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

Abstract

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.

1. 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]. At present, there are several methods to improve the ablative and oxidation resistance of fiber-reinforced composites. The first approach is the use of a ceramic fiber or other anti-ablative reinforcements, such as SiC fiber [17], which have better ablative properties, especially in oxygenated environments. When ceramic fiber-reinforced matrix composites are coated with environmental barrier coatings, they can be used in jet engines, as extensively and intensively reported in recent reviews by Fang et al. [18] and Tejero-Martin et al. [19]. The second approach is the optimization of the fiber structure (using 2.5D or 3D textiles). The third approach is an improvement of the interfacial bonding between the fiber and matrix, where pyrolytic carbon (PyC) is generally added between the fiber and matrix. The fourth approach is the addition of an ultrahigh-temperature ceramic into matrix. The fifth approach is coating the fibers or textile structure with ablative ceramics to suppress oxygen diffusion. By implementing the above measures, when ceramics with excellent ablative resistance are used as the matrix, and fibers with outstanding mechanical properties are used as reinforcement material, the composites can be applied to hypersonic vehicles or other high-temperature aerobic environments.

Silicon carbide (SiC) has been widely used as a high-temperature ceramic below 1800 °C in recent decades, since it possesses the merits of structural stability, oxidation resistance, excellent mechanical properties, etc. [20,21,22,23]. In order to expand its application field at higher temperatures, ultrahigh-temperature ceramics (UHTCs) of transition metals (Zr, Hf, Ta, Hf, etc.) [24] with melting points over 3000 °C have been used in combination with SiC ceramics [25,26,27,28,29], which are referred to as Si-based ceramics. Recently, the ablation and oxidation behaviors of fiber-reinforced Si-based ceramic composites with different fiber structures as reinforcement, including whiskers, particles, and preforms, have been investigated [30,31]. Figure 1a,b detail the optimal performance temperature and outstanding mechanical properties of carbon fiber-reinforced Si-based ceramic composites compared with conventional composites [32]. However, the ablative mechanism of C/Si-based ceramic composites has not been comprehensively reviewed, constituting Si-based ceramics as the matrix and different fiber structures for reinforcement.

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 transition metal Zr-supplemented C/SiM composites (C/SiZrM composites, where M refers to B, C, N, etc., typically ZrB₂, ZrC, ZrB₂, 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 3D-braided 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.

2. 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 fiber-reinforced 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.

3. Ablation Behaviors and Mechanisms of Pure C/SiM Composites

Table 1. Materials, methods, and ablation and oxidation properties of pure C/SiM composites.

Composites	Main Structure	Interphase	Matrix (Coating)	Preparing Method	Ablation Type	Mass Ablation Rate (mg∙s−1)	Liner Ablation Rate (mm∙s−1)	Ref.
C/SiC	4D axes carbon fiber preform		SiC coating		arcplasma torch	3	0.1311	[39]
	3D braid carbon		SiC coating		isothermal oxidation			[40]
	2D C/C needle	PyC	SiC	ICVI; PI;	engine torch			[41]
	carbon fiber (M30)	PyC	SiC	CVI	oxyacetylene torch			[42]
	3D C orthogonal structure		SiC	PIP, HP	oxyacetylene torch			[43]
	3D braid C/C	PyC	SiC	CVI, CVD	gas mixture (O2/H2O/Ar)			[44]
	3D needled (30 vol%)	PyC	SiC	CVI; LSI	oxyacetylene torch	1.6	0.0039	[45]
	3D braided	PyC	SiC	PIP	oxyacetylene torch			[46]
	2D plain woven carbon- fabric	PyC	Ph/SiC	LSI	oxyacetylene torch	1837		[47]
	needle punched disk felts	PyC	SiC	PIP; TCVI	oxyacetylene torch	1.53		[48]
	2.5D carbon fiber felts	PyC	SiC	PIP; TCVI	oxyacetylene torch			[49]
	2.5D needle puncher felts	PyC	SiC coating	TCVI; PC	oxyacetylene torch			[50]
	needle-carbon fiber felts	PyC	SiC	CVI; molten infiltration	oxyacetylene torch			[51]
	3D needled felt (T300)	PyC	SiC coating	CVI	plasma wind tunnel			[52]
	4D woven carbon preforms		SiC	impregnation	UH25 was used as fuel; N2O4 as oxidizer		0.005	[53]
	carbon fiber	PyC	SiC coating	CVD; ICVI	oxyacetylene torch			[54]
	2D C/C	PyC	SiC coating	slurry and sintering	isothermal			[55]
	2D C/C needle	PyC	SiC	CVI	hypersonic flowing propane flame			[56]
	2.5D needle punched carbon fiber felt	PyC	SiC	PI; PIP; CVI	plasma generator equipment		0.017	[57]
	2D carbon fiber	BN	SiC	CVD	temperature programmed oxidation			[58]
	carbon fibers (T-300)		SiC/SiC coating	PIP; CVD	air			[59]
	carbon fiber plain fabrics		ph/silicon	LPI	thermal plasma torch			[60]
	3D needle preform	PyC	SiC and Si	CVI; CVD	oxyacetylene torch	6.2		[61]
	carbon fiber	PyC	SiC	CVD	wind-tunnel			[62]
	carbon fiber cloth	PyC	SiC nanowires	CVD	oxyacetylene torch	0.400		[63]
	carbon preform	graphitized	SiC		oxy-kerosene hypersonic torch	0.09		[64]
	3D preform	PyC	SiC	CVI; PIP	plasma arc ablation	0.56	1.1 × 10−4	[65]
	2.5D preform	PyC	SiC		millisecond laser			[66]
	2D carbon fiber felts		SiC	CVI;	plasma stream			[67]
	carbon fiber	PyC	SiC coating	PC	furnace			[68]
	carbon fiber		SiC	CVI	continuous wave lasers			[69]
	2.5D needle punched preform	PyC	SiC	CVI; PIP	plasma generator equipment	0.133	0.0141	[70]
	3D needle-punched preform	PyC	SiC	CVI; PIP	plasma wind tunnel			[71]
	needled preform of carbon felt	PyC	SiC	PIP	oxy-acetylene torch			[72]
	2.5D carbon fiber felt		PyCx-SiCy	CVI;	oxyacetylene torch		0.0016	[73]
	carbon structure		PyCx-SiCy	CVI	oxyacetylene torch		0.0013	[74]
	2.5D needle punched preform	PyC	SiC coating	CVI; PC	oxyacetylene torch	0.0001	0.0003	[75]
	carbon fibre needled felts	PyC	SiC	RMI; CVI	oxyacetylene torch	0.75		[76]
Cf/Si3N4	needle preform		Si3N4	LPCVI; CVI	oxyacetylene torch			[77]
Cf/SiBCN	3D needled carbon fiber preform	(PyC/SiC)3	SiBCN	CVI; PIP	plasma ablation flame	0.0427	0.0017	[78]

