Coated Aluminum Substrates Research Articles

Humidity-induced curing and anti-corrosion properties of GPTMS-modified polyorganosilazane functionalized silica coating on AA2024-T3 aluminum alloy

Relative humidity (RH) is one of the key parameters that significantly affect the curing kinetics and final properties of polysilazane-based coatings. Thus, the paper discusses the effect of relative humidity during the curing process and the anti-corrosion properties of (3-glycidyloxypropyl) trimethoxysilane (GPTMS)-modified polyorganosilazane (OPSZ) functionalized silica coatings on AA2024-T3. Modified polyorganosilazane sol was prepared and then deposited on AA2024-T3 substrates varying the curing conditions. After the deposition, the coated aluminum substrates were exposed to different RH levels; 15%, 40% and 80%, and then cured at 120 °C for 2 h. Transparent and crack-free GPTMS-modified polyorganosilazane coatings with a thickness of around 15–17 μm were obtained. The exposure to the relative humidity increased the crosslinking and hydrolysis-condensation reactions of the OPSZ and GPTMS molecules, showing more Si-O-Si bonds. The incorporation of GPTMS affected the crosslinking structure, enhancing the corrosion protection properties of the coating. GPTMS-modified polyorganosilazane coatings cured at 40%RH had the best anti-corrosive properties after immersion in 3.5 wt% NaCl solution. The impedance modulus of ∼109 Ω.cm2 at a low frequency was obtained, which was five orders of magnitude higher than that for the AA2024-T3 alloy.Graphical

Evaluation of the effectiveness and durability of commercial non-stick coatings

In this study, four commercial non-stick coatings (two fluoropolymer and two ceramic) were characterized by micrography, Raman spectroscopy and energy dispersive X-ray spectroscopy (EDX). A pancake batter was then baked on coated aluminum substrates. In a first test, 90 consecutive baking cycles were performed on each coated specimen. In a second test, the coatings were abraded by sanding, followed by 30 baking cycles. In both cases, take-off pressure was evaluated to extract the pancake model, as well as water sliding angle, surface roughness and surface damage. The results have shown that perfluoroalkoxyalkane (PFA) and polytetrafluoroethylene (PTFE) were the most effective in terms of pull-off pressure (<3 kPa) versus ceramic siloxane coatings with values between 15 and 34 kPa after 90 firings. In the different experimental stages, a positive correlation between wettability and detachment force was observed, especially for fluoropolymer coatings. However, surface roughness did not strong correlate with the pull-off force, low values did not correlate with low values of pull-off force and vice versa. This paper proposes an objective and simple coating evaluation procedure for industrial environments. Although ceramic sol-gel coatings show potential, there is a need to improve their non-stick capability to bring them on par with traditional solutions.

Investigation of surface inclination effect during dropwise condensation of flowing saturated steam

When a pure vapor condenses over a surface, it can form a continuous liquid film or a multitude of discrete droplets, thus realizing the so-called dropwise condensation (DWC). In the literature, most of the experimental data refer to DWC on vertical condensing surfaces with quiescent vapor. However, in many applications, the condensing vapor usually has a non-zero flow velocity with a consequent effect on the sliding motion of droplets. Moreover, the drag force due to vapor velocity may be the only mechanism for liquid removal on a horizontal surface or in space applications. A systematic investigation of the effects of vapor drag and surface inclination on the heat transfer and droplet population during DWC is needed and is addressed in the present paper.Here, DWC of flowing steam is experimentally studied on sol-gel silica-based coated aluminium substrates at three different inclinations: vertical, inclined at 45°, and horizontal. Heat transfer coefficient (HTC) and droplet population measurements are performed in a wide range of heat flux (260–610 kW m−2) and average vapor velocity (3.3–13.8 m s−1). When decreasing the tilt angle, from vertical to horizontal, due to the lower contribution of the gravity force, the average droplet size increases, and a strong HTC reduction is observed above all at low vapor velocities. Because of the vapor drag force, the HTC increases with steam velocity and, at the highest mass velocity, the HTC is independent from the surface inclination. A model for the droplet departing radius in the presence of vapor velocity, initially proposed by the present authors for the sole case of vertical surfaces, is here modified to account also for the effect of surface inclination and then assessed against the present experimental data. Hence, we propose to predict the heat flux during DWC by coupling the new equation for the departing radius with the available models of heat transfer through a single droplet and drop-size distribution. The developed calculation method is found to provide satisfactory predictions of the HTC for the whole range of vapor velocity, heat flux and surface inclination.

