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

Deadline for manuscript submissions: closed (30 June 2020) | Viewed by 79137

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Prof. Dr. Rocco Lupoi

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Department of Mechanical and Manufacturing Engineering, Trinity College of Dublin, The University of Dublin, Parsons Building, Dublin 2, Ireland
Interests: cold spray; laser cladding; additive manufacturing; selective laser melting
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Dr. Shuo Yin

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Guest Editor

Department of Mechanical, Manufacturing and Biomedical Engineering, Trinity College Dublin, D02 PN40 Dublin, Ireland
Interests: additive manufacturing; 3D printing; cold spraying; digital light processing; selective laser melting; high velocity imaging; numerical simulation
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Prof. Dr. Guanjun Yang

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State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi, China
Interests: thermal spray; energy; solar cell
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Metallic coatings are primarily fabricated through the deposition of powders layer by layer onto a substrate using various spraying processes. They are widely used as a functional surface modification to protect the underlying substrate from damage under corrosion, erosion, high-temperature, and other aggressive environments. The emerging additive manufacturing processes can be utilised in a similar manner. Typically, these processes will build 3D components with geometric precision by direct material deposition onto a substrate or alternatively by selectively melting material layer by layer on a powder bed. Additive manufacturing offers flexibility in geometric design, rapid production of components with complex geometry and high spatial resolution, and customization of products at an acceptable cost, and has little material waste through the recycling of unprocessed powder. Today, with the rapid development of modern industry and the launch of industry 4.0, high value-added components that are difficult to produce using conventional manufacturing processes are in great demand; this is particularly the case within cutting-edge fields such as the aerospace, nuclear, and energy industry. Since novel metallic coating and additive manufacturing techniques will be critical to future developments within these industries and many others, this Special Issue is dedicated to the recent advances in metallic coating and, in particular, additive manufacturing technologies and the transition between the two. Original research and critical review articles in the relevant topics are welcomed.

In particular, the topics of interest include, but are not limited to:

Thermal spray, cold spray, laser cladding, PVD, CVD, and other coating technologies
Selective laser melting, laser metal deposition, electron beam melting, and other additive manufacturing processes
Numerical modelling on melting pool, material deposition, and manufacturing processes
New coating, additive manufacturing, and 3D printing technologies
Applications and case studies of additive manufacturing and 3D printing technologies

Prof. Dr. Rocco Lupoi
Dr. Shuo Yin
Prof. Dr. Guanjun Yang
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Coatings is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2600 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

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Published Papers (11 papers)

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Research

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16 pages, 16662 KiB

Open AccessArticle

Extreme High-Speed Laser Material Deposition (EHLA) of AISI 4340 Steel

by Tianci Li, Lele Zhang, Gregor Gilles Pierre Bultel, Thomas Schopphoven, Andres Gasser, Johannes Henrich Schleifenbaum and Reinhart Poprawe

Coatings 2019, 9(12), 778; https://doi.org/10.3390/coatings9120778 - 21 Nov 2019

Cited by 36 | Viewed by 7433

Abstract

A variant of conventional laser material deposition (LMD), extreme high-speed laser material deposition (German acronym: EHLA) is characterized by elevated process speeds of up to 200 m/min, increased cooling rates, and a significantly reduced heat affected zone. This study focuses on the feasibility of using EHLA to apply material onto Fe-based substrate materials with AISI 4340 as a filler material. We studied how three different build-up strategies—consisting of one, three, and five consecutive deposited layers and hence, different thermal evolutions of the build-up volume—influence the metallurgical characteristics such as microstructure, porosity, hardness, and static mechanical properties. We propose a thermo-metallurgical scheme to help understand the effects of the build-up strategy and the thermal evolution on the microstructure and hardness. The tensile strength of the build-up volume was determined and is higher than the ones of forged AISI 4340 material. Full article

(This article belongs to the Special Issue From Metallic Coatings to Additive Manufacturing)

