Boundary Friction Coefficient Research Articles

Study of the boundary friction of high-strength steel during the cold rolling

Skidding, which causes strip-surface defects between rolls and strips, is more likely to occur during the cold rolling of high-strength steel (HSS) than during that of ordinary steel. This occurs when the friction coefficient between them is low. However, the cause of the low friction coefficient during the cold rolling of HSS necessitates further investigation. This study conducted rolling tests using a synthetic hydrocarbon with oleic acid and methyl oleate as friction modifiers to compare the boundary friction coefficients of HSS and ordinary steel. Consequently, the friction coefficient of the HSS was lower than that of ordinary steel, regardless of the type of friction modifier applied. Furthermore, neutron reflectometry revealed that thicker adsorption films were formed on the HSS than on ordinary steel. Moreover, the thicker friction modifier film formation on HSS than on ordinary steel probably resulted in a lower friction coefficient during the cold rolling of HSS.

Smoothed Particle Hydrodynamics Approach for Simulation of Non-Newtonian Flow of Feedstocks Used in Powder Injection Molding

The present work aims to explore the ability to simulate flow patterns and the velocity field in the powder injection molding (PIM) process using the smoothed particle hydrodynamics (SPH) method. Numerical simulations were performed using the DualSPHysics platform. A feedstock formulated from 17-4 PH stainless steel powder (60 vol. % of powder) and a wax-based binder system was prepared to experimentally obtain its rheological properties that were implemented in DualSPHysics using two different viscosity models. The numerical simulations were calibrated, and then validated with real-scale injections using a laboratory injection press. During the calibration step, the feedstock flow momentum equation in the DualSPHysics code was modified and boundary friction coefficients at different injection rates were adjusted to create a frictional effect. During the validation step, these calibrated conditions were used to simulate the flow behavior into a more complex shape, which was compared with experimental measurements. Using an appropriate boundary friction factor, both the frictional effect of the boundaries and the stability of the numerical solution were taken into account to successfully demonstrate the ability of this meshless SPH method. The flow front length and feedstock velocity obtained in a complex cavity were satisfactorily predicted with relative differences of less than 15%.

Macroscale Superlubricity of Hydrated Anions in the Boundary Lubrication Regime.

Cations can achieve excellent hydration lubrication at smooth interfaces under both microscale and macroscale conditions due to the boundary layer composed of hydration shells surrounding charges, but what about anions? Commonly used friction pairs are negatively charged at the solid/solution interface. Achieving anionic adsorption through constructing positively charged surfaces is a prerequisite for studying the hydration lubrication of anions. Here we report the hydration layer composed of anions adsorbed on the positively charged polymer/sapphire interface at acidic electrolyte solutions with pH below the isoelectric point, which contributes to the hydration lubrication of anions. Strongly hydrated anions (for the case of SO42-) exhibit stable superlubricity comparable to cations, with strikingly low boundary friction coefficient of 0.003-0.007 under contact pressures above 15 MPa without a running-in period. The hydration lubrication performance of anions is determined by both the ionic hydration strength and ion adsorption density based on the surface potential and tribological experiments. The results shed light on the role of anions in superlubricity and hydration lubrication, which may be relevant for understanding the lubrication mechanism and improving lubrication performance in acidic environments, for example, in acid pumps, sealing rings of compressors for handling acidic media, and processing devices of nuclear waste.

In vitro oral simulation based on soft contact: The importance of viscoelastic response of the upper jaw substitutes.

Real oral processing is the squeezing and shearing between two soft surfaces. The importance of soft palate surface cannot be ignored while focusing on tongue substitutes. Thus the effects of viscoelasticity, roughness of upper jaw substitutes, and fluid rheological properties on lubrication properties were explored by in vitro oral tribology experiments. Different palate substitutes significantly changed the friction curves of pure water, milk, and yogurt. The boundary friction coefficients of pure water and milk are higher under softer or smooth palate substitutes due to stronger viscoelastic responses of friction pairs. Their boundary friction coefficients are lowest at rigid upper jaw substitutes owing to smaller contact angles and deformation. However, the boundary friction coefficient of yogurt is lower owing to its high viscosity, low loss factor, and large particle size under soft friction pairs. In addition, it is highest at rigid palate friction pair because a smaller contact area reduces the entrainment of yogurt, resulting in poor lubricating performance.

