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Coatings, Volume 10, Issue 6 (June 2020) – 98 articles

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Cover Story (view full-size image) Wood was the second material to be considered for building. In addition to its good technical [...] Read more.
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Open AccessArticle
Femtosecond Laser-Induced Damage Characterization of Multilayer Dielectric Coatings
Coatings 2020, 10(6), 603; https://doi.org/10.3390/coatings10060603 - 26 Jun 2020
Viewed by 455
Abstract
The laser-induced damage threshold (LIDT) of optical components is one of the major constraints in developing high-power ultrafast laser systems. Multi-layer dielectric (MLD) coatings-based optical components are key parts of high-power laser systems because of their high damage resistance. Therefore, understanding and characterizing [...] Read more.
The laser-induced damage threshold (LIDT) of optical components is one of the major constraints in developing high-power ultrafast laser systems. Multi-layer dielectric (MLD) coatings-based optical components are key parts of high-power laser systems because of their high damage resistance. Therefore, understanding and characterizing the laser-induced damage of MLD coatings are of paramount importance for developing ultrahigh-intensity laser systems. In this article, we overview the possible femtosecond laser damage mechanisms through damage morphologies in various MLD optical coatings tested in our facility. To evaluate the major contributions to the coating failure, different LIDT test methods (R-on-1, ISO S-on-1 and Raster Scan) were carried out for a high reflective hybrid Ta2O5/HfO2/SiO2 MLD mirror coating at a pulse duration of 37 fs. Different LIDT test methods were compared due to the fact that each test method exposes the different underlying damage mechanisms. For instance, the ISO S-on-1 test at a higher number of laser pulses can bring out the fatigue effects, whereas the Raster Scan method can reveal the non-uniform defect clusters in the optical coating. The measured LIDT values on the sample surface for the tested coating in three test methods are 1.1 J/cm2 (R-on-1), 0.9 J/cm2 (100k-on-1) and 0.6 J/cm2 (Raster Scan) at an angle of incidence of 45 deg. The presented results reveal that the performance of the tested sample is limited by coating defects rather than fatigue effects. Hence, the Raster Scan method is found to be most accurate for the tested coating in evaluating the damage threshold for practical applications. Importantly, this study demonstrates that the testing of different LIDT test protocols is necessary in femtosecond regime to assess the key mechanisms to the coating failure. Full article
(This article belongs to the Special Issue Visual Effects Coatings)
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Open AccessArticle
Effects of Graphene-Based Fillers on Cathodic Delamination and Abrasion Resistance of Cataphoretic Organic Coatings
Coatings 2020, 10(6), 602; https://doi.org/10.3390/coatings10060602 - 26 Jun 2020
Viewed by 318
Abstract
This study aims to demonstrate the excellent protective performance of functionalized graphene oxide (fGO) flakes in acrylic cataphoretic coatings. The filler content provides an important contribution in improving the chemical and mechanical resistance of the acrylic matrix. The morphology of the fillers was [...] Read more.
This study aims to demonstrate the excellent protective performance of functionalized graphene oxide (fGO) flakes in acrylic cataphoretic coatings. The filler content provides an important contribution in improving the chemical and mechanical resistance of the acrylic matrix. The morphology of the fillers was first investigated by optical and electron microscopy, analysing the distribution of the fGO flakes within the polymer matrix. After that, the flakes were added to the cataphoretic bath in different concentrations, resulting in four series of samples. The cathodic delamination of the coatings was assessed with cathodic polarization cycles and with measurements carried out with a scanning Kelvin probe. Finally, the abrasion resistance at the macroscopic and microscopic level was studied by scrub testing and scratching atomic force microscopy analysis, respectively. The incorporation of fGO at the optimized concentration of 0.2 wt.% greatly increases the cathodic delamination resistance of the acrylic matrix, resulting in an effective barrier against the effects of absorbed aggressive substances. Graphene-based fillers also enhance abrasion resistance, thanks to their high mechanical strength. Thus, this work demonstrates the great protective benefits that can be obtained when using fGO flakes as reinforcing fillers in cataphoretic coatings. Full article
(This article belongs to the Special Issue Advanced Hybrid Coatings and Thin Films for Surface Functionalization)
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Open AccessArticle
Corrosion Resistance of Hard Coat Anodized AA 6061 in Citric–Sulfuric Solutions
Coatings 2020, 10(6), 601; https://doi.org/10.3390/coatings10060601 - 26 Jun 2020
Viewed by 346
Abstract
Aluminum is a material widely used in aeronautical and transport industries due to its excellent mechanical and corrosion resistance properties. Unfortunately, aluminum alloys are susceptible to corrosion, which limits their use in some corrosive environments. The aim of this work is to characterize [...] Read more.
Aluminum is a material widely used in aeronautical and transport industries due to its excellent mechanical and corrosion resistance properties. Unfortunately, aluminum alloys are susceptible to corrosion, which limits their use in some corrosive environments. The aim of this work is to characterize hard coat film fabricated by anodizing in a citric–sulfuric acid system using electrochemical techniques. The anodization process was carried out using an aluminum alloy AA 6061 anodization bath: a mix of citric and sulfuric acid solutions were used. For the anodizing process, two current densities were used, 1 and 7.2 A·cm−2. Anodized specimens obtained under different conditions were exposed to a 3.5 wt.% NaCl solution, and their electrochemical behavior was studied by electrochemical impedance spectroscopy (EIS) and cyclic potentiodynamic polarization (CPP) according to ASTM G106-15 and ASTM G5-13, respectively. Scanning electron microscopy (SEM) was employed to determinate the morphology and thickness of coatings. The results showed improved corrosion resistance in 6061 aluminum anodized in citric–sulfuric acid electrolyte compared to those anodized in sulfuric acid solution. Full article
(This article belongs to the Special Issue Hard Coatings in Research and Industry)
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Open AccessArticle
Effect of Laser Processing on Surface Properties of Additively Manufactured 18-Percent Nickel Maraging Steel Parts
Coatings 2020, 10(6), 600; https://doi.org/10.3390/coatings10060600 - 26 Jun 2020
Viewed by 340
Abstract
In the present work, the experimental study on laser processing of additively manufactured (AM) maraging steel part surface was conducted. Nanosecond pulsed laser at ablation mode was used for surface modification in oxidizing atmosphere. The morphology, roughness, elemental and phase composition, microhardness and [...] Read more.
