Sliding Contact Interface Research Articles

Finite Element Analysis of Current Density Distribution in the Sliding Contact Interface of Electromagnetic Railguns: A Literature Review

Finite Element Analysis of Current Density Distribution in the Sliding Contact Interface of Electromagnetic Railguns: A Literature Review

The influence of proton radiation on the friction and wear of Ti:WS2/P201 composite lubricating materials for space application

The effects of proton radiation (PR) irradiation on the components, friction and wear of Ti:WS2/P201 composite lubricating system were mainly investigated. PR irradiation has a great effect on the physicochemical properties of P201 oil, which can not only decompose oil into small molecular but also make it cross-linking and polymerization. More seriously, oil layer with good wettability transfers to a solid-like organic film on sample surfaces after PR irradiation. In addition, PR irradiation has a slight effect on the microstructure of Ti:WS2 film due to coverage of oil layer. Meanwhile, Ti:WS2/P201 system obviously increases the friction coefficient and wear rate, especially extremely shortened life at high irradiation dose. It attributes to P201 oil occurred degradation when suffers from high PR energy, forming polymer with higher molecular weight. The polymer has poor replenishment on sliding contact interface, so destroying endurance of transfer film, which deteriorates the component and property of the system. At the high irradiation dose, the oil nearly loss fluidity, causing severe abrasive wear and tribo-oxidation, further accelerating the failure of the solid-oil materials.

Analysis of Sliding Contact of Armature Inside the Vertical Curved Rails in Electromagnetic Railgun

In previous studies, the armature–rail sliding contact interface is usually considered to be an ideal slide in which both rails are parallel and symmetric. However, due to the influence of structural deformation of the barrel affected by boundary constraints and the manufacturing errors of rails and armature, it often leads to a variety of nonideal the armature–rail matching such as space curved or twisted rails, offset or deflected armature, and so on. This article focuses on the common vertical curved rail in electromagnetic railgun. By introducing centrifugal force effects and coupling electromagnetic force, the armature–rail contact pressure is analyzed theoretically. According to the 3-D finite-element simulation results, the effect of centrifugal force on sliding contact pressure should not be ignored. The effects of different curved radii, velocities, and densities of armature on the contact pressure are compared and analyzed. The influence law on the contact pressure is obtained, which provides a theoretical basis for the design of the railgun.

Macroscale superlubricity and durability of in situ grown hydrogenated graphene coatings

Although macroscale superlubricity reduces the friction of graphene materials, industrial application has been limited to special environments or complex strain engineering. Herein, high-quality hydrogenated graphene coatings (0 < ID/IG < 0.21) are synthesized on industrial materials in situ by hot-filament chemical vapor deposition. The ball-on-disk friction tests demonstrate that the hydrogenated graphene coatings have superior lubricating properties and long service lifetime at a macro-load of 1 N and high sliding speed (0.6 m/s) under ambient conditions (23–25 °C, 50–55 %RH). The hydrogenated graphene coatings deposited with moderate hydrogen supplies show ultra-low friction and even localized superlubricity (∼0.009) due to the critical factor of surface defect density. Long-term stability is observed after more than 24,840 laps. Molecular dynamics simulation is performed to explain the self-passivating and self-healing effects on the atomic level. The macroscale lubrication mechanism involves H atoms capturing carbon dangling σ-bonds. The high-quality and chemically inert graphene surface facilitates the formation of a thick solid tribofilm at the sliding contact interface. The chemisorbed H atoms on the graphene layer lead to continuous healing and stabilization of the defective structure during the macroscale friction test. Our new findings provide insights into the lubrication mechanism and reveal a new approach to achieving macroscale superlubricity and high durability for hydrogenated graphene coatings.

Impact of Fatty Triamine on Friction Reduction Performance of MoDTC Lubrication Additive

The impact of a fatty triamine (Triameen YT) additive was investigated on the friction performance and stability of molybdenum dithiocarbamate (MoDTC) in the formulations containing polyalphaolefin synthetic base oil (PAO) and zinc dialkyldithiophosphate (ZDDP). Triamine has no significant effect when mixed with MoDTC and ZDDP, but it improves the performance of MoDTC alone. However, in the MoDTC—Triamine—PAO solutions, a chemical reaction easily occurred and a reddish precipitate was formed upon storage. According to IR, XPS, TEM, and XAS characterizations, this precipitate is poorly crystalline layered alkylammonium oxothiomolybdate. Formation of the precipitate impaired the tribological performance by decreasing the number of active species delivered at the sliding contact interface. However, low friction coefficients were recovered by redispersion of the precipitate in PAO.

