Conventional Coatings Research Articles

Self-Healable Superomniphobic Surfaces for Corrosion Protection.

Corrosion-protective surfaces are of the utmost relevance to ensure long-term stability and reliability of metals and alloys by limiting their interactions with corrosive species, such as water and ions. However, their practical applications are often limited either by the inability to repel low surface tension liquids such as oils and alcohols or by poor mechanical durability. Here, a superomniphobic surface is reported that can display very high contact angles for both high and low surface tension liquids as well as for concentrated acids and bases. Such extreme repellency allowed for approximately 20% of the corrosion rate compared to the conventional superhydrophobic corrosion protective coatings. Furthermore, the superomniphobic surface can autonomously repair mechanical damage at an elevated temperature (60 °C) within a short period of time (60 s), and the surface can restore its intrinsic corrosion protection performance. Such superomniphobic surfaces thus offer a wide range of potential applications, including pipelines, with sustainable corrosion protection and rust inhibitors for steel in reinforced concrete.

Thermal damage of conventional hard chromium coatings on 416 stainless steel

Hard chromium coatings have a wide range of application due to their resistance to corrosion and wear. In the present work, chromium was commercially electroplated on 416 stainless steel. Thermal damage of the chromium coatings was investigated as a function of heating temperature and cooling rate. Specimens were heated to various temperatures and then cooled down either by air cooling or water quenching. An optical microscope was employed to assess the damage due to heating. No thermally-induced cracks were observed for specimens heated up to 600 °C. At 800 °C and higher, thermally-induced cracks were observed and increased with increasing temperature. Cracks generated during heating were likely due to thermal expansion coefficient mismatch between the coating and the substrate. Heating was found to significantly alter the residual stresses state in the coating. Residual stress increased when heated to 600 °C and dropped at 1000 °C. Spherical indentations were employed to evaluation the effects of heating on the mechanical damage of the coatings. Ring, cone, surface radial and radial bend cracks were observed on as-deposited and heated specimens up to 600 °C. Specimen heated up to 800 °C showed partially developed ring and radial bend cracks, while specimen heated up to 1000 °C displayed no indentation related cracks.

Microstructure and wear characterization of AA2124/4wt.%B4C nano-composite coating on Ti−6Al−4V alloy using friction surfacing

Abstract This work is focused on developing AA2124/4wt.%B 4 C nano-composite coatings on Ti−6Al−4V using friction surfacing to improve the wear resistance. The composite was produced using conventional stir casting method and coatings were laid using an indigenously-developed friction surfacing machine. The rotational speed of the mechtrode was varied. The microstructure of the composite coating was observed using conventional and advanced microscopic techniques. The sliding wear behavior was evaluated using a pin-on-disc apparatus. The coating geometry (thickness and width) increased with increased rotational speed. The interface was straight without thick intermetallic layer. Homogenous distribution of nano B 4 C particles and extremely fine grains was observed in the composite coating. The interfacial bonding between the aluminum matrix and B 4 C particles was excellent. The composite coating improved the wear resistance of the titanium alloy substrate due to the reduction in effective contact area, lower coefficient of friction and excellent interfacial bonding.

Assessment of Tribological Properties of Low Friction Thin Layers Produced by Vacuum Methods

Low friction thin layers are an excellent alternative for conventional coatings. They provide increased life of the elements, to which they were applied, due to enhancing the hardness or chemical and electrochemical resistance. They help to avoid the cracks, oxidation, as well as possible structural changes during the element's work. However, they primarily improve tribological properties by increasing wear resistance and reducing the friction. This also applies to components operating under variable conditions such as load, speed, temperature. The presented article analyzes the properties of various low-friction thin layers deposited by vacuum methods on the steel substrates. DLC, TiC, MoS2, CrCN thin layers were chosen, as they achieve the lowest possible coefficient of friction. In the framework of this work the measurements of adhesion of the investigated layers to the substrate as well as the friction coefficient, chemical analysis, microstructure and topographic analysis of the low-friction layers were carried out.

