Dynamic responses of a precision positioning table impacted by a soft-mounted piezoelectric actuator

The piezoelectric actuator, lead zirconate titanate (PZT) actuator, has been used for precision positioning from micrometer down to nanometer scale. In this paper, a soft-mounted PZT actuator is designed with a low-stiffness spring element to achieve a high-accuracy and large-displacement characteristic in precision positioning motion. The motion of the sliding table, the contact force between the hammer and the sliding table, and the stick-slip frictional force caused by the grinded groove are investigated. The governing equations of the distributed and lumped parameter systems are formulated to obtain the dynamic responses, which agree well with the experimental results.     

Construction and testing of a nanomachining instrument

This paper presents a nanomachining instrument that was developed for conducting nanocutting, nanoscratching, and nanoindentation experiments. A piezoelectric tube scanner (PZT) is employed to generate three-dimensional machining motions. The sample is moved by the PZT, and the tool is kept stationary during machining. The machining forces are measured by force sensors with a resolution of sub-milliNewtons. The instrument is compact and can be used inside optical microscopes and scanning electron microscopes. In this paper, depth-sensing indentation experiments were performed to test the basic performance of the instrument. The indentation displacement was measured by a capacitance probe situated inside the PZT tube. An experimental system was constructed to locate and image indentations. The system consists of a high magnification microscope to measure coordinates of the indentation relative to a reference corner point on the sample, and an AFM equipped with an on-axis optical imaging system for locating the indentation. A technique was also employed to establish the tool-sample contact to nanometer accuracy. Indentation experiments were carried out on three kinds of materials with different hardness. Experimental results demonstrated the instrument has the ability of performing depth-sensing indentations. The frame compliance was also evaluated from the indentation results.     

Adhesion Mechanics between Nanoscale Silicon Oxide Tips and Few-Layer Graphene

Finite element method (FEM) simulations of the adhesive contact between a nanoscale tip and a silicon oxide substrate covered with graphene were performed, modelling experimental atomic force microscopy pull-off measurements. Simulations showed a slight increase in the pull-off force as layer number increased. This small enhancement was within reported experimental error, agreeing with the experimental findings of layer-independent adhesion forces. Pull-off forces did not vary with the elastic strain in the system for a given number of layers, but were influenced by the greater adhesive stresses for tip–graphene interaction compared with tip–substrate interactions. FEM simulations were also performed on suspended graphene and showed that the adhesive forces increased slightly beyond one layer of graphene, but then varied little from two to four layers of graphene. The results indicate that while there is some local delamination of the graphene sheets from the substrate, the adhesive stresses between the graphene layers in multilayer graphene effectively prevent out-of-plane mechanical deformation of the graphene layers that could result from tip–graphene interactions. Thus, the increased pull-off forces observed beyond one monolayer results from a change in the amount of material between the tip and substrate, or in this case the number of graphene layers, thus increasing the van der Waals force between tip and graphene.     

A micro-model for elasto-plastic adhesive–contact in micro-switches: Application to cyclic loading

Stiction is a major failure mode in micro-electromechanical systems. In previous works, a statistical rough surfaces interaction model, for which only elastic adhesive–contact has been considered, was developed for multiscale analyzes.However, during the impact between rough surfaces, plastic deformations of asperities cannot always be neglected. In the present work, the adhesion between rough surfaces is studied considering the elasto-plastic deformations of the asperities, and a model predicting the resulting micro-adhesive–contact forces is derived.For illustration purpose, an electrostatic-structural analysis is performed on a micro-switch. To determine the degree of plasticity involved, the impact energy of the movable electrode at pull-in is estimated. Thus the maximal adhesive force evolution during cyclic loading is predicted using the developed model.     

Effect of capillary formation on friction and pull-off forces measured on submicron-size asperities

Asperities with hemispherical peaks were fabricated on a silicon substrate using a focused ion beam. Pull-off and friction forces were measured on each asperity using atomic force microscopy (AFM) in high vacuum (HV) of 2 × 10^–5 Pa. The probe of the AFM cantilever had a flat square tip, approximately 1 × 1 m² in area. The radius of curvature of the asperity peaks ranged from 70 to 610 nm. The results showed that the pull-off force was roughly proportional to this radius. The friction force was proportional to the pull-off force. Effects of the substrate temperature on pull-off force on a plane (the flat substrate) and friction force on an asperity were also examined. The pull-off force on the flat substrate increased with increasing contact time at a substrate temperature of 100 °C or lower, but was independent of contact time at 190 °C or higher. This suggests that the capillary cannot form at a substrate temperature of 190 °C or higher. The friction force increased with lower sliding velocities at 100 °C or lower, suggesting the capillary has a lubricating effect that prevents direct solid contact.     

