Maximum Film Thickness Research Articles

Slot die stripe coating of low viscous fluids

Slot die coating is applied to deposit thin and homogenous films in roll-to-roll and sheet-to-sheet applications. The critical step in operation is to choose suitable process parameters within the process window. In this work, we investigate an upper limit for stripe coatings. This maximum film thickness is characterized by stripe merging which needs to be avoided in a stable process. It is shown that the upper limit reduces the process window for stripe coatings to a major extent. As a result, stripe coatings at large coating gaps and low viscosities are only possible for relatively thick films. Explaining the upper limit, a theory of balancing the side pressure in the gap region in the cross-web direction has been developed.

Impinging jet flow and hydraulic jump on a rotating disk

The free-surface flow formed by a circular jet impinging on a rotating disk is analysed theoretically. The study explores the effects of rotation and inertia on the thin-film flow. Both boundary-layer height and film thickness are found to diminish with rotation speed. A maximum film thickness develops in the supercritical region, which reflects the competition between the convective and centrifugal effects. Unlike the flow on a stationary disk, an increase in the wall shear stress along the radial direction is predicted, at a rate that strengthens with rotating speed. Our results corroborate well existing measurements. The location and height of the hydraulic jump are determined subject to the value of the thickness at the edge of the disk, which is established first for a stationary disk based on the capillary length, and then for a rotating disk using existing analyses and measurements in spin coating. The case of a stationary is revisited in an effort to predict the location and height of the jump uniquely. The formulated value of the height at the edge of the disk seems to give excellent results for a jet at moderately high flow rate (or low viscosity) where the jump structure is well identifiable in reality.

Effects of pH on CdO films deposited onto ITO coated glass substrates by electrodeposition

Thin films of CdO were deposited onto indium tin oxide (ITO) coated glass substrates using electrodeposition method. The pH of the final solutions was adjusted from 4 to 8 and effects of pH on the films were investigated. The maximum film thickness was reached at pH 5 and XRD studies showed that this film had relatively high peak intensities. Optical studies showed that band gap varies between 2.04 and 2.41 eV and depend on pH. The surface morphologies were analysed by SEM images and they revealed that films were deposited compact and there were no voids, cracks or pinholes.

Математическая модель процесса дегазации полимерного покрытия в условиях открытого космоса

A half-empiric mathematical model was elaborated to describe polymeric cover outgassing process in open space conditions. The calculation was carried out to estimate film thickness that arises as a result of gaseous products nonequilibrium condensation at a sensitive surface. Source data were: a) experimentally obtained time functions of polymeric cover mass loss at different temperatures; b) the construction element temperature regime data; c) the construction element temperature changes according to its location towards the Sun. To carry out the calculation the construction element and the sensitive surface geometry is considered to be axisymmetric. As a result, the numerical values of the average as well as maximum and minimum film thickness for the whole exploitation period of the spacecraft.

The role of frequency on fretting corrosion of Alloy 690TT against 304 stainless steel in high temperature high pressure water

In high temperature high pressure (HTHP) water, the role of frequency in fretting corrosion behavior and mechanism of Alloy 690TT against Type 304 stainless steel were investigated. Results indicated that wear mechanisms of delamination, material transfer and oxidation collectively dominated the fretting corrosion behavior. The delamination was related to pure mechanical wear to produce fresh surface exposed to HTHP water for oxidation, resulting in the increase of wear depth and volume. The increasing frequency played little role in oxide type of worn surfaces, which consisted of (Ni,Fe)(Fe,Cr)2O4 and Cr2O3. However, it enhanced the maximum thickness of oxide film in the worn surface. The increasing frequency resulted in the acceleration of the formation of nanostructured tribologically transformed structure (TTS) layer because it could aggravate the plastic deformation and increase the contact temperature for dynamic recrystallization. The increasing frequency accelerated the escape of the ultra-fine grained wear debris, which originated from not only the damage of TTS layer but also the crushed oxide particles generated in the contact gap.

Mixed-lubrication analysis of misaligned bearing considering turbulence

Abstract Some bearings operate in mixed-lubrication regime due to the low rotational speed and the heavy load and misalign due to shaft's gravity. For large bearings, the maximum film thickness is much larger than minimum nominal film thickness and turbulence may locally develop. This paper proposes an approach to analyze misaligned mixed-lubrication bearing considering turbulence. Based on the average flow model proposed by Patir and Cheng and the Ng-Pan turbulence model, a generalized average Reynolds equation is derived. The calculation procedure is established by finite difference method. The results show that turbulence remarkably increases friction coefficient, slightly increases the minimum nominal film thickness, and decreases the transition speed from mixed-lubrication regime to hydrodynamic lubrication regime.