Plasma-Sprayed Bioactive Ceramic Coatings with High Resorption Resistance Based on Transition Metal-Substituted Calcium Hexaorthophosphates.

Calcium (titanium, zirconium) hexaorthophosphates with a [NZP] (sodium zirconium phosphate) structure belonging to the NaSiCon (Na Superionic Conductor) family were deposited by atmospheric plasma spraying onto the surfaces of Ti6Al4V substrates. (NaSiCon sensu strictu refers to solids with the chemical formula Na1+xZr2SixP3−xO12, 0 < x < 3. In a broader sense, it is also used for similar compounds where Na, Zr and/or Si are replaced by isovalent elements). The microstructure of the coatings revealed the incongruent melting of the precursor material as ascertained by electron probe microanalysis (EPMA). The adhesion of the coatings to the substrate surface was within the limits specified for biomedical coatings. The solubility of the coatings was tested by immersion in 0.2 molar tris–hydroxymethyl–amino–methane–HCl (TRIS–HCl) buffer and found to be at least one order of magnitude lower than that of conventional hydroxylapatite coatings deposited under comparable conditions. In vitro biocompatibility tests with primary rat bone marrow cells (BMCs) showed a substantial cell proliferation in the presence of fetal bovine serum. Animal tests confirmed that coatings based on calcium (titanium, zirconium) hexaorthophosphates applied to Ti6Al4V rods implanted in the femoral medulla of sheep led to the strong neoformation of dense bone at a stable interface implant-bioceramic coating without coating delamination. Hence, based on their multifarious advantageous properties in the biomedical context, CaTi4-xZrx(PO4)6 ceramics may be considered the ‘Sleeping Beauty’ of osseoconductive coatings for the stem of hip endoprostheses and dental root implants, osteosynthetic fixation devices, and bioelectric devices including bone growth stimulators.

Physical-Mechanical and Antifungal Properties of Pectin Nanocomposites / Neem Oil Nanoemulsion for Seed Coating

Biodegradable polymers, when reinforced with nanostructures, are considered good sustainable coatings and viable alternatives to replace conventional coatings. In addition, biopesticides are also considered safe, biodegradable and environmentally friendly; therefore there is a growing interest in nanoemulsions based on phytochemical mixtures. In this context, the aim of this study is to aggregate Neem oil nanoemulsions and pectin matrices to produce nanocomposite films, as well as evaluate the nanoemulsions effect on the film properties for coating soybean seeds. Nanoemulsions were characterized assessing their average diameter and stability, while the nanocomposite antifungal, morphology, mechanical and barrier properties were analyzed. In general, the nanoemulsions had an average diameter close to 59 ± 0.61 nm, showed good stability and its addition improved film mechanical properties: reduced stiffness, resistance, and water vapor permeability, and increased extensibility. In addition, Neem oil provided antifungal properties against Aspergillus Flavus and Penicillium Citrinum. The seed coatings promoted a positive effect on the germination process of soybean seeds. Thus, antifungal nanocomposite films from renewable sources were successfully produced. The fungicidal inhibition of Neem oil as a nanoemulsion makes these new materials promising for the production of seed coatings.

A Brief Overview on the Anticorrosion Performances of Sol-Gel Zeolite Coatings

Research activity concerning nanoporous zeolites has grown considerably in recent decades. The structural porosity of zeolites provides versatile functional properties such as molecular selectivity, ion and molecule storage capacity, high surface area, and pore volume which combined with excellent thermal and chemical stability can extend its application fields in several industrial sectors. In such a context, anti-corrosion zeolite coatings are an emerging technology able to offer a reliable high performing and environmental friendly alternative to conventional chromate-based protective coatings. In this article, a focused overview on anti-corrosion performances of sol-gel composite zeolite coatings is provided. The topic of this review is addressed to assess the barrier and self-healing properties of composite zeolite coating. Based on results available in the literature, a property–structure relationship of this class of composites is proposed summarizing, furthermore, the competing anti-corrosion active and passive protective mechanisms involved during coating degradation. Eventually, a brief summary and a future trend evaluation is also reported.

