Friction Coefficient In Lubrication Research Articles

Effect of Ellipsoidal Particle Shape on Tribological Properties of Lubricants Containing Nanoparticles

Adding nanoparticles can significantly improve the tribological properties of lubricants. However, there is a lack of understanding regarding the influence of nanoparticle shape on lubrication performance. In this work, the influence of diamond nanoparticles (DNPs) on the tribological properties of lubricants is investigated through friction experiments. Additionally, the friction characteristics of lubricants regarding ellipsoidal particle shape are investigated using molecular dynamics (MD) simulations. The results show that DNPs can drastically lower the lubricant's friction coefficient μ from 0.21 to 0.117. The shearing process reveals that as the aspect ratio (α) of the nanoparticles approaches 1.0, the friction performance improves, and wear on the wall diminishes. At the same time, the shape of the nanoparticles tends to be spherical. When 0.85 ≤ α ≤ 1.0, rolling is ellipsoidal particles' main form of motion, and the friction force changes according to a periodic sinusoidal law. In the range of 0.80 ≤ α < 0.85, ellipsoidal particles primarily exhibit sliding as the dominant movement mode. As α decreases within this range, the friction force progressively increases. The friction coefficient μ calculated through MD simulation is 0.128, which is consistent with the experimental data.

Numerical calculation of barrel chamber mechanical wear based on the principle of dynamic pressure lubrication

Thermal-chemical erosion and mechanical wear are the main factors restricting the service life of barrel weapons. Considering the lack of systematic research on the mechanical wear of the barrel weapons, this paper gives the display expression of the dynamic pressure lubrication friction coefficient in projectile-barrel clearance based on the principle of dynamic pressure lubrication. The trend of friction coefficient is basically consistent with the friction experiment data. In the framework of dynamic pressure lubrication friction, the mechanical wear model of artillery barrel is derived, and the mechanical wear model in the whole life cycle is calculated by using the ALE mesh adaptive technology. The results show that the derived mechanical wear model can accurately describe the material degradation of the barrel chamber. The mechanical wear is mainly concentrated at the beginning of the rifling and the muzzle. After 500 shots, the mechanical wear of the beginning of the rifling and the muzzle are generally 0.3mm and 0.4mm.

Preparation and Modification of Graphite-based and Coal-based Graphene and its Tribological Properties in Lubricants

Graphite-based graphene oxide (GO) and coal-based graphene oxide (GO/SiO2, which we call graphene oxide/SiO2 because the surface of the resulting coal-based GO is linked with SiO2 from the coal structure) were prepared by the conventional Hummers oxidation method and ultrasound-assisted Hummers oxidation method, respectively, using natural graphite powder and Xinjiang Wucaiwan coal as carbon sources. Furthermore, the structures of the two were studied comparatively. Afterwards, the two lamellar structure products were hydrophilically modified with octadecylamine and dicyclohexylcarbodiimide by a hydrothermal method. And then, added these products (graphite-based GO-ODA and coal-based GO/SiO2-ODA) to No.32 cycloalkyl base oils in order to comparatively investigated their tribological properties as lubricant additives. The results showed that the dispersion stability of coal-based GO/SiO2 in the lubricant was significantly better than that of graphite-based GO; meanwhile, the tribological performance of GO/SiO2-ODA in the lubricant better than that of graphite-based GO-ODA due to the surface linkage of coal-based GO/SiO2 with SiO2 particles. After 1 h - long grinding in a four-ball friction test, the lubricant friction coefficient of 0.03 wt% coal-based GO/SiO2-ODA oil reached a minimum of 0.118, which reduced 58.1% compared to pure base oil and reduced 2.8 times compared to the counterpart using graphite-based GO-ODA under the same experimental conditions.

