Foreign Object Damage of Environmental Barrier Coatings Subjected to CMAS Attack

Abstract

Foreign object damage (FOD) and attack by calcium–magnesium–aluminosilicates (CMAS) have been identified as two key damage and failure mechanisms in environmental barrier coatings (EBCs), which are being developed as an enabling technology for silicon carbide (SiC) based composites in gas turbine engines. CMAS exposure at elevated temperatures has been shown to cause premature failure in ytterbium disilicate (Yb2Si2O7) based EBCs. Previously, the effects of foreign object damage were studied on NASA's Yb2Si2O7-based EBC deposited on SiC. This study aims to understand the synergistic effects of CMAS attack and FOD on the same EBC system. CMAS was loaded onto the EBC at either 2 mg/cm2 or 4 mg/cm2 prior to being fully reacted for 4 h in a 1316 °C furnace. Samples were then impacted at room temperature using a 1.59 mm hardened steel ball with projectile velocities of 50–100 m/s. The impact damage was characterized by depth of the impact crater and length of the EBC delamination as determined by optical profilometry and scanning electron microscopy (SEM) of the cross-sectioned samples. The results of the samples subjected to CMAS attack were then compared to that of the previous study on NASA's baseline Yb2Si2O7-based EBC. A mechanical assessment of the coatings was also made. Vickers indentation was used to determine coating hardness as well as estimate indentation fracture toughness, and nano-indentation was used to estimate the Young's modulus.