Interfacial friction of vdW heterostructures affected by in-plane strain

Interfacial properties of van der Waals (vdW) heterostructures dominate the durability and function of their booming practical and potential applications such as opoelectronic devices, superconductors and even pandemics research. However, the strain engineering modulates of interlayer friction of vdW heterostructures consisting of two distinct materials are still unclear, which hinders the applications of vdW heterostructures, as well as the design of solid lubricant and robust superlubricity. In the present paper, a molecular model between a hexagonal graphene flake and a rectangular SLMoS2 sheet is established, and the influence of biaxial and uniaxial strain on interlayer friction is explored by molecular dynamics. It is found that the interlayer friction is insensitive to applied strains. Strong robustness of superlubricity between distinct layers is owed to the structure’s intrinsic incommensurate characteristics and the existence of Moiré pattern. In engineering practice, it is of potential importance to introduce two distinct 2D materials at the sliding contact interface to reduce the interfacial friction of the contact pair and serve as ideal solid lubricants. Our research provides a further basis to explore the nanotribology and strain engineering of 2D materials and vdW heterostructures.

Synergetic effect of surface connectivity and functional parameters on the friction characteristics of a sliding contact interface

The characteristics of surface morphology markedly affect the tribological properties of contact interface such as lubrication and seal performance. In present work, the synergetic effect of surface connectivity and some three-dimensional topography parameters (ISO 25178) on the friction characteristics of a sliding contact interface, based on mathematical morphology and reciprocating linear tribotesting, is explored. The results indicate that the friction properties of sliding contact interface was significantly affected by the variation of surface connectivity. The increase in connectivity index would improve the relative contact area on the isotropic surfaces and affect the distribution and storage characteristics of the fluid, while this enhancement would be blocked on the anisotropic surface and the variation of connectivity index is obviously nonlinear with a shocking feature. The friction tests confirm that the friction coefficient of sliding contact interface decreases with the increasing surface connectivity index, which is mainly related to the enhancement of lubricant storage and flow in each void region and improving the lubrication behavior. Furthermore, the same variation law is applicable to the relationship between the functional morphology parameters and the friction property. The increasing Vvv and Sku, would contribute to a lower friction coefficient at the interface. Our work provides deep insights into regulating the tribological property of contact interfaces through surface topography control.

Experimental estimation of transient evolution of three thermal parameters characterizing a dry friction interface

Several studies have been performed to understand the occurred phenomena in a sliding contact interface. Due to a generated heat flux, the sliding contact necessarily induces an increase of the temperature. These temperatures could reach critical value for the system. Thus, it is imperative to anticipate the temperature increase to avoid severe issues. The aim of this study is to be able to determine the evolution of the thermal parameters characterizing sliding interfaces. In order to do this, we develop a new methodology which involves three transient parameters, the intrinsic partition coefficient, the thermal contact resistance and the heat flux generated by friction. The estimation of time dependent thermal sliding contact parameters is performed by an inverse heat conduction problem (IHCP) from temperature measurements. A parametrization technic with a constrained Levenberg Marquardt algorithm is used to regularize and decorrelate the IHCP. An experimental estimation of the evolution of the thermal contact parameters is performed. Several friction scenarios are investigated and show the relevance of the developed methodology.