Polyethylene–Carbon Nanotube Composite Film Deposited by Cold Spray Technique

Carbon nanotubes (CNTs) are high-performance materials because of their superior electrical conductivity, thermal conductivity, and self-lubrication, and they have been studied for application to polymer composite materials as fillers. However, the methods of fabricating polymer composites with CNTs, such as injection molding, are too complicated for industrial applications. We propose a simple cold spray (CS) technique to obtain a polymer composite of polyethylene (PE) and CNTs. The composite films were deposited by CS on polypropylene and nano-porous structured aluminum substrates. The maximum thickness of the composite film was approximately 1 mm. Peaks at G and D bands were observed in the Raman spectra of the films. Scanning electron microscopy images of the film surface revealed that PE particles were melted by the acceleration gas and CNTs were attached with melted PE. The PE particles solidified after contact with the substrate. These results indicate that PE–CNT composite films were successfully deposited on polypropylene and nano-porous structured aluminum substrates by CS.

The Research on the Performance of Gear Lubrication Based on Romax

The gears of a gear box were modeled and analyzed through Romax, and the effects of pressure angle on their lubrication properties were studied. The studies had shown that as the increase of pressure angle, the safety factor against micropitting had significantly increased. The gears were less prone to micropitting. The minimum film thickness and the maximum film thickness of the gears had also significantly increased, so the gear lubrication performance had been improved. It had important significance to improve the carrying capacity and prolong the service life of the gears.

Elastohydrodynamic lubrication of coated finite line contacts

In this work, a finite element-based model is presented that simulates elastohydrodynamic lubrication in coated finite line contacts. Using this model, the film thickness and pressure distributions, between a straight roller with rounded edges on a plate, were analyzed. The model was successfully validated against representative results reported in literature. Parameter studies were conducted to study the influence of varying operating conditions, axial surface profile parameters and coating mechanical properties on the overall elastohydrodynamic lubrication behavior of the contact. It was found that in contrast with typical elastohydrodynamic lubrication behavior, the maximum pressure and minimum film thickness, which are located at the rear of the contact, are largely influenced by variations in load. Results also reveal that axial surface profile parameters and coating mechanical properties may act as amplifiers to the effect of load on pressure and film thickness distribution and can thus, if smartly chosen, significantly enhance lubrication performance.

Numerical simulation of wickless gravity-assisted two-phase cooling system used in heavy duty extrusion pelleting line

Efficient cooling system is an essential part of heavy duty extrusion pelleting line which plays an important role in 300kt/a production engineering of polyolefin. In this paper, a wickless gravity-assisted two-phase cooling system (WGATPCS) used in heavy duty extrusion pelleting line was presented. The volume of the fluid (VOF) model in FLUENT was used to simulate the two-phase flow and heat transfer characteristic during the operation of WGATPCS by solving the equations of continuity, momentum and energy. The vapor velocity and temperature distribution of cooling system were obtained. The operating conditions applied in this paper are the followings: the filling ratio is 0.3, cooling water flow rate is 180L/h, and heating power is 11kW. The predicted temperature profile showed a good agreement with experimental data and the maximum relative error was below 0.99%. Therefore, it was concluded that the CFD model successfully reproduced the heat and mass transfer processes in proposed cooling system. The variation of liquid film thickness, the characteristics of liquid film and vapor-liquid interface were also analyzed. The maximum liquid film thickness was 0.017mm at the bottom of condenser pipe, the velocity of vapor and liquid at vapor-liquid interface were 0.00785m/s and 0.09m/s respectively. The average condensation and evaporation heat transfer coefficients were 68083W/(m2K) and 3929W/(m2K), respectively.

3-D simulation of flow behaviour and film distribution outside a horizontal tube

The behaviour of falling film has significant effects on the efficiency of heat and mass transfer. Therefore it is important to study on the distribution of the film thickness around a tube in both the circumferential and axial directions. A 3-D model was applied in this study to simulate the falling film flow process of a column flow outside a horizontal tube, of which the Reynolds number varied from approximately 171 to 368. The results showed that with a single liquid column, a saddle-shaped spreading liquid lamella was formed along the axial direction, and the maximum elongation distance of the liquid film was obtained in the axial direction. In contrast, with two adjacent liquid columns, the flow behaviour changed a lot due to the interaction between the columns. A crest was formed between the two columns, and the maximum film thickness was more than three times of the minimum in the axial direction. In addition, the film thickness in the central section of the liquid column was extracted. It rapidly decreased as the circumferential angle varied from 20° to 60°. Then it changed little in the range of 60–120°, and sharply increased in the range of 120–150°.

