The Frictional Response of VC(100) Surfaces: Influence of 1-Octanol and 2,2,2-Trifluoroethanol Adsorption

In this report, we present ultrahigh vacuum (UHV) atomic-scale measurements of the frictional response of the VC(100) surface and the influence on friction through the adsorption of 1-octanol (CH₃(CH₂)₇OH) and 2,2,2-trifluoroethanol (CF₃CH₂OH). Atomic force microscopy (AFM) has been used to determine the changes in interfacial friction and adhesion, while scanning tunneling microscopy (STM) has revealed changes in surface morphology upon adsorption. X-ray photoelectron spectroscopy (XPS) has been utilized to determine the composition of the surface formed through the reaction of these adsorbates with VC. Adsorption of 1-octanol on the VC(100) surface at room temperature causes a 15% reduction in the friction measured between a clean VC surface and a silicon nitride AFM tip. STM images, combined with XPS results, reveal that 1-octanol does not completely cover the surface and that saturation occurs approximately at a 500L exposure. Adsorption of 2,2,2-trifluoroethanol on the VC(100) surface at room temperature produces a significant increase in friction while at the same time producing a decrease in adhesion. These contrasting results are interpreted in terms of differences in interfacial shear strength, chemical composition, and the molecular details of the adsorbed layer.     

Influence of protein conformation on frictional properties of poly (vinyl alcohol) hydrogel for artificial cartilage

Poly (vinyl alcohol) (PVA) hydrogel is one of the anticipated materials for artificial cartilage. In our previous studies, wear of PVA hydrogel depended on content of proteins in lubricants. The secondary structures of bovine serum albumin (BSA) and human gamma globulin (HGG) were investigated in circular dichroism spectroscopy to clarify the influence of the proteins on frictional properties. BSA and HGG were mainly composed of the α-helix and the β-sheet, respectively. BSA containing the α-helix structure showed low friction compared to HGG composed of the β-sheet structure in mixed or boundary lubrication mode. The α-helix structure forms low shear layer because the α-helix structure is easily released from surfaces and low cohesive strength. HGG forms uniform adsorption layer, but showed higher friction than BSA in the rubbing with single protein. In the repeated rubbing with changing of lubricants from HGG to BSA, however, the final friction was reduced, because an optimum layered structure of proteins was formed. Hence, layered structure of proteins appears to play an important role to protect rubbing surfaces and to reduce friction. In heat treatment tests, heat-induced BSA showed very low friction because of reduction of the α-helix structure. Heat-induced HGG did not show large differences from native HGG, but could not bring low friction with heat-induced BSA. Thus it was shown that the protein conformation has effective influences on friction.     

Macro- and nanotribological properties of organosilane monolayers prepared by a chemical vapor adsorption method on silicon substrates

Organosilane monolayers are novel ultrathin films used to control the physicochemical properties, such as friction and wear, of solid surfaces. In this study, the authors prepared alkylsilane and fluoroalkylsilane monolayers with a series of chain lengths by a chemical vapor adsorption method. The monolayers tribological properties were investigated by lateral force microscope (LFM) and friction tester. LFM nanoscale measurements of tribological properties showed that alkylsilane monolayer gave lower lateral force than the Si substrate surface. The lateral force decreased as the length of the alkyl chain increased. On the macroscale, friction test revealed that the organosilane monolayers gave lower dynamic friction coefficients than the Si substrate surface in air at room temperature. The longer the alkyl chain, the greater the wear resistance of the organosilane monolayers. Friction experiments using tetradecane as a lubricant showed better tribological properties than were obtained in air. Furthermore, microscopically line-patterned two-component organosilane monolayers were prepared and their macroscopic friction behavior was investigated. Even though the height difference between the two-components was less than 1 nm, friction force anisotropy between the parallel and perpendicular directions against the line pattern was observed.     

