Smaller Friction Coefficient Research Articles

Numerical study on tribology properties of textured surfaces based on a new contact model

The frictional properties of textured surfaces are closely related to the contact between rough peaks in the mixed lubrication. A model for rough peak contact between polyurethane and metal was already developed in a previous study. This study aims to further analyze the influence of various parameters on the friction properties of textured surfaces based on the previously developed theoretical model. As found, the friction coefficient increases with the increase of the root mean square roughness, and the increasing magnitude shows an upward trend. The friction coefficient varies with different texture shapes, and among the four shapes, the spherical texture has the smallest friction coefficient. In addition, either small or large texture depths can adversely affect the friction coefficient. The friction coefficient decreases and stabilizes with the increase of area ratio. The effect of working parameters on the friction coefficient coincides with the Stribeck curve.

Influences of Al particles on structure and properties of Ni–Al composite coatings with nanoscale twin structure

Nickel-aluminum (Ni–Al) composite coatings containing nickel nanoscale twins have been prepared by co-electrodeposition on copper substrate. Effects of Al particles on microstructure, chemical composition, friction properties, and corrosion resistance of Ni–Al coatings were investigated. It presents an even and compact structure with granular surface that Al are evenly dispersed in pyramidal-like nickel lattice. Adding Al could alter crystal orientation, refine grains and promote strengthening effects induced by nanoscale twinning. It can enhance anti-corrosion properties and decrease the coefficient of friction (COF). The minimum COF was obtained at 10 g L−1 Al. The larger amounts and even dispersion of Al, the better corrosion resistance and the smaller COF. Increasing Al from 20 to 40 g L−1, incorporated Al in top surface rose from 20.86 to 30.13 wt%, which was larger than in cross section from 13.93 to 18.86 wt%. As the incorporated depth increased, the particle color changed from grey to dark. Nano-twins were generated around enveloped Al particles. The roughness increased as Al rose from 10 to 40 g L−1. The average roughness (Ra) was of 191–231 nm. Ni–Al-5g and Ni–Al-80g have excellent anti-corrosion features, owning the maximum Rct of 1437.2 and 1565.1 kΩ cm2, respectively, associated with the amount and distribution of Al particles, preferred orientation, grain sizes, and twinned structure. The enhanced corrosion resistance might be due to the generation of uniform, compact aluminum oxide layer by doping Al in nickel.

Anisotropic tribology and electrification properties of sliding-mode triboelectric nanogenerator with groove textures

Sliding-mode triboelectric nanogenerator (S-TENG) is based on the coupling of triboelectrification and electrostatic induction, converting electrical energy from sliding motion. Introducing micro-textures into the sliding surface, and adjusting the angle between the texture and sliding direction (direction angle) may achieve performance anisotropy, which provides novel ideas for optimizing the tribology and electrification performance of S-TENG. To guide the performance optimization based on the anisotropy, in this paper, groove micro-textures were fabricated on the surface of S-TENG, and anisotropic tribology and electrification performance were obtained through changing the direction angle. Based on the surface analysis and after-cleaning tests, the mechanism of the anisotropy was explained. It is shown that the anisotropy of friction coefficient can be attributed to the changes of texture edge induced resistance and groove captured wear debris, while the voltage anisotropy is due to the variations of debris accumulated on the sliding interface and the resulting charge neutralization. Among the selected 0°–90° direction angles, S-TENG at angle of 90° exhibits relatively small stable friction coefficient and high open-circuit voltage, and thus it is recommended for the performance optimization. The open-circuit voltage is not directly associated with the friction coefficient, but closely related to the wear debris accumulated on the sliding interface. This study presents a simple and convenient method to optimize the performance of S-TENG, and help understand the correlation between its tribology and electrical performance.

