Carbon–Carbon Composite Metallic Alloy Joints and Corresponding Nanoscale Interfaces, a Short Review: Challenges, Strategies, and Prospects

Abstract

Brazing of carbon–carbon (C/C) composites with metallic materials currently faces a series of difficulties, such as the poor wettability of metallic materials on the surface, the nanoscale interface bonding of C/C composites and metallic materials, thermal stress problems for these different materials, etc. Especially, the practical problems, including the low joint strength and insufficient reliability, still limit the large-scale practical application of brazing technology for C/C composites and metal materials. Herein, in order to guide the fabrication of high-quality joints, we present a brief discussion on the latest research progress in the joints of C/C composites and metallic materials, including current challenges, solution methods, mechanisms, and future prospects. More importantly, the nanoscale interface in the carbon–carbon composites and metallic alloys is paid very little attention, which has been critically discussed for the first time. Then, we further outline the possible solutions in joint problems of C/C composites and metallic materials, proposing feasible strategies to control the reaction in the brazing process, such as surface treatments, the addition of reinforcing phases, a transition layer sandwiched between the base material and the intermediate layer, etc. These strategies are being envisioned for the first time and further contribute to promoting the converged applications of C/C composites and metallic materials.

1. Introduction

Carbon–carbon (C/C) composite material is a new type of composite material, with carbon fiber or graphite fiber as the reinforcement material and carbonized or graphitized resin or carbon, made by chemical vapor deposition, as the matrix. It has a series of excellent properties, such as lightweight, high strength, an outstanding modulus, a low coefficient value of thermal expansion, a high resistance temperature, corrosion resistance, a high fracture toughness, a low creep property, etc. C/C composite material is considered a novel material, which has simultaneous structural and functional properties [1,2,3,4].

The density of C/C composites is low (<2.2 g/cm³), only 1/4 that of nickel-based superalloys and 1/2 that of ceramic materials [3]. The C/C composite is an ultra-high temperature composite that can be used at temperatures higher than 3000 °C. As the temperature increases, its strength not only does not decrease, but is even higher than that at room temperature [3]. Therefore, C/C composites are widely used in aerospace, marine, nuclear energy systems, biomedical, and other high-tech fields, such as space shuttle engines, aircraft brake discs, the International Thermonuclear Experimental Reactor (ITER) divertor, artificial joints, etc. [4,5,6,7]. It is worth mentioning that China plans to develop C/C composites for the Thorium Molten Salt Reactor Nuclear Energy System (TMSR) during the 15th Five-Year Plan [8]. However, the expensive preparation process and long production cycle of C/C composites make it difficult to prepare large and complex components. At the same time, C/C composites are susceptible to oxidation and have weaker mechanical properties than metallic materials at high temperatures. The individual use of C/C composites is almost impossible and usually requires a composite structure with other materials (especially metallic materials) to be used. In order to exploit their respective advantages, high-quality joining between C/C composites and metallic materials needs to be achieved. This is one of the key technologies that must be solved to promote the application of C/C composites.

To date, the joining methods of C/C composites mainly include mechanical joining [9,10], adhesive bonding [11,12,13,14], brazing [15,16,17,18,19], diffusion joining [20,21], and high-temperature self-propagation reactions. There are two problems with forming mechanical joints by using the difference in roughness between the C/C composite and the material to be joined. The first is the low interface strength of the mechanical joint, and the second is that the notch and tip effects caused by the notches and cracks introduced by the mechanical joint can greatly weaken the C/C composite and the joint. It is also possible to join C/C composites using adhesive joining, but the physical bond between the interfaces is weak. It is difficult to guarantee the quality of the joint. Meanwhile, the adhesive is usually a high polymer, which faces the aging problem when joining C/C composites. C/C composites are often used in extreme environments, such as high-temperature environments or friction fields, and the aging rate will seriously limit the service life of the joints. Due to the high melting point of C/C composites (3600~3800 °C), they cannot be joined by traditional methods of fusion welding. Brazing, as an economical and reliable material joining method, is one of the most researched, mature, and widely used methods for joining C/C composites to metals in recent decades.

