Low Normal Force Research Articles

Effect of cooling rate on the structure and nanotribology of Ag–Cu nano-eutectic alloys

Wear mechanisms of three Ag–Cu eutectic alloy samples cooled at different rates from the melt have been investigated by friction force microscopy. The eutectic phase exhibits a lamellar structure where the interlamellar thickness decreases with increasing cooling rate. The hardness of the samples decreases with decreasing thickness of the lamellae. In the low normal force regime (Fn ≤ 1000 nN) friction is governed by shearing and the relevant contact area can be well described by the Johnson–Kendal–Roberts model. At higher normal force values, the surface is worn, and friction can be described by the ploughing friction coefficient. A ploughing friction coefficient is determined, which is positively correlated with the hardness of the Ag–Cu eutectic alloy samples cooled at different rates, while the wear volume negatively correlates with the hardness.

Micromechanical behaviour of a polymer-coated sand

Polymers, such as polydimethylsiloxane, can be utilised as functional coatings with water repellent properties in granular materials. These new surface-modified granular materials have potential applications in barriers to control wetting. However, limited knowledge exists on the mechanical behaviour of particles with synthetic coatings. Here, a fundamental insight is provided by investigating the micromechanical behaviour of a coated sand with extreme water repellency by means of a custom-built inter-particle loading apparatus. To synthesize the water repellent coatings, an industrial sand was subjected to 3% mass ratio of dimethyldichlorosilane. In a comparative investigation between coated and uncoated (natural) sand particles, the results revealed (1) a decrease of the normal stiffness and (2) a dependency of the inter-particle friction coefficient on the normal force for the coated sand particles. In particular, the inter-particle friction coefficient significantly increased at low normal forces (lower than 2 N) with this effect decreasing as the coatings were damaged under a high normal force (over 5 N), and with the micromechanical behaviour of the coated sand converging towards that of the uncoated (natural) sand. The results are compared to theoretical models and mechanisms are proposed to explain the findings. By changing the nature of the coatings, this paper provides a basis for future research on granular materials with functional coatings for other applications.

TRACKED ROBOT FOR CLEANING OF PIPE

Urgency of the research. Service robot is as device which is currently used for standard works as cutting the grass or vacuum cleaning and also teleoperation robot which is remote controlled by human for special tasks as exploring, bomb removing, rescue action etc. Tendency is substitute human in dangerous or monotonous works via using of robots. Target setting. Tracked robot is designed for cleaning of pipe, which has inner pipe wall covered by sediments. The typical example is chimney, where carbon particles cover the pipe wall. Carbon particles can start burning process with result of total damaging of chimney and also building. Pipe robot can be used as practical aid for cleaning and inspection of pipes. Actual scientific researches and issues analysis. Other similar task is repairing of damaged inner pipe wall. Robot which repair pipe from inside pipe saves the costs for site excavation works. Uninvestigated parts of general matters defining. The questions of the design of pipe repairing robots are uninvestigated, because the next research will be focused to this. The research objective. In-pipe robot is as device for locomotion inside pipe with aim to make inspection or cleaning of inner surface of pipe wall. Tracked robot is designed because of better properties as overcoming of problematic places inside pipe and also lower normal force between the tracks and inner pipe wall. The statement of basic materials. Tracked segments are pressed to inner pipe wall and normal force is controlled by controller on the base of measurement of normal force. Cleaning brush module is connected to robot for dirties removing. CCD camera for inspection can be also connected to robot. Conclusions. The cleaning robot is important device for service of pipe systems as prevention of pipe damage of other negative phenomena. Contribution of this robot is significant, because it saves money and time.