Enhanced output of ZnO nanosheet-based piezoelectric nanogenerator with a novel device structure

We report a double-fold enhancement of piezoelectric nanogenerator output voltage with a simple design strategy. The piezoelectric nanogenerator is fabricated with ZnO nanosheets coated on both sides of the aluminum substrate in this new design strategy with necessary electrodes. The cost-effective hydrothermal method is employed to synthesize two-dimensional (2D) ZnO nanosheets on both sides of the aluminum substrate at a low growth temperature of 80 °C for 4 h. The ZnO nanosheets were characterized for their morphology, crystallinity, and photoluminescence property. The performance of nanogenerator fabricated with double-side coated aluminum substrate was compared to single-side coated aluminum substrate. The nanogenerators fabricated only with one side coating produced an output voltage of ∼170 mV. In contrast, the nanogenerators fabricated with double side coating produced an output voltage of ∼285 mV. The nanogenerator with double-side coating produced ∼1.7 times larger output voltage than that of single-side coated one. The enhancement in the output voltage is mainly due to ZnO nanosheet deformation along both sides and the electric field-induced synergetic effect between two front and back sides of piezoelectric nanogenerators. This nanogenerator fabrication technology has the potential to be scaled up for industrial production of piezoelectric energy collecting devices because of its simplicity and high output gain.

Ablation behavior of SiC whisker and ZrB2 particle-filled ZrO2 sol-gel composite coating under high-intensity continuous laser irradiation

In this study, laser ablation-resistant composite coating consisting of zirconium diboride (ZrB2) particulate and silicon carbide (SiC) whisker with zirconia (ZrO2)-sol binder was prepared to meet requirements of anti-laser ablation ability. Laser ablation behavior of as-prepared coating was investigated under varying laser powers. Results revealed that the highest back-surface temperature of coated aluminum substrate was only 150 °C when irradiated at a power density of 6.3 kW/cm2 for 10 s, which is below the maximum allowable temperature of the aluminum alloy substrate (200 °C). After ablation for 10 s, the mass ablation rate of the composite coating was 4.45 mg/s. Apparently, the composite coating exhibited outstanding ablation resistance against high-intensity continuous laser irradiation. At initial stage of ablation, combination of self-healing effect of liquid ZrO2–SiO2 layer and volatilization of B2O3 provide effective joint protecting mechanism. With increasing ablation time, ZrO2 ceramic barrier layer, product of the pyrolysis of ZrO2 gel and ZrB2 filler, becomes progressively more important in maintaining excellent laser ablation resistance behavior, which is normally effective at around ablation pit.

A promising self-assembly PTFE coating for effective large-scale deicing

Ice accretion is detrimental for the safety of transmission lines, aircraft, and buildings, which may also result in economic loss. In this study, a thin polytetrafluoroethylene (PTFE) coating on the aluminum substrate was fabricated by electrostatic attraction self-assembly technique and proposed for large-scale deicing application. The shear-force Fice analysis on deicing and related ice adhesion is conducted while interfacial toughness between solid-ice interfaces was also measured. Moreover, the surface morphology, wettability of the PTFE coated aluminum plate was also characterized. It was observed that the critical bonded length Lc between ice and PTFE coated surface is about 2.85 cm. When the bonded length L was higher than this value, the shear-force of dislodging ice could stay constant at about 50 N for the first deicing cycle. From the second deicing cycle, the constant shear-force is increased to 70 N and kept constant for at least ten cycles. Meanwhile, the surface roughness and water contact angle kept on increasing along with the loss of PTFE nanoparticles during icing-deicing cycles. Ultimately, An ice layer with the size of 98 cm × 96 cm×1 cm (L × w×h) was developed on the surface of PTFE coated aluminum substrate for the demonstration of its fracture and falling off under ice’s weight.