In the present work, the experimental study on laser processing of additively manufactured (AM) maraging steel part surface was conducted. Nanosecond pulsed laser at ablation mode was used for surface modification in oxidizing atmosphere. The morphology, roughness, elemental and phase composition, microhardness and tribological properties of the processed surfaces were investigated. The obtained results revealed that pulsed laser processing under the ablation mode in air allows obtaining modified surface with uniform micro-texture and insignificant residual undulation, providing 3 times lower roughness as compared with the as-manufactured AM part. The intensive oxidation of surface during laser processing results in formation of the significant oxides amount, which can be controlled by scanning speed. Due to the presence of the oxide phase (such as Fe2CoO4 and Ti0.11Co0.89O0.99), the hardness and wear resistance of the surface were significantly improved, up to 40% and 17 times, respectively. The strong correlation between the roughness parameter Ra and mass loss during the tribological test testifies the significant role of the obtained morphology for the wear resistance of the surface. Full article
(This article belongs to the Section Surface Characterization, Deposition and Modification)
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Open AccessArticle
Experimental Observation of Flow Reversal in Thin Liquid Film Flow Falling on an Inclined Plate
Coatings 2020, 10(6), 599; https://doi.org/10.3390/coatings10060599 - 26 Jun 2020
Viewed by 383
Abstract
A customized particle image velocimetry/planar laser induced fluorescence (PIV/PLIF) experimental method, aiming to capture transient hydrodynamics of solitary waves of inertia-dominated falling liquid films, is presented in this paper. A novel PIV/PLIF technique, which only uses one camera to capture simultaneously both particle [...] Read more.
A customized particle image velocimetry/planar laser induced fluorescence (PIV/PLIF) experimental method, aiming to capture transient hydrodynamics of solitary waves of inertia-dominated falling liquid films, is presented in this paper. A novel PIV/PLIF technique, which only uses one camera to capture simultaneously both particle image and fluorescence, and meanwhile a post-processing imaging method is also developed, which is able to simultaneously measure both internal velocity field in film and its topology. To validate the fidelity of the novel PIV/PLIF technique, a comparison between experimental results of streamwise velocity profile and film thickness and that of the Nusselt’s prediction at low Reynold number is carried out, and in addition, integral continuity is checked for transient wavy film, both of which shows that they are in good agreement. Based on experimental velocity fields and film topology, pressure distribution inside film is derived with the Poisson equation. Considering characteristics of traveling waves, the experimental results are presented respectively in spatial mode and temporal mode. In spatial mode, capillary wave dynamics are demonstrated out of velocity field, film topology and pressure distribution, which reveals that flow reversal occurs at capillary troughs. In temporal mode, the mechanism of flow reversal at capillary troughs is scrutinized on the basis of high-frequency velocity sampling and the derived pressure gradient. It is shown that flow reversal at capillary troughs is triggered due to occurrence of positive pressure gradient at the back side of the capillary wave crest, rather than the trough upstream as stated by the previous researchers. By elucidating the dynamics of flow reversal, mechanisms for the upper limit of Reynold number with respect to flow reversal underneath capillary wave trough were proposed, which might be the gradually saturated deceleration from the capillary curvature and shorten deceleration duration determined by the wave speed and capillary wave length. Our results should be of interest for optimization of the mass transport model of falling liquid films and shed light on the revealing of flow reversal mechanism. Full article
(This article belongs to the Section Thin Films)
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Open AccessArticle
Effects of NiSO4 Concentration on the Coloring Performance and Corrosion Resistance of the Colored Film on 304 Stainless Steel
Coatings 2020, 10(6), 598; https://doi.org/10.3390/coatings10060598 - 25 Jun 2020
Viewed by 420
Abstract
The colored films were successfully prepared on the 304 stainless steel surfaces in coloring solutions with different NiSO4 contents. The purpose of this study was to investigate the effects of NiSO4 in the coloring solution on the coloring performance of 304 [...] Read more.
The colored films were successfully prepared on the 304 stainless steel surfaces in coloring solutions with different NiSO4 contents. The purpose of this study was to investigate the effects of NiSO4 in the coloring solution on the coloring performance of 304 stainless steel and corrosion resistance of the obtained colored film in NaCl solution. The coloring rate was determined from coloring potential-time curve, and the protection properties of the color films in a 3.5% NaCl solution were characterized by potentiodynamic polarization scan, electrochemical impedance spectroscopy, and wear resistance test. The results showed that adding NiSO4 could accelerate the coloring process but brought about a negative impact on the surface’s corrosion resistance. Full article
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Open AccessCommunication
Nonuniform Slip Effect in Wetting Films
Coatings 2020, 10(6), 597; https://doi.org/10.3390/coatings10060597 - 25 Jun 2020
Viewed by 404
Abstract
The slip effect in wetting films is theoretically studied, and a nonlinear dependence of the hydrodynamic velocity on the slip length is discovered. It is demonstrated that the hydrodynamic flow is essentially affected by the presence of a nonuniform slip length distribution, leading [...] Read more.
The slip effect in wetting films is theoretically studied, and a nonlinear dependence of the hydrodynamic velocity on the slip length is discovered. It is demonstrated that the hydrodynamic flow is essentially affected by the presence of a nonuniform slip length distribution, leading also to enhancement of the energy dissipation in the films. This effect could dramatically slow the usually quick hydrodynamic flows over superhydrophobic surfaces, for instance. Full article
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Open AccessArticle
Adhesion Behavior of Textured Electrosurgical Electrode in an Electric Cutting Process
Coatings 2020, 10(6), 596; https://doi.org/10.3390/coatings10060596 - 25 Jun 2020
Viewed by 333
Abstract
Soft tissue adhesion on the electrosurgical electrode has been a major concern in clinical surgery. In order to improve the adhesion property of the electrode, micro-textures with different morphologies including micro-dimples, longitudinal micro-channels, and lateral micro-channels were created on the electrode surface by [...] Read more.
