Advances in Ablation or Oxidation Mechanisms and Behaviors of Carbon Fiber-Reinforced Si-Based Composites

Composites with excellent thermomechanical and thermochemical properties are urgently needed in the aerospace field, especially for structural applications under high-temperature conditions. Carbon fiber-reinforced Si-based composites are considered the most promising potential high-temperature materials due to their excellent oxidation resistance and ablative behaviors, good structural designability, and excellent mechanical properties. The reinforcement of the relevant composites mainly involves carbon fiber, which possesses good mechanical and temperature resistance abilities. In this paper, the ablation behaviors and mechanisms of related composites are reviewed. For carbon fiber-reinforced pure Si-based composites (C/SiM composites), the anti-ablation mechanism is mainly attributed to the continuous glassy SiO2, which inhibits the damage of the substrate. For C/SiM composite doping with refractory metal compounds, the oxides of Si and refractory metal together protect the main substrate from ablation and oxidation. Moreover, in addition to thermochemical damage, thermophysical and thermomechanical behavior severely destroy the surface coating of the substrate.


Introduction
In the field of aerospace, advanced thermal protection systems related to aerospace flight and rocket propulsion require some special materials, which have not only excellent thermal shock resistance, light weight, and high strength, but also excellent ablative resistance [1][2][3]. In practical application, ablation and active oxidation are severe problems, which must be avoided [4]. The mechanical properties of these materials have also been widely researched [5,6]. Therefore, it is urgent to develop ultrahigh-temperature materials with a high melting point to meet the application requirements in extreme-temperature environments above 1600 • C, especially when they are used as the leading edges and nose cones of hypersonic aircraft [7][8][9].
Fiber-reinforced composites with carbon fiber as the reinforcement material possess not only excellent mechanical properties, but also good thermal shock and ablative resistance [10]. They can be applied in extreme environments such as ultrahigh-temperature structures [11][12][13]. However, as the reinforcing material, carbon fiber is susceptible to oxidization above 450 • C in aerobic environments, which limits its application [14][15][16].
(a) (b) Figure 1. The optimal performance temperature and outstanding mechanical properties of carbon fiber-reinforced Si-based ceramic composite compared with conventional composites: (a) optimal performance temperature; (b) outstanding mechanical properties. Reprinted with permission from Ref. [32]. Copyright 2016, Springer Nature.
In this paper, the ablation and oxidation mechanisms, as well as the behaviors, of Sibased-ceramics coated or modified carbon fiber-reinforced composites, with different structures, are thoroughly reviewed. In Section 2, the preparation of C/Si-based composites is described. In Section 3, the ablation and oxidation behaviors, as well as the mechanisms, of carbon fiber-reinforced pure Si-based ceramic matrix composites (C/SiM composites, where C is carbon fiber, and M refers to B, C, N, etc.) are introduced. Section 4 provides a comprehensive review of the ablation and oxidation behaviors and The optimal performance temperature and outstanding mechanical properties of carbon fiber-reinforced Si-based ceramic composite compared with conventional composites: (a) optimal performance temperature; (b) outstanding mechanical properties. Reprinted with permission from Ref. [32]. Copyright 2016, Springer Nature.
In this paper, the ablation and oxidation mechanisms, as well as the behaviors, of Si-based-ceramics coated or modified carbon fiber-reinforced composites, with different structures, are thoroughly reviewed. In Section 2, the preparation of C/Si-based composites is described. In Section 3, the ablation and oxidation behaviors, as well as the mechanisms, of carbon fiber-reinforced pure Si-based ceramic matrix composites (C/SiM composites, where C is carbon fiber, and M refers to B, C, N, etc.) are introduced. Section 4 provides a comprehensive review of the ablation and oxidation behaviors and mechanisms of Molecules 2023, 28, 6022 3 of 30 transition metal Zr-supplemented C/SiM composites (C/SiZrM composites, where M refers to B, C, N, etc., typically ZrB 2 , ZrC, ZrB 2 , and ZrC). In Section 5, the ablation and oxidation behaviors, as well as the mechanisms, of C/SiM-Z composites are reviewed (Z refers to other transitional metals, i.e., Ta, Hf, Y, Ti, Mo, Cr, La, etc.). In these sections, the structures of the reinforced materials such as dispersive fibers, needle preforms, and 3Dbraided performs are presented, and the methods of ablation or oxidation are also described. Lastly, Section 6 focuses on the challenges and future prospects in the development of carbon fiber-reinforced Si-based ceramic composites in order to promote their application fields. This paper can provide a reference for the preparation of anti-ablative composites, with explanations of their ablative mechanisms.

