Evolution Of Contact Resistance Research Articles

Electrical measurement method of static friction force on rough surface

Friction plays a key role in the assessment of the safety and stability of mechanical systems (such as superconducting magnet quench explosion, aerospace vehicle bearing wear, etc.). Due to the closeness of the interfaces in engineering structures and the randomness of the contact surfaces, existing methods for measuring static friction force are unable to measure it at the contact interfaces of engineering structures under service conditions. In this paper, a new method for measuring the static friction force at the interface based on electrical signals is proposed. This method enables the measurement of the static friction force at interfaces of complex engineering structures under service conditions solely through electrical signals. The results indicate that the contact resistance gradually decreases with the increase in tangential load during the static friction stage until a monotonic behavior of macroscopic sliding occurs. The evolution of contact resistance is linked to the evolution of the real contact area, and this monotonic behavior can be explained as the deformation form of contact points. The accuracy of the proposed electrical measurement method is verified by comparison with experimental results (with an error of less than 9%). The indirect measurement method of friction force proposed in this paper can effectively measure the static friction force at the interfaces of engineering structures under service conditions, and it is expected to be applied to the detection of friction performance at engineering structure interfaces in extreme service environments.

Electrical Contact Resistance Evolution and Interface Fretting Wear Characteristics of Conductor-Parallel Groove Clamp System Response to Breeze Vibration Environment

Electrical Contact Resistance Evolution and Interface Fretting Wear Characteristics of Conductor-Parallel Groove Clamp System Response to Breeze Vibration Environment

Wear Reduction via CNT Coatings in Electrical Contacts Subjected to Fretting

Carbon nanotubes (CNT) are of great interest to the research community due to their outstanding mechanical, transport, and optical properties. These nanoparticles have also shown exceptional lubricating capabilities, which coupled with their electrical conductivity show promising results as solid lubricants in electrical contacts. In this study, three different CNT coatings were deposited over copper platelets via electrophoretic deposition and subsequently tribo-electrically characterized including electrical contact resistance evolution during fretting wear, wear protection, chemical analysis of fretting marks, as well as influence of CNT coating thickness, duration and normal load applied during fretting, and atmospheric humidity. Thicker CNT coatings show improved wear protection while retaining similar electrical behavior as uncoated copper, or even improving its electrical contact resistance. Moreover, the compaction of the porous CNT coating is crucial for optimal electrical performance at low humidity. For longer fretting tests (150,000 and 500,000 cycles), the coatings are displaced thus affecting the wear protection offered. However, the coatings stabilize and reduce ECR compared to uncoated samples. Furthermore, thicker CNT coatings can bear higher loads during fretting due to the increased lubricant reservoir, with carbonaceous tribofilm remaining at the contacting interface after 5,000 fretting cycles regardless of normal load.Graphical

Resistive-nanoindentation on gold: Experiments and modeling of the electrical contact resistance.

This paper reports the experimental, analytical, and numerical study of resistive-nanoindentation tests performed on gold samples (bulk and thin film). First, the relevant contributions to electrical contact resistance are discussed and analytically described. A brief comparison of tests performed on gold and on natively oxidized metals highlights the high reproducibility and the voltage-independence of experiments on gold (thanks to its oxide-free surface). Then, the evolution of contact resistance during nanoindentation is fully explained in terms of electronic transport regimes: starting from tunneling, electronic transport is then driven by ballistic conduction before ending with pure diffusive conduction. The corresponding analytical expressions, as well as their validity domains, are determined and compared with experimental data, showing excellent agreement. From there, focus is made on the diffusive regime. Resistive-nanoindentation outputs are fully described by analytical and finite-element modeling. The developed numerical framework allows a better understanding of the main parameters: it first assesses the technique capabilities (validity domains, sensitivity to tip defect, sensitivity to rheology, effect of an oxide layer, and so on), but it also validates the different assumptions made on current line distribution. Finally, it is shown that a simple calibration procedure allows a well-resolved monitoring of the contact area during resistive-nanoindentation performed on samples with complex rheologies (ductile thin film on an elastic substrate). Comparison to analytical and numerical approaches highlights the strength of resistive-nanoindentation for continuous area monitoring.

ЧИСЛЕННАЯ РЕАЛИЗАЦИЯ РЕШЕНИЯ ДВУМЕРНЫХ ЗАДАЧ ТЕРМОВЯЗКОУПРУГОСТИ

This paper considers the numerical implementation of two-dimensional thermoviscoelastic waves. The elastic collision of an aluminum cylinder with a two-layer plate of aluminum and iron is considered. In work [1] the difference schemes and algorithm of their realization are given. The most complete reviews of the main methods of calculation of transients in deformable solids can be found in [2, 3, 4], which also indicates the need and importance of generalized studies on the comparative evaluation of different methods and identification of the areas of their most rational application. In the analysis and physical interpretation of numerical results in this work it is also useful to use a priori information about the qualitative behavior of the solution and all kinds of information about the physics of the phenomena under study. Here is the stage of evolution of contact resistance of collision – plate, stress profile.

