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A special issue of Materials (ISSN 1996-1944). This special issue belongs to the section "Thin Films and Interfaces".

Deadline for manuscript submissions: closed (20 September 2024) | Viewed by 16100

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Dr. Tao Wang

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Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences (CAS), Shenzhen 518055, China
Interests: functional coatings; diamond and composite films; micro-nanostructured surfaces; robust superhydrophobic coatings

Special Issue Information

Dear Colleagues,

Coating and surface engineering play important roles in the development of industry and society. The performance of critical raw materials has been improved in many conventional and industrial areas. With the emergence of newly developing fields, e.g., ultraprecision tools, biomedical devices, and marine instruments, coatings with remarkable functional properties are in high demand. For example, high hardness and a low friction coefficient are necessary for tool coatings. Biomedical implants require chemical inertness, biocompatibility, and antibacterial efficiency. Moreover, corrosion-resistant coatings and biofilm adhesion are needed for marine instruments.

Notably, physical vapor deposition (PVD) and chemical vapor deposition (CVD) processes, which are often used for hard (ceramic) coatings, suffer from poor nucleation and lack adhesion. These issues arise from the incompatibility of the substrate material and the coating system. Therefore, key issues for coating systems concern nucleation, growth, and the adhesion of the coating to substrates and, concomitantly, implementing deposition procedure strategies that enable the multifunctionality of the coating, i.e., by tailoring the topography (roughness) and electrical conductivity. Facile and economically viable coating deposition processes combined with excellent adhesion, resistance, and multifunctionality will improve current industrial processes and enable novel applications in the future.

This Special Issue aims to address the latest trends in advanced coatings and surface engineering. The coating technology and excellent properties are highlighted with a focus on applications in various fields, including, but not limited to, precision machining, sensors, implants, electronics, and marine instruments.

Dr. Tao Wang
Guest Editor

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Keywords

thin films
micro/nanostructured surfaces
nucleation and film growth
protective coatings
wear and friction
anti-corrosion
anti-biofouling
electrical and electrochemical properties
superhydrophobic
film and coating characterization

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Published Papers (12 papers)

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Research

17 pages, 8855 KiB

Open AccessArticle

Microstructure and Hardness of Nickel-Based Coatings Prepared by Laser Additive Manufacturing on Water-Cooled Substrate: An Experimental and Numerical Study

by Mingjun Yan, Ruifeng Li, Jiajunqi Guo, Bin Liu, Xiaoqiang Zhang, Yue Zhao, Taotao Li, Lei Qiao and Seyed Reza Elmi Hosseini

Materials 2024, 17(23), 5692; https://doi.org/10.3390/ma17235692 - 21 Nov 2024

Viewed by 304

Abstract

The excess heat generated during the laser additive manufacturing process is prone to cause coating defects; a water-cooled substrate can effectively remove the excess heat and improve the hardness of the coating. In this study, the effects of water-cooled substrate on the microstructure and hardness of laser additive manufactured nickel-based coatings were investigated by experimental and numerical simulations. The results showed that the water-cooled substrate decreased the size of columnar crystals and increased the number as well as the length of secondary dendrite crystals at the bottom of the nickel-based coatings. There was also a noticeable increase in the size of equiaxed grains and the quantity of the solid solution in the middle of the coatings. The hardness value of the coating increased at the water velocity of 200 mL/s and 500 mL/s and finally decreased at 700 mL/s. A finite element model was established by ABAQUS software to numerically simulate the temperature field of the laser additive manufactured nickel-based coating with the water-cooled substrate. The results revealed significant differences in the temperature distribution of the coatings with different velocities. As the water velocity increased, the peak temperature at the center of the coating’s molten pool gradually decreased. In addition, the cooling rate of the specimens increased with the application of the water cooling, leading to a more concentrated temperature distribution near the laser heat source. Full article

(This article belongs to the Special Issue Advanced Multifunctional Coatings for New Applications)