2. Difficulties in Brazing C/C Composites and Metals

In order to obtain high-quality brazed joints of C/C composites and metals, it is crucial to solve the following problems: improving the wetting of the brazing material with the joined material, eliminating the thermal stresses due to the coefficient mismatch of thermal expansion for the matrix, and therefore, suppressing the generation of brittle phases in the process of their joining [18,22,23].

2.1. Improving the Wettability

The behavior of a non-reactive liquid wetting a solid can be described by the classical Young’s equation [4]:

$$\theta =\arccos \left(\frac{{\sigma }_{sv}-{\sigma }_{sl}}{{\sigma }_{lv}}\right)$$

(1)

where ${\sigma }_{sv},$ ${\sigma }_{sl},$ and ${\sigma }_{lv}$ denote the surface tension between a solid and gas, solid and liquid, and gas and liquid. The angle of $\theta$ is called the wetting angle and is a physical quantity used to characterize the wettability of two materials. The correspondence between its size and wettability is shown in

Table 1. Correspondence between the wetting angle and the wettability.

$\mathit{\theta }$	$\mathbf{c}\mathbf{o}\mathbf{s} \mathit{\theta }$	Wettability
$\theta =0$	1	completely wettable
$0<\theta <90°$	$0<\mathrm{c}\mathrm{o}\mathrm{s} \theta <1$	wettable
$90°\le \theta \le 180°$	$-1\le \mathrm{c}\mathrm{o}\mathrm{s} \theta \le 0$	non-wettable

.

C/C composites greatly differ from metals in terms of the crystal structure and physical–chemical properties, and it is difficult to wet metals and to not wet C/C composites. Meanwhile, some active metallic elements, such as Ti, Cr, Mo, and W, can form carbides with C in a liquid state. Based on this, many scholars have carried out a lot of research in improving the wettability of metal and C/C composites [24,25,26,27,28,29], and there are mainly two methods, as follows:

(1)

The surface of the C/C composites is pre-treated with a layer of active metallic elements’ powder by plating, sintering, or deposition, and then conventionally brazed. Kong [27] introduced a simple ammonium dichromate solution immersion method to synthesize copper wettable Cr₃C₂ coatings on and inside C/C preforms. The wetting angle of molten copper with C/C decreased from 140° to 60°, indicating a significant improvement in the wettability. Wenyan Zhou et al. [28] fabricated Mo₂C layers on the entire inner surface of C/C preforms in molten salt (NaCl-KCl) with the addition of ammonium paramolybdate. An approximately 1 μm-thick Mo₂C layer that uniformly covered the inner surface of the C/C preforms was obtained. The formation of the Mo₂C layer shows a transformation from the non-wettable interface of Cu/C into the wettable interfaces Mo₂C/C and Cu/Mo₂C, respectively. Good interface bonding was observed between Mo₂C/C and Cu/Mo₂C.

(2)

An appropriate amount of the active metal element is added to the brazing material to make an active brazing material. The most commonly used active metallic element is titanium (Ti). This method is called active metal brazing. Huang Wu et al. [29] investigated the effect of Ti on improving the wettability of liquid Cu with C/C composites by using the sessile drop method. The wetting angle of Cu–10 wt% Ti dropped on the C/C composite at 1300 °C for 60 min was measured as ~70°, which was smaller than that of a Cu drop on the composite. This indicates that the addition of Ti can effectively improve the wettability of liquid Cu on a C/C composite.

There are three types of active metal brazing: (1) brazing by placing titanium or zirconium directly between C/C composites in the form of shims, (2) pre-coating the surfaces to be joined with titanium or zirconium fine powder or titanium or zirconium hydride, and then brazing with brazing material, and (3) brazing directly with an active brazing material containing titanium and zirconium. Titanium-containing brazing materials are brittle and difficult to process and shape and are often made into double- or multi-layer brazing materials. For example, Cu-Ti brazing materials are made into bimetallic sheets, and Ag-Cu-Ti brazing materials are made into wires with titanium as the core and covered with silver-copper alloy. Titanium-containing brazing materials are also often used in powder form.

