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Coatings, Volume 7, Issue 12 (December 2017)

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Cover Story (view full-size image) We investigated Interpenetrating Polymer Networks (IPN) for high temperature applications. The [...] Read more.
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Open AccessArticle Surface Roughness Reduction of Additive Manufactured Products by Applying a Functional Coating Using Ultrasonic Spray Coating
Coatings 2017, 7(12), 208; doi:10.3390/coatings7120208
Received: 20 October 2017 / Revised: 14 November 2017 / Accepted: 20 November 2017 / Published: 23 November 2017
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Abstract
To reduce the high surface roughness of additive manufactured (AM) products, typically a post-treatment is required. Subtractive post-treatments are often performed by hand and are therefore expensive and time consuming, whereas conventional additive post-treatments, such as pneumatic spray coating, require large quantities of
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To reduce the high surface roughness of additive manufactured (AM) products, typically a post-treatment is required. Subtractive post-treatments are often performed by hand and are therefore expensive and time consuming, whereas conventional additive post-treatments, such as pneumatic spray coating, require large quantities of coating material. Ultrasonic spray coating, in contrast, is an additive post-treatment technology capable of applying coatings in an efficient way, resulting in less material usage. In this paper, we investigate the application of the ultrasonic spray coating process and the final properties of the coated AM part by applying a thin coating to reduce surface roughness of the AM substrate and to impart hydrophobic functionality. The hydrophobic coating is applied onto flat selective laser sintered (SLS) surfaces prepared from polyamide 12 (PA12) having a surface roughness of Ra = 20 µm. The hydrophobic coating consists of 5 wt % polyvinylidene fluoride (PVDF) in acetone. The coated substrates are analyzed for roughness using a profilometer, a contact angle using a goniometer, and a coating uniformity and thickness using light and scanning electron microscopes. The layer formation applying the ultrasonic spray coating is studied and compared with layer formation using pneumatic spray coating. It is found that a roughness reduction down to 5 µm was achieved via an ultrasonic spray coating with 30 layers of PVDF solution. It is shown in cross-section electron microscopy pictures that, due to the nature of the ultrasonically generated droplets, the rough and porous surface of the SLS surface is filled with the PVDF material after which the roughness is reduced by adding a thin layer on top. In comparison to a standard industry-applied pneumatic spray coating process, the results obtained from ultrasonic spray coating show less material usage, a reduced roughness, and a better filling of the pores, obviously resulting in optimized adhesion. Full article
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Open AccessArticle Microstructure and Mechanical Properties of TaN Thin Films Prepared by Reactive Magnetron Sputtering
Coatings 2017, 7(12), 209; doi:10.3390/coatings7120209
Received: 7 October 2017 / Revised: 15 November 2017 / Accepted: 20 November 2017 / Published: 23 November 2017
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Abstract
Reactive magnetron sputtering was used to deposit tantalum nitride (Ta–N) thin films on Si substrate. The effect of varying the N2 percentage in the N2/Ar gas mixture on the Ta–N film characteristics was investigated. Mechanical and tribological properties were studied
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Reactive magnetron sputtering was used to deposit tantalum nitride (Ta–N) thin films on Si substrate. The effect of varying the N2 percentage in the N2/Ar gas mixture on the Ta–N film characteristics was investigated. Mechanical and tribological properties were studied using nanoindentation and pin-on-disc wear testing. Decreasing the N2 content in the gas mixture was found to change the film structure from face centered cubic (fcc) TaN (from 25% to 10% N2) to highly textured fcc TaN (at 7% N2) to a mixture of fcc TaN1.13 and hexagonal Ta2N (at 5% N2), and finally to hexagonal Ta2N (at 3% N2). A high hardness of about 33 GPa was shown by the films containing the hexagonal Ta2N phase (5% and 3% N2). Decreasing the N2 content below 7% N2 was also found to result in microstructural refinement with grain size 5–15 nm. Besides the highest hardness, the film deposited with 3% N2 content exhibited the highest hardness/modulus ratio (0.13), and elastic recovery (68%), and very low wear rate (3.1 × 10−6 mm3·N−1·m−1). Full article
(This article belongs to the Special Issue Design and Synthesis of Hard Coatings)
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Open AccessArticle Long-Term Corrosion Protection of a Cupro-Nickel Alloy Due to Graphene Coating
Coatings 2017, 7(12), 210; doi:10.3390/coatings7120210
Received: 20 September 2017 / Revised: 28 October 2017 / Accepted: 13 November 2017 / Published: 23 November 2017
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Abstract
This study demonstrates the corrosion resistance of a Cu-Ni alloy coated with a multi-layer graphene. This is one of the first demonstrations of long-term corrosion resistance due to graphene coating, which is crucial since earlier studies have suggested graphene-coated copper to be considerably
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This study demonstrates the corrosion resistance of a Cu-Ni alloy coated with a multi-layer graphene. This is one of the first demonstrations of long-term corrosion resistance due to graphene coating, which is crucial since earlier studies have suggested graphene-coated copper to be considerably inferior to bare copper in terms of corrosion during long-term exposure to a corrosive environment. The inferior corrosion resistance of graphene-coated copper arises due to defects and poor surface coverage by graphene. Since it is prohibitively difficult to develop defect-free graphene on metals at a commercially feasible scale, this study investigated the hypothesis for an alternative approach, i.e., to develop multi-layer graphene with a reasonable assumption that the areas of defects/poor coverage of a layer will be masked by the subsequent over-layer(s). This study has validated this hypothesis. Electrochemical investigations have demonstrated multi-layer graphene to improve the corrosion resistance of a Cu-Ni alloy by an order of magnitude. However, the most striking finding of this study is that the improvement in corrosion resistance due to the multi-layer graphene coating sustained the entire duration of a long-term test (~350 h). Full article
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Open AccessArticle Flexible Thermoelectric Composite Films of Polypyrrole Nanotubes Coated Paper
Coatings 2017, 7(12), 211; doi:10.3390/coatings7120211
Received: 16 October 2017 / Revised: 12 November 2017 / Accepted: 13 November 2017 / Published: 24 November 2017
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Abstract
Flexible thermoelectric composite films of polypyrrole (PPy) nanotubes coated paper were fabricated by an in-situ polymerization procedure using methyl orange as a template and paper as the substrate for the first time. Both the electrical conductivity and Seebeck coefficient of the polypyrrole nanotubes
