Double-layer Coatings Research Articles

Analytical testing and methods for increasing the durability of plunger hydraulic cylinders

The article deals with the important problem of increasing the operating life of roll-balance system plunger hydraulic cylinders and describes the solution to this problem. The durability of plunger hydraulic cylinders determines not only the overall reliability of a hydraulic system, but also the technical and economic efficiency of the whole wide-strip hot rolling mill. Hydraulic cylinder durability depends primarily on the wear resistance of sealing units. Therefore in order to provide an effective approach to increasing the hydraulic cylinder life we have developed an analytical method for determining wear resistance of sealing units (i.e. determining a class or category) and calculating their expected operating life. The method is based on the solution of the fundamental equation of energy and mechanical theory of stationary units wear-fatigue as well as the theoretical provisions describing the strain pattern, initial and limiting values pertaining to plunger cylinders sealing units operating conditions. We have performed a computer experiment to study the behavior of various sealing materials and plungers with different antifriction coatings. As the result of the experiment we have established a number of ways to improve wear resistance of sealing units and durability of hydraulic cylinders by utilizing more durable elastomers and application of double-layer metal-polymer coatings. The most reliable design options of sealing units have been pilot tested and put into operation.

High temperature corrosion resistant coatings for gas flare systems

Gas flaring systems are used in processing plants to eliminate excess gases while serving as a safe pressure-relieving system. These systems are built using 310 stainless steel (SS310) thanks to its mechanical properties and performance under high temperature operating conditions. However, the findings have revealed that SS310 is susceptible to high temperature sulfidation when exposed to a corrosive environment. In the present work, SEM analysis has been conducted to investigate the causes of this type of failure. To tackle this problem, high velocity oxygen fuel (HVOF) thermal spraying was used to deposit a double-layer thermal barrier coatings, including a top yttria stabilized zirconia layer (TBC) and a bond coating CoNiCrAlY layer (BC) on SS310 substrates. The performance of the coatings was tested by exposing it to a high temperature corrosive environment. As a result, an improved resistance to corrosion was observed. This improved performance can be attributed to the absence of tetragonal to monoclinic phase change transformation, and to a decrease in the volume fraction of the monoclinic phase in yttria stabilized zirconia top coat, as indicated by XRD analysis.

Optimal design of quadruple-layer antireflection coating structure for conversion efficiency enhancement in crystalline silicon solar cells

In this paper, we employed a quadruple-layer antireflection coating structure with Al2O3 deposited by DC reactive magnetron sputtering and SiO2 deposited by PECVD (plasma enhanced chemical vapor) deposition as the bottom layer, and proposed an optical optimization program for quadruple-layer antireflection coatings to obtain the structure parameters. In the experiment, two types of optimization quadruple-layer antireflection coatings and typical SiNx:H double-layer antireflection coatings were deposited on the crystalline silicon solar cells. The reflectance of the silicon solar cell with quadruple-layer antireflection coatings was lower than that of the solar cell with SiNx:H double-layer antireflection coatings. These results demonstrated the possibility of substituting quadruple-layer antireflection coatings for double-layer antireflection coatings, as well as the feasibility of the application in photovoltaic.

Design and analysis of light trapping in thin-film gallium arsenide solar cells using an efficient hybrid nanostructure

Light trapping in thin-film solar cells is important for improving efficiency and reducing cost. We propose a hybrid nanostructure based on the anodic aluminum oxide grating and Si 3 N 4 double-layer antireflection coatings combined with Ag nanoparticles to achieve advanced light trapping property in gallium arsenide (GaAs) solar cells with 500-nm thickness. The finite-element method is used to study the relationship between geometrical parameters of hybrid nanostructure and optical characteristics of thin-film GaAs solar cells. The light trapping ability is systematically studied by COMSOL multiphysics. The simulation results show that the hybrid nanostructure can highly increase the light absorption in the wavelengths from 300 to 860 nm. The average absorption in 500-nm-thick GaAs layer is 96.7%. The short circuit current density in 500-nm-thick GaAs layer is 30.2 mA / cm 2 , representing a 58.9% enhancement compared with that ( 19.0 mA / cm 2 ) of the reference cell. Research paves the way for designing highly efficient light trapping structures in thin-film GaAs solar cells.

