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Coatings, Volume 7, Issue 9 (September 2017)

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Cover Story We propose here to revisit a classical CVD process with a design of experiment approach to reveal [...] Read more.
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Open AccessArticle Design of Nano-Porous Multilayer Antireflective Coatings
Coatings 2017, 7(9), 134; doi:10.3390/coatings7090134
Received: 29 July 2017 / Revised: 18 August 2017 / Accepted: 24 August 2017 / Published: 26 August 2017
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Abstract
We present an overview of the design potential of nano-porous anti-reflection coatings (ARCs) and the associated challenges in its fabrication. Genetic algorithm optimization for the most effective ARCs on glass for normal and all incident angles is carried out and an admittance loci-based
[...] Read more.
We present an overview of the design potential of nano-porous anti-reflection coatings (ARCs) and the associated challenges in its fabrication. Genetic algorithm optimization for the most effective ARCs on glass for normal and all incident angles is carried out and an admittance loci-based pictorial is used to visualize their performance. We then describe the advantages of using nano-porous multi-layers vis-à-vis other types of moth-eye type nano-grating films: Principally trading high performance and high cost of fabrication for good performance and lower cost facile synthesis. We describe some of the issues involved in the fabrication of such multi-layer stacks on glass and polymeric substrates and provide suggestions for overcoming these limitations. Full article
(This article belongs to the Special Issue Antireflective Coatings for Glass and Transparent Polymers)
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Open AccessArticle Microstructure and Mechanism of Grain Raising in Wood
Coatings 2017, 7(9), 135; doi:10.3390/coatings7090135
Received: 8 August 2017 / Revised: 22 August 2017 / Accepted: 24 August 2017 / Published: 29 August 2017
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Abstract
Grain raising, the lifting of fibres when water is applied to wood surfaces, is a reason why some companies are reluctant to finish wood products with water-borne coatings. However, the elements that lift-up and cause grain raising have not been identified, and the
[...] Read more.
Grain raising, the lifting of fibres when water is applied to wood surfaces, is a reason why some companies are reluctant to finish wood products with water-borne coatings. However, the elements that lift-up and cause grain raising have not been identified, and the relationship between wood density and grain raising has not been clarified. Our work sought answers to both questions. We planed or sanded different woods using aluminum oxide abrasive paper, and characterized surfaces using profilometry and SEM. Surfaces were re-characterized after wetting and drying. Grain raising is inversely related to wood density. In particular, very low-density woods are highly susceptible to grain raising, whereas grain raising does not occur in high-density woods or planed woods. In low-density woods, sanding tears cell walls creating loosely-bonded slivers of wood that project from surfaces, particularly after wetting and drying. This mechanism for grain raising was confirmed by modelling the action of abrasives on wood cell walls using an array of hollow tubes and a serrated tool. Less commonly, fibres and fibre-bundles project from surfaces. We observed that grain raising was correlated with the coarseness of the abrasive and conclude that it can be reduced in severity by tailoring sanding to account for the density and surface microstructure of wood. Full article
(This article belongs to the Special Issue Wood Coatings)
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Open AccessArticle Epitaxial Growth of AlN on (0001) Sapphire: Assessment of HVPE Process by a Design of Experiments Approach
Coatings 2017, 7(9), 136; doi:10.3390/coatings7090136
Received: 27 July 2017 / Revised: 13 August 2017 / Accepted: 14 August 2017 / Published: 1 September 2017
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Abstract
This study aims to present the interest of using a design of experiments (DOE) approach for assessing, understanding and improving the hydride vapor phase epitaxy (HVPE) process, a particular class of chemical vapor deposition (CVD) process. The case of the HVPE epitaxial growth
[...] Read more.