Soft tissue adhesion on the electrosurgical electrode has been a major concern in clinical surgery. In order to improve the adhesion property of the electrode, micro-textures with different morphologies including micro-dimples, longitudinal micro-channels, and lateral micro-channels were created on the electrode surface by laser surface texturing (LST). Electric cutting experiments were then performed to investigate the adhesion behavior of different electrodes. Experimental results showed that the textured electrode surfaces could reduce the soft tissue adhesion significantly due to the effect of air in micro-textures and the reduction of contact area between the electrode and the soft tissue. Moreover, the temperature distribution of the electric cutting process was simulated through COMSOL to verify the effect of different micro-textures on adhesion behavior. It was demonstrated that the better anti-adhesion property could be obtained at a large area density combined with lateral micro-channels. Full article
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Open AccessArticle
Vertical Guided Bone Augmentation Using Titanium Mesh Domes Coated with Natural Latex Extracted from Hevea brasiliensis
Coatings 2020, 10(6), 595; https://doi.org/10.3390/coatings10060595 - 25 Jun 2020
Viewed by 363
Abstract
The subject of this work is the evaluation of the use of titanium mesh domes coated with latex extracted from Hevea brasiliensis to promote vertical guided bone augmentation (GBA), above the normal limits of the skeleton. Twenty-four New Zealand rabbits were used, in [...] Read more.
The subject of this work is the evaluation of the use of titanium mesh domes coated with latex extracted from Hevea brasiliensis to promote vertical guided bone augmentation (GBA), above the normal limits of the skeleton. Twenty-four New Zealand rabbits were used, in which a circular groove of eight millimeters in diameter and nine holes in the internal region reaching the medulla were made with a trephine drill, in the calvaria. The dome, four millimeters in height, was fixed above this defect. The animals were divided into four groups (N = 6). The first (control) received a titanium dome not covered by the periosteum, and the second received a titanium dome that was covered by the periosteum. For the third, a dome with a latex coating was used and was not covered by the periosteum, and for the fourth, a titanium dome with a latex coating was used and was covered by the periosteum. After 90 days, the animals were euthanized. Computerized tomography imaging demonstrated that vertical bone augmentation was achieved in the groups with titanium domes coated with latex. Microscopic evaluation showed that there were no differences between the control group and Group 2, or between Groups 3 and 4. The other comparisons showed statistically significant differences (p < 0.05, ANOVA-Tukey). Full article
(This article belongs to the Special Issue Bioactive Surfaces and Coatings for Bone Regeneration)
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Open AccessCommunication
Tribological and Anticorrosion Performance of Electroplated Zinc Based Nanocomposite Coatings
Coatings 2020, 10(6), 594; https://doi.org/10.3390/coatings10060594 - 24 Jun 2020
Viewed by 392
Abstract
This paper presents the results of corrosion and tribological analysis of pure and nanocomposite zinc coatings. Coatings were electroplated using commercially available products—zinc acidic bath and a nanoparticle carrying plating additive. Electrochemical measurements were carried out to compare the anticorrosion performance of coatings. [...] Read more.
This paper presents the results of corrosion and tribological analysis of pure and nanocomposite zinc coatings. Coatings were electroplated using commercially available products—zinc acidic bath and a nanoparticle carrying plating additive. Electrochemical measurements were carried out to compare the anticorrosion performance of coatings. An investigation into the influence of nanoparticles on the mechanical properties was performed. The zinc nanocomposite coating exhibited better wear resistance and higher hardness than the plain zinc coating. The application of confocal laser scanning microscopy (CLSM) allowed the detailed description of friction/wear marks. The electrochemical studies revealed that the introduction of nanoparticles into the coating did not compromise its protective properties—a similar resistance of plain and nanocomposite coatings to corrosion was obtained. Full article
(This article belongs to the Section Surface Characterization, Deposition and Modification)
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Open AccessArticle
Biofunctionalization of Textile Materials. 3. Fabrication of Poly(lactide)-Potassium Iodide Composites with Antifungal Properties
Coatings 2020, 10(6), 593; https://doi.org/10.3390/coatings10060593 - 24 Jun 2020
Viewed by 428
Abstract
The paper presents a method of obtaining poly(lactide) (PLA) nonwoven fabrics with antifungal properties using potassium iodide as a nonwoven modifying agent. PLA nonwoven fabrics were obtained by the melt-blown technique and subsequently surface modified (PLA→PLA-SM-KI) by the dip-coating method. The analysis of [...] Read more.
The paper presents a method of obtaining poly(lactide) (PLA) nonwoven fabrics with antifungal properties using potassium iodide as a nonwoven modifying agent. PLA nonwoven fabrics were obtained by the melt-blown technique and subsequently surface modified (PLA→PLA-SM-KI) by the dip-coating method. The analysis of these PLA-SM-KI (0.1%–2%) composites included Scanning Electron Microscopy (SEM), UV/VIS transmittance, FTIR spectrometry and air permeability. The nonwovens were subjected to microbial activity tests against Aspergillus niger fungal mold species, exhibiting substantial antifungal activity. The studies showed that PLA-KI hybrids containing 2% KI have appropriate mechanical properties, morphology and demanded antimicrobial properties to be further developed as a potential antimicrobial, biodegradable material. Full article
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Open AccessArticle
Rebound Dynamics of Two Droplets Successively Impacting an Inclined Surface
Coatings 2020, 10(6), 592; https://doi.org/10.3390/coatings10060592 - 24 Jun 2020
Viewed by 363
Abstract
The dynamic behaviors of two droplets successively impacting inclined surfaces are simulated by a three-dimensional pseudopotential lattice Boltzmann model based on multi-relaxation-time. The effect of velocity ratio of two successive droplets on the contact time is investigated and two rebounding regimes are identified [...] Read more.