Preparation of hydrophobic broadband antireflective SiO2 coating on flexible poly(methyl methacrylate) substrates

The double-layer broadband antireflective coatings is crucial for optical devices, e.g. plastic eyeglass lenses and the solar cell system. Among optical situation, the wide wavelength range of light is one of the most demanded. Here we designed a double-layer broadband antireflective (AR) coating with excellent average transmittance at visible region for poly (methyl methacrylate) (PMMA) using thin film design software. A template-free sol–gel process was developed to prepare the SiO2/PMHS coating with ultralow refractive index of 1.13 for top layer. By combining the colloidal silica and poly (methyl hydrogen) siloxane (PMHS), the coating with adjustable refractive index that vary between 1.13 and 1.37 was obtained. The formation mechanism of the ultralow refractive index was proposed. Meanwhile, using PMHS to partially substitute the hydrophilic groups in the coating, the hydrophobicity of the coatings was improved, and its static water contact angle was as high as 126°. Besides, the coating had excellent mechanical property and refractive index of 1.22–1.42 can be well prepared by combining base-catalyzed silica sol and acid-catalyzed silica sol together for bottom layer. Then, using the two raw materials base/acid -catalyzed mix SiO2 sol and the PMHS-SiO2 sol and optimizing the thickness, the prepared double-layer coatings with average transmittance of 99.30% in the 400nm to 800nm wavelength on PMMA substrate was successfully obtained. Those prepared hydrophobic broadband AR coatings represented excellent transmittance and had enormous potential application in the optical devices.

High-performance hybrid coatings based on diamond-like carbon and copper for carbon steel protection

High performance double-layer coatings composed of copper (Cu) interlayer and the Cu/diamond-like carbon (DLC) composite top layer were prepared via magnetron sputtering for protection of carbon steel. The effect of Cu content in the composite layer on the internal stress, contact angle, mechanical, tribological, and corrosion performance of coatings was discussed. Raman spectroscopy confirmed formation of DLC phase. Double-layer Cu-rich coatings exhibited high degree of sp2 carbon clustering, but low diamond-like sp3 bonding. The internal stress of double layer Cu-(Cu/DLC composite) coatings was decreased with Cu content increase. The double-layer Cu-(Cu/DLC composite) thin films with low Cu content did not pose a significant effect on hardness and Young's modulus of the steel substrate, while increase of Cu content enhanced hardness of coating up to 28GPa. The contact angle of steel substrate showed an increase when coated with double-layer Cu-(Cu/DLC composite) thin film, but not that much at such inadequate Cu content. Inclusion of Cu interlayer improved to some extent corrosion properties, where increase of Cu content assisted in protection. Double-layer coatings with the highest performance were compared with single-layer Cu/DLC coating. Elimination of interlayer deteriorated mechanical properties, but increased electrochemical performance, signifying the importance of controlling Cu/DLC composition and Cu interlayer inclusion.

Study on properties of double-layered Ni–P–Cr composite coating prepared by the combination of electroless plating and pack cementation

The method of pack cementation was applied to prepare a chromized layer on the surface of an electroless Ni-P coating to further improve its mechanical property and corrosion resistance, and then the microstructure, chemical composition, and performance of the achieved coatings were investigated. The results indicated that the double-layered Ni-P-Cr composite coating was composed of an inner Ni-P layer and an outer Cr-Ni-P layer. Meanwhile, the thickness of the composite coating become larger and more Cr was found as chromizing time increased. By comparison, the double-layered coating with the chromizing time of 9 h showed a higher hardness than electroless Ni-P coatings and double-layered coatings with a shorter chromizing time. In addition, all the double-layered coatings generally displayed higher corrosion resistance than electroless coatings. Also, increasing chromizing time was beneficial to reduce the defects of the double-layered coatings ascribed to the dense oxide film, and thus improved corrosion resistance.