Experimental analysis of the surface roughness in the coefficient of friction test

Mechanical wear has its origin in the friction of surfaces under contact and it is characterized for low normal forces by the mean coefficient of friction (COF) test on tribometers. The roughness of the worn surfaces is well established that is independent of the wear. Nevertheless a main hypothesis of this work is the research for evidences of the friction test variability influence on the surface roughness of worn materials in the test. The experimental setup includes the COF test ball-on-flat with steel on titanium and the surface roughness measured with an optical profilometer (confocal microscope) is analyzed. The space domain study of roughness parameters is developed on profiles (1D) and the surface (2D). The correlation study of the variability of the COF test with roughness metrics seeks the most significant roughness parameters. Its coherence and significance to respect the friction process are discussed. The similarities in the surface topography and its link with the COF signal are analyzed in the frequency domain based on power spectral analysis (PSA) from the fast Fourier transformation (FFT). Next, the whole profiles are also decomposed in frequency ranges by wavelet filtering that reveal the traces of the COF test on the surfaces and their similarities in the frequency domain.

Algorithm of Linear Induction Motor Control for Low Normal Force of Magnetic Levitation Train Propulsion System

This paper presents the analyses and experimental results of a single-sided linear induction motor (SLIM) thrust and normal force for the propulsion of semi-high-speed magnetic levitation (maglev) trains. These trains are composed of a levitation system that uses electromagnetic suspension and a propulsion system that uses SLIMs. The propulsion system of maglev trains using SLIMs has better low noise and dynamic characteristics compared with those using rotators. However, it has nonlinear characteristics due to the effect of slip that occur in the secondary eddy-current induction process; the normal force generated by the SLIM can negatively affect the levitation control. Therefore, a new slip-control algorithm is proposed for the safe operation of maglev trains, reflecting the normal force of the motor in propulsion control. First, the SLIM thrust and normal force are analyzed through the finite-element method (FEM) for a precise analysis of the slips. Furthermore, a slip range with a low normal force was derived based on the FEM analysis results, and these results are reflected in the propulsion control algorithm. Finally, the new algorithm was validated by an application to a full-sized testing apparatus.

Influence of morphology on the micro-mechanical behavior of soil grain contacts

The development of the discrete element method (DEM) has provided important insights into the micro-scale behavior of geological materials with major applications in geotechnical, geological and petroleum engineering. Mechanical response of grain contacts including normal and tangential behavior are given as input quantities in various DEM applications. However, it was only achieved in recent years to properly capture the real soil grain behavior in the laboratory and provide useful input parameters to be further utilized in DEM analyses. In this paper, an experimental study was conducted investigating the normal and tangential contact behavior of soil grains composed of limestone sand (denoted as LS), which is a material of biogenic origin and it may find applications, for example, in offshore engineering. In their previous work (Senetakis et al. in Tribol Int 111:1–8, 2017), the authors reported on the dynamic inter-particle friction of this material and concluded that due to the smooth surface of the brand of grains they tested, the inter-particle friction was found lower in comparison to quartz grains of Leighton Buzzard sand (LBS). That study quantified friction at relatively large displacements of shearing, of the order of 80 μm or above, where a steady state sliding was observed. In the present work, the mechanical behavior of LS sand grains was enhanced with additional test data to further explore normal and tangential behavior. The focus of the study was to investigate the differences in inter-particle friction at different reference displacements, termed as small displacement range, where the behavior exhibited a peak, and large displacement range, where a fairly steady state sliding was occurred as well as to stress the differences in the observed responses between LS and LBS grains in terms of tribological and contact mechanics behavior. It is shown in the study that very high friction angles may be observed at displacements of about 10–30 μm for LS grain contacts when sheared under low normal forces, with a substantial drop of the mobilized friction leading to a steady state at much greater sliding displacements. This drop of friction with shearing displacement was not observed when the grains were sheared at higher normal forces. The morphological effects on friction were more evident at low normal forces for LS grains. In the normal loading tests, the LS sand grains exhibited significant plastic behavior. After the conduction of a number of normal loading–unloading cycles, the observed plastic deformations continued to increase, even though this observed trend was dominant at the first cycle. The observed apparent contact Young’s moduli of LS grains were found to be much lower than those of LBS grains. These observations should not be ignored by DEM modelers when the numerically created sample targets to represent a granular material such as crushed LS.