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/Si₃N₄ with the same Si₃N₄ 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.

3.1. 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:

$$2\mathrm{C}\left(\mathrm{s}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to 2\mathrm{C}\mathrm{O}\left(\mathrm{g}\right)$$

(1)

$$\mathrm{C}\left(\mathrm{s}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to {\mathrm{C}\mathrm{O}}_2\left(\mathrm{g}\right)$$

(2)

$$\mathrm{C}\left(\mathrm{s}\right)+{\mathrm{C}\mathrm{O}}_2\left(\mathrm{g}\right)\to 2\mathrm{C}\mathrm{O}\left(\mathrm{g}\right)$$

(3)

Reactions of silicon carbide:

$$\mathrm{S}\mathrm{i}\mathrm{C}\left(\mathrm{s}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to \mathrm{S}\mathrm{i}\mathrm{O}\left(\mathrm{g}\right)+\mathrm{C}\mathrm{O}\left(\mathrm{g}\right)$$

(4)

$$2\mathrm{S}\mathrm{i}\mathrm{C}\left(\mathrm{s}\right)+3{\mathrm{O}}_2\left(\mathrm{g}\right)\to 2\mathrm{S}\mathrm{i}{\mathrm{O}}_2\left(\mathrm{l}\right)+2\mathrm{C}\mathrm{O}\left(\mathrm{g}\right)$$

(5)

$$\mathrm{S}\mathrm{i}\mathrm{C}\left(\mathrm{s}\right)+3{\mathrm{C}\mathrm{O}}_2\left(\mathrm{g}\right)\to \mathrm{S}\mathrm{i}{\mathrm{O}}_2\left(\mathrm{l}\right)+4\mathrm{C}\mathrm{O}\left(\mathrm{g}\right)$$

(6)

$$2\mathrm{S}\mathrm{i}\mathrm{C}\left(\mathrm{s}\right)+3{\mathrm{O}}_2\left(\mathrm{g}\right)\to 2\mathrm{S}\mathrm{i}\mathrm{O}\left(\mathrm{g}\right)+2{\mathrm{C}\mathrm{O}}_2\left(\mathrm{g}\right)$$

(7)

$$\mathrm{S}\mathrm{i}\mathrm{C}\left(\mathrm{s}\right)+2{\mathrm{O}}_2\left(\mathrm{g}\right)\to \mathrm{S}\mathrm{i}\mathrm{O}\left(\mathrm{l}\right)+{\mathrm{C}\mathrm{O}}_2\left(\mathrm{g}\right)$$

(8)

Reactions of silicon dioxide and silicon monoxide:

$$\mathrm{S}\mathrm{i}{\mathrm{O}}_2\left(\mathrm{l}\right)\to \mathrm{S}\mathrm{i}{\mathrm{O}}_2\left(\mathrm{g}\right)$$

(9)

$$2\mathrm{S}\mathrm{i}\mathrm{O}\left(\mathrm{g}\right)\to \mathrm{S}\mathrm{i}{\mathrm{O}}_2\left(\mathrm{s}\right)+\mathrm{S}\mathrm{i}\left(\mathrm{s}\right)$$

(10)

$$\mathrm{S}\mathrm{i}{\mathrm{O}}_2\left(\mathrm{s}\right)+\mathrm{C}\left(\mathrm{s}\right)\to \mathrm{S}\mathrm{i}\mathrm{O}\left(\mathrm{g}\right)+\mathrm{C}\mathrm{O}\left(\mathrm{g}\right)$$

(11)

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 SiO₂. Below this temperature, SiO₂ 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 SiO₂, 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 SiO₂ 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 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.

3.2. 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 B₂O₃ gas [80]. The evaporation of B₂O₃ 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 B₂O₃ 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:

$${\mathrm{S}\mathrm{i}{\mathrm{O}}_2\left(\mathrm{l}\right)+\mathrm{B}}_2{\mathrm{O}}_3\left(\mathrm{g}\right)\to \mathrm{B}\mathrm{o}\mathrm{r}\mathrm{o}\mathrm{s}\mathrm{i}\mathrm{l}\mathrm{i}\mathrm{c}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{g}\mathrm{l}\mathrm{a}\mathrm{s}\mathrm{s}\left(\mathrm{l}\right)$$

(12)

$$4\mathrm{B}\mathrm{N}\left(\mathrm{s}\right)+3{\mathrm{O}}_2\left(\mathrm{g}\right)\to {2\mathrm{B}}_2{\mathrm{O}}_3+2{\mathrm{N}}_2\left(\mathrm{g}\right)$$

(13)

$${\mathrm{B}}_2{\mathrm{O}}_3\left(\mathrm{l}\right)\to {\mathrm{B}}_2{\mathrm{O}}_3\left(\mathrm{g}\right)$$

(14)

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.

