Coatings — Open Access Journal

In this paper, a comparison of TiO₂ thin films prepared by magnetron sputtering with a continuous and pulsed gas flow was presented. Structural, surface, optical, and mechanical properties of deposited titanium dioxide coatings were analyzed with the use of a wide range of measurement techniques. It was found that thin films deposited with a gas impulse had a nanocrystalline rutile structure instead of fibrous-like anatase obtained with a continuous gas flow. TiO₂ thin films deposited with both techniques were transparent in the visible wavelength range, however, a much higher refractive index and packing density were observed for coatings deposited by the pulsed gas technique. The application of a gas impulse improved the hardness and scratch resistance of the prepared TiO₂ thin films. Full article

Using organic coatings helps to protect PET (polyethylene terephthalates) surfaces, improve surface hardness, scratch resistance, and solvent resistance, prolong the service life of PET film, and to expand their scope of applications. There were some disadvantages, including poor flexibility and impact resistance in high-hardness coatings; organic coatings should also be modified to improve the toughness. Herein, a UV (ultraviolet curing) curable high-hardness organic coating used in PET surface protection was prepared and modified with inorganic nanoparticles, such as OVPOSS (octavinyl-polyhedral oligomeric silsesquioxane). The effects of the categories of nanoparticles on the coating performance were studied. UV-Vis spectra (ultraviolet visible light spectra), FT-IR (Fourier transform infrared spectrometer), TGA (thermogravimetric analysis), DMA (dynamic-mechanical), SEM (field emission scanning electron microscope), and AFM (atomic force microscope) were used to characterize the properties of the coatings. The results showed that the addition of eight-vinyl POSS to the organic coating significantly increased its glass transition temperature (T_g) from 100 to 120 °C, improved its storage modulus from 167.6 MPa to 258.9 MPa, and raised its impact resistance and flexibility. The SEM and AFM images displayed that the eight-vinyl POSS particles were dispersed homogeneously in the coating, arranged in an ordered network, and had good compatibility with organic components. The film displayed excellent properties, including 4 H of the pencil hardness, 100 g cm of impact resistance, excellent flexibility, and 90% of light transmittance, with the addition of 0.3 wt % OVPOSS. TGA analysis revealed that the coating had good thermal stability, with 5% weight loss temperature up to 335 °C. Full article

The technological evolution in the last century also required an evolution of materials and coatings. Therefore, it was necessary to make mechanical components subject to heavy wear more reliable, improving their mechanical strength and durability. Surfaces can contribute decisively to extending the lifespan of mechanical components. Chemical vapor deposition (CVD) and physical vapor deposition (PVD) technologies have emerged to meet the new requirements that have enabled a remarkable improvement in the morphology, composition and structure of films as well as an improved adhesion to the substrate allowing a greater number of diversified applications. Thin films deposition using PVD coatings has been contributing to tribological improvement, protecting their surfaces from wear and corrosion, as well as enhancing their appearance. This process can be an advantage over other processes due to their excellent properties and environmental friendly behavior, which gives rise to a large number of studies in mathematical modelling and numerical simulation, like finite element method (FEM) and computational fluid dynamics (CFD). This review intends to contribute to a better PVD process knowledge, in the fluids and heat area, using CFD simulation methods focusing on the process energy efficiency improvement regarding the industrial context with the sputtering technique. Full article

When a droplet impinges on a solid surface, its kinetic energy is mainly converted to capillary energy and viscous dissipation energy, the ratio of which depends on the wettability of the target surface and the liquid properties. Currently, there is no experimental or theoretical evidence that suggests which types of liquids exhibit the capillary energy-dominated impingement behavior. In this paper, we reported the droplet impingement behavior for a wide range of liquid viscosities, surface tensions and target surface wettabilities. Then, we showed that a recently developed energy balance equation for the droplet impingement behavior can be universally employed for predicting the maximum spreading contact area diameter of a droplet for Newtonian liquids in deposition process by modelling the droplet surface deformation. Subsequently, applicability limitations of recent existing models are discussed. The newly developed model demonstrated that the capillary energy-dominated impingement behavior can be observed at considerably low viscosities of liquid droplets such as that of the superfluid of liquid helium. Full article