This study aims to present the interest of using a design of experiments (DOE) approach for assessing, understanding and improving the hydride vapor phase epitaxy (HVPE) process, a particular class of chemical vapor deposition (CVD) process. The case of the HVPE epitaxial growth of AlN on (0001) sapphire will illustrate this approach. The study proposes the assessment of the influence of 15 process parameters on the quality or desired properties of the grown layers measured by 9 responses. The general method used is a screening design with the Hadamard matrix of order 16. For the first time in the growth of AlN by CVD, a reliable estimation of errors is proposed on the measured responses. This study demonstrates that uncontrolled release of condensed species from the cold wall is the main drawback of this process, explaining many properties of the grown layers that could be mistakenly attributed to other phenomena without the use of a DOE. It appears also that the size of nucleation islands, and its corollary, the stress state of the layer at room temperature, are key points. They are strongly correlated to the crystal quality. Due to the intrinsic limitations of the screening design, the complete optimization of responses cannot be proposed but general guidelines for hydride (or halogen) vapor phase epitaxy (HVPE) experimentations, in particular with cold wall apparatus, are given. Full article
(This article belongs to the Special Issue Chemical Vapor Deposition)
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Open AccessArticle Research on Microstructure, Mechanical and Tribological Properties of Cr-Ti-B-N Films
Coatings 2017, 7(9), 137; doi:10.3390/coatings7090137
Received: 30 June 2017 / Revised: 24 August 2017 / Accepted: 29 August 2017 / Published: 3 September 2017
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Abstract
In this study, the Cr-Ti-B-N composite thin films with different Ti and B contents in the films were fabricated by a reactive magnetron sputtering system using chromium (Cr) and TiB2 targets. The microstructure, mechanical properties, and friction properties over broad ranges of
[...] Read more.
In this study, the Cr-Ti-B-N composite thin films with different Ti and B contents in the films were fabricated by a reactive magnetron sputtering system using chromium (Cr) and TiB2 targets. The microstructure, mechanical properties, and friction properties over broad ranges of temperature were detected by XRD, SEM, TEM, nano-indentation, 3D profilometer, and tribometer. It was found that the Cr-Ti-B-N films exhibited a double face-centered cubic (fcc)-CrN and amorphous-BN structure. As the Ti and B elements were introduced, the hardness and elastic modulus of Cr-Ti-B-N films increased from 21 and 246.5 GPa to a maximum value of approximately 28 and 283.6 GPa for Cr38.7Ti3.7B6.4N51.2 film, and then decreased slightly with a further increase of Ti and B contents. The hardness enhancement was attributed to the residual compressive stress. The friction and wear resistance of the film was improved obviously by the addition of Cr and B, as compared with the CrN film under 200 °C. Full article
(This article belongs to the Special Issue Coatings Tribology)
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Open AccessArticle Thermal Transport on Graphene-Based Thin Films Prepared by Chemical Exfoliations from Carbon Nanotubes and Graphite Powders
Coatings 2017, 7(9), 138; doi:10.3390/coatings7090138
Received: 13 July 2017 / Revised: 30 August 2017 / Accepted: 31 August 2017 / Published: 3 September 2017
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Abstract
Thermal conductivities (k) of different graphene nanosheet (GN)-based heat sinks are investigated within the temperature range of 323–423 K. One- and two-step modified Hummers’ methods are adopted to chemically exfoliate GNs from two kinds of carbon precursors: carbon nanotubes (CNTs) and
[...] Read more.
Thermal conductivities (k) of different graphene nanosheet (GN)-based heat sinks are investigated within the temperature range of 323–423 K. One- and two-step modified Hummers’ methods are adopted to chemically exfoliate GNs from two kinds of carbon precursors: carbon nanotubes (CNTs) and graphite powders. The two-step method offers an improved exfoliation level of GN products, especially for the CNT precursor. Experimental results show that the GN-based heat sink—exfoliated from graphite powders after the two-step approach—delivers an enhanced k value to 2507 W/m K at 323 K, as compared to the others. The k value is found to be a decreasing function of the porosity of the heat sink, revealing the importance of solid/void fraction (i.e., volumetric heat capacity). The improved thermal efficiency mainly originates from the long phonon mean free path and the low void fraction of GN-based heat sinks, thus inducing highly efficient thermal transport in the GN framework. Full article
(This article belongs to the Special Issue Nanostructured Functional Coatings)
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Open AccessArticle Texture Analysis of Hydrophobic Polycarbonate and Polydimethylsiloxane Surfaces via Persistent Homology
Coatings 2017, 7(9), 139; doi:10.3390/coatings7090139
Received: 1 June 2017 / Revised: 15 August 2017 / Accepted: 28 August 2017 / Published: 3 September 2017
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Abstract
Due to recent climate change-triggered, regular dust storms in the Middle East, dust mitigation has become the critical issue for solar energy harvesting devices. One of the methods to minimize and prevent dust adhesion and create self-cleaning abilities is to generate hydrophobic characteristics
[...] Read more.
