Papers by Keyword: Yttrium Silicate

Abstract: A novel celsian/yttrium silicate coating was deposited on carbon fiber reinforced silicon carbide matrix (C/SiC) composite, using BaO-Al₂O₃-SiO₂ (BAS) glass andY₂O₃ powder as starting materials. The effects of sintering temperatures on microstructure and performance of coatings were studied. The results show that the final phases of sintered coatings are composed of celsian, yttrium silicate and remnant glass. The coating are dense, crack-free and pore-free in macroscopic scales when sintered at temperatures above 1400°C. The crystal grains in the coating grow too large, and the coating is loose in microstructure when sintered at temperatures higher than 1450°C. The coated samples sintered at 1450°C for 30min, which have the densest morphology and microstructure, have the lowest weight loss of 0.13 % after oxidation at 1500°C for 90 min.

Abstract: Continuous carbon fibre reinforced silicon carbide (C/SiC) composites were fabricated by precursor infiltration and pyrolysis (PIP) process, a mullite/yttrium silicate multilayer coating was prepared by plasma spray method as the oxidation protective coating. The efficiency of the coating against oxidation was characterized by means of heat treatment of the C/SiC composites at 1500 °C in static air for 1 hour. The results indicated that the weight loss of the coated composites was only 3.4 %, and the flexural strength of the composites retained 95.3 % of the original strength.

Abstract: . In the present work we used the sol-gel process to prepare Y2SiO5 precursor sols suitable for electrophoretic deposition (EPD). The sol synthesis was performed through the controlled hydrolysis of alkoxide solutions of tetraethoxysilane and yttriumoxoisopropoxide. During sol development emphasis was put on characterization of particles size and zeta potential of the formed aggregates. We succeeded in synthesizing a clear sol containing polymeric aggregates with acceptable particle charge. The electrophoretic deposition on glassy carbon or C/C-SiC slabs led to homogenous layers. At low sol concentrations micro cracks in the deposited layers were observed whereas higher concentrations led to thin and dense layers. During constant current EPD a constant voltage was recorded indicating that the deposited layer does not lead to an increase in resistivity in this kind of EPD system.

Abstract: The use of carbon fibre reinforced carbon composites in oxidizing atmospheres is limited to temperatures below 400 °C. To benefit from their excellent mechanical strength that is still preserved at high temperatures, suitable oxidation protection coating systems have to be developed. Composites which are capillary infiltrated with Si and coated with SiC via chemical vapour deposition show significantly enhanced oxidation resistance. For the increase of service temperature above 1300 °C, high temperature stable materials with low oxygen diffusivities such as yttrium silicates have to complement the SiC coating. The electrophoretic deposition performed under constant current conditions leads to relatively high green densities and therefore good sinterability of the applied coatings. In this work we present the preparation of suspensions, their characterization regarding particle size and electrophoretic mobility for yttrium silicate powder prepared by the solid state method. Depending on particle charge and conductivity of the investigated suspensions iodine is employed to increase particle charge. The use of current densities between 0.5-5mA/cm² leads to smooth and homogeneous layers. Layers sintered as low as 1400 °C for 2h already show promising protection of the C/C-Si-SiC substrate during thermogravimetric analysis.

Abstract: Yttrium silicates coatings were deposited on SiC-C/C composites surface by a hydrothermal electrophoretic process using yttrium silicates nano-crystallites, isopropanol and siodine as source materials, solvent and charging agent, respectively. The yttrium silicates nano-crystallites were preprepared by a sonochemical process. The influence of deposition temperatures on the microstructures and crystallization of the yttrium silicates coatings were investigated. Surface and cross-section microstructures of the as-prepared multi-coatings were characterized by XRD and SEM. Results show that the coatings are composed by yttrium silicates crystallites with the main phase of Y2Si2O7 and a little Y2SiO5. The thickness and the density of the yttrium silicates coatings increases with the increase of deposition temperatures; and the coating process has little influence on the phase compositions of the coatings.

Abstract: Due to favorable chemical and mechanical properties of yttrium silicate coatings (low Young’s modulus, low thermal expansion coefficient, low evaporation rate and oxygen permeability, good erosion resistance), this material is a promising complement to SiC coatings for protecting C/C-Si-SiC composites. For the preparation of silicate powders, the Pechini method was used. As a coating method, electrophoretic deposition from stable suspensions based on isopropanol was chosen. Under controlled deposition voltage and duration, coatings of various thicknesses were deposited. The deposited layers were characterized by SEM and EDX analysis. The protectiveness of these coatings was tested by isothermal thermogravimetry.