Double-layer Coatings Research Articles

Corrosion Resistance and Cathodic Disbondment of Epoxy Coating Applied on Zinc Phosphate Conversion Coating Containing Different Amounts of Cobalt Ions

In this study, the influence of phosphate surface pretreatment containing a cobalt ion additive on the anti-corrosion features of epoxy coating was investigated. Phosphate conversion coating (PCC) specimens were prepared. Then the epoxy coating was electrostatically sprayed on the phosphated substrate. The surface morphology and the composition of a zinc phosphate conversion coating with a cobalt ion additive were studied by scanning electron microscopy and X-ray diffraction, respectively. Also, the corrosion protection properties of specimens were studied by the polarization potentiodynamic test in 3.5% NaCl solution. Protective performance of double-layer coatings was studied using electrochemical impedance spectroscopy during 30 days in the mentioned solution. In addition, the impact of phosphate chemical treatment on increasing the adhesion strength of the powder coating was studied using the pull-off adhesion test and cathodic disbodnment test in the same solution. As a result, it was found that the corrosion resistance of PCC containing a cobalt additive enhanced significantly and that the PCC containing 3 g/L Co is less porous than other coatings. Also, the protection properties, adhesion strength, and cathodic disbondment resistance of the double-layer coatings with phosphate pretreatment containing 3 g/L Co increased significantly.

Microstructure and Thermal Properties of Double Rare-Earth Co-doped SrZrO3 Coating by the Solution Precursor Plasma Spray

Double rare-earth (Yb/Gd) co-doped SrZrO3 (SZYG) coatings were prepared by solution precursor plasma spray (SPPS) using an aqueous solution precursor. The SZYG coating is characterized as two phases of SrZrO3 and t-ZrO2 with interpass boundaries structure, nano- and micrometer porosity and through-thickness vertical cracks, analyzed by x-ray diffraction (XRD) and the scanning electron microscopy. XRD results showed that SrZrO3 and t-ZrO2 are very stable after heat treatment at 1400 °C for 360 h due to the doping of rare-earth elements. By comparing the thermal cyclic durability of the SZYG single-layer and the SZYG/YSZ double-layer coatings, the thermal lifetime of the double-layer coating is 650 cycles, which is 40% longer than that of the single-layer coating. The thermal conductivity of the as-sprayed SZYG coating prepared by SPPS is 0.83 W m−1 K−1 at 1000 °C, which is ~ 34% lower than that of SrZrO3 coating prepared by SPPS (~ 1.25 W m−1 K−1, 1000 °C). The superior performance of the SZYG coating is attributed to the co-doping of Yb2O3 and Gd2O3.

Improved corrosion resistance of AZ91D magnesium alloy coated by novel cold-sprayed Zn-HA/Zn double-layer coatings

In order to improve the corrosion resistance and bioactivity of biodegradable Mg alloy substrate, novel Zn-HA/Zn double-layer coatings with different HA/Zn ratios in weight were deposited on AZ91D substrates by cold spraying. Phase compositions and microstructures of as-sprayed coatings and coatings after corrosion tests were analyzed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). Electrochemical corrosion behaviors of both Zn-HA/Zn double-layer coatings were investigated in Hanks’ simulated body fluid using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Results showed that both the pure Zn coating and HA/Zn composite coatings presented the similar phase compositions with their primary powders in addition to Zn oxidizations. Zn powders were plastically deformed and partially oxidized due to its low melting point, while HA powders were mainly crashed into fragments and hill-like splats. Both Zn under layer and HA/Zn upper layer were well bonded and presented dense structures, differences in HA/Zn upper layers were related to the HA/Zn ratios. Potentiodynamic polarization and EIS measurements illustrated that the cold-sprayed Zn-HA/Zn double-layer coatings not only improve the corrosion resistance of Mg alloy substrates, but also enhance its bioactivity due to the HA existed in composite upper layer.

