Boundary Film Thickness Research Articles

A computational convection analysis of SiO2/water and MoS2‐SiO2/water based fluidic system in inverted cone

AbstractA complete shape factor investigation of water‐based mixture type hybrid nanofluid in a permeable boundary with the impact of magnetic field, thick dissemination, and warm radiation is presented in this article. A computational convection analysis of an inverted semi vertical cone with a porous surface in the form of /water nanofluid and /water hybrid nanofluid transport is developed. The system of differential equations is presented and solved numerically by the Lobatto IIIA method. The temperature distributions and fluid velocity are studied along with the coefficient of skin friction and the Nusselt number, taking into account the form of distinct nano‐particles. The flow problem's results are approximated by using several embedding variables. Tables and graphs are constructed for a variety of scenarios including maximum residual error, mesh points, and Nusselt numbers. We conclude that boundary film thickness reduces and the fluid flow is resisted by magnetic field presence. Fluid flow slows down as increases, and this reduction is more evident in nanofluids than in hybrid nanofluids. With an increment in , velocity drops. A detailed analysis of the proposed ordinary differential equations, boundary conditions, and numerical data of skin friction is given both in tabular and graphical forms. Additionally, it is observed that the fluid flow slows down more for the hybrid nanofluid than for the /water nanofluid. Moreover, it is clear that the temperature increase for the /water nanofluid is substantially greater. The authors deduce that the existence of a magnetic field resists fluid flow for hybrid nanofluid forms and decreases the thickness of the viscous boundary layer.

Research on impact reduction of flexible boundary particle damping honeycomb plate based on discrete element multi body dynamics coupling

<abstract> <p>The exposed platform is the main component of precision instruments such as pyrotechnic separation devices and load adapters, and its dynamic response characteristics are a key factor affecting the stability of aircraft. When the pyrotechnic separation device explodes, the exposed platform will be subjected to high-frequency, transient and high-order pyrotechnic impacts. High order pyrotechnic impacts can easily cause damage to sensitive components in onboard equipment, resulting in incalculable losses. Filling the honeycomb core cavity inside the exposed platform with a flexible boundary particle damper can effectively attenuate the pyrotechnic impact load after experiencing a large number of discontinuous structures. A numerical model of a flexible boundary particle damper and an exposed platform honeycomb panel was established through the coupling of discrete element and multi body dynamics. The effects of particle material, particle size, filling rate and flexible boundary film thickness on the impact reduction performance of the flexible boundary particle damper were analyzed, and the optimal characteristic parameters of the flexible boundary particle damper were obtained. The impact test was conducted on the honeycomb panel of the exposed platform using a light gas gun impact experiment, verifying the impact reduction effect of the flexible boundary particle damper and the accuracy of the numerical model. Finally, a pyrotechnic impact experiment was conducted on the exposed platform honeycomb panel with the optimal parameter flexible boundary particle damper. The experimental results showed that the impact reduction effect of the flexible boundary particle damper with this parameter reached 55.52%.</p> </abstract>

Effects of UHMWPE viscoelasticity on the squeeze‐film lubrication of hip replacements

AbstractMany studies have been performed to analyse the lubrication of artificial joints since the pioneer work by the late Professor Duncan Dowson. However, the viscoelastic deformation of one of the most widely used bearing materials, ultra‐high‐molecular‐weight polyethylene (UHMWPE), has only been considered recently. The described study attempted to investigate the effect of UHMWPE viscoelasticity on the elastohydrodynamic lubrication of such a soft artificial hip replacement under squeeze‐film motion. A transient viscoelastic squeeze‐film lubrication model of a typical hip implant was developed and solved to obtain the film thickness and pressure distributions. A boundary film thickness was adopted to consider the direct and indirect lubricant contact conditions. The results showed that the viscoelasticity had marked effects on the squeeze‐film lubrication performance of UHMWPE artificial hip joints. The minimum film thickness in the viscoelastic model was smaller than that of the elastic model, causing an earlier direct contact. However, the film thickness within the central contact region in the viscoelastic model was greater than that of the elastic model due to the restricted flow of the lubricant, therefore enhancing the lubricating effect and particularly with a short relaxation time and mechanical loss factor.