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 SiO₂-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.

4. 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 ZrB₂ and ZrC. The addition remarkably improves the ablative resistance of composites at elevated temperature [81,82,83].

Table 2. Materials and methods, as well as ablation and oxidation properties of C/SiZrM composites.

Composites	Main Structure	Interphase	Matrix (Coating)	Preparing Method	Ablation Method	Mass Ablation Rate (mg∙s−1)	Liner Ablation Rate (mm∙s−1)	Ref.
C/ZrB2-SiC	2D plain woven carbon cloth	PyC	ZrB2-SiC	CVI; SP	oxyacetylene torch			[84]
	needle punched carbon fiber webs	PyC	ZrB2-SiC	CVI; HCVI	arc-heated wind tunnel			[85]
	2D needle punched carbon fiber preform	PyC	ZrB2-SiC	CVI	arc-heated wind tunnel			[86]
	2D C/C composites		ZrB2-SiC	SAPS	oxyacetylene torch		1.7 × 10−4	[87]
	2D C/C composites		ZrB2-SiC	pack-cementation	oxyacetylene torch	0.062	0.0044	[88]
	needle punched integrated felt	PyC	ZrB2-SiC	TCVI; PIP	oxyacetylene torch			[89]
	2D C/C composites		ZrB2-SiC	SAPS; PC; SI	oxyacetylene torch			[90]
	2D needled carbon fiber preform		ZrB2-SiC	slurry-sintering; CVR	plasma generator			[91]
	needle-punching carbon fiber preform		ZrB2-SiC	TCVI; PIP	oxyacetylene torch			[92]
	needled integrated preform		ZrB2-SiC	pressing, pyrolysis; RSI	oxyacetylene torch	1.3		[93]
	C/C composites	PyC	SiC-ZrB2	CVD; CVI	oxyacetylene torch			[94]
	3D braided C/SiC preform		ZrB2-SiC	painting slurry; CVD; PIP	oxyacetylene torch	22.9	0.0236	[95]
	2D SiC-coated C/C preform		ZrB2-SiC	TCVI; PC; SAPS	oxyacetylene torch			[96]
	2D SiC-coated C/C preform		ZrB2-SiC-Si	PC	oxyacetylene torch	1.5	0.00021	[97]
	3D braided SiC-coated C/C preform		ZrB2-SiC	CVD; slurry painting; PIP	oxidation in air			[20]
	carbon fiber		ZrB2-SiC	slurry infiltration; HP	homemade testing chamber			[16]
	short random/aligned continuous carbon fiber		ZrB2-SiC	HP; SPS	arc-jet plasma			[35]
	porous C/C preform		SiC-ZrB2	RMI; ICVI	oxyacetylene torch	0.61	0.00672	[98]
	short carbon fiber		phenolic-ZrB2-SiC		oxyacetylene torch	14	0.000168	[99]
	needle-punched carbon preform	PyC	ZrB2-SiC-Si	CVI	oxyacetylene torch			[100]
	C/C preform		ZrB2-SiC	HPPS	oxyacetylene torch	2.46		[101]
	2D C/SiC preform		ZrB2-SiC	CVI; CVD	oxyhydrogen torch			[102]
	PAN-based carbon fiber	PyC	ZrB2-SiC	PIP	arc-jet wind tunnel			[103]
	C/C carbon fabric		ZrB2-SiC	LSI;	oxyacetylene torch		217	[104]
C/ZrC-SiC	3D 4-directional carbon fiber preform		ZrC-SiC	CVD; PIP	oxyacetylene torch	0.69	0.026	[79]
	3D 4-directional carbon fiber preform		ZrC-SiC	PIP	plasma wind tunnel	0.7	0.0009	[105]
	3D needle-punched carbon fabrics	PyC	ZrC-SiC	CVI; SI; RMI; PIP	plasma wind tunnel			[106]
	2D C/C carbon felts		ZrC-SiC	ICVI; RMI	oxyacetylene torch	0.24	0.00133	[107]
	2D C/C carbon felts		ZrC-SiC	ICVI; RMI	oxyacetylene torch	0.21	0.00144	[108]
	2D needled C/C carbon fiber felts		ZrC-SiC	TCVI; PIP	oxyacetylene torch	0.40	0.00102	[109]
	porous C/C preform		ZrC-SiC	PIP	oxyacetylene torch	2.29	0.0003	[110]
	2.5D carbon fiber felts	PyC	ZrC-SiC	TCVI; PIP	oxyacetylene torch	1.9	0.012	[111]
	2.5D needled carbon felts	PyC	ZrC-SiC	TCVI; PIP	oxyacetylene torch	0.585	0.00133	[112]
	2.5D needled integral C/C preform		ZrC-SiC	CVD; RMI	oxyacetylene torch	0.02	3.3 × 10−4	[113]
	2.5D needled C/C felts		ZrC-SiC	CVI; PIP	plasma generator	1.57	3.7 × 10−4	[114]
	3D orthogonal braided carbon fiber preform	PyC	ZrC-SiC	CVI; RMI	oxyacetylene torch			[115]
	2D needled C/C perform		ZrC-SiC	CVI; PIP; RMI	plasma generator	2.6	3.7 × 10−3	[116]
	C/C preform		ZrC-SiC	RMI; PIP	plasma generator	0.0045	4.8 × 10−3	[117]
	3D braided carbon fibers		SiC/Zr-Si-C/SiC	PIP; CVD	oxyacetylene torch	27.4	0.0255	[118]
	3D carbon fiber preform		ZrC-SiC	CVD; PIP	oxyacetylene torch			[119]