The dynamic behaviors of two droplets successively impacting inclined surfaces are simulated by a three-dimensional pseudopotential lattice Boltzmann model based on multi-relaxation-time. The effect of velocity ratio of two successive droplets on the contact time is investigated and two rebounding regimes are identified depending on whether the coalesced droplet retouches the surface or not. Increasing the velocity ratio leads to a stronger interaction between the two droplets and the phenomenon of coalesced droplet retouching the surface is observed when the velocity ratio exceeds a threshold, resulting in a longer contact time. An outcome map of droplet rebounding is obtained at various velocity ratios and contact angles of surface. It is found that the coalesced droplet cannot rebound from the surface at a larger velocity ratio and a lower contact angle of surface. Furthermore, the effect of the length between impact points on the contact time is exhibited, and a longer length is beneficial to coalesced droplet rebounding. Full article
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Open AccessArticle
Analysis of the Actual Contact Surface of Selected Aircraft Tires with the Airport Pavement as a Function of Pressure and Vertical Load
Coatings 2020, 10(6), 591; https://doi.org/10.3390/coatings10060591 - 24 Jun 2020
Viewed by 418
Abstract
The contact surface of the wheel with the airport surface is important for the safety of flight operations in the ground manoeuvring area. The area of the contact surface, its shape and stress distribution at the pavement surface are the subject of many [...] Read more.
The contact surface of the wheel with the airport surface is important for the safety of flight operations in the ground manoeuvring area. The area of the contact surface, its shape and stress distribution at the pavement surface are the subject of many scientists’ considerations. However, there are only a few research studies which include pressure and vertical load directly and its influence on tire-pavement contact area. There are no research studies which take into account aircraft tires. This work is a piece of an extensive research project which aims to develop a device and a method for continuous measurement of the natural airport pavement’s load capacity. One of the work elements was to estimate the relationship between wheel pressure and wheel pressure on the surface, and the area of the contact surface. The results of the research are presented in this article. Global experience in this field is cited at the beginning of the article. Then, the theoretical basis for calculating the wheel with the road surface contact area was presented. Next, the author’s research views and measurement method are presented. Finally, the obtained test results and comments are shown. The tests were carried out for four types of tires. Two of them were airplane tires from the PZL M28 Skytruck/Bryza and Sukhoi Su-22 aircraft. Two more came from the airport ASFT (airport surface friction tester) friction tester-one smooth ASTM; the other smooth retreaded type T520. The tires were tested in a pressure range from 200 to 800 kPa. The range of vertical wheel load on the pavement was 3.23–25.93 kN for airplane tires, and 0.8–4.0 kN for friction tester tires. The tests proved a significant influence of the wheel pressure value and wheel pressure on the surface on the obtained contact surface area of the wheel with the surface. In addition, it was noted that the final shape and size of the contact surface is affected by factors other than wheel pressure, tire pressure and dimensions. Full article
(This article belongs to the Special Issue Pavement Surface Coatings)
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Open AccessArticle
Precision Machining of Nimonic C 263 Super AlloyUsing WEDM
Coatings 2020, 10(6), 590; https://doi.org/10.3390/coatings10060590 - 24 Jun 2020
Viewed by 346
Abstract
Wire electrical discharge machining (WEDM) is an unconventional and very efficient technology for precision machining of the Nimonic C 263 super alloy, which is very widespread, especially in the energy, aerospace and automotive industries. Due to electrical discharge, defects in the form of [...] Read more.
Wire electrical discharge machining (WEDM) is an unconventional and very efficient technology for precision machining of the Nimonic C 263 super alloy, which is very widespread, especially in the energy, aerospace and automotive industries. Due to electrical discharge, defects in the form of cracks or burned cavities often occur on the machined surfaces, which negatively affect the correct functionality and service life of the manufactured components. To increase the efficiency of the machining of Nimonic C 263 using WEDM, in this study, extensive design of experiments was carried out, monitoring input factors in the form of machine parameters like Pulse off time, Gap voltage, Discharge current, Pulse on time and Wire feed, the output of which was comprehensive information about the behaviour of such machined surfaces, which allowed the optimization of the entire machining process. Thus, the optimization of the Cutting speed was performed in relation to the quality of the machined surface and the machining accuracy, as well as an analysis of the chemical composition of the machined surfaces and a detailed analysis of the lamella using a transmission electron microscope. A detailed study of the occurrence of surface or subsurface defects was also included. It was found that with the help of complex optimization tools, it is possible to significantly increase the efficiency of the machining of the Nimonic C 263 super alloy and achieve both financial savings in the form of shortened machine time and increasing the quality of machined surfaces. Full article
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Open AccessReview
Surface Modifications for Implants Lifetime extension: An Overview of Sol-Gel Coatings
Coatings 2020, 10(6), 589; https://doi.org/10.3390/coatings10060589 - 24 Jun 2020
Viewed by 368
Abstract
The limited lifetime of implants entails having patients undergo replacement surgeries, several times throughout life in young patients, with significant risks for them and extensive cost for healthcare service. The overcoming of such inconvenience is still today a hard challenge for the scholars [...] Read more.
The limited lifetime of implants entails having patients undergo replacement surgeries, several times throughout life in young patients, with significant risks for them and extensive cost for healthcare service. The overcoming of such inconvenience is still today a hard challenge for the scholars of the biomedical and biomaterial fields. The improvement of the currently employed implants through surface modification by coatings application is the main strategy proposed to avoid implants failure, and the sol-gel coating is an ideal technology to achieve this goal. Therefore, the present review aims to provide an overview of the most important problems leading to implant failure, the sol-gel coating technology, and its use as a strategy to overcome such issues. Full article
(This article belongs to the Special Issue Design of Functional Coatings by Chemical Methods)
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Open AccessArticle
Influence of the PVC of Glass Fiber Powder on the Properties of a Thermochromic Waterborne Coating for Chinese Fir Boards
Coatings 2020, 10(6), 588; https://doi.org/10.3390/coatings10060588 - 24 Jun 2020
Viewed by 379
Abstract
A thermochromic waterborne coating with thermal insulation efficacy was prepared by adding thermochromic microcapsules and glass fiber powder. The influence of the pigment volume concentration (PVC) of a glass fiber powder on the performance of the thermochromic coating for Chinese fir boards was [...] Read more.
