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Results of multiscale modeling simulations of the nonlinear response of SiC/SiC ceramic matrix composites are reported, wherein the microstructure of the ceramic matrix is captured. This micro scale architecture, which contains free Si material as well as the SiC ceramic, is responsible for residual stresses that play an important role in the subsequent thermo-mechanical behavior of the SiC/SiC composite. Using the novel Multiscale Generalized Method of Cells recursive micromechanics theory, the microstructure of the matrix, as well as the microstructure of the composite (fiber and matrix) can be captured.

Plasma Spray- Physical Vapor Deposition (PS-PVD) environmental barrier coatings (EBCs) of Yb2Si2O7 were deposited on SiC and exposed in a steam environment (90% H2O/O2) at 1426°C to form a thermally grown oxide (TGO) layer between the substrate and EBC. In advanced ceramic material systems such as coated ceramic matrix composites (CMCs), the TGO layer is the weak interface in coated CMC systems and directly influences component lifetimes. The effect of surface roughness and TGO thickness on the adhesion strength were evaluated by mechanical testing of the coatings after exposure. Morphology and oxide layer thickness were analyzed with electron microscopy while the composition and crystal structure were tracked with X-ray diffraction. The strength of the system is evaluated with respect to oxidation rate to give a qualitative understanding of coating durability.

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The stress—strain behavior at room temperature and at 1100° C (2000°F) is measured for two carbon fiber-reinforced silicon carbide (C/SiC) composite materials: a two dimensional (2D) plain-weave quasi-isotropic laminate and a 3D angle interlock woven composite. Previously developed micromechanics-based material models are calibrated by correlating the predicted material property values with the measured values. Four-point beam-bending subelement specimens are fabricated with these two fiber architectures and four-point bending tests are performed at room temperature and at 1100°C. Displacements and strains are measured at the mid-span of the beam and recorded as a function of load magnitude. The calibrated material models are used in concert with a nonlinear finite-element solution using ABAQUS to simulate the structural response of the two materials in the four-point beam bending tests. The structural response predicted by the nonlinear analysis method compared favorably with the measured response for both materials and both test temperatures. Results show that the material models scale-up fairly well from coupons to subcomponent level.