Change in the electrical resistance of the metallic composite material-steel contact under friction and electric current

The I–V characteristics of the sliding contact of metallic composites of grade 45 steel without a lubricant are presented. Steel-based composites are shown to increase the actual electric-contact area due to the appearance of electric discharges, which provide the main passage of an electric current with a density up to 300 A/cm². Copper-based composites cannot initiate electric-discharge conduction because of the fracture of the contact zone material at a current density higher than 50 A/cm². The electrical resistivity of the contact layer of metallic composites is calculated. It is found that, during friction with a high current density, the electrical resistivity of the contact layer approaches the electrical resistivity of graphite. It is experimentally shown that the actual electric-contact area can be increased by the introduction of a Pb-Sn melt into the friction zone and reaching a current density higher than 300 A/cm² in the contact.     

Evaluation of surface characteristics under fretting of electrical contacts: Removal behaviour of hot dipped tin coating

The fretting corrosion behaviour of hot dipped tin coating is investigated at low fretting cycles at ±25 μm displacement amplitude, 0.5N normal load, 3 Hz frequency, 45-50% relative humidity, and 25 ± 1 °C temperature. The typical characteristics of the change in contact resistance with fretting cycles are explained. The fretted surface is examined using laser scanning microscope, scanning electron microscope and energy dispersive X-ray analysis to assess the surface profile, extent of fretting damage, extent of oxidation and elemental distribution across the contact zone. The interdependence of extent of wear and oxidation increases the complexity of the fretting corrosion behaviour of tin coating. The variation of contact resistance clearly revealed the fretting of tin coating from 50 to 1200 cycles and the fretting of the substrate above 1200 cycles. The observed low and stable contact resistance region and the fluctuating resistance region at various fretting cycles are explained and substantiated with Scanning electron microscopy (SEM), laser scanning microscope (LSM) and energy dispersive analysis of X-rays (EDAX) analysis results of the fretted surface.     

Tribological Properties of Surface-Modified Graphene Filled Carbon Fabric/Polyimide Composites

To improve the wear resistance of carbon fabric reinforced polyimide (CF/PI) composite, surface-modified graphene (MG) was synthesized and employed as a filler. The flexural strength, Rockwell hardness and thermal properties of the composites were tested. The composites were also evaluated for their tribological properties in a ring-on-block contact mode under dry sliding conditions. The results showed that the wear rate of MG reinforced CF/PI composites was reduced when compared to unfilled CF/PI composite. It was found that the 1?wt% MG filled CF/PI composites exhibited the optimal tribological properties. The worn surface, wear debris and transfer films were analyzed by scanning electron microscopy (SEM) and optical microscopy (OM) with the results helping to characterize the wear mechanism.     

Effects of Aramid Fiber and Polytetrafluoroethylene on the Mechanical and Tribological Properties of Polyimide Composites in a Vacuum

Polyimide composites filled with aramid fiber (AF) and polytetrafluoroethylene (PTFE) were prepared by hot press molding. The thermal, mechanical, and tribological properties of the composites were studied systematically. The friction and wear behavior, sliding against GCr15 steel balls, were evaluated in a ground-based wear in space simulation facility using a ball-on-disk tribosystem. The morphologies of the worn surfaces during the sliding process of the composites were analyzed by scanning electron microscopy to reveal the wear mechanism. It was found that the heat-resisting performance and the hardness of the composites were minimally affected by the additives. The flexural strength of polyimide/AF/PTFE (PI-3) decreased when PTFE was added. The wear resistance increased and the coefficient of friction decreased due to the effect of both fillers. In vacuum, the friction coefficients of polyimide (PI-1), polyimide/AF (PI-2), and PI-3 increased slightly with sliding velocity, while the opposite results were obtained in air. With the increase of air pressure the friction coefficients of the samples increased.     

