Simulational study of electrical contact degradation under fretting corrosion

The simulation model of electrical contact resistance variation under various oxide fractions is constructed considering thermal-electrical coupled effects. The copper oxide is allocated on the contact area with various fractions by random distribution technique with finite element method. The contact degradations of experimental and analytical results are compared. The quantitative relation between insulation fraction and electrical resistance increment is analyzed.     

Cutter mark cross method for improvement of contact stiffness by controlling distribution of real contact area

Contact surfaces do not make contact perfectly because such surfaces have a lot of asperities. The real contact area is much smaller than the nominal contact area, and the real contact areas has a non-uniform distribution because of the waviness in the contact surface. The contact stiffness is influenced not only by the deformation of the asperities, but also by the distribution of the real contact areas. In general, a contact surface with a uniform distribution of the real contact areas has greater contact stiffness. However, this requires a grinding finish and costs more than the cutting finish. In this study, a method for uniformly distributing the real contact areas easily, is proposed to improve the contact stiffness of a contact surface finished by cutting. The method is called the cutter mark cross (CMC) method. The allowable waviness in the CMC method is shown. In addition, the effect of the CMC method is investigated by experimentation. The results show that the real contact areas can be distributed uniformly using the CMC method. The horizontal and vertical contact stiffness can also be improved.     

隔离开关自力型触头通流能力改进与优化

以某816kV特高压直流隔离开关触头为研究对象,通过分析、计算,提出了增加接触点数量和保持适当径向压力的改进方法.试验结果表明,按照该思路确定的改进方案可以明显降低接触部位的接触电阻、提高触头通流能力.     

Observation of proportionality between friction and contact area at the nanometer scale

The nanotribological properties of a hydrogen‐terminated diamond(111)/tungsten‐carbide interface have been studied using ultra‐high vacuum atomic force microscopy. Both friction and local contact conductance were measured as a function of applied load. The contact conductance experiments provide a direct and independent way of determining the contact area between the conductive tungsten‐carbide AFM tip and the doped diamond sample. We demonstrate that the friction force is directly proportional to the real area of contact at the nanometer‐scale. Furthermore, the relation between the contact area and load for this extremely hard heterocontact is found to be in excellent agreement with the Derjaguin–Müller–Toporov continuum mechanics model. This revised version was published online in September 2006 with corrections to the Cover Date.     

Experimental setup for non-destructive measurement of tunneling currents in semiconductor devices

A new experimental setup used to perform non-destructive measurement of electrical quantities on semiconductor devices is described in this paper. The particular case of tunneling current measurement in n-type semiconductor–oxide–semiconductor (SOS) capacitors, whose dielectrics play a crucial role in non-volatile memories, has been investigated. When the gates of such devices are polarized with a sufficient bias voltage while the other terminals are grounded, tunnel conduction of electrons through the thin oxide layer is allowed. Typical tunneling current measurements obtained with this advanced setup are presented and compared to the results yielded by older standard experimental protocols. An application to the experimental observation of the temperature dependence of the tunneling current is proposed. Conclusions about the benefits of this kind of electrical measurements are then drawn.     

Evaluation of the real contact areas, pressure distributions and contact temperatures during sliding contact between real metal surfaces

A three-dimensional elastic contact algorithm has been developed to analyse the normal contact problems of bodies having rough surfaces. The algorithm can evaluate the real contact areas and contact pressure distributions using measured surface roughness data.

Following an approximate elastic-plastic contact solution the analysis produces more realistic elastic and plastic contact areas; in addition results of contact pressure distributions can be predicted according to a given maximum plastic limit pressure.

The technique can simulate (in an approximate way) the elastic-plastic sliding contact behaviour in the vicinity of asperities or concentrated contact areas by ignoring the effect of the tangential forces on the vertical displacement.

Assuming a certain sliding speed and a particular coefficient of friction the local temperature distribution due to the heat generation over the real contact areas can also be calculated for 'slow sliding' problems.

The results show the moving real contact areas and the contact temperature fields for an electric spark mechanical steel surface moving over a planed bronze surface. Changes of the rigid body displacement, as well as the average and maximum pressures are also presented during sliding.

The micro-contact or asperity contact behaviour for bodies having large nominal contact area and the macro-contact behaviour for bodies being in 'concentrated contact' are also compared. In the latter case an ideal smooth steel ball was slid over the previously mentioned bronze surface.     

Design, manufacture, and analysis of metal foam electrical resistance heater

This paper presents a novel concept using the radial heating element made from porous Fe–Cr–Al metal foam in an air heater. Electrical resistance heating has been used extensively to convert the electrical energy into thermal energy. An analytic heat transfer model is first developed to estimate dimensions of the heating element. Four prototype Fe–Cr–Al metal foam electrical heaters with different levels of porosity and density are built. A more detailed computational fluid dynamics modeling of prototype heaters to include the temperature loss to the surroundings is developed. Experiments are conducted to evaluate the effects of airflow rates and electrical current and measure the change of air inlet and outlet temperatures. The experimental temperature measurements show reasonably good agreement with modeling predictions. Finally, improvements to the initial concept are discussed.     