Influence of thickeners (microfibrillated cellulose, starch, xanthan gum) on rheological, tribological and sensory properties of low-fat mayonnaises

Microfibrillated cellulose (MFC) is obtained by high-shear treatment of cellulose. MFC is suitable for use as clean-label, low-calorie thickener in semi-solid foods such as mayonnaises due to its high water holding capacity. The aim of this study was to determine the effect of type and concentration of thickener on rheological, tribological and sensory properties of low-fat mayonnaises. Low-fat mayonnaises were prepared with four types of thickeners (MFC, chemically modified starch, native waxy corn starch, xanthan gum) at three concentrations. Higher biopolymer concentrations resulted in increased shear viscosities, G′ and G″, yield stress and enhanced lubrication (i.e. lower friction coefficients). Mayonnaises with modified starch and xanthan gum generally had higher shear viscosity and yield stress compared to mayonnaises with comparable concentrations of MFC and waxy corn starch. MFC-thickened mayonnaises had highest G’, G” and boundary friction coefficients. Sensory properties of mayonnaises were determined using the Rate-All-That-Apply (RATA) method (n = 80). Addition of xanthan gum induced high sliminess and pulpiness, and low melting, creaminess and smoothness. Sensory properties of mayonnaises with MFC were generally similar to those with modified and waxy corn starch, despite differences in appearance (increased yellowness and slightly lower glossiness). Multiple Factor Analysis revealed that more shear-thinning mayonnaises were perceived as slimy. Boundary friction was negatively correlated with stickiness, while friction at the start of the hydrodynamic regime was positively correlated with melting sensations. We conclude that microfibrillated cellulose can be used as a thickener in low-fat mayonnaise as an alternative to commercially used chemically modified starch without considerably affecting its sensory texture properties.

Stability, Structure, Rheological Properties, and Tribology of Flaxseed Gum Filled with Rice Bran Oil Bodies.

In this study, rice bran oil bodies (RBOBs) were filled with varying concentrations of flaxseed gum (FG) to construct an RBOB-FG emulsion-filled gel system. The particle size distribution, zeta potential, physical stability, and microstructure were measured and observed. The molecular interaction of RBOBs and FG was studied by Fourier transform infrared spectroscopy (FTIR). In addition, the rheological and the tribology properties of the RBOB-FG emulsion-filled gels were evaluated. We found that the dispersibility and stability of the RBOB droplets was improved by FG hydrogel, and the electrostatic repulsion of the system was enhanced. FTIR analysis indicated that the hydrogen bonds and intermolecular forces were the major driving forces in the formation of RBOB-FG emulsion-filled gel. An emulsion-filled gel-like structure was formed, which further improved the rheological properties, with increased firmness, storage modulus values, and viscoelasticity, forming thermally stable networks. In the tribological test, with increased FG concentration, the friction coefficient (μ) decreased. The elasticity of RBOB-FG emulsion-filled gels and the ball-bearing effect led to a minimum boundary friction coefficient (μ). These results might contribute to the development of oil-body-based functional ingredients for applications in plant-based foods as fat replacements and delivery systems.

Numerical and Experimental Investigation of Quasi-Static Crushing Behaviors of Steel Tubular Structures.

In this study, a numerical and experimental investigation of the quasi-static crushing behavior of steel tubular structures was conducted. As the crushing failure behavior involves a high level of nonlinearity for the numerical simulations, these were compared with previous experimental works, including crushing tests of steel square tubes to calibrate the numerical results. Six parameters for the numerical simulations, namely (1) loading boundary condition, (2) geometrical imperfection, (3) friction coefficient, (4) element size, (5) element type, and (6) material nonlinearity model, were examined using a series of finite element analyses. Through the sensitivity study for each parameter, the deformation and crushing load of the steel tube were investigated, and the value that best matched the experimental results was selected. The results of the numerical analysis for the determined model were compared with the experimental results. Finally, the authors provided recommendations that should be considered when performing nonlinear finite element simulations of crushing failure events.