Molecular dynamics simulation of friction coefficient of Fe-Al during lubrication

Reducing wear and friction coefficient in the process of metal plate processing can save energy. In this work, the quantum chemical parameters of n-hexadecane, tetradecyl alcohol, triphenyl thiophosphate (T309) and dialkyl dithiophosphate (EAK) and the adsorption process on Al (111) plane were calculated based on density functional theory. The confined shear process of the lubricant system mixed with the above four molecules in Fe-Al system was simulated by molecular dynamics method, and the friction coefficient of the lubricant was calculated. The friction coefficients of lubricants were measured by reciprocating friction and wear test. The results show that T309 has the strongest reactivity and the most stable adsorption, and its adsorption energy on Al (111) plane is −12.08 eV. Shear force will affect the adsorption morphology of molecules, and long chain molecules will have a certain degree of crimp. When the pressure was 0.5 GPa, the minimum friction coefficient of lubricant was 0.24, mainly because the molecules would gather and adsorb to the metal wall during the shear process. With the increase of pressure perpendicular to the shear plane, the lubricant molecules on the metal wall would gather less, and the distribution of lubricant molecules between the metal walls was gradually uniform. The tested minimum friction coefficient (∼0.12) of the compound lubricant is attributed to synergistic friction reduction among molecules. Based on molecular dynamics simulation, the friction coefficient of aluminum lubricants can be predicted, and the simulated data are accurate and reliable.

Evaluation of Relation between Friction Coefficient and Oil Film Thickness of Ground Gear

Matsumoto's equation, which determines the ratio of the friction coefficient between boundary lubrication and fluid lubrication by the ratio of the maximum height roughness and minimum oil film thickness, is often used to predict the friction coefficient on gear tooth surfaces. Recently, however, several papers using gears with highly smoothed tooth surfaces have reported discrepancies between the measured friction coefficient of tooth surface and Matsumoto's equation. It seems that the problem of how to determine the boundary lubrication friction coefficient lies behind these discrepancies. In this paper, the coefficient of tooth friction is determined under a wide range of lubrication conditions using ground spur gears, and the experimental results are compared with the calculation results of Matsumoto's equation to reevaluate the relationship between the two. The friction coefficient of boundary lubrication was found to vary with surface roughness of tooth surface.

Effect of friction coefficient on the mixed lubrication model of rotary lip seals

This study is devoted to the effect of different coefficients on the calculation results of the rotary lip seal mixed lubrication model. It was proved by experiments that the dry friction coefficient used in the previous models was quite different from the boundary lubrication friction coefficient, which was theoretically more accurate. The pumping rate, friction torque, pressure distribution, and oil film thickness were calculated using both the dry friction coefficient and the boundary lubrication coefficient and the results were quite different. A friction coefficient test method under boundary lubrication condition for numerical simulation of rotary lip seals and an improved method for numerical calculation using the boundary lubrication coefficient instead of the dry friction coefficient were proposed. It was verified that the accuracy of numerical calculation can be improved, and the calculation result was closer to the actual working state.

Mapping of the lubrication regimes in rough surface EHL contacts

Abstract Understanding film formation in rough surface elastohydrodynamically lubricated (EHL) contacts have been an ongoing pursuit in lubrication science for more than half a century. This study furthers that quest by establishing a single combined friction and electrical contact resistance map that forms a clear and comprehensive overview of the lubrication performance. A ball-on-disc machine was operated under a wide variety of heavily loaded rolling/sliding contact conditions. Results show that while sweeping the contact over the SRR- and entrainment speed-domain, the primary sweep direction significantly affects running-in and consequently the transition from EHL to the mixed lubrication regime. Such knowledge sheds new light into the mechanisms that governs EHL film formation and the concurrent interplay with the mixed lubricated friction coefficient.

