A torsional resonance mode AFM for in-plane tip surface interactions

Changing the method of tip/sample interaction leads to contact, tapping and other dynamic imaging modes in atomic force microscopy (AFM) feedback controls. A common characteristic of these feedback controls is that the primary control signals are based on flexural deflection of the cantilever probes, statically or dynamically. We introduce a new AFM mode using the torsional resonance amplitude (or phase) to control the feedback loop and maintain the tip/surface relative position through lateral interaction. The torsional resonance mode (TRmode™) provides complementary information to tapping mode for surface imaging and studies. The nature of tip/surface interaction of the TRmode facilitates phase measurements to resolve the in-plane anisotropy of materials as well as measurements of dynamic friction at nanometer scale. TRmode can image surfaces interleaved with TappingMode™ with the same probe and in the same area. In this way we are able to probe samples dynamically in both vertical and lateral dimensions with high sensitivity to local mechanical and tribological properties. The benefit of TRmode has been proven in studies of water adsorption on HOPG surface steps. TR phase data yields approximately 20 times stronger contrast than tapping phase at step edges, revealing detailed structures that cannot be resolved in tapping mode imaging. The effect of sample rotation relative to the torsional oscillation axis of the cantilever on TR phase contrast has been observed. Tip wear studies of TRmode demonstrated that the interaction forces between tip and sample could be controlled for minimum tip damage by the feedback loop.     

Dual resonance excitation system for the contact mode of atomic force microscopy

We propose an improved system that enables simultaneous excitation and measurements of at least two resonance frequency spectra of a vibrating atomic force microscopy (AFM) cantilever. With the dual resonance excitation system it is not only possible to excite the cantilever vibrations in different frequency ranges but also to control the excitation amplitude for the individual modes. This system can be used to excite the resonance frequencies of a cantilever that is either free of the tip-sample interactions or engaged in contact with the sample surface. The atomic force acoustic microscopy and principally similar methods utilize resonance frequencies of the AFM cantilever vibrating while in contact with the sample surface to determine its local elastic modulus. As such calculation demands values of at least two resonance frequencies, two or three subsequent measurements of the contact resonance spectra are necessary. Our approach shortens the measurement time by a factor of two and limits the influence of the AFM tip wear on the values of the tip-sample contact stiffness. In addition, it allows for in situ observation of processes transpiring within the AFM tip or the sample during non-elastic interaction, such as tip fracture.     

The Beneficial Effect of High-Temperature Oxidation on the Tribological Behaviour of V and VN Coatings

This paper investigates the influence of the applied load and sliding velocity on the microfrictional properties of native oxide-covered Si(100) and Si(100) coated with octadecyltrichlorosilane (OTS) and perfluorodecyltrichlorosilane (FDTS) self-assembled monolayers (SAMs) using a precision microtribometer. Microfriction was investigated as a function of the applied load and sliding velocity. As has been confirmed in earlier studies, in the microtribological regime, OTS and FDTS significantly reduce the friction force in comparison to the bare native oxide-covered (hydrophilic) silicon surface. The friction versus applied load curve of the substrate material as well as the SAMs-covered surfaces can be described by a model based on contact mechanics. For the native oxide surface, microfriction is reduced with increasing sliding speed. The friction force of the OTS- and FDTS-covered surfaces increases with load and is proportional to the natural logarithm of sliding speed. The increase with sliding velocity gets larger for higher normal loads. It can be shown that this increase is proportional to the contact area of the counter sample with the SAMs.     

考虑摩擦力及试样表面倾角的原子力显微镜扫描模式

原子力显微镜(Atomic force microscopes,AFM)接触模式下的测量结果因受样本表面倾角和针尖一样本表面间摩擦力的影响而存在较大的测量误差.为避免针尖-表面间的摩擦力对AFM测量试样表面形貌的影响,并能够准确测量表面倾角,提出了一种新的AFM工作模式--消除倾角和摩擦力影响模式.在这种工作模式中,扫描方向垂直悬臂的长轴方向,通过测量悬臂的竖向和横向偏转而得到针尖所受的竖向和横向力,并计算得到针尖-试样表面间的van der Waals力及试样表面局部倾角,然后结合针尖项点和扫描器的位置及针尖-试样表面间距可以得到试样表面形貌的测量结果.在上述工作模式下,针尖-试样表面间的摩擦力是可控的,能够避免针尖或试样的损伤.仿真结果证明了这种方法的可行性.     