Mechanistic insights into the effect of structural factors on film formation and tribological performance of organic friction modifiers

The influence of molecular structure of organic friction modifiers (OFMs) on their adsorption behaviour on iron oxide surface immersed in PAO oil was investigated using neutron reflectometry (NR). The effects of carbon chain length and levels of unsaturation on film formation was studied using a series of fatty acids with 18–20 carbon atoms in the temperature range between 25 and 100 °C. The results revealed that saturated OFM molecules generate thicker films compared to its unsaturated counterparts. Further, a decrease in film thickness was observed with the degree of unsaturation with the exception of linolenic acid. The film thickness and grafting density of the OFM increased with temperature. A decreased film thickness for the binary additive system comprising GMO and saturated molecules suggested that the adsorption of OFMs on the surface is hindered in the presence of glycerol mono oleate (GMO). A lower boundary friction coefficient and wear exhibited by OFMs with longer chain and linear structure suggest that their tribofilms are more effective in withstanding shear. The tribofilm thickness measured using optical interferometry showed no correlation with the adsorbed layer thickness determined from NR data; this lack of correspondence is attributed to the load and shearing forces experienced by the OFM molecules at the contact surfaces during friction measurements.

Influence of Base oil Polarity on the Tribological Performance of Surface-Active Engine Oil Additives

Friction, wear and tribofilm growth of organic friction modifiers (glycerol monooleate and oleamide), anti-wear additive (ZDDP) and binary additive system comprising the organic friction modifiers and ZDDP were studied in polyalphaolefin (PAO) and ester oil. The mechanisms underlying base oil polarity-dependent frictional performance of the OFM and AW additives at high temperature (140 ℃), either singly or in combination, were investigated in the light of chemical composition analysis of the tribofilms post friction measurements using energy dispersive X-ray spectroscopy (EDX), static and dynamic time-of-flight secondary ion mass spectrometry (ToF–SIMS). Depending on the rubbing conditions, the boundary friction coefficient of the binary additive systems was found to be either lower than that of individual additives or to lay between the values for the individual additives. Chemical composition analysis of the tribofilms indicated that the nature of base oil controlled interactions between ZDDP and OFM and consequently adsorption and reactive tribofilm formation in the boundary lubrication layer. Surface roughness and wear scar width measured post tribological tests using 3D surface profiler showed improved wear performance in both PAO and ester-based additive formulations.

Ice speed skating: Onset of lubrication by frictional heating

In ice speed skating, the friction between the steel skate blade and the ice stems from boundary friction where the temperature of the interface is below zero and ice surface molecules exhibit unconventional mobility, and hydrodynamic friction where the ice melts and a thin water layer between the blade and the ice forms. An analytical solution of the steady-state lubrication equation shows that the boundary friction only plays a role at the tip of the skate blade over an extremely short contact length between the skate blade and the ice. Albeit its negligible contribution to total friction, boundary friction generates enough heat to melt the ice which allows the skater to slide smoothly on a thin layer of melt water. It is also shown that the precise value of the boundary friction coefficient is not important for speed skate friction because it works only over an extremely short contact length.

The method for leakage detection of urban natural gas pipeline based on the improved ITA and ALO

To solve the problems of the difficulty in early leakage monitoring and larger positioning error for urban hazardous chemicals pipelines, the optimized method based on the improved Inverse Transient Analysis (ITA) and Ant Lion Optimizer (ALO) was proposed. Firstly, based on the obtained experiment's results of leakage of natural gas in the non-metallic pipeline, the segment classification method was incorporated into the pressure gradient calculation. The modified method can adapt to the multi-node characteristics of urban pipe networks and help to obtain the preliminary positioning calculation results after optimization. Then the calculation results were embedded in the ITA calculation model. The input parameters of the gas pipeline such as boundary conditions, leakage rate and friction coefficient were used to establish the characteristic linear equations. Then the objective function of the least-squares criterion was defined, and the improved ITA model suitable for leakage detection of urban natural gas pipeline networks was constructed. Finally, the ALO was used to optimize the calculation process of the improved ITA model, and iteratively optimize the optimal friction coefficient and its corresponding minimum objective function (OF) value. As a result, a more precise location of the leakage source was calculated. The validation of the modified method is conducted by comparing the calculated values with the experiment's results. The results show that the method can accurately predict the location where the pipeline leakage occurs. The minimum error is 3.17%. Compared with the traditional ITA, this method not only accelerates the convergence speed of the objective function, but also improves the accuracy of location calculation.