Due to recent climate change-triggered, regular dust storms in the Middle East, dust mitigation has become the critical issue for solar energy harvesting devices. One of the methods to minimize and prevent dust adhesion and create self-cleaning abilities is to generate hydrophobic characteristics on surfaces. The purpose of this study is to explore the topological features of hydrophobic surfaces. We use non-standard techniques from topological data analysis to extract morphological features from the AFM images. Our method recovers most of the previous qualitative observations in a robust and quantitative way. Persistence diagrams, which is a summary of topological structures, witness quantitatively that the crystallized polycarbonate (PC) surface possesses spherulites, voids, and fibrils, and the texture height and spherulite concentration increases with the increased immersion period. The approach also shows that the polydimethylsiloxane (PDMS) exactly copied the structures at the PC surface but 80 to 90 percent of the nanofibrils were not copied at PDMS surface. We next extract a feature vector from each persistence diagram to show which experiments hold features with similar variance using principal component analysis (PCA). The K-means clustering algorithm is applied to the matrix of feature vectors to support the PCA result, grouping experiments with similar features. Full article
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Open AccessArticle The Effect of Glancing Angle Deposition Conditions on the Morphology of a Silver Nanohelix Array
Coatings 2017, 7(9), 140; doi:10.3390/coatings7090140
Received: 13 July 2017 / Revised: 24 August 2017 / Accepted: 31 August 2017 / Published: 4 September 2017
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Abstract
Silver nanohelices were grown on smooth substrates using glancing angle deposition and substrate cooling. Various nanohelix arrays were deposited under different deposition conditions—different deposition rates, substrate spin rates, deposition angles, and substrate temperatures. The effect of deposition conditions on the morphology of each
[...] Read more.
Silver nanohelices were grown on smooth substrates using glancing angle deposition and substrate cooling. Various nanohelix arrays were deposited under different deposition conditions—different deposition rates, substrate spin rates, deposition angles, and substrate temperatures. The effect of deposition conditions on the morphology of each nanohelix array in terms of pitch angle, pitch length, wire diameter, and radius of curvature was investigated. The dependence of circular dichroism on the size of the nanohelix arrays was also measured and demonstrated. Full article
(This article belongs to the Special Issue Nanostructured Functional Coatings)
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Open AccessArticle Zircon-Based Ceramic Coatings Formed by a New Multi-Chamber Gas-Dynamic Accelerator
Coatings 2017, 7(9), 142; doi:10.3390/coatings7090142
Received: 26 July 2017 / Revised: 4 September 2017 / Accepted: 5 September 2017 / Published: 8 September 2017
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Abstract
In this work, dense zircon-based ceramic coatings were obtained from inexpensive zircon powder on a steel substrate by using a new multi-chamber gas-dynamic accelerator. The microstructure and phase composition of the coating were characterized by scanning electron microscopy, optical microscopy, and X-ray diffraction.
[...] Read more.