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16 pages, 6481 KiB

Open AccessArticle

Deposition of Diamond Coatings on Ultrathin Microdrills for PCB Board Drilling

by Shuangqing Zhou, Stephan Handschuh-Wang and Tao Wang

Materials 2024, 17(22), 5593; https://doi.org/10.3390/ma17225593 - 15 Nov 2024

Viewed by 343

Abstract

The drilling of State-of-the-Art printed circuit boards (PCBs) often leads to shortened tool lifetime and low drilling accuracy due to improved strength of the PCB composites with nanofillers and higher thickness-to-hole diameter ratio. Diamond coatings have been employed to improve the tool lifetime and drilling accuracy, but the coated microdrills are brittle and suffer from coating delamination. To date, it is still difficult to deposit diamonds on ultrathin microdrills with diameters lower than 0.2 mm. To avoid tool failure, the pretreatment was optimized to afford sufficient fracture strength and enough removal of cobalt. Further, the adhesion of the diamond coating was improved by employing an interlayer comprising SiC/microcrystalline diamond, which mitigates stress accumulation at the interface. By these means, microdrills with diameters of 0.8 and 0.125 mm were coated with adherent diamonds. In this context, the composite coating with the diamond/SiC interlayer and a nanodiamond top layer featured enhanced adhesion compared to single nano- or microdiamond coatings on the WC-Co microdrills. The composite diamond-coated WC-Co microdrills featured improved wear resistance, resistance to delamination of the diamond coating, and improved performance for drilling PCBs compared to micro- and nanodiamond-coated microdrills without interlayer. In addition, a higher hole quality was achieved when the diamond-coated microdrills were used. These results signify that the composite/nanodiamond coating features the highest bonding strength and best drilling performance. Full article

(This article belongs to the Special Issue Advanced Multifunctional Coatings for New Applications)

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17 pages, 14819 KiB

Open AccessArticle

Study of Coating Growth Direction of 6061 Aluminum Alloy in Soft Spark Discharge of Plasma Electrolytic Oxidation

by Wenqiang Wang, Yifeng Yang, Cancan Liu, Bo Chen, Xuanyu Chen, Hao Wang, Rui Tong and Shiquan Zhou

Materials 2024, 17(12), 2947; https://doi.org/10.3390/ma17122947 - 16 Jun 2024

Viewed by 760

Abstract

Conventional plasma electrolytic oxidation treatments produce oxide coatings with micron-scale discharge pores, resulting in insulation and wear and corrosion resistance far below that expected of highly dense Al₂O₃ coatings. The introduction of cathodic polarization during the plasma electrolytic oxidation process, especially when the applied cathode-to-anode current ratio (Rpn) is greater than 1, triggers a unique plasma discharge phenomenon known as “soft sparking”. The soft spark discharge mode significantly improves the densification of the anode ceramic layer and facilitates the formation of the high-temperature α-Al₂O₃ phase within the coating. Although the soft spark discharge phenomenon has been known for a long time, the growth behavior of the coating under its discharge mode still needs to be studied and improved. In this paper, the growth behavior of the coating before and after soft spark discharge is investigated with the help of the micro-morphology, phase composition and element distribution of a homemade fixture. The results show that the ceramic layer grows mainly along the oxide–electrolyte direction before the soft spark discharge transformation; after the soft spark discharge, the ceramic layer grows along the oxide–substrate direction. It was also unexpectedly found that, under soft spark discharge, the silicon element only exists on the outside of the coating, which is caused by the large size and slow migration of SiO₃²⁻, which can only enter the ceramic layer and participate in the reaction through the discharge channel generated by the strong discharge. In addition, it was also found that the relative phase content of α-Al₂O₃ in the coating increased from 0.487 to 0.634 after 10 min of rotary spark discharge, which is an increase of 30.2% compared with that before the soft spark discharge transition. On the other hand, the relative phase content of α-Al₂O₃ in the coating decreased from 0.487 to 0.313 after 20 min of transfer spark discharge, which was a 55.6% decrease compared to that before the soft spark discharge transformation. Full article

(This article belongs to the Special Issue Advanced Multifunctional Coatings for New Applications)

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16 pages, 7796 KiB

Open AccessArticle

Effect of Negative Pulse on the Stability of Black Electrolytes for Magnesium Alloy Microarc Oxidation

by Bo Chen, Rui Tong, Hongtao Li, Wenqiang Wang, Xuanyu Chen, Hao Wang, Yifeng Yang and Shiquan Zhou

Materials 2024, 17(11), 2654; https://doi.org/10.3390/ma17112654 - 31 May 2024

Viewed by 576

Abstract

The correlation between negative pulse and the black electrolyte properties of magnesium alloy micro-arc oxidation and the treated area was investigated by introducing a negative pulse electric field. The physical phase composition, microstructure, elemental distribution, and content of the coating were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). The results showed that the introduction of negative pulses favored the generation of MgO and MgSiO₃ contents in the coatings, and an increase in the MgO phase was found in the coatings formed in the failed electrolytes; the microporous size and microcracks of the coatings were gradually and significantly reduced; the average consumption of Cu ions was 0.0453 g/L·dm², which is only 26% of that in the unipolar condition; the introduction of the negative pulses significantly improved the “anomalous consumption” of Cu ions. The introduction of negative pulse can significantly improve the “abnormal consumption” of copper ions, which is attributed to the change in the electric field by negative pulse, which makes the cathode-enriched Cu ions migrate to the anode and reduces the reduction and precipitation of Cu ions at the cathode. Full article

