Electrical Contact Resistance Measurements Research Articles

Local and quantitative measurement of mechanical properties by electrical-nanoindentation in situ SEM: Application to a multi-phase AgCuPd alloy

Nanoindentation has now become the key technique for measuring the mechanical properties of materials at small scales. However, the quantitative and accurate processing of nanoindentation data relies on a physical quantity that is not directly available: the contact area (Ac) between the indenter tip and the sample under test. In complex systems, determining Ac is challenging due to the limitations of standard methods: analytical models have restricted validity domains (sample homogeneity and rheology), and post-mortem observations of residual imprints are time-consuming, do not appraise property gradients and cannot be applied to materials with significant elastic recovery.In this paper, a comprehensive methodology is proposed to continuously measure contact area during indentation. The proposed methodology, referred to as electrical-nanoindentation (ENI), is based on real-time monitoring of the electrical contact resistance (ECR). The protocol only requires mechanical and electrical calibrations of the indenter tip on reference materials, leading to one-to-one relationship between ECR and contact area. An original approach is also proposed to deal with the presence of surface passivating layers that generally disturb ECR measurements. As an illustration, the methodology is applied to the characterization of a multi-phase alloy (MPA) composed of silver, copper and palladium. This alloy raises the same challenges as those usually faced by nanoindentation in advanced metallurgy: heterogenous distribution of individual phases at the micro-scale, composite response of a complex mixture of hard/stiff and ductile/soft phases, … In addition, the ohmicity of contact is disturbed by surface passivating layers. Despite these numerous hindrances, the proposed methodology is successfully applied to this material. The evolution of contact area is compared with standard methods: an impressive accuracy of <2% standard-deviation is achieved when compared to post-mortem observations. The elastic moduli and hardnesses of individual phases are then accurately extracted. In addition, in order to gain in spatial definition, the ENI set-up is integrated into a scanning electron microscope (SEM), enabling indent positioning with a precision close to 100 nm.Two challenges are successfully met with the ENI methodology. On a mechanical point of view, the response of individual phases can be identified despite the complex rheology of heterogeneous materials, proving the approach applies to all mechanical behaviors (sink-in or pile-up rheologies, homogeneous or heterogeneous materials, with or without elastic recovery, …). On an electrical point of view, even if contact ohmicity is the only requirement of the methodology, it is possible to identify and overcome deviations from contact ohmicity induced by surface passivation. In particular, the non-linear resistive contribution of insulating layers fades during indentation thanks to its dependence as the reciprocal of the square of contact radius. The present work provides the keys to monitoring the contact area on any metallic sample, whether oxide-free or oxidized, making this methodology a promising alternative to standard methods.

Comprehensive mechanical and tribological characterization of metal-polymer PTFE+Pb/Bronze coating by in-situ electrical contact resistance measurement augmented tribo-mechanical tests

This work thoroughly mechanically characterizes the DU® metal-polymer coating from GGB Bearing Technology by augmented multi-scale Instrumented Indentation Test (IIT). Nano-IIT will evaluate the uniformity of the Pb particles dispersion in the outermost layer. Dynamic nano-IIT will investigate the damping properties of the material as a function of load frequency. Micro-IIT will tackle the layer thickness evaluation and the gradient of mechanical properties through the layers by continuous multi-cycle indentations. Data augmentation provided by synchronous electric contact resistance (ECR) measurements will support the identification of the layer transitions. The coefficient of friction and the wear coefficient will also be obtained by linear reciprocating pin-on-disc testing with focus on the specific application to guide bushings for linear pneumatic actuators.

Determination of the Corrosion–Oxidation Turning Point at the Field’s Metal/Chromium Interface by Electrical Contact Resistance Measurement

The correlation of the electrical contact resistance measurement and microanalysis of the Field’s metal (FM)/Cr interface was used to determine the temperature-dependent reactions at the interface. Oxidation of Cr by oxygen contained in FM was identified through a sharp increase of the contact resistance around a certain temperature, above which the following resistance decrease was associated to a corrosion process. The corrosion–oxidation turning point was observed around 390 ± 1.7 °C. Cr saturation in FM combined with complete consumption of the oxygen contained in the FM induced a stabilization of the interface giving a specific contact resistance of about 30 µΩ·cm2.