Study of the sensitivity of the first four flexural modes of an AFM cantilever with a sidewall probe

The resonant frequency and sensitivity of flexural vibration for an atomic force microscope (AFM) cantilever with a sidewall probe have been analyzed. A closed-form expression for the sensitivity of vibration modes has been obtained using the relationship between the resonant frequency and contact stiffness of cantilever and sample. The results show that a sidewall scanning AFM is more sensitive when the contact stiffness is lower and that the first mode is the most sensitive. However, the high-order modes become more sensitive than the low-order modes as the contact stiffness increases. The resonance frequency of an AFM cantilever is low when contact stiffness is small. However, the frequency rapidly increases as contact stiffness increases. In addition, it can be found that the effects of the vertical extension on the sensitivity and the resonant frequency of an AFM cantilever are significant. Decreasing the length of vertical extension can increase the resonance frequency and sensitivity of mode 1 when the contact stiffness is small. However, the situation is reverse when the contact stiffness becomes large.     

Nanomechanical characteristics at an ultra-small particle-surface contact interface

Atomic force microscopy (AFM) measurements have shown that nanoscale interfaces in sliding contact frequently exhibit atomic lattice stick-slip friction. Using various material surfaces and AFM tips, including colloidal probes, and systematically varying applied load and lever stiffness, it is demonstrated that transitions can be repeatedly observed from smooth sliding to single unit-cell slips and then multiple slips. The behavior is dependent on the interplay between the stiffness of the contact zone, the measurement system (i.e., the AFM cantilever), and the interfacial potential. Atomic lattice stick-slip occurs with colloidal particle tip orders of magnitudes larger than those previously used. Stable atomically corrugated sliding in ambient conditions that cannot be seen elsewhere is reported. The generality of these conditions suggests that atomic-scale stick-slip behavior may be far more prevalent than previously appreciated. In addition, the friction-stiffness maps of various material surfaces in contact with a colloidal particle were reported, and the complex effects of system stiffness and pressure were discussed for chemical-mechanical polishing applications.     

Novel operation mode for eliminating influence of inclination angle and friction in atomic force microscopy

The accuracy of topography imaging in contact force mode of atomic force microscopy (AFM) depends on the one-to-one corresponding relationship between the cantilever deflection and the tip–sample distance, whereas such a relationship cannot be always achieved in the presence of friction and incline angle of sample surface. Recently, we have developed a novel operation mode in which we keep the van der Waals force as constant instead of the applied normal force, to eliminate the effect of inclination angle and friction on topography imaging in the contact force mode. We have improved our AFM to enable the new operation mode for validation. Comparative experiments have been performed and the results have shown that the effect of friction and inclination angle on topography imaging in contact mode of AFM can be eliminated or at least decreased effectively by working in the new operation mode we present.     

A New AFM Nanotribology Method Using a T-Shape Cantilever with an Off-Axis Tip for Friction Coefficient Measurement with Minimized Abbé Error

A new AFM (atomic force microscopy) nanotribology method using a T-shape cantilever with an off-axis tip (Nat Nanotechnol 2:507–514, 2007) has been developed for measuring friction coefficient at nanometer scale. In this method, signals due to both bending and twisting of the T-shape AFM cantilever are detected simultaneously. For a T-shape AFM cantilever, the bending is caused by the normal load and the twisting is caused by both the normal and the lateral loads. The twisting generated by the normal load is calibrated in advance. Consequently, the twisting only due to the lateral load can be decoupled from the total lateral voltage signal. And the friction coefficient can be finally determined based on a conversion relationship between the normal and lateral voltage signals of the AFM photodetector. A practical procedure for minimizing Abbé error in friction coefficient measurement has also been introduced. The proposed new method is simple and accurate, and requires the least operation for friction coefficient measurement at nanometer scale.     