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12 pages, 13808 KiB

Open AccessArticle

The Evaluation of Durability of Plasma-Sprayed Thermal Barrier Coatings with Double-layer Bond Coat

by Xiao Li, Xin-Yu Peng, Hui Dong, Yong Zhou, Tao Wang, Kang Ren and Liang Sun

Coatings 2019, 9(4), 241; https://doi.org/10.3390/coatings9040241 - 9 Apr 2019

Cited by 25 | Viewed by 3835

Abstract

The durability of atmospheric plasma-sprayed thermal barrier coatings (APS TBCs) with a double-layer bond coat was evaluated via isothermal cycling tests under 1120 °C. The bond coat consisted of a porosity layer deposited on the substrate and an oxidation layer deposited on the porosity layer. Two types of double-layer bond coats with different thickness ratios of the porosity layer to the oxidation layer (type A: 1:2 and type B: 2:1, respectively) were prepared. The results show that the porosity layer was oxidation free, the oxidation layer included a fraction of well-distributed α-Al₂O₃. The coefficient of thermal expansion of the oxidation layer was about 11.2 × 10⁻⁶ K⁻¹, which was rather lower than that of the porosity layer. Thus, the oxidation layer can be regards as a secondary bond coat between ceramic topcoat and traditional bond coat. The thermal cyclic lifetime of type A TBCs was about 60 cycles, which exceeded 1.2 times the durability of type B TBCs. The delamination cracks in both TBCs all propagated in the ceramic topcoat, which were all identical to those in traditional TBCs. Therefore, the design of the double-layer bond coat affected the stress level rather than the stress distribution in TBCs. Full article

(This article belongs to the Special Issue From Metallic Coatings to Additive Manufacturing)

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11 pages, 7161 KiB

Open AccessArticle

Erosion Resistance and Damage Mechanism of TiN/ZrN Nanoscale Multilayer Coating

by Jiao Chen, Mingrui Geng, Yuqin Li, Zhufang Yang, Yan Chai and Guangyu He

Coatings 2019, 9(2), 64; https://doi.org/10.3390/coatings9020064 - 22 Jan 2019

Cited by 31 | Viewed by 4744

Abstract

Ceramic coating is an effective method for improving the erosion resistance of a material, particularly for titanium alloys. In this study, a TiN/ZrN (ceramic/ceramic) nanoscale multilayer coating is designed and prepared on the Ti6Al4V titanium alloy surface by the physical vapor deposition (PVD) process. The cross-sectional microstructure and phase composition are measured using SEM and XRD, respectively. The hardness, elastic modulus, and adhesion of the coating are measured by the nano-indentation and scratch method. The erosion test is conducted at a 45° angle with 100 m/s velocity using self-developed erosion equipment. The erosion resistance mechanisms of both the substrate and the coating are revealed more intuitively through a single sand particle impact test. The results show that the erosion resistance rate of the coating is 15.5 times higher than that of the titanium alloy substrate. The damage mechanisms of material removal of the coating include crack deflection, crack branching, and succeeding interaction between them when suffering an impacting load. These cracks are started from the droplets and the stress concentrations on the coating surface during the preparation of coating. They are the primary reasons for the decrease in the erosion resistance of the coating. This research is important for the optimization of the erosion-resistant coating structure. Full article

(This article belongs to the Special Issue From Metallic Coatings to Additive Manufacturing)

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12 pages, 2740 KiB

Open AccessArticle

Tuning Nucleation Sites to Enable Monolayer Perovskite Films for Highly Efficient Perovskite Solar Cells

by Yan Li, Xiaolei Li, Qianqian Chu, Hui Dong, Jiantao Yao, Yong Zhou and Guanjun Yang

