Transient Friction Research Articles

Linking macroscale frictional properties to nanoscale water capillary bridges

Friction and unloading experiments of an ox-Si3N4 ball against a PDMS film were conducted using an in-situ optical microtribometer. The interface was first wetted with 0.1 mL water and allowed to dry out during the sliding. A transient friction coefficient 60% greater than in dry condition always occurred in accordance with discretely distributed nanoscaled water bridges, increased pull-off force, stick-slip amplitude, and contact deformation. In summary, this study provides experimental support for the hypothesis that multi-asperity capillary bridges could significantly impact the macroscale frictional behavior of solid-solid contacts.

Transient mixed-lubrication analysis of low-viscosity lubricated bearings under impact load with consideration of turbulence

For heavy-duty bearings with low-viscosity lubricant, asperities may contact near the minimum nominal film thickness, while turbulence may develop in the area with large film thickness. When bearings encounter transient impact or unsteady loads, the influence of turbulence and surface roughness is relatively complex. To investigate transient lubrication and dynamic characteristics of mixed-lubricated bearings with turbulent flow, a transient mixed-lubrication model considering turbulence is proposed in this paper. A transient generalized average Reynolds equation is derived based on the Ng-Pan turbulence model. The transient journal center positions are obtained by solving the journal's dynamic equation. The numerical procedure is established. Based on the proposed model, the effect of turbulence, surface roughness, and transient impact load direction as well as the magnitude on transient lubrication and dynamic characteristics of mixed-lubrication bearings is analyzed. The results show that turbulence increases the transient minimum nominal film thickness and may decrease the transient friction force in the mixed-lubrication regime. Surface roughness modifies the dynamic trajectory of the journal center and increases both the transient minimum nominal film thickness and the friction force in the mixed-lubrication regime. The impact load direction significantly affects transient characteristics of the bearing. An increase in the load-deflection angle destabilizes the bearing operation state.

Numerical investigation on transient third-grade magnetized nanofluid flow and radiative convection heat transfer from a stationary/moving cylinder: nanomaterial and nanoparticle shape effects

In this study, a mathematical model is developed for analyzing the time-dependent magneto-convective flow and heat transfer characteristics of an electrically conducting (functional) third-grade Reiner-Rivlin non-Newtonian nanofluid from a moving or stationary hot cylinder in the presence of magnetic field and thermal radiation. A well-tested convergent Crank–Nicolson type finite difference algorithm is employed to solve the transformed, nonlinear boundary value problem. The Tiwari-Das nanofluid volume fraction model is adopted for nanoscale effects and the Rosseland algebraic flux model for radiative heat flux effects. It has been shown that the shape of nanoparticles remarkably contributes to the enhancement of heat transfer. Several metallic nanoparticle types such as Al2O3, Cu, and TiO2 are examined. It is found from the investigation that the viscoelastic nanofluid with TiO2 nanoparticles results in more heat transfer than the other nanoparticles. Lower velocity and higher temperature values are computed at transient conditions with a higher third-grade fluid parameter for the flow of nanofluid (Al2O3-SA). The plots of transient friction and heat transfer coefficients are visualized at the surface of a hot cylinder. The tabulated heat transfer coefficient is comparatively more for the moving cylinder than the stationary cylinder. Detailed validation of results of the numerical scheme with previous studies is included. The simulations find applications in coating deposition (enrobing) of magnetic nanomaterial at high temperatures, functional nanomaterial synthesis, etc.

On the transient thermomechanical contact simulation for two sliding bodies with rough surfaces and dry friction

A transient thermomechanical contact model for two sliding bodies with rough surfaces is presented. The underlying equations are solved for mechanical, thermal and thermoelastic submodels from which dimensionless influence functions for a loaded rectangular element on a halfspace are derived. They can be used to calculate displacements, stresses as well as temperature distributions for arbitrarily shaped and time dependent heat sources or stress distributions on the surface. All formulas are verified by comparison to solutions from the literature. The submodels are afterwards combined into one thermomechanical simulation model by coupling the heat flux to the frictional power while avoiding the use of a prescribed friction coefficient. The solution process is described in detail. Calculations for two nominally flat but periodic rough surfaces with 256 × 128 elements are performed for a range of sliding velocities and indentations. Simulation results are discussed in particular with regard to the obtained transient friction coefficient.