Microscopic Studies of Friction and Wear at the Benzotriazole/Copper Interface

Interfacial friction and topographic changes at copper surfaces have been measured in situ with AFM in nitric acid and in acidic and neutral solutions of benzotriazole (BTA), a known corrosion inhibitor. In addition, changes in the thickness of the copper film have been measured ex situ using stylus profilometry as a function of solution treatment. These measurements demonstrate isotropic dissolution in 0.10 M HNO₃ solutions and little change in neutral solutions of BTA. However in acidified solutions of BTA, the formation of a substantial reaction overlayer, the presence of higher interfacial friction, and the tip-mediated, localized dissolution of copper interface is observed. These measurements indicate that BTA plays a multifunctional role under acidic conditions. In the absence of mechanical action, BTA acts to passivate the surface from isotropic etching under acidic conditions. In regions of interfacial contact, the dissolution of a BTA-Cu reaction layer leads to the localized removal of copper through a tip-mediated process.     

Friction and wear effects on a micro/nano‐scale

In this paper are described tribological effects which can be found in micro‐tribological systems, and in those macro‐systems which can be analysed by micro‐methods, e.g., by atomic force microscopy (AFM) or related methods. Micro‐tribology systems have friction contacts with loads in the micro/nano‐newton range and/or dimensions in the micro/nanometre range. Experiments on the micro/nano‐scale should be easier to explain by theoretical modelling due to their simpler system structure. An example is discussed of adhesion and friction measurements between AFM tips and clean, flat, solid surfaces in ultra‐high vacuum, which shows some of the special aspects of micro/nano‐tribology. Surprising friction characteristics on surfaces with an artificial micro‐structure can be explained by skilled and careful topographical analysis of the friction path with an AFM. In micro‐sensor contacts, ‘single wear events’ can be detected using AFM analysis of the contact region. For ceramic compounds, different friction levels for the components of the material can be found. The problems, difficulties, and dangers of misinterpretation are also discussed.     

Comparison of friction measurements using the atomic force microscope and the surface forces apparatus: the issue of scale

Results are presented of lateral force measurements using the atomic force microscope (AFM) and the surface forces apparatus (SFA). Two different probes are used in the AFM measurements; a sharp silicon nitride tip (radius R20 nm) and a glass ball (R15 m). The lateral force is measured between the (silicon nitride or glass) probe and a mica surface which has been coated by a thin lubricant film. In the SFA, a thin lubricant film separates two molecularly smooth mica surfaces (R1 cm) which are slid relative to each other. Perfluoropolyether (PFPE) and polydimethylsiloxane (PDMS) were used as the lubricant films. In the SFA where the contact diameter is largest, the PFPE film shows much lower friction than PDMS. As the size of the probe decreases, the difference in the measured friction decreases. For sharp AFM tips, no clear distinction between the tribological properties of the films can be made. Hence, the measured coefficient of friction varies according to the length scale probed, at least for small dimensions.     

Differences in the nanoscale electrical properties of GaN films grown on sapphire and ZnO substrates by molecular beam epitaxy

Gallium nitride (GaN) films were grown on sapphire and zinc oxide (ZnO) single crystal substrates using plasma‐assisted molecular beam epitaxy. As ZnO for GaN have a better lattice match, the coverage ratio of the GaN (002) plane on the ZnO substrate was significantly higher by about 45%. According to conducting atomic force microscopy and scanning surface potential microscopy measurements, the surface of GaN films grown on the ZnO substrate had two excellent physical characteristics: (a) an 18% reduction of the high contact current region, and (b) a highly uniform work function distribution. Therefore, for future applications in GaN‐based light‐emitting diodes, the use of ZnO as a substrate will prolong the luminescence lifetime and enhance the luminescent monochromaticity.     

