Frictional contacts lead to the formation of a surface layer called the third body, consisting of wear particles and structures resulting from their agglomerates. Its behavior and properties at the nanoscale control the macroscopic tribological performance. It is known that wear particles and surface topography evolve with time and mutually influence one another. However, the formation of the mature third body is largely uncharted territory and the properties of its early stages are unknown. Here we show how a third body initially consisting of particles acting as roller bearings transitions into a shear-band-like state by forming adhesive bridges between the particles. Using large-scale atomistic simulations on a brittle model material, we find that this transition is controlled by the growth and increasing disorganization of the particles with increasing sliding distance. Sliding resistance and wear rate are at first controlled by the surface roughness, but upon agglomeration wear stagnates and friction becomes solely dependent on the real contact area in accordance with the plasticity theory of contact by Bowden and Tabor.
We report piezo-response force microscopy studies of the static and dynamic properties of domain walls (DWs) in 11 to 36 nm thick films of crystalline ferroelectric poly(vinylidene-fluoride-trifluorethylene). The DW roughness exponent ζ ranges from 0.39 to 0.48 and the DW creep exponent μ varies from 0.20 to 0.28, revealing an unexpected effective dimensionality of ~1.5 that is independent of film thickness. Our results suggest predominantly 2D ferroelectricity in the layered polymer and we attribute the fractal dimensionality to DW deroughening due to the correlations between the in-plane and…
Nanoscale, single-asperity wear of single-crystal silicon carbide (sc-SiC) and nanocrystalline silicon carbide (nc-SiC) is investigated using single-crystal diamond nanoindenter tips and nanocrystalline diamond atomic force microscopy (AFM) tips under dry conditions, and the wear behavior is compared to that of single-crystal silicon with both thin and thick native oxide layers. We discovered a transition in the relative wear resistance of the SiC samples compared to that of Si as a function of contact size. With larger nanoindenter tips (tip radius around 370 nm), the wear resistances of both…
Building on an analogy to ductile fracture mechanics, we quantify the size of debris particles created during adhesive wear. Earlier work suggested a linear relation between tangential work and wear debris volume, assuming that the debris size is proportional to the micro contact size multiplied by the junction shear strength. However, the present study reveals deviations from linearity. These deviations can be rationalized with fracture mechanics and imply that less work is necessary to generate debris than what was assumed. Here, we postulate that the work needed to detach a wear particle is…
In the present work we show that the correct determination of surface morphology using Scanning Force Microscopy (SFM) imaging and Power Spectral Density (PSD) analysis of the surface roughness is an extremely demanding task that is easily affected by experimental parameters such as scan speed and feedback parameters. We present examples were the measured topography data is significantly influenced by the feedback response of the SFM system and the PSD curves calculated from this experimental data do not correspond to that of the true topography. Instead, either features are "lost" due to low …
To prolong lifetime and reduce energy consumption, a thorough understanding of abrasive wear is essential. The potentially crucial influence of surface topography intricacies on tribological behavior have been obscured, since roughness and waviness are often considered simple scalar quantities. Here, the complete waviness profile of the sliding track was used to shed light on the influence of surface topography on abrasive wear. Bearing steel pins and disks were tribologically tested with Al2O3-based slurries as interfacial medium. It was found that even small surface deviations (albeit minimi…