Temperature induced friction increase in friction test and forming demonstrator for sheet metal forming

High process stability is needed in sheet metal forming industry. This can be achieved by predicting and controlling the transient process and temperature variation, especially at start of production. In this connection, the temperature induced friction changing plays a significant role because it leads to product failures. The handling of the transient friction effects is currently done reactively, based on the individual experience of the machine operators. In future, those transient effects need to be controlled. This paper shows initially an analysis of the temperature induced friction increase in a well-known and proven flat strip drawing test. Different tribological systems were tested at tool temperatures between 20 and 80 °C. The temperature increase results in a higher friction of up to 77 %. Several influences on friction increase will be presented. These friction influences were verified afterwards with a heated forming demonstrator under laboratory conditions.

A High Fidelity Model Based Approach to Identify Dynamic Friction in Electromechanical Actuator Ballscrews using Motor Current

An enhanced model based approach to monitor friction within Electromechanical Actuator (EMA) ballscrews using motor current is presented. The research was motivated by a drive in the aerospace sector to implement EMAs for safety critical applications to achieve a More Electric Aircraft (MEA). Concerns in reliability and mitigating the single of point of failure (ballscrew jamming) have resulted in consideration of Prognostics and Health Monitoring (PHM) techniques to identify the onset of jamming using motor current. A higher fidelity model based approach is generated for a true representation of ballscrew degradation, whereby the motor is modelled using ‘dq axis’ transformation theory to include a better representation of the motor dynamics. The ballscrew kinematics are to include the contact mechanics of the main sources of friction through the Stribeck model. The simulations demonstrated feature extraction of the dynamic behaviour in the system using Iq current signals. These included peak starting current features during acceleration and transient friction variation. The simulated data were processed to analyse peak Iq currents and classified to represent three health states (Healthy, Degrading and Faulty) using k-Nearest Neighbour (k-NN) algorithm. A classification accuracy of ~74% was achieved.

Study on the tribo-dynamic performances of water-lubricated microgroove bearings during start-up

In the present works, a tribo-dynamic model considering the transient friction force is proposed to study the tribo-dynamic performances of the Water-Lubricated Microgroove Bearings (WLMB) during start-up under three acceleration modes. Based on the developed model, the tribo-dynamic performances of WLMB between three bottom shapes, i.e. triangle, left triangle, right triangle, are compared. In addition, the effects of the key factors, including the acceleration time, bearing shell thickness, surface parameters and dry friction coefficient, on the tribo-dynamic performances of WLMB are identified. Numerical results demonstrate that at the initial stage of start-up, the transient horizontal friction force drives the rotor along the opposite direction of rotation. The comparative analysis indicates that the sinusoidal acceleration mode and left triangle bottom shape may be recommended to yield the optimal tribo-dynamic performances of WLMB during start-up. The parametric studies reveal that during start-up, the acceleration time and the surface roughness significantly affect the tribo-dynamic performances, whereas the asperity curvature slightly affects the tribo-dynamic performances. Furthermore, the numerical results demonstrate that the dry friction coefficient plays an important role in tribo-dynamic analysis for WLMB during start-up.

Determination of Wear, Friction Behavior and Characterization of Carbon Fiber Reinforced Epoxy Composites for Transport Applications

Abstract Thermoplastics are finding the place in the current industrial sector due to its load bearing capacities. In this research, With the aid of pin on disc test set up, adhesive and abrasive wear behavior of leaf spring materials 30% short carbon fiber reinforced epoxy (SF), 30% long carbon fiber reinforced epoxy (LF) as well as Unreinforced epoxy (UF) are evaluated for automobile applications. Under multi pass abrasive wear condition, the effect of fiber reinforcement on plastic energy of deformation, matrix crystallinity and clogging behavior were investigated. The transient friction of Leaf spring materials was carried out under the load condition of 19.62N and 28.43N. During the friction test, the effect of fiber length, fiber loading condition on the co-efficient of friction and its specific wear rate of the composite materials are also investigated. The increase in load during adhesive mode for all the materials, specific wear rate and wear volume of the test materials also increased. Furthermore, the fiber- matrix interface, fractured surfaces were observed thoroughly through Scanning Electron microscopic (SEM) morphology.