Heat transfer during dropwise condensation of steam over a mirror polished sol-gel coated aluminum substrate

Dropwise condensation (DWC) is a promising heat transfer mechanism in new trends of thermal management and power generation systems to enhance the heat transfer during condensation. Creation of surfaces which can promote dropwise condensation is one of the main issues. For this purpose, a heat transfer surface that can maintain stable dropwise condensation, using hybrid organic-inorganic sol-gel silica coatings functionalized with methyl groups over an aluminum substrate, is developed and tested. This coating displays mildly hydrophilic behavior. Condensation of steam flowing on this surface occurs in dropwise mode with heat transfer coefficient values equal to 150–180 kW m−2 K−1 in the heat flux range between 150 and 510 kW m−2. The importance of the coating thermal resistance is discussed in the paper. The measured heat transfer coefficient is high compared to previous studies of DWC on metallic – and in particular aluminum - substrates. This type of surfaces paves the way to a cheap and green route to promote stable DWC on aluminum substrates without using fluorocarbons or controlled roughness patterns.

Enhancing the corrosion resistance of aluminum by graphene oxide and reduced graphene oxide films

This study is carried out with the goal of improving the corrosion resistance of aluminum in a saline solution through coating with graphene oxide (GO) and reduced GO (ERGO). The GO layer was applied on an aluminum substrate and then electrochemically reduced by cyclic voltammetry in 0.01 M K2HPO4 solution to form a reduced graphene oxide layer. The surface morphology, structure, and crystallinity of the coated materials were characterized using scanning electron microscope (SEM), x-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and Raman spectroscopy techniques. To study the electrochemical corrosion behavior of the coated aluminum substrate, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) methods were performed in 3.5 wt% NaCl solution. The results showed that both GO and ERGO films uniformly covered the surface of the aluminum substrates with layered structures. A noteworthy elimination of oxygen-containing functional groups (OFGs) occurred upon the electrochemical reduction of the film, yielding a highly-reduced ERGO layer on the substrate. Furthermore, the electrochemical reduction process significantly increased the thickness of the GO coating. The electrochemical test results exhibited that both GO, and ERGO layers improved the corrosion resistance of the bare aluminum substrate. However, electrochemical reduction of the GO layer had adverse effects on the corrosion barrier properties of the GO coating.

Acoustic Emission Feasibility Study for Carbon Epoxy Coated Aluminum Corrosion Monitoring

In the presented research effort, cyclic polarization tests were conducted coupled with acoustic emission measurements, in order to monitor the corrosion phenomena. Previous studies have shown that epoxy composite coatings offer satisfactory corrosion protection to aluminum substrates minimizing and even eliminating some degradation phenomena. In the present effort, acoustic emission was applied during potentiodynamic cyclic polarization testing on as received, anodized and carbon epoxy coated aluminum substrates in order to correlate corrosion phenomena, like oxide layer development and localized corrosion, with characteristic acoustic emission signal descriptors. The epoxy coating was used both as received and as reinforced with various carbon fillers i.e. carbon nano-tubes, amorphous graphite or a combination of them. Acoustic emission detected signals due to hydrogen bubble related processes during cathodic polarization and signals due to deposition of thick soluble films during the final stage of anodic polarization. The acoustic profile between coated and uncoated aluminum substrates, exhibited substantial changes dependent both on the surface preparation and the employed reinforcing filler. The overall outcome of the experimental results, indicate that with further research, acoustic emission can be employed as an Aluminum Corrosion Monitoring tool.

The Effect of Cerium Ions on the Structure, Porosity and Electrochemical Properties of Si/Zr-Based Hybrid Sol-Gel Coatings Deposited on Aluminum

This study was focused on the synthesis and characterization of Si/Zr-based hybrid sol-gel coatings with and without the addition of cerium(III) ions. The coatings were deposited on aluminum aiming to act as an effective and ecologically harmless alternative to toxic chromate coatings. The chemical composition, structure, thermal properties and porosity of the non-doped and Ce-doped coatings containing various Zr contents were examined by Raman spectroscopy and photothermal beam deflection spectroscopy. The corrosion properties of the coated aluminum substrates were studied using AC and DC electrochemical methods in 0.1 M NaCl electrolyte solution. Barrier and protecting properties of the coatings were monitored upon long-term immersion in chloride solution using electrochemical impedance spectroscopy. The effect of cerium ions was two-fold: on the formation of a more condensed Si−O−Zr network structure and on the formation of Ce-based deposits, which diminish the rate of cathodic reaction at the coating/metal interface. These effects acted synergistically and resulted in the creation of the coatings with effective barrier and active corrosion protection.

