C/C composites have been widely used in many fields because of their advantages, such as high specific strength and modulus, high temperature mechanical property, low thermal expansion coefficient, etc . Due to their low atomic number and low neutron absorption cross section, C/C composites have also been considered to be used as thermal structural materials in nuclear reactors [2–3]. However, oxidation is expected at high temperature in air. As we know, C/C composite is a kind of porous materials. Oxygen can penetrate into the composite and lead to catastrophic degradation of the material under certain conditions. Therefore, a good knowledge of oxidation process is necessary to utilize it [4–5].
The oxidation of carbon has been widely studied in the last few decades [6–8]. The results showed the mechanism and microstructure of oxidation were highly dependent on the type of carbon and the types of manufacturing processes. Therefore, the quantity and the size of the pores, the fibrous architecture and the nature of fibers are key factors that can have an impact on the oxidation of these materials. Some studies showed oxidation behaviors were strongly temperature-dependent. The 800 ℃ C/C composite material showed uniform attack, suggesting reaction control of the oxidation process; whereas the 1100 ℃ sample showed attack at the edges, suggesting diffusion control of the oxidation process. Also, some authors found that surface reaction rates depend on the type of carbon, inducing selective surface attacks as oxidation is preferably carried out along the fiber axis at the fiber/matrix interface [9–10]. Therefore, the oxidation is a complex problem. Optical and scanning electron microscopies are common methods to evaluate the impact of oxidation on the microstructure of materials [11–12]. However, comparing with them, non-destructive techniques such as tomography have many advantages, such as non-invasiveness, 3D imaging and quantitative analysis [13–14]. This technique has been used extensively by few authors to characterize the microstructure of composites during in-situ mechanical tests, different stages of the manufacturing process and after oxidation of the material [15–17].
A special C/C composite material for molten salt reactor (MSR) was developed several years ago. The density of the C/C composite material used should be as high as possible to prevent molten salt penetration. Considering the seriousness of oxidation, it is necessary to study the problem of the special material. As the oxidation mechanism and the resultant microstructure are closely connected, the objective of this paper is to characterize the microstructure during oxidation process. For this purpose, the C/C composite was oxidized repeatedly and characterized at different stages using SR-µCT. Finally, the microstructure evolution of the C/C composite material is obtained through comparing the 3D results by tomography at different oxidation stages.