Fabrication of ceramic composite coatings using electrophoretic deposition,reaction bonding and low temperature sintering

We have developed a novel combination of electrophoretic deposition (EPD), reaction bonding and low temperature sintering techniques for the fabrication of yttria stablised zirconia (YSZ)/alumina composite coatings on Fecralloys. A mixture of ethanol and acetylacetone solvent was found to be an effective medium for YSZ and aluminium particle suspension. With the particle size of YSZ and aluminium being significantly reduced during ball milling. By using the EPD process, uniform green form coatings containing YSZ and aluminium particles were produced on Fecralloys. After oxidation of aluminium at 500°C and sintering at 1200°C, a dense and adherent YSZ/Al₂O₃ coating was produced. The presence of aluminium in the green form coatings not only contribute to the bonding between the coating and the metal substrate, but also compensate for the volume shrinkage of the coatings during sintering by the volume expansion arising from oxidation of aluminium to alumina.     

Electrophoretic deposition of ceramic coatings on ceramic composite substrates

Abstract

The applicability of electrophoretic deposition (EPD) for the fabrication of single layer and multilayer ceramic coatings on dense ceramic composite materials has been examined. Al₂O₃/Y-tetragonal zirconia polycrystal (TZP) functionally graded composites of tubular shape were successfully coated with a two layer coating comprising porous alumina and dense reaction bonded mullite layers. The dual layer coating structure was designed to eliminate the numerous cracks caused by volume shrinkage during sintering of the individual EPD formed layers. In another example, mullite fibre reinforced mullite matrix composites were coated with a thin layer of nanosized silica particles using EPD. The aim was to achieve a compressive residual stress field in the silica layer on cooling from sintering temperature, in order to increase composite fracture strength and toughness. The EPD technique proved to be a reliable method for rapid preparation of single layer and multilayer ceramic coatings with reproducible thickness and microstructure on ceramic composite substrates.     

Ceramic Coatings on Ceramic Substrates by Liquid Metal Transfer

A variety of ceramic coatings have been put on ceramic substrates by the molten metal transfer technique. In the diffision coating process a reactant metal dissolved in a molten "transfer agent" metal reacts with a substrate to form the coating. Titanium dissolved in molten tin has been reacted with silicon carbide, aluminum nitride, and silicon nitride substrates to produce titanium carbide and titanium nitride coatings, respectively. Similarly tantalum metal has been reacted to form carbide or nitride coatings. The unreacted tin is finally removed by decantation followed by acid leaching.     

Electrophoretic deposition of electroless nickel coated YSZ core-shell nanoparticles on a nickel based superalloy

In this work, yttria-stabilized zirconia (YSZ) nanoparticles were covered by a thin Ni layer with approximately 10 nm thickness by electroless deposition method to reduce sintering temperature of the ceramic coating which was applied on a Ni based superalloy via electrophoretic deposition (EPD). Suspensions containing the processed Ni-YSZ core-shell nanoparticles in acetone and isopropyl alcohol solvents were stabilized by addition of 0.4 wt% iodine and 1.5 wt% polyethylenimine, respectively, to find more effective stabilization method for EPD. It was seen that the presence of the Ni layer on YSZ nanoparticles improved performance and sticking factor of EPD and uniform coatings were obtained in both suspensions. The Ni-YSZ green coating which was produced by EPD at voltage of 35 V and deposition time of 30 min in acetone with thickness of 41 μm was sintered in 1100 °C and finally a uniform NiO-YSZ coating was formed on the metallic surface.     

Shrinkage-free ZrSiO4-ceramics: Characterisation and Applications

Dense, shrinkage-free ZrSiO₄-ceramics can be produced by a reaction-bonding process using ZrSi₂, ZrO₂, and a polysiloxane as starting materials. Sinter shrinkage is compensated by the volume increase during oxidation of ZrSi₂. In addition, the use of a Si-containing so called low-loss-binder [polymethylsilsesquioxane (PMSS)] reduces shrinkage further. Near net-shape ceramic compacts can be produced by an embossing process. As the green bodies are extremely dense but not brittle, they can be mechanically machined. The reaction-bonding process has to be controlled in a way that first the pyrolysis of PMSS, followed by the oxidation of ZrSi₂, and finally the formation of ZrSiO₄ and the sintering to dense compacts take place. Due to the absence of shrinkage and the good mechanical properties obtained, these ceramics open up new applications in fields requiring high dimensional accuracy such as microsystem engineering or dentistry. ©     