Preparation of C/Si-Based Composites
Ablation is an erosive phenomenon characterized by the removal of raw or oxidized materials through a combination of thermo-mechanical and thermo-physical as well as thermo-chemical factors resulting from high temperature, high pressure, and velocity of combustion flame. The primary methods to test the ablative or oxidation properties of the composites include plasma arc ablation, oxyacetylene flame ablation, etc. During ablation, the high heat flux of the combustion gas with high pressure and speed leads to chemical erosion and mechanical scouring, resulting ultimately in the damage and failure of composites. These factors are extrinsic elements impacting ablation mechanisms. Numerical simulation and evaluation system can also be established for diagnosing the flame during ablation [33].
The preparation process for C/Si-based composites is complex and expensive, mainly due to selection of reinforcement and the recombination of reinforcement and matrix. Carbon fiber has been widely utilized as a reinforcement material for composite structures in the aerospace field due to its high strength and high modulus as well as high melting point, etc. [34,35]. The development of textile technology has resulted in the existence of reinforced fiber in three primary forms. They are dispersive fibers, needled structure, as well as 2.5D or 3D structure with outstanding structural integrity. The third structure is commonly employed as high-reliability aircraft components and nose cones of missile warheads, as it can be woven into an integrated structure, and the preform can subsequently serve as reinforcements directly [36]. This structure has a more prominent ablative resistance than its 2D prefabricated counterpart.
Moreover, Si-based ceramics have traditionally been used as the matrix of fiberreinforced composites or as a protective layer for these composites to improve the ablative or oxidation resistance of the fibers [37]. Additionally, there are a variety of densification methods for preparing fiber-reinforced Si-based ceramics [38]. These mainly include hot pressing (HP), polycarbosilane infiltration pyrolysis (PIP), pressureless infiltration (PI), thermal gradient chemical vapor infiltration technique (TCVI), chemical vapor infiltration (CVI), chemical vapor deposition (CVD), pack cementation (PC), isothermal chemical vapor infiltration (ICVI), reactive melt infiltration (RMI), slurry infiltration (SI), etc. Typically, different methods are used together to improve the densification of composites. Preliminary investigations show that fiber-reinforced SiC-based composites, especially those with carbon fiber as reinforcement, are prone to forming cracks. This leads to carbon fibers becoming susceptible to oxidation through ingress of air when exposed to high temperatures. Therefore, a graphitic carbon interphase or BN interphase is applied to the surface of carbon fibers to create a weak bond between the fiber and the matrix, promoting the toughening behavior of the composite. Table 1 provides the ablation and oxidation properties of pure C/SiM composites in recent years, including preparing and ablation methods. Meanwhile, based on the different matrix, the corresponding ablative-resistance composites are classified into two types. These are, respectively, C/SiC with the same matrix of SiC or SiC coating, and C/Si 3 N 4

Ablation Behaviors and Mechanisms of Pure C/SiM Composites
Molecules 2023, 28, 6022 4 of 30 with the same Si 3 N 4 matrix. The ablation and oxidation mechanisms of C/SiC have been more extensively investigated because SiC represents a promising ablation inhibitor, owing to its effective specific weight and low cost. In subsequent sub-sections, the ablation and oxidation mechanisms of these composites are separately discussed in detail.

Ablation Behaviors and Mechanisms of C/SiC Composites
The ablation and oxidation mechanisms of C/SiC composites are similar. The carbon fiber can be protected by the outer interphase and matrix. These mechanisms are mainly centered on the outer materials. During the ablation process, the composites primarily undergo the following chemical reactions.
Reactions of carbon fiber or PyC: Molecules 2023, 28, 6022 6 of 30 Reactions of silicon carbide: SiC(s) + 2O 2 (g) → SiO(l) + CO 2 (g) (8) Reactions of silicon dioxide and silicon monoxide: The ablation and oxidation behaviors of C/SiC composites are predominately controlled by oxidation, thermal decomposition, and sublimation, which are chiefly affected by the external ambient temperature [68,79]. Figure 2 illustrates the ablative progress and protective mechanism of C/SiC. Under a temperature of 1500 • C, oxygen reacts with SiC to yield SiO 2 . Below this temperature, SiO 2 possessing a high fluidity, quickly seals any existing cracks of a certain width. The healed original cracks can be observed in the figure.
In conjunction, the composites are shielded by solid or liquid SiO 2 , demonstrating effective ablative resistance performances without catastrophic repercussions. However, when the temperature surpasses 1500 • C, SiC oxidizes to generate gaseous SiO and CO. Concurrently, liquid SiO 2 is easy to gasify and vaporize in abundance, leading to the creation of surface cracks. This allows oxygen to react with SiC, further permeating the coating thickness to form a penetrating crack. Consequently, the matrix loses its protective capacities over the fibers, with oxygen traveling to the fiber surface through these defects and inducing a reaction. As a result, gas holes, surface cracks, and skeleton structures are formed, initiating erosion of the composites [54]. Moreover, ablation behaviors of C/SiC composites are also affected by the ablated method. When the composites undergo ablation in a plasma wind tunnel, both heat flux and stagnation pressure collectively control the ablation behaviors [52]. With low heat flux and stagnation pressure, SiO2 can deposit itself on the surface of the composites, causing minimal erosion and effectively protecting the fibers. In contrast, with high heat flux and Moreover, ablation behaviors of C/SiC composites are also affected by the ablated method. When the composites undergo ablation in a plasma wind tunnel, both heat flux and stagnation pressure collectively control the ablation behaviors [52]. With low heat flux and stagnation pressure, SiO 2 can deposit itself on the surface of the composites, causing minimal erosion and effectively protecting the fibers. In contrast, with high heat flux and stagnation pressure, SiC coatings are rapidly consumed due to sublimation and decomposition, resulting in quick exposure of fibers to plasma flow after the consumption of SiC coatings. When the composites are subjected to ablation under an oxyacetylene torch, the heat flux also affects the ablative behaviors [49]. The higher the heat flux, the faster the erosion rate of SiC, ensuing consequential defects in the matrix. As a result, the residual matrix shows reduced ablation resistance under mechanical denudation. In addition, these defects also loosen the surface, thereby enlarging the interface area between the oxidizing components and composites. Correspondingly, the oxidation rate would accelerate. The final ablative morphology of the composite can be observed in Figure 3, demonstrating obvious ablative characteristics and appearance of large surface ablation holes ( Figure 3a). Simultaneously, the fibers in the central zone endure severe ablation and noticeable erosion at the fiber tip due to an extremely high temperature and heat flux. Moreover, ablation behaviors of C/SiC composites are also affected by the ablated method. When the composites undergo ablation in a plasma wind tunnel, both heat flux and stagnation pressure collectively control the ablation behaviors [52]. With low heat flux and stagnation pressure, SiO2 can deposit itself on the surface of the composites, causing minimal erosion and effectively protecting the fibers. In contrast, with high heat flux and stagnation pressure, SiC coatings are rapidly consumed due to sublimation and decomposition, resulting in quick exposure of fibers to plasma flow after the consumption of SiC coatings. When the composites are subjected to ablation under an oxyacetylene torch, the heat flux also affects the ablative behaviors [49]. The higher the heat flux, the faster the erosion rate of SiC, ensuing consequential defects in the matrix. As a result, the residual matrix shows reduced ablation resistance under mechanical denudation. In addition, these defects also loosen the surface, thereby enlarging the interface area between the oxidizing components and composites. Correspondingly, the oxidation rate would accelerate. The final ablative morphology of the composite can be observed in Figure 3, demonstrating obvious ablative characteristics and appearance of large surface ablation holes ( Figure 3a). Simultaneously, the fibers in the central zone endure severe ablation and noticeable erosion at the fiber tip due to an extremely high temperature and heat flux.   Therefore, the ablation mechanisms of C/SiC composites are correlated not only with external ablative temperature, but also with the ablative method.