	2D needled C/C felts		ZrC-SiC	PIP	oxyacetylene torch	37.5	2.48 × 10−3	[120]
	C/C felt preform		SiC-ZrC	CVI; PIP	Developed personally		3 × 10−3	[121]
	needled carbon fiber integer preform		ZrC-SiC	CVI; PIP	plasma flame	1.73	1.94 × 10−4	[122]
	porous needling C/C preform		SiC-ZrC	RMI; CVI	oxyacetylene flame	1.18	2.47 × 10−3	[123]
	3D needle-punched carbon fiber fabrics		SiC-ZrC	slurry impregnation; CVI	arc-heated air plasma		0.039	[124]
	2D needle-punched carbon felt	PyC	SiC-ZrC	CVI; PIP; ICVI; TCVI	oxyacetylene torch	2.95	0.015	[125]
	C/C preform		SiC-ZrC	RMI; ICVI	oxyacetylene torch	1.21	5.9 × 10−3	[126]
	C/C preform		SiC-ZrC	CVI; PIP	oxyacetylene torch			[127]
	needle-punched carbon felt	PyC	SiC-ZrC	ICVI; PIP; ECVI	oxyacetylene flame	0.04	3.7 × 10−4	[128]
	2.5D carbon fiber preforms	PyC	ZrC-SiC	CVI	oxyacetylene torch	0.147	9.8 × 10−3	[129]
	2D needled carbon fiber preform	PyC	ZrC-SiC	TCVI	oxyacetylene flame	0.298	8.2 × 10−4	[130]
	needled carbon felt		ZrC-SiC	CVI; PIP	plasma generator	0.558	0.01633	[131]
	2D needled carbon felts	PyC	ZrC-SiC	CVI; PIP	oxyacetylene torch	0.46	6.7 × 10−4	[132]
	needled felt-structured C/C preform		SiC-ZrC	RMI	oxyacetylene torch	0.29	2.48 × 10−3	[133]
	C/C preform		ZrC-SiC	liquid sintering; RIM	oxyacetylene torch	0.87	2.8 × 10−4	[134]
	C/C preform		ZrC-SiC	RIM	oxyacetylene torch	0.8	3.85 × 10−3	[135]
	T300 fiber cloth		ZrC-SiC	PIP	laser ablation		0.0748	[136]
	2D C/C preform		SiC/ZrC-SiC		oxyacetylene flame	1.2		[137]
	carbon felts		SiCnw/PyC/ZrC-SiC	CLVD	oxyacetylene torch	0.47	7.3 × 10−4	[138]
	2.5D needling carbon felt		ZrC-SiC	CLVD; PIP	oxyacetylene flame	1.22	1.07 × 10−3	[139]
	2.5D needled carbon fiber felts		ZrC-SiC	CLVD	oxyacetylene torch	0.39	5.2 × 10−4	[140]
	2D carbon fiber cloths	PyC	SiC-ZrC	CVI	oxyacetylene ablator	1.17	7.5 × 10−3	[141]
	2D needled C/C preform		SiC-ZrC	CVI	oxyacetylene torch	0.29	4.2 × 10−4	[142]
	2D needle-punched C/C preform		SiC-ZrC	PC	oxyacetylene flame	1.378	1.928 × 10−3	[143]
	3D carbon fiber	PyC	SiC-ZrC	CVD	oxyacetylene flame			[13]
	3D needle-woven carbon fiber felt		SiC-ZrC	CVI	oxyacetylene torch	7.1	4.7 × 10−3	[144]
	3D needle- carbon fiber felt	PyC-SiC	SiC-ZrC	CVI; RMI	plasma torch			[145]
	2.5D needled C/C preform		ZrC/SiC	CVD	oxyacetylene torch	0.84		[146]
	2D needle-punched carbon felts	PyC	ZrC-SiC	CVI	oxyacetylene torch	0.343	4.67 × 10−4	[147]
C/ZrB2-ZrC-SiC	3D carbon fiber preform	PyC	ZrB2-ZrC-SiC	CVI; PIP	oxyacetylene; plasma torch	0.5; 0.13	1 × 10−3; 4 × 10−5	[81]
	needled C/ZrB2 preform		ZrC-SiC	vacuum impregnation; PIP	plasma generator	5.09	2.61 × 10−3	[148]
	2D C/C preform		ZrB2-ZrC-SiC	CVD; PC; SAPS	oxyacetylene torch	0.23	6.5 × 10−5	[149]
	2D needle punched carbon fiber fabric	PyC	SiC-ZrB2-ZrC	TCVI; PIP	oxyacetylene torch			[150]
	needle punched carbon fiber felts	PyC	SiC-ZrB2-ZrC	PIP; TCVI	oxyacetylene torch			[151]
	2D carbon fiber reinforcement felts	PyC	SiC-ZrB2-ZrC	CVI; PIP	oxyacetylene torch	0.0252	4.15 × 10−4	[152]
	carbon felts	PyC	ZrB2-ZrC-SiC	TCVI; PIP	oxyacetylene torch			[153]
	pitch-based carbon fibers		ZrB2-ZrC-SiC	HP	oxyhydrogen torch			[14]
	plain weave carbon fiber		ZrB2-SiC-ZrC	Silicon melt-infiltration	oxyhydrogen torch			[15]
	2.5D needle punched carbon fiber fabric		SiC-ZrB2-ZrC	TCVI; PIP	plasma and compressed air			[25]
	Carbon fiber cloth		ZrB2-SiC/ZrC	HP	oxyhydrogen torch	2.8		[154]
	2D C/C preform		ZrB2-SiC-ZrC	SAPS; RMI	oxyhydrogen torch	0.016	1.3 × 10−3	[155]
C/SiC-ZrSi2	3D needled carbon felts	PyC	SiC-ZrSi2	CVI; RMI	oxyacetylene torch			[156]
C/Zr2Si	3D needled carbon fiber felts	PyC	Zr2Si	RMI; CVI; arc melting	economical laser beam			[157]
C/SiC-Si-Zr	3D needled carbon fiber felts	PyC	SiC-Si-Zr	RMI; CVI	economical laser beam		0.0407	[158]
C/SiC/ZrO2	carbon fabric		Ph/SiC/ZrO2	ball milling	oxyacetylene flame	70.848	0.031	[159]

