Relative Film Thickness Research Articles

Intergranular Corrosion in Sensitized AA5083 Under Alternating Wet and Dry Conditions

AA5XXX aluminum-magnesium alloys have become a popular choice for high performance marine structures, especially for the US Navy. The combination of their high strength-to-weight ratio, uniform corrosion resistance, and low cost make these alloys well-suited for shipbuilding. Despite their excellent resistance to uniform corrosion, these alloys can become susceptible to intergranular corrosion (IGC) due to sensitization or precipitation of the more anodic β-phase (Al3Mg2) along the grain boundaries after long periods of exposures to service temperatures of as low as 50oC.[1-2] A significant amount of research has been conducted on the behavior of AA5083 in the context of full immersion exposure in marine environment.[3-6] IGC propagation depths were found to be influenced by the exposure time, applied potential, degree of sensitization (DoS), and orientation of propagation. These studies also revealed that an incubation period is necessary for β-phase pitting to develop into IGC in chloride-containing solutions under potentiostatic conditions. Although during service some areas of these marine structures will be constantly submerged, other parts will only be exposed to atmospheric exposure, and certain surfaces will be subjected to alternating conditions of wetting and drying. This study examines the IGC behavior of AA5083 under various alternating exposure conditions. Specifically, whether these alternating exposures may accelerate or impede IGC propagation in comparison with continuous full immersion was investigated. Because environmental conditions such as relative humidity, film thickness, and electrolyte concentration influence atmospheric corrosion, it is imperative to define a dry cycle environment. Preliminary tests involved a series of 4 sets of alternating wet and dry cycles on AA5083-H131 specimens sensitized to 50 mg/cm2. Every wet or dry cycle lasted for 25 h, for a cumulative wet or full immersion of 100 h. During each wet cycle, the specimens were potentiostatically held at -0.73 VSCE in 0.6 M NaCl solution, pH 8.3. The specimens were each subjected to a different dry cycle environment: (a) no electrolyte in 10% RH, (b) no electrolyte in 90% RH, and (c) fresh 0.6 M NaCl solution at open circuit potential. As compared to that continuously exposed to -0.73 VSCEfor 100 h, the IGC depths after the cumulative alternate immersion exposures, in Figure 1, are substantially lower, which could be due incubation at the beginning of each wet cycle. Additionally, these results also implied that during dry cycle, IGC can still propagate given the appropriate RH levels and/or electrolyte conditions. To avoid damage contributions from these factors, the 10% RH was chosen as the most conservative dry cycle environment. In order to study the influence of cycle time on the IGC damage depths, additional experiments involving various resident times in wet and dry environment were conducted. For each test, the resident time was equal for every wet and dry cycle. For all the tests, the total cumulative wet or full immersion time was 100 h. Initial results, in Figure 2, show that the shorter cycles achieved lower steady state current densities that decreased with each cycle. Moreover, an incubation time to reach a steady state current density was observed at each cycle. To probe the time to reach steady state current density, experiments with dry cycles as short as 30 min in between 25-h wet cycles were conducted. IGC depths were characterized at the end by using suitable metallographic techniques, and image analyses to track effects of cycle time and incubation on the physical damages. The results of this study provide insights into the importance of alternating wet and dry cycles on the IGC propagation of sensitized Al-Mg alloys. The study is part of a larger effort to understand IGC and IGSCC damage in AA5XXX series applied in marine environments. Acknowledgements Office of Naval Research under the contract ONR: N00014-08-10315 References [1] G.M. Scamans, N.J.H. Holroyd, C.D.S. Tuck, Corros. Sci., 27 (1987) 329-347. [2] R.H Jones, D.R. Baer, M.J. Danielson, J.S. Vetrano, Metall. Mater. Trans A, 32 (2001) 1699-1711 [3] S. Jain, M.L.C. Lim, J.L. Hudson, J.R. Scully, Corros. Sci., 59 (2012) 136-147. [4] M.L.C. Lim, J.R.Scully, R.G. Kelly, Corrosion, 69 (2013) 35-47. [5] M.L.C. Lim, R.G. Kelly, J.R. Scully, “Critical Electrochemical Conditions for Intergranular Corrosion in Sensitized AA5083-H131”, in NACE Department of Defense Virtual Corrosion Conference 2013. [6] Y. Yuan, “Localized Corrosion and Stress Corrosion Cracking of Aluminum-Magnesium Alloys” (Ph.D. diss., University of Birmingham: Birmingham, U.K., 2005) Figure 1