Prefinished Metal Polymer Hybrid Parts

In this study, the shaping and assembly behavior of adhesive polymer-metal-composites was investigated in an international cooperation using two step curable uretdione-polyester-based powder coatings (IPF development) which acts simultaneously as a reactive adhesive agent and as a high quality surface finish. To create the composite, a thermoplastic polyurethane (TPU) layer with good compatibility to the powder coating was over-molded onto a powder coated aluminum substrate. A polyamide (PA6) layer was over-molded on to the TPU layer to create a stiff composite structure with possibilities for further functionalization. The TPU-layer in between the metal substrate and the polymer top layer acts as a stress and strain compensation layer. These loads are caused by thermal expansion (under fluctuating temperatures) and external forces/deformation. Another key feature of the composite is the innovative process chain. The powder coating can resist high deformation and therefore the coating is suitable for a future application on to a metal substrate using a coil coating procedure. In addition, the coil could be easily implemented into a production line as a semi-finished product. The prefinished coated metal substrate could be formed (e.g. incremental forming, deep drawing) and inserted in the over-molding procedure. This overall shortened process chain allows not only an effective fabrication of pre-coated semi-finished materials and polymer-metal-joints in high quantities by saving process steps (e.g. cleaning steps, glue application) but also a higher versatility in the following composite production.

Nanoclay-based self-healing, corrosion protection coatings on aluminum, A356.0 and AZ91 substrates

Investigations were carried out to explore the feasibility of using Ce3+/Zr4+ encapsulated in environmental friendly nanoclay containers for prolonged corrosion protection of pure aluminum, aluminum alloy A356.0 and Mg alloy AZ91. Dimensions of clay nanotubes were studied using SEM and TEM analysis. They were subjected to pore volume and surface area analysis to confirm the loading of inhibitor into their lumen. The smart nanocontainers loaded with active cationic inhibitor were dispersed in an organic–inorganic hybrid sol–gel matrix synthesized from the hydrolysis and condensation of glycidoxypropyltrimethoxysilane and tetraethoxysilane. The loading of nanocontainers in the matrix sol was optimized. Coatings were generated on pure Al, A356.0, AZ91 substrates using a dip-coating technique and cured at 130°C for 1 h in air. The coated substrates were characterized for their corrosion resistance using potentiodynamic polarization and electrochemical impedance spectroscopic analysis by exposing them to 3.5 wt% NaCl solution for time intervals varying from 1 to 120 h. Micro-Raman spectroscopic studies were carried out to analyze the chemical composition of phases in the scribed area after exposure to corrosive medium. A few coated aluminum substrates were subjected to scanning vibrating electrode technique experiments to study the self-healing mechanism.

Evaluation of mechanical performance of NiCo nanocoated aerospace aluminum alloy using quantitative photo-thermo-mechanical radiometry as a non-contact strain gauge

Non-contact photo-thermo-mechanical radiometry (PTMR) was introduced to evaluate the mechanical properties of an aerospace aluminum alloy with Nickel Cobalt (NiCo) coatings. A home-made small-scale tensile rig was used to apply static uniaxial tensile load on the samples. The strength of nanocoated samples was recorded in terms of strain by an adhesive strain gauge also acting as a PTMR calibration device. For the purpose of non-destructive evaluation, the tests were limited within the elastic regime. Two experimental modalities were used: frequency scan at fixed load and strain scan at fixed frequency. The test results were analyzed with both a three-layer and a more detailed five-layer thermo-mechanical-wave (TMW) model. Both theoretical and experimental results indicated that the presence of the NiCo nanocoating can significantly strengthen the mechanical properties of the coated aluminum substrate. The coating can also provide protection to defective substrates and enhance their mechanical stiffness (strength).