Engineering wear models are generally empirical and lack connections to the physical processes of debris generation at the nanoscale to microscale. Here, we thus analyze wear particle formation for sliding interfaces in dry contact with full and reduced adhesion. Depending on the material and interface properties and the local slopes of the surfaces, we find that colliding surface asperities can either deform plastically, form wear particles, or slip along the contact junction surface without significant damage. We propose a mechanism map as a function of material properties and local geometry…
Artificially induced asymmetric tribological properties of ferroelectrics offer an alternative route to visualize and control ferroelectric domains. Here, we observe the switchable friction and wear behavior of ferroelectrics using a nanoscale scanning probe where down domains having lower friction coefficient than up domains can be used as smart masks as they show slower wear rate than up domains. This asymmetry is enabled by flexoelectrically coupled polarization in the up and down domains under a sufficiently high contact force. Moreover, we determine that this polarization-sensitive tribol…
Most natural and man-made surfaces appear to be rough on many length scales. There is presently no unifying theory of the origin of roughness or the self-affine nature of surface topography. One likely contributor to the formation of roughness is deformation, which underlies many processes that shape surfaces such as machining, fracture, and wear. Using molecular dynamics, we simulate the bi-axial compression of single-crystal Au, the high-entropy alloy Ni$_{36.67}$Co$_{30}$Fe$_{16.67}$Ti$_{16.67}$, and amorphous Cu$_{50}$Zr$_{50}$, and show that even surfaces of homogeneous materials develop …
Roughness-controlled Tribocharging Governs Friction in Dry Glass Contacts
University of Amsterdam（阿姆斯特丹大学）；Van’t Hoff Institute for Molecular Sciences（范特霍夫分子科学研究所）；Advanced Research Center for Nanolithography (ARCNL)（先进纳米光刻研究中心）
Tribology, which studies surfaces in contact and relative motion, includes friction, wear, and lubrication, straddling across different fields: mechanical engineering, materials science, chemistry, nanoscience, physics. This short review restricts to the last two disciplines, with a qualitative survey of a small number of recent progress areas in the physics of nanofriction.
Here we quantitatively evaluate the early stages of mechanical and morphological changes of polished human enamel surfaces induced by soft drinks using atomic force microscopy. With an increase of the immersion time in soft drinks, we found a significant increase of surface roughness (Ra) and a considerable decrease of elastic modulus (E) of the enamel. The prismatic structure of enamel was clearly observed after a one-hour immersion in Coca-Cola, which shows its strong erosion effect. A high surface roughness of enamel results in a high chance of cavities due to easier bacterial adhesion on r…
The origin of ultralow friction and high wear resistance in ultrananocrystalline diamond (UNCD) films is still under active debate because of the perplexed tribochemistry at the sliding interface. Herein, we report a comparative study on surface topography and nanoscale friction of tribofilms, in wear tracks of two sets of UNCD films having different structural characteristics. Despite both the films display ultralow coefficient of friction, the UNCD films grown under Ar atmosphere (UNCDAr) exhibit a high wear resistance while the wear rate is higher for the films grown in N2 (UNCDN). Friction…
High quantum yield, low transverse energy spread and prompt response time make GaAs activated to negative electron affnity, an ideal candidate for a photocathode in high brightness photoinjectors. Even after decades of investigation, the exact mechanism of electron emission from GaAs is not well understood. We show that a nanoscale surface roughness can affect the transverse electron spread from GaAs by nearly an order of magnitude and explain the seemingly controversial experimental results obtained so far. This model can also explain the measured dependence of transverse energy spread on the…
We used the Coherent Grazing Incidence Small Angle X-Ray Scattering (Co-GISAXS) technique to study the average kinetics and nanoscale dynamics during early-stage a-WSi$_2$ sputter deposition. The kinetic and dynamic properties are examined as a function of pressure, which is known to be a critical factor in determining final surface roughness. Surface growth kinetics and dynamics are characterized by time parameters extracted from the height-height structure factor and correlation functions. The roughness at a given length scale reaches a maximum before relaxing down to a steady state. The lat…
