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Advances in Hard Coatings: Production, Properties and Applications
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Advances in Hard Coatings: Production, Properties and Applications

A special issue of Coatings (ISSN 2079-6412). This special issue belongs to the section "Ceramic Coatings and Engineering Technology".

Deadline for manuscript submissions: closed (31 July 2021) | Viewed by 21877

Special Issue Editor

Prof. Dr. Cristina Santos Louro

E-Mail Website
Guest Editor
CEMMPRE, Centre of Mechanical Engineering, Materials and ProcessesUniversity of Coimbra, 3030-177 Coimbra, Portugal
Interests: hard coatings; biomedical coatings; sputtering; structural stability; oxidation/corrosion behaviour; mechanical properties; lightweight ceramics; polymer composites

Special Issue Information

Dear Colleagues,

Nearly 50 years have passed since the so-called 1st generation of hard coatings, correlated with the well-known Ti-based thin films, was industrially implemented in high-performance mechanical applications, particularly in cutting tools.

During this time, it soon became clear that the superior hardness of hard coatings came extrinsically from a combination of dissimilar features: phases, structures, and layers. Thus, modern surface architectures emerged, involving duplex, multiplex, and hybrid coatings. Moreover, control of structures and compositions of the coatings on the nanometer scale has revealed fascinating challenges, leading to nanocrystalline, nanocomposite, gradient, and superlattice coatings.

These achievements resulted in significant hardness enhancement, and its scale has been redone. In addition to hard coatings (Hv > 15GPa), superhard and ultrahard coatings (40 < Hv < 80 GPa) are acclaimed by various research works.

But increasing hardness and strength is often concomitant with loss of other properties. For this reason, the actual potential offered by modeling and simulation coupled with recent advances in deposition methodologies have brought hard coatings research to the point where realistic, model-based optimization of a coated surface, from mechanical to medical applications, is a reality.

The actual 4th generation of coatings, the smart or chameleon hard coatings, comprises those that change their properties as required to meet the specific necessities for a given application, surpassing the traditional metallic substrates by glass, ceramics, and polymers. Furthermore, many coatings systems are in commercial use, but there are many others that are still at the development stage and will hopefully contribute to the next generation of hard coatings.

Thus, it is a great pleasure to announce contributions to this Special Issue. All topics on surface modification and analysis are welcomed, emphasizing the newer processes, properties, and applications of hard coatings, and contributions may include, but are not limiting to, the following:

  • protective and tribological coatings;
  • smart coatings;
  • functionally graded hard coatings;
  • biomedical coatings;
  • optical coatings;
  • space coatings technology;
  • wind and energy conversion coatings;
  • numerical modeling on functional properties and manufacturing processes;
  • chemical, ion beam, plasma, and hybrid deposition technologies;
  • devices and industrial applications

Prof. Dr. Cristina Santos Louro
Guest Editor

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.

Published Papers (6 papers)