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 aims to model two-dimensional, incompressible asymmetric peristaltic propulsion coated with Synovial fluid (“non-Newtonian model”) with mass transport. Due to the coating of the same base-fluid at the surface of the channel, the boundaries become non-porous and exert no slip on the fluid particles. Two illustrative models for the viscosity, namely, shear-thinning (Model 1) and shear-thickening (Model 2), are considered, which reveal the presence and integrity of coating. The perturbation method has been applied to linearize the complicated differential equations. Model 1 predicted higher viscosity values and more significant non-Newtonian behavior than Model 2. It is also observed that the shear-thinning model behaved in quite the opposite manner for the shear thickening model. The converse behavior of Models 1 and 2 occurs due to a curvature of the flow domain. Moreover, Model 1 is not able to capture the correct exponential viscosity dependence on concentration for the whole range of shear rates. On the other hand, the second model shows a strong relationship with accurate power. Solutions are attained for velocity field, concentration profile, and pressure gradient. The novelty of all the essential parameters is analyzed through graphical results. Furthermore, streamlines are also drawn to determine the trapping mechanism. The present analysis is beneficial in the study of intrauterine fluid dynamics; furthermore, it is applicable in vivo diagnostic; drug delivery; food diagnostics; protein chips; and cell chips and packaging, i.e., smart sensors. Full article

Further strong growth of solar energy conversion based on PV (photovoltaic) technology requires constant improvement to increase solar cell efficiency. The challenge in front-side metallization of Si-solar cells is to print uniform fine lines with a high aspect ratio to achieve higher efficiencies simultaneously with a reduced consumption of raw materials. An in-depth understanding of the relationship between paste composition, rheology and screen-printed line morphology is essential. Three model pastes with similar silver content and corresponding vehicles differing in their thixotropic agent content were investigated. Rheological properties (yield stress, viscosity, wall slip velocity, structural recovery, and fracture strain) were determined using steady and oscillatory shear, as well as elongational flow rheometry. Pastes were screen-printed at various speeds through a layout screen including line widths between 20 and 55 µm. Dried fingers were analyzed with respect to line width, aspect ratio (AR) and cross-sectional area. Our investigations reveal that minor changes of thixotropic agent result in substantial variations of the paste’s flow properties. However, this only weakly affects the line morphology. Irrespective of printing speed or finger opening, AR is slightly increasing; i.e., the screen-printing process is robust against changes in paste rheology. Full article

Automotive conversion coatings consist of layers of materials that are chemically applied to the body structures of vehicles before painting to improve corrosion protection and paint adhesion. These coatings are a consequence of surface-based chemical reactions and are sandwiched between paint layers and the base metal; the chemical reactions involved distinctly classify conversion coatings from other coating technologies. Although the tri-cationic conversion coating bath chemistry that was developed around the end of the 20th century remains persistent, environmental, health, and cost issues favor a new generation of greener methods and materials such as zirconium. Environmental forces driving lightweight material selection during automobile body design are possibly more influential for transitioning to zirconium than the concerns regarding the body coating process. The chemistry involved in some conversion coatings processing has been known for over 100 years. However, recent advances in chemical processing, changes in the components used for vehicle body structures, environmental considerations and costs have prompted the automobile industry to embrace new conversion coatings technologies. These are discussed herein along with a historical perspective that has led to the use of current conversion coatings technologies. In addition, future directions for automobile body conversion coatings are discussed that may affect conversion coatings in the age of multi-material body structures. Full article

Linseed oil undergoes an oxidative drying reaction upon exposure to air, resulting in a soft film. The reaction conversion after 48 h reached 88% and 59% when it reacted at room temperature and −20 °C, respectively. Linseed-oil-loaded microcapsules were prepared using a urea-formaldehyde polymer as the shell wall material. The microcapsules were integrated into a commercially available protective coating formulation to prepare self-healing coating formulations with different capsule loadings. The coating formulations were applied on mortar specimens to prepare self-healing coatings. The effect of capsule loading on adhesion strength of the self-healing coating was studied. The self-healing function of the coating was investigated by SEM, a water sorptivity test and an accelerated carbonation test. Successful self-healing was demonstrated for both scratch and crack damage in the coatings. Low-temperature self-healing was demonstrated with a saline solution sorptivity test conducted at −20 °C. The linseed-oil-based microcapsule-type self-healing coating system is a promising candidate as a protective coating for cementitious materials. Full article

