C/C composite oxidation: Several previous research efforts have been made to understand the oxidation kinetics of unprotected C/C composites. However, the oxidation behavior of C/C composites varies depending on the desired weave geometry, type of carbon materials, microstructure and processing condition. Luthra summarizes the oxidation behavior of unprotected C/C composites with three main stages. First, oxidizing gas starts diffusing across the boundary layer at the surface of the composite. Following this, oxygen can either react chemically with carbon materials located at the surface of the composite, producing gases in form of CO and CO2, or oxygen can diffuse through existing cracks in the composite. Luthra states that at low temperatures, gas diffusion is the main mode of oxidation, whereas at high temperatures, both chemical reaction and diffusion of gaseous species through cracks in the composite control the oxidation process. At this stage, oxidation attack occurs at the surface and within the interior of the composite.
In addition to this, fiber/matrix interface along the fiber tows possesses preferential oxidation due a mismatch of coefficient of thermal expansion between the matrix and fiber, as explained by Crocker and Mcenaney. Also, the fiber edges are more sensitive to oxidize than the center or bulk of the fibers, creating a pointed morphology at the exposed ends of the fibers.
Moreover, Bacos affirms that for 2D C/C composites, carbon matrix degradation prevails during an oxidation process, since the reactivity of the carbon matrix is higher than that of the carbon fibers. This author also states that at low temperatures, oxidation damage is distributed uniformly throughout the interior of the composite and swollen cracks/voids are observed more frequently in the tows due to gaseous species transport. This oxidation behavior usually leads to the propagation of consumed channels along the fiber tows. On the other hand, at high temperatures, Bacos declares that only the first layers of the composite are extensively oxidized, while the fiber tows and matrix within the exposed surfaces show minimal evidence of oxidation.
C/C composite compressive response: Though there are no ASTM standards available to perform through-thickness compression tests on 2D fiber-reinforced composites, through-thickness compressive stiffness of fiber-reinforced composites has been measured by Lodeiro et al.. Difficulties faced during determining the compressive strength of 2D fiber-reinforced composites are reported in the literature. For instance, high stresses develop at the ends of the specimens during compression test, strength is sensitive to changes in the cross-sectional area, friction within the loading plates and the specimen affects the strength values, strain measurements from crosshead displacements and strain gauges provide different strength results and the strength is easily affected by the specimen geometry, among others. Conversely, Hodgkinson affirms the feasibility of testing unidirectional carbon/epoxy square specimens with a minimum thickness of 6mm to determine through-thickness compressive stiffness.
Furthermore, there is seldom information available regarding the failure analysis of 2D C/C composite specimens under compressive loads. Park and Lee observed two modes of failure in carbon-phenolic woven materials, i.e., horizontal and angular splitting of layers within the laminate. They suggest that during compression tests, matrix cracks propagate through the thickness of the composite leading to fiber breakage and consequently, complete failure.