Fabrication of composite coatings using a combination of electrochemical methods and reaction bonding process

The major difficulty in fabricating ceramic coatings on metal substrates using the electrophoretic deposition process (EPD) is problems caused by the volume shrinkage during the sintering of the green form ceramic coatings produced by EPD. Numerous cracks normally form in the EPD coating during sintering. In this work, we have developed the reaction bonding process to fabricate crack-free and dense ceramic coatings, where the volume shrinkage is compensated by the volume expansion due to the oxidation of aluminium in the green form coatings during sintering in air. Both EPD and electroplating were used here to produce green form coatings which contain aluminium particles and, in some cases, an intermediate nickel layer. During the subsequent heat treatment, melting and oxidation of the metals in the green form coating promote densification during sintering. By these means, relatively dense composite coatings have been fabricated on metal substrates.     

Fabrication technologies for oxide–oxide ceramic matrix composites based on electrophoretic deposition

Electrophoretic deposition (EPD) was used to fabricate alumina matrix composites with high volume fraction of woven fibre mat (Nextel™ 720) reinforcement in a multilayer structure. Colloidal suspensions of Al₂O₃ nanoparticles in ethanol medium with addition of 4-hydrobezoic acid were used for EPD. Two different techniques were developed for fabrication of Al₂O₃ matrix/Nextel™ 720 fibre composites. The first method is a combination of standard EPD of single fibre mats with a subsequent lamination procedure to fabricate the multilayered composite. The second method involves the simultaneous infiltration of several (three or more) Nextel™ 720 fibre mats by EPD in a tailor-made cell. The composites exhibit a homogeneous matrix microstructure, characterised by a very high particle packing density and relatively low porosity after sintering at 1300 °C. The EPD cell allows production of relatively large bodies (10 cm diameter). By combination of the multilayer EPD infiltration and lamination processes developed here, thick ceramic matrix composite components (>10 mm thickness) can be fabricated, which opens the possibility of greater industrial application of the materials.     

Development and characterisation of high-density oxide fibre-reinforced oxide ceramic matrix composites with improved mechanical properties

A low cost and reliable ceramic matrix composite fabrication route has been developed. It involves the coating of 2D woven ceramic fibres (Nextel? 720) with oxide nano-size ceramic particles by electrophoretic deposition (EPD) followed by impregnation of the coated fibres with ceramic matrix and warm pressing at 180 °C to produce the “green” component ready for pressureless sintering. The effects of two different weak interface materials, NdPO₄ and ZrO₂, on the thermomechanical properties of the composites are also examined. Damage mechanisms, such as debonding, fibre fracture, delamination and matrix cracking within the composite plates subjected to tensile loading are analysed using acoustic emission technique and correlated with microstructure. It is shown that the composites with NdPO₄ interface, 10% porosity and 40 vol.% fibre loading have superior themomechanical properties in terms of strength and damage-tolerant behaviour in multilayer plate form. The improved sinterability and microstructure stability at moderate temperatures ensure both the fibre integrity and load transfer efficiency resulting in high strength damage-tolerant composites. The final components produced are considered to be suitable for use as shroud seals and insulating plates for combustor chambers in aircraft engines.     

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.     

Nickel–alumina graded coatings obtained by dipping and EPD on nickel substrates

Nickel substrates have been coated by Ni/Al₂O₃ composite films by a dipping process using aqueous suspensions that contain a temporary binder. Two-layer and three-layer graded coatings have been produced, consisting of pure Ni powder and Ni/Al₂O₃ composites with Al₂O₃ contents of 15 and 30 vol.% as intermediate layers to release sintering and thermal stresses. The laminates were further coated with a ceramic layer of Al₂O₃/ZrO₂ that was deposited by electrophoretic deposition using a non-aqueous suspension. A continuous, thin Al₂O₃ layer surrounding Ni grains developed at the intermediate composite layer of Ni/Al₂O₃ allows the ceramic coating to maintain strongly adhered to the nickel substrate by means of a porous substrate/coating interface.     