Charge Contribution to the Adhesion Performance of Polymeric Microstructures

Strong attachment of many insects with microstructured attachment pads is due to the Van der Waals interactions or/and the capillary forces between the pads and substrates. To establish initial contact between two surfaces a certain normal force should be applied. The presence of the charges on surfaces could facilitate or impede the initial contact formation. In this study, forces appearing due to the contact electrification of microstructured material mimicking beetle adhesive pads were measured and their influence on the contact formation was discussed. The experiments have clearly demonstrated that static charges contribute to an initial contact establishment in materials with the mushroom-shaped microstructure, whereas Van der Waals interactions or/and capillary forces have the main contribution at pull-off. A simple model was successfully used for data analysis and extraction information about the charge distribution. The effect of the jump-in due to the electrostatic interaction has to be considered during the development of further implementation of biologically inspired microstructured adhesives.     

The Relationship Between Contact Mechanics and Adhesion in Nanoscale Contacts Between Non-Polar Molecular Monolayers

Atomic and friction force microscopy were employed to examine adhesion and friction between dodecanethiol self-assembled monolayers in pure media as well as in two-component heptane/acetone mixtures. In media that did not contain hydrogen bond donors, the pull-off forces were found to be in very good agreement with theoretic predictions based on the Lifshitz theory. As the hydrogen bond donor ability of the medium increased, the adhesion energy was found to be increasingly underestimated by the model, illustrating the importance of the medium–medium interactions outside the contact area in determining the adhesive properties of the contact at the nanoscale. Exceptionally, in n-octanol, the pull-off forces were considerably lower than predicted and a dual slope linear friction–load relation was observed. These observations were rationalized by the formation of physisorbed layers of octanol on the surfaces. The friction–load relationship in the other media was found to be dependent on the magnitude of adhesion. For weakly adhering systems, the friction–load relationship was linear, but as adhesion increased, a sublinear relationship was observed. The data were rationalized by treating the friction as the sum of an adhesion-dependent shear term characterized by a surface shear strength τ and a molecular plowing term characterized by a coefficient of friction μ. Thus, Amontons’ law appears to describe the limiting case of very weak adhesion where viscoelastic plowing is primarily responsible for energy dissipation, while a sublinear friction–load relationship emerges in other situations due to the dissipation of energy in shearing adhesive contacts.     

Mechanical Property Investigation of Soft Materials by Cantilever‐Based Optical Interfacial Force Microscopy

Cantilever‐based optical interfacial force microscopy (COIFM) was applied to the investigation of the mechanical properties of soft materials to avoid the double‐spring effect and snap‐to‐contact problem associated with atomic force microscopy (AFM). When a force was measured as a function of distance between an oxidized silicon probe and the surface of a soft hydrocarbon film, it increases nonlinearly in the lower force region below ∼10 nN, following the Herzian model with the elastic modulus of ∼50 MPa. Above ∼10 nN, it increases linearly with a small oscillatory sawtooth pattern with amplitude 1–2 nN. The pattern suggests the possible existence of the layered structure within the film. When its internal part of the film was exposed to the probe, the force depends on the distance linearly with an adhesive force of −20 nN. This linear dependence suggests that the adhesive internal material behaved like a linear spring with a spring constant of ∼1 N/m. Constant‐force images taken in the repulsive and attractive contact regimes revealed additional features that were not observed in the images taken in the noncontact regime. At some locations, however, contrast inversions were observed between the two contact regimes while the average roughness remained constant. The result suggests that some embedded materials had spring constants different from those of the surrounding material. This study demonstrated that the COIFM is capable of imaging mechanical properties of local structures such as small impurities and domains at the nanometer scale, which is a formidable challenge with conventional AFM methods. SCANNING 35:59‐67, 2013. © 2012 Wiley Periodicals, Inc.     