Analysis of double‐sided sandwiched linear flux‐switching permanent‐magnet machines with staggered stator teeth for urban rail transit

Linear flux-switching permanent-magnet (LFSPM) machines exhibit significant potential in many applications. A double-sided sandwiched LFSPM (DSSLFSPM) machine with high thrust force capability is investigated in this study. After being optimised for optimal thrust force using Maxwell 2D, this machine is found to deliver higher thrust force when compared with an optimised conventional 12/14 LFSPM machine. However, low thrust force ripple is also required in many applications. Thus, in order to reduce the thrust force ripple, the DSSLFSPM machine is improved by a staggered stator teeth structure and an end permanent-magnets method, then, two improved machines are obtained. The improved machines can reduce the thrust ripple to a very low level without thrust force density decrease. Further, the stator yoke thickness of the improved machines is optimised to reach optimal thrust force density. Finally, the performance of the DSSLFSPM-ST machines is analysed including the normal force, the efficiency and the power factor. It shows that the improved DSSLFSPM machines exhibit relatively low normal force and high thrust force density as well as low thrust ripples, indicating that they are suitable for long-stroke applications.

Influence of Calf Serum on Fretting Behaviors of Ti–6Al–4V and Diamond-Like Carbon Coating for Neck Adapter–Femoral Stem Contact

Influences of newborn calf serum on the fretting behaviors of Ti–6Al–4V and diamond-like carbon coating were investigated using a fretting-wear test rig with a cylinder-on-flat contact. The results indicated that, for the Ti–6Al–4V/Ti–6Al–4V contact, the friction coefficients were high (0.8–1.2) and the wear volumes presented an increase with the increase in the displacement amplitude under dry laboratory air conditions. Under serum-liquid conditions, the Ti–6Al–4V/Ti–6Al–4V contact presented significantly larger wear volumes under the displacement of ± 40 µm; however, it presented significantly lower friction coefficients (0.25–0.35) and significantly smaller wear volumes under the displacement of ± 70 µm. For the DLC coating/Ti–6Al–4V contact, the coating response wear maps could be divided into two areas: the coating working area (low normal force conditions) and the coating failure area (high normal force conditions). In the coating working area, the DLC coating could protect the substrate with low friction, low wear volume, and mild damage in the coating. The presence of serum had a positive influence on the tribological performance of the DLC coating. Furthermore, the positive influence was more significant under larger displacement amplitudes condition.

Reduction of the Adhesive Friction of Elastomers through Laser Texturing of Injection Molds

It is well known that elastomers usually possess poor dry sliding friction properties due to their highly adhesive character. In order to overcome this problematic behavior in industrial applications, interfacial materials such as oils, greases, coatings, or lacks are normally used in order to separate or to functionalize the contact surfaces of elastomers. Alternatively, the high adhesion tendency of elastomers may be explicitly reduced by modifying the elastomer composition itself or by enabling a reduction of its effective contact area through, for example, surface laser texturing. This second approach, i.e., the reduction of the adhesive character of elastomers through laser structuring, will be the main topic of the present study. For this purpose, different micro-sized grooved structures were produced on flat injection molds using an ultra-short pulsed laser. The micro-structured molds were then used to produce injection molded micro-ridged Liquid Silicone Rubber (LSR) sample pads. The investigations consisted firstly of determining the degree of replication of the mold micro-structures onto the surface of the LSR pads and secondly, to ascertain the degree of reduction of the friction force (or coefficient of friction) of these micro-ridged LSR pads in comparison to the benchmark (unstructured LSR pads) when tested under dry conditions against Aluminum alloy (Al-6082) or PA6.6-GF30 plates. For this second part of the investigation, the normal force (or contact pressure) dependency of the coefficient of friction was determined through stepwise load increasing friction tests. The results of these investigations have shown that the production of micro-ridged surfaces on LSR pads through laser structuring of the injection molds could be successfully achieved and that it enables a significant reduction of the friction force for low normal forces (or contact pressures), where the component of adhesion friction is playing an important and determining role in the overall friction behavior of the LSR elastomer.