Mixed lubrication of coated angular contact ball bearing considering dynamic characteristics

AbstractA mixed elastohydrodynamic lubrication (EHL) model for a coated angular contact ball bearing is developed by coupling the influence coefficients with the unified deterministic mixed EHL model. Based on the bearing quasi‐dynamics analysis, the influences of the surface roughness, coating thickness, and boundary friction coefficient on the mixed EHL performance of the coated bearing are discussed. The results reveal that high surface root‐mean‐square roughness can be detrimental to bearing lubrication performance and increase the risk of coating failures. Coating thicknesses on the order of a few microns have a limited effect on the contact load ratio, film thickness ratio, and friction coefficient while having a significant effect on the maximum surface tensile stress and interfacial shear stress, especially for high‐friction low‐speed conditions. A low‐friction self‐lubricating coating can effectively improve the tribological performance and prolong the service life of the bearing, especially in the bad lubrication state with severe asperity contacts.

Oral tribology, adsorption and rheology of alternative food proteins

Mechanistic knowledge using tribology and adsorption may help to screen various proteins with better lubrication; aiding the fast tracking of new ingredient formulations for use in low-fat/high protein food development. The aim of this study was to compare the lubrication, adsorption and physicochemical properties of alternative proteins (pea, potato, lupin and insect proteins) with whey protein isolate (WPI) as the control. Soluble fractions (1–10 wt%) of pea protein concentrate (PPCsol), insect protein concentrate (IPCsol), potato protein isolate (PoPIsol) and lupin protein isolate (LPIsol) were chosen as the alternative proteins. All proteins were negatively-charged at neutral pH and showed various degrees of aggregation (hydrodynamic diameters ranging from 25 nm for PoPIsol to 244 nm for PPCsol). The boundary friction coefficient (μ) at 5 wt% protein followed the trend as PPCsol > LPIsol > IPCsol > PoPIsol > WPIsol, highlighting excellent lubrication performances of PoPIsol, IPCsol and WPIsol. At higher protein concentrations (10 wt%), μ significantly increased for LPIsol, PoPIsol and IPCsol, while decreasing for WPIsol. Quartz crystal microbalance with dissipation monitoring (QCM-D) results revealed formation of rigid elastic films on hydrophobic surfaces by PoPIsol and WPIsol giving rise to low μ while more viscous films by PPCsol led to high μ. PPCsol had the highest hydrated mass (11.0 mg m−2) as compared to WPIsol (8.0 mg m−2) with lower values reported for other proteins (5.0–5.4 mg m−2). Strong correlations existed between μ scaled to viscosity, size and hydrated mass and viscoelasticity of films in alternative proteins, validating the surface-linked phenomena in frictional response.