Abrasive Wear Behavior of LDPE Filled with Silane Treated Flyash Cenospheres

Lightweight, high mechanical strength insulating materials exhibiting high resistance to corrosion, solvents and abrasive wear are desired for wire and cable insulation as well as protection. Polyethylenes are generally used for such applications owing to their good electrical insulation properties and being inert to solvents at room temperature. However, their abrasion resistance is quite poor. Hence, in the present work, an attempt has been made to improve the abrasive wear resistance of low-density polyethylene (LDPE) by incorporating hollow microspheres, known as cenospheres, in the base polymer to form composites. These cenospheres are obtained from flyash particles, a thermal power plant waste, and do not tend to increase the weight of the polymer composite when used as a filler. The composites were developed by changing the weight fraction of untreated as well as silane treated cenospheres to the extent of 5 wt%. Tribological characterization of these composites was done in abrasive wear mode by varying the operating parameters, such as speed and sliding distance against silicon carbide paper. It was found that 10 wt% silane treated cenosphere filled LDPE composite showed the maximum wear resistance (～×10^?11 m³/N m) among the six composites. However, a further increase in filler concentration decreased the wear resistance. The improvement in wear resistance was supported by scanning electron microscopy and attributed to the strong interaction between silane treated cenosphere and LDPE molecules which resisted the elongation and shearing of polymer chains by the abrasive grits.     

Superconducting properties of tin embedded in nanometer-sized pores of glass

The electrical resistance of tin embedded from a melt in porous glasses with an average pore diameter of ??7 nm has been investigated at low temperatures in magnetic fields up to 2 T. The temperatures of the transition to the superconducting state for nanocrystalline tin have been determined in magnetic fields of 0, 0.3, 0.5, 1.0, 1.5, and 2.0 T. It has been found that the temperature and magnetic-field dependences of the electrical resistance of the nanocomposite under investigation exhibit two transitions to the superconducting state. The nature of the double superconducting transitions has been discussed. The H _c -T _c phase diagram has been constructed using the entire set of data on the magnetic-field and temperature dependences of the electrical resistance of nanostructured tin. This phase diagram indicates that the upper critical magnetic field H _c2(0) for nanostructured tin is almost two orders of magnitude higher than the corresponding field for bulk tin.     

高温超导直接冷却的氮化铝与无氧铜接触界面热阻的实验研究

氮化铝(AlN)具有高热导性、高电绝缘性,是超导二元电流引线热截流结构中常用的材料之一。根据稳态导热法建立低温真空实验装置,实验研究了超导冷却系统热截流结构中,界面温度和接触压力对AlN块材与无氧铜(OFHC-Cu)块材间接触界面热阻的影响。在实验温度(90K-210K)和压力(0.273MPa-0.985MPa)条件下,AlN/OFHC-Cu接触界面热阻随接触压力的提高而降低,而当界面温度上升时界面热阻由于热载子热运动的强化而降低,温度较高时,接触界面热阻随压力变化的速率趋缓。低温下AlN/OFHC-Cu间的接触界面热阻是直接冷却超导系统的设计和超导系统的热稳定性方面必需解决的问题。     

ITO退火对GaN基LED电学特性的影响

近些年来,越来越多的发光二极管采用铟锡氧化物(ITO)作为电流扩展层,但是如果不对其进行任何处理,得到的发光二极管的电学特性很差,要得到好的电学特性需要对长有铟锡氧化物的发光二极管进行退火处理.针对不同的退火时间和退火温度对发光二极管的电学特性影响不同的问题,通过测量不同条件下退火得到的发光二极管的理想因子和串联电阻, 根据Shah等人提出的模型进行分析,推测出铟锡氧化物和P型氮化镓的接触特性.结果表明:发光二极管的电学特性开始随着退火温度的升高和时间的增加到达一个优值,如果继续增加温度或者时间都会导致发光二极管电学特性的下降.这样有利于优化退火温度和时间, 得到电学性能较好的器件.     

Simultaneously Improving Electrical Properties and Stabilizing Contact Resistance of Electrically Conductive Adhesives by Using Aminoaldehydes

In order to simultaneously improve the conductivity and stabilize the contact resistance of electrically conductive adhesives (ECAs) containing silver flakes, different types of aminoaldehydes as multifunctional additives, N, N-dimethyl-4-aminobenzaldehyde (DABA), benzaldehyde (BA) as a comparison, and formamide (FA), were introduced into ECAs formulations. The results showed that when the optimal levels of FA, BA, and DABA were individually added into ECAs, the decrease in the resistivities of the ECAs were 34.8%, 67.2%, and 41.7%, respectively, compared to the control sample. This is because, during the curing process, the aldehyde acts as a reducing agent and reduces the oxidized silver flakes. Furthermore, the ECAs with FA or DABA had better contact resistance stability than that of the control sample under the condition of 85°C/85% relative humidity, but ECAs with BA could not stabilize the contact resistance on tin finish, The results indicate that FA and DABA can be adsorbed onto the tin surface and inhibit the occurrence of the galvanic corrosion on the interface between ECA and tin finish. The rheological results showed the processability of ECAs with FA, BA, and DABA were better than that of the control sample. However, the shear strength of ECAs with BA and DABA slightly declined.     