Surface and Tribological Characteristics of Tribofilms Formed in the Boundary Lubrication Regime with Application to Internal Combustion Engines

One of the biggest challenges in engine tribology is to formulate appropriate lubricants, which will increase fuel efficiency by reducing friction, yet still provide good wear resistance. The lubricant should also be formulated to limit particulate and gaseous exhaust emissions to the levels allowed by current regulations. In real lubricant formulations there can be 10–15 additives and the interactions between additives must be taken into account. The effects of eliminating the friction modifier and friction modifier plus anti-wear additive zinc dialkyl dithiophosphate (ZDDP) from the additive package of fully formulated lubricants on friction, wear and wear film forming characteristics have been examined. Tests have been conducted under lubricated wear conditions at bulk oil temperatures of 20, 50, and 100 °C using a reciprocating pin-on-plate tribometer. Boundary lubrication conditions were varied according to the value of starting lambda ratio. The wear film has been examined by Energy Dispersive X-ray analysis (EDX) and X-ray Photoelectron Spectroscopy (XPS). In order to investigate the morphology of the reaction films formed by the additive packages of these lubricants, Atomic Force Microscopy (AFM) was used. In this paper it has been shown that tribofilms, derived from ZDDP/surface interactions, affect friction, the extent of which is determined by tribological conditions. Detergent interactions with ZDDP enhance the complexity of the tribofilm and enrich the level of C in the film whilst affecting the friction and wear response. Through integration of tribological measurements and surface analysis, progress towards improving the nature of interactions is made and forms the focus of the paper.     

Comparison of Two Methods of Methyl Group Grafting to the Silica Thin Film Surface and Its Tribological Properties Measured by Atomic Force Microscopy

Two different methods are used to modify silica surfaces and the results of the different modification methods on the frictional properties are presented in this paper. Methyl groups were introduced to the silica film directly during the synthesis step with the use of triethoxymethylsilane or by post-synthesis grafting from solution using trimethylchlorosilane. Atomic Force Microscopy (AFM) was used to compare the frictional behaviour of samples after modifiying the silica surface using the two methods. It was found, that depending on the presence of methyl groups on the surface or in bulk, the frictional properties are strongly influenced not only by methyl groups but also by elasticity of final material.     

ELID超精密磨削钢结硬质合金及其表面质量分析

运用铁基结合剂金刚石砂轮与专用高频脉冲电源相配合,对钢结硬质合金（ＧＴ３５）进行在线电解修整（ＥＬＩＤ）精密磨削。分析了ＥＬＩＤ超精密镜面磨削异相非均质材料钢结硬质合金表面的磨削机理,并通过原子力显微镜对其表面进行了微观表面形貌的分析。     

二氧化硅微米球与纳米粒子膜间的摩擦研究

通过旋涂法制备表面粗糙度为(57±1.7)nm的SiO2纳米粒子膜来模拟纳米器件的基底。采用原子力显微镜胶体探针,分别对有无正辛基三氯硅烷修饰时,该体系的纳米摩擦学性能进行评价。结果表明:二氧化硅胶体探针与SiO2纳米粒子膜之间的黏着力和摩擦力均随着法向载荷增加而增大,而经正辛基三氯硅烷修饰后,该自组装膜能显著改善SiO2表面间的摩擦学性能,起到明显减摩擦抗黏的效果。     

Adhesion and friction properties of hydrogenated amorphous carbon films measured by atomic force microscopy

Atomic force microscopy has been used to measure adhesion and friction forces at the interface between an oxidized metal probe tip and amorphous carbon films of varying hydrogen contents (12.3–39.0 atomic percent hydrogen). The interface of an oxide surface and a hard carbon coating models the unlubricated head-disk interface of current hard disk products. Adhesion forces normalized by the radius of curvature of the contacting tip range from 1.09 to 8.53 N/m. Coefficients of friction values, measured as the slope of the friction versus load plot, range from 0.33 to 0.87. A trend of increasing adhesion forces and coefficients of friction is observed for increasing hydrogen content in the films. We attribute the increase in adhesion and friction to increases in the surface free energy of the carbon films with the incorporation of hydrogen.     

