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Conductive edge-warping graphite mesas for robust structural superlubricity

Science China Technological Sciences 2024年第7期67卷 2040-2049页

作者： FENG WeiJia LIU Ying MA Ming PENG DeLi NIE JinHui State Key Laboratory of Tribology in Advanced Equipment & Department of Mechanical Engineering Tsinghua University Center for Nano and Micro Mechanics Tsinghua University Institute of superlubricity Technology Research Institute of Tsinghua University in Shenzhen

structural superlubricity(SSL) refers to a state of ultralow friction and zero wear when two solid surfaces slide against each other. Recent investigations have identified amorphous carbon at the edge of the graphite mesa as the primary source of friction in such SSL systems. Here, the tensile stress of metal thin film is exploited to engineer vertically conductive edge-warping graphite mesas(EWGM). Through this approach, robust SSL performance is realized, demonstrated by sliding an 8 μm side length square EWGM on an atomically smooth Au substrate for 10000 cycles at a constant voltage of 1 m V. In this SSL system,differential friction coefficients lower than 1.5 × 10-4are achieved, with static contact resistance between EWGM and Au substrate as low as 28 ? and sliding contact resistance as low as 32 ?. Moreover, the EWGM exhibits SSL behavior on polished Si wafer substrates. Furthermore, because of the no-edge contact with the substrate during sliding, friction is independent of the sliding speed of the EWGM. This study presents the first successful fabrication of conductive EWGM. Remarkably, in both EWGM-Au and EWGM-Si SSL systems, the measured frictions are more than one order of magnitude lower than those of ordinary self-retracting graphite mesas with no-edge warping, and no wear is observed during extended current-carrying sliding.Overall, these findings establish a solid groundwork for the future realization of macroscale conductive SSL systems.

关键词： structural superlubricity graphite mesa contact resistance tensile stress edge warping

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Robustness of structural superlubricity beyond rigid models

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Friction 2022年第9期10卷 1382-1392页

作者： Shizhe FENG Zhiping XU Applied Mechanics Laboratory Department of Engineering Mechanics and Center for Nano and Micro MechanicsTsinghua UniversityBeijing 100084China

structural superlubricity is a theoretical concept stating that the friction force is absent between two rigid,incommensurate crystalline surfaces.However,elasticity of the contact pairs could modify the lattice registry at interfaces by nucleating local slips,favoring commeasure.The validity of structural superlubricity is thus concerned for large-scale systems where the energy cost of elastic distortion to break the lattice registry is low.In this work,we study the size dependence of superlubricity between single-crystal graphite flakes.Molecular dynamics simulations show that with nucleation and propagation of out-of-plane dislocations and strained solitons at Bernal interfaces,the friction force is reduced by one order of magnitude.Elastic distortion is much weaker for non-Bernal or incommensurate ones,remaining notable only at the ends of contact.Lattice self-organization at small twist angles perturbs the state of structural superlubricity through a reconstructed potential energy surface.Theoretical models are developed to illustrate and predict the interfacial elastoplastic behaviors at length scales beyond those in the simulations.These results validate the rigid assumption for graphitic superlubricity systems at microscale,and reveal the intrinsic channels of mechanical energy dissipation.The understandings lay the ground for the design of structural superlubricity applications.

关键词： structural superlubricity lattice registry elastic deformation strained solitons crystal plasticity molecular dynamics simulations

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Research on the mechanism of the two-dimensional ultrasonic surface burnishing process to enhance the wear resistance for aluminum alloy

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Friction 2024年第3期12卷 490-509页

作者： Zhen-Yu ZHOU Qiu-Yang ZHENG Yu LI Cong DING Guang-Jian PENG Zhong-Yu PIAO College of Mechanical Engineering Zhejiang University of TechnologyHangzhou 310023China Key Laboratory of Special Purpose Equipment and Advanced Processing Technology Ministry of Education and Zhejiang ProvinceZhejiang University of TechnologyHangzhou 310014China