In recent years, scholars have reported some Fe-based and high-entropy alloy brazing materials that can be used for the brazing of C/C composites. However, the related studies are relatively few and need to be further developed [30,31].

2.2. Eliminating the Thermal Stresses

In addition to the wettability, how to relieve the thermal stress due to the coefficient mismatch in thermal expansion for the current composite materials is also a key issue to be solved in brazing. The coefficient of thermal expansion of 2D C/C composites is generally around: (2~3) × 10⁻⁶/K, and that of 3D C/C composites is generally around: (7~8) × 10⁻⁶/K, depending on the temperature range and tissue shape. The coefficient of thermal expansion of most metals and alloys is generally larger than 1 × 10⁻⁵/K depending on the temperature range and tissue shape, which results in a large amount of thermal stress in the joint during cooling. He Peng et al. [32,33] used finite element analysis to analyze the fracture of the joint and found that most of the residual thermal stresses in the joint are concentrated between the material with a smaller coefficient of thermal expansion and the joint layer. The authors further proposed the concepts of a residual stress factor in the connection layer, R_f, and a thickness factor in the connection layer, T_f:

$$R_f=\mathsf{\Delta }\alpha \cdot E\cdot {\sigma }_{0.2}\cdot \mathsf{\omega }$$

(2)

$$T_f=E\cdot {\sigma }_{0.2}\cdot \mathsf{\omega }\cdot \sqrt{\mathsf{\Delta }{\alpha }_1\cdot \mathsf{\Delta }{\alpha }_2}$$

(3)

In Equations (2) and (3), $\mathsf{\Delta }\alpha$ represents the difference in the coefficients of thermal expansion between the intermediate layer material and the base material with a smaller coefficient of thermal expansion. E is the modulus of elasticity of the material, ${\sigma }_{0.2}$ is the yield limit of the material, $\mathsf{\omega }$ is the coefficient of work hardening of the material, and $\mathsf{\Delta }{\alpha }_{\mathrm{1,2}}$ are the differences in the coefficients of thermal expansion between the intermediate layer and the two base materials.

After data simulation by the computer, the authors concluded that the connection layer should be selected as small as possible for T_f and R_f. Moreover, the thickness of the connecting layer should be as small as possible under the premise of ensuring sufficient physical contact.

2.3. Suppressing the Generation of Brittle Phases

The active element added to the brazing material can improve the wetting of the brazing material on the C/C composite. However, excessive active elements can form brittle carbides with C/C composites, which in turn reduce the joint strength [34]. At the same time, the active brazing material also forms some brittle intermetallic compounds with the metal base material, leading to a reduction in joint strength [35]. In order to reduce the formation of brittle phases, it is not advisable to add large amounts of reactive elements to the brazing material. For example, the mass fraction of Ti in the commonly used AgCuTi brazing materials is usually below 15 wt% [25,36,37].

In recent years, research to improve the brazing performance of C/C composites with metals has focused on both surface modification and the study of composite brazing materials. The more representative studies are shown in Figure 1.

3. Brazing of C/C Composites and Metals

Many studies have been conducted on the brazing between C/C composites and metals or non-metals. The brazing system of C/C composites and metals mainly includes C/C composites with copper and copper alloys (pure copper, oxygen-free Cu, CuZrCr alloy, CuW alloy, etc.), titanium and titanium alloys (TC4 alloy, TC17 alloy, Ti600 alloy, etc.), nickel-based superalloys (GH3044 alloy, GH3128 alloy, GH99 alloy, etc.), intermetallic compounds (TiAl alloy, Ti₃Al alloy), stainless steel, Nb, W, TZM alloy, and so on (as shown in Figure 2).