Development of a high-sensitivity plasticizer sensor based on a quartz crystal microbalance modified with a nanostructured nickel hydroxide film

Nanostructured nickel hydroxide (nano-Ni(OH)2) was synthesized at a low temperature without annealing. Accordingly, a plasticizer sensor based on a quartz crystal microbalance (QCM) modified with the nano-Ni(OH)2 sensing film was fabricated to detect dibutyl phthalate (DBP) and its relative film thickness was optimized. The sensor worked at room temperature and exhibited a high sensitivity of 4.91 Hz ppb−1 to DBP in a low concentration range of 5–20 ppb, and an ultra-low detection limit of 5 ppb was achieved. In addition, the sensor maintained good repeatability as well as stability shown by the experimental data. The responses to five possible interferences and four other plasticizers were also measured, which indicated the excellent selectivity of the sensor and its potential use in monitoring plasticizers in a gaseous state.

Tarnishing and Cu Ion release in Selected Copper-Base Alloys: Implications towards Antimicrobial Functionality

Copper corrosion, tarnishing, release and color stability were investigated in a series of candidate antimicrobial copper alloys Cu, Cu–5Sn, Cu–5Zn–5Al–1Sn, Cu–10Ni–1.5Fe–1Zn, Cu–18Ni–17Zn–1Mn, Cu–20Zn–3Si, and Cu–30Zn (wt% indicated). High Cu content, often utilized as an indicator of antimicrobial efficacy brought about by copper release, is shown to be insufficient alone to forecast Cu ion release tendencies during exposure. Alloys were exposed under full immersion in a standardized artificial perspiration solution. Copper release, relative film thickness, the molecular identity of the tarnish, and optical wavelength reflectivity were assessed to determine the fate of copper during oxidation. Copper release did not generally correlate with bulk Cu alloy content, tarnish thickness, or overall corrosion rate due to the unique effects of both major and minor alloying elements. Mechanisms for high or low corrosion rates, tarnishing and Cu release as a function of alloying content are discussed.

Friction reduction of highly-loaded rolling-sliding contacts by surface modifications under elasto-hydrodynamic lubrication

Due to the operation in mixed and boundary friction, minimization of friction losses in automobile powertrains offers high potential for lowering of greenhouse gas emissions and saving fossil fuels. In cooperation between Surface Engineering Institute (IOT) and Gear Research Center (FZG), surface modifications in terms of stochastic and deterministic structures, diamond-like carbon (DLC) and nitride hard physical vapor deposition (PVD) coatings were tested in lubricated rolling-sliding contacts in twin-disk and gear efficiency test-rig. It was found that deterministic structured surfaces provide for reduction of friction losses by up to 7% under extreme boundary friction conditions at high Hertzian stresses and small relative lubricant film thicknesses. Application-oriented tests using DLC coated gear wheels in gear efficiency test-rig exhibited a reduction of friction losses between 10% and 21% compared to uncoated gear wheels. The DLC coating ZrCg developed at IOT provided for friction reduction of 15% for small circumferential speeds in boundary friction. Friction losses were reduced by up to 39% at higher circumferential speeds in spite of fully separated surfaces in the fluid friction regime. Due to their thermophysical properties, it is assumed that DLC coatings have an impact on viscosity and shear resistance of the lubricant.

A novel FSI–thermal coupled TEHD model and experimental validation through indirect film thickness measurements for the lubricating interface in external gear machines

A novel fluid–structure interaction–thermal coupled model for the lateral lubricating gaps between gears and lateral bushes in external gear machines (EGM) is presented in this study. The model is the first of the lateral lubricating interface in EGMs capable of considering heat transfer, thermal effects in the fluid film and solid components as well as thermo-elastic deformation of the solids enabling prediction of the film thickness in EGM lateral gaps considering thermoelastohydrodynamic lubrication. Model validation is performed using lateral gap leakage measurement which is dependent on absolute film thickness. A novel relative lubricant film thickness measurement method in gear machines using capacitive sensors is also presented, and used to validate the spatial film thickness predictions from the model.