Influence of topography on electrical contact resistance of copper-based materials

The design of an electrical contact is crucial to ensure optimal performance, reliability, and efficiency of connectors. One key aspect is the surface quality of the contacting bodies—among other factors such as material selection, contact geometry, etc. In this work, we evaluated the influence on electrical contact resistance (ECR) of a smooth copper-based surface (brass, bronze, and tin-plated copper) when contacted against surfaces with different degrees of roughness. Furthermore, a carbon nanotube (CNT) coating was proposed with the goal of mitigating the topography-induced influence of the textured counter electrodes. The electrodes and counter electrodes were thoroughly characterized to understand the contacting mechanisms through numerical modeling,—namely, Greenwood-Williamson and Jackson-Green models—as well as using a practical-oriented slope analysis. Load-dependent ECR measurements were carried out to quantify the effect of roughness on contact resistance. When contacting against brass and bronze, a clear correlation between roughness and ECR can be established, with higher roughness equating to lower ECR In tin-plated copper, on the other hand, this hierarchy is not as well defined due to the ease with which the tin plating deforms, thus enabling the penetration of outstanding asperities and consequently establishing a better electrical contact. CNT-coated counter electrodes showed promising results, partially confirming the hypothesis proposed. However, unforeseen topography-related interactions with the CNT coating produced exceptions in the ECR measurements. Nonetheless, for most cases studied the coating did mitigate the influence of roughness.

Effect of Lubricant Properties and Contact Conditions on False Brinelling Damage

False brinelling is a type of fretting wear that occurs in rolling bearings when the contacts of bearing rings and rolling elements are subjected to small-amplitude oscillatory motion. It manifests itself as deep craters with relatively polished bottoms that are evenly spaced around the bearing rings. This work investigates the influence of lubricant properties and contact conditions, including oil viscosity, applied load, and amplitude and frequency of oscillations, on the onset and progression of false brinelling damage. Experiments were conducted with AISI 52100 bearing steel specimens in a ball-on-flat configuration under small-amplitude oscillating conditions. A set of base oils and a group of custom-made greases with systematically varying properties were employed. Importantly, the same ball-on-flat setup was adapted to allow optical access to the contact by employing a sapphire disc instead of a steel one. This allowed direct observation of the damage onset and progression to better elucidate the mechanisms taking place. Additional experiments were conducted with the same lubricants on a thrust ball bearing rig and the observed trends were compared to those from the ball-on-disc setup. Results show that false brinelling damage initiates locally at discrete asperity microcontacts and then spreads relatively quickly with continued rubbing to cover the whole contact area. Oscillation amplitudes shorter than the Hertz contact width (A/D < 1) were shown to be most damaging, while the damage reduces drastically for amplitudes larger than the Hertz contact width (A/D > 1). For the more important case of A/D < 1, oils and greases with lower viscosities produced less damage. Oils in general produced somewhat less damage than the equivalent lithium- or urea-thickened greases with the same base oil. Lower oscillation frequencies and lower contact pressures also produced less damage. The corresponding friction and electrical contact resistance measurements indicate that under these conditions of A/D < 1 the contact operates under boundary lubrication at best but can intermittently approach effectively dry conditions, depending on lubricant properties and applied conditions. These observations indicate that the key variable that controls damage when the oscillation amplitude is less than the contact width is the ability of the lubricant to penetrate and remain in the oscillating contact. Therefore, lubricant properties and contact conditions that promote this penetration, which may be different from those that promote hydrodynamic film formation, can mitigate against false brinelling.