In this work, dense zircon-based ceramic coatings were obtained from inexpensive zircon powder on a steel substrate by using a new multi-chamber gas-dynamic accelerator. The microstructure and phase composition of the coating were characterized by scanning electron microscopy, optical microscopy, and X-ray diffraction. The mechanical properties of the coatings were evaluated using microindentation, wear tests and bonding strength tests. The results showed that the obtained zircon-based ceramic coatings were continuous without cracks and bonded well with substrate without a sublayer. The zircon-based ceramic coatings consisted of c-ZrO2 (major phase), m-ZrO2 and SiO2. The zircon-based ceramic coatings had a porosity of 0.1%, hardness of 526 ± 65 HV0.2, and a fracture toughness of 2.5 ± 0.6 (МPа∙m1/2). The coatings showed the low specific wear rate and average erosion rate. The failure mode occurring in the tested coatings was cohesive. Full article
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Open AccessArticle A Comparative Study on the Elastic Characteristics of an Aluminum Thin-Film Using Laser Optical Measurement Techniques
Coatings 2017, 7(9), 143; doi:10.3390/coatings7090143
Received: 7 August 2017 / Revised: 25 August 2017 / Accepted: 7 September 2017 / Published: 10 September 2017
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Abstract
The increase of a surface area-to-volume ratio with the reduction of material dimensions significantly alters the characteristics of materials from their macroscopic status. Therefore, efforts have been made to establish evaluation techniques for nanoscale films. While contact mechanics-based techniques are conventionally available, non-contact
[...] Read more.
The increase of a surface area-to-volume ratio with the reduction of material dimensions significantly alters the characteristics of materials from their macroscopic status. Therefore, efforts have been made to establish evaluation techniques for nanoscale films. While contact mechanics-based techniques are conventionally available, non-contact and nondestructive methods would be preferable in case damages left on a sample after testing are not desirable, or an in situ assessment is required. In the present study, the Young’s modulus of an aluminum thin-film was evaluated using two different laser optical measurement techniques. First, microscale beam testing has been performed so that the resonant frequency change of a microfabricated cantilever beam induced by coating of a 153 nm thick aluminum layer on its top surface can be detected using a laser interferometer in order to evaluate the mechanical property through modal analysis using the finite element method. Second, picosecond ultrasonics were employed for cross-verification so that the mechanical characteristics can be evaluated through the investigation of the longitudinal bulk wave propagation behavior. Results show that the Young’s moduli from both measurements agree well with each other within 3.3% error, proving that the proposed techniques are highly effective for the study of nanoscale films. Full article
(This article belongs to the Special Issue Thin Film Deposition and Characterization Techniques)
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Open AccessArticle Microstructure and Wear Resistance of AlCoCrFeNiTi High-Entropy Alloy Coatings Produced by HVOF
Coatings 2017, 7(9), 144; doi:10.3390/coatings7090144
Received: 28 July 2017 / Revised: 26 August 2017 / Accepted: 5 September 2017 / Published: 12 September 2017
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Abstract
The investigation of high-entropy alloys (HEAs) has revealed many promising properties. HEAs with a high share of Al and Ti are suitable for the formation of lightweight materials. Investigations of the alloy system AlCoCrFeNiTi showed high strength, hardness, ductility, and wear resistance, which
[...] Read more.
The investigation of high-entropy alloys (HEAs) has revealed many promising properties. HEAs with a high share of Al and Ti are suitable for the formation of lightweight materials. Investigations of the alloy system AlCoCrFeNiTi showed high strength, hardness, ductility, and wear resistance, which makes this special alloy interesting for surface engineering and particularly for thermal spray technology. In this study, the suitability of inert gas-atomised HEA powder for high-velocity-oxygen-fuel (HVOF) thermal spray is investigated. This process allows for high particle velocities and comparatively low process temperatures, resulting in dense coatings with a low oxidation. The microstructure and phase composition of the atomised powder and the HVOF coating were investigated, as well as the wear behaviour under various conditions. A multiphase microstructure was revealed for the powder and coating, whereas a chemically ordered bcc phase occurred as the main phase. The thermal spray process resulted in a slightly changed lattice parameter of the main phase and an additional phase. In comparison with a hard chrome-plated sample, an increase in wear resistance was achieved. Furthermore, no brittle behaviour occurred under abrasive load in the scratch test. The investigation of wear tracks showed only minor cracking and spallation under maximum load. Full article
(This article belongs to the Special Issue High Entropy Alloy Coatings)
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Open AccessArticle In Vitro Assessment of Sericin-Silver Functionalized Silk Fabrics for Enhanced UV Protection and Antibacterial Properties Using Experimental Design
Coatings 2017, 7(9), 145; doi:10.3390/coatings7090145
Received: 22 July 2017 / Revised: 30 August 2017 / Accepted: 8 September 2017 / Published: 13 September 2017
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Abstract
Silk sericin (SS) was used as both a 3-dimensional matrix and reductant for the in situ synthesis of silver nanoparticles (AgNPs) finished on silk fabrics. We demonstrated enhanced UV protection and antibacterial properties using this synthesis which was an environmental friendly approach. Development
[...] Read more.