Experimental analysis of the thermal annealing of hard a-C:H films

The a-C:H layers were deposited on silicon substrates in 100 kHz bipolar-pulsed discharges from a fixed mixture of acetylene and argon. Three types of a-C:H material with different hydrogen contents and hardness were obtained by adjusting the pressure during deposition to 2 Pa (hardness ≈ 23 GPa; hydrogen concentration ≈ 19 at.%), 4 Pa (20 GPa; 20 at.%) and 8 Pa (17 GPa; 24 at.%). Annealing was performed in high vacuum at a heating rate of 3 K/min up to a maximum temperature, varied between 200 °C and 900 °C. The annealing process was investigated in situ by mass spectrometric measurement of the effusion products as a function of temperature. After cooling down in high vacuum, ex situ measurements revealed changes in layer thickness (profilometer), hardness (nanoindentation), residual stress (from the curvature of the silicon substrates), elemental composition (elastic recoil detection analysis and Rutherford backscattering), UV/VIS optical properties (variable angle spectroscopic ellipsometry), and bonding (Raman spectroscopy and Fourier transform infrared spectroscopy). The films retained their hardness, level of compressive stress, and elemental composition at least up to 500 °C. The variation of the film thickness with the annealing temperature was systematically analysed. Up to 625 °C, the a-C:H thickness increased by 8.5% without measurable difference between the three layer types nor any influence of the initial a-C:H thickness. With further annealing the increase of the film thickness passed a maximum, the magnitude and temperature-position of which increased with decreasing pressure during deposition. The highest relative film thickness increase of 14% was found for a-C:H deposited at 2 Pa and annealed to 725 °C. Based on the results of the complementary characterisation methods, the effects of annealing in high vacuum on film structure and properties are discussed and fundamental processes, prevailing in characteristic annealing-temperature ranges, are derived. • Effect of annealing on structure and mechanical properties of hard a-C:H is studied. • Increase of film thickness with annealing temperature is analysed in very detail. • Fundamental atomic processes, prevailing in three temperature ranges, are derived.

A METHOD FOR DEFINING INITIAL STRESS IN PROGRESSIVELY DEPOSITED FILM ON A SUBSTRATE

The initial stress is induced during film formation and is partially counterbalanced through curvature changes. Therefore, it is commonly evaluated by the measured residual stress. Initial stress may directly affect the film formation rather than residual stress in progressively deposited films. In the present work, we introduced a multiple layer model for progressively deposited films to obtain a quantitative solution for estimating the initial stress. The results showed that residual stress in the last layer is equal to the initial stress when layer number approaches infinity. In particular, the initial stress, σi in deposited film could be determined using the equation, σi = σSt/β, in which σSt is the averaged residual stress in films calculated by Stoney formula and β is the correction factor. The value of β varied between 0 and 1, depending on relative modulus and relative thickness of film and substrate. Finally, using element birth and death technique, a finite element model was presented to verify the analytical multiple layer model. Good agreement was obtained between the analytical and FE results.

Scanning probe microscopy based electrical characterization of thin dielectric and organic semiconductor films

Scanning probe microscopy (SPM) techniques offer various characterization methods for thin organic films. However, the majority of the electrical SPM measurements is currently performed in contact mode operation and may lead to severe damage at the surface of soft organic materials. This work shows the electrical characterization of organic insulator and semiconductor films by use of two SPM techniques operating with reduced lateral forces between SPM tip and sample. The first one is intermittent-contact scanning-capacitance-microscopy (IC-SCM) which is used for the detection of the local surface capacitance. The second one is torsional resonance tunneling-atomic-force-microscopy (TR-TUNA) which shows the local conductivity respectively relative film thickness of the sample. It is found that the tunneling current distribution across 50nm thick organic insulating films is very homogeneous and that inhomogeneities in P3HT and Pentacene films can be pinpointed even if no topographical variations are observable.