Sensitivity and resonant frequency of an AFM with sidewall and top-surface probes for both flexural and torsional modes

The resonant frequencies and flexural sensitivities of an atomic force microscope (AFM) with assembled cantilever probe (ACP) are studied. This ACP comprises a horizontal cantilever, a vertical extension and two tips located at the free ends of the cantilever and the extension, which makes the AFM capable of simultaneous topography at top surface and sidewalls of microstructures especially microgears, which consequently leads to a time-saving swift scanning process. In this work, the effects of the sample surface contact stiffness and the geometrical parameters such as the ratio of the vertical extension length to the horizontal cantilever length and the distance of the vertical extension from clamped end of the horizontal cantilever on both flexural and torsional resonant frequencies and sensitivities are assessed. These geometrical effects are illustrated in some figures. The results show that the low-order vibration modes are more sensitive for low values of the contact stiffness, but the situation is reversed for high values.     

Quantitative elastic modulus measurement by magnetic force modulation microscopy

It is experimentally demonstrated that magnetic force modulation microscopy (MFMM) is a technique allowing quantitative elastic modulus measurements. A model of the cantilever–tip–sample interaction taking into account the lateral contact stiffness (i.e., the friction effects at the level of the tip–sample contact), the position of the magnetic force applied to the cantilever with respect to the tip position, as well as the inclination of the cantilever arm with respect to the sample surface is presented. The model shows that MFMM is much less sensitive to lateral force than the other modulation techniques and thus, in contrast to the latter, that the contrast of the stiffness images can be interpreted as a true elasticity contrast and not as a mixture of friction and elasticity. Thanks to the study of the normal contact stiffness versus normal load that allows the characterization of contact between tip and sample, it is possible to perform quantitative elastic modulus measurements with a dynamic modulation method.     

Mapping of lateral vibration of the tip in atomic force microscopy at the torsional resonance of the cantilever

Lateral vibration of the tip in atomic force microscopy was mapped at the torsional resonance of the cantilever by attaching a shear piezo element at the base of the cantilever or under the sample. Fixed frequency excitation and self-excitation of torsional motion were implemented. The lateral vibration utilized as measured by an optical lever was in the order of 10 pm to 3 nm, and its frequency approximately 450 kHz for a contact-mode silicon nitride cantilever. The amplitude and phase of the torsional motion of the cantilever was measured by a lock-in-amplifier or a rectifier and plotted in x and y as the sample was raster scanned. The imaging technique gave contrast between graphite terraces, self-assembled monolayer domains, silicon and silicon dioxide, graphite and mica. Changing contrast was observed as silicon islands oxidized in atmosphere, showing that the imaging technique can detect change in lateral tip mobility due to changes occurring near the surface. Torsional self-excitation showed nanometric features of self-assembled monolayer islands due to different lateral dissipation. Dependence of torsional resonance frequency on excitation amplitude, and contrast change due to driving frequency around resonance were observed.     

激光检测摩擦力显微镜的定量标定

本文简要描述了激光检测摩擦力显微镜的工作原理，探索出一种横向力标定的有效方法，可以从横向力信号中提取摩擦力信号，从而能够定量地对试样表面的形貌和力学性质进行纳米量级的评定，以获取微观表面真实的三维形貌图和微观摩擦系数等信息，为纳米摩擦学设计提供依据。实验结果表明，用该方法测得未清洗单晶硅表面的微观摩擦系数约为０．０６，和Ｂｈｕｓｈａｎ等人的结果吻合的很好。     

Study of the sensitivity and resonant frequency of the torsional modes of an AFM cantilever with a sidewall probe based on a nonlocal elasticity theory

A relationship based on a nonlocal elasticity theory is developed to investigate the torsional sensitivity and resonant frequency of an atomic force microscope (AFM) with assembled cantilever probe (ACP). This ACP comprises a horizontal cantilever and a vertical extension, and a tip located at the free end of the extension, which makes the AFM capable of topography at sidewalls of microstructures. First, the governing differential equations of motion and boundary conditions for dynamic analysis are obtained by a combination of the basic equations of nonlocal elasticity theory and Hamilton's principle. Afterward, a closed‐form expression for the sensitivity of vibration modes has been obtained using the relationship between the resonant frequency and contact stiffness of cantilever and sample. These analysis accounts for a better representation of the torsional behavior of an AFM with sidewall probe where the small‐scale effect are significant. The results of the proposed model are compared with those of classical beam theory. The results show that the sensitivities and resonant frequencies of ACP predicted by the nonlocal elasticity theory are smaller than those obtained by the classical beam theory. Microsc. Res. Tech. 78:408–415, 2015. © 2015 Wiley Periodicals, Inc.     

