Diffusivity of Oxygen in Yttrium Silicates Estimated By Using High-Temperature Oxidation of Their Composites Dispersed with SiC Particles

Abstract

Yttrium silicates, Y2SiO5 and Y2Si2O7, have been studied as candidate materials for the environmental barrier coating (EBC) system on SiC/SiC composites for advanced gas turbine blades because of low oxygen permeability, low volatility in steam conditions, and comparable thermal expansion with SiC/SiC composites. In order to design the EBC system, diffusion properties of yttrium silicates are important. In this work, diffusivity of oxygen in yttrium silicates is estimated by using kinetics of high-temperature oxidation of their composites dispersed with SiC particles. Assuming oxidation of SiC particles dispersed in yttrium silicates is promoted mainly with inward diffusion of oxygen, diffusion of oxygen in yttrium silicates can be estimated. Y2SiO5 powder was synthesized by using the solid-state reaction technique with Y2O3 and SiO2 powders at 1400ºC for 20 h in air. Y2Si2O7 powder was prepared with the solid-state reaction process from powder mixture of Y2O3 and SiO2 at at 1500ºC for 24 h in air. Densified bulks of yttrium silicates dispersed with 5 vol% SiC particles were obtained with the pulsed electric current sintering of powder mixture of yttrium silicate and SiC. The bulk samples were oxidized at 1300ºC in air. Depth of the zone in which SiC particles are oxidized was measured by observing the cross-section with scanning electron microstructure. Thickness of the oxidized zone obeys the parabolic law. In the both cases, the oxidation product layers were not observed significantly on the surface of the composites after high temperature oxidation. These results mean that outward diffusion of Y or Si did not occur significantly for oxidation of SiC dispersed in the yttrium silicates. That is, the oxidation of the SiC dispersoid was conducted with inward diffusion of oxygen. Diffusion coefficient of oxygen in yttrium silicates can be obtained as shown in Table 1. The value for Y2SiO5 is larger than that for Y2Si2O7. This fact means that the protection performance of oxygen diffusion on Y2Si2O7 is better than that on Y2SiO5 for the EBC applications.Table 1 Diffusion coefficients of oxygen in yttrium silicates at 1300°C. Y2SiO5 Y2Si2O7 D O/ m2s-1 2.8×10-16 1.3×10-18