Steam oxidation of an yttrium disilicate environmental barrier coating (EBC) depending on various corrosive environment : kinetics
QUEMERAS L. 1, COUEGNAT G. 1, VANDELLOS T. 2, REBILLAT F. 1
1 Thermostructural Composites Laboratory, LCTS, UMR 5801, Université de Bordeaux, Pessac, France; 2 Safran Ceramics, Le Haillan, France
SiCf/SiCm Ceramic Matrix Composites are currently used in certain parts of aircraft engines where the environment is considered as severe. Due to this environment, parts undergo a significant surface recession by reactions with moisture at high temperature, reducing their lifetime. To protect them, an Environmental Barrier Coating (EBC) is deposited on their surfaces.
During the thermal cycles of engines, various physicochemical and thermomechanical mechanisms damage the coating. These damages lead to the loss of the protection efficiency of the EBC up to its spallation. This latter is the most critical damage for the system as it exposes the CMC to the environment. This work is focused on two mains types of damages: the bond coat oxidation, which causes local stresses induced by volume variations with the formation of SiO2 (TGO : thermal growth oxide), and the thermomechanical stresses across this layered system : bond coat/TGO/EBC.
The studied system consists of the CMC successively coated with a silicon bond coat and an yttrium disilicate top coat. Despite the EBC, the diffusion O2 and H2O of gaseous species through this latter at high temperature, allows the oxidation of the silicon bond coat oxidizes with the formation of a silica layer under the EBC.
Oxidation/corrosion tests (moist air) were carried out on polished Si samples and coated samples to discuss the effect of interface roughness and the presence of the top coat on kinetics and the stress generation. Those tests were conducted at different temperatures and oxidant pressure in order to describe, according to the environment: (i) the protection efficiency of the EBC and (ii) the roles of each gaseous species, on the oxidation mechanism of the bond coat, underneath the EBC. The growth of this layer leads to the generation of mechanical stresses near the SiO2/EBC interface. Oxidation kinetics and diffusion data are then used to predict the CMC/EBC cracking process in function of time, temperature and composition of work environment.