Synergy of Al2O3/GO hybrid nanofluids: Molecular dynamics perspective on dispersion/adsorption evolution and its tribological validation

A synergistic Al2O3/GO hybrid nanofluid was developed, aiming to attain prominent tribological properties for application in mechanical engineering. On the support of a molecular dynamics approach, amorphous models were simulated, thus illustrating the agglomeration or the adsorption in nanofluids, as well as revealing the evolution of the laminated adsorbed structure. Further, single-component or hybrid nanofluids were characterized and their tribological validations were performed. The results showed that the modified nanofluids realized dispersion equilibrium and suspension stabilization at the molecular level, and their wettability or thermo-physical indexes were improved, which was beneficial for the tribological performance. Especially in hybrid nanofluids, the synergistic effect further enhanced these capabilities, thereby maintaining a small friction coefficient and a flat wear region.

Confinement enhanced viscosity vs shear thinning in lubricated ice friction

The ice surface is known for presenting a very small kinetic friction coefficient, but the origin of this property remains highly controversial to date. In this work, we revisit recent computer simulations of ice sliding on atomically smooth substrates, using newly calculated bulk viscosities for the TIP4P/ice water model. The results show that spontaneously formed premelting films in static conditions exhibit an effective viscosity that is about twice the bulk viscosity. However, upon approaching sliding speeds in the order of m/s, the shear rate becomes very large, and the viscosities decrease by several orders of magnitude. This shows that premelting films can act as an efficient lubrication layer despite their small thickness and illustrates an interesting interplay between confinement enhanced viscosities and shear thinning. Our results suggest that the strongly thinned viscosities that operate under the high speed skating regime could largely reduce the amount of frictional heating.

The Effects of Indium Additions on Tribological Behavior of Spark Plasma Sintering-Produced Graphene-Doped Alumina Matrix Composites for Self-Lubricating Applications

Alumina (Al2O3) ceramics are interesting for low-weight and mid-high temperature applications. The addition of indium (In) and graphene nanoplatelets (GNPs) can be used to reduce the density and modify the functional properties and mechanical performance of the ceramic matrix. GNP and In-reinforced Al2O3 matrix composites were prepared by the spark plasma sintering (SPS) technique. Monolithic Al2O3 and Al2O3 matrix composites with either 5 or 10 wt.% of In and 2 wt.% of GNPs (Al2O3-5In-2GNPs and Al2O3-10In-2GNPs) were compacted into disc-shaped samples. The microstructure was studied and characterized with light-optical microscopy (LOM) and scanning electron microscopy (SEM). Hardness was determined using the Vickers technique and tribological properties were studied by the ball-on-disk method. The coefficient of friction (COF) and specific wear rates were evaluated from tribological tests. Worn surfaces were studied by SEM and confocal microscopy. Interdiffusion transition regions were formed among individual microstructural constituents (Al2O3, In, GNPs) under high sintering temperatures, which were responsible for the balanced hardness and low porosity of the produced composites. The addition of In and graphene nanoplatelets resulted in smaller COF and wear rates indicating good improvement in the tribological behavior. The prepared Al2O3-5In-2GNP and Al2O3-10In-2GNP composites represent promising nanocomposites for self-lubricating applications.

Discrete element modeling of rock-filled gabions under successive boulder impacts

A discrete element model consisting of irregular crushable filling particles and a flexible wire mesh is established. The numerical model is validated against the static net punching test and the dynamic pendulum impact test to ensure that the large irreversible plastic deformations and the impact forces during successive impacts can be captured. The impact force reduces at small friction coefficients, which is associated with the significant particle rearrangement during the impact process. The important role of friction is further confirmed by the energy evolution, showing that friction is the dominant mechanism for energy dissipation, instead of the more intuitionistic collision or particle crushing. Besides, the increase of impact force with the number of impacts becomes more significant at a higher impact energy due to the faster rate of momentum exchange. A bounce-back behavior of the boulder is observed when the impact energy is low, which may attenuate the impact force like the reflection waves in real debris flow events where multiple boulders are present. Our results highlight the combined effects of contact properties and impact energy, which are valuable in the design of rigid barriers shielded by rock-filled gabions for hazard mitigation in engineering practice.

