A preliminary study of a promising bi-layer environmental barrier coating (EBC) designed to reduce the susceptibility of SiC composites to hot water vapor erosion is reported. The EBC system consisted of a silicon bond coat and a pore-free ytterbium disilicate (YbDS; Yb 2 Si 2 O 7 ) topcoat. Both layers were deposited on -SiC substrates using a recently optimized air plasma spray method. The two layers of the coating system had coefficients of thermal expansion (CTE) that were well matched to that of the substrate, while the YbDS has been reported to have a moderate resistance to silicon hydroxide vapor forming reactions in water vapor rich environments. Thermal cycling experiments were conducted between 110 °C and 1316 °C in a flowing 90 % H 2 O/10 % O 2 atmospheric pressure environment, and resulted in the formation of a thermally grown (silica) oxide (TGO) at the silicon-ytterbium disilicate interface. The TGO layer exhibited linear oxidation kinetics consistent with oxidizer diffusion through the ytterbium silicate layer controlling its thickening rate. The effective diffusion coefficient of the oxidizing species in the YbDS layer was estimated to be 2x10 -12 m 2 s -1 at 1316 o C. Slow steam volatilization of the YbDS topcoat resulted in the formation of a thin, partially protective, high CTE ytterbium monosilicate layer on the outside of the YbDS coating. Progressive edge delamination of the coating system was observed with steam exposure time, consistent with water vapor volatilization of the TGO edges that were directly exposed to the environment. This was aided by outward bending of the delaminated region to relax TGO and YbMS surface layer stresses developed during the cooling phase of each thermal cycle.

The development of new generations of propellants with better energetic properties may be hampered by unsatisfactory mechanical behaviors in terms of strength and toughness. A micromechanical approach is adopted to provide a better understanding of the existing links between the constitutive phase behaviors and the local damage, and the macroscopic mechanical behavior of these materials. Three model materials have been made and tested in uniaxial tension. The stress-strain responses were recorded while monitoring their volume changes that quantify the macroscopic damage. A qualitative description of the local damage was obtained thanks to scanning electron microscopy images of samples under loading. The micromechanical approach consists in finite elements analyses on periodic microstructures of non-regular polyhedral particles embedded in a soft matrix. An original microstructure generation tool has been developed specifically in order to obtain highly filled isotropic microstructures. Debonding at the matrix/filler interface was taken into account with a cohesive-zone model (CZM). The impact of the CZM parameters is discussed, in an effort to make the link between the CZM parameters and how the local damage appears and develops, and between the cohesive behavior and the shape of the macroscopic stress-stretch responses of the heterogeneous materials.