gives the recent ablation and oxidation properties of C/SiZrM composites.

4.1. Ablation Behaviors and Mechanisms of C/ZrB₂-SiC Composites

In order to improve the ablation-resistant and oxidation-resistant properties of C/SiC composites under complex circumstances and at elevated temperatures, ZrB₂ is incorporated into the composite as the secondary phase of the matrix, which is called as C/ZrB₂-SiC composites [160]. During the ablation process, the composites primarily undergo the following chemical reactions, aside from those outlined in Equations (1)–(11).

$$2\mathrm{Z}\mathrm{r}{\mathrm{B}}_2\left(\mathrm{s}\right)+5{\mathrm{O}}_2\left(\mathrm{g}\right)\to 2\mathrm{Z}\mathrm{r}{\mathrm{O}}_2\left(\mathrm{s}\right)+2{\mathrm{B}}_2{\mathrm{O}}_3\left(\mathrm{l}\right)$$

(15)

$$2\mathrm{Z}\mathrm{r}{\mathrm{B}}_2\left(\mathrm{s}\right)+5{\mathrm{O}}_2\left(\mathrm{g}\right)\to 2\mathrm{Z}\mathrm{r}{\mathrm{O}}_2\left(\mathrm{l}\right)+2{\mathrm{B}}_2{\mathrm{O}}_3\left(\mathrm{g}\right)$$

(16)

$$\mathrm{Z}\mathrm{r}{\mathrm{O}}_2\left(\mathrm{s}\right)\to \mathrm{Z}\mathrm{r}{\mathrm{O}}_2\left(\mathrm{l}\right)$$

(17)

$$\mathrm{Z}\mathrm{r}{\mathrm{O}}_2\left(\mathrm{l}\right)\to \mathrm{Z}\mathrm{r}{\mathrm{O}}_2\left(\mathrm{g}\right)$$

(18)

$$\mathrm{Z}\mathrm{r}{\mathrm{O}}_2\left(\mathrm{s}\right)+{\mathrm{S}\mathrm{i}\mathrm{O}}_2\left(\mathrm{l}\right)\to \mathrm{Z}\mathrm{r}\mathrm{S}\mathrm{i}{\mathrm{O}}_4\left(\mathrm{s}\right)$$

(19)

The ablation and oxidation behaviors of C/ZrB₂-SiC composites are primarily affected by complex chemical erosion and mechanical denudation [86,161]. Figure 5 provides cross-section images of the C/ZrB₂-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, ZrB₂ and SiC are oxidized to form SiO₂, ZrO₂, B₂O_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 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-C_f 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.

4.2. 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/cm³), 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.

$$2\mathrm{Z}\mathrm{r}\mathrm{C}\left(\mathrm{s}\right)+3{\mathrm{O}}_2\left(\mathrm{g}\right)\to 2\mathrm{Z}\mathrm{r}{\mathrm{O}}_2\left(\mathrm{s}\right)+2\mathrm{C}\mathrm{O}\left(\mathrm{g}\right)$$

(20)

$$\mathrm{Z}\mathrm{r}\mathrm{C}\left(\mathrm{s}\right)+3\mathrm{C}{\mathrm{O}}_2\left(\mathrm{g}\right)\to \mathrm{Z}\mathrm{r}{\mathrm{O}}_2\left(\mathrm{l}\right)+4\mathrm{C}\mathrm{O}\left(\mathrm{g}\right)$$

(21)

$$\mathrm{Z}\mathrm{r}\mathrm{C}\left(\mathrm{s}\right)+2{\mathrm{O}}_2\left(\mathrm{g}\right)\to \mathrm{Z}\mathrm{r}{\mathrm{O}}_2\left(\mathrm{s}\right)+\mathrm{C}{\mathrm{O}}_2\left(\mathrm{g}\right)$$

(22)

$$\mathrm{Z}\mathrm{r}{\mathrm{O}}_2\left(\mathrm{s}\right)\to \mathrm{Z}\mathrm{r}{\mathrm{O}}_2\left(\mathrm{l}\right)\to \mathrm{Z}\mathrm{r}{\mathrm{O}}_2\left(\mathrm{g}\right)$$

(23)

$$\mathrm{S}\mathrm{i}{\mathrm{O}}_2\left(\mathrm{s}\right)\to \mathrm{S}\mathrm{i}{\mathrm{O}}_2\left(\mathrm{l}\right)\to \mathrm{S}\mathrm{i}{\mathrm{O}}_2\left(\mathrm{g}\right)$$

(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 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 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 SiO₂-rich 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].

4.3. 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/cm³ and 6.7 g/cm³, 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 °C. Concurrently, the evaporation and fusion of gases (CO_n, 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.

5. 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. Materials, methods, ablation, and oxidation properties of the C/SiZM composites.