Silk sericin (SS) was used as both a 3-dimensional matrix and reductant for the in situ synthesis of silver nanoparticles (AgNPs) finished on silk fabrics. We demonstrated enhanced UV protection and antibacterial properties using this synthesis which was an environmental friendly approach. Development and optimization was achieved using a central composite design (CCD) in conjunction with the response surface methodology (RSM). The goal was to identify the concentrations of SS and AgNO3 that produced the optimal balance between UV protection and antibacterial activity, when tested against E. coli and S. aureus. The SS-AgNP bio-nanocomposites were characterized using Scanning Electron Microscope (SEM-EDX), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR). Statistical analyses indicated an empirical second-order polynomial could accurately model the experimental values. To confirm that the optimal levels from RSM worked in practice, performance evaluations were conducted, including tests of cytotoxicity, of the durability and stability of UV protection, as well as of the antibacterial activity of the functionalized fabrics after repeated standard washing. The results suggest that these bio-nanocomposites have great potential for multi-functionalization on silk fabrics. Our method has been shown to convert the waste material (SS) to a fabric with high added value. Full article
(This article belongs to the Special Issue Fabric Coatings)
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Open AccessArticle Effect of Waveform and Heat Treatment Processes on the Performance of Electrodeposited Co-P Coating
Coatings 2017, 7(9), 146; doi:10.3390/coatings7090146
Received: 3 March 2017 / Revised: 6 August 2017 / Accepted: 5 September 2017 / Published: 15 September 2017
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Abstract
Cobalt-phosphorus (Co-P) alloy is a promising material for the replacement of traditional hard chromium alloy of high hardness. In this paper, the cobalt-phosphorus alloy layer with high phosphorus content was formed by electrodeposition in a cobalt sulfate solution system under direct current (DC),
[...] Read more.
Cobalt-phosphorus (Co-P) alloy is a promising material for the replacement of traditional hard chromium alloy of high hardness. In this paper, the cobalt-phosphorus alloy layer with high phosphorus content was formed by electrodeposition in a cobalt sulfate solution system under direct current (DC), single pulse (SP) current and double pulse (DP) current, separately. Surface morphology, structure and properties of the deposited layer were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), Vickers microhardness and a neutral salt spray test, respectively. The results showed that the dense Co-P coatings could be obtained by DC, SP and DP with P content of 9.6, 8.9 and 9.1 wt %, respectively. After 30 min heat treatment at 400 °C, coatings deposited under DC, SP and DP currents transformed from an amorphous to a nanocrystalline state, while the grain size was 12–13 nm, 10–12 nm and 8–10 nm, respectively. Among all these conditions, the microhardness of coatings deposited under DP current was the highest, which was 1211 HV, while the microhardness of coatings deposited under DC current was the lowest but higher than that of hard chromium. The wear rate of Co-P coatings was 4 × 10−6–5 × 10−6 mm3/N m with Si3N4 ball as bearing material, which was lower than that of hard chromium. In coatings deposited under different currents with a thickness of ca. 40 μm, no visible corrosion area appeared after 1000 h of a neutral salt spray test. Coatings heated at 300 and 400 °C reached the corrosion grade 7 and grade 4–5, respectively after 1000 h of a neutral salt spray test, so the wear resistance of Co-P coatings was better than that of hard chromium. Full article
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Open AccessArticle Particle Velocities near and along the Electrode during Electrophoretic Deposition: Influence of Surfactant Counter-Ions
Coatings 2017, 7(9), 147; doi:10.3390/coatings7090147
Received: 11 July 2017 / Revised: 4 September 2017 / Accepted: 11 September 2017 / Published: 14 September 2017
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Abstract
Research into the micro-nanostructured coatings process has made a variety of new applications available. Electrophoretic deposition (EPD) is an efficient and attractive technique to produce coated materials. Therefore the understanding of the formation and growth mechanism of the coating process continues to be
[...] Read more.