Ablation Behaviors and Mechanisms of C/SiBCN Composites
Compared with a traditional ceramic composite, during the ablation process, the BNC phase in a fiber-reinforced SiBCN composite can react with oxygen, thus generating B 2 O 3 gas [80]. The evaporation of B 2 O 3 gas can lower the surface temperature. Consequently, the B/Si ration in the glass decreases, with B being preferentially volatilized compared to Si, leading to an increase in the viscosity of the glass. Meanwhile, the low-viscosity B 2 O 3 liquid can seal cracks and enhance the ablation resistance of composites, owing to its liquidity feature. Furthermore, in order to further study the effects of ultra-high temperature ceramics (UHTCs) as the secondary phase and SiBCN as the first phase in composites, it is imperative to discuss the basic ablation properties and mechanisms of fiber-reinforced SiBCN composites. It is found that, in addition to the reactions in Equations (1)-(11), the following chemical reactions of Equations (12)- (14) also occur during the ablation of the composites: Molecules 2023, 28, 6022 Figure 4 provides the ablative surface and mechanisms of C/(Pyc/sic) 3 SiBCN composites [78]. The morphology of the ablation surface can be classified into the ablation center, transition zone, and ablation edge (Figure 4a). The ablative mechanism of fiber-reinforced SiBCN is related to the additional reaction of BN, as shown in Equation (13). The reaction yield of B 2 O 3 can react with SiO 2 to further form borosilicate glass. As the low-viscosity borosilicate glass diffuses, microcracks may be healed, significantly reducing the volatilization of B 2 O 3 , and enhancing the ablation resistance of composites. The volatilization of CO and N 2 results in the appearance of variously sized bubbles on the ablated surface of the composite.
to its liquidity feature. Furthermore, in order to further study the effects of ultra-high temperature ceramics (UHTCs) as the secondary phase and SiBCN as the first phase in composites, it is imperative to discuss the basic ablation properties and mechanisms of fiberreinforced SiBCN composites. It is found that, in addition to the reactions in Equations (1)-(11), the following chemical reactions of Equations (12)-(14) also occur during the ablation of the composites: Figure 4 provides the ablative surface and mechanisms of C/(Pyc/sic)3SiBCN composites [78]. The morphology of the ablation surface can be classified into the ablation center, transition zone, and ablation edge (Figure 4a). The ablative mechanism of fiber-reinforced SiBCN is related to the additional reaction of BN, as shown in Equation (13). The reaction yield of B2O3 can react with SiO2 to further form borosilicate glass. As the low-viscosity borosilicate glass diffuses, microcracks may be healed, significantly reducing the volatilization of B2O3, and enhancing the ablation resistance of composites. The volatilization of CO and N2 results in the appearance of variously sized bubbles on the ablated surface of the composite.  . Schematic of (a) ablation surface and diagram of (b) mechanisms of C/(Pyc/sic)3SiBCN composites. Reprinted with permission from Ref. [78]. Copyright 2021, Elsevier.
In addition to fiber-reinforced SiC or SiBCN matrix, composites with another matrix, such as Si3N4 ceramics with high strength, high thermal shock resistance, and good wear resistance are also used. During the ablation of the C/Si3N4 composite, carbon fiber is ablated in the central region, producing a large number of SiO2 droplets. Within the ring region, spherical solid SiO2 particles are formed, protecting the carbon fiber from further ablation [77].
In summary, the ablative mechanism of C/SiM composites primarily relies on a SiO2rich layer protecting the fiber from ablation. Gas evolution happens sooner due to the higher volatility of boron-containing species. Additionally, the evaporation of gases can . Schematic of (a) ablation surface and diagram of (b) mechanisms of C/(Pyc/sic)3SiBCN composites. Reprinted with permission from Ref. [78]. Copyright 2021, Elsevier.
In addition to fiber-reinforced SiC or SiBCN matrix, composites with another matrix, such as Si 3 N 4 ceramics with high strength, high thermal shock resistance, and good wear resistance are also used. During the ablation of the C/Si 3 N 4 composite, carbon fiber is ablated in the central region, producing a large number of SiO 2 droplets. Within the ring region, spherical solid SiO 2 particles are formed, protecting the carbon fiber from further ablation [77].
In summary, the ablative mechanism of C/SiM composites primarily relies on a SiO 2 -rich layer protecting the fiber from ablation. Gas evolution happens sooner due to the higher volatility of boron-containing species. Additionally, the evaporation of gases can carry away energy and reduce the surface temperature-both processes inhibit oxidation and ablation within the composites.