3.1. Brazing of C/C Composites with Copper and Copper Alloys

Since 1998, C/C composites have been selected as the first-wall protection material for the internal copper alloy cooling tubes in the cooling system protecting the vertical target of the International Thermonuclear Experimental Reactor (ITER) deflector due to their excellent overall performance, which are showed in

Table 2. Summary of the brazing of C/C composites to copper alloys.

Base Materials	Composition of Brazing Filler Metal (wt%)	Brazing Temperature (°C)/ Holding Time (min)	Strength (MPa)	References
C/C-OF Cu	Cu-28Ag-2Ti	850/10	22 (Shear)	[38]
C/C-OF Cu	Cu-2Al-3Si-2.3Ti	1030 ± 2	20.2 (Tensile)	[39]
C/C-Cu	Cu-50Pb	1150/40	1.5 (Shear)	[40]
C/C-Cu	Ti-15Cu-15Ni	1008/10	24 (Shear)	[41]
C/C-Cu	Ag-68.8Cu-4.5Ti	910/10	Bending: 14 (Flat), 52 (Conical interface)	[42]
C/C-Cu	Cu-30Ti	930	7 (Shear)	[43]
C/C-Cu	Ni-33Cr-24Pd-4Si	1210	6 (Shear)	[43]
C/C-Cu	Ni-11Cr-10P	935	7 (Shear)	[43]
C/C-CuCr	AgCu-2Ti	850/10	16 (Shear)	[43]
C/C-CuW	AgCu-2Ti	850/10	13 (Shear)	[43]
C/C-CuMoCu	68.8Ag-26.7Cu-4.5Ti	(915–920)/5	/	[44,45]
C/C-CuMoCu	63Ag-34.3Cu-1Sn-1.75Ti	(821–826)/5	/	[44]
C/C-CuMoCu	92.8Cu-3Si-2Al-2.25Ti	(1040–1045)/5	/	[44]
C/C-CuMoCu	70Ti-15Cu-15Ni	(975–980)/5	/	[44]
C/C-CuMoCu	63Ag-35.3Cu-1.75Ti	(830–835)/5	/	[45]

[38,39,40,41,42,43,44,45]. The ITER’s engineering team achieved a reliable connection between the C/C composite and OF-Cu by brazing.

Appendino et al. [41] brazed C/C composites and copper with Ti/15Cu/15Ni brazing material and performed mechanical shear tests on the joined specimens, which are showed in Figure 3A. The results show that the best results were for the brazing process at ~1008 °C for 10 min (1 kPa, Ar flow). The average shear strength of the six specimens was 24 MPa, and the dispersion of the mechanical results was small. The analysis of the fracture surfaces revealed that the cracks propagated through the joint region, while the TiCuNi/Cu, C/C/TiCuNi, and their interfaces did not fail, indicating a strong adhesion of the brazing alloy for these two materials (composite and the copper). It is well known that brazing the carbon-fiber-reinforced C/C composite and copper has gained increasing interest because of its important application in thermal management systems in nuclear fusion reactors and in the aerospace industry [42]. Figure 3B shows the high-resolution TEM images, revealing the microstructure and interfacial micro-chemistry of C/C composite/AgCuTi/Cu brazed joints [42]. Milena Salvo used a slurry technique to deposit metallic Cr on the surface of C/C composites. C/C/Cu and Cu/CuCrZr were joined using Gemco brazing material (87.75 wt% Cu, 12 wt% Ge, and 0.25 wt% Ni) (Figure 3C) [46]. This study achieved brazing of C/C composites with CuCrZr alloys by using a braze without reactive elements and prevented interfacial cracking and debonding by modulating the thermal stresses through the large ductility of the braze and pure copper. The presence of Ge_xCr_y intermetallic phases was not confirmed by XRD analysis or the Cr diffusion pattern. It was confirmed that the Cu interlayer can prevent the brittle intermetallic compounds’ formation, and then improve the joint strength.