Power law in swelling of ultra-thin polymer films

Steady state and dynamic behaviour of swelling of spin coated polyacrylamide films have been studied at room temperature in a precisely controlled environment of 12–99% relative humidity (RH). Swelling of the films was monitored by measuring the thickness of the films using X-ray reflectivity and Atomic Force Microscopy. Swellibility of the films was observed to be independent of thickness of the films indicating no significant influence of substrate polymer interaction or confinement effect in swelling. The water content of the films swelled at different humidity conditions was calculated comparing the electron density of the dry and the swelled films. The volume fraction of water in the swelled films and their swellibility were found to follow double power law behaviour as a function of relative humidity and the transition from one regime to the other was observed around 75% RH value. The behaviour was explained in terms of transition from short to long diffusing channels in the films. Comparison of time evolution of relative humidity of the experimental chamber with that of the dynamics of swellings of the films shows that diffusion of water into the environment was several orders of magnitude faster than the swelling dynamics. The observed values of the diffusion coefficients for the films at different humidity were same whereas the excluded volume parameter, that determine the saturated thickness of the films, was found to have dependence on relative humidity and film thickness.

A Theory of the Hydrodynamic Film Thickness in Centrally Pivoted Oil-Lubricated Plane-Pad Thrust Bearings

An analytical model is developed in this article for pivoted, oil-lubricated rectangular plane-pad thrust bearings considering the thermal effects of oil viscosity. The model is governed by three dimensionless parameters: the pad length-to-width ratio, the location of the pad pivot, and the bearing load parameter that prescribes lubricant thermal properties and applied load. Analyses were carried out for centrally pivoted bearings of a number of pad length-to-width ratios and a number of bearing load parameters. The results suggest a theory that, for bearings with a square pad or wider, the oil viscosity thermal effect enables a centrally pivoted bearing to develop a hydrodynamic film thickness of about one half of the film thickness that would be developed under isothermal conditions with the pad pivoted at its maximum-load-capacity optimal position. This level of relative film thickness could be developed under all design conditions of practical interest. The theory is fairly general and may be applicable to fan-shaped thrust bearing assemblies with the concept of equivalent length-to-width ratio of the bearing pad. The validity of the theory is modestly evaluated and should be further validated in time through various means and in various settings.

Mechanical properties of a high amylose content corn starch based film, gelatinized at low temperature

In the present study, a better knowledge of the influence of plasticizer content, storage relative humidity and film thickness on the mechanical properties of high amylose corn starch based films gelatinized at low temperature, is presented. The mechanical properties, tensile strength and percentage of elongation at break of high amylose corn starch films plasticized with glycerol were evaluated using tension tests. The films exhibited an increase in elongation and a decrease in tensile strength with increasing plasticizer concentration. When the glycerol level was high, some fissures were detected in the dry films, possible due to phase separation (starch-glycerol) phenomena. Film crystallinity is related to the reorganization capacity of the polymer chain, and thus the relative film crystallinity should increase with plasticizer content (glycerol and water). The mechanical properties were found to be strongly dependent on the water content due to the hydrophilic nature of starch films. The influence of moisture sorption on tensile strength was similar to that of plasticization with glycerol. The relationship between polymer chain mobility and water content explained this behavior. Elongation suffered a different effect and maximum values were reached at 45% relative humidity. The final drop in elongation was due to a softening of the structure at high relative humidity. The thicker the film the longer the drying time required, leading to greater relative crystallinity due to the corresponding increase in the possibility for chain reorganization. As a consequence, linear increases in tensile strength and elongation were observed with film thickness over the whole range studied (30 to 100 µm).

Experimental Investigation of Lubricant Flow Properties Under Micro Oil Supply Condition

Lubricant flow properties of polyalphaolefin (PAO) oil have been experimentally investigated based on a ball-on-disc configuration under micro oil supply condition. The oil pool shape and central film thickness in the contact region were obtained using fluorescence microscopy and optical interferometry, respectively. It has been found that the relative length between the inlet meniscus and Hertzian center point in the oil pool to Hertzian radius was much larger than 1 in a smaller lubricant supply of 20 μl, and the corresponding contact region initially entered the elastohydrodynamic lubrication (EHL) region and then became starved with the increasing speed. The variations of the relative film thickness as a function of starvation degree and the ratio of relative length to Hertzian radius were proposed to explain the obtained results. Besides, the fluorescence technique was used to directly observe the inlet meniscus position of the oil pool and helped to gain more understanding of the lubricant flow properties under micro oil supply condition.