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Flexible thermoelectric composite films of polypyrrole (PPy) nanotubes coated paper were fabricated by an in-situ polymerization procedure using methyl orange as a template and paper as the substrate for the first time. Both the electrical conductivity and Seebeck coefficient of the polypyrrole nanotubes coated paper composite films have been enhanced (from ~0.048 S/cm to ~0.068 S/cm and from ~5.34 μV/K to ~8.21 μV/K for the average value for three measurements, respectively) as the temperature increased from ~300 K to ~370 K, which lead to the same trend of the power factor. The thermal conductivity of the polypyrrole nanotubes coated composite films was very low (~0.1275 W·m−1·K−1 at ~300 K), and a highest ZT (material’s dimensionless figure of merit (S2σT/κ)) value of 3.2 × 10−7 was obtained at ~370 K. Full article
(This article belongs to the Special Issue Novel Thin Film Materials for Thermoelectric Applications)
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Open AccessArticle In-Situ Growth and Characterization of Indium Tin Oxide Nanocrystal Rods
Coatings 2017, 7(12), 212; doi:10.3390/coatings7120212
Received: 31 October 2017 / Revised: 16 November 2017 / Accepted: 22 November 2017 / Published: 25 November 2017
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Abstract
Indium tin oxide (ITO) nanocrystal rods were synthesized in-situ by a vapor-liquid-solid (VLS) method and electron beam evaporation technique. When the electron-beam gun bombarded indium oxide (In2O3) and tin oxide (SnO2) mixed sources, indium and tin droplets
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Indium tin oxide (ITO) nanocrystal rods were synthesized in-situ by a vapor-liquid-solid (VLS) method and electron beam evaporation technique. When the electron-beam gun bombarded indium oxide (In2O3) and tin oxide (SnO2) mixed sources, indium and tin droplets appeared and acted as catalysts. The nanocrystal rods were in-situ grown on the basis of the metal catalyst point. The nanorods have a single crystal structure. Its structure was confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The surface morphology was analyzed by scanning electron microscopy (SEM). During the evaporation, a chemical process was happened and an In2O3 and SnO2 solid solution was formed. The percentage of doped tin oxide was calculated by Vegard’s law to be 3.18%, which was in agreement with the mixture ratio of the experimental data. The single crystal rod had good semiconductor switch property and its threshold voltage of single rod was approximately 2.5 V which can be used as a micro switch device. The transmission rate of crystalline nanorods ITO film was over 90% in visible band and it was up to 95% in the blue green band as a result of the oxygen vacancy recombination luminescence. Full article
(This article belongs to the Special Issue Functional Oxide and Oxynitride Coatings)
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Open AccessArticle IPN Polysiloxane-Epoxy Resin for High Temperature Coatings: Structure Effects on Layer Performance after 450 °C Treatment
Coatings 2017, 7(12), 213; doi:10.3390/coatings7120213
Received: 23 October 2017 / Revised: 21 November 2017 / Accepted: 22 November 2017 / Published: 28 November 2017
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Abstract
Coatings for high temperatures (HT > 400 °C) are obtained from interpenetrating polymer network (IPN) binders formed by simultaneous polymerization of silicone and epoxide pre-polymers. A ceramic layer; mainly composed of silica and fillers; remains on the metal surface after a thermal treatment
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Coatings for high temperatures (HT > 400 °C) are obtained from interpenetrating polymer network (IPN) binders formed by simultaneous polymerization of silicone and epoxide pre-polymers. A ceramic layer; mainly composed of silica and fillers; remains on the metal surface after a thermal treatment at 450 °C. The layer adhesion and the inorganic filler’s distribution have been investigated by, firstly, exchanging the organic substituents (methyl and phenyl) of the silicone chains and, secondly, by adding conductive graphene nanoplatelets with the aim to assure a uniform distribution of heat during the thermal treatment. The results are evidence that different substituent ratios affect the polymer initial layout. The adhesion tests of paint formulations are analysed and were related to instrumental analyses performed using glow discharge optical emission spectroscopy (GDOES); thermal analyses (TG/DTA and DSC); electron microscopy with energy dispersive X-ray analysis (SEM-EDX). A greater resistance to powdering using phenyl groups instead of methyl ones; and an improved distribution of fillers due to graphene nanoplatelet addition; is evidenced. Full article
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Open AccessArticle Graphene Coating on Copper by Electrophoretic Deposition for Corrosion Prevention
Coatings 2017, 7(12), 214; doi:10.3390/coatings7120214
Received: 1 October 2017 / Revised: 20 November 2017 / Accepted: 22 November 2017 / Published: 30 November 2017
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Abstract
In this paper, we report the use of a simple and inexpensive electrophoretic deposition (EPD) technique to develop thin, uniform, and transparent graphene oxide (GO) coating on copper (Cu) substrate on application of 10 V for 1 s from an aqueous suspension containing
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In this paper, we report the use of a simple and inexpensive electrophoretic deposition (EPD) technique to develop thin, uniform, and transparent graphene oxide (GO) coating on copper (Cu) substrate on application of 10 V for 1 s from an aqueous suspension containing 0.03 wt % graphene oxide. GO was partially reduced during the EPD process itself. The GO coated on Cu was completely reduced chemically by using sodium borohydride (NaBH4) solution. The coatings were characterized by field emission scanning electron microscope (FESEM), Raman spectroscopy, Fourier-transform infrared spectroscopy (FTIR), XRD, and UV/VIS spectrophotometry. Corrosion resistance of the coatings was evaluated by electrochemical measurements under accelerated corrosion condition in 3.5 wt % NaCl solution. The GO coated on Cu and chemically reduced by NaBH4 showed more positive corrosion potential (Ecorr) (−145.4 mV) compared to GO coated on Cu (−182.2 mV) and bare Cu (−235.3 mV), and much lower corrosion current (Icorr) (7.01 µA/cm2) when compared to 15.375 µA/cm2 for bare Cu indicating that reduced GO film on copper exhibit enhanced corrosion resistance. The corrosion inhibition efficiency of chemically reduced GO coated Cu was 54.40%, and its corrosion rate was 0.08 mm/year as compared to 0.18 mm/year for bare copper. Full article
(This article belongs to the Special Issue Electrophoretic Deposition)
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Open AccessArticle The Effect of Post-Baking Temperature and Thickness of ZnO Electron Transport Layers for Efficient Planar Heterojunction Organometal-Trihalide Perovskite Solar Cells
Coatings 2017, 7(12), 215; doi:10.3390/coatings7120215
Received: 31 October 2017 / Revised: 10 November 2017 / Accepted: 25 November 2017 / Published: 30 November 2017
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Abstract
Solution-processed zinc oxide (ZnO)-based planar heterojunction perovskite photovoltaic device is reported in this study. The photovoltaic device benefits from the ZnO film as a high-conductivity and high-transparent electron transport layer. The optimal electron transport layer thickness and post-baking temperature for ZnO are systematically