Electrical characteristics and discharge properties of hybrid plasma electrolytic oxidation on titanium

In this study double-layered plasma electrolytic oxidation coatings are synthesized by coating in two different electrolytes. The effects of electrical properties of first fabricated layer (in first electrolyte) on morphology and properties of total hybrid coating are investigated. First layers were applied in four different basic electrolytes consisting of sodium phosphate/citrate (PC), sodium phosphate/silicate (PS), sodium phosphate/tungstate (PT), and sodium phosphate/aluminate (PA) solutions and the second layer was synthesized in calcium acetate/sodium dihydrogen phosphate hydrate solution. The top surface and cross sectional morphology, composition and corrosion resistance properties were thoroughly studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS) and Mott-Schottky analysis (M-S) methods. It was found that the morphology of the second layer depends on the morphology of the first layer, in terms of shape and size of the pores. However, XRD analysis revealed that there was no significant difference between the compositions of the oxide layers. Application of the second layer over PS sample reduced corrosion resistance from 163 to 95 KΩ cm2 while application of the second layer over PT sample led to increase of corrosion resistance from 31 to 126 KΩ cm2. The constructive or destructive role of two layers was correlated to discharge energy of the second layer which is in turn determined by the required energy for electrical charging of the first layer. The energy of electrical charging of PS and PT samples are 0.04 and 1.17 J, respectively. Therefore, plasma discharges over PS sample have higher energy, compared to PT sample, and as a result, corrosion resistance of barrier layer of PS sample drops. Electrical parameters extracted from M-S tests also verified our theory that donor density and capacitance of the first layer affect discharge behavior and intensity of the second layer. M-S results are also in conformance with evaluated corrosion resistances measured by EIS method.

Improved Mechanical Compatibility and Cytocompatibility of Ta/Ti Double-Layered Composite Coating

In order to improve the mechanical compatibility and cytocompatibility of titanium implants, a composite coating with double layers composed of tantalum and titanium was designed and prepared using plasma spraying technology. In the composite coating, the upper tantalum layer provides a good biocompatibility, and the sublayer of titanium with a porous structure ensures the low elastic modulus. Results show that the fabricated composite coating exhibits a relatively low elastic modulus of 26.7 GPa, which is close to the elastic modulus of human cortical bone. In vitro cytocompatibility evaluation of the composite coating shows that the human bone marrow stromal cells exhibit enhanced adhesion and spreading performance on the double-layered composite coating in comparison with the single-layered titanium coating. In order to eliminate the misgivings of chemical stability of the composite coating in clinical application, electrochemical corrosion of the coating was examined. The results obtained revealed a very weak galvanic corrosion between the tantalum and titanium in the composite coating, which would ensure the safety of the coating in vivo.

Mechanical and thermal properties of double-layer and triple-layer thermal barrier coatings with different ceramic top coats onto polyimide matrix composite