Chemical vapour deposition (CVD) is a promising method for producing large-scale graphene (Gr). Nevertheless, microscopic inhomogeneity of Gr grown on traditional metal substrates such as copper or nickel results in a spatial variation of Gr properties due to long wrinkles formed when the metal substrate shrinks during the cooling part of the production cycle. Recently, molybdenum (Mo) has emerged as an alternative substrate for CVD growth of Gr, mainly due to a better matching of the thermal expansion coefficient of the substrate and Gr. We investigate the quality of multilayer Gr grown on Mo…
Several issues concerning the applicability of the temperature coefficient of the resistivity (TCR) method to scaled interconnect lines are discussed. The central approximation of the TCR method, the substitution of the interconnect wire TCR by the bulk TCR becomes doubtful when the resistivity of the conductor metal is strongly increased by finite size effects. Semiclassical calculations for thin films show that the TCR deviates from bulk values when the surface roughness scattering contribution to the total resistivity becomes significant with respect to grain boundary scattering, an effect …
Traditional laws of friction believe that the friction coefficient of two specific solids takes constant value. However, molecular simulations revealed that the friction coefficient of nanosized asperity depends strongly on contact size and asperity radius. Since contacting surfaces are always rough consisting of asperities varying dramatically in geometric size, a theoretical model is developed to predict the friction behavior of fractal rough surfaces in this work. The result of atomic-scale simulations of sphere-on-flat friction is summarized into a uniform expression. Then, the size depend…
Time-domain extreme ultraviolet diffuse scattering spectroscopy of nanoscale surface phonons
Elettra Sincrotrone Trieste（的里雅斯特同步辐射中心）；Massachusetts Institute of Technology（麻省理工学院）；Ca’ Foscari University of Venice（威尼斯卡福斯卡里大学）；Max Born Institute（马克斯·玻恩研究所）；University of Bern（伯尔尼大学）；Università degli Studi di Trieste（的里雅斯特大学）；University of Erlangen-Nürnberg（埃尔朗根-纽伦堡大学）；University of Siegen（锡根大学）；Synchrotron SOLEIL（SOLEIL同步辐射中心）；Lawrence Berkeley National Lab（劳伦斯伯克利国家实验室）；Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory（斯坦福材料与能源科学研究所，SLAC国家加速器实验室）；Stanford PULSE Institute, SLAC National Accelerator Laboratory（斯坦福PULSE研究所，SLAC国家加速器实验室）
We demonstrate the stencil growth of nanoscale patterns using molecular dynamic simulation. A comparison has been made to a film grown by identical conditions without a stencil. It is shown that in the case of nanoscale proximity between mask and substrate, patterns of the same dimension as the mask can be obtained. The results also indicate that the obtained nanorod presents a higher surface area than the corresponding thin film. It is demonstrated that nanorod surface roughness decreases by merging adjacent surface irregularity during the deposition.
Saarland University（萨尔兰州立大学）；Forschungszentrum Jülich（于利希研究中心）；Chinese Academy of Sciences（中国科学院）；Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences（中国科学院兰州化学物理研究所）；MultiscaleConsulting（多尺度咨询公司）
As mechanical devices in the nano/micro length scale are increasingly employed, it is crucial to understand nanoscale friction and wear especially at technically relevant sliding velocities. Accordingly, a novel technique has been developed for Friction Coefficient Mapping (FCM), leveraging recent advances in high speed AFM. The technique efficiently acquires friction versus force curves based on a sequence of images at a single location, each with incrementally lower loads. As a result, true maps of the coefficient of friction can be uniquely calculated for heterogeneous surfaces. These param…
Two-level modeling for nanoscale pattern formation on silicon target by Ar$^+$ ion sputtering is presented. Phase diagram illustrating possible nanosize surface patterns is discussed. Scaling characteristics for the structure wavelength dependence versus incoming ion energy are defined. Growth and roughness exponents in different domains of the phase diagram are obtained.