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Solution-processed zinc oxide (ZnO)-based planar heterojunction perovskite photovoltaic device is reported in this study. The photovoltaic device benefits from the ZnO film as a high-conductivity and high-transparent electron transport layer. The optimal electron transport layer thickness and post-baking temperature for ZnO are systematically studied by scanning electron microscopy, photoluminescence and time-resolved photoluminescence spectroscopy, and X-ray diffraction. Optimized perovskite solar cells (PSCs) show an open-circuit voltage, a short-circuit current density, and a fill factor of 1.04 V, 18.71 mA/cm2, and 70.2%, respectively. The highest power conversion efficiency of 13.66% was obtained when the device was prepared with a ZnO electron transport layer with a thickness of ~20 nm and when post-baking at 180 °C for 30 min. Finally, the stability of the highest performance ZnO-based PSCs without encapsulation was investigated in detail. Full article
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Open AccessArticle Influence of Power Pulse Parameters on the Microstructure and Properties of the AlCrN Coatings by a Modulated Pulsed Power Magnetron Sputtering
Coatings 2017, 7(12), 216; doi:10.3390/coatings7120216
Received: 15 July 2017 / Revised: 9 November 2017 / Accepted: 27 November 2017 / Published: 30 November 2017
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Abstract
In this study, AlCrN coatings were deposited using modulated pulsed power magnetron sputtering (MPPMS) with different power pulse parameters by varying modulated pulsed power (MPP) charge voltages (350 to 550 V). The influence of power pulse parameters on the microstructure, mechanical properties and
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In this study, AlCrN coatings were deposited using modulated pulsed power magnetron sputtering (MPPMS) with different power pulse parameters by varying modulated pulsed power (MPP) charge voltages (350 to 550 V). The influence of power pulse parameters on the microstructure, mechanical properties and thermal stability of the coatings was investigated. The results indicated that all the AlCrN coatings exhibited a dense columnar microstructure. Higher charge voltage could facilitate a denser coating microstructure. As the charge voltage increased up to 450 V or higher, the microvoids along the column boundaries disappeared and the coatings became fully dense. The main phase in the AlCrN coatings was the c-(Al, Cr)N solid solution phase with NaCl-type phase structure. A diffraction peak of the h-AlN phase was detected at a 2θ of around 33°, when the charge voltage was higher than 500 V. The hardness of the AlCrN coatings varied as a function of charge voltage. The maximum value of the hardness (30.8 GPa) was obtained at 450 V. All the coatings showed good thermal stability and maintained their structure and mechanical properties unchanged up to 800 °C during vacuum annealing. However, further increasing the annealing temperature to 1000 °C resulted in apparent change in the microstructure and decrease in the hardness. The charge voltages also showed a significant influence on the high-temperature tribological behavior of the coatings. The coating deposited at the charge voltage of 550 V exhibited excellent tribological properties with a low friction coefficient. Full article
(This article belongs to the Special Issue Mechanical Behavior of Coatings and Engineered Surfaces)
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Open AccessArticle Contamination-Free Graphene Transfer from Cu-Foil and Cu-Thin-Film/Sapphire
Coatings 2017, 7(12), 218; doi:10.3390/coatings7120218
Received: 7 November 2017 / Revised: 24 November 2017 / Accepted: 27 November 2017 / Published: 2 December 2017
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Abstract
The separation of graphene grown on metallic catalyst by chemical vapor deposition (CVD) is essential for device applications. The transfer techniques of graphene from metallic catalyst to target substrate usually use the chemical etching method to dissolve the metallic catalyst. However, this causes
[...] Read more.
The separation of graphene grown on metallic catalyst by chemical vapor deposition (CVD) is essential for device applications. The transfer techniques of graphene from metallic catalyst to target substrate usually use the chemical etching method to dissolve the metallic catalyst. However, this causes not only high material cost but also environmental contamination in large-scale fabrication. We report a bubble transfer method to transfer graphene films to arbitrary substrate, which is nondestructive to both the graphene and the metallic catalyst. In addition, we report a type of metallic catalyst, which is 700 nm of Cu on sapphire substrate, which is hard enough to endure against any procedure in graphene growth and transfer. With the Cr adhesion layer between sapphire and Cu film, electrochemically delaminated graphene shows great quality during several growth cycles. The electrochemical bubble transfer method can offer high cost efficiency, little contamination and environmental advantages. Full article
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Open AccessArticle Surfaces Bearing Fluorinated Nucleoperfluorolipids for Potential Anti-Graffiti Surface Properties
Coatings 2017, 7(12), 220; doi:10.3390/coatings7120220
Received: 9 October 2017 / Revised: 22 November 2017 / Accepted: 24 November 2017 / Published: 4 December 2017
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Abstract
Graffiti can sometimes be a problem when put in an inappropriate place. We looked at a means to prevent such inconvenience. In this work, we explore the possibility of developing surfaces with controlled wettability. If the paint does not spread, the graffiti does
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Graffiti can sometimes be a problem when put in an inappropriate place. We looked at a means to prevent such inconvenience. In this work, we explore the possibility of developing surfaces with controlled wettability. If the paint does not spread, the graffiti does not stay. Here, the synthesis and electrodeposition of original 3,4-ethylenedioxythiophene (EDOT) with perfluorinated nucleolipids (ante approach) is reported. The elaboration of similar surfaces using post functionalization is also described. All the prepared surfaces were then investigated for their roughness, wettability, and morphology. Highly hydrophobic features are reported (θ = 137°) and oleophobic properties are also reported (θ = 110°) showing real potential for the control of surface wettability and for potential anti-graffiti applications, consequently. The surfaces obtained with the ante approach are rougher and more hydrophobic. Full article
(This article belongs to the Special Issue Anti-Graffiti Coatings)
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Open AccessArticle Effect of Hydrogen Concentration on the Growth of Carbon Nanotube Arrays for Gecko-Inspired Adhesive Applications
Coatings 2017, 7(12), 221; doi:10.3390/coatings7120221
Received: 18 October 2017 / Revised: 25 November 2017 / Accepted: 2 December 2017 / Published: 5 December 2017
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Abstract
Vertically-aligned carbon nanotubes (VACNTs) have extraordinary structural and mechanical properties, and have been considered as potential candidates for creating dry adhesives inspired by adhesive structures in nature. Catalytic chemical vapor deposition is widely used to grow VACNTs; however, the influential mechanism of VACNT
[...] Read more.