In this work, the entire manufacturing process of electrostatic supercapacitors using the atomic layer deposition (ALD) technique combined with the employment of nanoporous anodic alumina templates as starting substrates is reported. The structure of a usual electrostatic capacitor, which comprises a top conductor electrode/the insulating dielectric layer/and bottom conductor electrode (C/D/C), has been reduced to nanoscale size by depositing layer by layer the required materials over patterned nanoporous anodic alumina membranes (NAAMs) by employing the ALD technique. A thin layer of aluminum-doped zinc oxide, with 3 nm in thickness, is used as both the top and bottom electrodes’ material. Two dielectric materials were tested; on the one hand, a triple-layer made by a successive combination of 3 nm each layers of silicon dioxide/titanium dioxide/silicon dioxide and on the other hand, a simple layer of alumina, both with 9 nm in total thickness. The electrical properties of these capacitors are studied, such as the impedance and capacitance dependences on the AC frequency regime (up to 10 MHz) or capacitance (180 nF/cm²) on the DC regime. High breakdown voltage values of 60 V along with low leakage currents (0.4 μA/cm²) are also measured from DC charge/discharge RC circuits to determine the main features of the capacitors behavior integrated in a real circuit. Full article

Physical vapour deposition (PVD) is a well-known technology that is widely used for the deposition of thin films regarding many demands, namely tribological behaviour improvement, optical enhancement, visual/esthetic upgrading, and many other fields, with a wide range of applications already being perfectly established. Machining tools are, probably, one of the most common applications of this deposition technique, sometimes used together with chemical vapour deposition (CVD) in order to increase their lifespan, decreasing friction, and improving thermal properties. However, the CVD process is carried out at higher temperatures, inducing higher stresses in the coatings and substrate, being used essentially only when the required coating needs to be deposited using this process. In order to improve this technique, several studies have been carried out optimizing the PVD technique by increasing plasma ionization, decreasing dark areas (zones where there is no deposition into the reactor), improving targets use, enhancing atomic bombardment efficiency, or even increasing the deposition rate and optimizing the selection of gases. These studies reveal a huge potential in changing parameters to improve thin film quality, increasing as well the adhesion to the substrate. However, the process of improving energy efficiency regarding the industrial context has not been studied as deeply as required. This study aims to proceed to a review regarding the improvements already studied in order to optimize the sputtering PVD process, trying to relate these improvements with the industrial requirements as a function of product development and market demand. Full article

Ceramic-to-metal heterojunctions have been established to improve high-temperature stability for applications in aerospace and harsh environments. In this work, we employed low-temperature diffusion bonding to realize an alumina/Cu heterogeneous joint. Using a thin layer of lanthanum-doped titanium (La-doped Ti) to metallize the alumina surface, we achieved the bonding at a temperature range of 250–350 °C. We produced a uniform, thermally stable, and high-strength alumina/Cu joint after a hot-press process in vacuum. Signals from X-ray diffraction (XRD) suggested the successful diffusion of Ti and La into the alumina substrate, as Ti can easily substitute Al in alumina, and La has a better oxygen affinity than that of Al. The transmission electron microscopy and XRD results also showed the existence of Cu_xTi_yO phases without Cu_xTi_y or LaO_x. In addition, the bonding strength of alumina/copper hot-pressed at 250, 300, and 350 °C were 7.5, 9.8 and 15.0 MPa, respectively. The process developed in this study successfully lowered the bonding temperature for the alumina/copper joint. Full article

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

Coatings are normally employed to meet some functional requirements. There is a kind of self-generated coating during use, such as the transfer layer during sliding, which may greatly affect the tribological behavior. Although the transfer layer has aroused much attention recently, the formation of the transfer layer closely depends on the service conditions, which need to be further studied. In this paper, the effects of sliding speed, normal load, and duration of wear test on the transfer layer thickness during friction of Ni/PTFE (Polytetrafluoroethylene) composites were experimentally investigated. The formation mechanism of transfer layer and the relationships between tribological properties and transfer layer thickness were analyzed in detail. It was found that the transfer layer thickness increased with increases of sliding speed and normal load; and after a period of wear test, the transfer layer thickness remained stable. The transfer layer thickness correlates linearly with the friction coefficient and wear volume of the PTFE composites. With the increase of the transfer layer thickness, the friction coefficient decreased, while the wear volume increased, which means that a uniform, thin, and stable transfer layer is beneficial for the reduction of friction and wear of the polymeric composites. Full article