Boundary Lubrication Mechanisms for High-Performance Friction Modifiers.

We recently reported a new molecular heterocyclic friction modifier (FM) that exhibits excellent friction and wear reduction in the boundary lubrication regime. This paper explores the mechanisms by which friction reduction occurs with heterocyclic alkyl-cyclen FM molecules. We find that these chelating molecules adsorb onto (oxidized) steel surfaces far more tenaciously than conventional FMs such as simple alkylamines. Molecular dynamics simulations argue that the surface coverage of our heterocyclic FM molecules remains close to 100% even at 200 °C. This thermal stability allows the FMs to firmly anchor to the surface, allowing the hydrocarbon chains of the molecules to interact and trap base oil lubricantmolecules. This results in thicker boundary film thickness compared with conventional FMs, as shown by optical interferometry measurements.

Hierarchically porous zeolite X composites for manganese ion-exchange and solidification: Equilibrium isotherms, kinetic and thermodynamic studies

Abstract This paper deals with the kinetic and isotherm studies of manganese ion removal by zeolite X and its hierarchically porous composites as ion-exchange materials. A hydrothermal treatment was applied to grow a layer of zeolite X over diatomite and carbon surfaces. The hierarchically porous composites used for Mn2+ ion removal showed higher ion-exchange capacity when compared to pure zeolite X under same conditions. A thermodynamic study of the rate of ion-exchange revealed that the intra-particle diffusion rate constant of zeolite X/carbon and zeolite X/diatomite was higher than that of pure zeolite X indicating that the intra-particle diffusion was enhanced when zeolite was prepared in form of hierarchically porous composites. The study showed that the thickness of boundary film of zeolite X/carbon and zeolite X/diatomite was less than that of pure zeolite X indicating ion diffusion resistance to the active sites was reduced when the composites were utilised. The experimental data showed good agreement with Freundlich model. The calculated thermodynamic parameters such as ΔG°, ΔH° and ΔS° indicated the ion-exchange process of Mn2+ ion by zeolite X and its composites was spontaneous, endothermic and the randomness increased at the liquid/solid interface under the conditions studied. The results of kinetic study showed that the ion-exchange of Mn2+ ion by zeolite X and its composites followed a pseudo second order kinetic model indicating Mn2+ ion removal is a chemical reaction related to valence forces. The encapsulation of Mn2+ ion was successfully achieved by vitrification and geopolymerization processes demonstrating the efficacy of both processes for environmental management.

The Assessment of Friction Process of Model Kinematic Pair Worked under DC Voltage / Ocena Procesu Tarcia W Modelowym Weźle Kinematycznym Pracującym Pod Napięciem Elektrycznym

Abstract The subject of this paper is assessment of friction process of model kinetic pair worked under DC voltage. The friction under this conditions may take place in case of triboelectrical phenomena and in friction pairs of electrical motors. The significant influence of electric current flow on friction resistance and boundary film thickness was presented. Results of the research were compared to the results of research under load and without electric current applied.

Research on the Friction Behaviors of Two Rough Surfaces Covered with Boundary Film

Boundary film plays an important role in improving the lubricating characteristics of mixed lubrication. But due to the complication of surface profile and the microscale of boundary film, progress made on the boundary lubrication is relatively less than that of fluid film lubrication. A friction model of two rough surfaces covered with the boundary film is studied in the paper. The rough surface is modeled by a collection of spherical asperities with the same curvature radius and the Gaussian height distribution, the shearing force of asperities over the boundary film is expressed by the linear equation, and the plowing force of asperities over the solid flat is modeled by considering the possibility of asperities being sheared off. Then, the friction behaviors are researched by analyzing the effects of boundary film and surface topography in different values. The results demonstrate that the effects of boundary film on the friction coefficient are enhanced with the increase in the thickness of boundary film and are weakened with the increase in the contact load. And surface profile parameters also affect the friction behaviors of boundary lubrication, the smoother the surface is, the more significant the boundary lubricating effect is.