Improvement of EDM efficiency of silicon single crystal through ohmic contact

This paper describes the improvement of the machining rate of electrical discharge machining (EDM) for silicon single crystals by reducing the contact resistance between the silicon single crystal and metal electric feeder. To decrease the resistance of the rectifying contact between a p-type silicon wafer and the metal feeder, attempts to achieve ohmic contact were made by plating the contact surface of the silicon wafer with aluminum by vacuum evaporation, followed by the diffusion process. To accomplish an ohmic contact between n-type silicon and metal, gold–antimony alloy was used in place of aluminum. The influence of polarity on the machining rate is also discussed from the viewpoint of the rectifying nature of the interface between the arc plasma and silicon single crystal.     

Flexible contact stamp for electrical conductivity measurements of a soft matter

In this communication, a simple homemade four probe conductivity setup has been presented to measure the electrical conductivity of molecularly grafted and passivated solid surfaces. Setup was also extended for the temperature dependent conductivity measurements by designing a small furnace using nichrome heating element. The importance of prepared setup lies in its easy designing ability, portability and reliability to measure the conductivity of soft matters. Setup was designed for four probe conductivity measurements which can be used for two probes conductivity measurements also, depending upon the requirements under investigation. Setup was tested using its four probes for electrochemically grafted indium tin oxide surfaces with thiol containing organic molecules and trichloroethylene passivated Si surface. It was found very sensitive even for observing small changes in current–voltage values whereas recorded curves were quite reproducible.     

A numerical-analytical approach to determining the real contact area of rough surface contact

ABSTRACT

The objective of this study is to improve the accuracy of the real contact area calculated by the semi-analytical method (SAM). Two types of surface pairs are investigated: an analytically generated sinusoidal wavy surface against a rigid flat, and a pair of real rough surfaces. The results suggest that the real contact area calculated by the SAM is extremely sensitive to the resolution of input, i.e. the grid size. The SAM results of the real contact areas show poor convergence, especially in the case of the real rough surfaces. The main reason for this difference is the ‘over-covering’ effect when SAM calculates the real contact area. An exponential extrapolation technique is proposed to predict the real contact area values when further refinement of the grid resolution is unfeasible.     

Effect of electrical current on the tribological behavior of the Cu-WS2-G composites in air and vacuum

As the traditional graphite-based composites cannot meet the requirement of rapid developing modern industry, novel sliding electrical contact materials with high self-lubricating performance in multiple environments are eagerly required. Herein a copper-based composite with WS2 and graphite as solid lubricant are fabricated by powder metallurgy hot-pressed method. The friction and wear behaviors of the composites with and without current are investigated under the condition with sliding velocity of 10 m/s and normal load of 2.5N/cm 2 in both air and vacuum. Morphologies of the worn surfaces are observed by optical microscope and compositions of the lubricating films are analyzed by XPS. Surface profile curves and roughness of the worn surfaces are obtained by 2205 surface profiler. The results of wear tests show that the friction coefficient and wear volume loss of the composites with current are greater than that without current in both air and vacuum due to the adverse effects of electrical current which damaged the lubricating film partially and roughed the worn surfaces. XPS results demonstrate that the lubricating film formed in air is composed of oxides of Cu, WS2 , elemental S and graphite, while the lubricating film formed in vacuum is composed of Cu, WS2 and graphite. Because of the synergetic lubricating action of oxides of Cu, WS2 and graphite, the composites show low friction coefficient and wear volume loss in air condition. Owing to the fact that graphite loses its lubricity which makes WS2 become the only lubricant, severe adhesive and abrasive wear occur and result in a high value of wear rate in vacuum condition. The formation of the lubricating film on the contact interface between the brush and ring is one of the factors which can greatly affect the wear performance of the brushes. The low contact voltage drop of the composites in vacuum condition is attributed to the high content of Cu in the surface film. This study fabricated a kind of new sliding electrical contact self-lubricating composite with dual-lubricant which can work well in both air and vacuum environments and provides a comprehensive analysis on the lubrication mechanisms of the composite.     