Abstract The present study has discussed the deposited thermal barrier coatings of Cu-Sn/mullite and Cu-Sn/NiCrAlY/YSZ onto a carbon-fiber reinforced bismaleimide (BMI) matrix composite. The thicknesses of the metallic bonding layers and ceramic top coats were 100 µm and 300 µm, respectively. The microstructural, mechanical and thermal properties of the TBC systems were evaluated through x-ray diffraction (XRD), scanning electron microscopy (SEM), tensile adhesion strength testing (TAT), micro-hardness tests, roughness measurement, thermal shock testing and laser flash analysis (LFA). A comparison of the microstructure of the ceramic top coats showed that the YSZ had a homogeneous structure with high crystallinity while the mullite exhibited a multi-phase structure containing mullite, alumina (γ-Al2O3) and amorphous phases. The mullite layer recorded higher residual stress than that the YSZ layer because of its higher Young's modulus and the transformation of the mullite phase to alumina and amorphous phases during deposition. The adhesion strength of the sample with a triple-layer coating was greater than that with a double-layer coating because it exhibited a milder residual stress gradient across the coating. The thermal shock resistance of the triple-layer coated sample was 1.5-fold greater than that of the double-layer coated sample. This could be due to the existence of an oxidation-resistant layer of NiCrAlY and smaller differences in the coefficients of thermal expansion between coating layers in the triple-layer coating system. Thermal conductivity of the triple-layer coating sample was much less than that of the double-layer coating which can increase the operating temperature of the polymeric substrate to a higher value.

Biodegradation of hydroxyapatite coated Rex-734 alloy with silver and selenium/chitosan substitutions: In vitro analysis

Abstract Hydroxyapatite (HA) based bioceramic coatings; such as single-HA, double layered with Silver-Ag, Selenium-Se and Chitosan substitutions on Rex-734 alloy were executed by using the sol-gel method. Surface morphologies and adhesion strengths of the obtained coatings were investigated and coating material weight losses and metal ion releases from the substrates were determined in SBF (Simulated Body Fluid) environment. The coatings were characterized by SEM, EDS, XRD and adhesion strengths between the substrate and bioceramic coatings were measured with adhesion tests. SEM analyses revealed that crack-free surface morphologies were obtained in all bioceramic coatings. The coatings were kept in SBF for immersion periods from one to four weeks and the weight loss (%) in coatings and changes in ion release rates were determined. HA/Ag double layered coatings applied on the Rex-734 alloy increased the adhesion strength and accordingly reduced the weight losses (%) of coating materials in SBF. Se/Chitosan double layered coatings were found to decrease the adhesion strength and increase the weight losses (%) in SBF. Additionally, all bioceramic coatings were observed to prevent the elemental ion release by physical barrier effect for the release of metal ions during the four weeks immersion period in SBF.

Hollow Rodlike MgF2 with an Ultralow Refractive Index for the Preparation of Multifunctional Antireflective Coatings.

Antireflective coatings with superhydrophobic, self-cleaning, and wide-spectrum high-transmittance properties and good mechanical strength have important practical value. In this research, hollow nanorod-like MgF2 sols with different void volumes were prepared by a template-free solvothermal method to further obtain hollow nanorod-like MgF2 crystals with an ultralow refractive index of 1.14. Besides, a MgF2 coating with an adjustable refractive index of 1.10-1.35 was also prepared by the template-free solvothermal method. Then through the combination of base/acid two-step-catalyzed TEOS and hydroxyl modification on the surface of nanosilica spheres, the SiO2 coating with good mechanical strength, a flat surface, and a refractive index of 1.30-1.45 was obtained. Double-layer broadband antireflective coatings with an average transmittance of 99.6% at 400-1400 nm were designed using the relevant optical theory. After the coating thickness was optimized by the dip-coating method, the double-layer antireflective coatings, whose parameters were consistent with those designed by the theory, were obtained. The bottom layer was a SiO2 coating with a refractive index of 1.34 and a thickness of 155 nm, and the top layer was a hollow rodlike MgF2 coating with a refractive index of 1.10 and a thickness of 165 nm. The average transmittance of the obtained MgF2-SiO2 antireflective coatings was 99.1% at 400-1400 nm, which was close to the theoretical value. The hydrophobic angle of the coating surface reached 119° at first, and the angle further reached 152° after conducting surface modification by PFOTES. In addition, because the porosity of the coating surface was only 10.7%, the pencil hardness of the coating surface was 5 H and the critical load Lc was 27.05 N. In summary, the obtained antireflective coatings possessed superhydrophobic, self-cleaning, and wide-spectrum high-transmittance properties and good mechanical strength.