In vitro evaluation of the influence of velocity on sliding resistance of stainless steel arch wires in a self-ligating orthodontic bracket.

Of the variables used by in vitro studies of resistance to sliding (RS) in orthodontics, sliding velocity (SV) of the wire is often the one farthest from its clinical counterpart. We investigated whether velocity influences the RS at values approximating the orthodontic movement. A SS self-ligating bracket with a NiTi clip was fixed onto a custom-made model. Different shaped orthodontic SS wires of four sizes and two types (round, 0.020″ and 0.022″; rectangular, 0.016″×0.022″ and 0.017″×0.025″) were tested using an Instron® testing machine. Wires were pulled at four velocities (1×10-2 mm/s, 1×10-3 mm/s, 1×10-4 mm/s, 1×10-5 mm/s). Shapiro-Wilk test was used to evaluate the normal distribution of the data; two-way ANOVA was performed to compare means in the RS with wire characteristics and SV. Significance level was set at P<.05. RS was higher for rectangular wires, and for those with larger diameters. Lower SV was associated with lower RS, with wire type and size having an interaction effect. The RS relatively to SV can be represented as: RS ∝ α[ln(SV)]+β, where α and β are constants. At very low SV and low normal forces, SV influences the RS of SS archwires in orthodontic brackets, and the proportionality is logarithmic. Although respecting these parameters in vitro is challenging, quantitative evaluations of RS should be carried out at clinically relevant velocities if aiming at translational application in the clinical scenario.

Resistance to moment-normal force interaction of I-shaped steel sections

This paper describes the assessment of the EN 1993-1-1 design rules for cross-section resistance to moment-normal force interaction (M-N interaction). Besides the fact that the Eurocode design rules show unconservative predictions of the reduced plastic moment capacity for the presence of relatively low normal force, the recommended partial factor is γM0=1.00 and relative large shear forces up to 50% of the shear resistance are allowed. This means that, neglecting a possible benefit from strain hardening, the EN 1993-1-1 design rules have no spare capacity available in the section.A few large scale experiments for M-N interaction were performed on HEA240 (S355JR) sections. In order to simulate the behaviour of a section to M-N interaction, a Finite Element Model was created in Abaqus/CAE. The model was validated against the experimental test results. Then, 1188 numerical test results were generated. A statistical evaluation to assess the design rule against these numerical test results showed that the recommended value γM=1.00 of the partial factor used in the EN1993-1-1 design rules is statistically acceptable for I-shaped sections in steel grades S235 and S355. For S460 it is necessary to use γM=1.10 based on the currently available statistical material data.

Discrete element simulation of railway ballast: modelling cell pressure effects in triaxial tests

The paper investigates reproducing the effects of confining pressure on the behaviour of scaled railway ballast in triaxial tests in discrete element models (DEM). Previous DEM work, using a standard Hertzian elastic contact law with an elastic–perfectly plastic tangential slip model, has been unable to replicate the behaviour observed in laboratory tests across a range of confining pressures without altering both the material stiffness and the inter-particle friction. A new contact law modelling damage at the contacts between particles is introduced. Particle contact is via spherically-capped conical asperities, which reduce in height if over-stressed. This introduces plasticity to the behaviour normal to the contact surface. In addition, the inter-particle friction angle is varied as a function of normalized contact normal force. At relatively low normal forces the friction angle must be increased for peak mobilized friction angles to match the laboratory data, an effect that is attributed to interlocking at the scale of surface roughness. Simulation results show close agreement with laboratory data.