Vertically-aligned carbon nanotubes (VACNTs) have extraordinary structural and mechanical properties, and have been considered as potential candidates for creating dry adhesives inspired by adhesive structures in nature. Catalytic chemical vapor deposition is widely used to grow VACNTs; however, the influential mechanism of VACNT preparation parameters (such as H2 concentration) on its adhesion property is not clear, making accurate control over the structure of VACNTs adhesive an ongoing challenge. In this article, we use electron beam-deposited SiO2/Al2O3 as a support layer, Fe as catalyst, and C2H4/H2 gas mixtures as a feed gas to prepare VACNTs, while varying the ratio of the reducing atmosphere (H2) from 0% to 35%. VACNTs synthesized at a 15% H2 concentration (5 mm × 5 mm in size) can support a maximal weight of 856 g, which indicates a macroscopic shear adhesive strength of 34 N/cm2. We propose a hydrogen-concentration-dependent model for the shear adhesive performance of VACNTs. By adjusting the amount of hydrogen present during the reaction, the morphology and quality of the prepared VACNTs can be precisely controlled, which significantly influences its shear adhesive performance. These results are advantageous for the application of carbon nanotubes as dry adhesives. Full article
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Open AccessArticle Identification of Intermetallic Compounds and Its Formation Mechanism in Boron Steel Hot-Dipped in Al-7 wt.% Mn Alloy
Coatings 2017, 7(12), 222; doi:10.3390/coatings7120222
Received: 10 November 2017 / Revised: 27 November 2017 / Accepted: 1 December 2017 / Published: 6 December 2017
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Abstract
In laser welding and hot stamping Al-Si-coated boron steel, there is a problem that the strength of the joint is lowered due to ferrite formation in the fusion zone. The purpose of this study is to develop an Al-7 wt.% Mn hot-dip coating
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In laser welding and hot stamping Al-Si-coated boron steel, there is a problem that the strength of the joint is lowered due to ferrite formation in the fusion zone. The purpose of this study is to develop an Al-7 wt.% Mn hot-dip coating in which Mn, an austenite stabilizing element, replaces the ferrite stabilizing element Si. The nucleation and formation mechanism of the reaction layer was studied in detail by varying the dipping time between 0 and 120 s at 773 °C. The microstructure and phase constitution of the reaction layer were investigated by various observational methods. Phase formation is discussed using a phase diagram calculated by Thermo-CalcTM. Under a 30 s hot-dipping process, no reaction occurred due to the formation of a Fe3O4 layer on the steel surface. The Fe3O4 layer decomposed by a reduction reaction with Al-Mn molten alloy, constituent elements of steel dissolved into a liquid, and the reaction-layer nucleus was formed toward the liquid phase. A coated layer consists of a solidified layer of Al and Al6Mn and a reactive layer formed beneath it. The reaction layer is formed mainly by inter-diffusion of Al and Fe in the solid state, which is arranged on the steel in the order of Al11Mn4 → FeAl3 (θ) → Fe2Al5 (η) phases, and the Fe3AlC (κ) in several nm bands formed at the interface between the η-phase and steel. Full article
(This article belongs to the Special Issue Hybrid Surface Coatings & Process (Selected Papers from HyMaP 2017))
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Open AccessArticle Insights on the Role of Copper Addition in the Corrosion and Mechanical Properties of Binary Zr-Cu Metallic Glass Coatings
Coatings 2017, 7(12), 223; doi:10.3390/coatings7120223
Received: 26 September 2017 / Revised: 8 November 2017 / Accepted: 17 November 2017 / Published: 5 December 2017
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Abstract
The effect of copper addition on the corrosion resistance and mechanical properties of binary Zr100–xCux (x = 30, 50, 80, 90 at.%) glassy coatings was investigated by means of electrochemical measurements, scanning electron microscopy (SEM), energy dispersive analysis
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The effect of copper addition on the corrosion resistance and mechanical properties of binary Zr100–xCux (x = 30, 50, 80, 90 at.%) glassy coatings was investigated by means of electrochemical measurements, scanning electron microscopy (SEM), energy dispersive analysis spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and nano-indentation techniques. The corrosion resistance in 0.01 M deaerated H2SO4 solution and the mechanical properties of the Zr-Cu glassy coatings depend considerably upon the copper content in the glassy matrix. The top surfaces of the Zr-Cu coatings with lower Cu content were covered by a compact protective ZrO2 passive film. The competition between the oxidation of Zr atoms (ZrO2 film formation) and the oxidation–dissolution of Cu atoms assumed the most important role in the electrochemical behavior of the Zr-Cu glassy coatings. The generation of ZrO2 on the surface benefited the formation of passive film; and the corrosion resistance of the metallic glass coatings depended on the coverage degree of ZrO2 passive film. The evolution of free volume affected both the mechanical and corrosion behaviors of the Zr-Cu glassy coatings. Full article
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Open AccessArticle Modified Starch-Chitosan Edible Films: Physicochemical and Mechanical Characterization
Coatings 2017, 7(12), 224; doi:10.3390/coatings7120224
Received: 27 October 2017 / Revised: 28 November 2017 / Accepted: 2 December 2017 / Published: 7 December 2017
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Abstract
Starch and chitosan are widely used for preparation of edible films that are of great interest in food preservation. This work was aimed to analyze the relationship between structural and physical properties of edible films based on a mixture of chitosan and modified
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Starch and chitosan are widely used for preparation of edible films that are of great interest in food preservation. This work was aimed to analyze the relationship between structural and physical properties of edible films based on a mixture of chitosan and modified starches. In addition, films were tested for antimicrobial activity against Listeria innocua. Films were prepared by the casting method using chitosan (CT), waxy (WS), oxidized (OS) and acetylated (AS) corn starches and their mixtures. The CT-starches films showed improved barrier and mechanical properties as compared with those made from individual components, CT-OS film presented the lowest thickness (74 ± 7 µm), water content (11.53% ± 0.85%, w/w), solubility (26.77% ± 1.40%, w/v) and water vapor permeability ((1.18 ± 0.48) × 10−9 g·s−1·m−1·Pa−1). This film showed low hardness (2.30 ± 0.19 MPa), low surface roughness (Rq = 3.20 ± 0.41 nm) and was the most elastic (Young’s modulus = 0.11 ± 0.06 GPa). In addition, films made from CT-starches mixtures reduced CT antimicrobial activity against L. innocua, depending on the type of modified starch. This was attributed to interactions between acetyl groups of AS with the carbonyl and amino groups of CT, leaving CT with less positive charge. Interaction of the pyranose ring of OS with CT led to increased OH groups that upon interaction with amino groups, decreased the positive charge of CT, and this effect is responsible for the reduced antimicrobial activity. It was found that the type of starch modification influenced interactions with chitosan, leading to different films properties. Full article
(This article belongs to the Special Issue Edible Films and Coatings)
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Open AccessArticle In-Situ Heat Treatment Study on the Nanocrystalline Cr2O3 Film Using an Environmental Scanning Electron Microscope
Coatings 2017, 7(12), 225; doi:10.3390/coatings7120225
Received: 9 November 2017 / Revised: 5 December 2017 / Accepted: 7 December 2017 / Published: 8 December 2017
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Abstract
In this work, the surface morphology changes of nanocrystalline Cr2O3 film deposited on Si wafer during the heating process were observed in-situ by means of an environmental scanning electron microscope (ESEM). The Cr2O3 film cracked at high
[...] Read more.