Fabrication of ceramic membranes on porous ceramic supports by electrophoretic deposition

Abstract

Nanoporous alumina membrane and continuous zeolite L membrane were fabricated on the inner surface of microporous alumina tubes. In the former case, an electrophoretic deposition (EPD) technique was used for the deposition of bimodal alumina particles for the subsequent low temperature sintering. In the latter case, the EPD was used for the seeding process of zeolite L particles for the subsequent hydrothermal synthesis. A thin layer of polypyrrole was synthesised on the inside wall of the porous tubes by the chemical polymerisation of pyrrole to give the wall electric conduction for the EPD electrode. The thickness of the coating layers was controlled by altering the applied voltage and deposition time. The interfacial connection of the alumina or zeolite coated layer and the substrate was evaluated by SEM observations before and after the thermal treatment. The nanoporous structure of the alumina membrane was also characterised by a pore size analyser.     

Preparation and high-temperature oxidation resistance of multilayer MoSi2/MoB coating by spent MoSi2-based materials

Spent MoSi₂ and MoB were used as raw materials to prepare multilayer MoSi₂/MoB coating on molybdenum by the two-step method of slurry deposition and spark plasma sintering. The results showed dense MoSi₂/MoB coating after sintering while penetrated cracks appeared in MoSi₂ coating due to coefficient of thermal expansion mismatch between the Mo substrate and coating. After the sintering of MoSi₂/MoB coatings, MoB and Mo₂B diffusion layers were formed between MoB transition layer and Mo substrate without defects, exhibiting good metallurgical bonding. The high-temperature oxidation behavior of coatings (1500°C) was also explored. After oxidation of 50 h at 1500°C, lowest mass gain (0.035 mg/cm²) was obtained for MoSi₂/MoB coating, and the oxide scale was dense and complete without voids, making the oxygen diffusion at elevated temperature inhibited. Compared with MoSi₂ coating under the same oxidation conditions, relatively thinner silica oxide scale was acquired by MoSi₂/MoB coating because of the reduction of cracks, and the multilayer coating exhibits better anti-oxidation properties at high temperature.     

Fabrication method and mechanical properties of biomimetic Cf/ZrB2-SiC ceramic composites with bouligand structures

Herein, biomimetic C_f/ZrB₂-SiC ceramic composites with bouligand structures are fabricated by combining precursor impregnation, coating, helical assembly and hot-pressing sintering. First, C_f/ZrB₂-SiC ceramic films are achieved through a precursor impregnation method using polycarbosilane (PCS). Second, the PCS-C_f/ZrB₂-SiC ceramic films are coated with ZrB₂ and SiC ceramic layers. Finally, hot-pressing sintering is employed to densify helical assembly C_f/ceramic films with a fixed angle of 30°. The microstructures and carbon fiber content on the mechanical properties of biomimetic C_f/ZrB₂-SiC ceramic composites are analyzed in detail. The results show that the coated ceramic layer on PCS-C_f/ZrB₂-SiC films can heal the cracks formed by pyrolysis of PCS, and the mechanical properties are obviously improved. Meanwhile, the mechanical properties could be tuned by the contents of the carbon fiber. The toughening mechanisms of C_f/ZrB₂-SiC ceramic composites with bouligand structures are mainly zigzag cracks, crack deflection, multiple cracks, carbon fiber pulling out and bridging.     

Electrophoretic deposition of carbon nanotube–ceramic nanocomposites

The purpose of this paper is to present an up-to-date comprehensive overview of current research progress in the development of carbon nanotube (CNT)–ceramic nanocomposites by electrophoretic deposition (EPD). Micron-sized and nanoscale ceramic particles have been combined with CNTs, both multiwalled and single-walled, using EPD for a variety of functional, structural and biomedical applications. Systems reviewed include SiO₂/CNT, TiO₂/CNT, MnO₂/CNT, Fe₃O₄/CNT, hydroxyapatite (HA)/CNT and bioactive glass/CNT. EPD has been shown to be a very convenient method to manipulate and arrange CNTs from well dispersed suspensions onto conductive substrates. CNT–ceramic composite layers of thickness in the range <1–50 μm have been produced. Sequential EPD of layered nanocomposites as well as electrophoretic co-deposition from diphasic suspensions have been investigated. A critical step for the success of EPD is the prior functionalization of CNTs, usually by their treatment in acid solutions, in order to create functional groups on CNT surfaces so that they can be dispersed uniformly in solvents, for example water or organic media. The preparation and characterisation of stable CNT and CNT/ceramic particle suspensions as well as relevant EPD mechanisms are discussed. Key processing stages, including functionalization of CNTs, tailoring zeta potential of CNTs and ceramic particles in suspension as well as specific EPD parameters, such as deposition voltage and time, are discussed in terms of their influence on the quality of the developed CNT/ceramic nanocomposites. The analysis of the literature confirms that EPD is the technique of choice for the development of complex CNT–ceramic nanocomposite layers and coatings of high structural homogeneity and reproducible properties. Potential and realised applications of the resulting CNT–ceramic composite coatings are highlighted, including fuel cell and supercapacitor electrodes, field emission devices, bioelectrodes, photocatalytic films, sensors as well as a wide range of functional, structural and bioactive coatings.     