Influence of process parameters on bond properties of Al laminated structure produced by ultrasonic consolidation

Purpose The purpose of this paper is to investigate the influence of key parameters on the bond strength and failure modes of laminated structures made of different aluminum alloys (i.e. Al 2024 and Al 7075) via the ultrasonic consolidation (UC) process. Design/methodology/approach The UC is used to fabricate laminated structures with various parameters. The push-pin tests were performed on the specimens of different materials and parameters, and the force and displacement were recorded during the tests. The peak punch force was used to represent the bond quality of the laminated structure, and the curves of force versus displacement were used to study the failure modes of the structures. Findings It is found that the lower normal force, the larger vibration amplitude and the lower travel speed can result in stronger bonding. Three different failure modes are observed in the tests, due to the different relations between the toughness of interface and raw materials. The process parameters have influence on the interface toughness of a laminated structure, which further leads to different failure modes. Originality/value The overall mechanical properties of a laminated structure highly depend on the bond quality between laminated layers. The push-pin test can easily and effectively evaluate the bond quality of the laminated structure. This paper not only focuses on the bond strength evaluation, but also analyzes the different failure modes of laminated structures made of different aluminum alloys, which can give an opportunity to optimize the parameters for different materials.

Investigation of Friction and Wear Mechanisms of TiAlV Coated Carbide Against Ti6Al4V Titanium Alloy Using Pin-On-Disc Tribometer

Abstract The tribological behavior of the PVD-TiAlN coated carbide inserts in dry sliding against two-phase (α-β) titanium alloy,Ti6Al4V grade, was investigated. A modified pin-on-disc device was used to conduct experiments under variable normal load and sliding speed. Scanning electron microscopy (SEM) and X-ray micro-analyses by EDS were applied for observations of wear scars and wear products. It was revealed that the increase of sliding speed contributes to decreasing the friction coefficient under a low normal force, whereas the increase of the normal loading causes the friction coefficient is less sensitive to changes in the sliding speed and its values are equal to μ = 0.26-0.34. The adhesive nature of wear along with severe abrasive action of the Ti alloy were documented.

Fundamental investigation of micro-scale wear characteristics of ultra-fine gold wires under low contact force and long sliding distance

Micro-scale wear characteristics of ultra-fine gold wires with a diameter of ~28μm was investigated under a low contact force over a long sliding distance up to ~200km. The tip of the wire was slid against a silicon wafer under a low normal force of 5μN. Wear of the gold wire was quantified from the images of the tip obtained by a 3D laser microscope and a scanning electron microscope before and after the wear test. The dimensionless Archard relationship wear coefficient was in the order of 10−7 which was extremely low considering that sliding was carried out in dry condition. Such a low wear characteristics was attributed to low contact pressure that could be achieved by formation of a conformal contact due to flattening of the wire tip as sliding wear progressed. Wear behavior of the gold wire could be characterized by formation of thin flakes of wear debris that were generated as a result of adhesive shear stress imparted at the sliding interface. As an effort to reduce the wear coefficient even further, octadecyltrichlorosilane self-assembled monolayer was deposited on the silicon to serve as a nano-lubricant. It was effective in improving the wear resistance by about two-fold.

Friction at single-asperity contacts between hydrogen-free diamond-like carbon thin film surfaces

This work investigates the friction and adhesive forces at the single-asperity contact between hydrogen-free diamond-like carbon (DLC) coated surfaces in humid air and in argon. High power impulse magnetron sputtering deposition has been used to produce hydrogen-free DLC thin films on commercial silicon AFM tips and silicon wafers. The structure and surface morphology of the deposited DLC films has been investigated by X-ray photoelectron spectroscopy and tapping AFM, investigations that showed very smooth DLC films with about 29% of sp3 C−C bonds. The as-deposited hydrogen-free DLC thin films were hydrophilic (water contact angle about 65°) due to incorporation of a small amount of oxygen (about 9% of C=O bonds) at the film surfaces. The friction force measurements show that at low normal loading force the contact friction is much lower in the humid air than in the argon, while at large values of the loading force the contact friction does not depend on the measurement medium. A qualitative friction model based on the concept of dry and wet friction regimes, when the layer of water adsorbed on DLC film surfaces plays the role of lubricator, is proposed to explain these findings.