Research into the micro-nanostructured coatings process has made a variety of new applications available. Electrophoretic deposition (EPD) is an efficient and attractive technique to produce coated materials. Therefore the understanding of the formation and growth mechanism of the coating process continues to be investigated. In this study, a home-made EPD laminar flow cell was used for in-situ investigation of the particle velocity and deposition of micronic particles on a cathode. Monodisperse polystyrene latex particles were functionalized with cationic surfactants: cetyltrimethyl ammonium bromide (CTAB) or cetyltrimethyl ammonium chloride (CTAC). The tangential velocity of the particles when they migrated to the electrode, the approach angle and the tangential velocity along the electrode were measured under a DC electric field. From the values of the velocities, the particle-electrode distance was evaluated in CTAB and CTAC solutions. The electrophoretic velocity was calculated from the electrophoretic mobility of the particles and the electric field applied to the particles. All these parameters depend on the type of surfactant counter-ions and influence the growth of the coating. Dense structures were obtained in CTAB solution while open structures were observed in CTAC solution. Full article
(This article belongs to the Special Issue Electrophoretic Deposition)
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Open AccessArticle Deposition and Patterning of Polycrystalline Diamond Films Using Traditional Photolithography and Reactive Ion Etching
Coatings 2017, 7(9), 148; doi:10.3390/coatings7090148
Received: 31 July 2017 / Revised: 7 September 2017 / Accepted: 8 September 2017 / Published: 15 September 2017
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Abstract
Given the exceptional characteristics of diamond films, they have become increasingly popular in the fields of medicine, microelectronics, and detector electronics. However, despite all the advantages, there are many technological problems that complicate their widespread application and impose limitations on diamond use in
[...] Read more.
Given the exceptional characteristics of diamond films, they have become increasingly popular in the fields of medicine, microelectronics, and detector electronics. However, despite all the advantages, there are many technological problems that complicate their widespread application and impose limitations on diamond use in technological processes. In this study, we proposed a new technique for obtaining a complex topology of polycrystalline diamond coatings by selective seeding of the substrate by nucleation centers and subsequent surface treatment with reactive ion etching to reduce the number of parasitic particles. As a result, diamond films were obtained with a high particle concentration in the film region and high repeatability of the pattern. Moreover, parasitic particles influenced neutralization in areas where film coverage was not needed. The effect of the diamond nanoparticle concentration in a photoresist and the effect of reactive ion etching on the formation of a continuous film and the removal of parasitic nucleation centers were examined. The relative simplicity, low power consumption, and high efficiency of this method make it attractive for both industrial and scientific applications. Full article
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Open AccessArticle Hybrid Ti-MoS2 Coatings for Dry Machining of Aluminium Alloys
Coatings 2017, 7(9), 149; doi:10.3390/coatings7090149
Received: 19 August 2017 / Revised: 5 September 2017 / Accepted: 13 September 2017 / Published: 16 September 2017
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Abstract
Combinatorial deposition, comprising filtered cathodic vacuum arc (FCVA) and physical vapor deposition (PVD) magnetron sputtering is employed to deposit molybdenum disulphide (MoS2) and titanium (Ti) thin films onto TiB2-coated tool inserts specifically designed for the dry machining of aluminium
[...] Read more.