Ablation of C/SiZrM Composites
Pure C/SiM composites can form a protective silica effectively at low temperature. It is difficult to meet the oxidizing and ablation atmosphere requirements above 2000 • C. SiC tends to oxidize and ablate at high temperatures (>1800 • C), coupled with chemical erosion. Hence, ultra-high temperature ceramics (UHTCs) have been introduced into C/SiM composites as the second phase of the matrix, encompassing many borides, carbides, and nitrides of early transition metals, particularly ZrB 2 and ZrC. The addition remarkably improves the ablative resistance of composites at elevated temperature [81][82][83]. Table 2 gives the recent ablation and oxidation properties of C/SiZrM composites.

Ablation Behaviors and Mechanisms of C/ZrB 2 -SiC Composites
In order to improve the ablation-resistant and oxidation-resistant properties of C/SiC composites under complex circumstances and at elevated temperatures, ZrB 2 is incorporated into the composite as the secondary phase of the matrix, which is called as C/ZrB 2 -SiC composites [160]. During the ablation process, the composites primarily undergo the following chemical reactions, aside from those outlined in Equations (1)- (11).
The ablation and oxidation behaviors of C/ZrB 2 -SiC composites are primarily affected by complex chemical erosion and mechanical denudation [86,161]. Figure 5 provides cross-section images of the C/ZrB 2 -SiC composite after ablation [35], illustrating the accumulation of a glassy layer on the outermost surface, which serves to protect the inner material. Figure 6 provides the detailed ablation mechanisms. In the ablative process, ZrB 2 and SiC are oxidized to form SiO 2 , ZrO 2 , B 2 O 3, and borosilicate, the evaporation of which ultimately results in the porous surface layer. Between 450 • C and 1100 • C, these low viscosity and fluid B 2 O 3 and borosilicate easily cover the external surface of the carbon fiber, forming a regular and dense oxidation scale. This is premised on the fact that the melting point of B 2 O 3 is 450 • C and its boiling point 1850 • C. However, at higher temperatures, owing to the vapor pressure of the B 2 O 3 , a significant amount of B 2 O 3 preferentially evaporates from the surface, forming an enriched SiO 2 scale, where the melting point of SiO 2 is 1670-1710 • C. Due to the lower oxidation temperature of ZrB 2 and PyC, oxygen tends to diffuse inward through the oxide scale and reacts with these elements. In addition, the gradient of chemical potential and temperature within the composite encourages the oxidation of internal ZrB 2 . The formed ZrO 2 provides a pinning effect, preventing the cracking and spalling of silica-scale glass. The formation of the SiO 2 -ZrO 2 structure and ZrSiO 4 glass can further obstruct oxygen diffusion and also have good adhesion to the fiber. Meanwhile, the formed gaseous B 2 O 3 will migrate through the outer SiO 2 -rich scale layer. Therefore, the SiO 2 -rich oxide scale layer and a porous ZrB 2 -SiC-C f layer are formed. As B 2 O 3 evaporates, heat is dissipated and surface temperature of the composite decreases. With the diffusion of oxygen, the final products of SiO 2 , ZrO 2 , borosilicate glass, ZrSiO 4 and continuous integrated SiO 2 -ZrO 2 -SiC ceramic layer prevent fiber structure from further ablation. Additionally, the escape of gaseous by-products, such as CO, CO 2 , SiO, and B 2 O 3 , produce a more pronounced thermal barrier effect. viscosity and fluid B2O3 and borosilicate easily cover the external surface of the carbon fiber, forming a regular and dense oxidation scale. This is premised on the fact that the melting point of B2O3 is 450 °C and its boiling point 1850 °C. However, at higher temperatures, owing to the vapor pressure of the B2O3, a significant amount of B2O3 preferentially evaporates from the surface, forming an enriched SiO2 scale, where the melting point of SiO2 is 1670-1710 °C. Due to the lower oxidation temperature of ZrB2 and PyC, oxygen tends to diffuse inward through the oxide scale and reacts with these elements. In addition, the gradient of chemical potential and temperature within the composite encourages the oxidation of internal ZrB2. The formed ZrO2 provides a pinning effect, preventing the cracking and spalling of silica-scale glass. The formation of the SiO2-ZrO2 structure and ZrSiO4 glass can further obstruct oxygen diffusion and also have good adhesion to the fiber. Meanwhile, the formed gaseous B2O3 will migrate through the outer SiO2-rich scale layer. Therefore, the SiO2-rich oxide scale layer and a porous ZrB2-SiC-Cf layer are formed. As B2O3 evaporates, heat is dissipated and surface temperature of the composite decreases.
With the diffusion of oxygen, the final products of SiO2, ZrO2, borosilicate glass, ZrSiO4 and continuous integrated SiO2-ZrO2-SiC ceramic layer prevent fiber structure from further ablation. Additionally, the escape of gaseous by-products, such as CO, CO2, SiO, and B2O3, produce a more pronounced thermal barrier effect.