A study of interfacial electrical contact resistances of thermoelectric generators for hypersonic vehicles by experimental measurements and two-scale numerical simulations

An enormous amount of aerodynamic heat is produced on the surface of hypersonic vehicles, and the thermoelectric generator (TEG) technique holds the potential to convert aerodynamic heat into electric energy. In a TEG, the micrometer-scale roughness on TE legs and electrodes surfaces leads to the constriction of interfacial electric current and brings about electrical contact resistance (ECR). The heat transfer and conversion processes of a TEG are greatly influenced by such type of ECR. In this work, a TEG includes two ceramic substrates and a middle TE layer is established, and the TE layer consists of insulation material, N- and P-type TE legs and electrodes to connect the TE legs. The ECR and its effects on the TEG performance are studied by two-scale numerical simulations and experimental measurements. First, the practical topography of rough surfaces for TE legs and electrodes is measured, and the micrometer-scale ECR prediction numerical models are established based on the reconstructed topography. An ECR measurement platform is developed and the platform consists of a pressure device, power supply and data acquisition system which are used to provide loading pressures, voltage, and acquisition of electric current and voltage signals, respectively. The ECRs under different temperatures, loading pressures and gap mediums are then predicted and the numerical models are validated by a series of measurements. Second, the TE conversion process of TEG is simulated by a millimeter-scale model including insulation materials, TE legs, electrodes, and equivalent interfacial layers of ECR. The influence of ECR on the TE performance of TEG for the hypersonic vehicle is thus clarified.

Fabrication and simulation study for vertical micro-TEGs based on printed circuit board manufacturing processes

The development of fabrication procedures for micro-thermoelectric generators (µTEGs) based on low-cost fabrication technologies, appropriate for mass production is discussed and demonstrated in this study. Simulations were carried out, two manufacturing processes were tested, and device performance is compared to simulation results for vertical µTEGs. The substrate for this device is a printed circuit board, and the thermoelectric materials are self-developed Bi0.5Sb1.5Te3 (p-type) and Bi2Te2.7Se0.3 (n-type) pastes. A square µTEG (15 mm × 15 mm × 500 µm) with eight thermocouples (TCs) was fabricated. The characterization of the fabricated 8-TC-µTEG was carried out and a power output of 1.23 µW was obtained for the fabricated 8-TC-µTEG. The measurement results of this 8-TC-µTEG closely match simulation results as well. Moreover, a novel vertical electrical contact resistance measurement setup is designed and implemented into the final evaluation of µTEG production to have a more accurate assessment. The simulation study is also applied for the designed electrical contact resistance measurement setup. As a result, the contact resistivity of Bi2Te3/Cu was calculated as 5.65 × 10−4 Ωcm2.

Effect of heat treatment on weldability of Al[sbnd]Si coated hot-stamped Usibor® 1500P

Aluminum‑silicon (AlSi) coating is used to protect Press Hardened Steel sheets against oxidation during the austenitization inside the furnace before the stamping stage. Heat treatment conditions, i.e. temperature and dwell times, influence the diffusion of elements between the AlSi coating and the substrate. During resistance spot welding, the occurrence of molten metal splashing at sheet-sheet interface increases with more severe heat transfer conditions, i.e. over-heated press hardened steel sheets or too long dwell times. The welding current range is dramatically reduced in this case. Measurements of electrical contact resistance at sheet-sheet interface reveal higher values, which could involve interfacial overheating. The AlSi coating, which has lower melting point compared with the substrate, is extruded early during welding stage at the faying interface and forms an accumulation at the notch root close to the nugget. The potential role of this accumulation to prevent the splashing phenomena is not clear. The stiffness of this accumulation is investigated through the analysis of the microstructure and the chemical composition of the coating. The results have suggested that the aluminum content in the interdiffusion layer can change with the heat treatment conditions and may modify its capacity to confine and seal the molten zone.