Boundary Friction of ZDDP Tribofilms

The frictional properties of ZDDP tribofilms at low entrainment speeds in boundary lubrication conditions have been studied in both rolling/sliding and pure sliding contacts. It has been found that the boundary friction coefficients of these tribofilms depend on the alkyl structure of the ZDDPs. For primary ZDDPs, those with linear alkyl chains give lower friction those with branched alkyl chain ZDDPs, and a cyclohexylmethyl-based ZDDP gives markedly higher friction than non-cyclic ones. Depending on alkyl structure, boundary friction coefficient in rolling-sliding conditions can range from 0.09 to 0.14. These differences persist over long duration tests lasting up to 120 h. For secondary ZDDPs, boundary friction appears to depend less strongly on alkyl structure and in rolling-sliding conditions stabilises at ca 0.115 for the three ZDDPs studied. Experiments in which the ZDDP-containing lubricant is changed after tribofilm formation by a different ZDDP solution or a base oil indicate that the characteristic friction of the initial ZDDP tribofilm is lost almost as soon as rubbing commences in the new lubricant. The boundary friction rapidly stabilises at the characteristic boundary friction of the replacement ZDDP, or in the case of base oil, a value of ca 0.115 which is believed to represent the shear strength of the bare polyphosphate surface. The single exception is when a solution containing a cyclohexylethyl-based ZDDP is replaced by base oil, where the boundary friction coefficient remains at the high value characteristic of this ZDDP despite the fact that rubbing in base oil removes about 20 nm of the tribofilm. XPS analysis of the residual tribofilm reveals that this originates from presence of a considerable proportion of C-O bonds at the exposed tribofilm surface, indicating that not all of the alkoxy groups are lost from the polyphosphate during tribofilm formation. Very slow speed rubbing tests at low temperature show that the ZDDP solutions give boundary friction values that vary with alkyl group structure in a similar fashion to rolling-sliding MTM tests. These variations in friction occur immediately on rubbing, before any measurable tribofilm can develop. This study suggest that ZDDPs control boundary friction by adsorbing on rubbing steel or tribofilm surfaces in a fashion similar to organic friction modifiers. However it is believed that, for primary ZDDPs, residual alkoxy groups still chemically bonded to the phosphorus atoms of newly-formed polyphosphate/phosphate tribofilm may also contribute to boundary friction. This understanding will contribute to the design of low friction, fuel efficient crankcase engine oils.Graphical

Wear and fluid-solid-thermal transient coupled model for journal bearings

In this study, the transient interactions between the sliding wear behaviour and fluid–solid–thermal (FST) characteristics of journal bearings are revealed using an established mathematical model. The calculated temperature distribution is validated by a comparison with experimental results from the literature. Furthermore, a wear test for lubricated journal bearings is conducted to verify the predicted wear rate. The time-varying wear and FST performances of the journal bearing, including the wear rate, wear depth, fluid pressure, contact pressure, and maximum temperature are calculated numerically. Through numerical simulations, the effects of the boundary friction coefficient and surface roughness on the wear and FST performances are evaluated. To demonstrate the importance of considering the three-dimensional (3D) thermal effect during the wear analysis of lubricated journal bearings, the numerical results predicted by the isothermal model and the thermal model are compared systematically within a wide range of operating conditions. The numerical results reveal that the worn surface profile slightly decreases the maximum temperature. Additionally, the worn region is primarily located at both edges of the bearing, and the time-varying worn surface profile may be beneficial for improving the hydrodynamic effect. Furthermore, the effect of the 3D thermal characteristics on the wear prediction of journal bearings cannot be ignored when the external load, boundary friction coefficient, surface roughness are relatively large.

Correlation of Barreling Effect with Boundary Friction Coefficient during Upsetting of Various Materials Workpieces under Processing Conditions

The necessity of taking into account the barreling effect at the design of die cavities, and open die and closed die forging processes on the basis of pre-forming of billets by upsetting is shown. The division of ideas about the indices of “bulk” and “linear” barreling (barrel shape), which is determined by the ratio of the bulk and linear dimensions of the workpiece after upsetting by flat dies, was made. The functional relation (correlation) between these indices and the friction coefficient on the boundary surface between flat dies and the upsetted workpiece has been determined. The qualitative and quantitative effect of workpiece dimensions (diameter-to-height ratio), strain range, temperature (conditions of cold and hot processing) and type of forming material (structural carbon steel, some of Cu-grades and Al-grades) on the barreling effect and friction coefficient during upsetting was found. The developed method makes it possible to predict a change in the friction coefficient during the upsetting of various material workpiece under given conditions of forming and to monitor their effect to the final shape.