A thermochromic waterborne coating with thermal insulation efficacy was prepared by adding thermochromic microcapsules and glass fiber powder. The influence of the pigment volume concentration (PVC) of a glass fiber powder on the performance of the thermochromic coating for Chinese fir boards was investigated. It was found that a coating with a PVC of glass fiber powder of 0–22.0% had better discoloration properties. When the PVC of the glass fiber powder was more than 4.0%, with the increase of the PVC, the gloss of the coating decreased gradually, while, the adhesion, impact resistance, and liquid resistance were not affected. When the PVC of the glass fiber powder was 10.0%–30.0%, it showed thermal insulation efficacy and high hardness. The coating with a PVC of 16.0% glass fiber powder had better wear resistance. The discoloration property of the coating with thermal insulation efficacy was not affected by time. These results exhibit great potential for the application of a wood surface thermochromic and thermal insulation coating. Full article
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Open AccessArticle
Direct Femtosecond Laser Fabrication of Superhydrophobic Aluminum Alloy Surfaces with Anti-icing Properties
Coatings 2020, 10(6), 587; https://doi.org/10.3390/coatings10060587 - 24 Jun 2020
Viewed by 344
Abstract
Ice formation is a serious issue in many fields, from energy to aerospace, compromising the devices’ efficiency and security. Superhydrophobicity has been demonstrated to be correlated to the anti-icing properties of surfaces. However, fabricating surfaces with robust water repellence properties also at subzero [...] Read more.
Ice formation is a serious issue in many fields, from energy to aerospace, compromising the devices’ efficiency and security. Superhydrophobicity has been demonstrated to be correlated to the anti-icing properties of surfaces. However, fabricating surfaces with robust water repellence properties also at subzero temperature is still a great challenge. In this work, femtosecond laser (fs-laser) texturing is exploited to produce superhydrophobic surfaces with anti-icing properties on Al2024, an aluminum alloy of great interest in cold environments, in particular for aircraft production. Our textured substrates present self-cleaning properties and robust water repellency at subzero temperatures. Moreover, outstanding anti-icing properties are achieved on the textured surfaces at −20 °C, with water droplets bouncing off the surface before freezing. Full article
(This article belongs to the Special Issue Anti-Adhesive Surfaces)
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Open AccessArticle
Investigation of Electrochemical Oxidation Behaviors and Mechanism of Single-Crystal Silicon (100) Wafer under Potentiostatic Mode
Coatings 2020, 10(6), 586; https://doi.org/10.3390/coatings10060586 - 24 Jun 2020
Viewed by 350
Abstract
Electrochemical oxidation (ECO) has been used widely to oxidize single crystal Si wafers. Aiming at optimizing the ECO assisted machining methods, the oxidation behaviors of single- crystal silicon (100) wafer under potentiostatic mode are experimentally investigated. It is shown that the Si wafer [...] Read more.
Electrochemical oxidation (ECO) has been used widely to oxidize single crystal Si wafers. Aiming at optimizing the ECO assisted machining methods, the oxidation behaviors of single- crystal silicon (100) wafer under potentiostatic mode are experimentally investigated. It is shown that the Si wafer can be electrochemically oxidized and the oxidized film thickness reaches to 239.6 nanometers in 20 min. The hardness of the oxidized surface is reduced by more than 50 percent of the original surface. The results indicate that the oxide thickness and the hardness can be controlled by changing the voltage. Based on the experimental findings, a hypothesis on the ECO mechanism under potentiostatic mode was proposed to explain the fluctuations of current density under specific applied voltage. The occurrence of the multiple peaks in the current density curve during the oxidation process is due to the formation of discharge channels, which was initiated from the defects at the interface between the oxide bottom and the substrate. This breaks the electrical isolation and leads to the discontinuous growth of the electrochemical oxide layer. The present work contributes to the fundamental understanding of the ECO behaviors for the single-crystal Si (100) wafer under potentiostatic mode. Full article
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Open AccessArticle
Effect of Nb Content on the Microstructure and Wear Resistance of Fe-12Cr-xNb-4C Coatings Prepared by Plasma-Transferred Arc Welding
Coatings 2020, 10(6), 585; https://doi.org/10.3390/coatings10060585 - 23 Jun 2020
Viewed by 370
Abstract
The Fe-Cr-C coatings with different levels of Nb addition were prepared on carbon steel by a plasma transferred arc (PTA) weld-surfacing process and their microstructure and properties were investigated. As the Nb content increases from 8.96% to 12.55%, the coating gradually changes from [...] Read more.
The Fe-Cr-C coatings with different levels of Nb addition were prepared on carbon steel by a plasma transferred arc (PTA) weld-surfacing process and their microstructure and properties were investigated. As the Nb content increases from 8.96% to 12.55%, the coating gradually changes from a hypereutectic structure (martensite, austenite matrix, primary NbC and eutectic γ+M7C3) to a near eutectic structure (γ+M7C3 and NbC) and finally a hypoeutectic structure (primary γ, γ+M7C3 and NbC). As the Nb content increases, the hardness and wear resistance of the coating first increase and then decrease, which is closely related to the NbC volume fraction first increasing and then the NbC size coarsening. The Fe-Cr-C coating with 11.65% Nb balances the NbC content and size, and has the highest hardness and best wear resistance. As the Nb content increases further, the formation and aggregation of coarse NbC carbides in the coating results in high brittleness of the coating, which may cause the carbide particles to peel off the coating during the wear process, thereby reducing wear resistance. Full article
(This article belongs to the Special Issue Surface Engineering for Friction and Wear Reduction)
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Open AccessArticle
Heat Transfer Enhancement by Shot Peening of Stainless Steel
Coatings 2020, 10(6), 584; https://doi.org/10.3390/coatings10060584 - 23 Jun 2020
Viewed by 395
Abstract
In heat exchange applications, the heat transfer efficiency could be improved by surface modifications. Shot peening was one of the cost-effective methods to provide different surface roughness. The objectives of this study were (1) to investigate the influences of the surface roughness on [...] Read more.