Coalescence of surfactant covered drops in extensional flows: Effects of the interfacial diffusivity

Boundary integral simulations and scaling theory were employed to study the effects of insoluble surfactant surface diffusivity Ds and concentration Γ on the coalescence process of two equal-sized viscous drops. The drops underwent head-on collisions in a biaxial extensional flow, in the Stokes flow limit and low capillary numbers. The simulations were compared with the drainage time experiments of Yoon et al. [Phys. Fluids 19, 023102 (2007)10.1063/1.2409735] concerned with a polymeric system, polybutadiene (PBd) drops in a polydimethylsyloxane (PDMS) matrix, stabilized by block-copolymers acting as insoluble surfactants to explain the mechanism underneath their findings. An ad hoc equation of state, derived by mean field theory, specific for the block-copolymers in the experiments of Yoon et al., able to match the experimental surface tension data without fitting parameters, was used. We were able to reproduce the experimental drainage time data, although an additional attractive force, besides the usual van der Waals interactions, had to be introduced for high block-copolymer concentrations, probably as a result of the entropic attraction between the two facing dry brushes formed in the thin film between the two drops. According to simulations, the puzzling experimental drainage time transition for low surfactant concentrations, from high drainage time to low drainage time as Ca increases, was a consequence of the oscillating behavior of the minimum film thickness, which takes place for Marangoni numbers Ma < 5 and surface Peclet number Pes > 60. In this regard, a master curve was obtained for the scaled relative minimum film thickness attained during the oscillation as a function of Ma. This enabled to determine both the minimum value of the dimensionless attractive forces to avoid coalescence for each concentration studied and the range of Ma that favors early coalescence. The coalescence process was found very sensitive to Pes and for Pes O(100–1000) even trace amounts of surfactants can be as effective stabilizers as high surfactant concentrations. Moreover, for the polymeric system of interest, the range of Ds in which the drainage time changes from the saturation value to the clean interface value was computed as a function of the surfactant concentration. In the specific, for the PDMS/PBd system of interest the Ds range studied was O(10−12–10−5 cm2 s−1). Additionally, our scaling analysis further validates our simulations, also highlighting the effect on the drainage process of the different parameters, in particular, of the external pushing force, which is increased compared to a clean interface system, as Pes is increased or as the surfactant concentration is increased, because of the reduction in the interfacial mobility of the drop. Finally, our study suggests that matching simulations with four-roll mill drainage time experiments can be an effective method to determine block-copolymer surface diffusivity.

Template Free Synthesis of Hollow Ball-Like Nano-Fe2O3 and Its Application to the Detection of Dimethyl Methylphosphonate at Room Temperature

This paper is focused on the template-free synthesis of nanosized ferric oxide (nano-Fe2O3) and its application in quartz crystal microbalance (QCM) resonators to detect dimethyl methylphosphonate (DMMP), a simulant of Sarin. The X-ray diffraction (XRD) patterns confirm that the synthesized samples are made of Fe2O3 and the scanning electron microscopy (SEM) pictures show that the samples have ball-like shapes. The DMMP sensors with a sensing film of hollow ball-like and solid ball-like Fe2O3 are fabricated and their sensing characteristics are compared. The sensitivity of the hollow ball-like Fe2O3 sensor is more than 500% higher than the one of the solid ball-like Fe2O3 sensor. The hollow ball-like nano-Fe2O3 can be synthesized by a novel low temperature hydrothermal method. The sensors with the hollow ball-like Fe2O3 film perform well in a range of 1 to 6 ppm, with a sensitivity of 29 Hz/ppm at room temperature, while the appropriate recoverability and selectivity are maintained. In addition, the performance of different thicknesses of the sensing film of the hollow ball-like nano-Fe2O3 is investigated and the optimized relative film thickness of the hollow ball-like nano-Fe2O3 is found to be 20 μg/mm2.

Thin liquid film morphology driven by electro-static field

The development of stationary patterns on a thin polymer surface subject to an electric field is studied by means of the hexagonal-planform weakly nonlinear stability analysis and numerical simulations. The time evolution of the interface between the air and the polymer film on the unbounded spatial domain is described by a thin film equation, incorporating the electric driving force and the surface diffusion. The nonlinear interfacial growth includes the amplitude equations and superposition of one-dimensional structures at regular orientations. The pattern selection is driven by the subcritical instability mechanism in which the relative thickness of the polymer film plays a critical role.