In this work, the surface morphology changes of nanocrystalline Cr2O3 film deposited on Si wafer during the heating process were observed in-situ by means of an environmental scanning electron microscope (ESEM). The Cr2O3 film cracked at high temperature due to the cause of thermal stress; the corresponding crack area percentages on the film surface were real-time evaluated using image analysis software (SISC IAS V8.0) based on the principle of gray value analysis. In the meantime, the effects of the heating temperature on the crack area percentage, phase constituents, and grain size of the Cr2O3 film were also studied in detail. The results showed that the percentage of crack area on film surface first increased with the heating temperature rise, and reached the maximum value at around 980 °C, and then gradually declined again. The above trend is closely related to the changes of thermal stress and grain growth in film. In addition, the heat treatment also had a strong influence on the grain size of the Cr2O3 film. Full article
(This article belongs to the Special Issue Hybrid Surface Coatings & Process (Selected Papers from HyMaP 2017))
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Open AccessArticle Plasma-Sprayed LSM Protective Coating on Metallic Interconnect of SOFC
Coatings 2017, 7(12), 226; doi:10.3390/coatings7120226
Received: 3 November 2017 / Revised: 30 November 2017 / Accepted: 9 December 2017 / Published: 11 December 2017
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Abstract
In this study, a (La0.8Sr0.2)0.98MnO3 protective layer was prepared on the C276, Crofer22 APU, SUS304, and SUS430 alloys by the atmospheric plasma spraying technique (APS). The oxidation behavior and electrical property of these metal alloys have
[...] Read more.
In this study, a (La0.8Sr0.2)0.98MnO3 protective layer was prepared on the C276, Crofer22 APU, SUS304, and SUS430 alloys by the atmospheric plasma spraying technique (APS). The oxidation behavior and electrical property of these metal alloys have been investigated isothermally at 800 °C in air for up to 300 h. Results showed that the ferritic steels transform into MnCr2O4 spinels and a Cr2O3 layer during isothermal oxidation. The C276 alloy formed NiCr2O4 and FeCr2O4 layers; these are protective and act as an effective barrier against chromium migration into the outer oxide layer, and the alloy demonstrated good oxidation resistance and a reasonable match to the coefficient of thermal expansion of the substrate and a low-oxide scale area-specific resistance. The ASR effects on the formation of oxide scale have been investigated, and the ASR of coated samples was below 0.024 Ω·cm2. It has good electrical conductivity for SOFC in long-term use. Full article
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Open AccessArticle Inkjet-Printed Chemical Solution Y2O3 Layers for Planarization of Technical Substrates
Coatings 2017, 7(12), 227; doi:10.3390/coatings7120227
Received: 2 October 2017 / Revised: 16 November 2017 / Accepted: 25 November 2017 / Published: 11 December 2017
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Abstract
The implementation of the Chemical Solution Deposition (CSD) methodology with the Drop on Demand (DoD) inkjet printing (IJP) technology has been successfully employed to develop a Solution Deposition Planarization (SDP) method. We have used nanocrystalline yttrium oxide (Y2O3) to
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The implementation of the Chemical Solution Deposition (CSD) methodology with the Drop on Demand (DoD) inkjet printing (IJP) technology has been successfully employed to develop a Solution Deposition Planarization (SDP) method. We have used nanocrystalline yttrium oxide (Y2O3) to decrease the roughness of technical metallic substrates by filling the surface imperfections and thus avoiding costly polishing steps. This alternative process represents an outstanding methodology to reduce the final cost of the second-generation coated conductors manufacturing. Two Y2O3 metalorganic precursor ink formulations were successfully developed and tested to obtain surfaces as smooth as possible with adequate mechanical properties to hold the internal stress developed during the growth of the subsequent layers. By using these inks as precursors for IJP and after a proper tuning of the rheological and wetting parameters, we firstly obtained centimeter length uniform 100 nm-thick SDP-Y2O3 films on unpolished stainless-steel substrate from Bruker HTS. The scalability of the roll to roll (R2R)-IJP process to 100 m is then demonstrated on metallic substrates as well. A complete characterization of the prepared SDP-Y2O3 inkjet-printed layers was carried out using optical microscopy, FIB-SEM (Focus Ion Beam coupled to Scanning Electron Microscopy), XRD (X-ray Diffraction), AFM (Atomic Force Microscopy), reflectometry and nanoindentation techniques. Then, the morphology, thickness, crystallinity and mechanical properties were evaluated, together with the surface roughness in order to assess the resulting layer planarity. The impact of planarity was additionally studied via growth of biaxially textured buffer layers as well as further functional layers. 1.1 µm-thick YSZ layers with in-plane textures better than the stainless steel (SS) polished reference were successfully deposited on top of 100 nm SDP-Y2O3 films yielding 50% of Ic in contrast to the standard SS reference. Full article
(This article belongs to the Special Issue Mechanical Behavior of Coatings and Engineered Surfaces)
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Open AccessArticle An Experimental Study on Nano-Carbon Films as an Anti-Wear Protection for Drilling Tools
Coatings 2017, 7(12), 228; doi:10.3390/coatings7120228
Received: 6 October 2017 / Revised: 1 December 2017 / Accepted: 7 December 2017 / Published: 11 December 2017
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Abstract
Carbon thin films of 50–100 nm thickness were synthesized by Pulsed Laser Deposition in vacuum at different laser fluences from 2 to 6 J/cm2. The deposited films were characterized by Raman spectroscopy for compositional assessment, scanning electron microscopy for morphology/thickness evaluations,
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Carbon thin films of 50–100 nm thickness were synthesized by Pulsed Laser Deposition in vacuum at different laser fluences from 2 to 6 J/cm2. The deposited films were characterized by Raman spectroscopy for compositional assessment, scanning electron microscopy for morphology/thickness evaluations, and X-ray reflectivity for density, thickness, and roughness determinations. The films were ~100 nm thin, smooth, droplet-free, made of a-C:H type of diamond-like carbon. The mechanical properties of synthesized films were studied by nanoindentation and adhesion tests. The films that were obtained at low laser fluences (2, 3 J/cm2) had better mechanical properties as compared to those synthesized at higher fluences. The mean values of hardness were around 20 GPa, while the friction coefficient was 0.06. The deposition conditions of carbon thin films that displayed the best mechanical properties were further used to coat commercial drills. Both uncoated and coated drills were tested on plates that were made of three types of steel: Stainless steel 304, general use AISI 572 Gr 65 steel (OL60), and AISI D3 tool steel (C120). All of the drill edges and tips were studied by optical and scanning electron microscopes. The coated samples were clearly found to be more resistant, and displayed less morphological defects than their uncoated counterparts when drilling stainless steel and OL60 plates. In the case of C120 steel, carbon coatings failed because of the high friction between drill and the metal plate resulting in tip edges blunting that occurred during processing. Full article
