Thermochemical stability of Y <sub>2</sub> Si <sub>2</sub> O <sub>7</sub> in high‐temperature water vapor
Robert A. Golden, Kendall Mueller, Elizabeth J. Opila
Abstract
Abstract The thermochemical stability of Y 2 Si 2 O 7 was assessed in a high‐temperature high‐velocity water vapor environment to improve the understanding of the mechanisms that lead to SiO 2 depletion. Spark plasma sintered Y 2 Si 2 O 7 specimens were exposed in a steam‐jet furnace at 1000°C and 1200°C for 3‐250 hours, steam velocities of 131‐174 m/s and at 1 atm H 2 O pressure. These exposures resulted in the selective volatilization of SiO 2 to form volatile Si(OH) 4 and porous Y 2 SiO 5 . Microstructural evolution from fine rectangular pores at short times to larger rounded pores at longer times was observed. Mechanisms contributing to the overall depletion reaction kinetics were evaluated and include the interface reaction to form Y 2 SiO 5 and Si(OH) 4 (g), Y 2 SiO 5 coarsening, development of tortuosity in the pore network and diffusion of H 2 O (g) and Si(OH) 4 (g) through pores by molecular diffusion and/or Knudsen diffusion. SiO 2 depletion was found to follow parabolic volatilization kinetics ( k p = 0.38 µm 2 /h) at 1200°C indicating the reaction is limited by a diffusion process, most likely the outward diffusion of Si(OH) 4 (g) through pores. Results are utilized to assess the viability of Y 2 Si 2 O 7 and other rare‐earth silicates as environmental barrier coating (EBC) materials for SiC ceramic matrix composites (CMCs).