Interfacial thin films rupture and self-similarity

Two superposed thin layers of fluids are prone to interfacial instabilities due to London-van der Waals forces. Evolution equations for the film thicknesses are derived using lubrication theory. Using the intrinsic scales, for a single layer, results in a system with parametric dependence of four ratios of the two layers: surface tension, Hamaker constant, viscosity, and film thickness. In contrast to the single layer case, the bilayer system has two unstable eigenmodes: squeezing and bending. For some particular parameter regimes, the system exhibits the avoided crossing behavior, where the two eigenmodes are interchanged. Based on numerical analysis, the system evolves into four different rupture states: basal layer rupture, upper layer rupture, double layer rupture, and mixed layer rupture. The ratio of Hamaker constants and the relative film thickness of the two layers control the system dynamics. Remarkably, the line of avoided crossing demarks the transition region of mode mixing and energy transfer, affecting the scaling of the dynamical regime map consequentially. Asymptotic and numerical analyses are used to examine the self-similar ruptures and to extract the power law scalings for both the basal layer rupture and the upper layer rupture. The scaling laws for the basal layer rupture are the same as those of the single layer on top of a substrate. The scaling laws for the upper layer rupture are different: the lateral length scale decreases according to (tr-t)1/3 and the film thickness decreases according to (tr-t)1/6.

Modeling of exact viscoelastic stresses in bilayer systems due to thermal and/or lattice mismatch: Maxwell model

Abstract

Approach to Knowledge of the Interaction between the Constituents of Contact Lenses and Ocular Tears: Mixed Monolayers of Poly(methyl methacrylate) and Dipalmitoyl Phosphatidyl Choline

Mixed monolayers of poly(methyl methacrylate) (PMMA), the main component of hard contact lenses, and dipalmitoyl phosphatidyl choline (DPPC), a characteristic phospholipidic constituent of ocular tear films, were selected as an in vitro model in order to observe the behavior of contact lenses on the eye. Using Langmuir monolayer and Brewster angle microscopy (BAM) techniques, the interaction between both components was analyzed from the data of surface pressure-area isotherms, compressional modulus-surface pressure, and relative film thickness versus time elapsed from the beginning of compression, together with BAM images. Regardless of the surface pressure at which the molecular/monomer areas (A(m)) were recorded, the A(m) mole fractions of PMMA (X(PMMA)) plots show that the experimental results match the theoretical values calculated from additivity rule A(m) = X(PMMA)A(PMMA) + X(DPPC)A(DPPC). The application of the Crisp phase rule to the phase diagram of the PMMA-DPPC system can explain the existence of a mixed monolayer made up of miscible components with ideal behavior at surface pressures below 25 mN/m. However, at very high surface pressures, when collapse is reached (at 60 mN/m), the single collapsed components are segregated into two independent phases. These results allows us to argue that PMMA hard contact lenses in the eye do not alter the structural characteristics of the phospholipid (DPPC) in tears.

Linear instability of ultra-thin liquid films flowing down cylindrical fibre

The stability characteristics of an ultra-thin layer of a viscous liquid flowing down a cylindrical fibre are investigated by a linear theory. The film with the thickness less than 100 nm is driven by an external force and under the influence of the van der Waals forces. The results show that, when the relative film thickness decreases, the curvature of the fibre depresses the development of the linear perturbations, whereas the van der Waals forces promote the instabilities. This competition results in a non-monotonous dependence of the growth rate on the relative film thickness. The critical curves are also obtained to describe the transition from the absolute instability to the convective instability, indicating that the van der Waals forces can enlarge the absolutely unstable region. Furthermore, the surface tension can cause the development of the absolute instability, whereas the external force has an opposite effect.

The Influence of Surface Tension on Lubrication Film Thickness and Pressure

Modified Reynolds equation is deduced considering surface tension in this paper. The influence of surface tension on lubrication and elastohydrodynamic lubrication is analyzed. Result shows surface tension has apparent influence on oil film thickness. It makes minimum film thickness increase under relative thin film thickness. The influence decreases rapidly with the increasing of film thickness. Surface tension has little influence on oil film pressure distribution.