Multilayer Coatings Research Articles

Effect of electrodeposited Cu interlayer thickness on characterizations and adhesion force of Ni/Cu/Ni coatings on polyetherimide composite substrates

Polyetherimide (PEI) composite is an engineering plastic with excellent performance. After metallization, the metalized PEI composite has some unique properties, such as electromagnetic shielding and high electrical conductivity. In this study, a composite coating of Ni/Cu/Ni was produced on the surface of a PEI composite substrate. Nickel and copper layers were plated by electroless and electrodeposition, respectively. The influences of copper interlayer thickness and heat treatment on the adhesion state of the multilayer coatings on the substrate were studied. The morphology and phase of the composite coatings were determined by optical microscopy, scanning electronic microscopy and X-ray diffraction, respectively. The surface roughness and contact angles of the samples before and after sandblasting were measured by surface profile-meter and goniometer, respectively. The electrical resistivity and adhesion state of the composite coatings were determined by multimeter and the tape test, respectively. The results show that Ni/7.54 μm Cu/Ni and Ni/58.6 μm Cu/Ni multilayer deposits were produced on the substrates. The top surface of the coating with a thin 7.54 μm-Cu interlayer was much rougher than that of a coating with a thick 58.6 μm-Cu interlayer. The initial alkaline electroless nickel layer with 5.8 ± 0.5% phosphorus was composed of a mixture of amorphous and nanocrystalline phases. The Cu interlayer consisted of a face-centered cubic crystalline structure. The phosphorus contents of the top nickel layers for samples #1 and #2 were composed of 6.53 ± 0.4 wt% and 3.52 ± 0.21 wt%, respectively. An increase in Cu interlayer thickness for the composite coating resulted in an increase in electrical resistivity. The average microhardness values of Ni/7.54 μm Cu/Ni PEI composite and the Ni/58.6 μm Cu/Ni PEI composite were about 300 HV0.1 and 244.2 HV0.1, respectively. The adhesion state of the Ni/7.54 μm Cu/Ni and Ni/58.6 μm Cu/Ni PEI composites could be classified as grade 5B and 3B, respectively. The adhesion force of the Ni/58.6 μm Cu/Ni multilayer on the substrate can be improved by heat treatment at 200 °C for 4 h.

Physicochemical Characterization and Immunomodulatory Activity of Polyelectrolyte Multilayer Coatings Incorporating an Exopolysaccharide from Bifidobacterium longum.

Immunoregulatory polysaccharides from probiotic bacteria have potential in biomedical engineering. Here, a negatively charged exopolysaccharide from Bifidobacterium longum with confirmed immunoregulatory activity (EPS624) was applied in multilayered polyelectrolyte coatings with positively charged chitosan. EPS624 and coatings (1, 5, and 10 layers and alginate-substituted) were characterized by the zeta potential, dynamic light scattering, size exclusion chromatography, scanning electron microscopy, and atomic force microscopy. Peripheral blood mononuclear cells (hPBMCs) and fibroblasts were exposed for 1, 3, 7, and 10 days with cytokine secretion, viability, and morphology as observations. The coatings showed an increased rugosity and exponential growth mode with an increasing number of layers. A dose/layer-dependent IL-10 response was observed in hPBMCs, which was greater than EPS624 in solution and was stable over 7 days. Fibroblast culture revealed no toxicity or metabolic change after exposure to EPS624. The EPS624 polyelectrolyte coatings are cytocompatible, have immunoregulatory properties, and may be suitable for applications in biomedical engineering.

Expanding the Scope of an Amphoteric Condensed Tannin, Tanfloc, for Antibacterial Coatings.