N,N-diethyl-N-(2-methancryloylethy)-N-methylammonium bis(trifluoromethylsulfonyl) imide polymer (DEMM-TFSI) brush coated specimens (substrate: glasses) and a liquid ion type of polymer brush coating were investigated for their antifouling effect on biofilms. Biofilms were produced by two kinds of bacteria, E. coli and S. epidermidis. They were formed on specimens immersed into wells (of 12-well plates) that were filled with culture liquids and bacteria. The biofilm formation was observed. Also, brush coated specimens and glass substrates were investigated in the same way. DEMM polymer brush coated specimens formed more biofilm than PMMA (polymethyl methacrylate) polymer brush coated specimens and glass substrates. A greater amount of polarized components of biofilms was also observed for DEMM polymer brush coated specimens. The polar characteristics could be attributed to the attraction capability of bacteria and biofilms on DEMM polymer brush coated specimens. When considering the ease of removing biofilms by washing it with water, the ionic liquid type polymer brush (coated specimens) could be used for antifouling applications. If an initial antifouling application is needed, then the polar characteristics could be adjusted (design of the components and concentrations of ionic liquids, etc.) to solve the problem. Full article

A chemical conversion coating on 5052 aluminum alloy was prepared by using K₂ZrF₆ and K₂TiF₆ as the main salts, KMnO₄ as the oxidant and NaF as the accelerant. The surface morphology, structure and composition were analyzed by SEM, EDS, FT–IR and XPS. The corrosion resistance of the conversion coating was studied by salt water immersion and polarization curve analysis. The influence of fluorosilane (FAS-17) surface modification on its antifouling property was also discussed. The results showed that the prepared conversion coating mainly consisted of AlF₃·3H₂O, Al₂O₃, MnO₂ and TiO₂, and exhibited good corrosion resistance. Its corrosion potential in 3.5 wt % NaCl solution was positively shifted about 590 mV and the corrosion current density was dropped from 1.10 to 0.48 μA cm⁻². By sealing treatment in NiF₂ solution, its corrosion resistance was further improved yielding a corrosion current density drop of 0.04 μA cm⁻². By fluorosilane (FAS-17) surface modification, the conversion coating became hydrophobic due to low-surface-energy groups such as CF₂ and CF₃, and the contact angle reached 136.8°. Moreover, by FAS-17 modification, the corrosion resistance was enhanced significantly and its corrosion rate decreased by about 25 times. Full article

In this study, a method that can simultaneously separate oil/water mixtures and water-in-oil emulsions were developed. Various substrates (synthetic polymers and stainless steel meshes) were coated by rough hydrophobic polymer films. The prepared materials possessed superhydrophobicity and superoleophilicity. These superhydrophobic sponges can isolate extensive amounts of oil from water when connected to a related vacuum framework. Moreover, the superhydrophobic meshes (SHM) can separate both surfactant-free and -stabilized water-in-oil emulsions via gravity with high separation efficiency (oil purity: >99.99%) and flux (up to 4760 L m⁻² h⁻¹). The extraordinary performance of our materials and their low-energy, efficient, low-cost preparation propose that they have great potential for real-time applications. Full article

The study deals with an increase in the tool life parameter for metal-cutting tools and efficiency of end milling for titanium alloys, due to the use of tools with multilayered composite nano-structured Zr–ZrN–(Zr,Al)N and Zr–ZrN–(Zr,Cr,Al)N coatings, deposited through the technology of the filtered cathodic vacuum arc deposition (FCVAD). The studies included the microstructured investigations using SEM, the analysis of chemical composition (Energy-dispersive X-ray spectroscopy, EDXS), the determination of the value of critical failure force (with the use of scratch testing), and the measurement of the microhardness of the coatings under study. The cutting tests were conducted in end milling of titanium alloys at various cutting speeds. The mechanisms of wear and failure for end milling cutters with the coatings under study were studied in milling. The studies determined the advantages of using a tool with the coatings under study compared to an uncoated tool, as well as to tools with the commercial Ti–TiN coating and the nano-structured Ti–TiN–(Ti,Al)N coating. Adding Cr to the composition of the coating can significantly increase the hardness, while the coating retains sufficient ductility and brittle fracture resistance, which allows for a best result when milling titanium alloys. Full article

A rapid surface-enhanced Raman scattering (SERS) method for Pb²⁺ detection has been developed based on l-cysteine-modified Ag-coated Au nanoparticles with core-shell nanostructure. Specifically, l-cysteine-functionalized Au@Ag core-shell probes bearing Raman-labeling molecules (4-ATP) are used to detect Pb²⁺ upon the formation of nanoparticle aggregates. The proposed SERS-based method shows a linear range between 5 pM and 10 nM, with an unprecedented limit of detection (LOD) of 1 pM for Pb²⁺; this LOD shows the method to be a few orders of magnitude more sensitive than the typical colorimetric approach that is based on the aggregation of noble metal nanoparticles. Real water samples diluted with pure water have been successfully analyzed. This SERS-based assay may provide a general and simple approach for the detection of other metal ions of interest, and so could have wide-ranging applications in many areas. Full article