In Situ and Ex Situ Measurement of Stresses in Environmental Barrier Coatings

Abstract

Ceramic matrix composites (CMCs) offer a step improvement in temperature capability over the current best high-temperature superalloys for gas turbine applications. Silicon carbide (SiC)-based CMCs react with water vapor at temperature and therefore need an environmental barrier coating (EBC) for protection against the water vapor attack. EBC is typically deposited on a CMC component through air plasma spray (APS) process, which involves melting and fast quenching of molten ceramic powders (such as ytterbium disilicate and ytterbium monosilicate). The fast quenching and subsequent cool down to room temperature introduce residual stress in the EBC. In addition, EBC-coated CMC components often go through a heat treatment (HT) process afterwards. For Yb-silicates, this both stabilizes/crystalizes the EBC and increases hermiticity by healing cracks and pores. This heat treatment at elevated temperature changes the coating residual stress state. It is important to be able to measure the coating stresses during deposition and heat treatment so that APS process improvement and EBC coating life assessment can be made. This paper describes novel methods to measure coating stresses continuously during APS and during heat treatment up to 1400 °C, using an optical deformation and curvature-based approach.