Creation and investigation of chitin/HA double-layer coatings on AZ91 magnesium alloy by dipping method

Abstract

Research and analysis of stress distribution in multilayers of coated tools

Abstract The paper analyses stress distribution in mono/multi-layers of coated tools. It presents an analysis performed on mono-layer, double-layer and multi-layer coatings using numerical models to detect the developed stress distribution and the most stressed zones in these mostly 1–10 micron layers. With such thin layers, experimental detection of stress distribution would be very difficult. Moreover, coating thickness is distributed differently on flat surfaces from on shaped features in coated tools. This non-uniform coating results solely from the technology applied and such uneven deposition of the coating is critical in terms of how the tools themselves function. The paper provides an analysis of stresses in thinner or accumulated layers of coating on the rounded edge, for example in cutting. This analysis was initiated by ProTech Coating Service, Ltd., which manufactures physical vapour deposition coatings for cutting and forming tools.

Fabrication solar cell of $$\hbox {CdTe}_{0.65}\hbox {P}_{0.35}$$ CdTe 0.65 P 0.35 /Si with high efficiency using double-layer antireflection

In this work, solar cells that have double-layer coatings of $$\hbox {CdTe}_{0.65}\hbox {P}_{0.35}$$ /Si with high efficiency have been fabricated. The electrical and optical properties of $$\hbox {CdTe}_{0.65}\hbox {P}_{0.35}$$ /Si as transparent electrodes and antireflection (AR) coatings for Si-based solar cells are studied. This is done by adding fabricated graded refractive index AR coatings using $$\hbox {TiO}_{2}$$ and $$\hbox {SiO}_{2}$$ thin films kept over 80% of transmittance. As the AR coating with graded refractive indices using $$\hbox {TiO}_{2}$$ and $$\hbox {SiO}_{2}$$ layers was applied to generic silicon-based solar cell, the current level increases nearly twice more than that of bare silicon solar cell without AR coatings. The obtained results show that optimized double-layer ARCs can minimize reflectance within the spectral range of $$\sim $$ 400–945 nm, with the latter maintaining this performance over a broader spectrum of 390–1000 nm, in comparison with 45.05% reflectance for the bare solar cell.

Role of lanthanum nitrate in protective performance of PEO/epoxy double layer on AZ31 Mg alloy: Electrochemical and thermodynamic investigations

Abstract Direct current Plasma Electrolytic Oxidation (PEO) in a bath containing lanthanum nitrate, as pretreatment was performed on the AZ31 magnesium alloy. Incorporation of lanthanum in PEO coating was proved by SEM/EDS analysis. Fabricated oxide coatings were sealed with an epoxy layer. The role of lanthanum nitrate in the protective properties of the fabricated double top layer and its effective mechanism were investigated using electrochemical techniques and thermodynamic calculations. The double layer coatings containing lanthanum nitrate provided better protective properties in comparison to La-free double layer coating without any degradation after 42 days of immersion in 3.5% NaCl solution.

3D reticular pomegranate-like CoMn2O4/C for ultrahigh rate lithium-ion storage with re-oxidation of manganese

A three-dimensional (3D) reticular CoMn2O4/C anode with pomegranate-like configuration was synthesized via electrostatic spray deposition technique. It was found that the residual solvents which were generally considered as by-product also acted as a key role in the structure control. By simply varying the feeding rate at a certain temperature in the process, both the structure and the content of carbon in the products were controllable. The percolated reticular structure, accompanying with double-layer carbon coatings in the pomegranate-like construction, were considered to improve the electrochemical stability even for a longtime discharging/charging at high rate. Due to the re-oxidation of Mn during delithiation and the stable 3D reticular structure, the as-prepared 3D reticular anode with pomegranate-like configuration exhibited high specific capacities (1386.6-695.3mAhg−1 at 0.3–15Ag−1) and good cyclic stability (626.3mAhg−1 retained after 600 cycles at 12Ag−1). The approach in this study may provide a new strategy to fabricate other electrode materials based on transition metal oxides for energy storage application.