Transition Between Mixed Lubrication and Elastohydrodynamic Lubrication with Randomly Rough Surfaces

We are interested in understanding the effect of roughness on friction in mixed lubrication (ML) and elastohydrodynamic lubrication (EHL), notably in the case of realistic surfaces. Rolling–sliding experiments in piezoviscous base oil are conducted between rough steels obtained with different machining, polishing and coating processes in order to represent realistic surface conditions found typically in a car engine. The Reynolds number based on the maximum lubricant film thickness in our experiments is below \(R_{e}=4\cdot 10^{-3}\). Our experiments at moderate pressure (\({<}1\) GPa) and speeds (mm/s \(\rightarrow\) m/s) allow to reach EHL for all surface roughnesses which range from the nanometre up to the micron. These surfaces have a complex and multi-scale texture and require a statistical description. Using the most raw and simple filter on large topographic measurements, we correlate the most probable standard deviation of surfaces to friction during Stribeck experiments occuring both in ML and EHL without measurable wear. In order to predict the conditions leading to high friction in ML, we propose a friction-based definition of ML and describe the role played by pressure and roughness on the onset of ML.

Dip- and die-coating of hydrophilic squares on flat, hydrophobic substrates

We studied the selective deposition of a Newtonian liquid on the hydrophilic domains of chemically patterned substrates. We present experiments and numerical simulations of the dip-coating and self-metered die-coating of compact, geometrically convex patterns such as squares and diamonds. The coating of such patterns is intrinsically an instationary process. Nevertheless, the maximum film thickness entrained on diamonds that are either much smaller or larger than the lengthscale governing the coating process follows the theoretical predictions derived for steady-state coating of either narrow hydrophilic stripes or chemically homogeneous surfaces. The transition between these two regimes is determined by the ratio of the pattern dimensions and the characteristic length of the reservoir meniscus, i.e. the capillary length for dip-coating and the die-gap for die-coating processes. While the film thickness entrained on diamonds scales with the coating speed as a powerlaw, the entrained thickness on squares with sides oriented parallel and perpendicular to the coating direction appears to saturate for small coating speeds. This influence of the azimuthal pattern orientation is related to the capillary break-up of the coating meniscus. Residual satellite droplets observed underneath the hydrophilic patterns after coating are a consequence either of the break-up or the subsequent retraction of the coating meniscus.

Molecular dynamics simulation on pressure and thickness dependent density of squalane film

Molecular dynamics (MD) simulations using the polymer consistent force field (PCFF) were adopted to investigate the pressure and thickness dependent density of squalane film in a nanogap at 373 K, with three different initial film thicknesses, and for a wide range of pressures. The equivalent densities predicted by MD simulations were compared with the empirical data. Results show that the squalane atoms tend to form layers parallel to the confining substrates but the orientations of squalane molecules are irregular throughout the film. In addition, distinct excluded volumes are not found at the interfaces of the film and substrates. Furthermore, with the same initial film thickness h0, the film thickness h and compressibility decrease with increasing pressure, but the compressibility is similar for films with different initial film thicknesses. The equivalent densities predicted by MD simulations with the maximum initial film thickness (9.44 nm) are accurate to the values of Tait equation. The MD simulation with adequate initial film thickness can accurately and conveniently predict the bulk densities of lubricants.

On the crucial role of ellipticity on elastohydrodynamic film thickness and friction

The study reported here deals with elastohydrodynamic point contacts and it is focused on the influence of contact ellipticity. In five velocity–load reference cases, ellipticity was varied from slender to wide configurations, including the circular contact. For each case, Hertzian pressure, Hertzian area, load, and entrainment velocity were kept constant while the ellipticity was varied by changing the curvature radii. In this context, the maximum central film thickness did not occur for the infinitely wide contact, but for a slender configuration close to the circular case. Moreover, the minimum film thickness reached its optimum for a wide but finite elliptical contact. For low ellipticity ratios, specific film thickness features were obtained. In particular, very high central/minimum film thickness ratios are found. The cause of these behaviors was found in the change of the convergent shape. When the ellipticity was varied, the Poiseuille flows parallel and transverse to the entrainment direction were significantly modified and these modifications were quantitatively analyzed for the different cases. The competition between the Couette and the Poiseuille flows was totally different between the narrow and the wide elliptical contact, and this change was responsible for the film thickness variations with ellipticity. Ellipticity also had an effect on friction as it influenced the maximum pressure which in turn impacts the fluid viscosity.