Meta-Analysis Comparing Wettability Parameters and the Effect of Wettability on Friction Coefficient in Lubrication

This work presents a meta-analysis that compares the suitability of various parameters used to characterize wettability in tribological systems. It also examines the relationship between wettability and the friction factor for multiple lubricant-surface pairings. The characterization of wetting behavior was similar when using the contact angle between a lubricant and surface and various dimensional and dimensionless formulations of a spreading parameter. It was possible to identify hydrodynamic, boundary, and mixed lubrication regimes by combining a dimensionless wettability parameter with the specific film thickness for a variety of neat ionic liquids and magnetorheological fluids in contact with metallic, thermoplastic, and elastic surfaces. This characterization was possible using multiple dimensionless wettability parameters, but those that can be fully determined using only the contact angle may be preferred by experimentalists. The use of dimensional and dimensionless wettability parameters that included polar and disperse components of surface tension and surface energy did not appear to provide additional insight into the wettability or frictional performance for the tribological system examined here.

Friction Reduction by Micro-Textured Surfaces in Lubrication

Experimental investigations were carried out to verify if the friction reduction in lubrication can be expanded by a textured surface with sawtooth riblets. Sawtooth riblets were formed by ultraprecision diamond cutting, with a ridge angle of about 60°–90° and height of about 20–50 μm on the contact surface. Six types of textured surfaces with different ridge angles, heights, and sliding directions were tested and compared with the untextured surface. The tribological tests were conducted by a flat-on-flat tribometer in lubrication. The effects of the ridge angle, height, and relative sliding direction on the friction coefficient in lubrication were reported.

A Comparative Study on the Friction and Wear Properties of Three Different Copper Alloys

A comparative study on the friction and wear properties of three kinds of copper alloys, including Cu-Ni based, Cu-Al and Cu-Be alloys was carried out in this study. The friction pair was stainless steel, and both dry and MoS2 lubrication friction experiments were investigated. During the experiments, different loads were chosen for different alloys. It was found that under dry friction condition, the friction coefficients of both Cu-Ni based and Cu-Al alloys did not change as the loads changes, whereas the friction coefficient of Cu-Be alloy increased as the loads increases. Under lubrication friction condition, the friction coefficients of all three alloys did not change as the load changes. The results show that the dry friction coefficient of Cu-Ni based alloy was the largest (0.74), the Cu-Al alloy next (0.60), and the Cu-Be alloy had the smallest dry friction coefficient (0.54). The lubrication friction coefficient of Cu-Ni based and Cu-Be was equal and relatively smaller (0.12), whereas the Cu-Al alloy had a relative larger lubrication friction coefficient (0.27). The microstructure observations were consistent with the friction and wear performance, and the SEM results show that different wear mechanisms were dominated for different alloys.

Friction Characteristics of Ultrathin Perfluoropolyether Boundary Lubricant Films Subjected to Laser Irradiation Heating in Heat-Assisted Magnetic Recording

We investigated the laser-heating-temperature dependence of the friction characteristics of magnetic disks coated with boundary lubricant films by using a pin-on-disk tester and discussed the fundamental characteristics observed. In the contact area of the glass pin and disk, we observed the area that showed a buildup and depletion of the lubricant film thickness. The amount of buildup and depletion increased with increasing heating temperature. It was found that the friction coefficient of each lubricant film initially decreased as the temperature increased. Thereafter, the minimum value of the friction coefficient of each test lubricant occurred at a specific temperature, resulting in an increase in the friction coefficient inversely as the temperature further increased. It was suggested that the specific temperature corresponds to the transition temperature at which the test lubricant film evaporates. In addition, it was also suggested that the thermal durability of the lubricant film improved because of ultraviolet treatment.

Study of hydrodynamic behaviour of large bearings depending on the viscosity of the lubricant

The research refers to study of hydrodynamic behavior of large bearings equipped with automatic lubrication system, where the liquid used may have a viscosity with variable values depending on the temperature or depending on the additive used. The introduction of additives with nanoparticles causes modification of viscosity and a significant reduction of friction coefficient. This study establishes a direct link between the friction, viscosity and tensions that occur in the contact area. Experimental studies confirm improving tribological properties of the contact area between the bearing rollers and raceways, through attracting nanoparticles on surfaces in contact and reducing friction coefficient. Accomplished finite element analysis showed decrease of the contact stresses, therefore decrease of the wear and increase the life of the bearing. Research has concluded that influencing viscosity and friction coefficient of lubricant can result in reducing wear of the bearing components, as well as increase the lifetime of bearing.