Preparation of Al2O3 coating on TiN coating by polymer-assisted deposition to improve oxidation resistance in coking inhibition applications

In order to inhibit the metal catalytic coking and improve oxidation resistance of single TiN coating, the TiN/Al2O3 double layer coatings were designed as a chemically inert coating for methylcyclohexane supercritical pyrolysis. Internal TiN coatings were prepared by atmospheric pressure chemical vapor deposition using TiCl4–H2–N2 system. The external Al2O3 coatings with different thicknesses were prepared on the TiN surface by polymer-assisted deposition, and the coating with the most suitable thickness was further annealed at different temperatures of 600, 700, 800 and 900 °C. The morphology, elemental and phase composition of TiN/Al2O3 coatings were characterized by SEM, EDX and XRD respectively. The chemical state information of the coating elements was based on Ti 2p, Al 2p core level X-ray photoelectron spectroscopy (XPS) spectra. The results indicated that the external Al2O3 coating will partially peel off at 900 °C annealing temperature. The thermogravimetric analysis results indicated that all TiN/Al2O3 coatings show better oxidation resistance than single-layer TiN coating. The anti-coking test with methylcyclohexane supercritical pyrolysis showed that the TiN/Al2O3 coatings can effectively cover the metal catalytic sites and eliminate metal catalytic coking. However, the acid sites of external Al2O3 coating slightly promoted coking, so the anti-coking ratios of TiN/Al2O3 coatings were smaller than that of TiN. Thus, the addition of external Al2O3 coating can greatly improve the oxidation resistance of TiN coatings with little loss of coking resistance.

GaSe crystals with antireflection coatings for terahertz generation

Single- (SiO2) and double-layer (SiO2/Si3N4) antireflection coatings for wavelength region around 800 nm were deposited by magnetron sputtering onto GaSe crystals. The fabricated AR-coated crystals were used for terahertz generation by optical rectification of femtosecond laser pulses. The increase of generated terahertz power by about 28% for the GaSe crystals with the single-layer antireflection coatings was found as consequence of 27 percent increase in transmission through the front face of the crystal.

Efficiency and durability of g-C3N4-based coatings applied on mortar under peeling and washing trials

Durability of photocatalytic coatings is a major concern in engineering practice. Here, two types of novel visible light-responsive coatings, both consisting of vinyl chloride/vinyl ester/ethylene copolymer (as a binder) and graphitic carbon nitride (g-C3N4) but different in fabrication, are proposed and applied on the mortar surface. The first type is mono-layer coating (MC), where the g-C3N4 suspension containing the binder is directly sprayed on the mortar. The second type is double-layer coating (DC), where the binder layer is applied on mortar surface before spraying the g-C3N4 layer. Results show that the binder addition leads to a good anchorage of the coatings on both MC and DC mortar substrates, along with desirable resistance to peeling and washing, compared to the g-C3N4 coated mortar without the binder. The well-distributed binder in g-C3N4-based coating inevitably decreases the photocatalytic efficiency of the MC mortar due to masking effect of the binder on the coating surface. The DC mortar, on the contrary, takes full advantage of the binder adhesion by inserting a binder layer and therefore holds strong resistance to peeling and washing without compromising its photocatalytic efficiency. The proposed DC technique provides a promising strategy to fabricate highly cost-effective and durable photocatalytic coatings applied on cementitious materials.

Fabrication of Highly Reliable Joint Based on Cu/Ni/Sn Double-Layer Powder for High Temperature Application

Abstract A highly reliable three-dimensional network structure joint was fabricated based on Cu/Ni/Sn powder with double-layer coatings and transient liquid phase bonding (TLPB) technology for high temperature application. The Cu/Ni/Sn joint is characterized by Cu metal particles embedded in the matrix of (Cu,Ni)6Sn5/Ni3Sn4 intermetallic compounds (IMCs), with a low void ratio, and can be reflowed at low temperatures (<260°C), but it can reliably work at a high temperature up to 415°C. Cu/Ni/Sn double-layer powders with different Sn layer and Ni layer thickness were was fabricated and compressed as preform used for TLPB joint bonding. The microstructure and phase composition evolution for Cu/Sn and Cu/Ni/Sn systems during reflow and aging were comparatively studied. Two kinds of interfacial structure designs were made, and corresponding interfacial microscopic morphology was analyzed and compared under once and twice reflow soldering processes. The results indicated that the Sn-coating layer was completely consumed to form (Cu,Ni)6Sn5/Ni3Sn4 IMCs, and the Cu/Ni/Sn joint had a lower void ratio and a higher shear strength than those of Cu/Sn. The mechanism of the Ni-coating layer inhibiting phase transformation was studied. The high reliable three-dimensional network structure joint based on Cu/Ni/Sn double-layer powder was fabricated for high temperature application.