Flow transport modelling of feed species (water and salt) through a seawater RO membrane

Throughout recent years, there has been various research studies on seawater RO membrane modelling concerned with analysing RO membrane transport theories and touching on significant issues of the unique molecular separation mechanism of reverse osmosis that affect areas of our everyday life. However, much of this research could be largely characterised as being empirical in nature. The scope of this flow transport modelling analysis was to develop a mathematical model that would determine variations in the transport of water (solvent) and salt (solute) species within a seawater RO membrane module during reverse osmosis. Hence, the modelling analysis was performed in terms of flow, pressure and concentration of the seawater feed, whilst traversing through the module from point of entry at the axial–centre along the length of module (travelling as feed) to the point of exit at outer periphery (exiting as retentate). The modelling analysis in this paper has been devised based on a generalised capillary diffusion model for the transport of “water” and “salt” through a seawater RO membrane combined with relevant performance evaluation expressions for a SWRO membrane using actual operating data of a SWRO membrane module. For this purpose, the essential first year operating data for a new seawater RO membrane module (DuPont HFF membrane B-10 6835TR, operated at the Addur SWRO Desalination Plant) were compiled; (Note: the essential operating data for one seasonal year from this new SWRO membrane module were used so as to obtain more accurate results). This modelling analysis work was performed by the Author of this technical paper at the University of Newcastle Upon Tyne in the UK as part of the Author’s PhD research work during 2000 (modified in 2007). It is undeniable that concentration polarisation has detrimental effects on the performance of any RO membrane process, with its magnitude being of a vital importance (though not been determined before as no literature cited). Likewise, the resulting concentrated boundary film thickness and salt concentration of boundary solution on the high pressure side of the membrane were also determined in the overall application of this modelling analysis. Thus, the correlations developed for this model, the concept of the generated model and the entirety of merging the expressions (as well as the results achieved) are to provide significance in determining the performance of a seawater RO membrane and to add novelty with a sense of innovation to this modelling work.

The Study of the Dynamic Thickness of Organic Boundary Films Under Metallic Sliding Contact

The paper herein proposes a method to simultaneously measure the friction, wear, and thickness of boundary films under a pure sliding contact between metallic surfaces. The method is based on the capacitance method used by Crook and Dyson et al. to measure elastohydrodynamic film thicknesses. Pilot tests conducted on sunflower oil using the new method showed an appreciable growth in the boundary film thickness, with a corresponding drop in the coefficient of friction. Very little wear was observed during the film growth. It is postulated that this growth was due to the polymerization of the unsaturated fatty acid chains.

Boundary film shear elastic modulus effect in hydrodynamic contact. Part II: influence of operational parameters

The present paper is the subsequent research of the first part (Theor Comput Fluid Dyn, 2009). It investigates the boundary film shear elastic modulus effect in a hydrodynamic contact in different operating conditions. The hydrodynamic contact is one-dimensional, composed of two parallel plane surfaces, which are respectively rough rigid with rectangular micro projections in profile periodically distributed on the surface and ideally smooth rigid. The whole contact consists of cavitated area and hydrodynamic area. The hydrodynamic area consists of many micro Raleigh bearings which are discontinuously and periodically distributed in the contact. The hydrodynamic contact in a micro Raleigh bearing consists of boundary film area and fluid film area which, respectively, occur in the outlet and inlet zones. In boundary film area, the film slips at the upper contact surface due to the limited shear stress capacity of the film–contact interface, while the film does not slip at the lower contact surface due to the shear stress capacity of the film–contact interface large enough. In boundary film area, the viscosity, density, and shear elastic modulus of the film are varied across the film thickness due to the film–contact interactions, and their effective values are used in modeling which depends on the film thickness. In fluid film area, the film does not slip at either of the contact surfaces, and the shear elastic modulus of the film is neglected. It is found from the simulation results that the boundary film shear elastic modulus influences are normally negligible on the mass flow through the contact, the carried load of the contact and the overall film thickness of the contact, and the boundary film shear elastic modulus would normally influence the local film thickness in an elastic contact when the local film thickness is on the film molecule diameter scale. It is also found that the boundary film shear elastic modulus effect has the tendency of being increased with the reduction of the width of a micro contact. It is increased with the reduction of the boundary film–contact interfacial shear strength or with the increase of the critical boundary film thickness, while it is strongest at certain values of the contact surface roughness, the width ratio of fluid film area to boundary film area, and the lubricant film shear elastic modulus.