Frictional forces between cohesive powder particles studied by AFM

A range of commercially important powders (hydrated alumina, limestone, titania and zeolite) and glass ballotini were attached to atomic force microscope cantilevers, and inter-particle friction forces studied in air using lateral force microscopy (LFM). The in situ calibration procedure for friction forces is described. LF images, line profiles, LF histograms, surface roughness, pull-off forces, and the load dependence of friction in the range 0-25 nN were studied for both particle-particle and particle-wall (steel) contacts. The single-particle friction results are discussed in terms of contact mechanics theory. Particle-particle contacts showed load-dependent friction, involving single asperity contacts (non-linear behaviour) or multi-asperity contacts (linear behaviour). Particle-wall contacts usually showed little load dependence and were more adhesive. The results are also related to shear stress-normal stress data (yield loci) for the same materials from bulk shear testers.     

双测头复合型微纳米测量仪的研制

针对当前微纳米测量中存在的大范围高精度测量及复杂微结构几何参数表征难题,基于多测头传感和精密定位平台复用技术,开发了一台具有多种测量尺度和测量模式的复合型微纳米测量仪。为使其具备大范围快速扫描测量和小范围精细测量功能,仪器集成了白光干涉和原子力显微镜两种测头,通过设计适用于两种测头集成的桥架结构及宏/微两级驱动定位平台,实现整机的开发。为保证仪器测量结果的准确性和溯源性,利用标准样板对开发完成的仪器进行了校准。仪器搭载的白光干涉测头可以达到横向500 nm,纵向1 nm的分辨力;原子力显微镜测头横向和纵向分辨力均可达到1 nm。最后,利用目标仪器对微球样品进行了测量,通过大范围成像和小范围精细扫描,获得了微球的表面特征,验证了仪器对复杂微结构的测量能力。     

Stick–Slip Motion and Static Friction in a Nonlinear Deformable Substrate Potential

Nano-scale adhesive contact mediated by intermolecular van der Waals forces has become a typical fundamental problem in many areas. Interpretation and control of the strength and efficiency of the nano-scale adhesive contacts require a proper modeling considering the actual interfacial forces, the varying contact area, and clearance. In this article, the finite-element (FE) method is developed to model the nano-scale adhesive contact of elastic bodies with an adhesive pressure derived from the interatomic interaction Lennard-Jones potential, which permits numerical solutions for a variety of interface geometries. Compared with the analytical results from conventional Hertz, JKR, and DMT models, the validity of the FE model is verified. For nano-scale contact, the assumption of equivalent radius adopted in the Hertz model is initially investigated and proved to be improper for nano-scale adhesive contact due to the distribution variations of interfacial force caused by local contact geometry. Then adhesive contact behaviors of four typical nano-scale contact geometries inspired by tip shapes of bio-adhesive pads are investigated in detail, which are flat punch tip, sphere tip, mushroom tip, and empty cup tip. The simulation results indicate that the nano-scale tip geometry plays a dominant role on the pull-off strength. Within the investigated geometries, cup tip results in a highest adhesion efficiency followed by flat punch tip, sphere tip, and mushroom tip, respectively, which are highly geometry dependent and verified by former experimental results. The dominant effect is found coming from the contact area ratio of the adhesive area to the sticking area or the whole contact area. The FE modeling can serve a useful purpose in revealing the nano-scale geometry-based adhesion contact for surface topography design in MEMS to avoid stiction failure and for the artificial sticky feet in bionics to increase adhesion strength.     

Finite element modeling of adhesive contact using molecular potential

A finite element technique for analysis of adhesive contact is developed in which the adhesive force is modeled as a body force derived from Lennard–Jones 12–6 potential. Adhesive contact of an elastic hemispherical asperity with the plane surface of a semi-infinite rigid body is analyzed. Variations of the interaction force and contact radius during approach and withdrawal, and the dependence of pull-off force on the asperity radius are shown to be in good agreement with those of Maugis–Dugdale model. Analysis results reveal that smaller asperity is superior for preventing stiction and for reducing adhesive friction, but is subject to more severe adhesive wear. It is anticipated that this technique can be utilized in designing a low-adhesion surface profile for MEMS applications since the effect of various surface geometries can be examined.     

