A duplex bond coat approach to environmental barrier coating systems

SiC composites, when used in water vapor rich combustion environments, are vulnerable to volatilization of their protective SiO2 surface oxide by reaction with water vapor, and are typically protected by a silicon bond coat and ytterbium disilicate (Yb2Si2O7: YbDS) environmental barrier coating (EBC) system. Oxidation of the silicon bond coat at temperatures above 1200 degrees C results in formation of a ss-cristobalite SiO2 layer at the silicon-YbDS interface. Upon cooling through 240 degrees C, the ss ->alpha-cristobalite phase change results in a 4.9% SiO2 volume decrease, increasing the probability of delamination of the coating system as the oxide layer thickness increases. The use of a duplex silicon-HfO2 bond coat is investigated here in which m-HfO2 dynamically reacts with SiO2 to form HfSiO4 (hafnon), which is phase stable and thermomechanically compatible with silicon and YbDS. Thermal cycling of the duplex bond coat system between 110 and 1316 degrees C in a high-temperature steam environment for up to 1000 1 h cycles confirmed that growth of a ss-cristobalite SiO2 layer on the silicon outer surface was accompanied by simultaneous reaction with m-HfO2 to form hafnon, reducing the average SiO2 thickness by approximately 50% compared to a conventional silicon-YbDS coating system. A parallel reaction between the YbDS and m-HfO2 layers transformed the m-HfO2 to Yb2O3-stabilized HfO2 with a high coefficient of thermal expansion. Thermomechanical modelling indicated that the energy release rates remained insufficient to cause delamination. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.