Boundary film shear elastic modulus effect in hydrodynamic contact. Part I: theoretical analysis and typical solution

Boundary film shear elastic modulus effect is analyzed in a hydrodynamic contact. The contact is one-dimensional composed of two parallel plane surfaces, which are, respectively, rough rigid with rectangular micro projections in profile periodically distributed on the surface and ideally smooth rigid. The whole contact is consisted of cavitated area and hydrodynamic area. The hydrodynamic area consists of many micro Raleigh bearings which are discontinuously and periodically distributed in the contact. Analysis is thus carried out for a micro Raleigh bearing in this contact. The hydrodynamic contact in this micro Raleigh bearing consists of boundary film area and fluid film area which, respectively, occur in the outlet and inlet zones. In boundary film area, the film slips at the upper contact surface due to the limited shear stress capacity of the film-contact interface, while the film does not slip at the lower contact surface due to the shear stress capacity large enough at the film-contact interface. In boundary film area, the viscosity, density and shear elastic modulus of the film are varied across the film thickness due to the film-contact interactions, and their effective values are used in modeling, which depend on the film thickness. The analytical approach proposed by Zhang (J Mol Liq 128:60-64, 2006) and Zhang et al. (Int J Fluid Mech Res 30:542-557, 2003) is used for boundary film area. In fluid film area, the film does not slip at either of the contact surfaces, and the shear elastic modulus of the film is neglected. Conventional hydrodynamic analysis is used for fluid film area. The present paper presents the theoretical analysis and a typical solution. It is found that for the simulated case the boundary film shear elastic modulus effects on the mass flow through the contact, the overall film thickness of the contact and the carried load of the contact are negligible but the boundary film shear elastic modulus effect on the local film thickness of the contact may be significant when the boundary film thickness is on the 1 nm scale and the contact surfaces are elastic. In Part II will be presented detailed results showing boundary film shear elastic modulus effects in different operating conditions.

Tribological investigation of CaF 2 nanocrystals as grease additives

Calcium fluoride (CaF 2) nanocrystals with average grain size of 60 nm were synthesized via a precipitation method. The morphology and structure of nanocrystals were characterized by means of transmission electron microscopy (TEM) and X-ray powder diffraction (XRD). TEM and XRD showed that CaF 2 nanocrystals are cubic particles in submicron scale. The tribological properties of the prepared CaF 2 nanocrystals as an additive in lithium grease were evaluated with a four-ball tester. The results indicated that these nanocrystals exhibit excellent antiwear, friction reduction and extreme pressure (EP) properties. It was also found that the EP and antiwear capabilities of the grease are not proportional to the content of CaF 2 nanocrystals but there existed a certain value. The rubbed surface after friction test was investigated with X-ray photoelectron spectroscopy and scanning electron microscopy to understand the action mechanism. The results show that a boundary film mainly composed of CaF 2, CaO, iron oxide and some organic compounds was formed on the rubbed surface after friction test and the thickness of boundary film was about 12 nm. The disproportion of stoichiometric ratio of Ca and F in boundary lubrication film indicates that tribochemical reaction of CaF 2 nanocrystals occurred on the worn steel surface at severe tribological conditions.

Microstructure–stress relationships in liquid-phase sintered alumina modified by the addition of 5 wt.% of calcia–silica additives

Hot-pressed alumina samples, with 5 wt.% additions of CaO:SiO 2 possessing molar ratios ranging between 1:5 and 10:1 were studied by electron microscopy. Marked differences in microstructure (e.g. grain size, secondary crystalline phases and grain boundary film thickness) were observed, which depended on the composition of sintering additives. The compositions of glassy phases in triple pockets and in grain boundaries varied markedly depending on sintering additives, but also within individual specimens. High residual compressive stresses were measured in alumina grains of samples which contained low thermal expansion crystalline phases such as anorthite and grossite, while gehlenite-containing samples were correspondingly less stressed.