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Research

19 pages, 5863 KiB  
Article
Salivary pH Effect on Orthodontic Appliances: In Vitro Study of the SS/DLC System
by António Fróis, Manuel Evaristo, Ana Cristina Santos and Cristina Santos Louro
Coatings 2021, 11(11), 1302; https://doi.org/10.3390/coatings11111302 (registering DOI) - 27 Oct 2021
Cited by 7 | Viewed by 2139
Abstract
Stainless steels (SS) are the most-used alloys for manufacturing fixed orthodontic appliances due to their attractive set of mechanical properties, biocompatibility, and high corrosion resistance. Nevertheless, during regular orthodontic treatments–taking at least around 2 years–the intraoral environment inevitably degrades these bioalloys, releasing metallic [...] Read more.
Stainless steels (SS) are the most-used alloys for manufacturing fixed orthodontic appliances due to their attractive set of mechanical properties, biocompatibility, and high corrosion resistance. Nevertheless, during regular orthodontic treatments–taking at least around 2 years–the intraoral environment inevitably degrades these bioalloys, releasing metallic ions into the oral cavity. In the first part of this in vitro study, the corrosion resistance of commercial SS appliances (brackets, tubes, and bands) was evaluated in Fusayama-Meyer artificial saliva at pH values of 2.3 and 6.8 over the course of 30 days. As expected, the results corroborated that salivary pH highly influences corrosion behaviour. Released Ni, Cr, and Fe were within dietary intake values. In the second part, a novel approach for oral corrosion prevention based on the chemical inertness of DLC materials is presented. SS surfaces were functionalized with biocompatible a-C:H-sputtered coatings and submitted to the same experimental conditions. The anticorrosion ability of this system was demonstrated, preventing the pitting corrosion that occurred on the SS substrates. Despite the galvanic coupling effect due to the presence of the Cr-based interlayer, this study enhanced the potential use of the reactive sputter-deposited a-C:H coatings in orthodontics. Full article
(This article belongs to the Special Issue Advances in Hard Coatings: Production, Properties and Applications)
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19 pages, 64618 KiB  
Article
Can a-C:H-Sputtered Coatings Be Extended to Orthodontics?
by António Fróis, Ana Sofia Aleixo, Manuel Evaristo, Ana Cristina Santos and Cristina Santos Louro
Coatings 2021, 11(7), 832; https://doi.org/10.3390/coatings11070832 - 9 Jul 2021
Cited by 4 | Viewed by 2300
Abstract
Hydrogenated amorphous carbon (a-C:H) coatings are attractive materials for protecting metallic surfaces in extreme biological environments like the human oral cavity, due to the unusual combination of mechanical properties, superior bioinertness, and relative easier and cheaper production. In this work, two a-C:H coatings [...] Read more.
Hydrogenated amorphous carbon (a-C:H) coatings are attractive materials for protecting metallic surfaces in extreme biological environments like the human oral cavity, due to the unusual combination of mechanical properties, superior bioinertness, and relative easier and cheaper production. In this work, two a-C:H coatings were deposited on AISI 316L substrates by reactive magnetron sputtering with two CH4 flows to assess if this outstanding system could extend its application range to orthodontics. A 30-day immersion test in Fusayama-Meyer artificial saliva was conducted to mimic an extreme acidic intraoral pH. Extracts were quantified and used to perform in vitro assays with mono- and co-cultures of macrophages and fibroblast to assess cell viability, while mechanical and structural behaviors were studied by nanoindentation and visible Raman. The empirically estimated H contents of ~28 and 40 at.% matched the hard and soft a-C:H coating regimes of 18 and 7 GPa, respectively. After immersion, no important structural/mechanical modifications occurred, regardless of the H content, without corrosion signs, delamination, or coating detachment. However, the adhesion-promoting Cr-based interlayer seems to reduce corrosion resistance via galvanic coupling. The highest biocompatibility was found for a-C:H coatings with the lowest H content. This study indicates that sputtered a-C:H are promising surface materials in orthodontics. Full article
(This article belongs to the Special Issue Advances in Hard Coatings: Production, Properties and Applications)
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18 pages, 3994 KiB  
Article
Electrochemical Properties of TiWN/TiWC Multilayer Coatings Deposited by RF-Magnetron Sputtering on AISI 1060
