A predictive analytical friction model from basic theories of interfaces, contacts and dislocations

Friction between crystalline bodies is described in a model that unifies elements of dislocation drag, contact mechanics, and interface theory. An analytic expression for the friction force between solids suggests that dislocation drag accounts for many of the observed phenomena related to solid–solid sliding. Included in this approach are strong arguments for agreement with friction dependence on temperature, velocity, orientation, and more general materials selection effects. It is shown that calculations of friction coefficients for sliding contacts are in good agreement with available experimental values reported from ultrahigh vacuum experiments. Extensions of this model include solutions for common types of dislocation barriers or defects. The effects of third-body solid lubricants, superplasticity, superconductivity, the Aubry transition, and supersonic dislocation motion are all discussed in the framework of dislocation-mediated friction.     

Advances and problems with TEM characterization of Cr/CrN multilayer coatings

Multilayer Cr/CrN/Cr/Cr(N,C) and Cr/CrN with 8 and 32 layer coatings were deposited on austenite substrates using pulsed laser deposition (PLD) technique. The microstructure observations were performed using Philips CM20?, TECNAI G² F20 – TWIN? and JEOL EX4000? transmission microscopes. The performed experiments indicated that lowering the argon flow from 60 to 30 cm³/s during chromium ablation changes buffer layers microstructure from nearly amorphous to nano‐crystalline. The nitride or carbo‐nitride layers turned out to be less sensitive to changes in nitrogen flow during deposition. The columnar microstructure of Cr layers is coarser than those in CrN ones under the same deposition condition. This observation proved also that relying on PLD technique as thin as 30 nm layers might be formed within multilayer Cr/CrN coatings.     

Microscopy and formation of polymer/metal composites

A new class of materials, formed by dispersion of low-melting-point metal alloys in a polymer matrix, has been studied from the point of view of microstructure, interfacial interaction and mechanical properties. The phases in these composites were formed in the same way as for polymer blends and were thus dependent on viscosity ratio, concentration, surface tension and interfacial interactions. Metal alloys of tin and bismuth (Sn/Bi) were mixed with high-density polyethylene (HDPE) and polystyrene (PS) at elevated temperatures. Some preliminary investigations of lead and tin (Pb/Sn) alloys blended with HDPE, PS, polypropylene (PP), polyoxymethylene (POM), polyethylene-terephtalate (PET), polymethylmethacrylate (PMMA), polycarbonate (PC), polyvinylidenefluoride (PVDF) and Polyvinylchloride (PVC) were also undertaken. The composites were characterized by light and electron microscopy, image analysis, electrical conductivity measurements and impact testing. It is shown that the low-melting-point metal alloys can be dispersed in polymers to a submicrometre level by blending. The particle size distribution follows an exponential function, which means that very fine as well as large particles are present. The equilibrium between dispersion and coalescence is very rapidly established during mixing. The average particle size can be controlled by the properties of the matrix, concentration of the metal and processing conditions. An investigation of interfaces revealed that in some cases a chemical interaction between the metal and the polymer can occur. This is apparent by observation of degradation, fluorescence and changes in mechanical properties.     

Non-coating fixation techniques or redundancy of conductive coating, low kV FE-SEM operation and combined SEM/TEM of biological tissues

Non-coating fixation methods, in particular the tannic acid/arginine/osmium tetroxide procedure, are employed for a number of reasons on the guinea-pig organ of Corti hair cell stereocilia glycocalyx and the imprints of the stereocilia at the bottom side of the tectorial membrane, and on the rat and cat intestinal epithelial microvilli glycocalyx and mucus-producing goblet cells.
These methods are used firstly to confirm that non-coating prepared specimens can be embedded for TEM observation at 60–100 kV without loss of detail information, and these images can be compared with cryo-FE-SEM images of the same structure/tissue. Secondly, they show that specimens treated according non-coating techniques become optimally preserved and electrically conductive, so that no external conductive coating is required. In this way a comparison of images of subsequent fresh fracture faces is possible without a decrease in information on detail, which otherwise could happen after subsequent coating layers required after standard fixation. Thirdly, they show that non-coating methods can be used quite well with low accelerating voltages because the osmium–tannic acid complex in the specimen surface produces a large number of backscattered and secondary electrons in the surface layer, showing in particular surface phenomena. Fourthly, they show that with an optimal non-coating preservation, in combination with a well-balanced pre-fixation mixture, preparation artefacts due to extraction and even dehydration and drying are minimized. This is compared with images of the organ of Corti hair cells treated with a so-called three-aldehyde pre-fixation mixture, which causes disrupted stereocilia to cling onto the bottom side of the tectorial membrane.     