Effect of Organic-Mo on the Wear Behavior of Phenolic Resin Composites

Abstract

Organic molybdenum (e.g., molybdenum dialkyldithiocarbamate, Mo-DTC) is a typical additive for liquid lubricants which can produce a significant anti-wear role with only a minor addition. In this article Mo-DTC additives were used to enhance the wear resistance of a phenolic resin. Phenolic/Mo-DTC composites with various Mo-DTC filler ratios were prepared by hot-press molding. The hardness and wear performance of the composites were measured by a Shore durometer and by an M-2000 friction and wear tester, respectively. The results showed that after adding the Mo-DTC filler, both the hardness and the anti-wear properties of the composites materials were improved. Under the condition of high-speed with a smooth ring, the wear scar length for the sample with 1% Mo-DTC content decreased by 45.6% compared with that of the neat phenolic resin whereas for a wire rope ring a decrease of 16% was observed for 0.5% Mo-DTC. Based on the wear morphology of the composites, the wear mechanisms of the designed phenolic/Mo-DTC composites were determined.     

D2钢系列涂层磨损性能的数值模拟

采用真空离子镀的方式在AISI D2钢基体上制备渗N,TiN及复合涂层,并采用模拟和实验的方法研究涂层的磨损性能.将Archard经典粘着磨损模型离散化后用Fortran子程序的形式嵌入到商业化的有限元软件中,以求解接触摩擦过程中接触区每个节点或单元的磨损深度,定量分析了基体及三种涂层的磨损深度.研究结果表明,复合涂层具有最优耐磨性能.实验结果与模拟结果相符合. 关键词： 磨损 数值模拟 渗N TiN     

Current instability with an S -shaped current-voltage characteristic in thin films of composites based on polymers and inorganic particles

The switching and memory effects, which manifest themselves in the transition from a high-resistance state to a low-resistance state, are observed in thin films of composites based on polymers (carbazole derivatives) and silicon inorganic particles. It is established that, at small thicknesses of composite films, the resistance (with an initial value of no less than 100 MΩ) upon transition from the high-resistance state to the low-resistance state is changed by three or four orders of magnitude. This transition in the composites under investigation is accompanied by the appearance of an S-shaped current-voltage characteristic and its hysteresis. It is demonstrated that the observed effects are associated with the specific features of the thermomechanical and electrical properties of the polymer in the composite material. Original Russian Text ? é.A. Lebedev, E.L. Alexandrova, A.N. Aleshin, 2009, published in Fizika Tverdogo Tela, 2009, Vol. 51, No. 1, pp. 195–197.     

Raman and point contact current-voltage characterization of laser-induced diffusion in GaAs

Raman scattering and point contact current-voltage (PCIV) measurements were used as characterization tools of tin-diffused GaAs layers. Diffusion was induced by irradiating GaAs substrates covered with thin tin layers single pulses of a ruby laser. Samples processed with the lowest energies show strong damage and incomplete electrical activation as deduced from Raman and PCIV measurements, respectively. Raman microprobe in depth analysis and PCIV profiles also suggest the presence of a damaged region with incomplete electrical activation at the boundary between the molten layer and the solid substrate.     

Influence of laser treatment on the wear of friction surfaces of 40Cr steel

The friction wear of surfaces of 40Cr (40Khr) structural steel and 12Cr18Ni10Ti (12Kh18N10T) stainless steel is investigated. It is established, by comparison of the wear of initially annealed 40Cr steel after hardening by radiation from a CO₂ laser to various degrees of surface microhardness and separately after volume heat treatment, that the wear is due to fatigue in the entire range of microhardness, regardless of the dispersive properties of the structures. It is shown that the resistance to wear tends to increase with increase of the microhardness of the bearing surfaces. The dependence of the rate of wear on the surface microhardness is obtained. It is found that the wear process is accompanied by formation of a special structurally stressed state in the Saint-Venant region; this state is characterized by a constant hardness level independently of the preceding state of the material.Translation of Preprint No. 196, Lebedev Institute of Physics, Academy of Sciences, USSR, Moscow (1990).     

Point contact spectroscopy of U2Zn17

We report on point contact measurements on the heavy fermion system U₂Zn₁₇. It is shown that the first derivative of the current-voltage characteristic of low resistance contacts is very sensitive to the magnetic phase transition. A point contact theory applicable to heavy fermion compounds is still missing. The results are discussed in the frame of the heating model, which includes electrical and thermal resistivity as well as thermopower effects.     