(This article belongs to the Special Issue Advanced Multifunctional Coatings for New Applications)

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12 pages, 11237 KiB

Open AccessArticle

Influence of Additives on the Macroscopic Color and Corrosion Resistance of 6061 Aluminum Alloy Micro-Arc Oxidation Coatings

by Xuanyu Chen, Hao Wang, Cancan Liu, Wenqiang Wang and Bo Chen

Materials 2024, 17(11), 2621; https://doi.org/10.3390/ma17112621 - 29 May 2024

Viewed by 719

Abstract

In this study, we successfully employed the plasma electrolytic oxidation (PEO) technique to create a uniform white ceramic layer on the surface of the 6061 aluminum alloy using K₂ZrF₆ and Na₂WO₄ as colorants. A scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectrometer (EDS) and X-ray diffraction (XRD) were used to characterize the coatings, and we used an electrochemical workstation to test their corrosion protection properties. The corrosion resistance of the coatings was analyzed using potentiodynamic polarization curves. The results showed that K₂ZrF₆ addition whitened the coating with ZrO₂ as the main phase composition, inhibiting Al substrate depletion and enhancing coating corrosion resistance. A small amount of Na₂WO₄ decreased the coating’s L* value, successfully constructing ceramic coatings with L* (coating brightness) values ranging from 70 to 86, offering broad application prospects for decorative coatings. Full article

(This article belongs to the Special Issue Advanced Multifunctional Coatings for New Applications)

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11 pages, 4525 KiB

Open AccessArticle

Effect of Nitrogen on Growth and Optical Properties of Single-Crystal Diamond Synthesized by Chemical Vapor Deposition

by Ying Ren, Wei Lv, Xiaogang Li, Haoyong Dong, Nicolas Wöhrl, Xun Yang, Zhengxin Li and Tao Wang

Materials 2024, 17(6), 1311; https://doi.org/10.3390/ma17061311 - 12 Mar 2024

Cited by 1 | Viewed by 1801

Abstract

Concurrently achieving high growth rate and high quality in single-crystal diamonds (SCDs) is significantly challenging. The growth rate of SCDs synthesized by microwave plasma chemical vapor deposition (MPCVD) was enhanced by introducing N₂ into the typical CH₄-H₂ gas mixtures. The impact of nitrogen vacancy (NV) center concentration on growth rate, surface morphology, and lattice binding structure was investigated. The SCDs were characterized through Raman spectroscopy, photoluminescence (PL) spectroscopy, and X-ray photoelectron spectroscopy. It was found that the saturation growth rate was increased up to 45 μm/h by incorporating 0.8–1.2% N₂ into the gas atmosphere, which is 4.5 times higher than the case without nitrogen addition. Nitrogen addition altered the growth mode from step–flow to bidimensional nucleation, leading to clustered steps and a rough surface morphology, followed by macroscopically pyramidal hillock formation. The elevation of nitrogen content results in a simultaneous escalation of internal stress and defects. XPS analysis confirmed chemical bonding between nitrogen and carbon, as well as non-diamond carbon phase formation at 0.8% of nitrogen doping. Furthermore, the emission intensity of NV-related defects from PL spectra changed synchronously with N₂ concentrations (0–1.5%) during diamond growth, indicating that the formation of NV centers activated the diamond lattice and facilitated nitrogen incorporation into it, thereby accelerating chemical reaction rates for achieving high-growth-rate SCDs. Full article

(This article belongs to the Special Issue Advanced Multifunctional Coatings for New Applications)

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15 pages, 5238 KiB

Open AccessArticle

Comparison of Sublimation 3D Scanning Sprays in Terms of Their Effect on the Resulting 3D Scan, Thickness, and Sublimation Time

by Jakub Franke, Tomáš Koutecký and Daniel Koutný

Materials 2023, 16(18), 6165; https://doi.org/10.3390/ma16186165 - 11 Sep 2023

Cited by 1 | Viewed by 1379

Abstract

This study compared eight sublimation scanning sprays in terms of their effect on 3D scanning results, coating thickness, and sublimation time. The work used an automated spraying system to ensure the same deposition conditions for all tested materials. All experiments were performed under the same environmental conditions to exclude the influence of the ambient environment on the coatings. All tested scanning sprays created coatings with thicknesses in the order of tens of micrometers that were detectable by the 3D scanner Atos III Triple Scan. The coatings must be applied carefully when accurate measurements are required. All used materials enabled the capture of the highly reflective surface of the Si-wafer. However, the differences between some sprays were significant. Sublimation time measurements showed that all coatings disappeared from the Si-wafer surface completely. Nevertheless, all coatings left visible traces on the mirror-like surface. They were easily wiped off with a cloth. Full article