Added UHTC	Composites	Main Structure	Interphase	Matrix (Coating)	Preparing Method	Ablation Method	MR * (mg∙s−1)	LR * (mm∙s−1)	Ref.
Si-Hf	C/SiC-HfC	2D needled C/C felts		SiC-HfC	PIP; TCVI	oxyacetylene torch	2.5	1.2 × 10−4	[169]
		3D needle-punched felt	PyC	SiC-HfC	CVI; RMI	plasma wind tunnel			[170]
		2D carbon fabrics		SiC-HfC	SPS; PIP	CO2 laser	12.6		[171]
		C/C-HfC-SiC		SiC and HfC coating	CVR; VPS	ICP plasma wind tunnel			[172]
	C/C-HfB2-SiC	SiC-coated C/C preform		HfB2-SiC	PC; in situ reaction	oxyacetylene	0.147	2.67 × 10−4	[173]
	C/C-SiC-HfC	2.5D C/C preform		SiC-HfC	in situ reaction; CVD	oxyacetylene	2.05	1.93 × 10−3	[174]
	C/SiC-HfC	3D needle-punched preforms	PyC	SiC-HfC	CVI; RMI; PIP	oxyacetylene torch	1.5	4 × 10−3	[175]
	C/C-HfB2-SiC	2.5D needled carbon fiber felts	PyC	HfB2-SiC	CVI; PIP; HSLSI	oxyacetylene flame	0.5	4.15 × 10−4	[176]
	C/C-SiC-HfB2-Si	2.5D C/C preform		SiC-HfB2-Si	SP; GSI	oxyacetylene flame	0.07	7.2 × 10−4	[177]
	C/C-SiC-HfC	C/C preform		(SiC-HfC)4/SiC	LPCVD;	oxyacetylene torch	0.64	5.3 × 10−4	[178]
	C/SiHfBCN	2D carbon fabric		SiHfBCN	PIP	CO2 laser beam			[179]
	SiCf/HfC-SiC	2.5D SiC preform	PyC	HfC-SiC	CVI; PIP	oxyacetylene torch	1.32	7.37 × 10−3	[180]
Si-Ta	C/TaB2-SiC	2D-C/C preform		TaB2-SiC	PC; TCVI	oxyacetylene torch		4.2 × 10−3	[181]
	C/TaSi2	3D carbon fiber preform		TaSi2	pressure filtration	plasmatron			[182]
	C/SiCnw-TaSi2	carbon fiber preform		SiCnw-TaSi2	rapid sintering	oxyacetylene torch			[12]
	C/C-SiC-TaSi2	2D SiC-coated C/C preform		SiC-TaSi2	SAPS; PC	oxyacetylene torch	0.4	9 × 10−4	[183]
	C/C-SiC-TaC	needle-integrated C/C felts	PyC	C-SiC-TaC	CVI	oxyacetylene flame			[184]
Si-Zr-La	C/C/-ZrC-SiC-LaB6	2D C/C preform		ZrC-SiC-LaB6	SPS; SAPS	oxyacetylene torch			[185]
	C/C-SiC-ZrC-La	2D C/C preform		SiC-ZrC-La	PC; SAPS	oxyacetylene torch			[186]
	C/C-SiC-ZrB2-LaB6	3D C/C preform	PyC	SiC-ZrB2-LaB6	PIP; CVI;	plasma generator	0.38	3.7 × 10−4	[187]
	C/C-SiC-ZrB2-La2O3	2D C/C preform		SiC-ZrB2-La2O3	PC; SAPS	oxyacetylene flame	0.558	1.67 × 10−5	[188]
	C/C-ZrB2-SiC-La2O3	2D carbon fiber plain cloth	PyC	ZrB2-SiC-La2O3	CVI; SI; PIP	air plasma flame			[28]
	C/C-SiC-ZrC-La2O3	2D C/C preform		SiC-ZrC-La2O3	SAPS	oxyacetylene torch			[189]
Si-Zr-Hf	C/C-SiC-HfC-ZrC	2D C/C preform		SiC-HfC-ZrC	TCVI; PIP	oxyacetylene torch			[190]
	C/HfC-ZrC-SiC	2.5D needled C/C preform		HfC-ZrC-SiC	RMI	oxyacetylene torch	1.5	1.1 × 10−3	[191]
	C/C-HfC-ZrC-SiC	3D C/C preform		HfC-ZrC-SiC	CVI; PC; SAPS	oxyacetylene torch	0.017		[192]
	C/ZrC-SiC-HfB2	short carbon fiber		ZrC-SiC-HfB2	pressureless sintering	oxyacetylene flame	2.46	3.51 × 10−3	[193]
Si-Zr-Mo	C/C-ZrB2-MoSi2	C/C preform		ZrB2-MoSi2	plasma spraying	oxypropylene flame	1.91	4.8 × 10−4	[194]
	C/C-SiC-ZrB2/MoSi2	2.5D SiC-coated C/C preform		SiC-ZrB2/MoSi2	SAPS;	oxyacetylene torch	0.44	1.67 × 10−3	[195]
	C/C-Mo-ZrB2-MoSi2-SiC	2D C/SiC preform	PyC	SiC-ZrB2-MoSi2-SiC/Mo	HVOF; CVI; SAPS	CO2 laser beam			[196]
	C/SiOC-MoSi2-SiO2-SiC/ZrB2-MoSi2-SiC	carbon fiber needled felt		MoSi2-SiO2-SiC/ZrB2-MoSi2-SiC	PIP; slurry and precursor infiltration	oxyacetylene torch			[197]
Si-Ta-Hf	C/HfC-TaC/HfC-SiC	2D needled C/C preform		HfC-TaC/HfC-SiC	SAPS	oxyacetylene torch			[198]
	C/C-Hf-Ta-Si	2.5D C/C preform		Hf-Ta-Si-C	CVD;	oxyacetylene torch	0.03	1.17 × 10−4	[199]
	C/C-SiC-HfC-TaC	2D SiC-coated C/C preform		HfC-TaC	PC; SAPS	oxyacetylene torch	0.35	1.05 × 10−3	[200]
Si-Zr-Ti	C/C-ZrC-TiC-SiC	2.5D needled C/C preform	PyC	ZrC-TiC-SiC	reactive infiltration	oxyacetylene torch	2.6	8.2 × 10−4	[201]
	C/C-SiC-ZrC-TiC	needled C/C fabrics	PyC	SiC-ZrC-TiC	RMI; CVI	oxyacetylene torch	0.008		[202]