Spark Plasma Sintering of Graphene-Reinforced Inconel 738LC Alloy: Wear and Corrosion Performance

This study aims to investigate the microstructure, corrosion and tribological properties of spark plasma sintered graphene nanoplatelets (GNPs) reinforced Inconel 738 low carbon composites. The matrix and reinforcement were thoroughly milled in order to ensure homogeneity. Thereafter, the milled powders were consolidated by using spark plasma sintering. The microstructural evolution and the phases formed were examined by the using scanning electron microscopy and X-ray diffractometry techniques. The corrosion analysis was investigated in acidic and basic media, while the tribological test was conducted under dry sliding conditions at varying loads. The results show that the microhardness values were significantly influenced by varying the GNPs constituents in Inconel 738LC from 384 to 459 HV0.5, while the sintered density was influenced by the sintering parameters. The corrosion response of the sintered composites in both acidic and basic media are comparable, irrespective of the varying GNPs content in the matrix. The wear performance suggests that the addition of GNPs to IN738LC, greatly enhanced the wear resistance and reduced the friction coefficient of the sintered IN738LC-GNPs composites. The improvement is attributed to the influence of the graphene-based tribofilm that formed on the sliding contact interface, which reduced friction coefficient. Likewise, graphene has a slight potential of forming continuous tribofilms at the friction interface due to its lubricity. It is thought that the GNPs reinforcement reduced the pull-out tendency, during wear activities.

Research on Melting Erosion Law of a Multiturn Series–Parallel Railgun During Launching Period

The melting erosion law of the contact interface between armature and rails on the electromagnetic (EM) railgun is one of the key factors affecting the launching performance. In order to study the melting erosion law on the contact interface of a multiturn series-parallel (MSP) railgun, an EM structure thermal multiphysical field coupling model is built, and the armature velocity, contact force, and temperature of contact interface under different current peak values and shrink ranges are analyzed. Besides, the related verification experiments are carried out, and the melting erosion law of the contact interface is obtained. The results of simulations and experiments show the following conclusions: First, during the launching, the contact force between armature and rails varies with current fluctuation. Second, the friction heat power varies directly with the contact force, but Joule thermal power caused by contact resistance varies inversely with the contact force. Third, the ambient temperature of the sliding contact interface can be further improved by reasonable design of armature shrink range. Fourth, due to the inevitable wear during sliding, the melting area of the armature contact surface gradually transfers from the initial contact area to the tail. The above-mentioned conclusions can provide a technical reference for the high-speed experimental study of MSP railgun.

Identification of the mechanical moduli of flexible thermoplastic thin films using reflected ultrasonic waves: Inverse problem

A method for the identification of the mechanical moduli and density of flexible, supple thermoplastic thin films placed on elastic substrates using ultrasonic waves has been developed. The composite medium immersed in a fluid host medium (water) was excited using a 50MHz transducer operating at normal incidence in reflection mode. Inverse problems involving experimental data pertaining to elastic wave propagation in the thin films on their substrates and theoretical fluid-solid interaction models for stratified media using elasticity theory were solved. Two configurations having different interface boundary conditions (BC) were modeled, transverse slip for the sliding contact interface in the case where the thin films were placed on the substrate without bonding; a bonded interface condition. The inverse problem for the recovery of the mechanical parameters were solved for the thin films under the bonded and slip BCs. Substrates made of different elastic materials having different geometries were also evaluated and their advantages discussed.

Characterization of interface stresses and lubrication of rough elastic surfaces under ball-on-disc rolling

This paper presents a statistical method for characterizing the interface stresses between an elastic rough ball and a smooth rigid disc under lubrication by considering the percolation effect of lubricant flow. The statistical analysis was carried out to integrate the random, microscopic asperity deformation with the asperity–lubricant interaction. An efficient solution process with the aid of the fast Fourier transform was developed for the multi-scale analysis. To avoid numerical instability, the Poiseuille term of the average Reynolds equation was turned off when the average lubricant film thickness was below the percolation threshold and the average hydrodynamic pressure was solved by using the Couette term. This successfully overcomes the limitation in the conventional statistical treatment of mixed lubrication. Moreover, it enables a smooth transition from mixed to boundary lubrication and allows a large range of dimensionless rolling speed, which cannot be achieved by conventional approaches. Also, an empirical formula was developed to evaluate the average central and minimum film thicknesses between lubricated rough surfaces in contact. It was found that the method developed can precisely predict the effect of surface roughness on contact stresses, can effectively estimate the contribution of the direct asperity–disc contacts, and can be used to describe the lubricant performance at a rolling–sliding contact interface.