Coatings 2018, 8(11), 408; https://doi.org/10.3390/coatings8110408 - 18 Nov 2018

Cited by 10 | Viewed by 5672

Abstract

The nucleation site plays a critical role in achieving the full coverage of perovskite film at both the macroscopic and microscopic scales, and it is systematically investigated for the first time in this study. The results show that under natural conditions, the incomplete coverage of perovskite film is due to both heterogeneous nucleation and homogeneous nucleation. The established concentration field and temperature field in the precursor solution show that there are two preferential nucleation sites, i.e., the upper surface of the precursor solution (homogeneous nucleation) and the surface of the substrate (heterogeneous nucleation). The nucleation sites are tuned by decreasing the drying pressure from the atmosphere to 3000 Pa, and then to 100 Pa, and then the microstructures of the perovskite films change from an incomplete coverage state to a monolayer full coverage state, and then to a bilayer full coverage state. At last, when the full coverage perovskite films are assembled into perovskite solar cells, the photovoltaic performance of the monolayer perovskite solar cells is slightly greater than that of the bilayer perovskite solar cells. The electrochemical characterization shows that there is more restrained internal recombination of the monolayer perovskite solar cells compared with bilayer perovskite solar cells. Full article

(This article belongs to the Special Issue From Metallic Coatings to Additive Manufacturing)

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15 pages, 7518 KiB

Open AccessArticle

Corrosion Damage Mechanism of TiN/ZrN Nanoscale Multilayer Anti-Erosion Coating

by Mingrui Geng, Guangyu He, Zhiping Sun, Jiao Chen, Zhufang Yang and Yuqin Li

Coatings 2018, 8(11), 400; https://doi.org/10.3390/coatings8110400 - 13 Nov 2018

Cited by 19 | Viewed by 5584

Abstract

TiN/ZrN multilayers can effectively improve the erosion resistance of metals, particularly titanium alloys employed in aero engines. To explore the corrosion damage mechanism of TiN/ZrN nanoscale multilayers (nanolaminate), a novel [TiN/ZrN]₁₀₀ nanolaminate coating was deposited on Ti-6Al-4V alloys by multi-arc ion plating method. Salt spray corrosion tests and hot corrosion experiment were carried out to evaluate the corrosion resistance of the coating. The corrosion and damage mechanisms were explored with the help of detailed microstructure, phase composition and element distribution characterizations. The salt spray corrosion tests showed that the [TiN/ZrN]₁₀₀ nanolaminate coating possessed good corrosion resistance, which protected substrate against the corrosion. The low temperature hot corrosion tests showed that the oxidation occurred on the surface of the coating, which improved the oxidation resistance of the sample. However, the oxidized droplets squeezed the coating, and destroyed the oxidized layers. As a result, the coating was peeled off from the substrate. The research highlights the corrosion resistance of the novel TiN/ZrN nanolaminate coating and offers a support for their application in engine compressor blade. Full article

(This article belongs to the Special Issue From Metallic Coatings to Additive Manufacturing)

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9 pages, 6223 KiB

Open AccessArticle

Numerical and Experimental Investigation on Bonding Behavior of Cold Sprayed Porous WC-17Co Particles onto Different Substrates

by Xinkun Suo, Shuo Yin, Hua Li and Rocco Lupoi

Coatings 2018, 8(10), 367; https://doi.org/10.3390/coatings8100367 - 17 Oct 2018

Cited by 7 | Viewed by 3194

Abstract

Cold sprayed WC-Co metal matrix composite coatings have shown great potential in wear-resistance applications. This work aims to use experimental and numerical methods to clarify the deposition and particle–substrate bonding behavior of a single porous WC-17Co particle onto various substrates. To achieve this objective, porous WC-17Co particles were used as the feedstock; soft Al 2024 (Al alloy) and hard stainless steel 316 (SS) were used as the substrates. The experimental results revealed that brittle WC-Co particles tended to remain intact after depositing on a soft Al alloy substrate, but underwent serious fracture when impacting on a hard SS substrate. Further results indicated that the high energy dissipation and the consequent high stress concentration in the WC-Co particle was the main reason for inducing the particle fracture. In addition, two different mechanical interlocking mechanisms were identified during the WC-Co particle deposition process (namely WC reinforcement interlock and WC-Co particle interlock), dominating the particle-substrate bonding. A soft Al alloy substrate resulted in better interlocking than a hard SS substrate, thereby the corresponding particle bonding ratio was also much higher. Full article