ЧИСЛЕННАЯ РЕАЛИЗАЦИЯ РЕШЕНИЯ ДВУМЕРНЫХ ЗАДАЧ ТЕРМОВЯЗКОУПРУГОСТИ

This paper considers the numerical implementation of two-dimensional thermoviscoelastic waves. The elastic collision of an aluminum cylinder with a two-layer plate of aluminum and iron is considered. In work [1] the difference schemes and algorithm of their realization are given. The most complete reviews of the main methods of calculation of transients in deformable solids can be found in [2, 3, 4], which also indicates the need and importance of generalized studies on the comparative evaluation of different methods and identification of the areas of their most rational application. In the analysis and physical interpretation of numerical results in this work it is also useful to use a priori information about the qualitative behavior of the solution and all kinds of information about the physics of the phenomena under study. Here is the stage of evolution of contact resistance of collision – plate, stress profile.

Fretting wear behavior of brass/copper-graphite composites as a contactor material under electrical contact

In this study, copper-graphite composites were prepared by powder sintering. The wear resistance and electrical contact endurance of composites with different copper-graphite ratios were investigated and are discussed. The results illuminate that the high electrical contact resistance evolution during fretting wear is related to the graphite content. With a decrease in the copper content, the interaction between graphite and copper increases, and the conductivity decreases. The graphite particles of the flat sample delaminate and form a graphite friction film between the contact surfaces; this friction film has excellent lubricating properties. Thus, the copper-graphite composites with the highest graphite content herein demonstrate good behavior against fretting wear and exhibit poor electrical contact performance. In other words, a relatively high copper content could maintain a decreased and stable contact resistance. The comprehensive performance of composite materials can be effectively developed by designing the copper-graphite ratio.

Table top experimental setup for electrical contact resistance measurement during indentation

Indentation has become a useful technique for measuring the physical properties of nanomaterials, mechanical stability of beam structures and diaphragms, with the advancement of nanotechnology and MEMS based devices. When combined with electrical contact resistance measuring attachment, indentation tool can provide valuable new insights into contact resistance evolution and material deformation behavior at the nanoscale and obtain other physical properties of the materials. In this work, the design and development of a low-cost tabletop indenter setup with in-situ electrical contact resistance (ECR) measurement attachment is reported. The indenter consists of a piezo electric based positioner and a displacement sensor. A conductive diamond probe attached to the positioner monitors electrical resistance simultaneously. The functionality of the indenter is verified by indenting a porous-alumina-Ni nanocomposite material. It is found that resistance drops with increasing load as area of contact increases. Also, hardness of the composite is calculated from the indentation impression.

Contact behavior evolution induced by damage growth in radio-frequency microelectromechanical system switches

This study provides a two-contact-event model to explain the evolution of the contact behavior of microelectromechanical system (MEMS) switches through their lifetime. The succession of two dynamic contact events is carefully considered during actuation inspired by experimental observations. The contact between the MEMS switch tip and the drain can be treated as an effective contact between an elastic hemisphere and a rigid plane. If the first contact event results in elastic deformation, the effective hemisphere will fully recover. Consequently, the subsequent contact event also produces elastic deformation. If, on the other hand, the first contact event induces elastoplastic or plastic deformation, a residual depth will be produced between the hemisphere and the rigid plane. The contact force of the subsequent contact event can be significantly reduced due to this additional residual depth. With the growth of residual depth during the switch cycling process, the modeling results show three possible situations of contact radius evolution: (1) The contact radius increases to a maximum value and then decreases to zero; (2) the contact radius increases to one local maximum value; then decreases to a local minimum value; subsequently increases again to another maximum value, and finally decreases to zero; and (3) the contact radius increases to one maximum value and then decreases to zero; after an intermittent response, the contact radius increases again to another maximum value and finally decreases to zero. Furthermore, the Maxwell spreading formula is applied to determine the contact resistance which is inversely proportional to the contact radius. Three situations of contact resistance evolution corresponding to the evolution of contact radius are obtained. All three situations are also observed and validated by the experimental results.

Inter-layer thermal contact resistance evolution with the degree of intimate contact in the processing of thermoplastic composite laminates

This article focuses on the contact between layers in forming processes of composite laminate. The link between the degree of intimate contact and the consequent thermal contact resistance between layers is investigated. A hot plate forming process experiment allows to propose a relation and determine the missing parameter for APC2 thermoplastic prepreg composite. Besides the new proposed relation, this work showed that the internal thermal contact resistances in the laminate is significant. Therefore, thermal modeling of forming processes of composite laminate (such as automatic tape placement) should account for this phenomenon.