Bacterial infections are a common mode of failure for medical implants. This study aims to develop antibacterial polyelectrolyte multilayer (PEM) coatings that contain a plant-derived condensed tannin polymer (Tanfloc, TAN) with inherent antimicrobial activity. Tanfloc is amphoteric, and herein we show that it can be used as either a polyanion or a polycation in PEMs, thereby expanding the possibility of its use in PEM coatings. PEMs are ordinarily formed using a polycation and a polyanion, in which the functional (ionic) groups of the two polymers are complexed to each other. However, using the amphoteric polymer Tanfloc with weakly basic amine and weakly acidic catechol and pyrogallol groups enables PEM formation using only one or the other of its functional groups, leaving the other functional group available to impart antibacterial activity. This work demonstrates Tanfloc-containing PEMs using multiple counter-polyelectrolytes including three polyanionic glycosaminoglycans of varying charge density, and the polycations N,N,N-trimethyl chitosan and polyethyleneimine. The layer-by-layer (LbL) assembly of PEMs was monitored using in situ Fourier-transform surface plasmon resonance (FT-SPR), confirming a stable LbL assembly. X-ray photoelectron spectroscopy (XPS) was used to evaluate surface chemistry, and atomic force microscopy (AFM) was used to determine the surface roughness. The LDH release levels from cells cultured on the Tanfloc-containing PEMs were not statistically different from those on the negative control (p > 0.05), confirming their non-cytotoxicity, while exhibiting remarkable antiadhesive and bactericidal properties against Pseudomonas aeruginosa (P. aeruginosa) and Staphylococcus aureus (S. aureus), respectively. The antibacterial effects were attributed to electrostatic interactions and Tanfloc's polyphenolic nature. This work underscores the potential of Tanfloc as a versatile biomaterial for combating infections on surfaces.

The synergistic mechanism of transition layer and transfer layer on dry sliding wear of multi-layered diamond-like carbon (DLC) coating at wide temperature

The wear resistance of 17-4PH stainless steel was improved at wide temperature (25–200 °C) by depositing multi-layered Cr/CrC/DLC coating. The results showed that the lubricating effect of the transfer layer led to a reduction of the friction coefficient to 0.03. DLC coating had a protective effect on the 17-4PH substrate, and the surface of the coating was basically not worn at 25–100 °C. This study focused on the wear mechanism and frictional chemical reaction mechanism of multi-layered coatings at wide temperatures. With the increase of temperature, the wear mechanism of DLC coating changed from abrasive wear to adhesive wear, and finally delamination. The complex tribochemical reaction in the wear process led to the dynamic change of its graphitization transformation, which affected the formation of the transition layer and the change in surface mechanical properties of the coating. This was mainly because the transition layer Cr/CrC and Si3N4 played a role in inhibiting the graphitization transformation of DLC layer during the wear process, thus affecting the friction properties of DLC.

Eco-Friendly Multilayer Coating Harnessing the Functional Features of Curcuma-Based Pigment and Rice Bran Wax as a Hydrophobic Filler.

This work aims to highlight the multiple features shown by curcuma-based pigment and rice bran wax, which can be selectively employed as bio-based additives for the realization of multilayer wood coatings with multiple functionalities, harnessing the capabilities of the two environmentally friendly fillers, in line with current environmental sustainability trends. The role of the two green materials on the morphology of the composite layers was examined through observations employing scanning electron and optical microscopy, revealing a strong alteration of the film's appearance, both its color and reflectivity. Additionally, their influence on the paint's resilience was assessed by exposing the samples to UV-B radiation and consecutive thermal shocks. The coating displayed a clear and uniform change in color because of substantial curcuma powder photo-degradation but it remained exceptionally stable when subjected to thermal stresses. Moreover, the protective properties of the coatings were evaluated by conducting liquid resistance tests and water uptake tests, while the hardness and the abrasion resistance of the coatings were assessed to evaluate the effect of the additives on the mechanical properties of the coatings. In conclusion, this study showcases the promising joint action of curcuma-based pigment and rice bran wax in multilayer coatings. This combination offers vibrant yellow tones and an appealing appearance to the paint, enhances the surface's water-repellent properties, and improves the mechanical resistance of the coatings.