Development and characterization of alkali treated abaca fiber reinforced friction composites

Abaca fibers show tremendous potential as reinforcing components in composite materials. The purpose of this study is to investigate the effect of abaca fiber content on physical, mechanical and tribological properties of abaca fiber reinforced friction composites. The friction composites were fabricated by a compression molder and investigated using a friction test machine. The experiment results show that surface treatment of abaca fibers could improve the mechanical properties of abaca fiber and interface bonding strength of the abaca fiber and composite matrix. Density of friction composites decreased with the increasing of abaca fiber content (0 wt%–4 wt%). The different content of abaca fibers had less effect on hardness of specimens, whereas large of impact strength. The specimen F3 with 3 wt% abaca fibers had the lowest wear rate and possessed the best wear resistance, followed by specimen F4 with 4 wt% abaca fibers. The worn surface morphologies were observed using the Scanning Electron Microscopy for study the tribological behavior and wear mechanism. The results show that a large amount of secondary contact plateaus presented on the worn surface of specimen F3 which had relatively smooth worn surface.     

Switching and memory effects governed by the hopping mechanism of charge carrier transfer in composite films based on conducting polymers and inorganic nanoparticles

The switching and memory effects in composite films based on conducting polymers [poly(phenylenevinylene), thiophene, and carbazole derivatives] and inorganic nanoparticles (ZnO, Si) are investigated. It is established that the introduction of inorganic nanoparticles (ZnO, Si) exhibiting strong acceptor properties into polymer materials leads to the appearance of memory effects, which manifest themselves in the transition of the polymer from a low-conductivity state to a high-conductivity state. For a number of composites, this transition is accompanied by the formation of a region with a negative differential resistance and a hysteresis in the current-voltage characteristics. It is demonstrated that the observed effects are determined by the mechanism of charge carrier transfer in the composite. In particular, the main mechanism of transport in films based on thiophene derivatives is associated with electrical conduction due to the tunneling of charge carriers between conducting regions embedded in a nonconducting matrix, whereas the dominant mechanism of transport in “polymer-semiconductor nanoparticle” composite films is hopping conduction, which is responsible for the effects observed in these objects.     

Formation of metal and alloy surface layers at electroexplosive alloying

The structure-phase peculiarities of the surface layers, formed at electroexplosive alloying of steels and alloys with Al, Cu, B + Cu, C, and C + B have been studied by metallography and scanning and transmission electron microscopy. The treatment involving melt overheating under plasma jet pressure and subsequent cooling at a high rate is established to form nanosized phases. It is shown that different forms of electroexplosive treatment lead to the simultaneous increase in the microhardness, wear resistance, and heat resistance of the steel and alloy surface.     

Effect of the size misfit of atoms of binary copper alloys on the friction fracture strength

The fracture strength of copper-based binary substitutional solid solutions was studied under dry friction. It is shown that an essential factor influencing the wear resistance of copper alloys is a local internal distortion of the crystal lattice due to the difference between the atomic radii of the basis metal and the alloying element. It is found that the wear resistance of a substitutional solid solution can be increased by substituting an alloying element with a larger atomic radius for copper atoms and can be decreased in the opposite case. A model is proposed relating the experimental dependences of the wear characteristics of alloys to a change in the interatomic bonding strength caused by doping of the matrix with atoms of various radii.     

Studies on wettability, mechanical and tribological properties of the polyurethane composites filled with talc

A series of polyurethane (PU)/talc composites modified by a high molecular weight hydroxyl-terminated polydimethylsiloxane (HTPDMS) were prepared. The effect of the talc content on the mechanical, wettability and tribological properties of the PU composites was studied. Tensile strength of the PU composites reached to the maximum after adding 5% talc. The water contact angles (CA) of the original surfaces and worn surfaces of the polyurethane composites were measured. The experimental results indicated that the contact angles of the worn surface increased after friction. The friction and wear experiments were tested on a MRH-3 model ring-on-block test rig at different sliding speeds and loads under dry sliding and water lubrication. Experimental results revealed that the talc contributed to largely improve the tribological properties of the PU composites. The coefficient of friction (COF) of the composites increased with increasing talc. Scanning electron microscopic (SEM) investigations showed that the worn surfaces of the talc filled PU composites were smoother than pure polyurethane under given load and sliding speed.