Oxidation Kinetics and Stress Effects for the Oxidation of Continuous Carbon Fibers within a Microcracked C/SiC Ceramic Matrix Composite

Abstract

Carbon fiber‐reinforced silicon carbide (C/SiC) composites have the potential to be utilized in many high‐temperature structural applications, particularly in aerospace. However, the susceptibility of the carbon fibers to oxidation has hindered the composite's use in long‐term reusable applications. In order to identify the composites limitations, fundamental oxidation studies were conducted to determine the effects of such variables as temperature, environment, and stress. The systematic studies first looked at the oxidation of the plain, uncoated carbon fiber, then when fiber was utilized within a C/SiC composite, and finally when a stress was applied to the C/SiC composite (stressed oxidation). The first study, oxidation of just the carbon fibers, showed that the fiber oxidation kinetics occurs in two primary regimes: chemical reaction control and diffusion control. The second study, oxidation of the C/SiC composite, showed the self‐protecting effects from the SiC matrix at elevated temperatures when the composite was not stressed. The final study, stressed oxidation of the C/SiC composite, more closely simulated application conditions in which the material is expected to encounter thermal and mechanical stresses. The applied load and temperature will affect the openings of the as‐fabricated cracks, which are an unavoidable characteristic of C/SiC composites. The main objective of the paper was to determine the oxidation kinetic regimes for the oxidation of carbon fibers in a cracked silicon carbide matrix under stressed and unstressed conditions. The studies help to provide insights in to the protective approaches, that could be used to prevent oxidation of the fibers within the composite.

Oxidation Kinetics and Its Impact on the Strength of Carbon Short Fiber Reinforced C/SiC Ceramics

“…Samples with comparable size have been used for determination of oxidation kinetics for C/C or C/SiC composite. [20,29] The specimens with fibers mainly orientated in longitudinal direction to the beam axis are referred to as longitudinal orientation samples (LO-samples) and perpendicular orientation (PO-samples) when the fibers are perpendicular to the beam axis, respectively. Three to four samples were tested.…”

“…Crack closure effects like in continuous fiber reinforced C/SiC which induce a decrease of weight loss at high temperatures at 1000 8C are not observed here. [6][7][8][9][10][11]15,16,[20][21][22] In principle, two distinct temperature ranges are determined. In range I between 600 and 800 8C, the weight loss showed a strong dependence on temperature.…”

“…This can be explained by crack closing effects due to thermal expansion and silica formation. [6][7][8][9][10][11][15][16][17][20][21][22] The morphological and microstructural changes of the materials after the oxidation are generally examined by scanning electron microscope (SEM), transmission electron microscope (TEM), or X-ray diffraction (XRD). [15][16][17]23] Short fiber reinforced carbon composite materials are developed especially for applications where the strong anisotropy of continuous fiber reinforced composite materials is undesirable.…”

Creep of Hi-Nicalon S Ceramic Fiber Tows at 800 deg C in Air and in Silicic Acid-Saturated Steam

“…Several studies have investigated the thermal stability of SiC fibers in oxidizing environments and the effects of oxidation on SiC fibers and its relation to subcritical crack growth (delayed failure) [24,43,49,105,111,121,[123][124][125][126][142][143][144][145][146][147][148][149][150][151][152][153][154][155][156][157][158]. LaraCurzio modeled the effects of oxidation on the stress-rupture behavior of fiber bundles and presented results for Nicalon T M fibers.…”

Bending and Thermal Expansion Properties of 2.5D C/SiC Composites

“…[1][2][3][4][5][6] In particular, the mechanical properties of C/SiC CMCs are stable at high temperatures and under severe service environments. 7,8) Therefore, C/SiC composites are considered as promising candidates for thermal structural and thermal protection materials for many high temperature applications, such as rocket nozzles, aeronautic jet engines, heat shields and aircraft braking materials.…”