APS YAG Coatings for Al2O3/Al2O3 Ceramic Matrix Composites

Abstract

Al2O3/Al2O3 ceramic matrix composites (CMC) are candidate lightweight, damage-tolerant hot-gas leading components of gas turbines. The susceptibility of Al2O3/Al2O3 CMC to chemical degradation in combustion environments at high temperatures, however, requires the development of protective coatings. Owing to its thermodynamic compatibility and low CTE mismatch to Al2O3, Y3Al5O12 (YAG) is a promising material for such thermal and environmental barrier coatings (T/EBC). YAG coatings were deposited by means of air plasma spraying (APS) on Al2O3/Al2O3 CMC with a reaction-bonded Al2O3 bondcoat (RBAO). APS YAG coatings are mostly amorphous in the as-deposited state and exhibit a good adherence. Above 900°C amorphous APS coatings crystallize instantaneously to YAG without formation of intermediate metastable phases. Crystallization is accompanied by an anisotropic shrinkage and increase of thermal conductivity. APS YAG coatings exhibit a characteristic segmentation crack pattern which is considered beneficial with respect to coating strain tolerance and thermal shock resistance. The durability of APS YAG coatings in harsh, H2O-rich combustion environments could be demonstrated in a high-velocity burner-rig. However, APS YAG coatings are prone to infiltration by molten CMAS-type deposits which may limit their application in aeroengines.