(This article belongs to the Special Issue Advanced Multifunctional Coatings for New Applications)

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13 pages, 3067 KiB

Open AccessArticle

Surface Morphology and Optical Properties of Hafnium Oxide Thin Films Produced by Magnetron Sputtering

by José de Jesús Araiza, Leo Álvarez-Fraga, Raúl Gago and Olga Sánchez

Materials 2023, 16(15), 5331; https://doi.org/10.3390/ma16155331 - 29 Jul 2023

Cited by 4 | Viewed by 1924

Abstract

Hafnium oxide films were deposited on sapphire and silicon (100) substrates using the DC reactive magnetron sputtering technique from a pure hafnium target at different discharge power levels. The influence of the cathode power on the chemical composition, morphology, crystallographic structure and optical properties of the films was investigated. X-ray diffraction (XRD), energy dispersive X-ray analysis (EDX) and Fourier-transform infrared spectroscopy (FTIR) were employed to determine the chemical composition and bonding structure. In all cases, the films were found to be amorphous or nanocrystalline with increased crystalline content as the sputtering power was increased, according to XRD and FTIR. In addition, EDX showed that the films were oxygen-rich. The effect of power deposition on the surface topography and morphology of the films was studied using atomic force microscopy (AFM) and scanning electron microscopy (SEM). The AFM and SEM images revealed the emergence of mound morphologies as the cathode power was increased. These features are related to blistering effects probably due to the presence of stress and its promotion within the film thickness. Finally, the optical properties showed an average transmission of 80% in the visible range, and the refractive index determined by spectral ellipsometry (SE) was found to be in the range of 1.85–1.92, close to the reported bulk value. SE was also used to study the film porosity observed by SEM, which can be related to the oxygen-rich character of the films. Full article

(This article belongs to the Special Issue Advanced Multifunctional Coatings for New Applications)

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20 pages, 17490 KiB

Open AccessArticle

Evolution of the Microstructure and Mechanical Performance of As-Sprayed and Annealed Silicon Coating on Melt-Infiltrated Silicon Carbide Composites

by Mengqiu Guo, Yongjing Cui, Changliang Wang, Jian Jiao, Xiaofang Bi and Chunhu Tao

Materials 2023, 16(12), 4407; https://doi.org/10.3390/ma16124407 - 15 Jun 2023

Viewed by 1327

Abstract

In this study, silicon coating was deposited on melt-infiltrated SiC composites using atmospheric plasma spraying and then annealed at 1100 and 1250 °C for 1–10 h to investigate the effect of annealing on the layer. The microstructure and mechanical properties were evaluated using scanning electron microscopy, X-ray diffractometry, transmission electron microscopy, nano-indentation, and bond strength tests. A silicon layer with a homogeneous polycrystalline cubic structure was obtained without phase transition after annealing. After annealing, three features were observed at the interface, namely β-SiC/nano-oxide film/Si, Si-rich SiC/Si, and residual Si/nano-oxide film/Si. The nano-oxide film thickness was ≤100 nm and was well combined with SiC and silicon. Additionally, a good bond was formed between the silicon-rich SiC and silicon layer, resulting in a significant bond strength improvement from 11 to >30 MPa. Full article

(This article belongs to the Special Issue Advanced Multifunctional Coatings for New Applications)

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15 pages, 6203 KiB

Open AccessArticle

Enhanced Mechanical and Hydrophobic Antireflective Nanocoatings Fabricated on Polycarbonate Substrates by Combined Treatment of Water and HMDS Vapor