Combinatorial deposition, comprising filtered cathodic vacuum arc (FCVA) and physical vapor deposition (PVD) magnetron sputtering is employed to deposit molybdenum disulphide (MoS2) and titanium (Ti) thin films onto TiB2-coated tool inserts specifically designed for the dry machining of aluminium alloys. Titanium is deposited by FCVA while MoS2 is magnetron sputtered. The deposition set up allows several compositions of Ti-MoS2 to be deposited simultaneously, with Ti content ranging between 5 and 96 at. %, and their machining performances to be evaluated. Milling took place using a CNC Vertical Machining Center at a 877 mm/min feed rate. The effect of different coating compositional ratios on the degree of aluminium sticking when a milling insert is used to face mill an Al alloy (SAE 6061) was investigated using a combination of energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) analysis. XPS studies suggest that the greater degree of Al sticking on the rake face of the inserts is due to the formation of greater amounts of non-protective Ti-O phases. EDX mapping of the milling inserts after machining reveal that a Ti:MoS2 ratio of around 0.39 prevents Al from sticking to the tool edges. Since we prevent Al from sticking to the tool surface, the resultant machined surface finish is improved thus validating the machining performance of TiB2-coated tools using optimum compositions of Ti:MoS2 thin film coatings. Full article
(This article belongs to the Special Issue Coatings for Cutting Tools)
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Open AccessArticle Effect of Particle Size on the Thermal Shock Resistance of Plasma-Sprayed YSZ Coatings
Coatings 2017, 7(9), 150; doi:10.3390/coatings7090150
Received: 18 August 2017 / Revised: 12 September 2017 / Accepted: 18 September 2017 / Published: 19 September 2017
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Abstract
In this study, yttria-stabilized zirconia (YSZ) coatings were deposited by atmospheric plasma spraying (APS) using feedstocks with two different particle sizes. The effect of particle size on the pore structure and failure mechanism of the coatings was investigated. The evolution of the pore
[...] Read more.
In this study, yttria-stabilized zirconia (YSZ) coatings were deposited by atmospheric plasma spraying (APS) using feedstocks with two different particle sizes. The effect of particle size on the pore structure and failure mechanism of the coatings was investigated. The evolution of the pore structure of the two kinds of coatings during cyclic thermal shock test was described by quantitative metallography. The influence of pore orientation on the thermal stress of the coating system was analyzed by the finite element method. It was found that the coatings deposited using coarse particles show a high thermal shock life time. The orientation of the pores in the coatings prepared by different particle sizes was different. A structural parameter was proposed to effectively characterize the pore orientation of the coatings. Coatings prepared by coarse YSZ powder tend to form almost the same number of horizontal and vertical pores, while coatings prepared by fine powder tend to form horizontal ones parallel to the direction of the substrate. The simulation results revealed that the vertical pores can reduce the thermal stress in the coating. The results of this investigation are a benefit to the design and integrity of TBCs. Full article
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Open AccessArticle Microstructure and Wear Behavior of FeCoCrNiMo0.2 High Entropy Coatings Prepared by Air Plasma Spray and the High Velocity Oxy-Fuel Spray Processes
Coatings 2017, 7(9), 151; doi:10.3390/coatings7090151
Received: 31 July 2017 / Revised: 2 September 2017 / Accepted: 18 September 2017 / Published: 20 September 2017
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Abstract
In the present research, the spherical FeCoCrNiMo0.2 high entropy alloy (HEA) powders with a single FCC solid solution structure were prepared by gas atomization. Subsequently, the FeCoCrNiMo0.2 coatings with a different content of oxide inclusions were prepared by air plasma spraying (APS) and
[...] Read more.