(This article belongs to the Special Issue Nanostructured Thin Films)
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Open AccessArticle Photocatalytic Behavior of Water-Based Styrene-Acrylic Coatings Containing TiO2 Sensitized with Metal-Phthalocyanine Tetracarboxylic Acids
Coatings 2017, 7(12), 229; doi:10.3390/coatings7120229
Received: 6 November 2017 / Revised: 3 December 2017 / Accepted: 8 December 2017 / Published: 12 December 2017
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Abstract
The study presents the results regarding the photocatalytic behavior of some water-based styrene-acrylic coatings containing TiO2 nanoparticles sensitized with metal-phthalocyanine tetracarboxylic acids. Coating materials have been studied in terms of color characteristics, photocatalytic behavior, and resistance to self-degradation depending on the structure
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The study presents the results regarding the photocatalytic behavior of some water-based styrene-acrylic coatings containing TiO2 nanoparticles sensitized with metal-phthalocyanine tetracarboxylic acids. Coating materials have been studied in terms of color characteristics, photocatalytic behavior, and resistance to self-degradation depending on the structure of phthalocyanine sensitizers. Coatings that were exposed to Xenon light showed degradation of the organic sensitizer rather than of the binder. Photocatalytic tests using methylene blue as a standard contaminant indicated that the coating containing TiO2 nanoparticles sensitized with Fe(III) phthalocyanine tetracarboxylic acids showed the highest efficiency both in ultraviolet or visible light. In this case, the UV light induced a photodegradation rate that was greatly increased of about fifty times comparatively with that induced by LED light and was determined by two different mechanisms, but side reactions like methylene blue and sensitizer self destruction are possible to occur simultaneously. Photocatalytic materials of this type are suitable to be used as decorative coatings especially for indoor applications. Full article
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Open AccessArticle Simulation of Acoustic Wave Propagation in Aluminium Coatings for Material Characterization
Coatings 2017, 7(12), 230; doi:10.3390/coatings7120230
Received: 28 September 2017 / Revised: 23 October 2017 / Accepted: 4 December 2017 / Published: 14 December 2017
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Abstract
Aluminium coatings and their characterization are of great interest in many fields of application, ranging from aircraft industries to microelectronics. Here, we present the simulation of acoustic wave propagation in aluminium coatings via the elastodynamic finite integration technique (EFIT) in comparison to experimental
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Aluminium coatings and their characterization are of great interest in many fields of application, ranging from aircraft industries to microelectronics. Here, we present the simulation of acoustic wave propagation in aluminium coatings via the elastodynamic finite integration technique (EFIT) in comparison to experimental results. The simulations of intensity (I)–defocus (z) curves, obtained by scanning acoustic microscopy (SAM), were first carried out on an aluminium bulk sample, and secondly on a 1 µm aluminium coating deposited on a silicon substrate. The I(z) curves were used to determine the Rayleigh wave velocity of the aluminium bulk sample and the aluminium coating. The results of the simulations with respect to the Rayleigh velocity were corroborated by non-destructive SAM measurements and laser ultrasonic measurements (LUS). Full article
(This article belongs to the Special Issue Modelling and Simulation of Coating)
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Open AccessArticle Mechanical and Thermal Properties of Epoxy Composites Containing Zirconium Oxide Impregnated Halloysite Nanotubes
Coatings 2017, 7(12), 231; doi:10.3390/coatings7120231
Received: 9 November 2017 / Revised: 8 December 2017 / Accepted: 8 December 2017 / Published: 15 December 2017
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Abstract
Liquid epoxy resins have received much attention from both academia and the chemical industry as eco-friendly volatile organic compound (VOC)-free alternatives for applications in coatings and adhesives, especially in those used in households. Epoxy resins show high chemical resistance and high creep resistance.
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Liquid epoxy resins have received much attention from both academia and the chemical industry as eco-friendly volatile organic compound (VOC)-free alternatives for applications in coatings and adhesives, especially in those used in households. Epoxy resins show high chemical resistance and high creep resistance. However, due to their brittleness and lack of thermal stability, additional fillers are needed for improving the mechanical and thermal properties. Halloysite nanotubes (HNTs) are naturally abundant, inexpensive, and eco-friendly clay minerals that are known to improve the mechanical and thermal properties of epoxy composites after suitable surface modification. Zirconium is well known for its high resistance to heat and wear. In this work, zirconium oxide-impregnated HNTs (Zr/HNTs) were added to epoxy resins to obtain epoxy composites with improved mechanical and thermal properties. Zr/HNTs were characterized by field-emission transmission electron microscopy, transmission electron microscopy with energy-dispersive X-ray spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. Changes in the thermal properties of the epoxy composites were characterized by thermo mechanical analysis and differential scanning calorimetry. Furthermore, flexural properties of the composites were analyzed using a universal testing machine. Full article
(This article belongs to the Special Issue Hybrid Surface Coatings & Process (Selected Papers from HyMaP 2017))
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Open AccessArticle Multi-Walled Carbon Nanotube-Assisted Electrodeposition of Silver Dendrite Coating as a Catalytic Film
Coatings 2017, 7(12), 232; doi:10.3390/coatings7120232
Received: 17 November 2017 / Revised: 7 December 2017 / Accepted: 12 December 2017 / Published: 14 December 2017
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Abstract
A multi-walled carbon nanotube (MWCNT)-coated indium tin oxide (ITO) slide was used as a platform for the growth of a silver dendrite (Ag-D) film using cyclic voltammetry. The particular dendritic nanostructures were formed by the diffusion-limited-aggregation model due to the potential difference between
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A multi-walled carbon nanotube (MWCNT)-coated indium tin oxide (ITO) slide was used as a platform for the growth of a silver dendrite (Ag-D) film using cyclic voltammetry. The particular dendritic nanostructures were formed by the diffusion-limited-aggregation model due to the potential difference between the MWCNTs and the ITO surface. The Ag-D-coated ITO film was then used for the catalytic degradation of methyl orange (MO) and methylene blue (MB) under static aqueous conditions. The network structure of the Ag-D allows the efficient diffusion of MO and MB, and consequently enhances the catalytic performance. Since the thin film is much easier to use for the post-treatment of powder catalysts, the proposed method shows great potential in many catalytic applications. Full article
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Open AccessArticle Metakaolin-Based Geopolymer with Added TiO2 Particles: Physicomechanical Characteristics
Coatings 2017, 7(12), 233; doi:10.3390/coatings7120233