Ablation Behaviors and Mechanisms of C/ZrC-SiC Composites
Refractory carbide ZrC, with its high melting point of 3540 °C, relatively low density (6.7 g/cm 3 ), thermal shock resistance, and chemical inertness, etc., is regarded as an outstanding advanced ceramic, thus making it one of the most promising UHTCs. For this reason, it has been integrated into C/SiC composites and designed as C/ZrC-SiC composites suitable for application in extreme environments [162]. During the ablation process, the ablative performance of the C/ZrC-SiC composite is determined by a combination of chemical erosion, thermo-physical conditions, and mechanical denudation. Along with the reactions noted in Equations (1)- (11), there are other reactions that also occur in response to external environmental temperature when they are subjected to ablation. 2ZrC(s) + 3O 2 (g) → 2ZrO 2 (s) + 2CO(g) ZrC(s) + 3CO 2 (g) → ZrO 2 (l) + 4CO(g) ZrC(s) + 2O 2 (g) → ZrO 2 (s) + CO 2 (g) ZrO 2 (s) → ZrO 2 (l) → ZrO 2 (g) SiO 2 (s) → SiO 2 (l) → SiO 2 (g) Figure 7 depicts the ablation behaviors of the C/ZrC-SiC composite [13]. It is evident that the ZrC and SiC ceramics are evenly dispersed and sintered within closely braided carbon fibers. During the ablation of the composite, as shown in Figure 7b,c, intense oxidizing airflow persistently infiltrates through holes, exacerbating the oxidation reaction of ZrC and SiC, resulting in the gradual erosion and enlargement of the pores. Figure 8 presents the ablation mechanism of the C/ZrC-SiC composite. The combined ablative and oxidative behaviors of ZrC and SiC contribute to the self-healing feature of the composite. ZrC is the source of the refractory ZrO2 phase. The formed continuous liquid SiO2, SiO2-ZrO2 glassy layer, as well as ZrSiO4 act as effective barriers that obstruct the inward oxygen diffusion. The stable molten liquid ZrO2 scale can prevent the fiber from ablation when the temperature is above 2700 °C. It can cover and seal cracks as well as pores, hindering further in-depth oxygen diffusion into the oxidation-prone fiber. The singular ZrO2 layer features a weak interfacial bond and can easily fall off. However, the glassy silica phases can permeate the gaps in the ZrO2 skeleton, stick to the central ZrO2 layer, and facilitate the sintering of porous ZrO2, consequently strengthening its intact surface. Simultaneously, the formed glassy ZrO2-SiO2 layer is generated on the surface, and a porous Reprinted with permission from Ref. [101]. Copyright 2020, Elsevier.