Calibration of the scanning (atomic) force microscope with gold particles

Scanning force microscopy (SFM) holds great promise for biological research. Two major problems that have confronted imaging with the scanning force microscope have been the distortion of the image and overestimation in measurements of lateral size due to the varying geometry and characteristics of the scanning tip. In this study, spherical colloidal gold particles (10, 20 and 40 nm in diameter) were used to determine (1) tip parameters (size, shape and semivertical angle); (2) the distortion of the image caused by the tip; and (3) the overestimation or broadening of lateral dimensions. These gold particles deviate little in size, are rigid and have a size similar to biological macromolecules. Images of the colloidal gold particles by SFM were compared with those obtained by electron microscopy (EM). The height of the gold particles as measured by SFM and EM was comparable and was little affected by the tip geometry. The measurements of the lateral dimensions of colloidal gold, however, showed substantial differences between SFM and EM in that SFM resulted in an overestimate of the lateral dimensions. Moreover, the distortion of images and broadening of lateral dimensions were specific to the SFM tip used. The calibration of the SFM tip with mica provided little clue as to the type of distortion and the amount of lateral broadening observed when the larger gold particles were scanned. The SFM image also depended on the orientation of the tip with respect to the specimen. Our results suggest that quantitative SFM imaging requires calibration to identify and account for both the distortions and the magnitude of lateral broadening caused by the cantilever tip. Calibration with gold particles is fast and nondestructive to the tip. The raw imaging data of the specimen can be corrected for the tip effect and true structural information can be derived. In summary, we present a simple and practical method for the calibration of the SFM tip using gold particles with a size in the range of biomacromolecules that allows: (1) selection of a cantilever tip that produces an image with minimal distortion; (2) quantitative determination of tip parameters; (3) reconstruction of the shape of the tip at different heights from the tip apex; (4) appreciation of the type of distortion that may be introduced by a specific tip and quantification of the overestimation of the lateral dimensions; and (5) calculation of the true structure of the specimen from the image data. The significance is that such calibration will permit quantitative and accurate imaging with SFM.     

Influence of scanning rate on quality of AFM image: Study of surface statistical metrics

The purpose of this work is to study the dependence of AFM‐data reliability on scanning rate. The three‐dimensional (3D) surface topography of the samples with different micro‐motifs is investigated. The analysis of surface metrics for estimation of artifacts from inappropriate scanning rate is presented. Fractal analysis was done by cube counting method and evaluation of statistical metrics was carrying out on the basis of AFM‐data. Combination of quantitate parameters is also presented in graphs for every measurement. The results indicate that the sensitivity to scanning rate growths with fractal dimension of the sample. This approach allows describing the distortion of the images against scanning rate and could be applied for dependences on the other measurement parameters. The article explains the relevance and comparison of fractal and statistical surface parameters for characterization of data distortion caused by inappropriate choice of scanning rate.     

Periodicities in the properties associated with the friction of model self-assembled monolayers

The classical molecular dynamics simulations presented here examine the periodicities associated with the sliding of a diamond counterface across a monolayer of hydrocarbon chains that are covalently bound to a diamond substrate. Periodicities observed in a number of system quantities are a result of the tight packing of the monolayer and the commensurate structure of the diamond counterface. The packing and commensurability of the system force synchronized motion of the chains during sliding contact. This implies that the size of the simulations for this special case can be reduced so that the simulations can be conducted with sliding speeds and time durations that may bridge the gap between theory and experiment.     

Influence of Mg doping on the morphology and optical properties of ZnO films for enhanced H2 sensing