(This article belongs to the Special Issue From Metallic Coatings to Additive Manufacturing)

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22 pages, 5783 KiB

Open AccessArticle

Influence of Particle Velocity When Propelled Using N₂ or N₂-He Mixed Gas on the Properties of Cold-Sprayed Ti6Al4V Coatings

by Adrian Wei-Yee Tan, Jun Yan Lek, Wen Sun, Ayan Bhowmik, Iulian Marinescu, Xu Song, Wei Zhai, Feng Li, Zhili Dong, Chris B. Boothroyd and Erjia Liu

Coatings 2018, 8(9), 327; https://doi.org/10.3390/coatings8090327 - 18 Sep 2018

Cited by 34 | Viewed by 4922

Abstract

Cold-spraying is a relatively new low-temperature coating technology which produces coatings by the deposition of metallic micro-particles at supersonic speed onto target substrate surfaces. This technology has the potential to enhance or restore damaged parts made of light metal alloys, such as Ti6Al4V (Ti64). Particle deposition velocity is one of the most crucial parameters for achieving high-quality coatings because it is the main driving force for particle bonding and coating formation. In this work, studies were conducted on the evolution of the properties of cold-sprayed Ti64 coatings deposited on Ti64 substrates with particle velocities ranging from 730 to 855 m/s using pure N₂ and N₂-He mixture as the propellant gases. It was observed that the increase in particle velocity significantly reduced the porosity level from about 11 to 1.6% due to greater densification. The coatings’ hardness was also improved with increased particle velocity due to the intensified grain refinement within the particles. Interestingly, despite the significant differences in the coating porosities, all the coatings deposited within the velocity range (below and above critical velocity) achieved a high adhesion strength exceeding 60 MPa. The fractography also showed changes in the degree of dimple fractures on the particles across the deposition velocities. Finite element modelling was carried out to understand the deformation behaviour of the impacting particles and the evolutions of strain and temperature in the formed coatings during the spraying process. This work also showed that the N₂-He gas mixture was a cost-effective propellant gas (up to 3-times cheaper than pure He) to deliver the high-quality Ti64 coatings. Full article

(This article belongs to the Special Issue From Metallic Coatings to Additive Manufacturing)

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11 pages, 2691 KiB

Open AccessArticle

The Temperature Distribution in Plasma-Sprayed Thermal-Barrier Coatings During Crack Propagation and Coalescence

by Hui Dong, Yan Han, Yong Zhou, Xiao Li, Jian-Tao Yao and Yan Li

Coatings 2018, 8(9), 311; https://doi.org/10.3390/coatings8090311 - 4 Sep 2018

Cited by 13 | Viewed by 3472

Abstract

A Finite-Element Model (FEM) for thermal-barrier coatings was employed to elaborate the temperature distribution on yttria-stabilized zirconia (YSZ) free surface during cracks coalescing, then the influence of sintering of YSZ induced by heat-transfer overlapping on energy release rate was quantificationally evaluated. A three-dimensional model including three layers was fabricated. Two types of cracks, with and without depth variations in YSZ coating, were introduced into the model, respectively. The temperature rise of YSZ coating over the crack is independent of each other at the beginning of crack propagation. As crack distance shortens, the independent temperature-rise regions begin to overlap, while maximum temperature is still located at the crack center before crack coalescence. The critical distance that the regions of temperature rise, just overlapping, is the sum of half lengths of two coalescing cracks (i.e., a₁ + a₂), which is independent of cracking path. The maximum temperature in YSZ sharply increases once cracks coalesce. Compared with one delamination crack, the effective energy-release rate induced by heat-transfer overlapping increases in the range of 0.2%–15%, depending on crack length and crack distance, which is on some level comparable to that of deterioration of thermal expansion misfit induced by temperature jump between crack faces. Full article

(This article belongs to the Special Issue From Metallic Coatings to Additive Manufacturing)