Effect of ion structure on nanoscale friction in protic ionic liquids.

The effect of ionic liquid (IL) molecular structure on nanoscale friction has been investigated using colloidal probe Friction Force Microscopy (FFM). The ILs studied were ethylammonium formate (EAF), ethylammonium nitrate (EAN), propylammonium formate (PAF), propylammonium nitrate (PAN), dimethylethylammonium formate (DMEAF), and ethanolammonium nitrate (EtAN). ILs were confined between a silica colloid probe and a mica surface, and the friction force was measured as a function of normal load for sliding velocities between 10 and 40 μm s(-1). At low normal forces, multiple IL layers are found between the probe and the surface, but at higher force, in the boundary layer regime, a single ion layer separates the probe and the surface. In the boundary layer regime energy is dissipated by two main pathways. Firstly, the ionic liquid near the surface, with the exception of the boundary layer, is expelled from the advancing contact made by the probe on the surface. This disruption in the interactions between the boundary layer and the near surface multilayers, leads to energy dissipation and depends on the strength of the attraction between the boundary and near surface layers. The second pathway is via rotations and twists of ions in the boundary layer, primarily associated with the cation terminal methyl group. The friction coefficient did not vary over the limited range of sliding speeds investigated.

Development and Optimization of a Low Normal Force Contact

Throughout the technological advancements, electronic devices are getting high dense and less expensive in day by bay. Thus, significantly reducing the production cost is the vital issue in order to produce more competitive device with maintaining its standard in all expect. Therefore, introducing new compact contact in high-Hertz stress with low normal force (LNF) will be a key technology to achieve the future goal. In this research, it is identified that only a 5μm radial tip with 0.1N force contact provides an excellent electrical performance which is much sharper than conventional connectors. This invention gives a big hope to produce less expensive and high dense Low Normal Force contact. The contacts durability test were also successfully passed 0.30 million cycles of uses while the contact resistance were ≦50mΩ.

Design and fabrication stable LNF contact for future IC application

Enable the design of a small contact spring for applications requiring high density, high speed and high durability. A low normal force (LNF) contact spring with high performance is fabricated using a unique combined MEMS photo resist lithography and electro fine forming (EFF) technology. Reducing a total contact material cost of a connector, a high-Hertz stress with LNF contact will be a key technology in the future. Only radius R 5μm tip with 0.1N force contact provides an excellent electrical performance which is much sharper than conventional contact. 0.30million cycle's durability test was passed at 300μm displacement and the contact resistance was ≤50mΩ.

Maximum micro-slip in tangential contact of randomly rough self-affine surfaces

Many advanced laws of friction contain a characteristic length parameter defining the crossover from sticking to sliding. The physical sense and the scaling properties of this length, however, could not yet be clarified. In the present paper, the fact that any tangential contact of bodies with curved or rough surfaces naturally shows a preliminary slip at the micro scale is used. This is true even if the friction can be described locally by the simplest Coulomb law without preliminary micro-slip. This is suggested as the main physical mechanism for crossover from sticking to sliding. Under this assumption, it becomes possible to theoretically predict the value of the characteristic length of preliminary micro-slip. In this paper, based on recent advances in contact mechanics of self-affine fractal surfaces, the characteristic slip distance for such surfaces is estimated. We show that at low normal forces, the preliminary slip shows no dependence on the system size but only on the applied normal force. For the special case of randomly rough surfaces with a long wavelength roll-off or cut-off the preliminary slip length has the order of magnitude of the rms roughness multiplied with the coefficient of friction.