Optimising the design of textured surfaces for reducing lubricated friction coefficient

AbstractUnder operating conditions which are unfavourable for lubrication, such as high load and low velocity, the use of textured surfaces significantly promotes the formation of a thick lubricant film and an improvement of the friction coefficient. This paper relates to the manufacture of textures using a photolithography and chemical etching process. Different surface geometries, texturing densities and depths were designed to analyse the influence of these parameters. The friction coefficient was measured in a ball‐on‐disc tribometer under different lubrication regimes, and the results have been used to develop an artificial neural network with texturing optimisation potential. © 2016 The Authors Lubrication Science published by John Wiley &amp; Sons Ltd.

Microscopic mechanism for shear thickening of non-Brownian suspensions.

We propose a simple model, supported by contact-dynamics simulations as well as rheology and friction measurements, that links the transition from continuous to discontinuous shear thickening in dense granular pastes to distinct lubrication regimes in the particle contacts. We identify a local Sommerfeld number that determines the transition from Newtonian to shear-thickening flows, and then show that the suspension's volume fraction and the boundary lubrication friction coefficient control the nature of the shear-thickening transition, both in simulations and experiments.

Artificial neural network approach to predict the lubricated friction coefficient

ABSTRACTThis paper analyses the applicability of artificial neural networks for predicting the lubricated friction coefficient. We will consider their use as faster and simpler alternatives to simulations based on theoretical behaviour equations. The development of several different artificial neural networks is presented. They have been trained through tribological tests on a mini‐traction‐machine, which furnishes the friction coefficient in point contacts. Once the training has been completed the networks are applied as tools for predicting the results in different operating conditions. Their advantages and disadvantages are analysed compared with conventional simulation tools. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Optimising lubricated friction coefficient by surface texturing

Surface texturing has proved to be a very useful tool for expanding the behaviour under hydrodynamic and elastohydrodynamic regimes instead of mixed or boundary lubrication regimes, and therefore for reducing the friction coefficient under high-load low-speed conditions. This article presents the texturing of different copper test-samples using photolithography and chemical etching to measure the friction coefficient using a point contact machine. The effects of texture size, texturing density, the initial roughness of the samples and the operating conditions have all been studied. Some combinations of texturing density and texture size achieve up to 30% reduction in the friction coefficient. Taking into account experimental data, artificial neural networks are used as a tool for both predicting and optimising the friction coefficient on the textured surface for any given operating condition.

Modelling and simulation of thermal effects in wet clutches operating under boundary lubrication conditions

Wet clutches are frequently used in the drive trains of many modern vehicles. The behaviour of the clutches influences the behaviour of the whole drive train and therefore of the whole vehicle. The design of the clutch is very important because it operates in cooperation with the other parts of the drive train. The clutch also often has to work in the lubricant present in the transmission. To optimize the clutch for an application, properties such as disc geometry, materials, friction disc surface, and engagement axial force can be varied when designing the clutch. Today, the design process involves much testing, which is expensive and time consuming. There are no good hand-book solutions or engineering tools available, hence the designer has to be very experienced and often use trial and error methods in order to end up with a working clutch for an application. A simulation model is developed in this article, which in combination with a simple measurement technique for measuring the boundary lubrication friction coefficient is used to estimate temperature and torque transfer for a wet clutch working under limited slip conditions. The developed simulation model can be used as a design tool for wet clutches. The approach developed in this article can be used to investigate torque behaviour for wet clutches that have not been designed and is, therefore, suitable to use when optimizing the performance of a new clutch. The model includes fluid dynamics, contact mechanics, and temperature computations in the fluid film between the friction disc and the separator disc. Temperature computations in the clutch discs are also included in the model. The fluid dynamics calculations use homogenized flow factors to enable simulations of flow on a coarser grid and still include all surface roughness effects. The temperature distribution in the film in the sliding interface is approximated as a polynomial of the second order. The heat transfer in the grooves of the friction discs is solved by means of an equilibrium equation that includes radial cooling flow effects because of centrifugal flows. The temperature in the friction disc and separator disc is obtained from the solution of the full three-dimensional energy equation in polar cylindrical coordinates. The model is validated by measurements made in a test rig and good agreement between measurements and simulations is obtained, both with regard to temperature and transfered torque. The use of this model can reduce the time needed to develop a limited slip wet clutch application since the systematic way of finding the optimal clutch design will be more efficient than the often used Edisonian trial and error approach.