Electromagnetic and microwave absorption properties of coatings based on spherical and flaky carbonyl iron

Single-layer and double-layer polyurethane (PU) matrix coatings containing spherical carbonyl iron (SCI) and flaky carbonyl iron (FCI) were designed and prepared by using a simple and effective manufacturing method, and the thickness of the coatings was kept at 1.5 mm. The complex permittivity, complex permeability and absorption properties of the coatings were investigated in the frequency range of 2–18 GHz. The results indicate that all the single-layer and double-layer coatings exhibit excellent absorption properties and wide absorption bands. By optimizing the filler radio and coating structure, the optimal reflection loss (RL) value can reach − 35 dB at 8.6 GHz and make the widest absorption band reach 15.5 GHz (2.5–18.0 GHz) and 4.9 GHz (10.7–15.6 GHz) for RL < − 5 dB and RL < − 10 dB, respectively. The coatings of SCI/FCI/PU exhibit a broad effective absorption bandwidth, which can be effectively applied to radar signature reduction and electromagnetic interference suppression in military and civil fields.

Assessment of Tribological Properties of Single- and Double-Layer Pad Coatings Deposited onto Cultivator Points

Assessment of Tribological Properties of Single- and Double-Layer Pad Coatings Deposited onto Cultivator Points

Application of Electrospark Deposition and Modified SHS Electrode Materials to Improve the Endurance of Hot Mill Rolls. Part 2. Structure and Properties of the Formed Coatings

The electrospark deposition of coatings onto SPKhN–60 white cast iron samples has been undertaken in two stages. A barrier sublayer has been deposited at the first stage, using chromium and nickel electrodes, and a multifunctional protective coating was deposited at the second stage. The effect of the sublayer on coating properties upon application of STIM–40NAOKn (TiC–NiAl + $${\text{ZrO}}_{2}^{{{\text{nano}}}}$$ ) and STIM-11OKn (TiB2–NiAl + $${\text{ZrO}}_{2}^{{{\text{nano}}}}$$ ) electrodes is studied. The coating structures are investigated. The grain size of the refractory phase is found to be smaller than 100 nm. The application of double-layer coatings increased the wear and heat resistances of white cast iron samples. Pre-deposition of a nickel sublayer enhanced the heat resistance of STIM-11OKn coating over eightfold. The full-scale tests of the rolls strengthened using SHS electrodes were carried out and positive results were obtained.

Computational Design of Electrode-Electrolyte Interfaces for Solid-State Lithium Ion Batteries

All-solid-state lithium-ion batteries have attracted significant interest for their enhanced safety compared with conventional batteries employing an organic liquid electrolyte. However interfacial reactions between the electrodes and the electrolyte can increase the impedance of charge carrier transfer, degrading battery performance. In this work, we present a systematic high-throughput screening method to identify coatings for interfaces based on thermodynamic phase equilibria to maintain long-term interface stability during battery operation. Using Monte Carlo simulated ab initio phase diagrams, our approach can incorporate metastable phases in the coating materials screening and identify potential multi-layer coatings. We identified single- and double-layer coatings for 56 combinations of common electrode and electrolyte materials from existing databases of inorganic materials. Top protective coatings were further evaluated on their electrical resistance, ionic conductivity and electrochemical stability. Our work provides a general approach to mitigating interfacial reactions and optimizing battery performance by rational coating design.

Double layer graphene oxide-PVP coatings on the textured Ti6Al4V for improvement of frictional and biological behavior

In this study, double-layer coatings of graphene oxide (GO)-poly(vinyl pyrrolidone) (PVP) (PGO) were deposited on the micro-textured Ti6Al4V substrates by electrophoretic deposition. Results revealed that the combination of micro-dimples and bi-layer PGO coatings significantly reduced the friction coefficient of textured Ti6Al4V from 0.5 to <0.03. PGO layer provided both relatively smooth surface of PVP for suitable adhesion and a rigid layer of GO for load bearing. Moreover, the textured micro-dimples acted as sinks to preserve the PVP and GO fragments and wear debris leading to sustain low friction coefficient for a long period of operation. In addition, our results demonstrated that the bi-layer PGO coating on the micro-textured Ti6Al4V substrate resulted in significant improvement of MG63 cell attachment and proliferation. In summary, PGO coated textured Ti6Al4V with simultaneously improved wear resistance and biological function have the potential for orthopedic implants.

