RARE-EARTH SILICATES AS ENVIRONMENTAL BARRIER COATINGS FOR Si BASED NON OXIDE CERAMICS
Abstract :
Environmental barrier coatings (EBCs) are provided to protect the Si-based ceramics against water-vapour attack. Presently, the EBCs are based on rare-earth silicates which have higher temperature capability, lower volatility, and superior thermo-chemical compatibility compared to the earlier materials. In this work, yttrium silicates (Y2SiO5 and Y2Si2O7) and ytterbium silicates (Yb2SiO5 and Yb2Si2O7) are evaluated as potential EBC materials. Sol-gel synthesis of yttrium monosilicate is the main focus to study the phase selection. It is seen that acid catalyzed sol helps to maintain homogeneity in the synthesis of yttrium monosilicate. Presence of a thermodynamic miscibility gap around the monosilicate composition in the liquid phase indicates that phase separation occurs in the amorphous stage prior to crystallization. This phase separation leads to the formation of secondary phases upon crystallization. Yttrium disilicate also exhibits miscibility gap and is synthesized in base catalyzed condition since it maximizes the disilicate phase fraction upon calcination. This theory holds good for the ytterbium silicate compounds as well. It was also understood that in aqueous sol-gel syntheses involving silica, water and ethanol play a crucial role. The ratio of water/TEOS/ethanol affects the rates of hydrolysis and condensation influencing the phase formation upon calcination. The synthesized powders obtained from the sol-gel synthesis of the compounds were used to prepare dense pellets. The pellets were used to determine the CTE and Vickers hardness of the compounds.
Yttrium and ytterbium silicate in-situ coatings were developed by spraying their respective oxides on SiC substrates. The coatings were heated up to 1400 °C to allow the formation of yttrium/ytterbium silicate compounds. The coatings were exposed to oxy-acetylene flame to study their ablation behaviour at different temperatures. No significant mass loss was observed after exposure to the flame for 18 h for both yttrium and ytterbium silicate compounds. SEM, EDS and XRD were used to explain the changes in the microstructure and phase formation during exposure to the flame. Overall, it was seen that ytterbium silicate coatings showed better ablation resistance than yttrium silicate coatings.