Special Issue "High Entropy Alloy Coatings"

A special issue of Coatings (ISSN 2079-6412).

Deadline for manuscript submissions: closed (31 July 2017).

Special Issue Editor

Guest Editor
Prof. Dr. T.M. Yue Website E-Mail
Department of Industrial and Systems Engineering, The Hong Kong Polytechnic University, Hong Kong
Interests: laser surface modification; laser cladding; electrospark deposition; electroforming; electroplating; graded materials; composite materials; high-entropy alloys

Special Issue Information

Dear Colleagues,

Since the coining of ‘High-entropy Alloys’ (HEAs) by Professor Yeh some 10 years ago, the number of research papers published on these materials has steadily increased, with a noticeable upsurge in recent years. A quick search on the Web of Science showed that from 2004 to 2013, the number of SCI journal papers published with the topic of ‘High Entropy Alloys’ had increased fifty-fold. This is not surprising given the remarkable characteristics of HEAs, i.e. comprised of simple solid solution phases, good mechanical properties, excellent thermal stability, and resistance to wear and corrosion. Indeed, because of their unique properties, they are considered to be ideal coating materials for protecting components and products alike, particularly when harsh working environments are encountered. Recently, some exciting developments regarding HEA coatings have been reported. Various coating technologies, such as magnetron sputtering, plasma transfer, thermal spraying, laser cladding, vacuum evaporation, and electrospark deposition, have been successfully employed to fabricate a wide variety of HEA coatings on different substrate materials. These developments are expected to make inroads into the coating industry and to meet the challenges for different stress-bearing applications, including those for the sports and medical equipment industries. With this background in mind, we consider that it is timely for us to share the latest research developments on HEA coatings, not only among ourselves but to the scientific community as a whole. To this end, on behalf of the Journal, I take pleasure in inviting you to contribute to this Special Issue and to submit original research papers, which cover, but are not limited to the following topics

•           Coating technologies

•           Microstructures and properties

•           Modelling of microstructure

•           Design of new coatings

•           Industrial applications

•           Recent progress/Future development

Prof. Dr. T.M. Yue
Guest Editor

Manuscript Submission Information

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Published Papers (7 papers)