On the high temperature oxidation behavior of AlCrBN/TiAlNbSiN multilayer coatings with addition of boron and silicon

Developing a TiAlN-based hard coating with a multilayer structure comprising multiple nitride components shows potential to enhance the overall performance of high temperature applications. In this study, multi-target cathodic arc evaporation was utilized to deposit multilayer coatings comprising of nanostructured AlCrN/TiAlNbN and AlCrBN/TiAlNbSiN. The focus of this study is to analyze how the different coatings undergo structural changes due to the growth of oxide scale and diffusion processes that take place during oxidation when subjected to high temperatures of 900 °C, and the roles of B and Si addition in their oxidation resistance were examined. The deposited AlCrN/TiAlNbN coating showed a typical columnar structure with nanolayer stacking (average bilayer periodic thickness ~ 14.8 nm). The surface formed an oxide layer rich in titanium oxide, while the internal oxide layer of the AlCrN/TiAlNbN coating that underwent oxidation contained mixed metal oxides with a dominant composition of Al2O3, which hindered additional oxidation. The AlCrBN/TiAlNbSiN coatings, which had average bilayer periodic thickness 11.4 nm ~12.2 nm, showed fine-fibrous growth morphologies and refining effects of B and Si in the AlCrBN/TiAlNbSiN coatings, and they possessed better high temperature oxidation resistance thang that of AlCrN/TiAlNbN. Due to the formation of a protective oxidized layer with a combination of metal oxides, which minimizes the inward diffusion of oxygen during oxidation, the AlCrBN/TiAlNbSiN coating with a thicker AlCrBN layer exhibited the highest resistance to oxidation compared to the other coatings studied.

Change in Physicomechanical Properties and Stress–Strain State of Multilayer Coatings on Cyclic Heating

Change in Physicomechanical Properties and Stress–Strain State of Multilayer Coatings on Cyclic Heating

Nondestructive evaluation of aircraft stealth coating by Terahertz-time domain spectroscopy: experimental and numerical investigation

ABSTRACT Particle-reinforced polymer matrix composites (PMCs) are often used as radar-absorbing materials (RAM) in aircraft stealth applications. The thickness evaluation of such multilayered coatings with micrometre-level thickness is highly challenging from single-side access. In this study, we used Terahertz Time-domain Spectroscopy (THz-TDS) in the transmission mode to extract the refractive index of the dielectric coatings for measuring the thickness using the time-of-flight method. A numerical study based on the Finite Element method (FEM) has also been developed to validate the transmission experiments. A frequency-dependent complex dielectric parameters must be considered for coatings with high absorption in the THz regime. This was addressed by performing the finite element simulations in a frequency domain. The thickness of each layer of a multi-layered coating is estimated by carrying out the experiments in reflection mode. Since the interface echoes were overlapping, a deconvolution algorithm and frequency thresholding were employed to reconstruct the signals reflected from the different interfaces. Using this technique, the thickness of each layer of coating is estimated accurately in a single measurement, which was challenging to measure using other conventional non-destructive testing (NDT) methods.

Polyelectrolyte-based antifouling and pH-responsive multilayer coatings for reverse osmosis membrane

Membrane fouling, the accumulation of material on the membrane surface, has been a challenge that limits the application of reverse osmosis technology. Ordinary cleaning processes do not remove contaminants from the membrane surface. To effectively clean a membrane, we fabricated a sacrificial multilayer polyelectrolyte coating based on a layer-by-layer assembly approach using chitosan (CS) and tannic acid (TA) on the membrane surface. The CS/TA coated membranes represent a superior permeance flux of 64 L m−2 h−1 with a clear NaCl rejection of 98.89 %. Meanwhile, the membrane surface hydrophilicity was enhanced due to the abundant amino and hydroxyl groups, which shows superior anti-fouling properties against proteins, polysaccharides, and surfactant. Moreover, the coatings were unstable, which would decompose at pH 8.5. The degradation endows the coated membranes approving long-term anti-fouling performance, which the permeance recovery reaching up to 85 %. Constructing pH-responsive smart coatings provides a promising route to address membrane fouling. It can be upscaled in numerous practical applications like sensors, medical devices, and drug delivery.