Challenges and important considerations when benchmarking single-cell alkaline electrolyzers

This study outlines an approach to identifying the difficulties associated with the benchmarking of alkaline single cells under real electrolyzer conditions. A challenging task in the testing and comparison of different catalysts is obtaining reliable and meaningful benchmarks for these conditions. Negative effects on reproducibility were observed due to the reduction in conditioning time. On the anode side, a stable passivation layer of NiO can be formed by annealing of the Ni foams, which is even stable during long-term operation. Electrical contact resistance and impedance measurements showed that most of the contact resistance derived from the annealed Ni foam. Additionally, analysis of various overvoltages indicated that most of the total overvoltage comes from the anode and cathode activation overpotential. Different morphologies of the substrate material exhibited an influence on the performance of the alkaline single cell, based on an increase in the ohmic resistance.

Generic Approach for Contacting Thermoelectric Solid Solutions: Case Study in n- and p-Type Mg2Si0.3Sn0.7.

Metallization (known as contacting) of thermoelectric (TE) legs is vital to the long-term performance of a TE device. It is often observed that the compositional changes in a TE solid solution may render a given contact material unsuitable due to a mismatch in the thermal expansion coefficient values. Finding suitable contact materials for TE solid solutions (which often are the best TE materials) remains a challenge. In this work, we propose a multilayer single-step approach in which the same combination of contact materials can be used for a wide compositional range in a solid solution. The outer layer is a metal foil, which helps in creating an Ohmic contact with the interconnects. The intermediate layer is a mixture of the TE material and a metal powder, which results in the formation of the diffusion barrier. The innermost layer is the TE material, which is the active component of the device. The strategy was applied on n- and p-doped Mg2Si0.3Sn0.7 with elemental Cu and Ni providing the desired interface functionalities. Single-step compaction was carried out using the monoblock sintering technique. Microscopic investigation reveals the formation of a well-bonded crack-free interface. Various intermetallic phases were identified at the interface, and the formation of the MgNi2Sn phase was found to be critical to prevent any interdiffusion of elements. Electrical contact resistance (rc) measurements were conducted, and low values of 3 and 19 μΩ cm2 were measured in n- and p-type legs, respectively. The contacted TE legs were further annealed at 400 °C for 7 days to check their stability. Microstructural and electrical resistance measurements reveal minimal changes in the interface layer and rc values, indicating the workability of the multilayer technique.

Improving the reliability of conductive atomic force microscopy-based electrical contact resistance measurements

Electrical contact resistance (ECR) measurements performed via conductive atomic force microscopy (C-AFM) suffer from poor reliability and reproducibility. These issues are due to a number of factors, including sample roughness, contamination via adsorbates, changes in environmental conditions such as humidity and temperature, as well as deformation of the tip apex caused by contact pressures and/or Joule heating. Consequently, ECR may vary dramatically from measurement to measurement even on a single sample tested with the same instrument. Here we present an approach aimed at improving the reliability of such measurements by addressing multiple sources of variability. In particular, we perform current-voltage spectroscopy on atomically flat terraces of highly oriented pyrolytic graphite (HOPG) under an inert nitrogen atmosphere and at controlled temperatures. The sample is annealed before the measurements to desorb adsorbates, and conductive diamond tips are used to limit tip apex deformation. These precautions lead to measured ECR values that follow a Gaussian distribution with significantly smaller standard deviation than those obtained under conventional measurement conditions. The key factor leading to this improvement is identified as the switch from ambient conditions to a dry nitrogen atmosphere. Despite these improvements, spontaneous changes in ECR are observed during measurements performed over several minutes. However, it is shown that such variations can be suppressed by applying a higher normal load.

Effects of alternating dissolved oxygen and dissolved hydrogen on the corrosion behavior of alloy 52 in high temperature high pressure water

The effects of alternating dissolved oxygen (DO) and dissolved hydrogen (DH) on the corrosion behavior of Alloy 52 are studied by electric/electrochemical methods measured in situ including contact electric resistance (CER) and electrochemical impedance measurements (EIS) together with multiple ex-situ surface analysis techniques including scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) analysis. The film resistance changes cyclically in both DO and DH cycles. In DO cycle film resistance decreases with the increase of corrosion potential (Ecorr) while in DH cycle film resistance shows an opposite trend. The preformed oxide film in a low DO condition can suppress the oxidation rate in middle DO condition, whilst a high DO level can deteriorate the protectiveness of Cr enriched layer. In the DH cycle, the immersion in a low DH condition can increase the resistance in the following high DH condition. All the results indicate the characteristic of oxide formed in one water chemistry is highly influenced by the pre-immersion history, which should be paid much attention in the study of DO or DH effects on the properties of the oxide film and stress corrosion cracking (SCC) initiation.