Hot corrosion behaviour of atmospheric and solution precursor plasma sprayed (La0.9Gd0.1)2Ce2O7 coatings in sulfate and vanadate environments

Conventional and solution precursor plasma spraying (SPPS) techniques were employed for developing gadolinium oxide doped lanthanum cerate ((La0.9Gd0.1)2Ce2O7, Gd-LC) based double-layered thermal barrier coatings (TBCs). Hot corrosion studies of the above coatings were carried out in molten Na2SO4 + V2O5 (1:1) environment at 900 °C. The state-of-the-art yttria-stabilized-zirconia (YSZ) coating was found to be completely delaminated after 120 h by forming yttrium vanadate (YVO4) and m-ZrO2. The accelerated delamination of YSZ can be attributed to the undesired phase transformation during exposure to corrosive species. Double-layered APS coatings were found to last for more than 300 h without densification of underlying YSZ layer and also, show better adherence with bond coat. SPPS Gd-LC coatings were found to be completely delaminated on densifying the underlying YSZ within 300 h. Lanthanum vanadate (LaVO4) was found to be the main corrosive product along with minor amounts of gadolinium vanadate (GdVO4) in Gd-LC double-layer coatings.

The Influence of the Kaolin Content on the Thermal Protection Property of Double-Layer Coating of Flexible Composites

The comparison and analysis for the preparation of related performance indicators of the thermal protection such as the ablation resistance performance, thermal stability at high temperature and reflection ability of the heat ray of kaolin double-layer coated flexible composites were carried on. Because of the 2 d stratified structure of kaolin, and at the same time it processes the higher refractoriness, excellent resistance to the elevated temperature, the certain heat insulation and the reflection performance of the heat ray, the adequate padding of kaolin can improve the prepared thermal protection ability of double-layer coating of flexible composites.

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.

Investigation of functionally graded HA-TiO2 coating on Ti–6Al–4V substrate fabricated by sol-gel method

Abstract In this study, functionally graded hydroxyapatite-titanium dioxide coating (FGC HA-TiO2), with bottom-up composition: 100% TiO2, 50% TiO2 -50% HA, 100% HA, was produced by sol-gel method on the Ti–6Al–4V alloy substrate. The XRD, FTIR and XRF results signified heat treatment of each coating's layer at 450 °C leads to desired phase achievement. The coherence, strength and scratch resistance of FGC HA-TiO2 in comparison with control samples namely; single-layer HA and double-layer HA-TiO2 coatings, were assessed by cross-sectional SEM images and nano-scratch test. According to the results, the insertion of the TiO2 layer improves the bonding strength of the coating to the substrate and grading the coating composition plays an essential role in upgrading the integrity and coherency of the coating. The contribution of the TiO2 intermediate layer and composition gradient on developing coating strength was 22% and 36%, respectively. The immersion test in the simulated body fluid (SBF) suggested bioactivity of the Ti–6Al–4V substrate enhances by coating.

CMAS corrosion resistance in high temperature and rainwater environment of double-layer thermal barrier coatings odified by rare earth

Abstract In order to explore the difference of CMAS corrosion resistance in high temperature and rainwater environment of single-layer and double-layer thermal barrier coatings (TBCs), and further reveal the mechanism of CMAS corrosion resistance in above environment of double-layer TBCs modified by rare earth, two TBCs were prepared by air plasma spraying, whose ceramic coating were single-layer ZrO2–Y2O3 (YSZ) and double-layer La2Zr2O7(LZ)/YSZ, respectively. Subsequently, CMAS corrosion resistance tests at 1200 °C and rainwater environment of two TBCs were carried out. Results demonstrate that after high temperature CMAS corrosion for the same time, due to phase transformation, the volume of YSZ ceramic coating in single-layer TBCs shrank and surface cracks formed, which would lead to coating failure. When LZ ceramic coating of double-layer TBCs reacted with CMAS, compact apatite phases and fluorite phases formed, the penetration of CMAS into ceramic coating was inhibited effectively. Raman analysis and calculation results show that both of the surface residual stress of ceramic coating in two TBCs were compressive stress, and the residual stress of ceramic coating in double-layer TBCs were smaller than that of single-layer TBCs. Atomic force microscopy of TBCs after CMAS corrosion show that surface of double-layer TBCs was more uniform and compact than that of single-layer TBCs. The electrochemical properties in simulated rainwater of two TBCs after high temperature CMAS corrosion showed that double-layer TBCs possessed higher free corrosion potential, lower corrosion current and higher polarization resistance than those of single-layer TBCs. Consequently, the presence of LZ ceramic coating effectively improved CMAS corrosion resistance in high temperature and rainwater environment of double-layer TBCs.