Ablation Behaviors and Mechanisms of C/ZrC-SiC Composites
Refractory carbide ZrC, with its high melting point of 3540 • C, relatively low density (6.7 g/cm 3 ), thermal shock resistance, and chemical inertness, etc., is regarded as an outstanding advanced ceramic, thus making it one of the most promising UHTCs. For this reason, it has been integrated into C/SiC composites and designed as C/ZrC-SiC composites suitable for application in extreme environments [162]. During the ablation process, the ablative performance of the C/ZrC-SiC composite is determined by a combination of chemical erosion, thermo-physical conditions, and mechanical denudation. Along with the reactions noted in Equations (1)- (11), there are other reactions that also occur in response to external environmental temperature when they are subjected to ablation. 2ZrC(s) + 3O 2 (g) → 2ZrO 2 (s) + 2CO(g) (20) ZrC(s) + 3CO 2 (g) → ZrO 2 (l) + 4CO(g) ZrC(s) + 2O 2 (g) → ZrO 2 (s) + CO 2 (g) SiO 2 (s) → SiO 2 (l) → SiO 2 (g) (24) Figure 7 depicts the ablation behaviors of the C/ZrC-SiC composite [13]. It is evident that the ZrC and SiC ceramics are evenly dispersed and sintered within closely braided carbon fibers. During the ablation of the composite, as shown in Figure 7b,c, intense oxidizing airflow persistently infiltrates through holes, exacerbating the oxidation reaction of ZrC and SiC, resulting in the gradual erosion and enlargement of the pores. Figure 8 presents the ablation mechanism of the C/ZrC-SiC composite. The combined ablative and oxidative behaviors of ZrC and SiC contribute to the self-healing feature of the composite. ZrC is the source of the refractory ZrO 2 phase. The formed continuous liquid SiO 2 , SiO 2 -ZrO 2 glassy layer, as well as ZrSiO 4 act as effective barriers that obstruct the inward oxygen diffusion. The stable molten liquid ZrO 2 scale can prevent the fiber from ablation when the temperature is above 2700 • C. It can cover and seal cracks as well as pores, hindering further in-depth oxygen diffusion into the oxidation-prone fiber. The singular ZrO 2 layer features a weak interfacial bond and can easily fall off. However, the glassy silica phases can permeate the gaps in the ZrO 2 skeleton, stick to the central ZrO 2 layer, and facilitate the sintering of porous ZrO 2 , consequently strengthening its intact surface. Simultaneously, the formed glassy ZrO 2 -SiO 2 layer is generated on the surface, and a porous interlayer is formed by the ZrO 2 skeleton and a few silica glasses, which is due to the evaporation of CO, CO 2 , SiO, and SiO 2 . The ZrO 2 -melting layer, the porous layer, and SiO 2 -rich layer together constitute the comprehensive glassy ZrO 2 -SiO 2 , which inhibits the erosion of oxidative gas. Moreover, the formation of continuous integrated SiO 2 -ZrO 2 -ZrC-SiC layer safeguards the C/C preform from further ablation by acting as a thermal and oxygen diffusion barrier [114].
Molecules 2023, 28, 6022 14 of 29 interlayer is formed by the ZrO2 skeleton and a few silica glasses, which is due to the evaporation of CO, CO2, SiO, and SiO2. The ZrO2-melting layer, the porous layer, and SiO2rich layer together constitute the comprehensive glassy ZrO2-SiO2, which inhibits the erosion of oxidative gas. Moreover, the formation of continuous integrated SiO2-ZrO2-ZrC-SiC layer safeguards the C/C preform from further ablation by acting as a thermal and oxygen diffusion barrier [114].

Ablation Behaviors and Mechanisms of C/ZrB2-ZrC-SiC Composite
To further improve the ablation resistance of the C/SiC composite at elevated temperatures, UHTCs of ZrB2 and ZrC can be collectively incorporated into the composite due to their high melting points of 3250 °C and 3540 °C, along with their low densities of 6.1 g/cm 3 and 6.7 g/cm 3 , respectively, which will create a C/ZrB2-ZrC-SiC composite [163]. Compared to C/ZrC-SiC and C/ZrB2-SiC composites, this incorporation can yield better hardness, fracture toughness, and flexural strength. Moreover, Equations (1)-(24) will occur during ablation. Figure 9 details the morphology of the C/ZrB2-ZrC-SiC composite both before and after ablation, with a relatively uniform Zr element (Figure 9b). The vaporization of SiO escape promotes the development of poriferous and lax structure (Figure 9c). The ablation mechanisms of the C/ZrB2-ZrC-SiC composite are displayed in Figure 10. The ZrB2-ZrC-SiC matrix undergoes oxidation to form molten oxide scales of ZrO2-SiO2, thus developing a Zr-Si-O glass phase, which possesses high viscosity. This can flow and seal the pores on the ablated surface, meanwhile most of the oxidation product B2O3 evaporates above 1650 interlayer is formed by the ZrO2 skeleton and a few silica glasses, which is due to the evaporation of CO, CO2, SiO, and SiO2. The ZrO2-melting layer, the porous layer, and SiO2rich layer together constitute the comprehensive glassy ZrO2-SiO2, which inhibits the erosion of oxidative gas. Moreover, the formation of continuous integrated SiO2-ZrO2-ZrC-SiC layer safeguards the C/C preform from further ablation by acting as a thermal and oxygen diffusion barrier [114].

Ablation Behaviors and Mechanisms of C/ZrB2-ZrC-SiC Composite
To further improve the ablation resistance of the C/SiC composite at elevated temperatures, UHTCs of ZrB2 and ZrC can be collectively incorporated into the composite due to their high melting points of 3250 °C and 3540 °C, along with their low densities of 6.1 g/cm 3 and 6.7 g/cm 3 , respectively, which will create a C/ZrB2-ZrC-SiC composite [163]. Compared to C/ZrC-SiC and C/ZrB2-SiC composites, this incorporation can yield better hardness, fracture toughness, and flexural strength. Moreover, Equations (1)-(24) will occur during ablation. Figure 9 details the morphology of the C/ZrB2-ZrC-SiC composite both before and after ablation, with a relatively uniform Zr element (Figure 9b). The vaporization of SiO escape promotes the development of poriferous and lax structure (Figure 9c). The ablation mechanisms of the C/ZrB2-ZrC-SiC composite are displayed in Figure 10. The ZrB2-ZrC-SiC matrix undergoes oxidation to form molten oxide scales of ZrO2-SiO2, thus developing a Zr-Si-O glass phase, which possesses high viscosity. This can flow and seal the pores on the ablated surface, meanwhile most of the oxidation product B2O3 evaporates above 1650