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.

Resistive-nanoindentation: contact area monitoring by real-time electrical contact resistance measurement

Abstract

Measurement of electrical contact resistance at nanoscale gold-graphite interfaces

An approach to measuring electrical contact resistance as a direct function of the true contact size at the nanoscale is presented. The approach involves conductive atomic force microscopy (C-AFM) measurements performed on a sample system comprising atomically flat interfaces (up to several hundreds of nanometers in lateral size) formed between gold islands and a highly oriented pyrolytic graphite (HOPG) substrate. The method overcomes issues associated with traditional C-AFM such that conduction can be correlated with a measurable true, conductive contact area. Proof-of-principle experiments performed on gold islands of varying size point toward an increasing contribution of the island-HOPG junction to the measured total resistance with decreasing island size. Atomistic simulations complement and elucidate experimental results, revealing the maximum island size below which the electrical contact resistance at the island-HOPG junction can be feasibly extracted from the measured total resistance.

Practical evaluation of electrical contact resistance of thermoelectric legs at high operation temperature

The evaluation of the electrical contact resistances of thermoelectric legs is important to predict the performance of thermoelectric modules. Thermoelectric modules can be divided into devices operating at high and low temperatures. In the former, the electrical contact resistance can change with temperature. However, many research groups have only analyzed the electrical contact resistance at mainly room temperature. In this study, an electrical contact resistance measurement system for thermoelectric legs of thermoelectric devices at high operating temperature (400 to 600 °C) is proposed, which provides a more accurate performance prediction. The measurement conditions are determined according to the amplitude and frequency of the current to minimize the measurement errors caused by the Peltier effect. Skutterudite thermoelectric legs with a Ti metallization layer are used to understand the specific contact resistance at high temperatures. Based on the obtained results, the thermoelectric module is analyzed through a simulation to investigate the power outputs with the specific contact resistances at both room and high operation temperatures.

Investigation on metallic contact conditions using ECR technique by performing rolling/sliding experiments

Understanding asperity contact conditions in rough surface elastohydrodynamic lubrication (EHL) contacts is of particular interest in tribology due to the drastic change in asperity characteristics during the wear process leading to significant losses. This work attempts to understand in situ change in surface topography during the wear process without disturbing the contact configuration. Measurements of electrical contact resistance (ECR) are one of the promising techniques for achieving this. To understand the behavior of asperity-to-asperity contacts, twin disc experiments are performed under rolling/sliding conditions. The effect of load and speed on friction coefficient and contact potential is presented. It is shown that transient representation of contact potential and friction coefficient is more appropriate than the average value of the friction coefficient and contact potential in the mixed-elastohydrodynamic lubrication regime. The variation of average friction coefficient and contact potential for different film thickness with respect to surface roughness (lambda ratio) is presented. Semi-quantitative techniques are used for theoretical prediction of the electrical contact resistance, number of contacting asperity and the real contact area. The trend for variation of numerically determined contact resistance with lambda ratio is found to be same as for experimentally determined average contact potential. It is also observed that at low lambda ratio, significant real contact area is formed which causes severe asperity-to-asperity contacts. Whereas, small real contact area is obtained at high lambda ratio which shows less asperity interactions.

Shear-induced softening of nanocrystalline metal interfaces at cryogenic temperatures

We demonstrate inverse Hall-Petch behavior (softening) in pure copper sliding contacts at cryogenic temperatures. By kinetically limiting grain growth, it is possible to generate a quasi-stable ultra-nanocrystalline surface layer with reduced strength. In situ electrical contact resistance measurements were used to determine grain size evolution at the interface, in agreement with reports of softening in highly nanotwinned copper. We also show evidence of a direct correlation between surface grain size and friction coefficient, validating a model linking friction in pure metals and the transition from dislocation mediated plasticity to grain boundary sliding.