Sliding tribological properties of untreated and PIII-treated PETP

Poly(ethylene terephthalate) (PETP) was treated by plasma immersion ion implantation (PIII or PI 3) in nitrogen. The surface changes were characterised by XPS and water contact-angle measurements. Sliding tribological properties of untreated and nitrogen PIII-treated PETP against conventional low carbon structural steel S235 were studied under dry and water-lubricated conditions by a pin-on-disc tribometer. XPS results suggested the evolution of surface composition and bonding towards those of amorphous hydrogenated carbon-nitride. Water contact-angle decreased implying increased surface wettability. At a very low Pv factor (0.0075 MPa m s −1) for the nitrogen PIII-treated PETP the dry friction coefficient was smaller than, while the lubricated friction coefficient was similar to, the corresponding value of the untreated variant. At higher Pv factors (near 0.1 MPa m s −1), however, both the dry and lubricated friction coefficients were higher for the treated sample than for the untreated variant, suggesting an increased adhesion component of friction for the nitrogen PIII-treated PETP in this region.

Boundary mode frictional properties of engineered cartilaginous tissues

Despite the fact that lubrication is a primary function of articular cartilage, there is little information on the frictional properties of cartilaginous engineered tissues. A biochemical mediator of cartilage frictional properties in boundary lubrication, lubricin, has been shown to be secreted from chondrocyte-hydrogel constructs. In the current studies we utilized articular chondrocytes (CON), meniscal fibrochondrocytes (MEN), and mesenchymal stem cells (MSC) in alginate cultures to determine lubricin localization and the inherent boundary lubrication friction coefficient. Additionally, we investigated the ability of these tissues to be lubricated by synovial fluid and the reversibility of this lubrication. Cell-alginate constructs were cultured over six weeks, culture medium assayed for lubricin release by ELISA and constructs analyzed with immunohistochemical (IHC) methods to investigate the localization of lubricin. Engineered tissues were tested in a custom friction instrument to determine the equilibrium friction coefficient (microeq) in boundary lubrication mode, following incubation with equine synovial fluid (SF), and subsequent extraction in l.5M NaCl. MSCs released 10 fold more lubricin than CON or MEN cultures. IHC analysis showed no localization of lubricin to alginate, minimal focal staining of engineered constructs at six weeks in culture, and the ability of all engineered tissues to localize lubricin when exogenously treated with SF. Frictional characterization showed no difference in microeq over culture for all engineered tissues, while incubation in SF decreased microeq for all tissues over culture duration, and extraction of lubricin resulted in a loss of lubrication of all engineered tissues.

Surface-modification in situ of nano-SiO 2 and its structure and tribological properties

The preparation of a series of dispersible nano-SiO 2 by surface-modification in situ was described in this paper. It is found that some silane coupling agents can be combined with nano-SiO 2 by covalent bonds, which change the nanoparticle's surface properties and make nano-SiO 2 disperse well and steadily in many organic mediums. The structure of nanoparticles was characterized by transmission electron microscopy (TEM), infrared spectrum (IR), X-ray photoelectron spectra (XPS) and thermogravimetric analysis (TG). The dispersivity of these nanoparticles in organic solvents was measured by light transmittance. Considering such superior dispersion in oily solvents and very small size, we primarily investigated their tribological behaviors as additive in lubricant on wear testers. The results show that they can evidently increase anti-wear ability and reduce the friction coefficient of lubricant.