Diffusive properties of fluid-filled grain boundaries measured electrically during active pressure solution

Diffusion through ‘wetted’ grain boundaries is often the rate limiting process during rock deformation by intergranular pressure solution. However, the underlying processes operative within such boundaries are poorly understood. In this contribution we have studied the diffusive properties of wetted grain boundaries by measuring the electrical resistivity of single, annular halite–glass contacts undergoing active pressure solution. Optical observation shows continuous growth (i.e. widening) of the annular contacts by pressure solution. From the resistivity measurements and making use of the Nernst–Einstein equation, it was possible to calculate the apparent grain boundary diffusion coefficient Z= DδC (i.e. the product of grain boundary diffusion coefficient D, grain boundary film thickness δ and the solubility C of the diffusing species in the grain boundary fluid) during the pressure solution process. The Z-values obtained lie in the range 3×10 −20–2×10 −18 m 3/s, show an inverse dependence on normal stress ( σ n) and agree well with values inferred previously from single contact and polycrystalline compaction experiments.

Elastohydrodynamic properties and boundary lubrication performance of polyolester in a hydrofluoroether refrigerant environment

The electrohydrodynamic (EHD) properties of polyolester (POE) in refrigerant environments were determined with a ball-on-sapphire disc-type EHD tester. The film thickness and the traction were both lower for hydrofluoroether HFE-245mc than for hydrofluorocarbon (HFC) at an identical gas pressure. This is because of the higher refrigerant concentration (RC) for HFE-245mc than for HFC, which resulted in a lower atmospheric viscosity and a lower viscosity-pressure coefficient of the refrigerant/POE mixture for HFE-245mc. The boundary lubrication performance was studied with a block-on-ring type tribometer. The coefficient of friction in HFE-245mc was lower than that in HFC, and HFE-245mc was superior to HFC in terms of antiwear performance. X-ray photoelectron spectroscopy (XPS) of the surface film showed the presence of FeF2 and iron oxide whose concentration was much higher, and the boundary film thickness was much thicker for HFE-245mc than for HFC. Thus, it was inferred that HFE-245mc adsorbed on to the rubbing surface was decomposed to form an effective antiwear film having a low shear resistance which resulted in reduction in the friction and wear.

A model for viscous flow creep in ceramics containing secondary crystalline phases

Many structural ceramics (e.g., Si{sub 3}N{sub 4}) contain an intergranular amorphous phase which results from liquid phase sintering. This amorphous phase is present both at the multigrain junctions and as thin films at two-grain boundaries. High temperature creep deformation of these materials may occur by viscous flow of the amorphous phase from grain boundaries in compression to those in tension. Several models have been developed to describe this process, based on the assumption of either square or hexagonal grains. All these models assumed that the grain size and grain boundary film thickness were uniform throughout the material. In polycrystalline material, however, a large range of grain size distribution is usually observed and secondary crystalline phases (SP) may also exist. The effect of grain size distribution on the viscous flow process has been analyzed recently by Dey et al. They argued that the presence of large grains can cause very high local stress concentrations. Since a residual glass film always remains along matrix/SP grain boundaries (heterophase boundaries) and is generally 2 or 3 times thicker than that along homophase boundaries (between matrix grains), a new model considering this heterogeneity is required to describe viscous flow creep in ceramics containing secondary crystallinemore » phases.« less