Multiscale Numerical Model for Contact Resistance Prediction in the Proton Exchange Membrane Fuel Cell

Proton exchange membrane (PEM) fuel cell is considered to be a promising power source for vehicles, portable application and stationary by virtue of its zero pollution, low operating temperature, fast start-up and high efficiency. Contact behavior between gas diffusion layer (GDL) and bipolar plate (BPP) is of significant importance to the performance of PEM fuel cell. Most current studies for contact behavior are conducted by experiments and finite element analysis (FEA), which lead to high cost and huge computation for the whole fuel cell [1,2]. Comparatively speaking, the more generalized and effective analytical methods are deficient in this field because of multiscale features (from several-μm fiber to hundreds-mm fuel cell) and complex diverse mechanical properties of the components (such as the fibrous GDL and solid bipolar plate) [3]. The objective of this study is to build a numerical model to predict the contact behavior via a micro scale model for the “mechanical-electrical” relation between fibrous carbon paper and solid surface, as well as a macro scale model for the electrical contact resistance (ECR) of whole fuel cell. For the micro scale model, mathematical descriptions of solid surface topography and carbon fiber structure are established according to actual features of the components at the interface. Solid surface profile is modeled by the classic Greenwood & Willamson rough surface. Carbon paper is simulated by the random line network model based on the multi-layer construction. Contact pressure and resistance are calculated to generate the “mechanical-electrical” relation based on the Hertzian theory and statistical methods. In the micro model, the effects of carbon paper compression and internal fiber contact on contact resistance are given necessary attention, which are not considered in previous studies [4]. For the macro scale model, the ECR of whole fuel cell is calculated based on the predicted “electrical-mechanical” constitution relation. First, channel configuration of widespread metallic BPP is described according to the forming process and measurement of channel configuration. Then, nonlinear contact pressure distribution is obtained based on the interface deformation characteristic and Winkler surface model. Finally, ECR considering the channel configuration is calculated based on the “electrical-mechanical” constitution relation from micro model. Experiments of metallic BPP fabrication and ECR measurement are conducted to validate the multiscale numerical model. In the comparison, the model results show good agreement with the experimental results as shown in Fig.1 (b). During the experiments, the measurement results also show obvious dependence on the micro topography and channel configuration of the contacting BPP and GDL. Hence, influences of interface topography, material property, GDL deformation, and channel configuration are systematically studied based on the multiscale model. As shown in Fig.1 (c), it is found that lower ECR would be generated by reducing of surface roughness and softening of material, and significant increase occurs in ECR as the channel width decreases because of the growing contribution of channel gradient region. Size factor is proposed to evaluate the influence of various feature scale to the contact behavior in the fuel cell. Moreover, the assembly process of fuel cell could also be evaluated by the multiscale model.

Electrical contact resistance measurements of clamped battery cell connectors for cylindrical 18650 battery cells

The present study investigates the application of clamped cell connectors for 18650 battery cells with focus on the electrical contact resistance (ECR). The ECR, which occurs at the contact interface, is one of the crucial criterion to design a battery cell joint. A poor connection with an increased contact resistance can lead to a significant heat generation and can subsequently cause a heat transfer into the battery cell. A positive terminal of a Panasonic NCR18650B Li-Ion cell is used for ECR measurements and for an investigation of the mechanical properties. A load test indicates that the positive terminal undergoes an elastic deformation up to a force of about 400N and can therefore be used for clamped joints without any damage. To investigate the dependencies of ECR, the applied force on the joint is varied from 25 to 400N and five different materials with three different surface roughnesses are used as cell connectors. Furthermore, the influence of aged surfaces is investigated. The obtained data show that the ECR of clamped cell connectors is reduced to about 50μΩ. Thus, the ECR is in the range or even lower than for welded contacts.