Effects of Stress Distribution at the Contact Interface on Static Friction Force: Numerical Simulation and Model Experiment

This study develops a simplified model that can simulate the dynamics of a split contact interface at the transition from static to kinetic friction. The model has a contact interface formed by multiple contacting points connected to a rigid base via a spring. From a numerical analysis of this model, the effect of the stress distribution at the contact interface on the level of static friction force was investigated. Consequently, it was found that the existence of the stop-restart motion can act to increase the macroscopic (apparent) static friction force. Thus, the numerical analysis demonstrated that the macroscopic static friction coefficient could be changed without varying the local static friction coefficient. The type of tangential loading history, i.e., the existence of stop-restart motion, is an important factor for characterizing the level of the static friction force. In other words, this implies that we can adjust the level of the macroscopic static friction coefficient without changes to the local static friction. Furthermore, the above numerical prediction was confirmed by a model experiment focusing on the sliding contact interface of an object built from separated rubber blocks.

Electric and thermal characteristics of repeated sliding electric contact interface under high pulse current

In solid electromagnetic rail launcher, electric and thermal characteristics of sliding electric contact between armature and rails are related to contact status, current conduction and energy dissipation of sliding interface, influencing launcher system efficiency and rail lifespan. Many repeated launch test groups under different maximum linear current density were designed and carried out. Through iterative computation with test electric parameters, dynamic variation rule of sliding contact resistance and interface joule heat power were obtained, and effect of melt deposition evolvement and linear current density on electro-thermal properties were further analyzed. With inspection of melting deposition on rails after repeated tests, interface evolution process of sliding electric contact in gun bore was discussed. Results show that the steady critical point of sliding electric contact resistance and peak point of interface joule heat power both came up during current falling-edge. Contact resistance value of steady phase can be reduced in 10-2 m, and the maximum value of joule heat power can reach 10-1 MW. Contact resistance and heat power were affected hardly by repeated tests shows that deposition would re-melt and play positive role in sliding electric contact characteristic. Interface joule heat power was enlarged evidently by increasing linear current density even under the same input energy.

Reduced wear and friction enabled by graphene layers on sliding steel surfaces in dry nitrogen

We report on the friction and wear behavior of graphene-lubricated 440C steel test pairs in dry nitrogen under different loads. Tribological test results have revealed that a few-layer graphene is able to drastically reduce the wear and the coefficient of friction (COF) of 440C steel during the initial sliding regime and under low load conditions. Specifically, the COF has been reduced from ≈1 for bare steel to 0.15 for steel covered by a low concentration of graphene flakes. Such low COFs have persisted for thousands of sliding passes, even though the graphene layers formed on sliding surfaces have not been continuous or continuously replenished; they were made of a few sheets of graphene. The wear rates of the steel test pairs have been also reduced (by as much as two orders of magnitude), again despite the very sporadic and thin nature of the graphene layers. A possible explanation for the low friction and wear reduction is that graphene as a two-dimensional material shears easily at the sliding contact interface and, hence, provides low friction.

Analytical predictive modeling of scuffing initiation in metallic materials in sliding contact

Scuffing, described as sudden catastrophic tribological failure of a sliding surface, is a complex phenomenon and consequently still poorly understood. Its occurrence is influenced by many factors such as material microstructure, lubricant chemistry, operating environment, and contact parameters. Over the years, numerous attempts have been made at predictive modeling of scuffing occurrence, often based on the assumption that adhesion and consequent welding between contacting asperities is the basic mechanism of the failure. These previous attempts at scuffing prediction have not been satisfactory. A mechanism of scuffing based on adiabatic shear instability in the near-surface material of sliding interface was recently proposed by the authors. As a follow on to that work, a predictive analytical model is developed for scuffing with adiabatic shear instability as the basic mechanism of failure is presented in the current paper. The model expresses susceptibility of a sliding contact interface to scuffing in terms of material properties and contact conditions. Preliminary correlation was established between the model prediction and scuffing test results with 4340 steel. As modifiers to the shear instability mechanism, the effect of lubricant chemistry and surface reactions and numerous other factors that are known to affect scuffing damage can be incorporated.