Thick Protective UHTC Coatings for SiC-Based Structures: Process Establishment

A new process to form thick and dense ultra-high-temperature ceramic (UHTC) composite coatings over SiC surfaces is described. Coatings of ZrB₂/SiC/(ZrC) thicker than 100 μm are formed by a reaction-bonded SiC (RBSC) approach based on Si infiltration into ZrB₂/C preform coating. The residual Si, typically found in RBSC, can be eliminated efficiently to provide a coating material that performs at temperatures above 1500°C. The process is performed at 1500°C in Ar at ambient pressure. The interface between the in situ formed SiC and the Zr phases is very tight, as is the interface with the substrate.
The ZrB₂ particles used in this process are rearranged in their morphology and an additional new phase containing Zr–C is formed. The coatings exhibit excellent integrity, hardness, and bonding to the tested substrates. A preliminary oxidation study indicates good protection of substrates at 1500°C under both passive and active oxidation conditions, provided that the coatings have sufficient thickness.     

Constrained Drying of an Aqueous Yttria-Stabilized Zirconia Slurry on a Substrate II: Binary Particle Slurry

Binary-sized particle slurries (binary slurries) prepared by mixing micro-particles and electrostatically stabilized nano-particle slurries (nano-slurries) were cast onto metal substrates to form wet coatings and the drying process was studied. The highly charged nano-particles were found to deposit on the surface of the micro-particles, resulting in a decrease in the viscosity of the binary slurry. The drying kinetics were classified as an initial constant rate period (CRP), followed by a falling rate period. In the early stage of CRP, the movement of micro-particles induced coating shrinkage and tensile stresses developed due to the constraint of the substrate. Thereafter, the nano-slurry continued to dry and shrink around the static micro-particles without further coating shrinkage, but with an increase in the stress. The addition of micro-particles reduced the level of the maximum stress and prevented crack formation during drying. The microstructure of green coatings shows the presence of nano-particles between micro-particles that cement the micro-particles together.     

Constrained sintering of YSZ/Al2O3 composite coatings on metal substrates produced from eletrophoretic deposition

Yttria stabilized zirconia/alumina (YSZ/Al₂O₃) composite coatings were prepared from electrophoretic deposition (EPD), followed by sintering. The constrained sintering of the coatings on metal substrates was characterized with microstructure examination using electron microscopy, mechanical properties examination using nanoindentation, and residual stress measurement using Cr³⁺ fluorescence spectroscopy. The microstructure close to the coating/substrate interface is more porous than that near the surface of the EPD coatings due to the deposition process and the constrained sintering of the coatings. The sintering of the YSZ/Al₂O₃ composite coating took up to 200 h at 1250 °C to achieve the highest density due to the constraint of the substrate. When the coating was sintered at 1000 °C after sintering at 1250 °C for less than 100 h, the compressive stress was generated due to thermal mismatch between the coating and metal substrate, leading to further densification at 1000 °C because of the ‘hot pressing’ effect. The relative densities estimated based on the residual stress measurements are close to the densities measured by the Archimedes method, which excludes an open porosity effect. The densities estimated from the hardness and the modulus measurements are lower than those from the residual stress measurement and the Archimedes method, because it takes account of the open porosity.     

Conductive Polymer Coating on Nonconductive Ceramic Substrates for Use in the Electrophoretic Deposition Process

Uniform coating and line patterning of a conductive polypyrrole (Ppy) film on nonconductive ceramic materials were performed for use as substrates in the electrophoretic deposition (EPD) process. The Ppy was synthesized by chemical oxidation in the pyrrole solution. Direct shaping or line patterning of alumina or zirconia particles by EPD was carried out using the Ppy films as cathodes.     

Investigation of shrinkage control in Ni–Al2O3 (Metal–Ceramic) functionally graded materials

Nickel–alumina (metal–ceramic) graded composites were fabricated based on empirical relationships that were used to predict shrinkage expected during the sintering process. Compositions ranged from pure Al₂O₃ to nickel. Compositions with different sintering behaviors were composed of Ni powders having 3-μm particles and Al₂O₃ powders having 0.16-μm particles that were mixed in specific volume percentages. The green and sintered densities were measured for each layer of the multi-layered samples. Based on these measurements, an empirical formulation for shrinkage was derived to express the relationship between green and sintered densities. Shrinkage was predicted from the observed decrease in porosity during sintering. This was used to establish a relationship between shrinkage and composition. Dramatic shrinkage, which can lead to cracking of the sample, was resolved by minimizing shrinkage differences within the functionally graded materials using the derived empirical formulation for shrinkage; internal cracking was significantly reduced by maintaining a consistent shrinkage gradient.     