In heat exchange applications, the heat transfer efficiency could be improved by surface modifications. Shot peening was one of the cost-effective methods to provide different surface roughness. The objectives of this study were (1) to investigate the influences of the surface roughness on the heat transfer performance and (2) to understand how the shot peening process parameters affect the surface roughness. The considered specimens were 316L stainless steel hollow tubes having smooth and rough surfaces. The computational fluid dynamics (CFD) simulation was used to observe the surface roughness effects. The CFD results showed that the convective heat transfer coefficients had linear relationships with the peak surface roughness (Rz). Finite element (FE) simulation was used to determine the effects of the shot peening process parameters. The FE results showed that the surface roughness was increased at higher sandblasting speeds and sand diameters. Full article
(This article belongs to the Section Surface Characterization, Deposition and Modification)
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Open AccessFeature PaperArticle
Modulus, Strength and Cytotoxicity of PMMA-Silica Nanocomposites
Coatings 2020, 10(6), 583; https://doi.org/10.3390/coatings10060583 - 23 Jun 2020
Viewed by 421
Abstract
Key advantages of Poly(methyl methacrylate)—PMMA for denture application are related to aesthetics and biocompatibility, while its main deficiency is related to mechanical properties. To address this issue, SiO2 nanoparticle reinforcement was proposed, containing 0 to 5% nanosilica, to form nanocomposite materials. Flexural [...] Read more.
Key advantages of Poly(methyl methacrylate)—PMMA for denture application are related to aesthetics and biocompatibility, while its main deficiency is related to mechanical properties. To address this issue, SiO2 nanoparticle reinforcement was proposed, containing 0 to 5% nanosilica, to form nanocomposite materials. Flexural strengths and elastic moduli were determined and correlated to nominal nanoparticle content and zeta potential of the liquid phase nanoparticle solutions. Another issue is the biocompatibility, which was determined in terms of cytotoxicity, using L929 and MRC5 cell lines. The addition of nanoparticle was proved to be beneficial for increasing flexural strength and modulus, causing a significant increase in both strength and moduli. On the other hand, the formation of agglomerates was noted, particularly at higher nanoparticle loadings, affecting mechanical properties. The addition of nanosilica had an adverse effect on the cytotoxicity, increasing it above the level present in unmodified specimens. Cytotoxic potential was on the acceptable level for specimens with up to 2% nanosilica. Consequently, nanosilica proved to be an effective and biocompatible means of increasing the resistance of dental materials. Full article
(This article belongs to the Special Issue Antibacterial Surfaces, Thin Films, and Nanostructured Coatings)
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Open AccessArticle
Anti-Graffiti Coatings on Stones for Historical Buildings in Turin (NW Italy)
Coatings 2020, 10(6), 582; https://doi.org/10.3390/coatings10060582 - 22 Jun 2020
Viewed by 360
Abstract
The application of anti-graffiti products to stones belonging to architectural heritage is a common procedure that is currently complementary to traditional graffiti removal treatments, such as chemical and mechanical cleaning. In this study, two anti-graffiti coatings (a sacrificial product and a permanent one) [...] Read more.
The application of anti-graffiti products to stones belonging to architectural heritage is a common procedure that is currently complementary to traditional graffiti removal treatments, such as chemical and mechanical cleaning. In this study, two anti-graffiti coatings (a sacrificial product and a permanent one) were tested on four stones (with a different texture, mineralogy, and surface finish) commonly found in the historical city center of Turin (Italy). In order to evaluate the effectiveness of the anti-graffiti products, the removal of two graffiti paints with different compositions was tested. The results of the cleaning procedures performed on the surfaces coated with anti-graffiti products were evaluated, considering both the graffiti remains and the alterations induced on the surface. Chemical cleaning based on the use of a low-toxic ternary solvent mixture was applied on the unprotected stones for a comparison with the results obtained on the surfaces coated with anti-graffiti products. The samples were observed under stereomicroscopy and ultraviolet fluorescence photography and all of the treated surfaces were evaluated by roughness measurements, the contact sponge method, static contact angle measurements, and scanning electron microscopy. The composition of the anti-graffiti product, the graffiti paint to be cleaned, and the remover recommended by the manufacturer proved to be key factors for the cleaning effectiveness achieved on coated surfaces. Moreover, to a lesser extent, the mineralogy, texture, and surface finish of the stone also influenced the results of the cleaning procedures. The sacrificial anti-graffiti product enhanced the cleaning effectiveness on all stones if compared to uncoated surfaces; however, the permanence of coating remains on the surface after cleaning proved to be critical. Regarding the use of the permanent anti-graffiti products, intense disparate results were achieved, depending on the graffiti paint composition. Full article
(This article belongs to the Special Issue Looking for a Sustainable Cleaning of Cultural Heritage: Agenda 2030)
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Open AccessArticle
Heat-Treatment of Aluminium-Nickel Composite Cold Sprayed Coating
Coatings 2020, 10(6), 581; https://doi.org/10.3390/coatings10060581 - 22 Jun 2020
Viewed by 354
Abstract
Intermetallic compounds, especially aluminides, show good high-temperature strength, oxidation resistance, high melting points, and thus have received considerable attention as potential substitutes for superalloys in high-temperature applications. Aluminides are especially interesting because they are stable up to the critical temperature of ordering, which [...] Read more.