Scuffing mechanism of near-surface material during lubricated severe sliding contact

Abstract Scuffing, defined as a sudden catastrophic failure of lubricated sliding contact, is one of the least understood tribological failure processes, in spite of its great technological significance. It is usually accompanied by a large sudden rise in friction, contact temperature, and noise. Occurrence of scuffing results in loss of surface integrity and functionality of the tribological components. Although extensive phenomenological studies have been conducted on scuffing because of its implication regarding machine-element reliability, the basic mechanism of the failure process is still not well understood. Consequently, prediction and sometimes prevention of scuffing failure are difficult. Based on the recent observation of microstructural changes accompanying scuffing failure and various other phenomenological observations, a scuffing mechanism is proposed that involves adiabatic shear instability in the near-surface material at the sliding contact interface. Localized shear instability occurs when the rate of thermal softening due to the heat of plastic deformation exceeds the rate of work hardening—this is scuffing initiation. Propagation of scuffing to final catastrophic failure is determined by a balance between heat generation and heat dissipation. If the rate of heat generation by more plastic deformation exceeds the rate of heat dissipation, scuffing propagates; otherwise, the scuffing process is quenched. Once validated, this proposed mechanism will facilitate the prediction of scuffing based on material properties. It will also provide a mechanism-based strategy for scuffing failure prevention in tribological components.

Flash temperature during impact of layered elastic solids

An analytical model to calculate the maximum flash temperature during oblique spherical impact of an elastic single layered solid is presented. The model is developed by combining the contact parameters obtained from a spherical impact model and flash temperature solution for the sliding contact interface of a layered solid. Dynamic thermomechanical finite element analysis is used to examine the validity of the analytical model with good success. The flash temperature, obtained when the coating is accounted for, is different from that when homogeneous material only is considered, and needs to be accounted for estimating flash temperatures during impact of layered media.

Role of the Transfer Film on the Friction and Wear of Metal Carbide Reinforced Amorphous Carbon Coatings During Run-in

The relationship between friction, wear, and transfer films of three metal carbide-reinforced amorphous carbon coatings (TiC/a:C, TiC/a:C-H, and WC/a:C-H), sometimes referred to as metal-doped diamond-like carbon coatings, has been investigated. Tribological tests were performed in an in situ tribometer with sapphire or steel hemispheres run against coated flats in dry or ambient air. The sliding contact interface was observed and recorded by optical microscopy during reciprocating sliding tests. The friction and wear behavior during run-in depended on the number of sliding cycles to form a stationary transfer film on the hemisphere. Stationary transfer films formed rapidly (within ten cycles) and the friction coefficient fell to 0.2 (ambient air) or 0.1 (dry air), except with sapphire against WC/a:C-H in dry air; with the latter, a stationary transfer film required nearly 100 cycles to form, during which the friction remained high and the wear rate was from 10 to 100 times higher than the other two coatings. For all coatings, three velocity accommodation modes (VAM) were observed from run-in to steady-state sliding and were correlated with the friction and wear behavior. The delayed adherence of the transfer film to sapphire from WC/a:C-H coatings in dry air is discussed in terms of equilibrium thermochemistry. Friction and wear behavior during run-in, therefore, depended on transfer film adherence to the hemisphere and the VAM between transfer films and the coating.

Transmission Electron Microscopy Analysis of Mo–W–S–Se Film Sliding Contact Obtained by Using Focused Ion Beam Microscope and In Situ Microtribometer

To better understand the fundamentals of solid lubrication, microstructural analyses on the wear scar surface and contact interface of Mo–W–S–Se composite films produced by pulsed laser deposition were completed. Focused ion beam (FIB), transmission electron microscopy (TEM), and X-ray energy dispersive spectroscopy were employed to study the cross-sectional microstructure and chemistry of wear scars. In particular, a novel microtribometer was built for in situ tribological measurements within a FIB microscope. The sliding tip was welded in contact to the wear scar surface on the film under load by re-deposition of sputtering materials from the FIB cut of the tip. Using this technique, cross-sectional TEM specimens were prepared precisely at the contact point without tip/film separation. Here, the in situ FIB microtribometer is critically important for retaining the microstructure of lubricant films as formed at the sliding contact interface between the tip and film without separation. It provides the unique ability to stop sliding, section the contact, and reveal microstructural changes to that contact without disrupting the sliding interface. The cross-sectional TEM measurements were performed on the sliding contact interface for both the regions in contact and just past contact, and both the reorientation and recrystallization of lubricant films were revealed.