C/C composite materials have several advantages, i.e., low density, high thermal conductivity, low thermal expansion coefficient, and resistance to ablation, which can be used in various fields such as aerospace and nuclear chemical engineering. However, the susceptibility of C/C composite materials to oxidation in high-temperature environments is a primary concern for their further application. One effective solution is the fabrication of high-temperature oxidation-resistant ceramic coatings on the surface of C/C composite materials. The existing high-temperature oxidation-resistant ceramic coatings for C/C composites are gradually transited from single-layer to multi-layer. Typically, these composite coatings consist of three components, i.e., a mitigating layer, an intermediate layer and a sealing layer. The mitigating layer serves to alleviate the thermal expansion coefficient mismatch between the substrate and the coating, while an oxygen-blocking layer characterized by a low oxygen diffusion rate prevents external oxygen from penetrating the substrate. The sealing layer with low porosity, high-temperature fluidity, and self-healing properties protects against environmental medium erosion.

This review categorizes the existing high-temperature oxidation-resistant coatings of C/C composites based on different oxidation resistance mechanisms of ceramic coatings, i.e., ceramic coatings based on self-healing glass phase protection mechanisms, ceramic coatings based on enhanced glass phase protection mechanisms, and antioxidant coatings based on composite glass phases. The ceramic coatings based on enhanced glass phase protection mechanisms are further divided into ion-enhanced glass phases, microporous-inclusion structure enhanced glass phases, and particle-enhanced glass phases. In the self-healing glass phase protection mechanism coatings, SiO₂ and B₂O₃ are the most common glass phases. However, single-layer SiO₂ or B₂O₃ coatings are restricted in their application at > 1000 ℃ due to the volatilization of the generated B₂O₃ glass phase. It is thus necessary to incorporate both silicides and borides into the coating to facilitate the formation of a silicon-boron glass phase from their high-temperature oxidation products (i.e., SiO₂ and B₂O₃), thereby enhancing the oxidation resistance at > 1000 ℃. In addition, SiOC glass phases can also improve oxidation resistance at > 1000 ℃.

In ion-enhanced glass phases, Ta and Hf ions exhibit significant reinforcement effects on the glass phase. Specifically, Ta ions due to its intense ionic complexation form a coral-like oxide framework with Hf ions, acting as a supporting scaffold in the Ta-Hf-Si-O glass phase. This enhances the viscosity and strength of the glass layer and the oxygen barrier capability of the coating. Hf cations diffuse into the SiO₂ lattice, creating more stable chemical bonds that increase the thermal stability of SiO₂ glass, allowing the glass phase to continue protecting the substrate from oxidation in a wider temperature range for an extended period. In microporous-inclusion structure enhanced glass phases, the microporous structure can inhibit crack propagation, while the inclusion structure can suppress the tendency for coating cracking, thereby also enhancing oxidation resistance. A research indicates that SiC-Si coatings with microporous structures can protect C/C composites from oxidation at 1500 ℃ for 846 h, with a mass loss of only 0.16%. In particle-enhanced glass phases, the SiO₂ glass phase generated upon oxidation of the coating becomes stable due to the incorporation of Zr compound particles as reinforcing phases, reducing the formation of cracks in the glass phase. This results in superior high-temperature oxidation resistance. For instance, La₂O₃-modified ZrB₂-SiC coatings yield a Zr compound particle distribution in La-Si-O glass that forms a dense oxidation film, demonstrating the oxidation resistance at 1500 ℃ for 550 h and at 1600 ℃ for 107 h, while enduring 50 thermal cycles from 1500 ℃ to room temperature with a mass gain of 0.35%. Researchers develop the coatings that can resist oxidation at 1700 ℃ for 415 h based on the aforementioned antioxidant mechanism.

In coatings based on composite glass phases, silicate and aluminate composite glass phases synergistically utilize the advantages of different glass phases. In addition to exhibiting self-healing functions, they also possess a superior oxidation resistance, remaining stable under high-temperature and oxidative conditions. Moreover, elements such as Zr, Y, and Al can capture oxygen molecules, forming a stable oxide layer that further protects the C/C substrate from oxidation damage. In addition, this review also represents the specific preparation processes and oxidation resistance effects for coatings based on oxide-generated glass phase protection mechanisms, enhanced glass phase protection mechanisms, and silicate glass phase coatings.