Study on wear resistance and toughness of calcium ion crosslinked cellulose nanofibers/multi-walled carbon nanotubes/hexagonal boron nitride hybrid reinforced Ni[sbnd]B composite coatings on N80 steel

This study developed a novel NiB/Ca-CNFs/MWCNTs/h-BN coating on N80 steel. The disadvantages of hexagonal boron nitride (h-BN) were improved by creatively combining multi-walled carbon nanotubes (MWCNTs), cellulose nanofibers (CNFs), and calcium ion cross-linking strategies. This led to the creation of Ca-CNFs/MWCNTs/h-BN hybrids, which were then doped into NiB coatings by varying pulse current densities. The results demonstrated that the surface morphology, mechanical properties, and toughness of the composite coating are all influenced by the Ca-CNFs/MWCNTs/h-BN doped at various current densities. Meanwhile, the NiB/Ca-CNFs/MWCNTs/h-BN (4 A/dm2) coating has the greatest microhardness (902.42 Hv) and the smallest friction coefficient (0.487) under reciprocating dry sliding friction conditions of Si3N4 friction balls. In addition, the results of nanoindentation and tensile tests show that the elastic modulus of NiB/Ca-CNFs/MWCNTs/h-BN (4 A/dm2) composite coating is 194.096 MPa and the tensile elongation is 12.2 %.

Influence of deposition voltage on tribological properties of W-WS2 coatings deposited by electrospark deposition

Electrospark deposition coatings were prepared with different deposition voltage on CrNi3MoVA steel substrates by using a W-WS2 sintered electrode and their tribological properties were investigated. The microhardness, roughness and tribological properties of the coatings were tested by using Vickers hardness tester, confocal laser scanning microscope and tribometer, and the morphologies, composition and phase structure were obtained by utilizing scanning electron microscopy (SEM), energy dispersive X-ray spectrum (EDS) and X-ray diffraction(XRD). The results showed that with the increase of deposition voltage, the hardness and roughness of the coatings increase. The coatings remarkably increase the tribological properties of CrNi3MoVA steel, and among the three coatings deposited at 40 V, 60 V and 80 V, the coating deposited at 60 V has the smallest friction coefficient and the best wear resistance.

Application of Collins’ upper bound theorem to upsetting processes

The standard formulation of the upper bound theorem is incompatible with the Coulomb friction law. However, this law is widely used in metal forming processes analysis. An alternative formulation based on non-kinematically admissible velocity fields can evaluate the load required to deform the material. The present paper adopts this formulation for evaluating the compression force in solid and hollow cylinder compression processes. The trial velocity fields are continuous. The field for the solid cylinder involves no parameter for minimization, and the field for the hollow cylinder involves one parameter. Parametric analyses of the solutions are provided. The compression forces found are compared to available results at small friction coefficients. The solution is also in agreement with physical expectations. In particular, the solution for the hollow cylinder compression predicts that the radial velocity is everywhere positive in the friction coefficient is small enough. For larger friction coefficients, the radius where the radial velocity vanishes propagates from the inner radius as the friction coefficient increases. The final result can be used in conjunction with standard procedures for evaluating the friction coefficient.

Investigation on the effect and growth mechanism of two-stage MAO coating

As a more popular surface treatment method, micro-arc oxidation technology can well solve the problems of insufficient strength and poor corrosion resistance of aluminum alloy surface. In this paper, a composite coating was obtained by using a two-stage micro-arc oxidation method different from the conventional micro-arc oxidation treatment, which solved the problem of incompatibility between the two types of electrolytes and obtained a coating with superior effect. The microscopic morphology and elemental composition were characterized by using Energy Dispersive Spectrometer (EDS)/X-ray Diffraction (XRD)/Scanning Electron Microscopy (SEM). The coating thickness was tested using a coating thickness gauge, its tribological properties were tested by friction experiments, and its corrosion resistance was tested by electrochemical corrosion tests. The results show that the coating has the advantages of large thickness, dense, good anti-wear performance, small friction coefficient and excellent corrosion resistance, which enhanced the performance of MAO coating and was an important guide for further work.