In the present research, the spherical FeCoCrNiMo0.2 high entropy alloy (HEA) powders with a single FCC solid solution structure were prepared by gas atomization. Subsequently, the FeCoCrNiMo0.2 coatings with a different content of oxide inclusions were prepared by air plasma spraying (APS) and high-velocity oxy-fuel spraying (HVOF), respectively. The microstructure, phase composition, mechanical properties, and tribological behaviors of these HEA coatings were investigated. The results showed that both HEA coatings showed a typical lamellar structure with low porosity. Besides the primary FCC phase, a mixture of Fe2O3, Fe3O4, and AB2O4 (A = Fe, Co, Ni, and B = Fe, Cr) was identified as the oxide inclusions. The oxide content of the APS coating and HVOF coating was calculated to be 47.0% and 12.7%, respectively. The wear resistance of the APS coating was approximately one order of magnitude higher than that of the HVOF coating. It was mainly attributed to the self-lubricated effect caused by the oxide films. The mass loss of the APS coating was mainly ascribed to the breakaway of the oxide film, while the main wear mechanism of the HVOF coating was the abrasive wear. Full article
(This article belongs to the Special Issue High Entropy Alloy Coatings)
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Review

Jump to: Research

Open AccessReview Recent Development in Phosphonic Acid-Based Organic Coatings on Aluminum
Coatings 2017, 7(9), 133; doi:10.3390/coatings7090133
Received: 11 July 2017 / Revised: 7 August 2017 / Accepted: 21 August 2017 / Published: 23 August 2017
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Abstract
Research on corrosion protection of aluminum has intensified over the past decades due to environmental concerns regarding chromate-based conversion coatings and also the higher material performance requirements in automotive and aviation industries. Phosphonic acid-based organic and organic-inorganic coatings are increasingly investigated as potential
[...] Read more.
Research on corrosion protection of aluminum has intensified over the past decades due to environmental concerns regarding chromate-based conversion coatings and also the higher material performance requirements in automotive and aviation industries. Phosphonic acid-based organic and organic-inorganic coatings are increasingly investigated as potential replacements of toxic and inefficient surface treatments for aluminum. In this review, we have briefly summarized recent work (since 2000) on pretreatments or coatings based on various phosphonic acids for aluminum and its alloys. Surface characterization methods, the mechanism of bonding of phosphonic acids to aluminum surface, methods for accessing the corrosion behavior of the treated aluminum, and applications have been discussed. There is a clear trend to develop multifunctional phosphonic acids and to produce hybrid organic-inorganic coatings. In most cases, the phosphonic acids are either assembled as a monolayer on the aluminum or incorporated in a coating matrix on top of aluminum, which is either organic or organic-inorganic in nature. Increased corrosion protection has often been observed. However, much work is still needed in terms of their ecological impact and adaptation to the industrially-feasible process for possible commercial exploitation. Full article
(This article belongs to the Special Issue Surface Chemical Modification)
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Open AccessReview Interlayers Applied to CVD Diamond Deposition on Steel Substrate: A Review
Coatings 2017, 7(9), 141; doi:10.3390/coatings7090141
Received: 19 July 2017 / Revised: 1 September 2017 / Accepted: 4 September 2017 / Published: 8 September 2017
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Abstract
Academics and industry have sought after combining the exceptional properties of diamonds with the toughness of steel. Since the early 1990s several partial solutions have been found but chemical vapor deposition (CVD) diamond deposition on steel substrate continues to be a persistent problem.
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Academics and industry have sought after combining the exceptional properties of diamonds with the toughness of steel. Since the early 1990s several partial solutions have been found but chemical vapor deposition (CVD) diamond deposition on steel substrate continues to be a persistent problem. The main drawbacks are the high carbon diffusion from gas phase into substrate, the transition metals on the material surface that catalyze sp2 bond formation, instead of sp3 bonds, and the high thermal expansion coefficient (TEC) mismatch between diamond and steels. An intermediate layer has been found necessary to increase diamond adhesion. Literature has proposed many efficient intermediate layers as a diffusion barrier for both, carbon and iron, but most intermediate layers shown have not solved TEC mismatch. In this review, we briefly discuss the solutions that exclusively work as diffusion barrier and discuss in a broader way the ones that also solve, or may potentially solve, the TEC mismatch problem. We examine some multilayers, the iron borides, the chromium carbides, and vanadium carbides. We go through the most relevant results of the last two and a half decades, including recent advances in our group. Vanadium carbide looks promising since it has shown excellent diffusion barrier properties, its TEC is intermediary between diamond and steel and, it has been thickened to manage thermal stress relief. We also review a new deposition technique to set up intermediate layers: laser cladding. It is promising because of its versatility in mixing different materials and fusing and/or sintering them on a steel surface. We conclude by remarking on new perspectives. Full article
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