Custom-Made Chemically Modified Graphene Oxide to Improve the Anti-Scratch Resistance of Urethane-Acrylate Transparent Coatings

In this work, a thermoset ultraviolet (UV)-cured polyurethane-acrylate resin was doped with different chemically-modified graphene obtained from a commercial graphene oxide (GO): as-received GO, chemically reduced GO (rGO), GO functionalized with vinyltriethoxysilane (VTES) (GOvtes), and GO functionalized with VTES and subsequently reduced with a chemical agent (rGOvtes). Modified graphene was introduced in the oligomer component via solvent-assisted process using acetone, which was recovered after completion of the process. Results indicate that the GO-doped oligomers produce cured coatings with improved anti-scratch resistance (above the resistance of conventional coatings), without surface defects and high transparency. The anti-scratch resistance was measured with atomic force microscopy (AFM). Additionally, results are presented in terms of Wolf–Wilburn scale, a straightforward method widely accepted and employed in the coating industry.

Nanocrystalline Cermet Coatings for Erosion–Corrosion Protection

The processing techniques, microstructural characteristics, and erosion corrosion behaviour of Cr3C2–NiCr and tungsten carbide (WC)-based cermet coatings are reviewed in this work. Conventional and nanocrystalline Cr3C2–NiCr and WC-based cermet coatings are generally synthesized using thermal spray technique. The wear, erosion, and corrosion protection ability of conventional and nanocermet coatings are compared based on available literature. In Cr3C2–NiCr coatings, the corrosion resistance is offered by NiCr metal matrix while the wear resistance is provided by the carbide ceramic phase, making it suitable for erosion–corrosion protection. The nanocrystalline cermet coatings exhibits better erosion–corrosion resistance as compared to the conventional coatings. The nanocrystalline coatings reduces the erosion–corrosion rate significantly compared to conventional coatings. It is attributed to the presence of the protective NiCr metallic binder that allows easier and faster re-passivation when the coating is subjected to wear and the fine-grain structure with homogeneous distribution of the skeleton network of hard carbide phases. In addition, corrosion-accelerated erosion dominates the reaction mechanism of erosion–corrosion and, therefore, higher hardness, strength, and better wear resistance of nanocermet coating along with its faster repassivation kinetics accounts for improved corrosion resistance as compared to conventional coatings.

Thin Biobased Transparent UV-Blocking Coating Enabled by Nanoparticle Self-Assembly.

A waterborne, UV-blocking, and visually transparent nanocomposite coating was formulated with ZnO nanoparticles and 2-hydroxyethyl cellulose (HEC). The coating is highly effective (<5% UV and ∼65% visible transmittance), and the film thickness (0.2-2.5 μm) is ∼100 times thinner than the conventional coatings of similar UV-blocking performance. The superior properties are due to the fractal structures of ZnO nanoparticles assembled within the HEC matrix, revealed by scanning electron microscopy and small-angle X-ray scattering (SAXS). Changing the binder to 2-hydroxyethyl starch (HES) diminishes the UV-blocking performance, as ZnO nanoparticles form dense globular aggregates, with an aggregation number measured by SAXS 3 orders of magnitude larger than the HEC coating. Since HEC and HES share the same repeating glucose unit in the polymer backbone, it suggests that the conformational characteristics of the binder polymer have a strong influence on the nanoparticle aggregation, which plays a key role in determining the optical performance. Similar structures were achieved with TiO2 nanoparticles. This study not only offers a cost-effective and readily scalable method to fabricate transparent UV-blocking coating but also demonstrates that the unique fractal aggregation structures in a nanocomposite material can provide high performance and functionality without fully dispersing the nanoparticles.