Effects of Ni/MoS2, Ag and Cr2O3 on the Microstructure and Mechanical Performance of a CoCrFeNi High-Entropy Alloy over a Wide Temperature Range

In the field of aerospace, core components require excellent wear resistance, lubrication and mechanical properties over a wide temperature range. In this study, three groups of CoCrFeNi high-entropy alloy (HEA)-based self-lubricating composites were designed with the addition of Ag, Ni/MoS2 and Cr2O3 using discharge-plasma-sintering technology. Their microstructure, phase composition, mechanical properties, friction and wear properties were analyzed. The results showed that, with the addition of Ag, the hardness and yield stress of HEA-Ni/MoS2-Ag were reduced by 36 HV and 24 MPa, respectively, while the plastic strain was increased by 2%. With the addition of Cr2O3, the hardness (382 HV) and yield stress (430 MPa) of HEA-Ni/MoS2-Ag-Cr2O3 reached their highest values, but the plastic strain reached its lowest value. HEA-Ni/MoS2-Ag-Cr2O3 had the smallest friction coefficient in which the friction coefficient at 800 °C was only 0.42. Additionally, it had a small wear rate of 3.2 × 10−6 mm3/Nm over a wide temperature range. At lower temperatures, Ni/MoS2 and Ag were conducive to lubrication, and the wear resistance was improved by the presence of Cr2O3. At high temperatures, a nickel oxide phase and a variety of silver molybdate phases were formed via a tribochemical reaction, which was vital to the high-temperature tribological properties.

Supported fluorine-free ionic liquids with highly sensitive gas-sensing performance

Monitoring volatile organic compounds (VOCs) is of increasing significance in the fields of environmental protection, health and safety, economic issues in industries, etc. Ionic liquids (ILs) arouse a great interest as potential sensing materials for VOCs. Here, the gas-sensing ability of fluorine-free ILs such as [N4,4,4,4][MEEA], [P6,6,6,14][MEEA], and [P6,6,6,14][TpA] have been thoroughly investigated. These ILs with phosphonium and ammonium cations as well as fluorine-free anions were successfully supported on carboxyl and amino groups terminated surfaces (denoted as COOH and NH2) as electrical gas sensors to tailor the response to the electronic resistance after sensing of VOCs. The [P6,6,6,14][MEEA] IL supported by either COOH or NH2 exhibits the strongest electrical sensing response within five cycles at a 150 ppm concentration of acetone or toluene, followed by [P6,6,6,14][TpA], and [N4,4,4,4][MEEA] with the shorter ammonium cation exhibiting the weakest response. The achieved higher gas sensing capability of the supported [P6,6,6,14][MEEA] is strongly associated with the accelerated ion mobility reflected from the smaller friction coefficient (μ), while [N4,4,4,4][MEEA] possesses the highest μ and [P6,6,6,14][TpA] in between. The reduced friction coefficient of the supported [P6,6,6,14][MEEA] IL originates from the observed more ordered ion layering, speeding up the kinetics of gaseous molecules in the supported IL. The monitoring of VOCs in these supported ILs offers an excellent opportunity in e-nose sensing devices to easily and cost-effectively detect VOCs at ppm concentrations.

Effect of Reinforcement with Short Carbon Fibers on the Friction and Wear Resistance of Additively Manufactured PA12.

Reinforcing thermoplastic materials for additive manufacturing with either short, long, and continuous fibers or micro/nanoparticles is a sound means to enhance the mechanical/tribological properties of functional 3D printed objects. However, despite the fact that reinforced thermoplastics are being used extensively in modern applications, little data are found in open literature regarding the effect of such reinforcements on the friction and wear characteristics of additively manufactured objects. Therefore, this article presents a comparative study that aims to investigate the friction and wear behavior of carbon fiber-reinforced polyamide 12 (CF-PA12) as compared to pure polyamide 12 (PA12). The test specimens were prepared by selective laser sintering (SLS) at five different build orientations and examined using a pin-on-disc tribometer in dry sliding mode. The coefficient of friction (COF), interface temperature, friction-induced noise, and specific wear rate were measured. Scanning electron microscopy (SEM) was used to inspect the tribo-surfaces. The results revealed that both the COF and contact temperature of CF-PA12 are orientation-independent and are lower than those of pure PA12. Also, it was found that, compared with pure PA12, CF-PA12 has 25% smaller COF and 15-40% higher wear resistance. Further, the SEM of tribo-surfaces showed that adhesive wear dominates the surface of pure PA12, while both adhesive and abrasive wear patterns coexist in CF-PA12. Moreover, fiber crushing and thinning were observed, and this, under some circumstances, can result in a considerable increase in frictional noise.