Experimental study of transient friction characteristics and velocity distribution of pulsatile flow in rod bundles

Rod bundles are widely used in various devices and energy engineering including heat exchangers, nuclear fuel assembly and so on. The transient fluctuation of flow rate in the thermal devices is a common phenomenon under accident scenarios or ocean condition, the study of pulsatile flows has significant practical relevance for the design and safety of thermal devices. In order to determine the flow characteristics of pulsatile flows in rod bundles, the experimental investigations for transient friction loss and evolution of flow field of pulsatile flows of a 5 × 5 rod bundle (P/D = 1.326) are performed. The experiments were conducted under the ranges of time-averaged Reynolds number Reta = 2000–7000, pulsatile amplitude Au = 0.2–0.6 and pulsatile period T = 10 s–100 s. A comparison of friction factors under steady flows and pulsatile flows is performed to determine the influence of pulsatile flows on frictional characteristics. The results show that the transient friction factors versus flow rate under pulsatile flow presents an “oval” shape, and the “oval” shape has a variation with pulsatile parameters changing. To further reveal the details of pulsatile flow fields, the evolution of flow pattern in the pulsatile flow of Reta = 5200, Au = 0.6, T = 10 s is visualized in rod bundles by Particle Image Velocimetry (PIV). The velocity distribution in different stages of pulsatile flows are displayed and discussed. The influence of velocity gradient and turbulent shear stress in the whole pulsatile period for frictional loss is analyzed, and the variation of transient friction factors under different pulsatile parameters is revealed. The results describe transient behaviors to contribute a better understanding for transient flows in fuel assemblies.

Transient behaviors of friction, temperature and fatigue in different contact trajectories for spiral bevel gears

Abstract The dramatic changes in contact geometry, load, entraining and sliding velocity vectors along contact trajectories of spiral bevel gears may cause significant squeezing action. It suggests that the steady-state analysis at each discrete engaging point cannot effectively describe the tribological behavior in spiral bevel gears. The present study aims to systematically study the transient friction, temperature and contact fatigue behaviors in different contact trajectories for spiral bevel gears based on a newly developed transient mixed EHL model. It shows that the obtained transient friction coefficient is much higher than the steady-state when the gears mesh in the first half of the engagement period. During the entire engagement period, the predicted transient fatigue life is significantly longer than the predicted steady-state fatigue life, which is of importance to the integrated and economical design of spiral bevel gears. The comparison with single half-sine waveform asperity demonstrated that the transient asperity contact pressure is much lower than that in quasi-steady state. The variation of flash temperature appears to be insignificant. Moreover, in different contact trajectories, the weakest point in one engagement period always occurs at a certain point when one teeth pair just finished the engaging-in and another pair of teeth completed engaging-out, and the toe contact exhibits the maximum frictional energy consumption and shortest fatigue life among them.

Mechanical spectroscopy as an in situ tool to study anelasticity of martensitic transition in Fe-16Mn-8Cr-2Co alloy

Abstract Reversible martensitic transition in a homogenized FeMnCrCo (Fe-16Mn-8Cr-2Co) alloy is studied by mechanical spectroscopy with a focus on parameters of anelastic transient effects which accompany martensitic transition. SEM and XRD results reveal that microstructure of homogenized samples consists of α′-martensite (BCT), e-martensite (HCP) and retained γ-austenite (FCC). After first thermal cycle over the martensitic transition, the amount of transformed phase and height of corresponding transient internal friction peaks pronouncedly drop down. Three subsequent thermal cycles lead to minor changes, whereas phase transformation temperature remains the same. Upon heating, the relationship between transient peak height ( Q Tr - 1 ) and test parameters ( T – heating rate, f – measuring frequency and e0 – strain amplitude) can be expressed as Q Tr - 1 ≈ K( T /(f0.73 × e0)).

Assessing the diesel engine performance using piston with dissimilar material piston rings

Abstract Investigation displays that frictional loss in an I.C engine is the utmost important aspect in defining the fuel frugality and performance of an automobile. The behavior and transient friction features of a piston ring with dissimilar materials are assessed experimentally by an engine device. Outcomes are stated for a sequence of tests on piston rings with dissimilar groupings of dissimilar material. Evaluations were provided among three dissimilar kinds of materials for piston rings. A combination of the toroidal piston with aluminium-titanium piston ring has better performance over the other combination.