A macroscopically nominal flat surface is rough at the nanoscale level and consists of nanoasperities. Therefore, the frictional properties of the macroscale-level rough surface are determined by the mechanical behaviors of nanoasperity contact pairs under shear. In this work, we first used molecular dynamics simulations to study the non-adhesive shear between single contact pairs. Subsequently, to estimate the friction coefficient of rough surfaces, we implemented the frictional behavior of a single contact pair into a Greenwood-Williamson-type statistical model. By employing the present mult…
Accelerated Discovery of Topological Conductors for Nanoscale Interconnects
NORDITA, KTH Royal Institute of Technology and Stockholm University（北欧理论粒子物理研究所，皇家理工学院和斯德哥尔摩大学）；University of Connecticut（康涅狄格大学）；Northwestern University（西北大学）；IBM Thomas J. Watson Research Center（IBM托马斯·J·沃森研究中心）；National Yang Ming Chiao Tung University（国立阳明交通大学）；Academia Sinica（中央研究院）
The surface topography of diamond coatings strongly affects surface properties such as adhesion, friction, wear, and biocompatibility. However, the understanding of multi-scale topography, and its effect on properties, has been hindered by conventional measurement methods, which capture only a single length scale. Here, four different polycrystalline diamond coatings are characterized using transmission electron microscopy to assess the roughness down to the sub-nanometer scale. Then these measurements are combined, using the power spectral density (PSD), with conventional methods (stylus prof…
In this master thesis, diamond-like carbon DLC/Cr bilayer systems, with thickness up to 1278 nm were formed on ABS, glass and Si substrates. Substrates surface were prepared by oxygen plasma cleaning process. The chromium thin film, which acts as a buffer layer, was grown by magnetron sputtering deposition. Diamond-like carbon was deposited by pulsed-DC PECVD, with methane and hydrogen as reactants. A Plackett-Burman experimental design was carried out in order to determine the influence of technological parameters of the deposition process on the thickness, deposition rate, intrinsic stress, …
Two-dimensional (2D) transition metal dichalcogenides are potential candidates for ultrathin solid-state lubricants in low-dimensional systems owing to their flatness, high in-plane mechanical strength, and low shear interlayer strength. Yet, the effects of surface topography and surface chemistry on the tribological properties of 2D layers are still unclear. In this work, we performed a comparative investigation of nanoscale tribological properties of ultra-thin highly-ordered PtSe2 layers deposited on the sapphire substrates with the in-plane and out-of-plane crystallographic orientation of …
It is generally thought that capillary interactions in nanoscale contacts give rise to unwanted behaviour due to high adhesion. We show that this is not the case for sufficiently small contacts in ambient conditions. High resolution ambient atomic force microscopy AFM requires tip-sharpness, proximity and small forces, but the cantilever dynamics might not allow these three conditions to be met simultaneously. Hitherto, accepted dogma is that small drive amplitudes lead to either tip trapping or L mode (attractive) imaging, where proximity is inhibited. Here we show that the hydration layer mi…
Observation of robust macroscale structural superlubricity
Tsinghua Shenzhen International Graduate School, Tsinghua University（清华大学深圳国际研究生院）；Research Institute of Tsinghua University in Shenzhen（清华大学深圳研究院）；Center for Nano and Micro Mechanics, Tsinghua University（清华大学微纳力学中心）；Department of Engineering Mechanics, Tsinghua University（清华大学工程力学系）；International School for Advanced Studies (SISSA)（国际高等研究院）；College of New Materials and New Energies, Shenzhen Technology University（深圳技术大学新材料与新能源学院）；Suzhou Laboratory（苏州实验室）；School of Chemistry and The Sackler Center for Computational Molecular and Materials Science, Tel Aviv University（特拉维夫大学化学学院及萨克勒计算分子与材料科学中心）
Understanding the contact between solid surfaces is a long standing problem which has a strong impact on the physics of many processes such as adhesion, friction, lubrication and wear. Experimentally, the investigation of solid/solid interfaces remains challenging today, due to the lack of experimental techniques able to provide sub-nanometer scale information on interfaces buried between millimeters of materials. Yet, a strong interest exists improving the modeling of contact mechanics of materials in order to adjust their interface properties (e.g. thermal transport, friction). We show here …