Effect of corrosion on self-centering energy dissipative devices

This study reveals the effect of corrosion on the performance of typical self-centering energy dissipation devices for seismic resilience enhancement. Two types of SC devices, namely, high-strength steel ring spring-based SC damper (HSSR damper) and SMA ring spring-based SC damper (SMAR damper), are examined. These specimens were subjected to 720 h’ accelerated corrosion, corresponding to roughly 20 years’ exposure to city out-door environment. Numerical studies are carried out in ABAQUS to further interpret the test results. Among other important findings, the tests showed that the HSSR damper is likely to lose its self-centering capability after corrosion. The high-strength steel (HSS) rings were severely corroded, and the restoring force provided by the rings was insufficient to overcome the friction action. The SMAR damper showed much reduced sensitivity to corrosion. For both dampers, there was no damage to either the SMA/HSS rings or the other components under repeated loading, implying a low risk of corrosion-related low-cycle fatigue failure. The numerical study implies that the friction condition over the entire contact surface is not identical once the damper experiences corrosion. A “two-region modelling strategy” with a smaller friction coefficient and a larger friction coefficient leads to satisfactory agreements with the test results for the corroded HSSR damper. For the SMAR damper, a constant friction coefficient for the surfaces between inner and outer rings has been found to be appropriate for both non-corroded and corroded cases. Based on the limited experimental and numerical data, it is recommended that anti-corrosion measurement should be mandatory for HSSR dampers. Overstrength due to the long-term effect should also be considered in the design of HSSR dampers from a life-cycle design point of view. If no special anti-corrosion measurement is applied, an overstrength factor of 1.3 may be considered. Overstrength may be ignored for SMAR dampers. By representing the key components of SC devices as a group of springs, the analytical description of considered SC devices can be obtained.

Achieving nearly zero earing defects on cylindrical drawn cups with the zoning lubricant technique

Earing has been a major issue in the deep-drawing process, especially for cylindrical cups. The additional work required to fabricate the cylindrical cup according to its specifications is frequently caused by these defects. Only a few methods have been developed to eliminate these defects, despite knowing the origin of these defects to be the material's anisotropic mechanical property (R-value). In this study, we proposed a zoning lubricant technique to achieve practically zero earing defects by applying a varied friction coefficient in relation to the R-value. This technique was examined based on experimental work and the finite-element method (FEM). The deep-drawing process for achieving practically zero earing defects was evident based on the FEM simulation findings. This resulted in practically no earing defects being discovered, and earing defects were avoided as a result of these characteristics. However, to accomplish almost zero earing defects, the proper design of zoning lubricant uses connected to the R-value should be properly considered. According to these findings, a smaller friction coefficient should be placed along the plane and at 90° to the rolling direction, whereas a larger friction coefficient should be positioned at 45° to the rolling direction. Furthermore, it was initially discovered in the current investigation that the use of zoning lubricant could successfully execute the procedure with nearly zero earing defects.

2次均質化法を用いた多結晶材料の圧縮変形における寸法依存性と加工条件の関係評価

In a hot steel rolling process, the rolling force and the strain distribution in a rolled material is strongly affected by the friction coefficient between the work roll and the rolled material and the rolling shape factor which is the ratio of the contact length between the work roll and the material to the material thickness. In this respect, many numerical attempts have been made to reveal their relationship. These simulations often suppose the homogeneity of the material because of simplicity. However, the recent researchers have suggested that the heterogeneity in microstructure can cause size effect on deformation behavior of polycrystalline material. In order to clarify the size effect in the rolling process, this study investigates the size effect on the deformation force and the strain distribution in the plane strain compression which is simple and similar to the flat rolling in effects of the friction coefficient and the shape factor, using the second-order homogenization-based finite element (2nd HMFE) simulations. Simulations are performed by two-dimensional 2nd HMFE with different friction coefficients, shape factors, work hardening parameters and size ratios between micro and macrostructures, which are related to strain heterogeneity in a deformed material. As a result, in case of the small friction coefficient, relatively uniform deformation tends to occur in the material, which results in small effects of the size ratio on deformation behavior. In contrast, in case of the large friction coefficient, size effects on the compression force and the strain distribution appears to be large when the shape factor of a deformed material approaches 1. It is considered to be because high strain gradient tends to reduce the plastic work in the microstructure and a deformation band in which the strain gradient is higher is formed in the material in such a case.

