Carbon silicon-carbide composites (C/SiC) have been proposed for a variety of future spacecraft applications such as liquid engine combustion chambers, control surfaces, and thermal protection for reentry vehicles. The ability of C/SiC to maintain its strength and stiffness at high temperatures as well as its low density make it an attractive candidate material for these high-temperature applications. One shortcoming of C/SiC is that, in the presence of oxygen, the carbon fibers and pyrocarbon fiber coating will oxidize at elevated temperatures, limiting the durability and useful life of the C/SiC component. The susceptibility of C/SiC to oxidation degradation does not forbid its use in future launch vehicle applications, as long as it can be verified through testing and analysis that the component will maintain its strength and stiffness throughout its service life, with the demonstration of sufficient safety factors.
The physics of carbon oxidation is well understood, and equations to define the dependence of the rate of oxidation with temperature and oxygen partial pressure have been long established (refs. 1 and 2). However, since the temperature and stress state vary within the C/SiC component and since the local oxygen vapor pressure is tied to the stress state through the process of diffusion, the temperature and local oxygen partial pressure, and thus the oxidation rates, will vary within a C/SiC component. For designers to use C/SiC components in future spacecraft applications, they need a design analysis tool that can determine the spatial distribution of the extent of oxidation and the resulting residual strength and stiffness in the C/SiC component as a function of the time, temperature, and environmental oxygen concentrations to which the C/SiC structure is exposed.
A numerical method to predict the oxidation behavior and oxidation patterns in C/SiC composite structures was developed at the NASA Glenn Research Center from the mechanics of the flow of ideal gases through a porous solid(ref. 3). The application of flow-through-porous-media theory to the C/SiC oxidation problem results in a set of two coupled nonlinear differential equations written in terms of the partial pressures of the oxidant and oxide. The differential equations are solved simultaneously at discrete time steps, and utilizing suitable time-marching scheme, the partial vapor pressures of the oxidant and oxides are obtained as a function of the spatial location and time. The local rate of carbon oxidation is determined using the map of the local oxidant partial vapor pressure along with the Arrhenius rate equation. The nonlinear differential equations are cast into matrix equations by applying the Bubnov-Galerkin weighted residual method, allowing for the solution of the differential equations numerically.
Comparison of predicted and measured carbon weight fraction versus time for thermogravimetric analysis specimens at various temperatures.
Thus far, the numerical method has been utilized to model the carbon oxidation and weight loss behavior of C/SiC specimens during thermogravimetric experiments. It successfully reproduced the influence of temperature on the rate of carbon oxidation (see the preceding figure). In addition, it replicated the profound influence of temperature on the oxidation pattern (see the following figure), which has been observed in other experimental studies.
Spatial distribution of carbon volume fraction at various times within an interior section of a prismatic C/SiC specimen of rectangular cross section. Results show the effect of temperature on the carbon fiber volume distributions. Left: Test conducted at 700 °C. Right: Test conducted at 950 °C.
Last updated: October 11, 2006
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