by Andrés González-Hernández, Ana Beatriz Morales-Cepeda, Martín Flores, Julio C. Caicedo, William Aperador and César Amaya
Coatings 2021, 11(7), 797; https://doi.org/10.3390/coatings11070797 - 1 Jul 2021
Cited by 9 | Viewed by 2721
Abstract
Nitride and carbide ternary coatings improve the wear and corrosion resistance of carbon steel substrates. In this work, Ti-W-N and Ti-W-C coatings were deposited on AISI 1060 steel substrates using reactive radio frequency (RF) magnetron sputtering. The coatings were designed as monolayers, bilayers, [...] Read more.
Nitride and carbide ternary coatings improve the wear and corrosion resistance of carbon steel substrates. In this work, Ti-W-N and Ti-W-C coatings were deposited on AISI 1060 steel substrates using reactive radio frequency (RF) magnetron sputtering. The coatings were designed as monolayers, bilayers, and multilayers of 40 periods. The coatings were obtained with simultaneous sputtering of Ti and W targets. The microstructure, composition, and electrochemical properties were investigated by techniques such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM), electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization. XRD results shower a mix of binary TiN and W2N structures in the Ti-W-N layer, a ternary phase in Ti-W-C layers, in addition of a quaternary phase of Ti-W-CN in the multilayers. The analysis of the XPS demonstrated that the atomic concentration of Ti was more significant than W in the Ti-W-N and Ti-W-C layers. The lowest corrosion rate (0.19 mm/year−1) and highest impedance (~10 kΩ·cm2) out of all coatings were found in n = 40 bilayers. In the simulation of equivalent electrical circuits, it was found that the Ti-W-N coating presented three processes of impedance (Pore resistance + Coating + Inductance). However, the multilayer (n = 40) system presented a major dielectric constant through the electrolyte adsorption; therefore, this caused an increase in the capacitance of the coating. Full article
(This article belongs to the Special Issue Advances in Hard Coatings: Production, Properties and Applications)
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11 pages, 1923 KiB  
Article
Ultra-Short Pulse HiPIMS: A Strategy to Suppress Arcing during Reactive Deposition of SiO2 Thin Films with Enhanced Mechanical and Optical Properties
by Vasile Tiron, Ioana-Laura Velicu, Teodora Matei, Daniel Cristea, Luis Cunha and George Stoian
Coatings 2020, 10(7), 633; https://doi.org/10.3390/coatings10070633 - 30 Jun 2020
Cited by 18 | Viewed by 5533
Abstract
In this contribution, based on the detailed understanding of the processes’ characteristics during reactive high-power impulse magnetron sputtering (HiPIMS), we demonstrated the deposition of silicon oxide (SiO2) thin films with improved optical and mechanical performances. A strategy for stabilizing the arc-free [...] Read more.
In this contribution, based on the detailed understanding of the processes’ characteristics during reactive high-power impulse magnetron sputtering (HiPIMS), we demonstrated the deposition of silicon oxide (SiO2) thin films with improved optical and mechanical performances. A strategy for stabilizing the arc-free HiPIMS of Si target in the presence of oxygen was investigated. Arcing was suppressed by suitable pulse configurations, ensuring good process stability without using any feedback control system. It was found that arcing can be significantly alleviated when ultra-short HiPIMS pulses are applied on the target. The optical and mechanical properties of SiO2 coatings deposited at various pulsing configurations were analyzed. The coatings prepared by ultra-short pulse HiPIMS exhibited better optical and mechanical performance compared to the coatings prepared by long pulse HiPIMS. The optimized SiO2 coatings on quartz substrates exhibited an average transmittance of 98.5% in the 190–1100-nm wavelength range, hardness of 9.27 GPa, hardness/Young’s modulus ratio of 0.138, and critical adhesion load of 14.8 N. The optical and mechanical properties are correlated with the film morphology, which is inherently related to energetic conditions and process stability during film growth. Full article
(This article belongs to the Special Issue Advances in Hard Coatings: Production, Properties and Applications)
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15 pages, 3730 KiB  
Article
Room and High Temperature Tribological Behaviour of W-DLC Coatings Produced by DCMS and Hybrid DCMS-HiPIMS Configuration
by Manuel Evaristo, Filipe Fernandes and Albano Cavaleiro
Coatings 2020, 10(4), 319; https://doi.org/10.3390/coatings10040319 - 27 Mar 2020