3.2. Brazing of C/C Composites to Titanium and Titanium Alloys

Titanium alloy is a vital material in the aerospace field because of its high specific strength, good corrosion resistance, and excellent overall mechanical properties. Joining titanium alloys with C/C composites with excellent high-temperature properties can obtain a lightweight, high-temperature performance and meet certain structural strength requirements of the joint, showing a large number of applications in rocket, aerospace, and other fields. The titanium- and silver-based brazing materials are commonly used for brazing C/C composites to titanium alloys, which are shown in

Table 3. Summary of the brazing of C/C composites to titanium alloys.

Base Materials	Composition of Brazing Filler Metal (wt%)	Brazing Temperature (°C)/Holding Time (min)	Strength (MPa)	References
C/C-TC4	Ag-26.7Cu-4.6Ti	910/10	25 (Shear)	[47,48]
C/C-Graphite foam-Ti tube	63Ag-32.25Cu-1.75Ti	820/5	12.4 (Shear)	[49]
C/C-TiAl	Ag-26.7Cu-4.6Ti	900/10	12.9 (Shear)	[50]
C/C-TiAl	Ti-Ni-Cu (1:1:1)	980/10	18 (Shear)	[51]
C/C-Ti	68.8Ag-26.7Cu-4.5Ti	910/5	0.24 ± 0.09 (Tensile)	[52,53]
C/C-Ti	92.8Cu-3Si-2Al-2.2Ti	1040/5	0.27 ± 0.12 (Tensile)	[52,53]
C/C-Ti	70Ti15Cu15Ni	975/5	0.33 ± 0.13 (Tensile)	[52,53]
C/C-Ti	Ag-32.25Cu-1.75Ti	830/5	24 (Shear)	[54]
C/C-Ti	MBF-20 Amorphous (Ni-6.48Cr-3.13Fe-4.38Si-3.13B)	1045/8	/	[55]
C/C-Ti	MBF-30 Amorphous (Ni-4.61Si-2.8B-0.02Fe)	1080/8	/	[55]

clearly [47,48,49,50,51,52,53,54,55]. M. Singh et al. [53] used three brazing materials, Cu-ABA, TiCuNi, and TiCuSi, in the form of a foil with a thickness of about 50 μm, to braze C/C composites to pure titanium. The analysis revealed that metallurgical bonding due to redistribution and diffusion of solutes during the brazing process ensures the wetting of the brazing material to the substrate. The best joint performance was achieved with the Cu-ABA brazing material. The orientation of the C fiber bundles in the C/C composite affects the joint properties. The highest joint load capacity was obtained with the carbon fiber bundles oriented perpendicular to the Ti tube axis, and the lowest was obtained with the C fiber bundles oriented parallel to the Ti tube axis. Youqiong Qin et al. [47,48] brazed C/C composites and the TC4 titanium alloy by using a TiZrCuNi amorphous alloy braze, and they investigated the effect of the brazing process parameters on the microstructure of the joints. The Cu and Mo interlayers are added between the C/C composite and the TC4 titanium alloy to form the foil titanium-based brazing material. On the one hand, it relieves the residual stresses at the joint due to the differences in the thermal expansion coefficients between the base material and the brazing material; on the other hand, it avoids the formation of a large number of brittle intermetallic compounds, such as Ti_xCu_y and Ti_xNi_y, between the brazing material and the TC4 titanium alloy base material. The results demonstrate that this method can be used for brazing of C/C composite materials and TC4 titanium alloys. At lower brazing temperatures, the interface structure of this joint is: Cu/Cu₅₁Zr₁₄/Ti₂(Cu,Ni) + Ti(Cu,Ni) + TiCu + Cu₂TiZr/TiC/C/C composites. As the temperature increases, the Cu in the layer dissolves into the brazing material, forming Ti(Cu,Ni)₂ and Cu(s.s), and then the TiC reaction layer changes from intermittent to continuous. The end products in the center of the joint are Ti(Cu,Ni)₂ and Cu(s.s).