Ablation Behaviors and Mechanisms of C/ZrB 2 -ZrC-SiC Composite
To further improve the ablation resistance of the C/SiC composite at elevated temperatures, UHTCs of ZrB 2 and ZrC can be collectively incorporated into the composite due to their high melting points of 3250 • C and 3540 • C, along with their low densities of 6.1 g/cm 3 and 6.7 g/cm 3 , respectively, which will create a C/ZrB 2 -ZrC-SiC composite [163]. Compared to C/ZrC-SiC and C/ZrB 2 -SiC composites, this incorporation can yield better hardness, fracture toughness, and flexural strength. Moreover, Equations (1)-(24) will occur during ablation. Figure 9 details the morphology of the C/ZrB 2 -ZrC-SiC composite both before and after ablation, with a relatively uniform Zr element (Figure 9b). The vaporization of SiO escape promotes the development of poriferous and lax structure (Figure 9c). The ablation mechanisms of the C/ZrB 2 -ZrC-SiC composite are displayed in Figure 10. The ZrB 2 -ZrC-SiC matrix undergoes oxidation to form molten oxide scales of ZrO 2 -SiO 2 , thus developing a Zr-Si-O glass phase, which possesses high viscosity. This can flow and seal the pores on the ablated surface, meanwhile most of the oxidation product B 2 O 3 evaporates above 1650 • C. Concurrently, the evaporation and fusion of gases (CO n , SiO 2 and B 2 O 3 ) can dissipate the surface heat of the substrate. Resultantly, many small pores are formed in the glass layer owing to the gas diffusion and evaporation, while large pores are formed as a result of matrix ablation and possibly pre-existing pores before ablation. Therefore, oxygen diffuses into the interior via these channel pores. In addition, both the matrix and the molten oxidation product can be stripped away by a high-velocity and high-pressure flame [148]. The ablation of the C/ZrB 2 -ZrC-SiC composite predominantly rests on the oxidation process and the mechanical ablation triggered by the flame. °C. Concurrently, the evaporation and fusion of gases (COn, SiO2 and B2O3) can dissipate the surface heat of the substrate. Resultantly, many small pores are formed in the glass layer owing to the gas diffusion and evaporation, while large pores are formed as a result of matrix ablation and possibly pre-existing pores before ablation. Therefore, oxygen diffuses into the interior via these channel pores. In addition, both the matrix and the molten oxidation product can be stripped away by a high-velocity and high-pressure flame [148]. The ablation of the C/ZrB2-ZrC-SiC composite predominantly rests on the oxidation process and the mechanical ablation triggered by the flame.

Ablation of the C/SiZM Composites
To further improve the ablative resistance of C/SiM composites in complex and extreme circumstances, other transition metals except from Zr, such as Ta, Hf, Y, Ti, Mo, Cr, La, etc., are also incorporated into the C/SiM composite [164][165][166][167][168], which is called a C/SiZM composite (Z=Ta, Hf, Y, Ti, Mo, Cr, La, etc.). Table 3 showcases recent ablation °C. Concurrently, the evaporation and fusion of gases (COn, SiO2 and B2O3) can dissipate the surface heat of the substrate. Resultantly, many small pores are formed in the glass layer owing to the gas diffusion and evaporation, while large pores are formed as a result of matrix ablation and possibly pre-existing pores before ablation. Therefore, oxygen diffuses into the interior via these channel pores. In addition, both the matrix and the molten oxidation product can be stripped away by a high-velocity and high-pressure flame [148]. The ablation of the C/ZrB2-ZrC-SiC composite predominantly rests on the oxidation process and the mechanical ablation triggered by the flame.

Ablation of the C/SiZM Composites
To further improve the ablative resistance of C/SiM composites in complex and extreme circumstances, other transition metals except from Zr, such as Ta, Hf, Y, Ti, Mo, Cr, La, etc., are also incorporated into the C/SiM composite [164][165][166][167][168], which is called a C/SiZM composite (Z=Ta, Hf, Y, Ti, Mo, Cr, La, etc.). Table 3 showcases recent ablation