Modeling of thermo-mechanical coupling in linear friction welding of Ni-based superalloy

Thermo-mechanical coupling effect is the core of friction welding, which determines the microstructure evolution and mechanical performance of friction welded joints. However, the studies on thermo-mechanical fields during linear friction welding (LFW) based on rigid/plastic friction pair model in the past cannot reveal the real physical nature correctly. So in this study, the plastic/plastic friction pair model composed of two reciprocating workpieces was established to describe the friction interface condition, in which the modified Coulomb's model was used to calculate the friction stress. Transient friction heat generation and volumetric heat generation by plastic deformation were used to calculate the heat input. Results showed interface temperature varied with time periodically. Plastic deformation heat during LFW had significant influences on temperature and plastic deformation. Stress field distribution changed alternately with the reciprocate motion, and the variation of joint internal stress reached the quasi-steady state as well as thermal equilibrium. Also, the influences of welding parameters were studied quantitatively. Finally, simulated results were well validated by experimental temperature and shortening histories, as well as microstructure observation. The findings in these studies can help to understand the linear friction welding process, and provide a theoretic foundation on the control of linear friction welding process.

Temperature Chaos, Memory Effect, and Domain Fluctuations in the Spiral Antiferromagnet Dy

The spiral antiferromagnetic phase of polycrystalline dysprosium between 140 K and the Néel temperature at 178 K and its domain wall (DW) dynamics were investigated using high-resolution ultrasonic spectroscopy. Two kinetic processes of quasi-static DW motion occur under non-isothermal and isothermal conditions. A “fast” process is proportional to the rate of the temperature change and results in a new category of anelastic phenomena: magnetic transient ultrasonic internal friction (IF). This IF, related to fast moving magnetic DWs, decays rapidly after interruptions of cooling/heating cycles. A second, “slow” kinetic process is seen as logarithmic IF relaxation under isothermal conditions. This second process is glass-like and results in memory and temperature chaos effects. Low-frequency thermal fluctuations of DWs, previously detected by X-ray photon correlation spectroscopy, are related to critical fluctuations with Brownian motion-like dynamics of DWs.

Water hammer analysis using an implicit finite-difference method

The Implicit Finite-Difference Method (IFDM) for the solution of water hammer in pipe networks is presented. All the equations necessary to calculate the flow and pressure in each node of the network are shown in detail. Section-by-section coupling through the Karney equation allows obtain a system of equations for each pipe section which is easy to solve applying the Thomas' algorithm. Also, there are presented the original expressions in the form of finite differences are presented for: (1) the frictional term of the dynamics equation, (2) the transient friction factor proposed by Brunone-Vítkovsky, and (3) the short pipe replacement elements which allow increase the time step. It is demonstrated that the proposed methodology allows modelling the transient flow with higher level of stability and numerical accuracy in comparison to the Method of Characteristics (MOC), especially when the Courant number (Cn) is less than 1. However, because of IFDM works with weighting coefficients (θ 1 and θ2) which must adopt values generally close to 0.5 depending on the analyzed problem, the achievement of the best near-to-exact solution requires to analyze each case separately, being obligatory to apply a trial/error procedure that can make the analysis cumbersome and time consuming.

Effect of Untampered Plasma Coating and Surface Texturing on Friction and Running-in Behavior of Piston Rings

The running-in behavior and the associated transient friction characteristics of a piston ring with different surface treatments are experimentally evaluated using a custom-made engine testing apparatus. Results are reported for a series of running-in and steady-state experiments on piston rings with different combinations of coated and textured surfaces. Comparisons are provided between five different types of piston rings: (1) with no textures; (2) with textures only; (3) with coating only; (4) first textured and then coated; and (5) first coated and then textured. A combination of the texturing and coating showed 12.5% improvement in the frictional behavior and up to 50% improvement in break-in time compared to cases when only one surface treatment was applied.

Dynamic Sliding Enhancement on the Friction and Adhesion of Graphene, Graphene Oxide, and Fluorinated Graphene.

Graphene and functionalized graphene are promising candidates as ultrathin solid lubricants for dealing with the adhesion and friction in micro- and nanoelectromechanical systems (MEMS and NEMS). Here, the dynamic friction and adhesion characteristics of pristine graphene (PG), graphene oxide (GO), and fluorinated graphene (FG) were comparatively studied using atomic force microscopy (AFM). The friction as a function of load shows nonlinear characteristic on GO with strong adhesion and linear characteristic on PG and FG with relatively weak adhesions. An adhesion enhancement phenomenon that the slide-off force after dynamic friction sliding is larger than the pull-off force is observed. The degree of adhesion enhancement increases with the increasing surface energy, accompanied by a corresponding increase in transient friction strengthening effect. The dynamic adhesion and friction enhancements are attributed to the coupling of dynamic tip sliding and surface hydrophilic properties. The atomic-scale stick-slip behaviors confirm that the interfacial interaction is enhanced during dynamic sliding, and the enhancing degree depends on the surface hydrophilic properties. These findings demonstrate the adhesive strength between the contact surfaces can be enhanced in the dynamic friction process, which needs careful attention in the interface design of MEMS and NEMS.