Structure and tribology of κ-carrageenan gels filled with natural oil bodies

Natural oil bodies (OBs) are micron-sized oil droplets found in parts of plants. Due to their nutritional value and characteristic that they are naturally emulsified, OBs have potential applications in food industry. In this study, κ-carrageenan (KC, 1.0 wt %) gels filled with soybean OBs (up to 10 wt % oil content) bound or unbound with the matrix were constructed. Their structure, rheological and tribological properties were characterized. It was found that in the boundary lubrication regime, the lubrication properties of the OB cream and OB emulsion-gels were significantly improved compared with pure water and the KC gels, and are better than the emulsion-gels with medium chain triglycerides (MCT) droplets stabilized by Tween 80. An ultralow boundary friction coefficient (μ) of 0.038 was achieved. The viscosity of the OB cream, the elasticity of the OB emulsion-gels, as well as the ball-bearing and rolling effect of the OBs in and out the gel fragments under shear were believed to contribute to the promising lubrication properties. It was also found that there was no significant difference in μ between the bound and unbound emulsion-gels, although μ of both types of emulsion-gels decreased with increase of oil concentration. The fragmentation and structure failure of the emulsion-gels under shear were believed to weaken the difference.

A Dynamic Wear Model for Micro-Grooved Water-Lubricated Bearings Under Transient Mixed Lubrication Condition

Abstract The study presents a dynamic wear model for micro-grooved water-lubricated bearings considering the transient mixed elastohydrodynamic lubrication (mixed-EHL) condition. In the established model, the modified Archard wear model and the mixed-EHL model are bridged to study the transient interdependent relationship between the sliding wear behavior and the mixed-EHL performance. In order to consider the effect of the transient mixed-EHL performance on the sliding wear, the Archard model is extended to include the time-varying wear coefficient based on the fatigue concept. To verify the presented model, the comparisons with the experimental results available in the literatures have been conducted. In this study, the evolution of the wear and mixed-EHL performance distribution over time is predicted, and the impact of the radial clearance, boundary friction coefficient, and surface parameters on the numerical predictions is evaluated. The simulation results reveal that the worn region moves toward the rotational direction slowly. The simulation results also reveal that the wear rate and the wear coefficient first decrease considerably, and then decrease gently, and the sliding wear geometry promotes the hydrodynamic effects and reduces the asperity contact during the operation. Furthermore, the parametric study demonstrates that dynamic wear and mixed-EHL performance is sensitive to the radial clearance, boundary friction coefficient, and surface parameters.

Flash temperature and anti-wear tribofilm growth mechanisms by asperity contact in top-ring/liner conjunction of IC engines

Understanding the growth mechanism of tribofilms at the top of the liner in IC engines is of great importance to protecting the sealing function of the top ring. In present study, the transient heat transfer is investigated between the ring and the liner in the top dead center (TDC) area with considerations of asperity contact pressure, boundary friction coefficient and reciprocating piston motion. A 3D transient model is developed to predict the evolutions of effective heat partition and conduction between the contact surfaces. It is revealed that the flash temperature in the asperity contact pairs of the top-ring/liner conjunction is remarkably higher than the steady-state liner temperature, therefore significantly enhancing the growth rate of anti-wear film formation based on a stress-activated Arrhenius model. The results show that the asperity contact model and the flash temperature model can be employed as an engineering tool for anti-wear tribofilm prediction in top-ring/liner conjunction of IC engines.

Numerical investigation of mixed convection heat transfer behavior of nanofluid in a cavity with different heat transfer areas

The main purpose of this research is the numerical modeling of laminar mixed convection heat transfer inside an open square cavity with different heat transfer areas. In the considered geometry, cold fluid enters the cavity. At the middle of the cavity, there is a hot isothermal circular heat source. For increasing the heat transfer, solid silver nanoparticles with volume fractions (φ) of 0, 2 and 4% are added to water. Studied Re numbers are 10, 50, 120 and 200. The location of the hot zone changes the temperature distribution in the fluid layers. If the heat transfer area is located in an appropriate location, temperature distribution becomes more uniform. Increasing Re leads to smaller temperature gradients in regions near the hot surface and higher temperature at fluid layers close to the surface. By increasing fluid velocity, backflows do not improve heat transfer but it is able to change the heat transfer mechanism. By decreasing the fluid velocity, the effects of velocity gradients and extension of the velocity boundary layer increase and friction coefficient attains the maximum value.