Estimation of real contact area in a wheel-rail system by means of ultrasonic waves

The application of an ultrasonic technique to estimate the Real Contact Area (RCA) in a wheel-rail system is proposed. The method is based on the analysis of the reflection of ultrasonic waves which are sent over the contact region and reflected by it according to the state of contact. The interaction of the experimental data with a theoretical model formulated in the early 1990s by Krolikowski allows us to calculate the RCA for the area illuminated by the ultrasonic beam. By varying the external load and the surface conditions of the contacting elements, it is possible to obtain the trend of the RCA with increasing load (for a certain roughness) as well to explore changes in RCA for different roughness while the load is kept fixed. Results showed that RCA grows almost linearly with the applied load and that an increase of one order of magnitude in combined roughness of the wheel-rail system leads to a roughly sevenfold reduction in RCA under a given load.     

双圆弧齿轮齿腰曲率半径对齿腰应力的影响

基于任意转角位置的双圆弧齿轮齿廓数学模型,构建了双圆弧齿轮齿腰过渡曲线曲率半径的计算数学模型,提出了齿腰任一点局部应力的折截面法计算数学模型,验证了两数学模型的准确性,并应用这两个数学模型分析讨论了不同参数条件下曲率半径对齿腰应力的影响。其分析结果将为双圆弧齿轮设计、参数优化选择等提供参考依据。     

Real area of contact and boundary friction in metal forming

Upper-bound models for asperity flattening on a workpiece surface undergoing bulk plastic deformation are developed. It is found that the effective hardness of the surface can be greatly reduced by the presence of underlying plastic flow. Theoretical predictions of the variation of real area of contact with strain show excellent agreement with experiments using model asperities in rolling. Friction models which allow for the reduction in effective hardness are developed for cases in which roughness is concentrated on either the workpiece or tooling.     

Study of unidirectional conductivity on the electrical discharge machining of semiconductor crystals

This paper investigates the unidirectional conductivity of semiconductor crystals machined by electrical discharge machining (EDM) by analyzing the properties of current–voltage (I–V) curves of the equivalent circuit. The simplified equivalent circuit of a semiconductor EDM consists of reverse-biased diodes and linear resistance. The I–V curve has three typical parameters, namely, conduction angle, breakdown angle, and breakdown point. The values of the conduction angle and the breakdown point are determined by the contact area of the reverse-biased diode, and the breakdown angle is determined by the value of linear resistance. Two diodes exist in the model with two metal electric feeders. To increase the current in this model, the diode with larger contact area should be reverse-biased. If the work piece is connected to anode in semiconductor EDM, the diode in the conduction side is reverse-biased and the avalanche voltage is only 42 V. If the work piece is connected to the cathode in semiconductor EDM, then the arc plasma, which is a termination with a small area, becomes reverse-biased. The temperature in the arc plasma side is high, causing the breakdown voltage to be much higher than the theoretical calculation value 88.5 V. As a result, when the work piece is connected to the cathode, spark production is difficult. Holes are bored on the P-type semiconductor crystals by positive and negative polarity, which could prove that machining with positive polarity is suitable for P-type semiconductor crystals during EDM. When the no-load voltage is set to 150 V, the penetration speed by positive polarity can reach 533 μm/min.     

The dynamic contact area of elastomers at different velocities

The friction in tribo-systems that contain viscoelastic materials, such as elastomers, is relevant for a large number of applications. Examples include tyres, hoses, transmission and conveyor belts. To quantify the friction in these applications, one must first understand the contact behaviour of such viscoelastic materials, both in static and in dynamic situations. This work discusses an experimental study into the change of the contact area with the sliding velocity and relates the change in contact area with the mechanical properties of the elastomer. The results show that for a tribo-system containing an elastomer, there is a threshold velocity, above which the size of the contact area significantly reduces.     

Plastic deformation and contact area of an elastic–plastic contact of ellipsoid bodies after unloading

This paper presents theoretical and experimental results of the residual or plastic deformation and the plastic contact area of an elastic–plastic contact of ellipsoid bodies after unloading. There are three regime responses of the deformation and contact area: elastic, elastic–plastic and fully plastic. Experimental investigation is presented in order to validate the proposed model. A new technique is introduced to measure the plastic deformation and plastic contact area. Very good correlation is found between the theoretical prediction and the experimental results.     

Wear resistance of composites for sliding electrical contact from reprocessed bearing steel

The electrical contact characteristics are determined of model composites from bearing steel reprocessed from grinding waste. It is established that the tested sliding steel composites have a low contact electrical resistance and wear rate at current densities up to 300 A/cm². An assumption is made that efficient performance of these composites results from the superdispersed structure of the reprocessed bearing steel.     

对数螺旋锥齿轮齿面接触区的相关特性

介绍了对数螺旋锥齿轮轮齿接触检测的方法。在Y9550型锥齿轮滚动检查机上进行了对数螺旋锥齿轮样件齿面接触区的检测实验,通过实验获得了对数螺旋锥齿轮接触区的位置、形态和大小等结果,并将实验结果与格里森螺旋锥齿轮接触区进行了对比,得出了对数螺旋锥齿轮齿面接触区的相关特性。