Highly oriented ZnO and Mg doped ZnO thin films were fabricated on Al₂O₃ substrate by sputtering at room temperature. The effect of Mg doping on the structural, optical, and morphological properties of ZnO film was investigated. The intensity of (002) peak in X‐ray diffraction measurements revealed the influence of Mg doping on the crystallinity and orientation of ZnO film. Photoluminescence (PL) results show that the Ultraviolet (UV) emission peak was shifted to lower wavelength side for Mg:ZnO film indicating the possibility for quantum confinement. UV–vis–NIR optical absorption revealed an improvement in optical transmittance from 70 to 85%, and corresponding optical band gap from 3.25 to 3.54 eV. Atomic force microscope (AFM) images revealed the nano‐size particulate microstructure of the films. The surface topography of Mg doped ZnO film confirmed decreased grain size with large surface roughness and increased surface area, favorable for sensing. Pure ZnO and Mg doped ZnO film were used as active layer and tested for its sensing performance to hydrogen. Compared to undoped ZnO, 22 at.% Mg doped ZnO film showed much higher sensor response to H₂ at a concentration as low as 200 ppm and at a lower operating temperature of 180°C. A linear sensor response was observed for H₂ concentration in the range of 100–500 ppm. Microsc. Res. Tech. 76:1118–1124, 2013. © 2013 Wiley Periodicals, Inc.     

Transmission electron microscopy and scanning force microscopy of poly r(A-U) and poly r(A-U)-ethidium bromide

Transmission electron microscopy and scanning force microscopy of negative-stained, carbon-coated replica and mica-adsorbed preparations of 200 μM poly r(A-U) and 50 μM ethidium bromide/200 μM poly r(A-U) have been employed to evaluate ethidium-induced changes in poly r(A-U) topology. Poly r(A-U) alone exhibits elongated conformations 85–115 nm in length that possess a number of hairpin loops as well as single-stranded domains. While the double-stranded domains are found predominately at the base of the hairpin loops (diameter = 5–30 nm), other rod-like (presumably double-stranded) regions ranging from 25–80 nm in length are present in other portions of the poly r(A-U). In contrast with the poly r(A-U) alone, the EB/poly r(A-U) combination appears as a heterogeneous population of condensed structures whose lengths and widths vary from 12–88 nm and 15–45 nm, respectively. These conformational changes are due to a number of factors, including the displacement of ordered water surrounding the poly r(A-U) and charge shielding of the phosphate groups of the poly r(A-U) upon the binding of the ethidium.     

Influence of the artificial saliva storage on 3‐D surface texture characteristics of contemporary dental nanocomposites

The aim of this study was to analyse the influence of the artificial saliva on a three‐dimensional (3‐D) surface texture of contemporary dental composites. The representatives of four composites types were tested: nanofilled (Filtek Ultimate Body, FUB), nanohybrid (Filtek Z550, FZ550), microfilled (Gradia Direct, GD) and microhybrid (Filtek Z250, FZ250). The specimens were polymerised and polished by the multistep protocol (SuperSnap, Shofu). Their surface was examined, before and after 3 weeks’ exposure to artificial saliva storage. The surface texture was analysed using the atomic force microscope (AFM). The obtained images were processed to calculate the areal autocorrelation function (AACF), anisotropy ratio S_tr (texture aspect ratio), and structure function (SF). The log–log plots of SF were used to calculate fractal properties, such as fractal dimension D, and pseudo‐topothesy K. The analysis showed changes in surface anisotropy ratio S_tr values, which became higher, whereas the S_q roughness (root‐mean‐square) reduced after the artificial saliva storage. All the samples exhibited bifractal structure before the saliva treatment, but only half of them remained bifractal afterwards (GD, FZ250), whereas the other half turned into a monofractal (FUB, FZ550). The cube‐count fractal dimension D_cc was found to be material‐ and treatment‐insensitive.     

基于纳米压痕技术和AFM的单晶铝硬度测试实验研究

利用纳米压痕技术对单晶铝作压痕试验,获得载荷-压深的加载和卸载曲线。根据O liver-Pharr方法求出压头与测试材料之间接触表面的投影面积Ac和硬度值Hop。再利用原子显微镜(atom ic force m icroscopy,AFM)得到压痕的真实三维形貌图。结合M atlab对压痕进行分析,得到压痕的真实残余面积Aresidual,并计算出其硬度Hresidual。通过对两组单晶铝的硬度数据进行比较分析:在微纳米尺度下,两种方法计算得到的压痕硬度都存在压痕尺寸效应,Hresidual的压痕尺寸效应比Hop要更明显。