Dense Cr/GLC multilayer coating by HiPIMS technique in high hydrostatic pressure: Microstrusctural evolution and galvanic corrosion failure

The dense Cr/GLC multilayer coatings were fabricated by high power impulse magnetron sputtering (HiPIMS) with various duty cycles. HiPIMS-deposited GLC coatings with high density displayed the lower corrosion current density by five times than DC- deposited GLC, due to the higher C+ ionization rate and larger incident ions energy. Noted that, after 425 h immersion with 15 MPa, though a laminar-like spallation occurred in the Cr/GLC interfaces instead of the coating/substrate interfaces, the lower galvanic corrosion current density was particularly achieved at 2.04 × 10−9 A·cm−2 for the HiPIMS-coatings with reduced porosity, emulating an excellent tolerance of long-term corrosion resistance for deep-sea.

Effect of Surface Modification via Laser Irradiation on the Operability of Carbide End Mills When Cutting Aircraft Alloys

In modern aviation production, innovative hard-to-machine materials with unique physical and mechanical properties are being used increasingly. When processing such materials, the weakest link in the technological chain of production is the metal-cutting tool. In this paper, to improve the efficiency of the blade cutting of heat-resistant alloys, we propose the use of nanostructured multilayer wear-resistant coatings with subsequent laser processing of the cutting surfaces of the end milling cutters according to various schemes. In this case, an increase in the wear-resistant properties of the cutting edge by 15%–20% is provided due to the formation, at high temperatures, of secondary structures with increased wear resistance and a reduction of the temperature and force loading of contact processes. Methodologically, the work was carried out in several consecutive stages: the first stage was the determination of effective grades of wear-resistant coatings obtained via various installations with their subsequent laser processing during the «SharpMark™ Fiber» installation; at the second stage tribotechnical tests were carried out during the tribometer and adhesion installation; and in the third stage wear-resistant, temperature-force tests were carried out using milling machines in various cutting modes. According to the results of the field tests, the tool durability period was increased by 15%–20%.

Anti-corrosive yet color-tunable AlN/Si multilayer hard coatings on 304 stainless steel by magnetron sputtering

AlN based coatings are widely used to protect metal substrates from corrosion. However, synthesizing anti-corrosive yet color-tunable AlN based coatings is challenging. In this study, the AlN/Si multilayer coating was deposited on 304 stainless steel by embedding nine film layers of Si in an AlN monolayer coating using magnetron sputtering. The effects of embedded Si thin layers on the morphology, structure, composition, hardness, friction, optical and corrosion resistance of the AlN coatings were investigated. With the embedding of Si thin layers, the surface pinhole defects and columnar structure of the AlN monolayer coatings were significantly suppressed, and the surface particle size was reduced from 200 to 40 nm, and the average grain size of AlN was reduced from 19.7 to 12.9 nm. The hardness increased from 8.9 to 12.2 GPa, and the friction coefficient decreased from 0.96 to 0.75. The corrosion current density decreased from 6.81 × 10−8 to 2.57 × 10−10 A/cm2 in 3.5 wt% NaCl solution, and from 3.76 × 10−6 to 4.69 × 10−8 in 1 mol/L H2SO4 solution, which is about 4 orders of magnitude lower than the 304 steel. When the thickness of the AlN top layer was controlled to 220, 200, 185, 170, 160, 150 and 140 nm, the colors of the AlN/Si multilayer coatings were red, orange, yellow, green, blue, indigo and violet, respectively. The mechanism of anti-corrosion enhancement and color tunability of AlN/Si multilayer coatings was revealed in detail.

