We review the development of virtual tests for high-temperature ceramic matrix composites with textile reinforcement. Success hinges on understanding the relationship between the microstructure of continuous-fiber composites, including its stochastic variability, and the evolution of damage events leading to failure. The virtual tests combine advanced experiments and theories to address physical, mathematical, and engineering aspects of material definition and failure prediction. Key new experiments include surface image correlation methods and synchrotron-based, micrometer-resolution 3D imaging, both executed at temperatures exceeding 1,500°C. Computational methods include new probabilistic algorithms for generating stochastic virtual specimens, as well as a new augmented finite element method that deals efficiently with arbitrary systems of crack initiation, bifurcation, and coalescence in heterogeneous materials. Conceptual advances include the use of topology to characterize stochastic microstructures. We discuss the challenge of predicting the probability of an extreme failure event in a computationally tractable manner while retaining the necessary physical detail.

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Stochastic Virtual Tests for High-Temperature Ceramic Matrix Composites

The degradation of SiC-based ceramic matrix composites (CMCs) in conditions typical of gas turbine engine operation proceeds via the stress-rupture of fiber bundles. The degradation is accelerated when oxygen and water invade the composite through matrix microcracks and react with fiber coatings and the fibers themselves. We review micromechanical models of the main rate-determining phenomena involved, including the the diffusion of gases and reaction products through matrix microcracks, oxidation of SiC (in both matrix and fibers) leading to the loss of stiffness and strength in exposed fibers, the formation of oxide scale on SiC fiber and along matrix crack surfaces that cause the partial closure of microcracks, and the concomitant and synergistic loss of BN fiber coatings. The micromechanical models could be formulated as time-dependent coupled differential equations in time, which must be solved dynamically, e.g., as an iterated user-defined material element, within a finite element simulation. A paradigm is thus established for incorporating the time-dependent evolution of local material properties according to the local environmental and stress conditions that exist within a material, in a simulation of the damage evolution of a composite component. We exemplify the calibration of typical micromechanical degradation models using thermodynamic data for the oxidation and/or volatilization of BN and SiC by oxygen and water, mechanical test data for the rate of stress-rupture of SiC fibers, and kinetic data for the processes involved in gas permeation through microcracks. We discuss approaches for validating computational simulations that include the micromechanical models of environmental degradation. A special challenge is achieving validated predictions of trends with temperature, which are expected to vary in a complex manner during use..

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Modeling environmentally induced property degradation of SiC/BN/SiC ceramic matrix composites

The oxidation and scale crystallization kinetics of Hi-NicalonTM-S SiC fibers were measured after oxidation in dry air between 700° and 1400°C. Scale thickness, composition, and crystallization were characterized by TEM with EDS, supplemented by SEM and optical microscopy. TEM was used to distinguish oxidation kinetics of amorphous and crystalline scales. Oxidation initially produces an amorphous silica scale that incorporates some carbon. Growth kinetics of the amorphous scale was analyzed using the flat-plate Deal-Grove model. The activation energy for parabolic oxidation was 248 kJ/mol. The scales crystallized to tridymite and cristobalite, starting at 1000°C in under 100 h and 1300°C in under 1 h. Crystallization kinetics had activation energy of 514 kJ/mol with a time growth exponent of 1.5. Crystalline silica nucleated at the scale surface, with more rapid growth parallel to the surface. Crystalline scales cracked from thermal residual stress and phase transformations during cool-down, and during oxidation from tensile hoop growth stress. High growth shear stress was inferred to cause intense dislocation plasticity near the crystalline SiO2–SiC interphase. Crystalline scales were thinner than amorphous scales, except where growth cracks allowed much more rapid oxidation.

Hi‐NicalonTM‐SSiC Fiber Oxidation and Scale Crystallization Kinetics

Hi-Nicalon™-S SiC fiber strengths and Weibull moduli were measured after oxidation for up to 100 hours between 700°C and 1400°C in wet and dry air. SiO2 scale thickness and crystallization extent were measured by TEM. The effect of furnace environment on trace element levels in the SiO2 scales was characterized by secondary ion mass spectroscopy. Crystallization kinetics and Deal-Grove oxidation kinetics for glass and crystalline scale, and the transition between them, were modeled and determined. Crystallization retards oxidation kinetics, and scale that formed in the crystalline state was heavily deformed by the growth stress accompanying SiC oxidation volume expansion. Glass scales formed in dry air slightly increased fiber strength. Glass scales formed in wet air did not increase strength, and in some cases significantly decreased strength. Scales more than 200 nm thick were usually partially or completely crystallized, which degraded fiber strength. Contamination of scales by trace impurities such as Al and Ca during heat treatment inhibited crystallization. The oxidation kinetics and the strengths of oxidized Hi-Nicalon™-S fibers are compared with previous studies on SiC fibers, bulk SiC, and single-crystal SiC. Empirical relationships between oxidation temperature, time, scale thickness, and strength are determined and discussed.

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Oxidation kinetics strength of Hi‐NicalonTM‐S SiC fiber after oxidation in dry and wet air

A method to calculate growth stress in SiO2 scales formed during SiC fiber oxidation was developed. Calculations were done for Hi-NicalonTM-S SiC fibers using previously measured Deal-Grove oxidation kinetics parameters. Initial compressive stresses in SiO2 of ∼25 GPa from the 2.2× oxidation volume expansion are rapidly relaxed to lower levels by flow of silica with a shear-stress dependent viscosity. At >1200°, viscous flow of silica relaxes stress to negligible levels. At 700° – 900 °C, compressive axial and hoop stress at the GPa level persist, but radial stresses are much smaller. The decrease in growth stress with increase in temperature is a consequence of larger activation energy for silica viscosity than for oxidation kinetics. Radial expansion of the outer scale eventually causes hoop stress to become tensile in the outer scale, and axial stress becomes tensile from the Poisson effect. Tensile hoop stresses can be >1 GPa for thick scales formed at <1000 °C. Approximate analytical expressions for ...

M. Wilson

Papers

Hi‐NicalonTM‐SSiC Fiber Oxidation and Scale Crystallization Kinetics

Relationships Between Fiber Strength, Passive Oxidation and Scale Crystallization Kinetics of Hi-Nicalon™-S SiC Fibers