Silicon Nitride Based Ceramic Nanocomposites

Nanocomposites (Si3N4/SiC) were studied by combined high‐resolution transmission electron microscopy and electron energy‐loss spectroscopic imaging (ESI) techniques. In ESI micrographs three types of crystalline grains were distinguished: Si3N4 matrix grains (0.5 μΩ), nanosized SiC particles (<100 nm) embedded in the Si3N4, and large SiC particles (100–200 nm) at grain boundary regions (intergranular particles). Amorphous films were found both at Si3N4 grain boundaries and at phase boundaries between Si3N4 and SiC. The Si3N4 grain boundary film thickness varied from 1 to 2. 5 nm. Two kinds of embedded SiC particles were observed: type A has a special orientation with respect to the matrix, and type B possesses a random orientation with respect to the matrix. The surfaces of type B particles are completely covered by an amorphous phase. The existence of the amorphous film between the matrix and the particles of type A depends on the lattice mismatch across the interface. The mechanisms of nucleation and growth of Ω‐Si3N4 grains are discussed on the basis of these experimental results.

A model of the dynamics of boundary film formation

The dynamics of formation and loss of the boundary films formed during sliding on steel surfaces were investigated over a range of temperature. Tests are performed on a cylinder-on-disk machine using mineral oil with various concentrations of zinc dialkyldithiophosphate (ZDP). The thickness and refractive index of the boundary films during step load test were monitored in situ with an ellipsometer, and the composition of the films was analyzed by X-ray photoelectron spectroscopy (XPS). As temperature increases, chemical reactivity increases the film formation rate, while the film removal rate increases owing to (a) the decrease of durability of the boundary film material and (b) the reduction of hydrodynamic fluid film thickness due to decreasing viscosity of the lubricant. There is a balance between these two competing mechanisms, and this balance is reflected in the boundary film thickness. The boundary films consist of a film of oxide and metallic compound (OMM) covered by an organo-iron compound (OIC). Their relative effectiveness in preventing scuffing depends on temperature and composition. In particular, the OIC is effective in reducing wear of the opposing surfaces by covering the OMM.

Boundary lubrication of rubber by aqueous surfactant

An apparatus has been developed which uses a laser to measure the thickness of boundary films in lubricated rubber contacts. In earlier measurements using the same interferometric principle the active optical component was a two-piece beamsplitter. This has been replaced by a simple prism. The behaviour of surfactant solution boundary films in rubber/glass contact has been re-examined using this apparatus, and films in rubber/rubber contact have been explored in detail for the first time. Stable films in static contact have been recorded at thicknesses up to 25 nm at an average pressure of 10 kPa. At higher average pressures, up to 50 kPa, the collapse of these films has been observed, which brings into question the role of electric double layers in supporting the normal load. Introducing a surfactant solution to the region around a dry, static, loaded rubber/glass contact results in the separation of the surfaces and the formation of an equilibrium film. The thickness of the film formed is the same as by squeezing rubber and glass surfaces together under the same load when immersed in the surfactant solution. This observation indicates the presence of surface repulsive forces. The improved sensitivity of the new apparatus was demonstrated during the course of experiments. In earlier measurements it was difficult to resolve the thickness of films thinner than 4 nm. It appears that films can now be resolved down to 1 nm, which is of the molecular dimensions of surfactants.

Study on contact mechanism and electric resistance characteristics of twisted conductors

AbstractThis paper analyzes the mechanical and electrical characteristics of the hand‐twisted conductor splices in communication cables. the validity of the analysis is verified by experiment. Concerning the mechanical characteristics, it is shown that in the physical contact of the conductors the central axis is twisted with an arbitrary pitch angle on a quasi‐cylinder with the radius equal to that of the conductor. Based on such representation of the contact mechanism, the contact pressure and contact width of the twisted conductors are calculated. Concerning the electrical characteristics, it is assumed that there is a similarity between the current distribution through the twisted conductor contact and the electric lines of force produced by the charge on the surface of contact. Based on such assumption, the constriction resistance of the twisted contact is calculated with consideration of the contact pressure and contact width. Estimation is also made on the growth process of the film resistance and the boundary film thickness due to the chemical substances formed on the conductors in contact. the electrical equivalent circuit for the twisted conductor contact is derived, including the constriction resistance, film resistance and conductor resistance. Using the derived equivalent circuit, an expression is derived for estimation of the contact resistance of the twisted conductors.