Analysis of corrosion protection behavior of Al2O3-TiO2 oxide ceramic coating on carbon steel pipes for petroleum industry

Corrosion is the deterioration of materials by chemical interaction with their environment. In the oil and gas industry, corrosion of the pipelines and other equipment is one of the leading causes of failure and the corrosion-related costs are very high. Hence, corrosion protection is an essential requirement. In this study, the objective is to analysis of the corrosion protection behavior of spray Alumina-Titania (Al₂O₃-TiO₂) oxide ceramic coating on carbon steel pipes C45 using two different thermal spray coatings processes. These two different thermal spraying coating, High velocity oxy-fuel (HVOF) and plasma thermal spraying techniques can be used instead of extensive treatment by expensive chemical formation of coatings on pipelines and equipment to improve or restore a component's surface properties or dimensions and to protect them from corrosion. Molten or semi-molten ceramic composite powders are sprayed on the surface in order to produce a dense coating layer. FESEM of coated samples showed that a high temperature of plasma coating method end in melting the ceramic powders and creation of completely melted regions on the coated samples’ surface compared to HVOF coating techniques. Corrosion testing of coated samples in seawater (3.5% NaCl) was conducted within 30 days. Electrochemical impedance spectroscopy (EIS) as well as potentiodynamic polarization outcomes represented that the corrosion resistivity of plasma coating technique for this type of ceramic composite is better than HVOF coating technique. However, both types of coating techniques are protecting the substrate against seawater.     

High strength silica-based ceramics material for investment casting applications: Effects of adding nanosized alumina coatings

A nanosized alumina coating was synthesized on the surface of fused silica particles by electrostatic attraction. The effects of the coated fused silica particles on the cristobalite crystallization behavior, microstructure evolution, and flexural strength of silica-based ceramic cores were investigated. X-ray diffraction (XRD) was used to characterize phase transformations in the specimens, and the results indicated that the formed nanosized alumina coatings could retard cristobalite formation by inducing compressive stress on the fused silica particle surface. A mullite phase was also found due to the reaction of the nanosized alumina coating and the surface of the fused silica when the sintering temperature was increased to 1300 °C. Analysis using scanning electron microscopy equipped with energy dispersive spectrometry (SEM/EDS) suggested that alumina nanoparticles in the coated layer dispersed into a liquid phase and formed a barrier layer to impede the movement of the liquid phase, preventing the pore-filling process and increasing the open porosity of the ceramic specimens. Flexural strengths at room temperature were tested, indicating that increases in the sintering temperature of the specimens without coated fused silica powders had little effect on flexural strength. However, the flexural strength of the specimens with coated fused silica powders increased with increases in sintering temperature. The improvement in flexural strength was related to the reinforcement by sintering necks between particles and the improvement in the strength of the coated fused silica powder.     