	C/C-ZrC-SiC/TiC	2.5D needled C/C preform		ZrC-SiC/TiC	SAPS; SSP; CVI;	oxyacetylene flames		1 × 10−3	[203]
Si-Ti	C/SiC-Ti3SiC2	carbon cloths	PyC	SiC-Ti3SiC2	LSI; CVI; SI	oxyacetylene torch	6.3	0.024	[204]
	C/C-SiC-Ti3SiC2	C/TiC preform		SiC-Ti3SiC2	LSI	oxyacetylene flame	11.8	0.06	[205]
Si-Y	C/C-SiC-Y2SiO5	2D needle carbon fabric		SiC-Y2SiO5	TCVI; PC; SPS	oxyacetylene torch	0.031	2.6 × 10−3	[206]
	C/C-Y2SiO5-SiC	2D C/C preform		Y2SiO5-SiC	PC; SPS	oxyacetylene torch	0.035	3.43 × 10−3	[207]
Si-Zr-Cr	C/C-ZrB2-CrSi2-SiC-Si	2D C/C preform		ZrB2-CrSi2-SiC-Si/SiC	PC	corundum tube furnace			[208]
	C/C-SiC-Cr-ZrC	2D C/C preform		SiC-Cr-ZrC	TCVI; SAPS	oxyacetylene flame			[209]
Si-Hf-Ti	C/C-HfC-TiC-SiC	C/C		HfC, TiC and SiC coating	VPS; CVR	ICP plasma wind tunnel			[172]
Si-Ti-Ta	C/C-SiC-TiC-TaC	2/2 C/C twill carbon cloth		SiC-TiC-TaC	MI; SPS;	oxyacetylene flame	3.9	0.0022	[210]
Zr-Hf	C/C-HfC-ZrC	C/C preform		HfC-ZrC	CVD;	oxyacetylene torch			[211]
Hf-Ta-Zr	C/HfC-TaC(HfC-ZrC)	C/C preform		HfC-TaC/HfC-ZrC	CVD;	oxyacetylene torch	0.63	2.1 × 10−4	[212]
SiZrHfTiCr	C/C-(HfZrTiCr)B2-SiC-Si	C/C preform		(Hf1/4Zr1/4Ti1/4Cr1/4)B2-SiC-Si	SP; GRSI	oxyacetylene ablator	0.37	1.5 × 10−4	[29]
SiZrAlCr	C/C-ZrC-SiC-Al2O3-Cr	C/C-ZrC-SiC preform		Al2O3-SiC-ZrC-Cr	RMI; SI; plasma spray	oxyacetylene torch	0.52	4.7 × 10−4	[213]
Si-Zr-V	C/C-ZrC-SiC-V0.9-Si0.1	3D needled carbon preform		ZrC-SiC-V0.9-Si0.1	RMI;	oxyacetylene torch	0.25	4.3 × 10−4	[214]
Si-Mo/Ti	C/C-(Mo,Ti)Si2-SiC	porous C/C preform		(Mo,Ti)Si2-SiC	RMI	oxyacetylene torch	0.01	2 × 10−3	[215]
Si-Mo	C/C-SiC-MoSi2	porous C/C preform		SiC-MoSi2	VFI	oxyacetylene torch	1.34	3.5 × 10−3	[216]
SiZrMoTa	C/SiCO-TaSi2-MoSi2-ZrO2	carbon felts		TaSi2-MoSi2-ZrO2	sol-gel; pyrolysis	oxyacetylene flame	0.4	8.33 × 10−4	[217]
SiZrCrY	C/C-ZrSi2-CrSi2-Y2O3/SiC	2D SiC-coated C/C preform		ZrSi2-CrSi2-Y2O3/SiC	SAPS;	oxyacetylene torch	0.16	1 × 10−3	[218]
SiZrCrAl	C/C-ZrC-SiC-Al-Cr	2.5D needled C/C preform		ZrC-SiC-Al-Cr	CVD; RMI	oxyacetylene torch	0.02	2.5 × 10−4	[219]
Si-Zr-La/Y	C/SiC-ZrC-La2O3; C/SiC-ZrC-Y2O3	3D needled felt		SiC-ZrC-La2O3; SiC-ZrC-Y2O3	CVI; RMI; PIP	oxyacetylene torch	1.19; 4.52	9.93 × 10−3; 0.0178	[220]
Si-Mo-(Ti/Al)	C/C-MoSi2-SiC-(Ti/Al)	needle-punched C/C preform	PyC	MoSi2-SiC-(Ti/Al)	CVI	oxyacetylene torch	0.01	2 × 10−3	[221]
Si-Mo-Hf-W	C/ZrB2-SiC-MoSi2; C/ZrB2-SiC-HfSi2; C/ZrB2-SiC-WSi2;	short carbon fiber		ZrB2-SiC-MoSi2; ZrB2-SiC-HfSi2; ZrB2-SiC-WSi2;	ball-milling; hot-pressing	oxyacetylene torch			[222]
Si-Zr-Y	C/C-ZrB2-SiC-Y2O3/SiC	C/C preform		ZrB2-SiC-Y2O3/SiC	PC; APS	muffle furnace			[27]
Si-Zr-Sm	C/C-ZrB2/SiC-Sm2O3	C/C preform		ZrB2/SiC-Sm2O3	APS; IPS	plasma torch	0.319		[223]
Si-Cu	C/C-SiCW-Cu	carbon fiber bundle		SiCW-Cu	CVD; CVI;	oxyacetylene torch	4.56	8 × 10−3	[224]
Si-Nd	C/C-Si-SiC-SiO2-Nd2O3	SiC coated C/C preform		Si-SiC-SiO2-Nd2O3	CVI; laser cladding	laser-ablation			[225]
Si-Al	C/C-Al20Si/graphite	3D needled C/C preform		Al20Si/graphite	GCVI;	combustion chamber			[226]
Si-Zr-Ta	C/SiC-ZrB2-TaxCy	carbon fiber cloth mat		SiC-ZrB2-TaxCy	RHP; PIP	oxyacetylene torch	1.33	1.9 × 10−4	[227]
Si-Zr-Nb	C/SiC-NbC-ZrC	2D C/C preform		SiC-NbC-ZrC	SAPS	oxyacetylene torch	0.48	1.3 × 10−4	[228]
Si-La	C/C-SiC-La2O3	2.5D carbon fiber felts	PyC	SiC-La2O3	PIP; CVI;	plasma generator	0.722	0.0333	[229]
SiTiZrHfNbTa	C/(TiZrHfNbTa)C-SiC	3D-needled carbon fiber	PyC/SiC	(TiZrHfNbTa)C-SiC	PIP; CVI;	air plasma torch	2.60	2.89 × 10−3	[230]
Si-Zr-V	C/C-ZrC-SiC-V	C/C preform		ZrC-SiC-V	RIM	oxyacetylene torch	2	7 × 10−4	[231]
Si-Zr-Cu	C/C-SiC-ZrC-Cu	needled carbon fiber felts	PyC	SiC-ZrC-Cu	CVI; PIP; VPI	oxyacetylene flame	3.4	3.5 × 10−3	[232]