The Contact Resistance Performance of Gold Coated Carbon-Nanotube Surfaces under Low Current Switching

Multi-Walled CNT (MWCNT) are synthesized on a silicon wafer and sputter coated with a gold film. The planar surfaces are mounted on the tip of a piezo-electric actuator and mated with a gold coated hemispherical surface to form an electrical contact. These switching contacts are tested under conditions typical of MEMS relay applications; 4V, with a static contact force of 1mN, at a low current between 20-50mA. The failure of the switch is identified by the evolution of contact resistance which is monitored throughout the switching cycles. The results show that the contact resistance can be stable for up to 120 million switching cycles, which are 106 orders of higher than state-of-the-art pure gold contact. Bouncing behavior was also observed in each switching cycle. The failing mechanism was also studied in relation to the contact surface changes. It was observed that the contact surfaces undergo a transfer process over the switching life time, ultimately leading to switching failure the number of bounces is also related to the fine transfer failure mechanism.

Study of minimum fretting for connectors used in automotive applications

For automotive applications, the mechanical behaviour of the contact area under vibration is one of the key factors for connector reliability. Such vibrations on the bulk contact device are typically in the range of 10–2,000 Hz and result in displacements of only a few microns at the contact interface. In the present study, a bench test has been developed to control motions down to 1 $$\upmu $$ m. The objective is to determine the minimum amplitude for fretting-corrosion degradation based on the evolution of contact resistance and to study the effects of the material, the contact force, the coating, for these low displacement amplitudes. To obtain the limit of the appearance of fretting, a sub-micrometer incrementing displacement amplitude methodology was applied on a high stiffness bench test including a double piezoelectric actuator. It was found that the fretting degradation starts to occur from 2 to 6 $$\upmu $$ m when the contact force is from 0.5 to 2.5 N with a tin-coated terminal. Moreover, pure copper, tin and nickel have similar amplitude fretting limits while noble metals confirm the absence of fretting up to 10 $$\upmu $$ m amplitude and for a large number of operations (10 $$^{6 }$$ cycles). Best fitting of the obtained minimum fretting amplitude data to the Mindlin equation is discussed and improved by a correction factor.

Experimental Analysis of Heat Transfer in Rotational Molding Process

Abstract Rotational molding is a process by which powdered or liquid plastics are converted into hollow articles. This process is relatively simple in a technical point of view but it involves very complex physical phenomena. Constant quality in technical parts requires the mastery of the process by controlling on line these phenomena. One of these of first importance is heat transfer. This paper concerns the experimental analysis of heat transfer in rotational molding process with semi-crystalline polymer. By using an instrumented mold associated with an original radio transmission data acquisition system, we demonstrate that the rotational nature of the process implies complex heat transfer evolutions in the mold. The crystallization of the material is modeled with accuracy by coupling heat transfer equation to a kinetic model determined by calorimetry. It appears that thermal contact resistance evolution is a key point of heat transfer during the solidification of the polymer.

Evolution of Contact Resistance during the Bonding Process of NCA Flip-Chip Interconnections

Non-conductive adhesive (NCA) flip-chip interconnects are emerging as an attractive alternative to lead or lead-free solder interconnects due to their environmental friendliness, lower processing temperatures, and extendability to fine-pitch applications. The electrical connectivity of an NCA interconnect relies solely on the pure mechanical contact between the integrated circuit (IC) bump and the substrate pad; the electrical conductivity of the contact depends on the mechanical contact pressure, which in turns depends to a large extent on the cure shrinkage characteristics of the NCA. Therefore, it is necessary to monitor the evolution of the electrical conductivity which could reflect the impact of cure- and thermal-induced stresses during the curing and cooling process, respectively. In this article, in situ measurement of the development of contact resistance during the bonding process of test chips was developed by using a mechanical tester combined with a four-wire resistance measurement system. A drop of resistance induced by the cure stress during the bonding process is clearly observed. With decreasing bonding temperature, the drop of contact resistance induced by cure shrinkage becomes larger, while the cooling-induced drop of resistance becomes smaller. The evolution of contact resistance agrees well with experimental observations of cure stress build-up. It is found that vitrification transformation during the curing of the adhesive could lead to a large cure stress and result in the reduction of the contact resistance. Furthermore, no obvious changes were observed when the applied load was removed at the end of bonding.