Microtribological studies of unlubricated sliding Si/Si contact in air using AFM/FFM

Tribological properties of Si/Si contacts were measured on a microscale by using an atomic force/friction force microscope. Friction forces and pull-off forces between a Si tip and a polished surface of a Si(100) wafer were studied as a function of applied normal load and relative humidity of the surrounding air. The results show that pull-off forces and friction coefficients increased and were strongly influenced by capillary forces with increasing humidity. Tribological interactions during 20 passes of overlapping sliding contact at 50% relative humidity and very small loads of 70 nN were confined to the layer of adsorbates and chemical reactions, without measurable solid damage on the Si(100) wafer.     

转动C形腿与沙土作用建模及力学参数辨识

机器人足端与沙土相互作用力学模型的建立和参数辨识，是沙土表面步行移动机器人多模态感知和决策的重要约束条件和物理信息。C形腿构型在沙土表面具有高通过性和适应性，基于地面动力学中的抵抗力学理论，充分考虑C形腿在摆动步态条件下位姿和速度矢量对足地相互作用动态力学的影响，进而建立C形腿与沙土的相互作用力学模型。然后，通过三组宽度条件的C形腿与沙土表面的转动接触力学试验，提取数据并分析水平和竖直接触力随姿态角度的变化规律。然后，通过积分模型的解析推导获得线性表达形式，基于递归最小二乘算法对未知参数矩阵进行逐项推导。最后，基于逐项迭代输入输出矩阵函数，获得参数在已有数据样本容量空间内的辨识结果。与试验结果相比，辨识后的预测竖直力和水平力误差分别为4.05%和4.22%，验证参数辨识的准确性和有效性。辨识的参数能够反映沙土地面的部分物理特征，基准值则反映腿部几何构型对力学模型的影响。     

Topographic effects on adhesive force mapping of stretched DNA molecules by pulsed-force-mode atomic force microscopy

Adhesive interaction between a tip and a sample surface was examined on a microscopic scale by pulsed-force-mode atomic force microscopy (PFM-AFM). The signal measured by monitoring pull-off force is influenced by various factors such as topography, elasticity, electrostatic charges, and adsorbed water on surfaces. Here, we focus on the topographic effects on the adhesive interaction. To clarify the topographic influence, the adhesive force measurement of a stretched DNA molecule with a smaller radius of curvature than that of a tip was carried out at low relative humidity (RH) with an alkanethiol-modified tip. The experimental conditions such as low RH and the use of the alkanethiol-modified tip were required to minimise the influence of water capillary force on hydrated DNA strands. The hydrophobic modification of a substrate surface was also important to minimise the adsorbed water effect. The DNA molecules were stretched on the substrate surfaces by an immobilisation process called a dynamic molecular combing method. The two-component vapour-phase surface modification with an alkylsilane mixed with a silane derivative containing an amino end group enhanced the DNA adsorption due to the electrostatic interaction. The experimental results for the topographic effects on the adhesive force mapping were reproducible.     

Tribological properties of asperity arrays coated with self-assembled monolayers

The effects of a self-assembled monolayer (SAM) coating on the friction and pull-off forces were determined by using two-dimensional asperity arrays on silicon wafers. The arrays were coated with SAM composed of one of five different alkylchlorsilanes. First, two-dimensional asperity arrays were created by using a focussed ion beam (FIB) system to mill patterns on silicon plates. Each silicon plate had different patterns of equally spaced asperities. Each pattern (5 × 5 μm²) had a different radius of curvature of the asperity peaks, ranging from about 200 to 2500 nm. Then, each silicon plate was immersed in a solution of a different alkylchlorsilane in hexane (either hexyltrichlorosilane, octyltrichlorosilane, dodecyltrichlorosilane, tetradecyltrichlorosilane, or octadecyltrichlorosilane), thus coating the asperity arrays with SAM. The friction and pull-off forces on the SAM-coated arrays were measured by using an atomic force microscope (AFM) that had a square flat probe. The pull-off force for SAM-coated silicon was roughly proportional to the radius of curvature of the asperity peaks. The magnitude of the pull-off force corresponded approximately to the capillary force calculated by using the contact angle of water on the surface of SAM. The friction coefficient correlated with the inverse of the alkyl-chain length of the SAM.     