A numerical study on droplet-particle collision dynamics

The impact of liquid droplets onto spherical stationary solid particles under isothermal conditions is simulated. The CFD model solves the Navier-Stokes equations in three dimensions and employs the Volume of Fluid Method (VOF) coupled with an adaptive local grid refinement technique able to track the liquid-gas interface. A fast-marching algorithm suitable for the quick computation of distance functions required during the grid refinement in large 3-D computational domains is proposed. The numerical model is validated against experimental data for the case of a water droplet impact onto a spherical particle at low We number and room temperature conditions. Following that, a parametric study is undertaken examining (a) the effect of Weber number (= ρu2Do/σ) in the range of 8 to 80 and (b) the droplet to particle size ratio ranging in-between 0.31 and 1.24, on the impact outcome. This has resulted to the identification of two distinct regimes that form during droplet-particle collisions: the partial/full rebound and the coating regimes; the latter results to the disintegration of secondary satellite droplets from elongated expanding liquid ligaments forming behind the particle. Additionally, the temporal evolution of variables of interest, such as the maximum dimensionless liquid film thickness and the average wetting coverage of the solid particle by the liquid, have been quantified. The present study assists the understanding of the physical processes governing the impact of liquids onto solid spherical surfaces occurring in industrial applications, including fluid catalytic cracking (FCC) reactors.

Electrodeposition of ZnO thin films on conducting flexible substrates

In this work, we studied the DC electrochemical deposition of zinc oxide (ZnO) thin films on a conducting and flexible substrate, for their application in energy harvesting piezoelectric nanodevices. The deposition process was performed by varying the zinc nitrate concentration (c) in the electrolyte, its temperature (T), and the applied deposition potential (V), and subsequently tracing the influence of such parameters on the morphology (analyzed by scanning electron microscopy), crystallography (X-ray diffraction), and thickness (using the deposition current transient curves) of the ZnO thin films. The variation of the electrodeposition parameters led to the formation of different micro- and nano-structures, such as flat layers, microflowers, nanospheres, webs, and microramifications. Furthermore, the analysis of the deposited charge (by integrating the deposition current transients) illustrated an increase in the deposition rate with the increase of T and c, and a decrease of V. Finally, the maximum ZnO film thickness (~5 \(\upmu {\mathrm{m}}\)) was obtained for T = 80 °C, c = 0.1 M, and V = −1.5 V. This study provides us important tools to tune the electrochemical growth of ZnO thin films.

A numerical study of effects of counter-current gas flow rate on local hydrodynamic characteristics of falling films over horizontal tubes

A three-dimensional Computational Fluid Dynamics (CFD) model based on the VOF method is developed to investigate the hydrodynamic characteristics of falling film over horizontal tubes. The model not only considers gravity and surface tension, but also considers drag force when counter-current gas flow is imposed. For different liquid flow rates, laminar model, and k-ε turbulence model are employed to improve the model accuracy. Simulations are performed for the film Re varying from 100 to 1000 and gas flow rate ranging from 0m/s to 0.5375m/s. Based on the simulations, the gas flow effects on flow mode transitions agree well with the experimental data. In the circumferential direction, all bulk film thicknesses increase with the increased gas flow rate in the three basic flow modes — droplet, column, and sheet. In both the droplet and column flow mode, the position in which the thinnest film is located changes when gas flow is imposed. In the axial direction, the maximum film thickness is approximately twice that of the minimum in the droplet flow mode. Regular stable–crest–stable distribution can be observed in the column flow mode and the film distribution nearly has no variation in the sheet flow mode over time.

Simulation of the operation of the gas refrigerator recurrent complicated ethylene polymer deposits

The work based on the methods and equations of mechanics of multiphase media, the resulting system of ordinary differential equations, describing hydrodynamic and heat and mass transfer processes in counterflow heat exchangers type “pipe in pipe”, where the inner tube flows a mixture of ethylene and polyethylene, and in the annular space counter-current moves water. For gas mixtures is complicated by the “sticking” of liquid polyethylene on the walls of the pipe. In describing the process of formation of the polymer film is considered one of the possible mechanisms for the admission of liquid polyethylene particles from the turbulent core to the surface of the pipe, the turbulent diffusion. The results of studies on the dynamics of accumulation of the plastic film on the inner surface of gas refrigerator returnable ethylene. It is established that the formation of the profile of sediments is complex: the maximum film thickness is shifted to the output section of the channel.

Towards near-permanent CoCrMo prosthesis surface by combining micro-texturing and low temperature plasma carburising

An advanced surface engineering process combining micro-texture with a plasma carburising process was produced on CoCrMo femoral head, and their tribological properties were evaluated by the cutting-edge pendulum hip joint simulator coupled with thin film colorimetric interferometry. FESEM and GDOES showed that precipitation-free C S-phase with a uniform case depth of 10μm was formed across the micro-textures after duplex treatment. Hip simulator tests showed that the friction coefficient was reduced by 20% for micro-metre sized texture, and the long-term tribological property of microtexture was enhanced by the C-supersaturated crystalline microstructure formed on the surface of duplex treated CoCrMo, thereby enhancing biotribological durability significantly. In-situ colorimetric interferometry confirmed that the maximum film thickness around texture area was 530nm, indicating that the additional lubricant during sliding motion might provide exceptional bearing life.