Thermal stability and ablation properties study of aluminum silicate ceramic fiber and acicular wollastonite filled silicone rubber composite

The thermal stability and ablation properties of silicone rubber filled with silica (SiO₂), aluminum silicate ceramic fiber (ASF), and acicular wollastonite (AW) were studied in this article. The morphology, composition, and ablation properties of the composite were analyzed after oxyacetylene torch tests. There were three different ceramic layers found in the ablated composite. In the porous ceramic layer, the rubber was decomposed, producing trimers, tetramers, and SiO₂. ASF and part of AW still remained and formed a dense layer. The SiO₂/SiC filaments in the ceramic layer reduced the permeability of oxygen, improving the ablation properties of the composites. The resultant ceramic layer was the densest, which acted as effective oxygen and heat barriers, and the achieved line ablation rate of the silicone composite were optimum at the proportion of 20 phr/40 phr (ASF/AW). Thermogravimetric analysis (TGA) confirmed that thermal stability of the composites was enhanced by the incorporation of ASF and AW. The formation of the ceramic layer was considered to be responsible for the enhancement of thermal stability and ablation properties. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39700.     

Water vapor corrosion behavior and failure mechanism of plasma sprayed mullite/Lu2Si2O7-Lu2SiO5 coatings

A new coating with mullite as the inner layer and Lu₂Si₂O₇-Lu₂SiO₅ (lutetium disilicate-lutetium monosilicate, LuDS-LuMS) composite as the top layer was designed and fabricated on the surface of porous SiC substrate by atmospheric plasma spraying (APS). The microstructure evolution, phase transformation, inter-diffusion and failure mechanism of the coated samples during steam cycling at 1450?°C were systemically investigated. The results indicated that the coated samples maintain weight gain and the average weight gain rate is 1.12?×?10^?1 mg/cm² h. The thermal expansion coefficient (CTE) mismatches between the coating and the substrate as well as between the two ceramic layers have produced a thermal mismatch stress. In addition, the chemical reaction induced the sintering of LuDS-LuMS layer, the amorphous-crystalline phase transformation and the transformation between LuDS and LuMS phases could result in aging stress. With the accumulation of thermal mismatch stress and aging stress, the through and horizontal cracks have appeared in the coating, leading to the coating failure.     

Graphene added multilayer ceramic sandwich (GMCS) composites: Structure,preparation and properties

The multilayer ceramic composites (MLC) consist of two ceramic materials insoluble in each other and sequentially piled in a symmetric manner whereas they can be divided into two groups: multilayer composites with weak interfaces and composites with strong interfaces. The graphene added multilayer ceramic sandwich (GMCS) composite was developed. The multilayer stack of Si₃N₄ with 5 and 30 wt% graphene addition were stratified in sandwich structure. So formed multilayer stacks with 5 and 7 layers were sintered by hot issostatic pressing (HIP). The homogenity of graphene addition, the effect of layered structures and the position of layers with lower and higher graphene content on the final properties were studied.     

Fabrication of Mullite and Mullite-Matrix Composites by Transient Viscous Sintering of Composite Powders

Mullite was fabricated by a process referred to as transient viscous sintering (TVS). Composite particles which consisted of inner cores of α-alumina and outer coatings of amorphous silica were used. Powder compacts prepared with these particles were viscously sintered to almost full density at relatively low temperatures (∼1300°C). Compacts were subsequently converted to dense, fine-grained mullite at higher temperatures (∼1500°C) by reaction between the alumina and silica. The TVS process was also used to fabricate mullite/zirconia/alumina, mullite/silicon carbide particle, and mullite/silicon carbide whisker composites. Densification was enhanced compared with other recent studies of sintering of mullite-based composites. This was attributed to three factors: viscous flow of the amorphous silica coating on the particles, avoidance of mullite formation until higher temperatures, and increased threshold concentration for the development of percolation networks.     

Characterization of Chemical Vapor-Deposited (CVD) Mullite+CVD Alumina+Plasma-Sprayed Tantalum Oxide Coatings on Silicon Nitride Vanes After an Industrial Gas Turbine Engine Field Test

Silicon nitride ceramic vanes coated with chemical vapor-deposited (CVD) mullite, CVD alumina, and plasma-sprayed tantalum oxide were exposed to field tests in an industrial gas turbine engine. Results varied due to expected non-uniformities in the CVD coating microstructures, but dense CVD mullite/alumina showed excellent stability and protective capacity after 1148 h of engine testing. Surfaces without CVD coatings experienced massive intragranular subsurface oxidation and/or rapid recession of the ceramic substrate due to volatilization of silica species formed by oxidation. These results suggest that thin (<5 μm), dense, high-purity CVD mullite and CVD alumina are viable components for an environmental barrier coating system to protect structural ceramics in combustion environments.     

