simulation of real contact area between materials is foundationally important for the contact mechanics of mechanical structures. The Greenwood and Williamson(GW) model and the Majumdar(MB) model are the basic models in this field, which are widely accepted and proven to be valid in many experiments and engineering. Although the contact models have evolved considerably in recent years, the verifications of the models are most based on the indirect methods such as electrical conductivity and contact stiffness, because of the lack of effective methods to directly measure the variation of contact surface. In this paper, the total reflection(TR) method is introduced into the verification of contact *** experiment system based on TR method is constructed to measure the real contact area of two PMMA specimens. The comparison analysis between the results of experiment and models suggests that the experiment result has the same trend with simulation, the MB model has better agreement with the experimental result because this method can take into account the variation of radius and the merging of asperities, while the GW model has a huge deviation because of the dependence on resolution and the lack of considering the variation of radius and asperity's merging process. Taking the interaction of asperities into account could give a better result that is closer to the experiment. Our results and analysis prove that the experimental methods in this paper could be used as a more direct and valid method to quantitatively measure the real contact area and to verify the contact models.

关键词： real contact area Greenwood and Williamson (GW) model Majumdar (MB) model Persson’s model total reflection (TR) method

Design space of electrostatic chuck in etching chamber

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Journal of Semiconductors 2015年第8期36卷 94-100页

作者：孙钰淳程嘉路益嘉侯悦民季林红 State Key Laboratory of tribology Department of Mechanical Engineering Tsinghua University

One of the core semiconductor devices is the electrostatic chuck. It has been widely used in plasma- based and vacuum-based semiconductor processing. The electrostatic chuck plays an important role in adsorbing and cooling/heating wafers, and has technical advantages on non-edge exclusion, high reliability, wafer planarity, particles reduction and so on. This article extracts key design elements from the existing knowledge and techniques of electrostatic chuck by the method proposed by Paul and Beitz, and establishes a design space systematically. The design space is composed of working objects, working principles and working structures. The working objects in- volve electrostatic chuck components and materials, classifications, and relevant properties; the working principles involve clamping force, residual force, and temperature control; the working structures describe how to compose an electrostatic chuck and to fulfill the overall functions. The systematic design space exhibits the main issues during electrostatic chuck design. The design space will facilitate and inspire designers to improve the design quality and shorten the design time in the conceptual design.

关键词： design space electrostatic chuck clamping force temperature control