Yttrium Silicate Coatings for Oxidation Protection of Carbon–Silicon Carbide Composites

Yttrium silicate (Y₂SiO₅) coatings complement SiC coatings for protecting ceramic multilayer composite materials based on carbon-fiber-reinforced SiC composites (C-SiC). Thick (100 μm), dense Y₂SiO₅ coatings were prepared by dip coating, using concentrated aqueous slips. The resulting phases were studied by taking into account the simultaneous presence of oxide and non-oxide materials, which affected the chemical stability of the coatings. Thick, mechanically stable coatings were obtained by sintering in carbon crucibles and a SiC bed in an argon-flow furnace. Pure Y₂SiO₅ coatings completely separated from the SiC substrates. A high percentage of Y₂Si₂O₇ was necessary to fit the thermal expansion coefficients and ensure the stability of the coatings. Oxidation resistance of the coated substrates was investigated by isothermal and stepwise oxidation tests.     

High performance environmental barrier coatings, Part II: Active filler loaded SiOC system for superalloys

Polymer derived ceramic (PDC) matrix composite coatings are a promising candidate to be used as alternative environmental barrier coatings, such as in oxidation protection. This paper reports the processing of three PDC coating systems on a superalloy substrate, Inconel 617, using a simple dip coating method. The performance of SiON coating, particle filled SiOC coating, and their combined coating system is evaluated by long time static oxidation testing at 800 °C for 100–200 h. Two key parameters have been analyzed: the weight gain of the coating samples and the thickness of the thermally grown oxide (TGO) layer at the ceramic–metal interface. Results show that all three coating systems are able to significantly reduce the weight gain of metal substrates due to oxidation. However, the SiON bond coat + particle filled SiOC top coat double-layer coating system is most effective in maintaining the integrity of the substrate in the investigated 200 h. Based on the kinetics of oxidation, likely rate controlling steps are identified.     

High performance environmental barrier coatings, Part I: Passive filler loaded SiCN system for steel

A novel environmental barrier coating system for steel consisting of a perhydropolysilazane (PHPS) bond coat and a polysilazane-based glass/ceramic composite top coat has been developed. After stabilising the coating slurries, double layers were applied on mild and stainless steel substrates by the dip-coating technique. Parameters like pre-treatment of the steel substrates, filler systems, particle size of the fillers or coating thickness were varied to optimize the coatings. The thermal treatment was performed in air at temperatures up to 800 °C. Microstructural analysis by SEM and XRD revealed the formation of a coating system consisting of a SiNO bond coat and a ZrO₂-filled glass/ceramic top coat. A uniform, well adherent, dense and crack-free coating system with a noteworthy thickness up to 100 μm was achieved. Even after cyclic oxidation tests on coated samples at 700 °C the coating system was still undamaged and no oxidation occurred on the mild steel substrates.     

Bioactive glass coating using aerosol deposition

This work demonstrates the successful deposition of bioactive glass (BG) 45S5 coatings on various metallic and ceramic substrates at room temperature under low vacuum condition by using aerosol deposition (AD). This room temperature and particle impact consolidation-based deposition method enabled us to deposit well-adhered and dense BG coatings directly on metallic and ceramic substrates. In vitro tests with human osteoblast-like cells on substrates with a 45S5 BG coating demonstrated high cell activity on the surfaces. All tested materials exhibited high in vitro biocompatibility as no inhibition in cell proliferation could be observed. The utilization of AD process for achieving non-crystalline BG coatings is promising for practical bio-medical applications, e.g., bioactive coatings on bioinert metallic and ceramic substrates.     

Electrophoretic Deposition of YSZ Particles on Non-Conducting Porous NiO–YSZ Substrates for Solid Oxide Fuel Cell Applications

This paper reports a method of performing electrophoretic deposition (EPD) on non-conducting substrates overcoming the requirement of a conducting substrate through the use of porous substrates. The conductivity of the substrate is therefore no longer a limiting factor in the application of EPD. This method is applicable to the fabrication of thick or thin layers of ceramic or metal for various applications. As an example, thin and dense yttria-stabilized zirconia (YSZ) layers have been deposited on a non-conducting NiO–YSZ substrate by EPD from a non-aqueous suspension. A solid oxide fuel cell constructed on these sintered bilayers exhibited power densities of 384 and 611 mW/cm² at 750° and 850°C, respectively.     