Contact Mechanics and Friction on Dry and Wet Human Skin

The surface topography of the human wrist skin is studied using an optical method and the surface roughness power spectrum is obtained. The Persson contact mechanics theory is used to calculate the contact area for different magnifications, for both dry and wet condition of the skin. For dry skin, plastic yielding becomes important and will determine the area of contact observed at the highest magnification. The measured friction coefficient [M.J. Adams et al., Tribol Lett 26:239, 2007] on both dry and wet skin can be explained assuming that a frictional shear stress σ_f ≈ 15 MPa acts in the area of real contact during sliding. This frictional shear stress is typical for sliding on polymer surfaces, and for thin (nanometer) confined fluid films. The big increase in the friction, which has been observed for glass sliding on wet skin as the skin dries up, can be explained as resulting from the increase in the contact area arising from the attraction of capillary bridges. This effect is predicted to operate as long as the water layer is thinner than ～14 μm, which is in good agreement with the time period (of order 100 s) over which the enhanced friction is observed (it takes about 100 s for ～14 μm water to evaporate at 50% relative humidity and at room temperature). We calculate the dependency of the sliding friction coefficient on the sliding speed on lubricated surfaces (Stribeck curve). We show that sliding of a sphere and of a cylinder gives very similar results if the radius and load on the sphere and cylinder are appropriately related. When applied to skin the calculated Stribeck curve is in good agreement with experiment, except that the curve is shifted by one velocity-decade to higher velocities than observed experimentally. We explain this by the role of the skin and underlying tissues viscoelasticity on the contact mechanics.     

Macro- and Nanotribological Properties of Graphite Tribofilms: Influence of the Sliding Interface

Macrotribological studies of microcrystalline graphite powder reveal a drastic decrease in the friction coefficient when the experiments are carried out in the presence of low-viscosity liquids. The friction reduction is attributed to the simultaneous presence of particles and liquid in the sliding contact, but the mechanisms involved remain unclear. In order to contribute to the understanding of liquid action in friction reduction mechanisms, nanoscale investigations of the tribofilms have been performed using lateral force microscopy. Attention is devoted to the nanostructure of the film surfaces and their nanofriction behavior using an atomic force microscope. The influence of the tip/sample interfaces on friction properties is investigated by using AFM tips constituted of different compounds (silicon, gold/chromium alloy, silicon nitride or carbon-covered AFM tip) and by performing the nanofriction tests in air or liquid environments. The results indicate that the friction reduction observed at macroscale is attributed neither to the lowering of the shear strength of the carbon/carbon interface in the presence of liquid nor to the nanostructure of the film surface. Collective liquid/particles effects inside the contact during sliding are probably involved.     

Contribution of crystallographic texturing to the sliding friction behaviour of fcc and hcp metals

Dry sliding wear tests were performed on two polycrystalline materials representing fcc and hcp crystal structures, i.e. aluminum and titanium, respectively. A block-on-ring type wear machine with a rotating ring made of AISI 52100 type bearing steel was used and variation of coefficients of friction with sliding distance was measured at a sliding speed of 0.13 m s⁻¹ and normal load of 10 N. The texture was evaluated during wear using an X-ray diffraction inverse pole figure technique for a range of sliding distances. Pole density distributions for the [0 0 0 1] and [1 1 1] poles for of Ti and Al, respectively, were then determined from the inverse pole figures. The texture evolution during sliding wear was subsequently related to the friction and wear behaviour. For the aluminum sample, a (1 1 1) texture developed parallel to the worn surface with increasing sliding distance (a six-fold increase in the (1 1 1) pole density as the sliding distance increases from 0 to 2714 m). The titanium sample (normal section) which had a preferred orientation with the basal poles, [0 0 0 1], parallel to the contact surface prior to testing, an increase in wear, i.e. sliding distance, did not change the texture. However, for the transverse section of titanium, the basal pole, [0 0 0 1], density parallel to the worn surface increased with increasing sliding distance. The shape of the coefficient of friction versus sliding distance curve was strongly influenced by crystallographic texturing. A drop in the coefficient of friction with the progressive development of the [1 1 1] and [0 0 0 1] texture was observed for both Al and Ti (transverse section), respectively, as a result of easy glide planes becoming parallel to the sliding plane.     