Microstructure and Corrosion Behavior of Zinc/Hydroxyapatite Multi-Layer Coating Prepared by Combining Cold Spraying and High-Velocity Suspension Flame Spraying.

The study aims to enhance the corrosion resistance and bioactivity of Mg alloy substrates through the development of a zinc/hydroxyapatite multi-layer (Zn/HA-ML) coating. The Zn/HA-ML coating was prepared by depositing a cold-sprayed (CS) Zn underlayer and a high-velocity suspension flame sprayed (HVSFS) Zn/HA multi-layer and was compared with the CS Zn coating and the Zn/HA dual-layer (Zn/HA-DL) coating. Phase, microstructure, and bonding strength were examined, respectively, by X-ray diffraction, scanning electron microscopy, and tensile bonding testing. Corrosion behavior and bioactivity were investigated using potentiodynamic polarization, electrochemical impedance spectroscopy, and immersion testing. Results show that the HVSFS Zn/HA composite layers were mainly composed of Zn, HA, and ZnO and were well bonded to the substrate. The HVSFS HA upper layer on the CS Zn underlayer in the Zn/HA-DL coating exhibited microcracks due to their mismatched thermal expansion coefficient (CTE). The Zn/HA-ML coating exhibited good bonding within different layers and showed a higher bonding strength of 27.3 ± 2.3 MPa than the Zn/HA-DL coating of 20.4 ± 2.7 MPa. The CS Zn coating, Zn/HA-DL coating, and Zn/HA-ML coating decreased the corrosion current density of the Mg alloy substrate by around two-fourfold from 3.12 ± 0.75 mA/cm2 to 1.41 ± 0.82mA/cm2, 1.06 ± 0.31 mA/cm2, and 0.88 ± 0.27 mA/cm2, respectively. The Zn/HA-ML coating showed a sixfold decrease in the corrosion current density and more improvements in the corrosion resistance by twofold after an immersion time of 14 days, which was mainly attributed to newly formed apatite and corrosion by-products of Zn particles. The Zn/HA-ML coating effectively combined the advantages of the corrosion resistance of CS Zn underlayer and the bioactivity of HVSFS Zn/HA multi-layers, which proposed a low-temperature strategy for improving corrosion resistance and bioactivity for implant metals.

Heparin-Loaded Composite Coatings on Porous Stent from Pure Magnesium for Biomedical Applications.

Challenges associated with drug-releasing stents used in percutaneous transluminal coronary angioplasty (PTCA) encompass allergic reactions, prolonged endothelial dysfunction, and delayed stent clotting. Although absorbable stents made from magnesium alloys seem promising, fast in vivo degradation and poor biocompatibility remain major challenges. In this study, zirconia (ZrO2) layers were used as the foundational coat, while calcium phosphate (CaP) served as the surface layer on unalloyed magnesium specimens. Consequently, the corrosion current density was decreased to 3.86, from 13.3 μA/cm2. Moreover, a heparin-controlled release mechanism was created by co-depositing CaP, gelatin (Gel), and heparin (Hep) on the specimens coated with CaP/ZrO2, thereby boosting magnesium's blood compatibility and prolonging the heparin-releasing time. Techniques like X-ray diffractometry (XRD), focused ion beam (FIB) system, toluidine blue testing, UV-visible spectrometry, field emission scanning electron microscopy (FESEM), and surrogate tests for endothelial cell viability were employed to examine the heparin-infused coatings. The drug content rose to 484.19 ± 19.26 μg/cm2 in multi-layered coatings (CaP-Gel-Hep/CaP-Hep/CaP/ZrO2) from 243.56 ± 55.18 μg/cm2 in a single layer (CaP-Hep), with the controlled release spanning beyond 28 days. Also, cellular viability assessments indicated enhanced biocompatibility of the coated samples relative to those without coatings. This suggests the potential of magnesium samples after coating ZrO2 and CaP with Gel as candidates for porous biodegradable stents or even scaffolds in biomedical applications.