Effect of different-condition parameters on frictional properties of non-asbestos phenolic resin-based friction material

In allusion to WSM-3 non-asbestos phenolic resin-based friction material, the experimental devices are set up for studying tribological properties of the WSM-3 resin-based non-asbestos brake shoe braking on 16Mn steel. According to collected experiential data of physical test, the change rules of the transient/mean friction coefficient are studied with contact pressure, sliding velocity, and contact surface temperature, and then the curves are drawn. The changing law of the friction coefficient is explored with the different-condition parameters, and the variation mechanism of friction performance of WSM-3 brake shoe material is discussed. The experiment results show that the frictional properties of the materials have not only strong system dependency but also strong coupling with the working conditions of the system. It is required to consider the transient friction coefficient or the average coefficient of friction as a variable in the material tribological design.

Transient experimental and modelling studies of laser-textured micro-grooved surfaces with a focus on piston-ring cylinder liner contacts

This paper presents a comparison between the results from numerical modelling and experiments to shed light on the mechanisms by which surface texture can reduce friction when applied to an automotive cylinder liner. In this configuration, textured features move relative to the piston-liner conjunction and to account for this our approach is to focus on the transient friction response to individual pockets as they pass through, and then leave, the sliding contact. The numerical approach is based on the averaged Reynolds’ equation with the Patir & Cheng's flow factors and the p-θ Elrod-Adams mass-conserving cavitation model. The contact pressures that arise from the asperity interactions are solved simultaneously to the fluid flow solution using the Greenwood and Tripp method. The experimental data is produced using a pin-on-disc set up, in which laser textured pockets have been applied to the disc specimen. Under certain conditions in the mixed and boundary lubrication regimes, both model and experimental results show i) an increase in friction as the pocket enters the contact, followed by ii) a sharp decrease as the pocket leaves the contact, and then iii) a gradual decay back to the pre-entrainment value. From the evidence obtained for the first time from the proposed combined modelling and experimental investigation conducted under carefully controlled conditions, we suggest that these three stages occur due to the following mechanisms: i) a reduction in fluid pressure due to the increased inlet gap, ii) inlet suction as the cavitated fluid within the pocket draws lubricant into the contact, and iii) film thickness decay as oil is squeezed out of the contact. The interplay of these three mechanisms is shown to control the response of micro-textured surfaces under all lubrication regimes.

The Transient Friction Response of a Laser-Textured, Reciprocating Contact to the Entrainment of Individual Pockets

To shed light on the mechanisms with which surface texture improves the tribological performance of piston–liner contacts, we have measured the transient friction response as individual pockets pass through a reciprocating sliding contact. Tests were performed at different sliding speeds and results compared to those from a non-textured, reference specimen under different lubrication regimes. At low speed when the contact is in the boundary regime, friction force falls abruptly as each pocket leaves the contact zone, before gradually returning to an approximately steady-state value. This suggests that each pocket acts to temporarily increase the film thickness, which then decays to its non-textured value as oil is squeezed out. At higher speeds, friction is seen to reduce in a stepwise fashion, since the period between pockets being entrained is less than the time taken for the film to decay. In addition, friction results obtained when the contact is operating in the middle of the mixed regime point to a temporary film thickness collapse as the pocket enters the contact, and this agrees with recent modelling predictions. At higher speeds, the compound effect of successive pockets is to shift the contact to the right on Stribeck curve. These results imply that each pocket gives rise to an increase in film thickness that is both short-lived and small in magnitude (we estimate a few tens of nm). However, the resulting effect on friction can be significant (up to 82 % in this study) for two reasons: (1) provided the pocket frequency is sufficiently high, each successive pocket entrainment builds the film up without there being time for it to reduce back to its steady-state value; (2) when the contact is in the mixed regime, the Stribeck curve is at its steepest and friction is therefore most sensitive to film thickness changes. This has important practical implications in that pocket spacing on piston liners should be varied as a function of reciprocating sliding speed.