Ablation of the C/SiZM Composites
To further improve the ablative resistance of C/SiM composites in complex and extreme circumstances, other transition metals except from Zr, such as Ta, Hf, Y, Ti, Mo, Cr, La, etc., are also incorporated into the C/SiM composite [164][165][166][167][168], which is called a C/SiZM composite (Z=Ta, Hf, Y, Ti, Mo, Cr, La, etc.). Table 3 showcases recent ablation and oxidation properties of C/SiZM composites and provides a summary of both historical and recent ablation research results of C/C-SiC-Z composites.  Ref.
The ablation mechanisms of the C/SiC-HfC and C/SiC-HfB2 are provided in Figure  11. The formation of SiO2-HfO2 protects the fiber from ablation during the initial ablation. However, these ablative products lose their protective function over time owing to mechanical denudation and thermal chemical ablation damage. In fact, HfO2, SiHfxOy-based layers (SiHf-O glass) and liquid SiO2 can protect the fiber from ablation.  Figure 11. Ablation mechanisms of the (a) C/SiC-HfC, A,B,C refer to the center region, transitional region and fringe region, respectively, reprinted/adapted with permission from Ref. [171], Copyright 2019, Elsevier; and (b) C/SiC-HfB2 composite, ⅰ,ⅱ,ⅲ,ⅳ denote the oxidation process occurring at 1773 K over time, while ⅰ,ⅴ,ⅵ,ⅶ signify the same process at 1973 K as time progresses, reprinted with permission from Ref. [177], Copyright 2021, Elsevier.
In terms of the ablative mechanisms of the Ta-added C/SiM composite, the formation of a mosaic-structured Ta-Si-O glassy layer, alongside the SiO2 layer and Ta2O5 on the surface of the C/C composite, inhibits oxides from damaging the fibers. Ta2O5, acting as "pinning phases", is beneficial to maintain the stability of TaB2-SiC coating and augmenting its ablative resistance. In the case of the Zr-La added C/SiM composite, the oxide of La Figure 11. Ablation mechanisms of the (a) C/SiC-HfC, A,B,C refer to the center region, transitional region and fringe region, respectively, reprinted/adapted with permission from Ref. [171], Copyright 2019, Elsevier; and (b) C/SiC-HfB 2 composite, i,ii,iii,iv denote the oxidation process occurring at 1773 K over time, while i,v,vi,vii signify the same process at 1973 K as time progresses, reprinted with permission from Ref. [177], Copyright 2021, Elsevier.
In terms of the ablative mechanisms of the Ta-added C/SiM composite, the formation of a mosaic-structured Ta-Si-O glassy layer, alongside the SiO 2 layer and Ta 2 O 5 on the surface of the C/C composite, inhibits oxides from damaging the fibers. Ta 2 O 5 , acting as "pinning phases", is beneficial to maintain the stability of TaB 2 -SiC coating and augmenting its ablative resistance. In the case of the Zr-La added C/SiM composite, the oxide of La promotes the liquid phase sintering of ZrO 2 , and generates a molten phase of La 2 Zr 2 O 7 . Additionally, evolution of La 2 O 3 , La 2 Si 2 O 7 , La 0.71 Zr 0.29 O 1.65 , and micron-sized ZrO 2 -La 2 O 3 -SiO 2 liquid phase layers provide superb oxygen barrier protection for the composites. When Zr-Hf is incorporated into C/SiM composites, the consequent dense, compact, and continuously oxidized HfO 2 -ZrO 2 -SiO 2 mixture layer is helpful for ablation protection.

Conclusions and Future Perspectives
In this paper, ablation characteristics of carbon fiber-reinforced Si-based composites has been exhaustively reviewed. The ablation mechanisms were comprehensively provided. For the ablation of carbon fiber-reinforced Si-based materials, oxides of Si and other UHTCs (Zr, Ta, La, Hf, Mo, Ti, Y, Cr, Al, V, Mo, Sm, Cu, Nd, Nb, et al.) with high melting points can collaboratively protect the carbon fiber substrate from ablation, particularly at elevated temperatures. The synergistic effect of SiO 2 combined with the corresponding oxides of UHTCs can potentially extend their usage to environments with higher temperatures. In addition, over time, the gas oxidations gradually evaporate and a large number of pores and cracks are formed on the surface. Consequently, the oxygen diffuses into the carbon fiber substrate and causes oxidation. Ultimately, this results in the damage of the composites. Meanwhile, thermo-mechanically, this can also result in the depletion of surficial coating.
However, following issues need further attention in the study of carbon fiber-reinforced high-temperature ceramic composites to enhance their practical applications: (1) The mechanical properties When high-temperature materials are utilized in actual environments, consideration must be given not only to their anti-ablation properties, but also to their mechanical properties. Therefore, properties such as tension, compression and bending, etc. should all be studied concurrently to provide an accurate assessment of their comprehensive performance in the future.
(2) Selection of reinforcement In order to further optimize performance under high-temperature conditions for certain materials, an appropriate reinforcement structure can be reasonably selected. When they are used as the primary structural component, 2D or 3D preforms with an integral structure can be employed directly.
(3) Matrix modification The use of a Si-based matrix is selected primarily due to the formation of glassy and molten SiO 2 at temperatures approaching to 1800 • C. Coincidentally, when the temperatures exceed this figure, refractory metals can also be incorporated. This would result in the production of a Si-relevant oxide together with other refractory metals. As a result, the more stable and continuous protective dense oxide scale can be created, limiting the diffusion pathways, and ensuring the structural stability of the composites. Simultaneously, the metal size and ratios used should also be considered, as different ratios can produce different reaction products, and hence further affect the overall protective capacities of the composite.
Finally, further research is required to understand which type of refractory materials can offer the best oxidation resistance for the composite, and what kind of test methods are best to evaluate the anticipated resistance abilities for their intended applications.