by Yao Yan, Jia Liu, Bing Zhang, Ruohan Xia, Yuqi Zhang and Zisheng Guan

Materials 2023, 16(10), 3850; https://doi.org/10.3390/ma16103850 - 19 May 2023

Cited by 7 | Viewed by 1861

Abstract

Polycarbonate (PC) with high transmittance, stable mechanical performance and environmental resistance is crucial for practical applications. In this work, we report a method for the preparation of a robust antireflective (AR) coating by a simple dip-coating process of a mixed ethanol suspension consisting of tetraethoxysilane (TEOS) base-catalyzed silica nanoparticles (SNs) and acid-catalyzed silica sol (ACSS). ACSS greatly improved the adhesion and durability of the coating, and the AR coating exhibited high transmittance and mechanical stability. Water and hexamethyldisilazane (HMDS) vapor treatment were further employed to improve the hydrophobicity of the AR coating. The as-prepared coating exhibited excellent antireflective properties, with an average transmittance of 96.06% in the wavelength range of 400 to 1000 nm, which is 7.55% higher than the bare PC substrate. After sand and water droplet impact tests, the AR coating still maintained enhanced transmittance and hydrophobicity. Our method shows a potential application for the preparation of hydrophobic AR coatings on a PC substrate. Full article

(This article belongs to the Special Issue Advanced Multifunctional Coatings for New Applications)

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12 pages, 4731 KiB

Open AccessArticle

HD-SiO₂/SiO₂ Sol@PDMS Superhydrophobic Coating with Good Durability and Anti-Corrosion for Protection of Al Sheets

by Ruohan Xia, Bing Zhang, Kousuo Dong, Yao Yan and Zisheng Guan

Materials 2023, 16(9), 3532; https://doi.org/10.3390/ma16093532 - 5 May 2023

Cited by 6 | Viewed by 2304

Abstract

Superhydrophobic coatings with excellent water-repellent properties imply a wide range of application areas. However, improvements are needed in terms of stability and complex processing procedures. In the present study, a superhydrophobic coating on Al sheets was prepared by mixing hexadecyltrimethoxysilane (HDTMS)-modified SiO₂ nanoparticles and acid-catalyzed silica sols (HD-SiO₂/SiO₂ Sol) with polydimethylsiloxane (PDMS) binder. The HD-SiO₂ nanoparticles and acid-catalyzed silica sol (SiO₂ sol) form a binary graded micro-nanostructure, providing excellent superhydrophobicity (Water Contact Angle = 158.5°, Sliding angle = 0°). Superhydrophobic coatings with excellent water-repellent properties have potential for corrosion prevention. However the commonly used organic resins have poor chemical and mechanical properties. In the present study, the results of outdoor exposure for 30 days, immersion in acid and alkaline solutions for 24 h, grit abrasion, and water impact experiments, respectively, showed that the prepared superhydrophobic coating has good wear resistance. The integrated superhydrophobic coating on the Al sheets exhibited good corrosion inhibition with an efficiency (η) of 98.9%, which is much higher than that of the uncoated sheets. The present study provides a promising approach for producing stable superhydrophobic coatings at a low cost, with the potential to supplant conventional organic resin anti-corrosion coatings. Full article

(This article belongs to the Special Issue Advanced Multifunctional Coatings for New Applications)

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14 pages, 5953 KiB

Open AccessArticle

Highly Transparent and Zirconia-Enhanced Sol-Gel Hybrid Coating on Polycarbonate Substrates for Self-Cleaning Applications

by Bing Zhang, Ruohan Xia, Yao Yan, Jia Liu and Zisheng Guan

Materials 2023, 16(8), 3138; https://doi.org/10.3390/ma16083138 - 16 Apr 2023

Cited by 2 | Viewed by 1857

Abstract

To improve the efficacy of polymer-based substrate hybrid coatings, it is essential to simultaneously optimize mechanical strength and preserve the optical properties. In this study, a mixture of zirconium oxide (ZrO₂) sol and methyltriethoxysilane modified silica (SiO₂) sol-gel was dip-coated onto polycarbonate (PC) substrates to form zirconia-enhanced SiO₂ hybrid coatings. Additionally, a solution containing 1H, 1H, 2H, and 2H-perfluorooctyl trichlorosilane (PFTS) was employed for surface modification. The results show that the ZrO₂-SiO₂ hybrid coating enhanced the mechanical strength and transmittance. The average transmittance of the coated PC reached up to 93.9% (400–800 nm), while the peak transmittance reached up to 95.1% at 700 nm. SEM images and AFM morphologies demonstrate that the ZrO₂ and SiO₂ nanoparticles were evenly distributed, and a flat coating was observed on the PC substrate. The PFTS-modified ZrO₂-SiO₂ hybrid coating also exhibited good hydrophobicity (WCA, 113°). As an antireflective coating on PC, with self-cleaning capability, the proposed coating has application prospects in optical lenses and automotive windows. Full article

(This article belongs to the Special Issue Advanced Multifunctional Coatings for New Applications)

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