Elevated Temperature Fracture Properties Of Yb2Si2O7-Based Environmental Barrier Coatings

The room and elevated temperature fracture properties of NASA's Gen II Yb2Si2O7 (YbDS) environmental barrier coating (EBC) were measured on air plasma spray (APS) and field assisted sintering technology (FAST) samples. Noteworthy is the low fracture toughness and the physical and chemical instability of the APS material even after several hours of heat treatment. Heat treatments increase hardness, elastic properties, fracture strength, and fracture toughness. The values after any heat treatment depend on the initial as-sprayed density. At an as-sprayed bulk density of 5.48 g/cm3, APS YbDS exhibits room temperature Young's modulus and fracture toughness of 45 GPa and 0.92 MPa'm, respectively. With brief heat treatments (~4 hours), Young's modulus and the fracture toughness increase to 82 GPa and 1.36 MPa'm, respectively. The increases can be attributed to devitrification, phase changes, and splat bonding. Subcritical crack growth is exhibited at room temperature, implying water vapor induced stress corrosion. Elevated temperature subcritical crack growth measurements were limited by material instability and creep. In contrast to APS material, the FAST material exhibits similar properties both as-processed and after heat treatment, with Young's modulus and fracture toughness of 150 GPa and 1.36 MPa'm, respectively. The fracture toughness of the EBCs remained nominally constant up to 1316°C.