Thermal Shock and Oxidation Stability Tests to Grade Plasma Sprayed Functionally Gradient Thermal Barrier Coatings

Functionally graded layers in thermal barrier coatings reduce the stress gradient between the overlaid ceramic coatings and the underlying metallic component. Introduced to alleviate early onset of spallation of the coating due to thermal expansion mismatch, this facilitates improvement in the life of the component. Conventional thermal barrier coatings typically comprise of duplex layers of plasma sprayed 8% yttria stabilized zirconia (ceramic) coatings on bond coated (NiCrAlY) components/substrates (Inconel 718 for example). This work highlights the superiority of plasma sprayed coatings synthesized from blends of the intermetallic bond coat and ceramic plasma spray powders on Inconel 718 substrates in three-layer configuration over the duplex layered configuration. Assessed through (a) thermal shock cyclic tests (at 1200oC and 1400oC) in laboratory scale basic burner rig test facility and (b) oxidation stability test in high temperature furnace (at 800oC and 1000oC) the functionally graded coatings of certain configurations exhibited more than double the life of the conventional 8% yttria stabilized zirconia duplex (double layer) coatings. Micro- and crystal structure analysis support the findings and results are detailed and discussed.

Evaluating the efficiency of utilizing selectively optimized metamaterial nanostructures for passive radiative cooling of satellites and components

The need for efficient, smart radiators and thermal control technologies will be imperative to ensure the longevity of satellites and for carrying out temperature sensitive operations in space. Advancement in nanofabrication techniques has brought about the ability to create metamaterial nanostructures and selectively control their optical properties so that they reflect better in the visible spectrum and strongly emit in the infrared spectrum, which allows for better cooling. This meta-analysis looks at contemporary research that has utilised metamaterial nanostructures for passive radiative cooling attempting to identify the cooling trends among these structures. The absorbance, emissivity and reflection spectra of these structures are compared, and their effectiveness compared to conventional coolant coatings is critiqued upon. The defining thermodynamic parameters for this study were radiative cooling power and temperature reduction. Through inductive reasoning, we predict that the emissivity in the infrared of a pyramidal layered structure of Al2O3, TiO2 and SiO2 can outperform current material choices. Improving efficiency with the prediction outlined can provide increased radiative cooling.&#x0D; Keywords: Passive radiative cooling; thermal radiation; metamaterials; broadband optical filters; selective absorption and emission; two-dimensional thin film coatings; nanophotonic structures

Structural characteristics and high-temperature oxidation behavior of HVOF sprayed nano-CeO2 reinforced NiCoCrAlY nanocomposite coatings

Nano-CeO2 reinforced NiCoCrAlY nanocomposite coatings were applied by high velocity oxygen fuel spraying (HVOF) method. To prepare nano-CeO2 reinforced NiCoCrAlY nanocomposite powder feedstock, the high-energy planetary ball-milling process was used. The mixture ratios for nanocomposite coatings were selected 0.5, 1 and 2 wt% of nano-CeO2. For the measurement of oxidation kinetics, free-standing coating specimens were exposed to air at 1273 K and the thickness of the thermally grown oxide (TGO) scale was measured and analyzed at several exposure times. The microstructure of the coatings before and after oxidation test as well as prepared feedstock powders was evaluated by field emission scanning electron microscope (FESEM). In addition, the microhardness of different composition of nano-CeO2 reinforced NiCoCrAlY nanocomposite coatings were obtained and compared with the conventional NiCoCrAlY coating. The results indicated that the nanocomposite coating of NiCoCrAlY-1 wt% nano-CeO2 had better oxidation behavior. The mentioned coating also had higher hardness and lower porosity content in comparison with all other types of conventional and nanocomposite NiCoCrAlY coatings.

