Research Progress on Matrix Modification of SiC_f/SiC Composites for Oxidation and Corrosion Resistance

Continuous silicon carbide fiber reinforced silicon carbide ceramic matrix composites (referred to as SiC_f/SiC composites) become the preferred high-temperature and lightweight thermal structure materials for high thrust-to-weight ratio aero engines due to their excellent high-temperature properties. However, their poor high-temperature oxidation and corrosion properties limit their long-life use. It is effective to improve the oxidation and corrosion properties of SiC_f/SiC composite matrix through matrix modification, and the commonly used modified materials mainly include self-healing components, network intermediate oxides and rare-earth silicates.

The self-healing components form the liquid phase of B₂O₃ or borosilicate glass phase in the process of oxidative corrosion that has a certain fluidity and viscosity at a high temperature, which can fill the cracks and holes in the matrix. The volume expansion caused by oxidation makes the small defects heal when the oxidizing medium diffuses in the cracks as the oxidation phase, which can effectively block the diffusion of the oxidizing medium to the key areas that are easy to oxidize such as fibers and interfaces, thus enhancing the oxidation resistance of the material and extending its service life. The network intermediate oxide can absorb the "free oxygen" in the borosilicate glass melt and change its coordination from [MO₆] to [MO₄], reconstructing the silicate network damaged by water vapor erosion and maintain its integrity, and significantly improving the stability of the self-healing glass phase under high temperature water vapor conditions and the oxidation and corrosion properties of composites. Rare-earth silicate itself has excellent antioxidant corrosion properties, and the rare-earth elements migrate to the near surface of the matrix to form an antioxidant layer during the oxidative corrosion process, which inhibits the penetration of oxidative corrosion medium into the matrix and the reaction between borosilicate glass and water vapor, reduces the generation of gaseous substances, and slows down the oxidative corrosion of the matrix to a certain extent.

There are two main ways to introduce modified materials into the matrix, i.e., one refers to the direct introduction of antioxidant corrosive materials without changing their phases during use, such as directly introducing rare-earth silicates into the matrix, and another refers to the introduction of the precursor of the modified material or substances containing modified elements into the matrix, and the target modified material is obtained through chemical reactions during the oxidative corrosion process, usually in this way the introduction of self-healing components and oxidation corrosion phases into the matrix. In addition, the synthesis process of modified materials mostly adopts CVI, PIP, RMI, SI, etc., and their advantages and disadvantages of different modification processes are different. For instance, the preparation temperature of CVI process is low, but the densification cycle is long and the cost is high. The RMI process is simple, the cycle is short, and the composite material with a high density is prepared, but the melting temperature is high, which is easy to heat damage to the fiber and interface. Besides a single process, composite processes are mostly used to achieve complementary advantages, introducing modified materials into the matrix and realizing the densification of composite materials.

It is effective to introduce self-healing components, network intermediate oxides and rare-earth silicates into the SiC_f/SiC composite matrix to fill the pores and cracks of the material via generating self-healing components during the oxidative corrosion process, and forming a dense oxide layer on the surface of the material to resist further erosion by the oxidizing medium, thus improving the water and oxygen corrosion resistance of the composites. However, there are still some challenges. Firstly, the research on the oxidation corrosion mechanism of composite materials is still in-depth, and there is a lack of data on oxidation kinetics, oxidative corrosion rate and oxidative corrosion depth, and the basic research on the damage evolution mechanism of materials in different environments is still relatively weak. It is thus necessary to construct a complete and reliable performance database of SiC_f/SiC composites modified with different substrates, clarify the oxidative corrosion damage mechanism, and provide design parameters and theoretical support for the practical application of composites. Secondly, the effective temperature range of a single modified substance to improve the oxidative corrosion performance of composites is limited. The temperature range of B group is below 1000 ℃, and the temperature range of Al group is 1000–1300 ℃. The synergistic effect of multiple modification strategies is explored via introducing multiple modified substances into the matrix of composites at the same time. It is expected to achieve the oxidation and corrosion properties of SiC_f/SiC composites in a wide temperature range and achieve a long-life cycle protection. Finally, it is also worth to develop new preparation processes, such as nano-infiltration and transient eutectic (NITE), or use hybrid processes to achieve material densification and improve the performance of SiCf/SiC composite substrates while modifying them.