Comparing electron tomography and HRTEM slicing methods as tools to measure the thickness of nanoparticles

Nanoparticles’ morphology is a key parameter in the understanding of their thermodynamical, optical, magnetic and catalytic properties. In general, nanoparticles, observed in transmission electron microscopy (TEM), are viewed in projection so that the determination of their thickness (along the projection direction) with respect to their projected lateral size is highly questionable. To date, the widely used methods to measure nanoparticles thickness in a transmission electron microscope are to use cross-section images or focal series in high-resolution transmission electron microscopy imaging (HRTEM “slicing”). In this paper, we compare the focal series method with the electron tomography method to show that both techniques yield similar particle thickness in a range of size from 1 to 5 nm, but the electron tomography method provides better statistics since more particles can be analyzed at one time. For this purpose, we have compared, on the same samples, the nanoparticles thickness measurements obtained from focal series with the ones determined from cross-section profiles of tomograms (tomogram slicing) perpendicular to the plane of the substrate supporting the nanoparticles. The methodology is finally applied to the comparison of CoPt nanoparticles annealed ex situ at two different temperatures to illustrate the accuracy of the techniques in detecting small particle thickness changes.     

Platinum/iridium/carbon: a high-resolution shadowing material for TEM,STM and SEM of biological macromolecular structures

Thin Pt/Ir/C coating films (1.5 nm) show a fine granularity and provide a high structural resolution in the transmission electron microscope (TEM) when applied to freeze-dried biological macromolecules. They keep their structure when exposed to atmospheric conditions, without the need of an additional stabilizing carbon layer, in contrast to conventional high-resolution shadowing materials such as Ta/W and Pt/C. However, the correct ratio of the components has turned out to be crucial. When evaporating Pt/Ir/C from the source electrode in an electron-beam-heated evaporator, the ratio of the three elements changes progressively, and, consequently, the properties of such films depend strongly on the mass that has been pre-evaporated. In this paper we present a quantitative analysis of the composition of Pt/Ir/C films by wavelength-dispersive X-ray analysis (WDX) undertaken in association with TEM experiments. We applied Pt/Ir/C shadowing to two regular biological test specimens, the phage T4 type III polyhead and the HPI-layer of Deinococcus radiodurans. It turns out that Pt/Ir/C films containing at least 25% C are three-dimensionally stable on the freeze-dried macromolecular samples. By the dramatically improved resolution power of the latest scanning electron microscopes (SEM) and the invention of the scanning tunnelling microscope (STM), two new surface-sensitive tools for the investigation of biological macromolecular structures became available. The Pt/Ir/C coating has proved to be well suited for STM and SEM imaging of freeze-dried biological structures because of its good electrical conductivity and its direct three-dimensional stability. We compare STM, SEM and TEM images of freeze-dried and Pt/Ir/C-coated polyheads.     

基于Delmia/Quest的钣金零件生产线的仿真与分析

利用Delmia/Quest仿真平台,以某钣金厂蒙皮拉伸成形生产线为例阐述了仿真分析的方法和关键步骤,对其进行仿真与分析,找出生产中的瓶颈点,并进行改进方案的分析与验证。通过仿真直观地反映了生产线上制造资源的分配及利用率,为部门管理和决策提供了定量的分析依据。     

基于Internet/Intranet在线监测系统人机交互界面的设计与实现

结合基于Internet/Intranet在线监测系统远程人机交互图形界面特性与开发实例,该文着重阐述了在这种情况下用Flash技术进行在线监测系统人机交互图形界面设计的方法与实现方式,最后讨论了该技术的发展前景。     

Study on a linear relationship between limited pressure difference and coil current of on/off valve and its influential factors

On/off solenoid valves with PWM control are widely used in all types of vehicle electro-hydraulic control systems respecting to their desirable properties of reliable, low cost and fast acting. However, it can hardly achieve a linear hydraulic modulation by using on/off valves mainly due to the nonlinear behaviors of valve dynamics and fluid, which affects the control accuracy significantly. In this paper, a linear relationship between limited pressure difference and coil current of an on/off valve in its critical closed state is proposed and illustrated, which has a great potential to be applied to improve hydraulic control performance. The hydraulic braking system of case study is modeled. The linear correspondence between limited pressure difference and coil current of the inlet valve is simulated and further verified experimentally. Based on validated simulation models, the impacts of key parameters are researched. The limited pressure difference affected by environmental temperatures is experimentally studied, and the amended linear relation is given according to the test data.     

基于Pro/E的带线缝合针自动打孔夹持机械手设计及仿真分析

针对医用缝合针尾部同轴装线孔的孔径微小、同轴度要求高以及人工生产效率低下的问题,设计了一种由四槽轮驱动的间歇性同步送料及夹持机械手。给出了夹持机械手开合角度与自身结构参数以及凸轮廓线大、小半径差的关系。通过Pro/E软件进行机构三维建模并利用仿真功能进行了机构运动仿真,输出夹持机械手与送料盘及待打孔缝合针配合关键点位置坐标曲线,并输出具体曲线数据检查了系统设计误差,为带线缝合针自动打孔机整机设计奠定了基础。     

高速铣削高体积分数SiCp/Al复合材料表面形貌及切屑机制的研究

为了研究高体积分数SiCp/Al复合材料的切削机理及工件表面形貌,采用PCD刀具对干式切削和水溶性冷却液浇注冷却的湿式切削两种切削条件下的高速铣削进行了研究。结果表明,在对颗粒尺寸大、体积分数高的SiCp/Al复合材料进行高速铣削时,干式切削无论是在工件已加工表面形貌和微观结构,还是在切屑形成及形貌上,都好于湿式切削。两种切削条件下均可获得较理想的表面粗糙度。