Micro‐cone textures for improved light in‐coupling and retroreflection‐inspired light trapping at the front surface of solar modules

AbstractMicron‐scale textures at the front surface of solar modules have been reported to improve the current generation by both enhancing light in‐coupling as well as by reducing light out‐coupling via back‐reflection, similar to the retroreflective effect. Whereas the general working principle and advantages of these textures have been described previously, here, the interplay of the reflection properties of different substrates with the enhancement effects is analyzed for textures of conical geometry. The study takes into consideration the incident light of arbitrary angle of incidence as well as the overall energy yield. Supported by optical simulations, periodic micro‐cone textures were optimized and prototyped based on direct laser writing and a scalable replication process. Micron‐scale textures with cones of various aspect ratios were examined on mono‐crystalline silicon (c‐Si) solar cells; an optimum aspect ratio of 0.73 was identified. This moderate aspect ratio is suitable for large‐scale replication, while showing near‐zero surface reflection and excellent light trapping. An increase in energy yield of up to 8% was calculated for the case of micro‐cone textures at the front surface of commercial alkaline‐etched c‐Si solar cells. Moreover, the excellent optical properties of the micro‐cone textures were highlighted by improving the power conversion efficiency (PCE) of a Cu(In,Ga)Se2 (CIGS) thin‐film solar cells from 20.2% to 20.9%. A comparable PCE improvement has been achieved by conventional MgF2 antireflection coatings, but the angular stability and in turn the energy yield of the micro‐cone textures is much higher.

Evaluation of roughcast on the adhesion mechanisms of mortars on ceramic substrates

Buildings frequently suffer from the low durability of external coating layers that use mortar. Low durability can be caused by a low efficiency of the adhesion between the coating mortar and the substrate, which may be ceramic, concrete, natural stone or other material. The application of the roughcast is often used empirically to increase adhesion, however, there are few studies to support this practice. The objective of this study was to analyze the adhesion mechanisms between the substrate and the mortar layer, evaluating tensile bond strength implications beyond the microstructure in the transition zone. Two conventional mortar coatings were used, applied on two ceramic substrates fired at 700 °C and 950 °C, besides the application or not of the intermediate adhesion system (roughcast). Both mortars and ceramic blocks were characterized according to the main testing standards. The tensile bond strength was performed for all the combinations proposed. The heat of hydration of samples of the roughcast was also analyzed. The results indicated that the adoption of the intermediary adhesion mechanisms between the ceramic substrate and the mortar, referred to as “Roughcast”, significantly increased the tensile bond strength and altered the type of rupture in the test. The composition of the mortar as well as the firing temperature of the ceramic substrate also influenced the strength conditions.

TiO2–x-Enhanced IR Hot Carrier Based Photodetection in Metal Thin Film–Si Junctions

We investigate titanium nitride (TiN) thin film coatings on silicon for CMOS-compatible sub-bandgap charge separation upon incident illumination, which is a key feature in the vast field of on-chip photodetection and related integrated photonic devices. Titanium nitride of tunable oxidation distributions serves as an adjustable broadband light absorber with high mechanical robustness and strong chemical resistivity. Backside-illuminated TiN on p-type Si (pSi) constitutes a self-powered and refractory alternative for photodetection, providing a photoresponsivity of about ∼1 mA/W at 1250 nm and zero bias while outperforming conventional metal coatings such as gold (Au). Our study discloses that the enhanced photoresponse of TiN/pSi in the near-infrared spectral range is directly linked to trap states in an ultrathin TiO2–x interfacial interlayer that forms between TiN and Si. We show that a pSi substrate in conjunction with a few nanometer thick amorphous TiO2–x film can serve as a platform for photocurrent...

Impedance-Near-Zero Acoustic Metasurface for Hypersonic Boundary-Layer Flow Stabilization

The laminar-turbulent transition in hypersonic flows is associated with the acoustic-wave-like Mack second mode. Understanding the physics and suppression of this mode is not only of basic scientific interest, but is also essential to future high-speed transportation. This study reveals that, counterintuitively, an almost nondissipative boundary with near-zero surface acoustic impedance can suppress this mode, and performs even better than conventional dissipation-based ultrasonic absorptive coatings. A practical realization, based on an acoustic metasurface, is furthermore presented, pointing toward full control of the hypersonic boundary-layer transition. Full speed ahead!