Multilayer coatings based on cerium oxide and manganese cobaltite spinel for Crofer22APU SOC interconnects

The application of protective coatings on stainless-steel interconnects can improve the durability of the metallic component in Solid Oxide Cell devices. In this work, we explored the possibility of producing multilayered coatings based on cerium oxide deposited by electrolytic deposition and manganese cobaltite spinel deposited by electrophoretic deposition. Obtained coatings were oxidized at 850 °C in air for 1000 h and the oxidation rate was calculated in the range of 10-14 g2cm-4s−1. The morphological evolution of the oxide scale and the coating system after oxidation revealed that the ceria particles were located at the boundaries between the grains of the reaction layer and those of the MCO coating, as assessed by FESEM and TEM characterizations.

CAVITATION EROSION RESISTANCE OF VACUUM-ARC COATINGS BASED ON TiN

This review presents an examination of various studies investigating the impact of deposition process conditions and structural characteristics of TiN-based vacuum-arc coatings on their ability to withstand cavitation erosion in water. The structural and phase composition of TiN coatings is influenced by two key technological parameters: the nitrogen pressure in the vacuum chamber and the substrate bias potential. However, it has been observed that vacuum-arc alloy coatings such as TiSiN and TiAlYN exhibit notably lower resistance to cavitation erosion. Additionally, research on multilayer Ti-TiN coatings with varying numbers and thickness ratios of layers has not shown an improvement in resistance to cavitation wear when compared to single-layer coatings deposited under optimal conditions. On the other hand, single-phase stoichiometric TiN coatings deposited at a higher nitrogen pressure of 2 Pa and a bias potential of up to -300 V have demonstrated remarkable resistance to cavitation wear. These coatings could be effectively utilized to protect the titanium alloy Ti-6Al-4V against cavitation damage.

Structure and Properties of Multilayer Nanocomposite Coatings of Polyvinyl Alcohol with Aluminum Oxide Nanoparticles

The nanocomposite polymer – inorganic materials formation, the study of their morphology and mechanical properties at the nanolevel is acute in the development of new materials for various functional purposes, including medical ones. As a result of the research the technique for producing singleand multilayer films of polyvinyl alcohol and composite polymer coatings with aluminum oxide nanoparticles by the spin coating method has been developed. It is shown that the optimal mass content of aluminum oxide nanoparticles in suspension for the formation of uniform composite coatings is 0.625 %. Based on experimental data on the structuralmorphological and mechanical properties of the formed coatings obtained by atomic force microscopy, it has been found that an increase in the number of layers of composite coatings leads to an increase in the number of conglomerates which, in turn, increases the surface roughness of the films. The modulus of elasticity of single-layer films of polyvinyl alcohol is (509.5 ± 10 %) MPa. In the case of composite coatings with aluminum oxide nanoparticles, changes in the elastic modulus have been established for multilayer coatings: an increase to 559.0 MPa (5 layers) and a decrease to 415.2 MPa (10 layers). The modulus of elasticity of the investigated single-layer coatings is significantly reduced in the range of 20−40 ºС. The smallest values after exposure to temperatures have been determined for films with nanoparticles (236.2 ± 10 %) MPa. Nanocomposites demonstrate an increase in the contact angle with an increase in the number of layers of composite coatings up to 20. A subsequent increase in the thickness of the coatings (the number of layers) leads to an increase in the hydrophilicity of the nanocomposites. The developed compositions of nanocomposite films are promising as sorption coatings.

Fabrication and Characterization of a Multifunctional Coating to Promote the Osteogenic Properties of Orthopedic Implants.