Received: 26 October 2017 / Revised: 2 December 2017 / Accepted: 12 December 2017 / Published: 15 December 2017
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Abstract
The effect of the TiO2 addition on the physicomechanical properties of a geopolymer system based on metakaolin (MK) and hydroxide and potassium silicate as activators is presented in this article. Three different liquid-solid systems (0.35, 0.40, and 0.45) and two titanium additions
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The effect of the TiO2 addition on the physicomechanical properties of a geopolymer system based on metakaolin (MK) and hydroxide and potassium silicate as activators is presented in this article. Three different liquid-solid systems (0.35, 0.40, and 0.45) and two titanium additions were investigated (5% and 10% of the cement content). The flowability, setting time, and mechanical strength of the geopolymer mixtures and their microstructural characteristics were evaluated using techniques such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). It was concluded that a percentage of up to 10% TiO2 does not affect the mechanical properties of the geopolymer, although it does reduce the fluidity and setting times of the mixture. Full article
(This article belongs to the Special Issue Nanocoatings with Air-Purifying Properties)
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Open AccessArticle Stability of Aqueous Polymeric Dispersions for Ultra-Thin Coating of Bi-Axially Oriented Polyethylene Terephthalate Films
Coatings 2017, 7(12), 234; doi:10.3390/coatings7120234
Received: 24 August 2017 / Revised: 4 December 2017 / Accepted: 12 December 2017 / Published: 16 December 2017
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Abstract
The stability of polyacrylate and polyester based aqueous dispersions designed for ultrathin coating of extruded plastic films, especially bi-axially oriented polyethylene terephthalate (BOPET), was studied. Also, the effect of the gemini surfactant based defoaming/wetting agent on the properties of the dispersions was examined.
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The stability of polyacrylate and polyester based aqueous dispersions designed for ultrathin coating of extruded plastic films, especially bi-axially oriented polyethylene terephthalate (BOPET), was studied. Also, the effect of the gemini surfactant based defoaming/wetting agent on the properties of the dispersions was examined. The addition of the defoaming/wetting agent resulted in reducing the surface free tension of the polyacrylate and polyester dispersion by 15% and 20%, respectively and the initial foam height by 60% and 15%, respectively. At the same time, the agent addition did not compromise the temperature and pH stability of the dispersions. Such modified dispersion can be utilized for ultrathin coating of plastic film used for packaging, to improve their processability, printability, and metallization. Full article
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Open AccessArticle Effect of the Addition of Molybdenum on the Structure and Corrosion Resistance of Zinc–Iron Plating
Coatings 2017, 7(12), 235; doi:10.3390/coatings7120235
Received: 24 November 2017 / Revised: 11 December 2017 / Accepted: 14 December 2017 / Published: 16 December 2017
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Abstract
Zn–Ni plating is indispensable in various industries because of its high corrosion resistance. However, Ni has been reported to trigger allergies; thus, an alternative Ni-free plating is desired. Zn–Fe plating is considered to be a promising candidate, albeit its corrosion resistance still needs
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Zn–Ni plating is indispensable in various industries because of its high corrosion resistance. However, Ni has been reported to trigger allergies; thus, an alternative Ni-free plating is desired. Zn–Fe plating is considered to be a promising candidate, albeit its corrosion resistance still needs to be improved. The corrosion resistance of Zn–Fe plating is expected to increase by the addition of Mo as the third alloying element as it is more noble than Zn and Fe. In this study, Zn–Fe–Mo plating with a corrosion resistance nearly equivalent to that of the Zn–Ni plating was fabricated. Zn–Fe–Mo plating was electrically deposited from continuously-agitated plating baths prepared by mixing ZnSO4, FeSO4, Na2MoO4, Na3C6H5O7, and Na2SO4 using Fe or Ni plates as the substrate. The surface morphology, composition, crystal phase, and electronic state of Mo of the platings were investigated by scanning electron microscopy equipped with energy-dispersive spectroscopy (SEM-EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The anti-corrosion performance was evaluated by Tafel extrapolation method. Formation of plating comprising a Mo containing alloy phase was found to be crucial for improving corrosion resistance. The Zn–Fe–Mo plating demonstrates promise for replacing anti-corrosion Zn–Ni platings. Full article
(This article belongs to the Special Issue Hybrid Surface Coatings & Process (Selected Papers from HyMaP 2017))
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Open AccessArticle Impact Wear of Structural Steel with Yield Strength of 235 MPa in Various Liquids
Coatings 2017, 7(12), 237; doi:10.3390/coatings7120237
Received: 22 August 2017 / Revised: 24 October 2017 / Accepted: 20 November 2017 / Published: 20 December 2017
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Abstract
The wear of pipelines, used in slurry transport, results in high costs for maintenance and replacement. The wear mechanism involves abrasion, corrosion, impact, and the interaction among them. In this work, we study the effect of impact on the wear mechanism and wear
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The wear of pipelines, used in slurry transport, results in high costs for maintenance and replacement. The wear mechanism involves abrasion, corrosion, impact, and the interaction among them. In this work, we study the effect of impact on the wear mechanism and wear rate. Results show that when the effect of impact is small, the wear mechanism is dominated by electrochemically induced surface modification, which leads to a lower wear rate in a corrosive environment than in a non-corrosive environment. By contrast, when the effect of impact is large, the wear mechanism is drastically altered. In that regime plastic deformation is important. The influence of corrosion in the high impact regime can be neglected. Our findings show the importance of including impact effect in the distinction of wear of slurry pipes. Full article
(This article belongs to the Special Issue Manufacturing and Surface Engineering)
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Open AccessArticle Microstructure and Tribological Performance of TiB2-NiCr Composite Coating Deposited by APS
Coatings 2017, 7(12), 238; doi:10.3390/coatings7120238
Received: 19 October 2017 / Revised: 16 December 2017 / Accepted: 18 December 2017 / Published: 20 December 2017
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Abstract
Nickel chromium (NiCr) powders with different titanium diboride (TiB2) additions (20, 40 and 60 wt %) were prepared with a mechanical alloying method and then sprayed using an air plasma spraying technology. The microstructure and phase composite of the powders and