High-temperature oxidation and wear resistance of WC-Co based coatings with WB addition

A new type of WC-based coating with high oxidation- and wear-resistance at elevated temperature was fabricated by thermal spraying the pre-treated WC-Co powder doped with WB. Addition of WB led to in situ formation of WCoB, which acted as a substitute for Co in the powders and the resultant coatings. It was shown by thermal analysis that WCoB has obviously higher oxidation resistance at high temperatures than that of WC and Co. Thus, the oxidation of the WC-WCoB coating was mainly initiated from WC, rather than from Co in the conventional WC-Co coatings. Most of WCoB was preserved in the coating after high-temperature wear tests. Particularly, with an addition of 40 wt.% WB, the wear rates of the WC-Co coating were dramatically decreased by 90% and 77% at the room and elevated temperatures, respectively.

Anticipatory Life Cycle Assessment of sol-gel derived anti-reflective coating for greenhouse glass

Environmental analysis should be performed early in the Research and Development (R&D) phases of new technologies to timely determine potential impact and, thereby, include prevention and minimization of unfavorable ecological impact into the innovation process. Here, we demonstrate the application of anticipatory Life Cycle Assessment (LCA) on novel anti-reflective coatings used for greenhouses in the Netherlands. Currently, these coating technologies are developed at the laboratory scale (lab-scale), but they have the potential to be transferred to commercial scale on the short term. What-if scenarios have been used to scale-up the coating production process to pilot and industrial scales. The scenarios were developed by optimizing the laboratory scale coating production parameters. A cradle-to-grave LCA has been done to compare novel coatings with conventional coatings. The functional unit has been defined as the production of 1692.30 kg of tomatoes in greenhouses during 30 years. Results indicate that the novel coating manufactured at industrial scale can compete with conventional coatings in terms of the environmental performance. Furthermore, LCA shows that the novel coating assessed does not bring environmental benefits as compared to employing uncoated glass. However, the use of the glass coating in the greenhouse may bring economic benefits during functional lifetime by means of increased yield of crop (e.g. tomatoes). Different pathways of the technological development of the novel coating have been considered in the sensitivity analysis. The option that includes glass manufacturing in the Netherlands rather than China has led to the best environmental impact results.

Microstructural degradation and interdiffusion behavior of NiAl and Ge-modified NiAl coatings deposited on Alloy 602 CA

Ge-modified and conventional nickel aluminide coatings were deposited on Alloy 602 CA via a single-step slurry manufacturing process and diffusion heat treated at 800 °C for 5 h. The effect of Ge-addition to slurry nickel aluminide coatings was investigated with respect to the microstructural degradation of coatings during isothermal exposure at different temperatures for 100 h. Addition of Ge resulted in the formation of Cr- and Ge-rich precipitates distributed within a β-phase diffusion zone. The inter-diffusion zone (IDZ) consisted of (Ni,Cr)3Ge phase. On the other hand, the employment of pure Al powder led to the formation of Al-rich phases such as ζ and δ near the surface and a strong Al-gradient in the Ge-free β-phase coating. While the extent of microstructural degradation was similar for both coatings after 100 h at 900 °C, exposure at higher temperatures resulted in Kirkendall void formation in the NiAl coating, whereas the initial (Ni,Cr,Fe)3Ge layer transformed into chromium carbides, which in-situ formed a dense interlayer below the aluminide and obstructed interdiffusion in the Ge-containing coating. At 1050 °C, the NiAl coating delaminated due to the severe Kirkendall porosity, which resulted in the oxidation of the interface and nitridation of the alloy and at 1200 °C it led to the complete dissolution of the NiAl coating. The Ge-containing equivalent maintained its integrity and protective behavior in both cases.