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16 pages, 5509 KiB

Open AccessArticle

High-Temperature Oxidation Resistance of NiAl Intermetallic Formed In Situ by Thermal Spraying

by Qianqian Jia, Deyuan Li, Shumei Li, Zhuang Zhang and Nannan Zhang

Coatings 2018, 8(8), 292; https://doi.org/10.3390/coatings8080292 - 19 Aug 2018

Cited by 25 | Viewed by 5542

Abstract

An Al/Ni composite coating was deposited on the surface of a pure Ti substrate by arc spray technology and plasma spray technology. In order to enable the in-situ reaction between the Al/Ni composite coating and the specimen, they were heated under different conditions. In addition, oxidation testing was conducted to test the oxidation-resistant property of the coating. The phase transition regulation of the coating after heating, the influence of heating at different temperatures and time on the reaction depth, and the correlated theory of the in-situ formation of the NiAl intermetallic compounds were studied and analyzed. The results showed that after the heat treatment, a ragged wave-like morphology was exhibited in the diffusion front of Al, and a small amount of the Ni in the diffusion region did not participate in the reaction. The growth of the NiAl intermetallic layer in the diffusion region of the Al/Ni/Ti specimen was obviously slower compared with the Al/Ni specimen. Full article

(This article belongs to the Special Issue From Metallic Coatings to Additive Manufacturing)

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10 pages, 2747 KiB

Open AccessArticle

Preparation of Yttria-Stabilized Zirconia Hollow Sphere with Reduced Shell Thickness by Controlling Ambient Temperature during Plasma Process

by Lina Zhao, Zhi Zhang, Yugang Duan, Hong Cui and Yong Gao

Coatings 2018, 8(7), 245; https://doi.org/10.3390/coatings8070245 - 11 Jul 2018

Cited by 8 | Viewed by 4892

Abstract

Yttria-stabilized zirconia (YSZ) hollow sphere (HS) powder is a novel potential feedstock material for the plasma spraying of next generation advanced thermal barrier coatings with low thermal conductivity and high sintering resistibility. In this study, YSZ HS powders were prepared by plasma treatment with/without a heat preservation zone around the flying path of the particles during plasma flame. The results of the scanning electron microscopy of YSZ HS powders showed that HS prepared with a heat preservation zone during the plasma process exhibited a regular spherical morphology and a homogeneous thin shell structure. Due to the sufficient heating of the shell regions, the HS powder presented a well densified shell structure. Furthermore, the mechanism of formation of the HS powder with reduced shell thickness was also discussed based on the analysis of the evolution of the powder structure. This kind of hollow sphere powder with a very thin shell structure provides a new alternative feedstock material for the development of next generation high performance thermal barrier coatings. Full article

(This article belongs to the Special Issue From Metallic Coatings to Additive Manufacturing)

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Review

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26 pages, 2798 KiB

Open AccessReview

State of the Art in Directed Energy Deposition: From Additive Manufacturing to Materials Design

by Adrita Dass and Atieh Moridi

Coatings 2019, 9(7), 418; https://doi.org/10.3390/coatings9070418 - 29 Jun 2019

Cited by 384 | Viewed by 27348

Abstract

Additive manufacturing (AM) is a new paradigm for the design and production of high-performance components for aerospace, medical, energy, and automotive applications. This review will exclusively cover directed energy deposition (DED)-AM, with a focus on the deposition of powder-feed based metal and alloy systems. This paper provides a comprehensive review on the classification of DED systems, process variables, process physics, modelling efforts, common defects, mechanical properties of DED parts, and quality control methods. To provide a practical framework to print different materials using DED, a process map using the linear heat input and powder feed rate as variables is constructed. Based on the process map, three different areas that are not optimized for DED are identified. These areas correspond to the formation of a lack of fusion, keyholing, and mixed mode porosity in the printed parts. In the final part of the paper, emerging applications of DED from repairing damaged parts to bulk combinatorial alloys design are discussed. This paper concludes with recommendations for future research in order to transform the technology from “form” to “function,” which can provide significant potential benefits to different industries. Full article

(This article belongs to the Special Issue From Metallic Coatings to Additive Manufacturing)

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