Multiscale analysis on two-dimensional nanoscale adhesive contacts

Nanoscale adhesive contacts play a key role in micro/nano-electro-mechanical systems as the dimension of the components come to nanometer.Experimental studies on nanoscale adhesive contacts are limited by some uncertain factors and the cost of experiments is too high.Besides,nanoscale textured surfaces are difficult to process and nanoscale adhesive contacts of textured surfaces are still lack of investigation.By using multiscale method,which couples molecular dynamics simulation and finite element method,two-dimensional nanoscale adhesive contacts between a rigid cylindrical tip and an elastic substrate are investigated.For the contacts between the rigid cylindrical tip and smooth surface,Von Mises stress distributions,the maximum Von Mises stresses,and contact forces are compared for different radii to show the size effects and adhesive effects.The phenomena of hysteresis are observed and more obvious as the radii of the tip increase.The influences of indentation depth and indentation speed are also discussed.Then two series of textured surfaces are employed,and the influences of the texture asperity shape,asperity height,and asperity spacing on contact forces are studied.The contact forces comparisons show that textured surfaces can reduce contact forces effectively in the range of the two series.Contact forces of textured surfaces increase as the asperity heights increase,and textured surfaces with smaller asperity spacing will get higher contact forces.Contact forces may be controlled through textured surfaces in the future.The obtained results will help to improve contact condition and provide theory basis for texture design.     

Easy-to-Implement Equations for Determining Adhesive Contact Parameters with the Accuracy of Numerical Simulations

The numerical simulation based on the Lennard-Jones potential for the adhesive contact between spheres is employed. The result is compared with the Maugis-Dugdale model. An empirical formula is obtained for fitting the relation between the Tabor parameter and the pull-off force for the numerical simulation. By using the empirical formula, the equations for approach versus load and the Tabor parameter versus the contact radius at zero load can be found. All these equations are both simple and as accurate as the numerical simulation.     

Chemical force microscopy of microcontact-printed self-assembled monolayers by pulsed-force-mode atomic force microscopy

A novel chemically sensitive imaging mode based on adhesive force detection by previously developed pulsed-force-mode atomic force microscopy (PFM-AFM) is presented. PFM-AFM enables simultaneous imaging of surface topography and adhesive force between tip and sample surfaces. Since the adhesive forces are directly related to interaction between chemical functional groups on tip and sample surfaces, we combined the adhesive force mapping by PFM-AFM with chemically modified tips to accomplish imaging of a sample surface with chemical sensitivity. The adhesive force mapping by PFM-AFM both in air and pure water with CH3- and COOH-modified tips clearly discriminated the chemical functional groups on the patterned self-assembled monolayers (SAMs) consisting of COOH- and CH3-terminated regions prepared by microcontact printing (microCP). These results indicate that the adhesive force mapping by PFM-AFM can be used to image distribution of different chemical functional groups on a sample surface. The discrimination mechanism based upon adhesive forces measured by PFM-AFM was compared with that based upon friction forces measured by friction force microscopy. The former is related to observed difference in interactions between tip and sample surfaces when the different interfaces are detached, while the latter depends on difference in periodic corrugated interfacial potentials due to Pauli repulsive forces between the outermost functional groups facing each other and also difference in shear moduli of elasticities between different SAMs.     

The effect of water on friction of MEMS

Water plays a significant role in the performance of micro electro mechanical systems (MEMS). A special apparatus was employed to investigate the adhesive friction attributed to water at low coverages, i.e., in the nanometer range, where friction and adhesion are a function of the water layer thickness. In addition, the history of the sample surface also plays a significant role. The friction forces associated with hydrophobic samples are negligibly affected by humidity changes, whereas those of hydrophilic samples show a strong dependence. Sample coverage and the friction force are also influenced by the sample temperature. High forces were measured for high humidities at low sample temperatures, for hydrophilic silicon. In contrast, hydrophobic samples show an increase of the friction force with increasing temperature. Experiments performed under high vacuum demonstrated that decreasing the water layer thickness by desorption decreases the friction force with several sub‐minima and sub‐maxima. The friction signal is accompanied by sudden fluctuations. For submonolayer coverage the friction force starts to increase. This revised version was published online in July 2006 with corrections to the Cover Date.