Note *: MR refers to mass ablation rate; LR is liner ablation rate.

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.

Composites of C/SiM modified with Hf demonstrate remarkable thermal stability, where the melting point of Hf approaches 2227 °C. During the ablation process, the following chemical reactions take place:

$$2\mathrm{H}\mathrm{f}\mathrm{C}\left(\mathrm{s}\right)+3{\mathrm{O}}_2\left(\mathrm{g}\right)\to 2\mathrm{H}\mathrm{f}{\mathrm{O}}_2\left(\mathrm{s}\right)+2\mathrm{C}\mathrm{O}\left(\mathrm{g}\right)$$

(25)

$$\mathrm{H}\mathrm{f}\mathrm{C}\left(\mathrm{s}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to \mathrm{H}\mathrm{f}\mathrm{O}\left(\mathrm{g}\right)+\mathrm{C}\mathrm{O}\left(\mathrm{g}\right)$$

(26)

$$\mathrm{H}\mathrm{f}\mathrm{C}\left(\mathrm{s}\right)+2{\mathrm{O}}_2\left(\mathrm{g}\right)\to \mathrm{H}\mathrm{f}{\mathrm{O}}_2\left(\mathrm{s}\right)+\mathrm{C}{\mathrm{O}}_2\left(\mathrm{g}\right)$$

(27)

$$2\mathrm{H}\mathrm{f}{\mathrm{B}}_2\left(\mathrm{s}\right)+5{\mathrm{O}}_2\left(\mathrm{g}\right)\to 2\mathrm{H}\mathrm{f}{\mathrm{O}}_2\left(\mathrm{s}\right)+2{\mathrm{B}}_2{\mathrm{O}}_3\left(\mathrm{g}\right)$$

(28)

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, SiHf_xO_y-based layers (SiHf-O glass) and liquid SiO₂ can protect the fiber from ablation.

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 promotes the liquid phase sintering of ZrO₂, and generates a molten phase of La₂Zr₂O₇. Additionally, evolution of La₂O₃, La₂Si₂O₇, La_0.71Zr_0.29O_1.65, and micron-sized ZrO₂-La₂O₃-SiO₂ 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 HfO₂-ZrO₂-SiO₂ mixture layer is helpful for ablation protection. Moreover, the addition of Zr-Mo to the C/SiM composite leads to the formation of SiO₂-ZrO₂-Mo_4.8SiC_0.6 oxide protective barrier that impedes oxygen diffusion into the substrate interior. For the Ta-Hf modified C/SiM composite, an integral scale constituted by Hf-Ta-Si-O (HfO₂-Ta₂O₅-SiO₂ ceramic sheet) oxides act as oxygen insulator, and the formation of micro-cracks mitigates thermal stress. For the Zr-Ti enhanced C/SiM composite, the multiphase oxidation scale of Zr-Ti-Si-O glass provides exceptional resistance against the ablation of the substrate.

As for other Ti, Y, Cr, Al, V, Mo, Sm, Cu, Nd, Nb, et al., added C/SiM composites, the primary protective oxide layers TiO₂, Y₂O₃, Cr₂O₃, Al₂O₃, V₂O₃ (V₂O₅), MoSi₃ (Mo₅Si₃), Sm₂O₃, CuxO, Nd₂O₃, NbO (NbO₂ and Nb₂O₅) contribute to the anti-ablation resistance of the substrate.

6. 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 SiO₂ 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 SiO₂ 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.