Tribocorrosion behavior of high-entropy alloys FeCrNiCoM (M = Al, Mo) in artificial seawater

A corrosion, wear and synergism behaviors between them of the high-entropy alloys (HEAs) FeCrNiCo(I-0), FeCrNiCoAl(I-Al) and FeCrNiCoMo (I-Mo) in artificial seawater were studied quantitatively. The I-Al has small coefficient of friction and large wear rate because the Al dissolve preferentially. The wear rate of I-Mo is the smallest for the wear debris containing molybdate fills in the corrosion pits where the FCC phase is corroded firstly. The wear is main cause of the tribocorrosion compared with the corrosion. The corrosion rate increases due to the wear is 100 times larger than the wear rate increases due to the corrosion.

Study on the slip characteristics of rock inhomogeneous friction interface

Theory assumes that the friction along the discontinuous interface is uniform, and the selection of uniform distribution of friction resistance along the interface is a simplification of the actual physical process. This assumption leads to unsafe design. Therefore, the numerical simulation and experimental methods are used to study the slip of the discontinuous interface with non-uniform friction. This study used a numerical model comprised of two blocks with completely matching contact interfaces. The friction coefficient of one part of the contact surface differed from that of the other, resulting in a non-uniform friction interface. The model was simulated under biaxial compression. First, a normal load of 3 MPa was applied, followed by a shear load until the contact interface slipped. The initiation and propagation of slippage at the contact interface and the changes in the stress field at the slippage contact interface were monitored. The slip started from the area with low frictional strength and gradually expanded to the area with high frictional strength with increasing shear load. The transfer of the slip from an interface with a small friction coefficient to a high friction strength resulted in stress concentration at the interface of the non-uniform friction interface. Engraving using an engraving machine produces a discontinuous interface in which one part of the interface has high friction strength, whereas the other part has low friction strength. The shear load is applied using a shear instrument to cause the discontinuous interface to slip. At the same time, DIC monitored the displacement of the discontinuous interface. The comparison found that the rule of the experimental results is the same as that of the numerical simulation. Reinforcement measures (enhanced friction strength) for areas with low friction strength can effectively prevent slip damage. Based on the distribution of shear stiffness of the rock discontinuous interface, the friction properties of the entire fracture interface can be obtained to accurately identify areas with low frictional strength, and targeted reinforcement measures should be carried out to prevent slip damage.

Hysteretic performance of self-centering buckling-restrained braces with embedded friction spring

This paper proposes an alternative configuration of an all-steel assembly self-centering buckling-restrained brace (FSBRB) fabricated by combining a buckling-restrained brace and friction spring. The hysteretic performance of the FSBRB is investigated using low-cycle fatigue tests, and the accuracy and applicability of the theoretical hysteretic model are verified by comparing the theoretical, test, and simulation results. The test results show that the hysteresis curve of the FSBRB exhibits a symmetric flag shape with stable hysteresis performance under cyclic loading. The recommended minimum pre-pressure of the friction spring is not less than four times the yield force of the buckling-restrained brace to achieve a full self-reset. The friction spring also has non-negligible energy dissipation capacity, which accounts for 60% of the total energy dissipation of the FSBRB under the zero residual displacement condition. The equivalent viscous damping ratio of the brace is approximately 20%. The parameter analysis results obtained by simulation show that the change in taper angle has the greatest influence on peak force and residual displacement. Moreover, the friction coefficient mainly affects energy consumption, and pre-pressure ratio has a strong relationship with the yield force of the brace. Accordingly, to achieve the self-centering capacity, the FSBRB design scheme with small taper angle, friction coefficient, and pre-pressure ratio is selected priority to attain the performance objective and reduce the force requirement of connecting parts.