Oxygen tracer diffusion in yttrium silicates

Abstract

The diffusivity of oxygen in environmental barrier coating (EBC) materials must be sufficiently low to limit the oxidation of the underlying silicon bond coat and silicon carbide ceramic matrix composite (SiC CMC). Yttrium silicates have been proposed as candidate EBC materials however there are limited oxygen diffusivity data available. In this study, oxygen diffusion coefficients for polycrystalline Y₂Si₂O₇ and Y₂SiO₅ were determined using the oxygen tracer diffusion technique. The ¹⁸O diffusion concentration profiles were measured after exposure at temperatures of 1000 – 1300°C using time-of-flight secondary ion mass spectrometry (ToF-SIMS). Oxygen tracer diffusion and surface exchange coefficients were obtained by fitting the semi-infinite solution of the diffusion equation to the concentration profiles. Oxygen diffusion and surface exchange coefficients in yttrium silicates ranged from 10⁻¹⁴ – 10⁻¹² cm²/s and 10⁻¹⁰ – 10⁻⁸ cm/s respectively. Fast transport of ¹⁸O along grain boundaries, pores and in some grain orientations (anisotropic diffusion) was observed. Results are utilized to assess the viability of yttrium silicates and other rare-earth silicates as EBC materials for SiC CMCs.