Double-layer nanoparticle-based coatings for efficient terrestrial radiative cooling

One passive cooling approach is pumping energy to outer space through thermal radiation. Such a radiative cooling mechanism widely exists in nature and is important to maintain the temperature of the earth. However, natural materials generally have poor radiative cooling efficiency. To better utilize the radiative cooling for thermal management applications, the surface should be designed to have a high reflectivity in the solar spectrum and high emissivity in the "sky window" region (8–13µm in wavelength). In this work, we propose and demonstrate a highly scalable nanoparticle-based double-layer coating to achieve such selective radiative properties. Double-layer coatings consisting of a top reflective layer with high solar albedo and a bottom emissive layer are achieved by properly designed TiO2, SiO2, and SiC nanoparticles. These coatings were fabricated on both low- and high-emissivity substrates and their spectral radiative properties were characterized. The coating composed of TiO2 and SiO2 on a reflective substrate has excellent selective emission property for radiative cooling purpose. Under dry air conditions and assuming non-radiative heat transfer coefficient hc=4W/m2K, TiO2+SiO2 and TiO2+SiC can theoretically achieve about 17°C below ambient at night and 5°C below ambient under direct solar radiation (AM1.5). On-site measurements have also been conducted. Under direct solar irradiation, significant temperature reduction was observed for both aluminum and black substrate after the coating was applied. At nighttime, radiative cooling effect can cool the surface to a few degrees below ambient temperature. Although the theoretical cooling under dry weather condition is not observed, the experiment results can be well explained by theoretical calculations with the consideration of high humidity and non-radiative heat transfer. This nanoparticle-based approach can be easily applied to large area, which is a significant step of achieving large scale application of the radiative cooling technology.

Role of internal oxidation on the failure of air plasma sprayed thermal barrier coatings with a double-layered bond coat

Failure of air plasma sprayed (APS) thermal barrier coatings (TBCs) with a double-layered bond coat was investigated. The bond coat consists of a dense layer near the substrate side and a porous layer on the surface. Both were made of NiCoCrAlY alloy and deposited using high velocity oxygen-fuel technique. After thermal cycling, a large amount of internal oxides formed (up to 45% in volume fraction), introducing a significant volume expansion both in in-plane and perpendicular direction in the bond coat. However, the presence of the ceramic top coat can suppress the bond coat in-plane swelling thus lowering the internal oxidation rate. Compared with the fully dense bond coat, the interface roughness of the porous bond coat increases significantly when internal oxidation occurs. There is a strong correlation between the internal oxidation and the interface roughness. In addition, both the beneficial and detrimental effects of internal oxidation on TBC failure were discussed. It is proposed that a fully dense bond coat with a proper surface roughness should have a longer thermal cycling lifetime.

Evaluation of La containing PEO pretreatment on protective performance of epoxy coating on magnesium

Abstract The effect of plasma electrolytic oxidation (PEO) process was studied on magnesium surface preparation for epoxy coating implementation. In addition, the effect of lanthanum nitrate, current density, and process time was studied on the microstructure of the prepared oxide coating surface. Epoxy coating was applied on the substrate prepared by PEO using the electrostatic method. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to study the composition and morphology of PEO coatings, as well as the cross-section of the double-layer coatings. The protective performance of the coatings was evaluated using an electrochemical impedance spectroscopy (EIS) test for 28 days in a 3.5% NaCl solution. Finally, the Pull-off adhesion test was carried out to examine the impact of surface preparation in the improvement of the adhesion strength of the organic coatings. Based on the SEM micrographs, the density of PEO coating increases by adding lanthanum nitrate. Furthermore, the results indicate that preparation of magnesium surface by PEO process contributes to protecting organic coating placed 28 days in the corrosive solution without causing any localized corrosion at the interface of organic coating with PEO coatings.