Fabrication of yttria-stabilized-zirconia thick coatings via slurry process with pressure infiltration

Yttria-stabilized-zirconia (YSZ) coatings with thicknesses up to 420 μm have been prepared using a novel slurry process with pressure infiltration. Binary-sized particle slurries (binary-slurries), composed of nano-particle slurry (nano-slurry) and micro-sized preformed particles, were cast on metal substrates to form coatings. After sintering at 1150 °C for 1 h, preformed particles were cemented with nano-particles to form a porous YSZ coating. Subsequently, the nano-slurry was infiltrated into the porous coatings under pressure. The infiltrated nano-slurry filled the pores, and was sintered together with the porous coating, resulting in an increase in both density and mechanical properties of the coating. After 5–6 infiltration cycles, the coating reached 82% theoretic density and micro-hardness of 3.7 GPa. Such coatings could be used as thermal barrier coatings for high temperature applications.     

涂覆工艺对金属基陶瓷涂层高温抗氧化性能影响的研究

将陶瓷粘结料与Cr2O3或重烧MgO制成料浆,采用刷涂、浸渍和喷涂三种涂敷方法和高温熔烧工艺制备金属基Cr2O3或MgO质高温抗氧化陶瓷涂层。研究了三种涂敷方法、涂层厚度对涂层试样高温抗氧能力的影响规律。结果表明:当料浆组成、涂层厚度和熔烧制度相同条件下,采用喷涂工艺所制备的涂层试样具有最佳的抗氧化能力,且Cr2O3和MgO质陶瓷涂层在1200℃、30h的抗氧化能力分别是金属基体的约62倍和16倍。采用SEM对涂层的显微结构进行了表征,揭示涂覆工艺与涂层结构和抗氧化能力之间的关系。     

无溶剂环氧涂层体积收缩率的研究

涂料在聚合过程中会产生体积收缩,而体积收缩将导致涂层的附着强度大大降低,还可能加速涂层老化的进程。本研究采用三维数字显微镜测试、扫描电镜测试和电化学交流阻抗谱测试等检测方法,对不同体积收缩率的无溶剂环氧涂层进行了性能分析,明确了体积收缩率对无溶剂环氧涂层的微观结构及防腐性能的影响。研究结果表明,调整颜基比可以控制无溶剂环氧涂层体积收缩率的大小;具有较小体积收缩率的涂层其抗渗透能力较好,具有较好的涂层防腐性能。     

Design,thermal shock resistance of dense BaO-Al2O3-SiO2 glass/Si3N4-barium feldspar coating for porous Si3N4 ceramic

Silicon nitride-monoclinic barium feldspar (Si₃N₄-m-BAS) composite possesses great dielectric properties, low density, and low thermal expansion coefficient (CTE). Preparing dense Si₃N₄-m-BAS coating on porous Si₃N₄ ceramic is an effective strategy to improve its water resistance and ensure its dielectric performances. However, this promising coating has not been reported yet, because the synthesis of m-BAS is difficult, and the densification of Si₃N₄-BAS composite requires very high temperature. Here, the BaO-Al₂O₃-SiO₂ glass/Si₃N₄-BAS coating was first fabricated by a manual spray method and pressureless sintering at 1450°C. Combining the influence of Si⁴⁺ on the crystal phase composition of BAS and the volume expansion effect of silicon in N₂, an effective coating structure design scheme was proposed. By changing the content of silicon powder, the CTE and horizontal shrinkage of the coating during sintering were controlled. Besides, the prepared coatings exhibited low water absorption and high bonding strength. During the thermal shock tests, SiO₂ produced by the oxidation of Si₃N₄ healed the cracks in the coating, thus delaying the degradation of the properties. The coating prepared in this work is expected to be applied to radome in extreme service environments.     

Structural characterization of YSZ/Al2O3 nanostructured composite coating fabricated by electrophoretic deposition and reaction bonding

Suspension of YSZ and Al particles in acetone in presence of 1.2 g/l iodine as dispersant was used for electrophoretic deposition of green form YSZ/Al coating. Results revealed that applied voltage of 6 V and deposition time of 3 min were appropriate for deposition of green composite form coating. After deposition, a nanostructured dense YSZ/Al₂O₃ composite coating was fabricated by oxidation of Al particles at 600 °C for 2 h and subsequently sintering heat treatment at 1000 °C for 2 h. Melting and oxidation of Al particles in the green form composite coating not only caused reaction bonding between the particles but also lowered the sintering temperature of the ceramic coating about 200 °C. The EDS maps confirmed that the composition of fabricated coating was uniform and Al₂O₃ particles were dispersed homogenously in YSZ matrix.