Assessment of drilling characteristics of alumina coated on aluminium using CO2 laser

Abstract This paper reports experimental investigation of effect of laser parameters on the drilling responses of alumina coated aluminium substrates using 1.8 kW CO 2 pulsed mode laser. Alumina was deposited on 4 mm thick Al 6065 plates by plasma spray technique. The main influencing factors were: Laser power on spatter deposition and Heat Affected Zone; Pulse frequency on entrance and exit circularities; Piercing time on taper, heat affected zone and exit circularity. Genetic algorithm optimization results indicated simultaneous improvement of 5.2% in hole quality at 416.6 W laser power, 1.4 kHz pulse frequency and 0.4 s piercing time. Response Surface Methodology indicated minimum spatter deposition area of 11.57 mm 2 , Heat Affected Zone 0.589 mm, taper 2.35° and maximum circularity at entrance 0.88 and exit 0.905 at 400 W laser power, 1.2 kHz frequency and 0.48 s piercing time. These results were in close agreement with those of Genetic algorithm. The optimum values were validated through confirmatory experiments.

Nano-TiO2 coatings on aluminum surfaces by aerosol flame synthesis

Aluminum alloys are widely used in the aeronautic industry for their high mechanical properties; however, because they are very sensitive to corrosion, surface treatments are often required. TiO2 has excellent resistance to oxidation and it is often used to improve the corrosion resistance of aluminum surfaces. Several coating procedures have been proposed over the years, which are in some cases expensive in terms of production time and amount of deposited material. Moreover, they can damage aluminum alloys if thermal treatments are required. In this paper, a one-step method for the coating of aluminum surfaces with titania nanoparticles is presented. Narrowly sized, TiO2 nanoparticles are synthesized by flame aerosol and directly deposited by thermophoresis onto cold plates of aluminum AA2024. Submicron coatings of different thicknesses are obtained from two flame synthesis conditions by varying the total deposition time. A fuel-lean synthesis condition was used to produce 3.5nm pure anatase nanoparticles, while a mixture of rutile and anatase nanoparticles having 22nm diameter — rutile being the predominant phase —, was synthesized in a fuel-rich condition. Scanning electron microscopy is used to characterize morphology of titania films, while coating thickness is measured by confocal microscopy measurements. Electrochemical impedance spectroscopy is used to evaluate corrosion resistance of coated aluminum substrates. Results show an improvement of the electrochemical behavior of titania coated surfaces as compared to pristine aluminum surfaces. The best results are obtained by covering the substrates with 3.5nm anatase-phase nanoparticles and with lower deposition times, that assure a uniform surface coating.

Optical performance of high power LED on silver and nickel thin film coated aluminum substrates using sputtering process

Silver and Nickel thin films were prepared by sputtering technique and used as thermal interface materials to effectively improve the optical properties of LEDs. Different driving currents were used to test the performance of LED and measure the parameters such as color correlated temperature (CCT), Color Rendering Index and luminance LUX. The CCT value was found to be high for Ag (50 nm) buffered Ni (400 nm) thin film at 100 mA, which was subsequently decreased as the driving current increased. The results also showed that the lux value of the LED increased significantly for Ni (400 nm) thin film coated Al substrate boundary condition compared to Ag thin film with low thickness (50 and 100 nm) at 700 mA. Atomic Force Microscopy analysis of the thin film showed that Ag (100 nm) buffered Ni (200 nm) has the highest roughness compared to the other thin film coating. In general, the observed results showed that the Ni thin film and Ag (50 nm) buffered with Ni (400 nm) thin film can be effectively used as solid thin film thermal interface material, given that they improve the performance of the LED package significantly.