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Research

Open AccessArticle
Microstructure, Mechanical and Corrosion Behaviors of CoCrFeNiAl0.3 High Entropy Alloy (HEA) Films
Coatings 2017, 7(10), 156; https://doi.org/10.3390/coatings7100156 - 26 Sep 2017
Cited by 10
Abstract
The HEA-CoCrFeNiAl0.3 thin film in this study has been successfully developed by radio frequency (RF) magnetron sputtering to meet the increasing demand in engineering applications. Its microstructure and surface profile were investigated accordingly. The as-synthesized HEA film was found to have a [...] Read more.
The HEA-CoCrFeNiAl0.3 thin film in this study has been successfully developed by radio frequency (RF) magnetron sputtering to meet the increasing demand in engineering applications. Its microstructure and surface profile were investigated accordingly. The as-synthesized HEA film was found to have a homogeneous element distribution and ultra-smooth surface, exhibiting a typical face-centered cubic (FCC) solid solution. The film showed better mechanical properties than its bulk counterpart, with a Young’s modulus and hardness of ~201.4 GPa and ~11.5 GPa, respectively. Furthermore, corrosion tests demonstrated decreased sensitivity to localized corrosion in comparison to the commercial 304 stainless steel in NaCl solution. Full article
(This article belongs to the Special Issue High Entropy Alloy Coatings)
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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; https://doi.org/10.3390/coatings7090151 - 20 Sep 2017
Cited by 5
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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Open AccessArticle
Microstructure and Wear Resistance of AlCoCrFeNiTi High-Entropy Alloy Coatings Produced by HVOF
Coatings 2017, 7(9), 144; https://doi.org/10.3390/coatings7090144 - 12 Sep 2017
Cited by 7
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
Fabrication and Characterization of AlxCoFeNiCu1−x High Entropy Alloys by Laser Metal Deposition
Coatings 2017, 7(4), 47; https://doi.org/10.3390/coatings7040047 - 25 Mar 2017
Cited by 8
Abstract
High entropy alloys are multicomponent alloys that have at least five different principal elements as alloying elements. Each of these elements has an atomic percentage between 5% and 35%. Typically, they form body-centered cubic (bcc) or face-centered cubic (fcc) structure and are known [...] Read more.
High entropy alloys are multicomponent alloys that have at least five different principal elements as alloying elements. Each of these elements has an atomic percentage between 5% and 35%. Typically, they form body-centered cubic (bcc) or face-centered cubic (fcc) structure and are known to possess excellent mechanical properties, corrosion resistance, excellent electric and magnetic properties. Owing to their excellent corrosion and wear resistance, researchers are focusing on employing these materials as coatings. In this research, Laser Metal Deposition (LMD) was used to fabricate AlxCoFeNiCu1−x (x = 0.25, 0.5, 0.75) high entropy alloys from elemental powder based feedstocks. Thin wall claddings fabricated via LMD were characterized by a variety of techniques. Data from X-ray Diffraction (XRD) and Electron Back Scatter Diffraction (EBSD) suggested that with increase in Al content and decrease in Cu content, a change in crystal structure from a predominantly fcc to a combined fcc and bcc structure can be observed. The microstructure of the material was observed to be columnar dendritic. Data from standard less EDS analysis showed that the dendritic phase was Fe and Co enriched while the matrix was Cu and Al enriched in all the considered high entropy alloy fabrications. The Vickers hardness data was used to estimate the mechanical properties of these deposits. Results also showed that with the increase in aluminum content, AlxCoFeNiCu1−x displayed higher hardness. The high hardness values imply potential applications in wear resistant coatings. Full article
(This article belongs to the Special Issue High Entropy Alloy Coatings)
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Open AccessArticle
Effects of Different Levels of Boron on Microstructure and Hardness of CoCrFeNiAlxCu0.7Si0.1By High-Entropy Alloy Coatings by Laser Cladding
Coatings 2017, 7(1), 7; https://doi.org/10.3390/coatings7010007 - 11 Jan 2017
Cited by 8
Abstract
High-entropy alloys (HEAs) are novel solid solution strengthening metallic materials, some of which show attractive mechanical properties. This paper aims to reveal the effect of adding small atomic boron on the interstitial solid solution strengthening ability in the laser cladded CoCrFeNiAlxCu [...] Read more.
High-entropy alloys (HEAs) are novel solid solution strengthening metallic materials, some of which show attractive mechanical properties. This paper aims to reveal the effect of adding small atomic boron on the interstitial solid solution strengthening ability in the laser cladded CoCrFeNiAlxCu0.7Si0.1By (x = 0.3, x = 2.3, and 0.3 ≤ y ≤ 0.6) HEA coatings. The results show that laser rapid solidification effectively prevents brittle boride precipitation in the designed coatings. The main phase is a simple face-centered cubic (FCC) matrix when the Al content is equal to 0.3. On the other hand, the matrix transforms to single bcc solid solution when x increases to 2.3. Increasing boron content improves the microhardness of the coatings, but leads to a high degree of segregation of Cr and Fe in the interdendritic microstructure. Furthermore, it is worth noting that CoCrFeNiAl0.3Cu0.7Si0.1B0.6 coatings with an FCC matrix and a modulated structure on the nanometer scale exhibit an ultrahigh hardness of 502 HV0.5. Full article
(This article belongs to the Special Issue High Entropy Alloy Coatings)
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Open AccessArticle
Ab Initio Predicted Alloying Effects on the Elastic Properties of AlxHf1−xNbTaTiZr High Entropy Alloys
Coatings 2015, 5(3), 366-377; https://doi.org/10.3390/coatings5030366 - 29 Jul 2015
Cited by 6
Abstract
Using ab initio alloy theory, we investigate the equilibrium bulk properties and elastic mechanics of the single bcc solid-solution AlxHf1−xNbTaTiZr (x = 0–0.7, 1.0) high entropy alloys. Ab initio predicted equilibrium volume is consistent with the available [...] Read more.
Using ab initio alloy theory, we investigate the equilibrium bulk properties and elastic mechanics of the single bcc solid-solution AlxHf1−xNbTaTiZr (x = 0–0.7, 1.0) high entropy alloys. Ab initio predicted equilibrium volume is consistent with the available experiment. We make a detailed investigation of the alloying effect of Al and Hf on the equilibrium volume, elastic constants and polycrystalline elastic moduli. Results imply that the partial replacement Hf with Al increases the stability of the bcc phase and decreases the ductility of the AlxHf1−xNbTaTiZr HEAs. The inner ductility of Al0.4Hf0.6NbTaTiZr is predicted by the calculations of ideal tensile strength. Full article
(This article belongs to the Special Issue High Entropy Alloy Coatings)
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Open AccessArticle
Machining Performance of Sputter-Deposited (Al0.34Cr0.22Nb0.11Si0.11Ti0.22)50N50 High-Entropy Nitride Coatings
Coatings 2015, 5(3), 312-325; https://doi.org/10.3390/coatings5030312 - 23 Jul 2015
Cited by 9
Abstract
(Al0.34Cr0.22Nb0.11Si0.11Ti0.22)50N50 high-entropy nitride coatings prepared by reactive magnetron sputtering have been proved to have high hardness and superior oxidation resistance. Their thermal stability, adhesion strength, and cutting performance were investigated [...] Read more.
(Al0.34Cr0.22Nb0.11Si0.11Ti0.22)50N50 high-entropy nitride coatings prepared by reactive magnetron sputtering have been proved to have high hardness and superior oxidation resistance. Their thermal stability, adhesion strength, and cutting performance were investigated in this study. Hardness of the coating is 36 GPa, which only decreases slightly to 33 GPa after 900 °C annealing either in air or in vacuum for 2 h. No significant change in phase and microstructure were detected after annealing at 1000 °C. Rockwell C indentation and scratch tests shows that Ti interlayer provides a good adhesion between the nitride film and WC/Co substrates. In various milling tests, inserts coated with (Al0.34Cr0.22Nb0.11Si0.11Ti0.22)50N50 have evidently smaller flank wear depth than commercial inserts coated with TiN and TiAlN, even with their smaller thickness. Therefore, the (Al0.34Cr0.22Nb0.11Si0.11Ti0.22)50N50 coating has great potential in hard coating applications. Full article
(This article belongs to the Special Issue High Entropy Alloy Coatings)
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