Comprehensive tribological study of optimized N-DLC/DLC coatings fabricated by active screen DC-pulsed PACVD technique

Abstract The conventional DC-pulsed PACVD method was applied to deposit DLC and N-DLC coatings on AISI H13 substrate. ased on the optimized parameters achieved for single layer, N-DLC/DLC double layer coatings were deposited using two different techniques, namely: conventional and active screen. Chemical bindings and structural evolutions of double-layer coatings were investigated by visible Raman spectroscopy, and X-ray Photoelectron Spectroscopy (XPS). Accordingly, the friction coefficient of the active screen N-DLC/DLC coating at the end of the wear test was obtained about the value of 0.12 which was found to be the lowest among all samples.

High temperature oxidation resistance and thermal growth oxides formation and growth mechanism of double-layer thermal barrier coatings

In order to reveal the formation and growth mechanism of thermal growth oxides (TGO) in double-layer thermal barrier coatings (TBCs) during high temperature oxidation, and to further clarify the mechanism of lanthanum zirconate (LZ) as the outermost ceramic coating to improve the high temperature oxidation resistance of double-layer TBCs, two kinds of TBCs were prepared by air plasma spraying, whose ceramic coating were single-layer YSZ and double-layer LZ/8YSZ, respectively. Subsequently, the two TBCs were oxidized at 1100 °C, whereupon the microstructure and growth behavior of TGO during high temperature oxidation were studied comparatively. The results show that the weight gain of the two TBCs were faster in the early stage of oxidation, and obviously slowed down when the oxidation time exceeded 100 h. But for the whole process of high temperature oxidation, the oxidation weight gain of double-layer TBCs was obviously slower than that of single-layer TBCs. In the process of high temperature oxidation, TGO growth in double-layer TBCs was slower. After oxidation for the same time, not only the morphology of TGO in double-layer TBCs was thinner and more compact, but also the formation time of holes was delayed than in single-layer TBCs, besides, no obvious cracks appeared. With oxidation time extending, Al2O3, spinel oxides and complex compounds formed in TGO successively. Spinel oxides and complex compounds were collectively called SC, which was critical for cracks nucleation and propagation during high temperature oxidation of TBCs. Due to better ability to isolate external oxygen of LZ ceramic coating, the diffusion kinetics of elements in bonding coating of double-layer TBCs was changed. In the early stage of oxidation, the oxidation of Al3+, Ni3+, Co3+, Cr3+ metal cations was delayed. Accordingly, the formation of SC was postponed. In later stage of oxidation, with the gradual growth and thickening of Al2O3, it is difficult for O2− to diffuse to the interface between TGO and bonding coating through Al2O3 protective barrier, which also prevents the growth of SC. Consequently, the high temperature oxidation resistance of double-layer TBCs is superior to single-layer TBCs.

Designing anti-reflection coatings for optical surface

In this paper, we have analyzed the optical properties of single and double layer antireflection coatings with the help of influential characteristics matrix formulation method in the visible electromagnetic spectrum. We have studied deeply for the glass coated with MgF_2 as a single layer coating and the spectrum analysis of double layer coatings of glass with〖 MgF〗_2-Al_2 O_3. With the aid of the algebraic mathematics and MatLab program, we have developed antireflection coating designs which leads to an enhancement of material’s ability for transmission spectrum through it. According to the result we have obtained, it clearly seen that the maximum transmission power for single layer and double layer coatings are about 97.2% which occurs at wavelength of 450 nm and 100% occurs at 324 nm, respectively. It is also observed that the minimum value recorded in transmission spectra for double layer antireflection coating is 99.83% at a wavelength of 418 nm (for normal incidence) and 400 nm (for θ=60° angle of incident). In broadly speaking, our fundamental work shows double layer antireflection coating is more convenient and feasible than single layer antireflection coating.  Â