Effect of long-term neutral salt spray exposure on durability of adhesive-bonded Zr–Ti coated aluminum joint

In this study, the effect of long-term neutral salt spray (NSS) exposure (i.e., 50g/L concentration of salt solution at 25°C) on the retained strengths of Zr–Ti coated and bare lap-shear aluminum joints bonded with hem flange adhesive Henkel Terokal 8021 NB was investigated. A one-part toughened epoxide adhesive and Zr–Ti coated aluminum substrates (i.e., AA6014-T4 and AA6016-T4) were selected. Adhesive-bonded coated aluminum joints (ACJ) and bare aluminum joints (ABJ) were fabricated and exposed in NSS environment for various times. Quasi-static tests were conducted immediately following removal of the joints from the salt chamber at the ambient condition. It was found that while NSS exposure for 720h degraded little the strength of ACJ, it decreased the strength of ABJ. The Zr–Ti coating protected aluminum substrates from electrochemical reaction and consequently minimized the strength degradation. As the exposure time was prolonged to 1400h, the strength of ACJ was reduced drastically while the strength of ABJ was only degraded slightly. Fractography, differential scanning calorimetry, electrochemical potentiostatic polarization measurements, and X-ray photoelectron spectroscopy analyses revealed the strength degradation of ACJ was caused primarily by the corrosion of Zr–Ti coating. The passive film on bare aluminum provided a long time protection against NSS exposure, and consequently minimized the strength degradation for ABJ.

Improvement of Adhesion Strength of High Temperature Plasma Coated Aluminum Substrate with Aluminum-Alumina Powder Mixture

High temperature plasma coating technology has been applied to recover damaged aluminum dies from wear by spraying pure aluminum and alumina powder. However, the coated mixed powder layer composed of aluminum and alumina often undergoes a detachment from the substrate, making the coated substrate die unable to maintain its expected life span. In this study, in order to increase the bonding strength between the substrate and the coating layer, a pure aluminum layer was applied as an intermediate bond layer. In order to prepare the specimen with variable bond coating conditions, the bond coat layers with a various gun speed from 10 cm/sec to 30 cm/sec were prepared with coating cycle variations ranging from three to nine cycles. The specimen with a bond coat layer coated with a gun speed of 20 cm/sec and three coating cycles exhibited ~13MPa of adhesion strength, while the specimen without a bond coat layer showed ~6 MPa of adhesion strength. The adhesion strength with a variation of bond coat layer thickness is discussed in terms of coating parameters.

Enhancement of protection of aluminum through dopamine impregnation into hybrid sol–gel monolayers

The present work involves the incorporation of dopamine (DA) into the hybrid sol–gel monolayers (Hy) consisting of (3-glycidoxypropyl)methyldiethoxysilane and Tetraethyl orthosilicate. The resultant doped hybrid layers coated over aluminum surface was characterized by FT-IR, UV–Vis, XRD, EDX and SEM analyses. The corrosion behavior of bare and coated aluminum substrates in 3.5 % NaCl medium was examined by cyclic voltammetry, potentiodynamic polarization and electrochemical impedance spectroscopy techniques. Results from the electrochemical measurements showed that the DA impregnated Hy sol–gel monolayers offered better corrosion protection with superior coating thickness than the Hy sol–gel monolayers without DA. The enhancement of protection could be attributed to the fact that DA promote the formation of strongly bonded and densely packed monolayer films, which show better adhesion than the conventional silane systems.

Study of different aluminum alloy substrates coated with Ni–Co–P as metallic bipolar plates for PEM fuel cell applications

Pure Al and some of its alloys [Al alloy 6061 (AA6061), Al alloy 3004 (AA3004) and Al alloy 1050 (AA1050)] were coated with Ni–Co–P using electroplating power supply technique. Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) techniques were applied to study the surface morphology and chemical composition of coated aluminum substrates. Their performance against corrosion was examined using potentiodynamic polarization technique in (0.5 M H2SO4 + 2 ppm HF) solution. Corrosion potential values were shifted in the positive direction at all aluminum substrates after their coating with Ni–Co–P. Corrosion current density values at coated pure Al and AA1050 were decreased by ∼18.6 times, compared to those at bare substrates. The stability of coated aluminum alloys was investigated during long-time operation under cathodic environment in PEMFCs using potentiostatic polarization test at +160 mV (MMS) in air-saturated solution. Ni–Co–P/AA3004 substrate showed a high corrosion rate after short time, while coated AA6061 one slowly corroded. Interfacial contact resistance (ICR) values between metallic bipolar plates and gas diffusion layer were measured. Coating AA1050 with Ni–Co–P reduces its ICR value by 13 times. Accordingly, electroplated Ni–Co–P/AA1050 substrate can be chosen as an efficient bipolar plate material in PEMFCs.