Titanium-based alloys are used in orthopedic applications as fixation elements, hard tissue replacements in artificial bones, and dental implants. Despite their wide range of applications, metallic implant defects and failures arise due to inadequate mechanical bonding, postoperative clotting problems, aseptic loosening, and infections. To improve the surface bioactivity and reduce the corrosion rate of the Ti6Al4V alloy, multi-layered coatings (HAp, BG, Cs, and Hep) were applied via electrophoretic deposition (EPD). XRD images showed the presence of HAp within the coating. In vitro investigation: cell line NIH-3T3 fibroblasts were seeded on the non-coated and coated Ti6Al4V substrates, and their cellular behavior was evaluated. The results indicated that the HApBGCsHep coating could enhance the adhesion and proliferation of NIH 3T3 cells. In addition, the potentiodynamic polarization results are compatible with the in vitro outcome.

Thickness, Adhesion and Microscopic Analysis of the Surface Structure of Single-Layer and Multi-Layer Metakaolin-Based Geopolymer Coatings

This work is focused on creating coating layers made of a metakaolin-based geopolymer suspensions (GP)-formed Al matrix modified using H3PO4 acid with Al(OH)3 in isopropyl alcohol, named GP suspension I, and H3PO4 acid with nano Al2O3 in isopropyl alcohol, named GP suspension J. The selected GP suspensions were applied on aluminum and steel underlying substrates as single-layer coatings and multi-layer coatings, where multi-layer coatings included three and five layers that were polymerized by a curing process. Curing was divided into two types with every layer curing process and final layer curing process. For both GP suspensions I and J, the effect of the number of layers and the type of substrate on adhesion was investigated. The prepared samples on underlying substrates were characterized on the microscopy analysis including SEM for high-resolution images and 3D laser confocal microscopy (CLSM) for the 3D visualization of the coatings structure. Microscopy analysis showed structural defects such as porosity, cracks and peeling, which increase with a greater number of applied layers. However, these defects were only evident on a micro scale and did not seem to be fatal for the performance of the surface stability. The EDS mapping of the prepared layer showed inhomogeneity in the distribution of elements caused by the brush application. A grid test and thickness measurement were performed to complete the microscopy analysis. The grid test confirmed a very high adhesion of GP coatings on the aluminum substrate with a rating of one (only in one case was there a rating of two) and a lower adhesion on the steel substrate with the most frequent rating of three (in one case, there were ratings of two and one). The thickness measurement proved a noticeably thicker thickness of the prepared layer on the Fe substrate compared to the Al substrate by 20%–30% in the case of suspension I and by 70%–10% in the case of suspension J. The thickness of the layer also showed a dependence on the method of application and curing, as a thicker layer was always achieved when curing after the final layer of the GP suspension, compared to curing after each applied layer. The resulting single-layer and multi-layer thicknesses ranged from approx. 7 to 30 µm for suspension I and from approx. 3 to 11 µm for suspension J. A non-linear increase in thickness was also evident from the thickness measurement data.

Multilayer gradient rare earth disilicate coating prepared by in-situ melt infiltration and sintering method with high thermal shock resistance on porous Si3N4 ceramics

Dense multilayer gradient rare earth disilicate (γ-Y2Si2O7 /β-Yb2Si2O7 /β-Lu2Si2O7) coatings were in-situ prepared by melt-infiltration /sintering procedure on porous Si3N4 ceramics for water resistance. Experimental and numerical simulation methods were used to study their thermal shock behavior. As a control, thermal shock behavior of pure γ-Y2Si2O7 coatings were also compared. FEM results showed that the gradient design of modulus in ceramic coating can effectively avoid the mismatch of mechanical properties between coating layer and internal substrate, reducing the transient thermal stress in each layer during thermal shock. All of the pure γ-Y2Si2O7 coatings were failed after thermal shock tests with ΔT ≈ 1200 ℃. However, when sintering temperature of multi-layer disilicate coatings were higher than 1400 ℃, the water absorption rates after thermal shock were all less than 5%, still showing good waterproof performance. The gradient design of modulus could effectively improve the structural stability of ceramic coatings.