表面粗糙度对滑动电接触磨损率的影响

在电气化铁路弓网系统中,磨损率是衡量列车运行状态与接触导线使用状态的重要指标。为了充分模拟弓网系统中磨损率情况,利用自行搭建的滑动电接触摩擦磨损试验机对滑板和接触导线进行摩擦磨损试验,分析滑板表面粗糙度、法向压力、接触电流与运行速度对磨损率的影响。得出结论:滑板磨损率随滑板初始表面粗糙度、接触电流、法向压力、运行速度的增加而增加,而高载荷下粗糙度对于磨损率的影响降低;滑板摩擦从磨合期进入稳定摩擦期存在一个临界表面粗糙度,当滑板初始表面粗糙度值等于临界粗糙度值时,其磨损率最低;不同初始表面粗糙度的滑板在跑合期内磨损过程不同,在稳定摩擦期内磨损过程趋于一致,且摩擦试验后滑板表面粗糙度也接近。     

Surface analysis of laser cladded Stellite exposed to self-mated high load dry sliding

Materials from the Stellite family of Co-based alloys are commonly used as low friction, galling resistant materials in high load dry sliding contact applications.In the present investigation, the surface region of a Co-based material (Stellite 21) exposed to self-mated high load dry sliding at room temperature has been analysed in detail.During sliding, an approximately 30 nm thick Co-enriched tribofilm is created. It exhibits low friction properties and a high galling resistance. The transformation from an face-centred-cubic structure to easily sheared hexagonal-closed-packed basal planes in the tribofilm combined with the high load carrying capacity of the underlying deformation hardened zone is suggested to explain the excellent low friction properties and galling resistance of this material.     

AFM悬臂定位系统中光干涉误差及减小方法研究

光杠杆法是原子力显微镜（AFM）悬臂定位的主要方法。由于悬臂自身的尺寸和材料特性、检测光路系统等因素制约，悬臂弯曲测量时存在光泄露。被试样表面反射的部分泄露光与悬臂反射光产生干涉，在探针一试样接近曲线中产生光干涉误差。基于轻触模式AFM，分析了光干涉误差的产生原因，并对其引起的AFM测量误差进行了数学分析和仿真、提出了减小光干涉误差的方法。实验结果和理论分析表明，为了进一步提高AFM的测量精度，有必要克服定位系统中的光干涉误差。     

Characteristics of a dynamic atomic force microscopy based on a higher-order resonant silicon cantilever and experiments

In order to improve the sensitivity and scanning speed of the dynamic AFM, a surface scanning method using higher-order resonant cantilever is adopted and investigated based on the higher-order resonance characteristics of the silicon cantilever, and the theoretical analysis and experimental verification on the higher-order resonance characteristics of the corresponding dynamic AFM cantilever are given. In this method, the cantilever is excited to oscillate near to its higher-order resonant frequency which is several times higher than that of the fundamental mode. Then the characteristic changes a lot compared with the first-order resonant cantilever. Because of the changes of the quality factor, amplitude and the mode shape of the cantilever, the higher-order resonant AFM gets higher sensitivity and scanning speed. Based on the home-built tapping-mode AFM experiment system, the resolution and the response time of the first and second order resonance measured by experiment are respectively: 0.83 nm, 0.42 nm; 1265 μs, 573 μs. The higher-order resonance cantilever has higher sensitivity and the dynamic measurement performance of the cantilever is significantly improved from the experimental results. This can be a useful method to develop AFM with high speed and high sensitivity. Besides above, the surface profile of a grating sample and its three-dimensional topography are obtained by the higher-order resonant mode AFM.