Intermetallic compounds, especially aluminides, show good high-temperature strength, oxidation resistance, high melting points, and thus have received considerable attention as potential substitutes for superalloys in high-temperature applications. Aluminides are especially interesting because they are stable up to the critical temperature of ordering, which is close to the melting temperature. In the Al-Ni system, the most studied intermetallics are Ni3Al, NiAl and NiAl3. In the presented study, Al and Ni powders were mixed together with Al2O3 in various proportions to produce dense coatings by low-temperature cold spraying. Two types of post-deposition treatments were applied to produce aluminides, namely furnace heating and resistance spot welding. The former caused a long time diffusion while the latter a self-propagating high temperature synthesis. Both heating methods enabled formations of intermetallic phases. However, the furnace heating provides high porosity. The microstructure of the samples was analyzed by SEM (scanning electron microscope), EDS (energy dispersive X-ray spectroscopy) and XRD (X-ray diffraction) together with microhardness measurements. Full article
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Open AccessEditorial
Special Issue on Surface Treatment by Laser-Assisted Techniques
Coatings 2020, 10(6), 580; https://doi.org/10.3390/coatings10060580 - 22 Jun 2020
Viewed by 366
Abstract
Laser radiation is a powerful tool for surface modification owing to its spatial and material absorbance selectivity [...] Full article
(This article belongs to the Special Issue Surface Treatment by Laser-Assisted Techniques)
Open AccessArticle
Conductive Coatings of Cotton Fabric Consisting of Carbonized Charcoal for E-Textile
Coatings 2020, 10(6), 579; https://doi.org/10.3390/coatings10060579 - 21 Jun 2020
Viewed by 484
Abstract
Development of smart textiles is an emerging discipline in the last two decades where a conductive element is integrated into a textile material by some means. The purpose of this research was to develop a conductive textile fabric by coating with charcoal as [...] Read more.
Development of smart textiles is an emerging discipline in the last two decades where a conductive element is integrated into a textile material by some means. The purpose of this research was to develop a conductive textile fabric by coating with charcoal as a conductive element. The charcoal was produced by carbonizing the eucalyptus wood at a temperature of 928 °C for 37 min producing 59.17% w/w of fixed carbon yield and conductivity of 463.34 Sm−1 (Siemens per meter) compared to immeasurable conductivity of the wood. This was followed by characterization of physical and chemical properties of charcoal. Thereafter, a cotton fabric was pad-coated with a dispersion based on the charcoal. The paper herein reports the results of preparing different recipes using different quantities of charcoal particles with other components of the coating mixture, which was tested to obtain the best coating in terms of electrical conductivity. The optimal concentration of the conductive particles of the charcoal was studied. Performance evaluation of the coated fabric was assessed for the durability of fabric towards different fastness agents. The effect of charcoal loading on thermal and sensorial comfort of the fabric in addition to the air and water permeability was studied and a significant change was observed. Finally, a proof of concept was developed to demonstrate if the resulting pieces of information during the process were viable. As observed, the pad-coated cotton fabric using charcoal showed increased electrical conductivity from 1.58 × 10−12 Scm−1 (Siemens per centimeter) for the controlled sample to 124.49 Scm−1 for the coated sample designating that the resulting fabric is in a conductor category. Full article
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Open AccessFeature PaperArticle
Regenerable Superhydrophobic Coatings for Biomedical Fabrics
Coatings 2020, 10(6), 578; https://doi.org/10.3390/coatings10060578 - 20 Jun 2020
Viewed by 452
Abstract
Coatings with high water repellence represent a promising field for biomedical applications. Superhydrophobicity (SH) can be used for preventing adhesion, controlling cell deposition, and spreading by inhibition of adsorption processes at liquid–solid interfaces. The recyclability of medical aids like fabrics can open the [...] Read more.
Coatings with high water repellence represent a promising field for biomedical applications. Superhydrophobicity (SH) can be used for preventing adhesion, controlling cell deposition, and spreading by inhibition of adsorption processes at liquid–solid interfaces. The recyclability of medical aids like fabrics can open the way for lower cost and more environmentally-friendly solutions. In this case, two different coatings form recyclable and low global warming potential materials and green solvents have been prepared and characterized based on their wettability properties. The resulting substrates have been used for the adhesion and spreading of representative skin cell lines, both tumoral and non-tumoral, showing a strong decrease in cell viability with values < 10%. The coated substrates showed a complete recovery on initial SH properties after rinsing with suitable solvents. Full article
(This article belongs to the Section Surface Coatings for Biomedicine and Bioengineering)
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Open AccessArticle
Enhanced Electrochromic Properties by Improvement of Crystallinity for Sputtered WO3 Film
Coatings 2020, 10(6), 577; https://doi.org/10.3390/coatings10060577 - 19 Jun 2020
Viewed by 433
Abstract
Tungsten oxide (WO3) is widely used as a functional material for “smart windows” due to its excellent electrochromic properties, however it is difficult to overcome the conflict between its optical modulation and cyclic stability. In this work, WO3 thin films [...] Read more.
Tungsten oxide (WO3) is widely used as a functional material for “smart windows” due to its excellent electrochromic properties, however it is difficult to overcome the conflict between its optical modulation and cyclic stability. In this work, WO3 thin films with different crystal structures were prepared by DC reactive magnetron sputtering method. The effects of substrate temperature on the structure, composition, and electrochromic properties of WO3 films were investigated. The results show that the crystallinity of the WO3 film increases with increasing deposition temperature, indicating that temperature plays an important role in controlling the structure of the WO3 film. For WO3 thin films formed at a substrate temperature of 573 K, the film is in an amorphous state to a crystalline transition state. From X-ray diffraction (XRD) analysis, the thin film showed a weak WO3 crystallization peak, which was in the composite structure of amorphous and nanocrystalline. Which has the best electrochromic properties, with modulation amplitude of 73.1% and bleached state with a coloration efficiency of 42.9 cm2/C at a wavelength of 550 nm. Even after 1500 cycles, the optical modulation still contains 65.4%, delivering the best cyclic stability. Full article
(This article belongs to the Special Issue Advanced Thin Films Deposited by Magnetron Sputtering)
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Open AccessArticle
Facile Route for Synthesis of Novel Flame Retardant, Reinforcement and Antibacterial Textile Fabrics Coatings
Coatings 2020, 10(6), 576; https://doi.org/10.3390/coatings10060576 - 19 Jun 2020
Viewed by 397
Abstract
New and innovative textile fabrics coatings were facilely developed. The coating was developed based on synthesis of novel charring and antibacterial organic agent in conjunction with chitosan. N-[2,3-dibromo-4-(4-methoxy-3-methylphenyl)-4-oxobutanoyl]anthranilic acid was synthesized as organic antibacterial, reinforcement, and charring agent (OA) and then, dispersed [...] Read more.