The gradient nanostructure is machined on the aluminum(Al)alloy by the two-dimensional ultrasonic surface burnishing process(2D-USBP).The mechanism of why the gradient nanostructure enhances wear resistance is investigated.The mechanical properties and microstructure characterization for the gradient nanostructure are performed by operating a nanoindenter,transmission electron microscopy(TEM),and electron backscattered diffraction(EBSD).Dry wear tests are performed on the samples before and after machining to evaluate the wear resistance and mechanisms.The effect of the gradient nanostructure on the wear resistance is explored by developing the crystal plasticity(CP)finite element and molecular dynamics(MD)models.The characterization results show that the 2D-USBP sample prepared a gradient structure of~600μm thick on the aluminum surface,increasing the surface hardness from 1.13 to 1.71 GPa and reducing the elastic modulus from 78.84 to 70.14 GPa.The optimization of the surface microstructure and the increase of the mechanical properties effectively enhance the wear resistance of the sample,with 41.20%,39.07%,and 54.58% of the wear scar areas for the 2D-USBP treated samples to the original samples under 5,10,and 15 N loads,respectively.The gradient nanostructure hinders the slip of dislocations inside the sample during the wear process and reduces the size and scope of plastic deformation;meanwhile,the resistance to deformation,adhesion,and crack initiation and propagation of the sample surface is improved,resulting in enhanced wear resistance.

关键词： structural superlubricity lattice registry elastic deformation strained solitons crystal plasticity molecular dynamics simulations

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100 km wear-free sliding achieved by microscale superlubric graphite/DLC heterojunctions under ambient conditions

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National Science Review 2022年第1期9卷 17-24页

作者： Deli Peng Jin Wang Haiyang Jiang Shuji Zhao Zhanghui Wu Kaiwen Tian Ming Ma Quanshui Zheng Department of Engineering Mechanics Tsinghua University Center for Nano and Micro Mechanics Tsinghua University Institute of superlubricity Technology Research Institute of Tsinghua University in Shenzhen State Key Laboratory of Tribology Tsinghua University Department of Mechanical Engineering Tsinghua University

Wear-free sliding between two contacted solid surfaces is the ultimate goal in the effort to extend the lifetime of mechanical devices,especially when it comes to inventing new types of micro-ele ctromechanical systems where wear is often a major obstacle.Here we report experimental observations of wear-free sliding for a micrometer-sized graphite flake on a diamond-like-carbon(DLC) surface under ambient conditions with speeds up to 2.5 m/s,and over a distance of 100 km.The coefficient of friction(COF) between the microscale graphite flake,a van der Waals(vdW) layered material and DLC,a non-vdW-layered material,is measured to be of the order of 10-3,which belongs to the superlubric regime.Such ultra-low COFs are also demonstrated for a microscale graphite flake sliding on six other kinds of non-vdW-layered materials with sub-nanometer roughness.With a synergistic analysis approach,we reve al the underlying mechanism to be the combination of interfacial vdW interaction,atomic-smooth interfaces and the low normal stiffness of the graphite flake.These features guarantee a persistent full contact of the interface with weak interaction,which contributes to the ultra-low COFs.Together with the extremely high in-plane strength of graphene,wear-free sliding is achieve d.Our results broaden the scope of superlubricity and promote its wider application in the future.

关键词： structural superlubricity wear-free microscale graphite DLC

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Controlled Movements in Superlubric MEMS

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Journal of Harbin Institute of Technology(New Series) 2020年第3期27卷 45-57页

作者： Cangyu Qu Xiaojian Xiang Ming Ma Quanshui Zheng Institute of superlubricity Technology Research Institute of Tsinghua University in ShenzhenShenzhen 518057GuangdongChina Center for Nano and Micro Mechanics Tsinghua UniversityBeijing 100084China State Key Laboratory of Tribology&Department of Mechanical Engineering Tsinghua UniversityBeijing 100084China Department of Engineering Mechanics Tsinghua UniversityBeijing 100084China

structural superlubricity(SSL)refers to a state where the friction and wear between two directly contacted solid surfaces are virtually zero.The realization of microscale SSL in 2012 rapidly explored SSL technologies which hold great potential in the development of reliable and energy⁃efficient micro devices.A key to a successful superlubric device is to control the movements of the superlubric slider.To solve this challenge,here two general principles are shown to guide and control the motion of the slider,i.e.,by minimization of interfacial energy and minimization of electrostatic energy.When the shapes of the slider and substrate are designed appropriately,the excess interfacial energy of the contact⁃pair provides restoring and constraining forces to the slider.Similarly,tunable driving and constraining forces are enabled by the electric fields induced by the electrodes buried in the substrate.These concepts are demonstrated on the design of a superlubric resonator whose natural frequency of the lateral translational mode is well⁃defined and unfavorable rotation is constrained.The above design principles should be applicable to superlubric devices in general and help the development of future applications of structural superlubricity.

关键词： structural superlubricity MEMS resonator controlled movement

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