Modeling microvoiding kinetics of rare‐earth disilicates in flowing atmospheres containing water vapor

Abstract Rare‐earth disilicate (REDS, RE 2 Si 2 O7) layers that may be used as environmental barrier coatings (EBCs) on ceramic matrix composite (CMC) components in high‐temperature stages of turbine engines are subject to microvoiding to form porous rare‐earth monosilicate (REMS, RE 2 SiO 5 ) layers in flowing atmospheres containing water vapor. A simple microvoiding kinetic model that incorporates both mass transfer of the reaction product Si(OH) 4 (g) through the external gas phase boundary layer and pore diffusion of Si(OH) 4 (g) through the microvoided layer has been developed. Model predictions are in good agreement with measured growth rates of microvoided layers under low‐flowrate steam furnace test and high‐flowrate burner rig conditions. Since pore diffusion is generally the rate‐limiting step for EBC microvoiding in turbine engines, furnace testing under conditions of kinetic control by gas phase mass transfer is not generally capable of predicting REDS microvoiding rates under engine conditions. The kinetic model can be extended to incorporate changes in pore size and distribution, cracking of the microvoided layer, and introduction and cracking of an additional REMS topcoat to the system. The model can be used to generate a reasonable prediction of the time required to fully microvoid a REDS EBC layer on a CMC component in the hot gas path of an aircraft engine or stationary gas turbine.