New and innovative textile fabrics coatings were facilely developed. The coating was developed based on synthesis of novel charring and antibacterial organic agent in conjunction with chitosan. N-[2,3-dibromo-4-(4-methoxy-3-methylphenyl)-4-oxobutanoyl]anthranilic acid was synthesized as organic antibacterial, reinforcement, and charring agent (OA) and then, dispersed in chitosan solution followed by coating on textile fabrics using immersion route forming new flame retardant coating layer. The developed organic molecule structure was elucidated using spectroscopic techniques. The mass loadings of developed organic agent dispersed in chitosan solution were varied between 20–60 wt.%. The coated textile fabrics have special surface morphology of fiber shape aligned on textile fibers surface. The thermal stability and charring residues of the coated textile fabrics were enhanced when compared to blank and organic agent free coated samples. Furthermore, the flammability properties were evaluated using LOI (limiting oxygen index) and UL94 tests. Therefore, the coated textile fabrics record significant enhancement in flame retardancy achieving first class flame retardant textile of zero mm/min rate of burning and 23.8% of LOI value compared to 118 mm/min. rate of burning and 18.2% for blank textile, respectively. The tensile strength of the coated textile fabrics was enhanced, achieving 51% improvement as compared to blank sample. Additionally, the developed coating layer significantly inhibited the bacterial growth, recording 18 mm of clear inhibition zone for coated sample when compared to zero for blank and chitosan coated ones. Full article
(This article belongs to the Special Issue Novel Coatings for Smart Textile Fabrics for Enhanced Functions)
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Open AccessArticle
Double Glow Plasma Surface Metallurgy Technology Fabricated Fe-Al-Cr Coatings with Excellent Corrosion Resistance
Coatings 2020, 10(6), 575; https://doi.org/10.3390/coatings10060575 - 19 Jun 2020
Viewed by 361
Abstract
Double glow plasma surface metallurgy (DGPSM) technology was applied to obtain a Fe-Al-Cr coating on the surface of Q235 carbon steel. The influence of the sample temperature, gas pressure, the distance between the substrate, and the source electrode on the quality of the [...] Read more.
Double glow plasma surface metallurgy (DGPSM) technology was applied to obtain a Fe-Al-Cr coating on the surface of Q235 carbon steel. The influence of the sample temperature, gas pressure, the distance between the substrate, and the source electrode on the quality of the obtained Fe-Al-Cr coatings was systematically investigated. The results showed that the parameters for DGPSM have a significant effect on the uniformity, particle size, compactness, and thickness of the coating. Under the optimized parameters (sample temperature: 800 °C, gas pressure: 35 Pa, and electrode distance: 15 mm), the obtained Fe-Al-Cr coating contains Fe2AlCr, Fe3Al(Cr), FeAl(Cr), Fe(Cr) solid solution, Cr23C6, and α-Fe(Al), exhibiting excellent corrosion resistance in a 0.5 mol/L H2SO4 solution, which is even better than that of the 304 stainless steel. Full article
(This article belongs to the Section Corrosion, Wear and Erosion)
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Open AccessArticle
Architectured Cu–TNTZ Bilayered Coatings Showing Bacterial Inactivation under Indoor Light and Controllable Copper Release: Effect of the Microstructure on Copper Diffusion
Coatings 2020, 10(6), 574; https://doi.org/10.3390/coatings10060574 - 19 Jun 2020
Viewed by 391
Abstract
A Ti–23Nb–0.7Ta–2Zr–1.2O alloy (at %), called “gum metal”, was deposited by direct-current magnetron sputtering (DCMS) on an under layer of copper. By varying the working pressure during the deposition, columnar TNTZ (Ti–Nb–Ta–Zr) nanoarchitectures were obtained. At low working pressures, the upper layer was [...] Read more.
A Ti–23Nb–0.7Ta–2Zr–1.2O alloy (at %), called “gum metal”, was deposited by direct-current magnetron sputtering (DCMS) on an under layer of copper. By varying the working pressure during the deposition, columnar TNTZ (Ti–Nb–Ta–Zr) nanoarchitectures were obtained. At low working pressures, the upper layer was dense with a coarse surface (Ra = 12 nm) with a maximum height of 163 nm; however, the other samples prepared at high working pressures showed columnar architectures with voids and an average roughness of 4 nm. The prepared coatings were characterized using atomic force microscopy (AFM) for surface topography, energy dispersive X-ray spectroscopy (EDX) for atomic mapping, scanning electron microscopy (SEM) for cross-section imaging, contact angle measurements for hydrophilic/hydrophobic balance of the prepared surfaces, and X-ray diffraction (XRD) for the crystallographic structures of the prepared coatings. The morphology and the density of the prepared coatings were seen to influence the hydrophilic properties of the surface. The antibacterial activity of the prepared coatings was tested in the dark and under low-intensity indoor light. Bacterial inactivation was seen to happen in the dark from samples presenting columnar nanoarchitectures. This was attributed to the diffusion of copper ions from the under layer. To verify the copper release from the prepared samples, an inductively coupled plasma mass spectrometer (ICP-MS) was used. Additionally, the atomic depth profiling of the elements was carried out by X-ray photoelectron spectroscopy (XPS) for the as-prepared samples and for the samples used for bacterial inactivation. The low amount of copper in the bulk of the TNTZ upper layer justifies its diffusion to the surface. Recycling of the antibacterial activity was also investigated and revealed a stable activity over cycles. Full article
(This article belongs to the Special Issue Advances in Antimicrobial Coatings)
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