Influence of surface topography on the surface durability of steam oxidised sintered iron

Durability of surfaces has been reported as the main factor affecting tribological behavior of steam oxidised sintered iron. The presence of surface pores and their negative influence on load bearing capacity, suggest that surface topography might play an important role on the durability of the oxide layer. In this paper, the influence of compaction pressure and powder grade on surface topography, and as a consequence, its effect on the tribological behavior of steam oxidised sintered iron has been analysed. Specimens prepared from atomised iron powders with different sizes were compacted using 4 different pressures, sintered, and then subjected to steam treatment. Tribological characterisation was carried out in a reciprocating sliding wear test. Although the processing parameters affected the surface topography to a considerable extent, the main influence may be attributed to powder grade. A strong influence of surface topography on the durability distance, evaluated in terms of the evolution of contact resistance with total sliding distance, has been highlighted. Surfaces which were smoother and had high load-carrying capacity were always associated with a higher durability distance.

Evolution of contact resistance and coupling loss in prototype ITER PF NbTi conductors under transverse cyclic load

Cyclic energizing of a magnet coil with Cable in Conduit Conductors (CICC), as for fusion applications, results in an anomalous change of the interstrand contact resistance (R/sub c/) and coupling loss (n/spl tau/) due to the alternating transverse forces. Previously, three Nb/sub 3/Sn ITER conductors have been tested in a cryogenic press, up to 40 cycles. Now, for the first time, the behavior of NbTi conductors under cyclic load is investigated and results are presented for three full-size prototype ITER Poloidal Field (PF) Coil conductors. One conductor has bare copper strands and no petal wrapping while the others have a Cr and solder strand surface coating. The press can transmit a maximum transverse force of 800 kN/m directly to a cable section of 400 mm length at 4.2 K. Each conductor is tested up to 220 kN/m and 40,000 full loading cycles. The magnetization of the conductors and the R/sub c/ between combinations of strands and strand bundles is measured at various number of cycles. It appears that the R/sub c/ can vary for up to orders of magnitude during cyclic loading.

Thermal contact resistance and adhesion studies on thin copper films on alumina substrates

A new photothermal method for measuring the thermal contact resistance in the interfacial area is presented. Copper thin films were prepared on alumina substrates by physical vapour deposition. On the basis of a mathematical model developed here, thermal contact resistance was determined in samples of various thicknesses and processed under various argon pressures. The effects of these parameters on the films and interface properties are discussed. A correlation between the thermal contact resistance and the adhesion, as determined by the scratch test, is found. In order to understand the origin of the mean critical load and the thermal contact resistance evolution, observations were made by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The results obtained have shown that the change in stress level in the copper film and the formation of a new compound in the interfacial area seem to be the main reasons for the enhancement of heat transfer.

Correlation between the adhesion and the thermal contact resistance: effects of substrate surface ion bombardment etching

Earlier the effects of ion bombardment etching ceramic substrates (Al 2O 3) were studied on the adhesion strength and thermal contact resistance of sputtered copper films with the use of the scratch adhesion test and the thermal measurements, respectively. Most relations between thermal contact resistance and mean critical load use scratch tests to characterize adhesion. In order to understand the origin of the mean critical load and the thermal contact resistance evolution, observations were made by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The results obtained have shown that the structural change of the copper film and the formation of a complex interface seem to be one of the main reasons for this evolution in argon treated samples.

Effect of heat treatment on the electrical properties of Cr/Cu films

The low temperature (216–415 °C) interdiffusion of vacuum-deposited Cr/Cu bilayers has been investigated by measuring the evolution of contact resistance. The activation energy of the diffusion process was measured. TCR and electrical resistance of the couples after the heat treatment period were measured as a function of heat treatment temperature. Some conclusions concerning the size-effect were obtained.

A study of intermetallic compound formation in a copper–tin bimetallic couple

The formation of intermetallics in copper–tin bimetallic couples has been studied from room temperature to 183 °C by measuring the evolution of contact resistance and composite electrical resistance with time and temperature in order to assess the kinetic behavior of the system. X-ray diffractogram (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) studies have also been performed on the samples. As regards bulk diffusion, copper diffuses interstitially into tin rapidly at room temperature with the formation of η′-Cu6Sn5 intermetallic compound. Further diffusion through this phase as evaluated by composite electrical resistivity measurements is given by 0.40 eV, assuming a model of defect-assisted diffusion into the grains. The grain-boundary diffusion is found to occur with an activation energy of 0.78 eV as estimated from contact resistivity measurements. SEM confirms the presence of grain-boundary diffusion of tin in copper, whereas XRD and TEM measurements indicate the growth of η′ intermetallics at room temperature which exists up to 90 °C when it transforms to the ε-Cu3Sn phase. The presence of tin whiskers on the tin surface has also been occasionally observed.