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Abstract: The 2xxx aluminum alloys are largely used in aeronautical structural applications (fuselage, wings) due to their high mechanical strength/weight ratio, but have poor localized corrosion resistance. Their anticorrosive protection is generally ensured by multi-layered coatings based on inorganic and organic layers. The usual technique to evaluate the corrosion protection efficiency provided by the coatings is the salt spray test (ASTM B117). Nevertheless, though the test is employed worldwide, it presents some weak points, such as: it is a destructive test, subjectivity of test (only visual evaluation by the operator), low correlation between conditions of the test and real conditions, low reproductibility. The electrochemical impedance spectroscopy would be an alternative: non-destructive test, objectivity of the test (impedance measurement), the test can be performed in-situ. In this work, uncoated and coated AA2024-T3 alloys were tested in solutions of NaCl 3.5wt% at room temperature. The coatings were: (a): chromating conversion coating; (b): (a) + epoxy primer; (c): (b) + top coat; (d): chromic anodizing coating; (e): (d) + epoxy primer; (f): (e) + top coat. The specimens of AA 2024-T3 divided into seven groups of duplicates (each group representing a superficial state, non protected or a type of coating) were submitted to electrochemical impedance spectroscopy at corrosion potential using frequencies in the 0.1 to 100 kHz range. To ensure reliability of the results, a design of experiment based on a saturated factorial design was applied involving two control variables (treatment of surface and alternating current frequency), one response variable (impedance modulus) and two blocks (samples and repetitions). Some simulations of surface degradation via standardized accelerated tests (salt spray test) and simulations of common defects in process or assembly operations in aircraft manufacture according to valid standards in the aeronautical industry were also made on coated AA2024-T3 specimens. These surfaces were also tested using electrochemical impedance spectroscopy under the same experimental conditions (3.5 wt% NaCl, room temperature, 0.1Hz-100kHz). The statistical analysis of results showed the effectiveness of the application of electrochemical impedance spectroscopy to assist in the quality control of processes of surface treatments in the aerospace sector. The synergic effect involving the care taken in the experimental conditions, the type of experimental design and the sampling size were important to validate the results facing violations of hypotheses in the analysis of variance method. Each coating on AA2024-T3 alloy, and also the bare metal, is characterized by an Impedance versus Frequency Bode curve, sort of fingerprint, which can be used to identify the coating and to evaluate easily and accurately its quality. The results have demonstrated promising and allow to establish strategies for implementation of the electrochemical impedance spectroscopy technique in surface treatment processes. Keywords: AA2024 alloy, corrosion, EIS, non-destructive test, aeronautic industry. |
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Abstract: The aim of this work was to provide a comprehensive insight concerning coated films which might be used for first mirrors in ITER. The influence of the mirror crystallite size has been addressed as well as the coating techniques to provide nanocrystalline films. Tests of coated mirrors both in laboratories and in tokamaks are reviewed. For the tokamak tests a wide angle camera system has been installed in JET-ILW which is composed of a mirror box with 3 stainless steel mirrors coated with rhodium viewing the torus through a conically shaped aperture. The system delivered the required image quality for plasma monitoring and wall protection. No or insignificant degradation of the optical transmittance has been observed during the experimental campaign in 2014 with about 3000 plasma pulses in different magnetic field configurations. Keywords: First Mirrors, ITER, Mo, Rh coating, JET wide angle viewing system. |
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Abstract: Osteoblast viability, proliferation, protein expression and mineralization were studied on bare, micro- and nanoporous silicon (Si) substrates. Micro- and nano-porous-Si substrates were prepared by anodic etching of silicon in ethanolic hydrofluoric acid and characterized using scanning electron and atomic force microscopies. Mouse osteoblasts were cultured on these substrates and cellular response to these surfaces was assessed using the Live/Dead Cell Viability assay and the MTT assay for cell proliferation. Osteoblast functionality was assessed using immunohistochemistry for bone protein specific markers. Osteoblasts grew well on micro- and nanoporous silicon substrates over the twenty-one day experimental period supporting the assessment that these are suitable cell supportive surfaces. Cell proliferation rates on bare and nanoporous silicon were similar initially, however, nanoporous silicon displayed enhanced cell proliferation, in comparison to bare silicon, after 14 days in culture. Immunocytochemical assays, using bone specific markers, showed positive reactions for osteonectin and osteopontin expression on all substrates with staining intensity increasing over the 21-day experimental period. Calcium mineral deposits were quantified using the Alizarin Red histochemical assay and nanoporous silicon induced the highest level of calcium mineral production in comparison to bare and microporous silicon. The data supports the potential use of nanoporous silicon as a surface implant coating for dental and orthopedic applications. The ability to dope (and then release) drugs or growth factors from the silicon nanopores offers the potential for a multi-functional implant surface.. Keywords: Porous-Si, anodic etching, implant surfaces, osteoblasts. |
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Abstract: Zinc oxide (ZnO) is a wide band gap (~3.37 eV) semiconductor. Thin film ZnO has many attractive applications in optoelectronics and sensors. Recently, nanostructured ZnO (e.g. ZnO quantum dot) has been demonstrated as a hyperbolic material; its dielectric function has opposite signs along different crystal axes within the mid-infrared, making it an interesting material for metamaterials and nanophotonics. Conventional sputtering deposition usually leads to the formation of polycrystalline ZnO films with randomly oriented grains and rough surface. This work demonstrated a solution-based process to grow ZnO thin films with highly oriented nanocrystals. Low-temperature plasmas were employed to modulate the microstructure and optical properties of the films. Such highly anisotropic nanostructured transparent semiconductor films may lead to interesting material properties in developing new optoelectronic devices. Keywords: Zinc oxide, sol-gel, oxygen plasma, crystal size. |
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Abstract: Simple β-NiAl, Hf-modified β-NiAl, Pt-diffused, Pt-modified β-(Ni,Pt)Al + ξ-PtAl2, and Hf-Pt-modified β-(Ni,Pt)Al were cyclic oxidation tested at 1050°C in air on Inconel-718 substrates for up to 4370h. The Pt-diffused specimen failed most quickly, < 100 h, while the simple β-NiAl aluminide maintained a positive weight change for ~1300 h. The Pt-modified aluminides clearly improved the cyclic oxidation behavior of both simple and Hf-modified aluminides, sustaining a zero weight change only after 3600 and 4000 h, respectively. The Hf additions did not immediately appear to produce as strong an improvement as expected, however, they were more highly ranked when normalized by coating thickness. They also decreased surface rumpling, important for TBC durability. Hf-rich NiAl grain boundaries, formed during coating processing, resulted in HfO2 particles in the scales and oxide pegs at the metal interface, all suggesting some level of over-doping. The high sulfur content of the substrate influenced spalling to bare metal and re-healing to less protective Ni(Al,Cr)2O4 spinel-type and (Ti,Cr,Nb)O2 rutile scales. The evolution of these surface features have been documented over 100 to 4370 h of exposure. The coating aluminum content near failure was ~2-3 wt. %. Keywords: Hafnium, aluminide coatings, oxidation testing, Inconel 718, vapor phase process. |