Microstructure and anti-oxidation properties of multi-composition ceramic coatings for carbon/carbon composites

To protect carbon/carbon (C/C) composites from oxidation at elevated temperature, an effective WSi₂-CrSi₂-Si ceramic coating was deposited on the surface of SiC coated C/C composites by a simple and low-cost slurry method. The microstructures of the double-layer coatings were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy analyses. The coating exhibited excellent oxidation resistance and thermal shock resistance. It could protect C/C composites from oxidation in air at 1773 K for 300 h with only 0.1 wt.% mass gain and endure the thermal shock for 30 cycles between 1773 K and room temperature. The excellent anti-oxidation ability of the double-layer WSi₂-CrSi₂-Si/SiC coating is mainly attributed to the dense structure of the coating and the formation of stable vitreous composition including SiO₂ and Cr₂O₃ produced during oxidation.     

A new technique for preparation of PDMS/ceramic nanocomposite membrane for gaseous hydrocarbons separation

A novel composite membrane using polydimethylsiloxane (PDMS) as a top active layer and ceramic nanocomposite as the support layer was developed for the gaseous hydrocarbons separation. For the fabrication of hybrid membranes, nanocomposite technology applied for manufacturing ceramic supports with controllable microstructures. Also, a new method was used for coating a uniform and no penetrated polymeric layer. Top layer of ceramic support with nanocomposite microstructures was fabricated using 5 wt % α‐Al₂O₃‐SiO₂ bidispersed suspensions with optimum weight fraction of second phase (SiO₂) based on the fractional collision frequency theory. PDMS selective layer was coated on the outer surface of the porous ceramic nanocomposite support by dip‐coating method. In this respect, the effect of several parameters such as pretreatment temperature, PDMS solution concentration, and number of coated polymeric layers on prepared layers morphology and hybrid membrane performance in the separation of condensable hydrocarbons (iso and n‐butane) from hydrogen were investigated. The results showed that the membranes fabricated at 140°C as pretreatment temperature and three polymeric layers by 7, 15, and 15 wt % PDMS concentration, respectively, had a high selectivity (>25 at 2 bar)) in C₄H₁₀/H₂ separation. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Ablation behavior of a network interlacing ZrC-VC ceramic coating prepared by a pioneering spillover permeation

A novel network interlacing ZrC-VC ceramic coating was prepared by a pioneering spillover permeation. With the increase of Zr content in the blind vias, the content of ZrC in the coating and the density of the coating all decrease. The density of the coating on C/C–ZrC–SiC substrate is obviously higher than that on C/C substrate. The linear ablation rate of the novel ceramic coated C/C–ZrC–SiC composites was ?0.06 μm/s with about 20 and 1.56 times reduction than C/C composites and C/C–ZrC–SiC composites respectively. The improved ablation resistance was attributed to a dense honeycomb ZrO₂ layer filled with liquid vanadium oxide in the ablation center and the improved thermal radiation.     

Gradient MoSi2-borosilicate glass high emissivity coating with enhanced contact and impact damage resistance

High emissivity coatings on fibrous insulation tiles played an important role in thermal protection systems and thereby intrigued many researchers; however, there was little emphasis on the mechanical properties of the coatings. In this study, a gradient MoSi₂-borosilicate glass coating with a dense surface layer and a porous interlayer was designed for mullite fibrous ceramics. Mechanical properties and structure parameters of the coating layers and the substrate were studied. The gradient coating was compared with a monolayer dense coating of the same composition and same surface density in contact damage resistance, impact resistance and emissivity. Compared with monolayer dense coating coated substrates, the gradient coating coated ones exhibited two times higher load bearing capacity in Hertzian indentation test at the same displacement of 1?mm; they appeared to be stiffer and harder at constant load of 20?N, and showed better impact resistance at impact energy range of 0.25–0.75?J in the falling weight test; besides, fatal radial cracks were not observed in gradient coatings after the tests. In addition, the gradient coating had higher emissivity (0.838) than the monolayer dense coating (0.816) because of the significant absorptivity increase and reflectivity decrease by small gradual slopes in the rough surface.