Cited by 41 | Viewed by 4090
Abstract
Carbon-based coatings are used in many applications, particularly in sliding contacts to reduce friction and wear. To improve the tribological properties, these coatings are usually alloyed with metals; W is one of the most used since it helps improve the tribological performance at [...] Read more.
Carbon-based coatings are used in many applications, particularly in sliding contacts to reduce friction and wear. To improve the tribological properties, these coatings are usually alloyed with metals; W is one of the most used since it helps improve the tribological performance at high temperatures. In this work, we compared the tribological performance of Diamond-Like Carbon alloyed with tungsten (DLC-W) films deposited by direct current magnetron sputtering (DCMS) with films deposited in a hybrid configuration DCMS + high power impulse magnetron sputtering (HiPIMS). The DLC-W coatings were produced with approximately the same W content. One hydrogenated film was deposited with the hybrid configuration for comparison purposes. Microstructure, structure, mechanical properties, and tribological behaviour were used to compare the coatings. All the films displayed a low-order structure of tungsten carbide embedded in an amorphous carbon matrix. The use of the hybrid HiPIMS/DCMS results in coatings with more compact morphologies due to the high ionization fraction of the species produced on the W target (W and Ar ionized species), which primarily will oppose the shadowing effect as the ions will reach the substrate at angles close to 90°. HiPIMS non-hydrogenated film is the more tribological, performing either at room or high temperature (150 °C) due to the much more compact morphology, which avoids the detachment of hard W-C particles, which are responsible for more efficiently scratching the film surface. Experiments revealed that wear behaviour in all the films is governed by the contact of the tribolayer formed on the counterpart composed of W–C, C and W–O against the surface of the film. Full article
(This article belongs to the Special Issue Advances in Hard Coatings: Production, Properties and Applications)
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15 pages, 4747 KiB  
Article
Comparative Study of DC and RF Sputtered MoSe2 Coatings Containing Carbon—An Approach to Optimize Stoichiometry, Microstructure, Crystallinity and Hardness
by Talha Bin Yaqub, Todor Vuchkov, Pedro Sanguino, Tomas Polcar and Albano Cavaleiro
Coatings 2020, 10(2), 133; https://doi.org/10.3390/coatings10020133 - 3 Feb 2020
Cited by 22 | Viewed by 4215
Abstract
Low stoichiometry, low crystallinity, low hardness and incongruencies involving the reported microstructure have limited the applicability of TMD-C (Transition metal dichalcogenides with carbon) solid-lubricant coatings. In this work, optimized Mo–Se–C coatings were deposited using confocal plasma magnetron sputtering to overcome the above-mentioned issues. [...] Read more.
Low stoichiometry, low crystallinity, low hardness and incongruencies involving the reported microstructure have limited the applicability of TMD-C (Transition metal dichalcogenides with carbon) solid-lubricant coatings. In this work, optimized Mo–Se–C coatings were deposited using confocal plasma magnetron sputtering to overcome the above-mentioned issues. Two different approaches were used; MoSe2 target powered by DC (direct current) or RF (radio frequency) magnetron sputtering. Carbon was always added by DC magnetron sputtering. Wavelength dispersive spectroscopy displayed Se/Mo stoichiometry of ~2, values higher than the literature. The Se/Mo ratio for RF-deposited coatings was lower than for their DC counterparts. Scanning electron microscopy showed that irrespective of the low carbon additions, the Mo–Se–C coatings were highly compact with no vestiges of columnar growth due to optimal bombardment of sputtered species. Application of substrate bias further improved compactness at the expense of lower Se/Mo ratio. X-ray diffraction, transmission electron microscopy, and Raman spectroscopy confirmed the presence of MoSe2 crystals, and (002) basal planes. Even very low carbon additions led to an improvement of the hardness of the coatings. The work reports a comparison between RF and DC sputtering of MoSe2 coatings with carbon and provides a guideline to optimize the composition, morphology, structure, and mechanical properties. Full article
(This article belongs to the Special Issue Advances in Hard Coatings: Production, Properties and Applications)
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