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Nickel chromium (NiCr) powders with different titanium diboride (TiB2) additions (20, 40 and 60 wt %) were prepared with a mechanical alloying method and then sprayed using an air plasma spraying technology. The microstructure and phase composite of the powders and the cross-sections of deposited coatings were analyzed with a scanning electronic microscope and X-ray diffraction. The tribological performance of the coatings was studied using a pin-on-disk tribometer at room temperature. The weight loss of the as-sprayed coating was measured by using a high accuracy weighing balance. Cr3C2-25NiCr coating was produced and tested for comparison. The morphologies of the worn surface were then investigated. Parts of debris with some scratches were found, presenting typical signs of abrasive wear and showing slight adhesive wear on the surface. The 20 wt % additive TiB2 coating demonstrated the highest microhardness and the lowest coefficient of friction. The wear resistance of the metal-ceramic composites coatings was enhanced with the addition of TiB2. Full article
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Review

Jump to: Research

Open AccessReview Emerging Corrosion Inhibitors for Interfacial Coating
Coatings 2017, 7(12), 217; doi:10.3390/coatings7120217
Received: 5 September 2017 / Revised: 26 September 2017 / Accepted: 2 October 2017 / Published: 1 December 2017
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Abstract
Corrosion is a deterioration of a metal due to reaction with environment. The use of corrosion inhibitors is one of the most effective ways of protecting metal surfaces against corrosion. Their effectiveness is related to the chemical composition, their molecular structures and affinities
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Corrosion is a deterioration of a metal due to reaction with environment. The use of corrosion inhibitors is one of the most effective ways of protecting metal surfaces against corrosion. Their effectiveness is related to the chemical composition, their molecular structures and affinities for adsorption on the metal surface. This review focuses on the potential of ionic liquid, polyionic liquid (PIL) and graphene as promising corrosion inhibitors in emerging coatings due to their remarkable properties and various embedment or fabrication strategies. The review begins with a precise description of the synthesis, characterization and structure-property-performance relationship of such inhibitors for anti-corrosion coatings. It establishes a platform for the formation of new generation of PIL based coatings and shows that PIL corrosion inhibitors with various heteroatoms in different form can be employed for corrosion protection with higher barrier properties and protection of metal surface. However, such study is still in its infancy and there is significant scope to further develop new structures of PIL based corrosion inhibitors and coatings and study their behaviour in protection of metals. Besides, it is identified that the combination of ionic liquid, PIL and graphene could possibly contribute to the development of the ultimate corrosion inhibitor based coating. Full article
(This article belongs to the Special Issue New Generation Coatings for Metals)
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Open AccessReview Role of Moisture in the Failure of Coatings on Wood
Coatings 2017, 7(12), 219; doi:10.3390/coatings7120219
Received: 14 September 2017 / Revised: 27 November 2017 / Accepted: 27 November 2017 / Published: 2 December 2017
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Abstract
Most wood coating tests are done either in a short term artificial weathering chamber or long term on an outdoor rack/fence. In both cases, the coatings are exposed to both ultraviolet radiation and water. This study is focused on the influence of moisture
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Most wood coating tests are done either in a short term artificial weathering chamber or long term on an outdoor rack/fence. In both cases, the coatings are exposed to both ultraviolet radiation and water. This study is focused on the influence of moisture alone on wood opaque film forming coating failures. As moisture is sorbed into the wood structure, the wood swells in proportion to the volume of water sorbed. As moisture is lost, the wood shrinks in proportion to the volume of the water lost. Moisture in the wood end grain is responsible for coating failure in, for example, window corners and end to end siding. The wood cell wall moisture can be greatly reduced by a process known as acetylation which not only reduces the moisture sorbed in the cell wall but results in high levels of dimensional stability. The reduced moisture uptake along with the stability results in less stress created between the coating wood surface interface improving the performance of the coating and increasing its useful lifetime. Full article
(This article belongs to the Special Issue Wood Coatings)
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Open AccessReview Photonic Structures for Light Trapping in Thin Film Silicon Solar Cells: Design and Experiment
Coatings 2017, 7(12), 236; doi:10.3390/coatings7120236
Received: 27 October 2017 / Revised: 12 December 2017 / Accepted: 12 December 2017 / Published: 21 December 2017
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Abstract
One of the foremost challenges in designing thin-film silicon solar cells (TFSC) is devising efficient light-trapping schemes due to the short optical path length imposed by the thin absorber thickness. The strategy relies on a combination of a high-performance back reflector and an
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One of the foremost challenges in designing thin-film silicon solar cells (TFSC) is devising efficient light-trapping schemes due to the short optical path length imposed by the thin absorber thickness. The strategy relies on a combination of a high-performance back reflector and an optimized texture surface, which are commonly used to reflect and scatter light effectively within the absorption layer, respectively. In this paper, highly promising light-trapping structures based on a photonic crystal (PC) for TFSCs were investigated via simulation and experiment. Firstly, a highly-reflective one-dimensional photonic crystal (1D-PC) was designed and fabricated. Then, two types of 1D-PC-based back reflectors (BRs) were proposed: Flat 1D-PC with random-textured aluminum-doped zinc oxide (AZO) or random-textured 1D-PC with AZO. These two newly-designed BRs demonstrated not only high reflectivity and sufficient conductivity, but also a strong light scattering property, which made them efficient candidates as the electrical contact and back reflector since the intrinsic losses due to the surface plasmon modes of the rough metal BRs can be avoided. Secondly, conical two-dimensional photonic crystal (2D-PC)-based BRs were investigated and optimized for amorphous a-SiGe:H solar cells. The maximal absorption value can be obtained with an aspect ratio of 1/2 and a period of 0.75 µm. To improve the full-spectral optical properties of solar cells, a periodically-modulated PC back reflector was proposed and experimentally demonstrated in the a-SiGe:H solar cell. This periodically-modulated PC back reflector, also called the quasi-crystal structure (QCS), consists of a large periodic conical PC and a randomly-textured Ag layer with a feature size of 500–1000 nm. The large periodic conical PC enables conformal growth of the layer, while the small feature size of Ag can further enhance the light scattering. In summary, a comprehensive study of the design, simulation and fabrication of 1D-PC- and 2D-PC-based back reflectors for TFSCs was carried out. Total absorption and device performance enhancement were achieved with the novel PC light-trapping systems because of their high reflectivity or high scattering property. Further research is necessary to illuminate the optimal structure design of PC-based back reflectors and high solar cell efficiency. Full article
(This article belongs to the Special Issue Plasma Etching and Deposition)
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