Guo Wei et al. [35] studied the brazing of C/C composites and the TC4 titanium alloy using the AgCu brazing material (72Ag-28Cu, mass fraction, %), with a brazing temperature of 820~940 °C and a holding time of 3~30 min. It was demonstrated that the Ti in the TC4 titanium alloy diffused into the brazing material and the C/C composite during brazing to chemically react with the base and brazing materials, enhancing the wettability of the brazing material to the C/C composite. The interface structure of this joint is: TC4/Ti₂Cu + Ti(s.s)/Ti₂Cu/TiCu/Ag(s.s) + Ti₃Cu₄/TiC/C/C composites. The shear strength of the obtained joint is 33 MPa at 880 °C for 10 min.

Duo Liu and Kehan Zhao et al. introduced carbon fibers in the AgCuTi brazing material to braze C/C composites and the TC4 alloy [15,19,56]. It was demonstrated that the interfacial structure of the brazed joint obtained by using the AgCuTiC composite brazing material, with a CNTs content of 0.2% at a brazing temperature of 880 °C and a holding time of 20 min, was: TC4/diffusion layer/Ti₂Cu/TiCu/Ti₃Cu₄/TiCu₄/TiC + TiCu₂ + Ag(s.s) + Cu(s.s)/Ti₃Cu₄/TiCu₄/TiC/C/C composites. A moderate amount of CNTs can reduce the formation of brittle compounds at the interface and refine the interfacial organization, as well as alleviate the mismatch of thermal expansion coefficients between parent materials and reduce residual stresses. A high content of CNTs consumes a large amount of Ti in the brazing material, which weakens the bonding on one side of the C/C composite and reduces the shear strength of the joint. A maximum joint strength of 44 MPa was obtained at a 0.4% mass fraction of carbon fiber.

3.3. Brazing of C/C Composites to Nickel-Based Alloys

Nickel-based high-temperature alloys have good oxidation resistance, excellent thermal corrosion resistance, a high working temperature, stable organization, and less harmful phases, and can work under high-temperature and high-stress environments. They are widely used in high-tech fields, such as in aerospace and marine industries and nuclear energy systems. C/C composites are joined to nickel-based high-temperature alloys with a low mass, high load-bearing capacity, and a long high-temperature life. The nickel-based braze is the most commonly used braze for brazing C/C composites to nickel-based high-temperature alloys, which are shown in

Table 4. Summary of the brazing of C/C composites to nickel-based alloys.

Base Materials	Composition of Brazing Filler Metal (wt%)	Brazing Temperature (°C)/Holding Time (min)	Strength (MPa)	References
C/C-K24	Ag26Cu2Ti	880/10	16 (Shear)	[57]
C/C-GH3044	Ag-(10,20,30,40)Ti	(990~1080)/(10~90)	45.8 (Shear)	[58]
C/C-GH3044	80Ni-20Ti PTLP	1030/30	9.78 (Shear)	[59]
C/C-GH3044	Ni71CrSi	1080/30	35.08 (Shear)	[60]
C/C-GH3044	Ni71CrSi	1080/30	54.4 (Shear)	[61]
C/C-GH3044	Ti/Ni/Cu/Ni (1:1:20:1)	1030/30	32.09 (Shear)	[62]
C/C-GH3128	Ni-Si	1160	12.6 (Shear)	[63]
C/C-GH3128	BNi-2 (Ni-7.0Cr-3.0Fe-4.5Si-3.1B)	1170/60	Shear: 35.4 (RT), 15.3 (800 °C), 8.6 (1000 °C)	[64]
C/C-GH3128	Ni-Ti	1170	22.5 (Shear)	[65]
C/C-GH99	BNi-2 Ni-(6~8)Cr-(2.5~3.5)Fe-(4~5)Si-(2.75~3.5)B	1170/60	16 (Shear)	[34]
C/C-GH600	BNi68CrWB	1150~1200/10	Shear: 49.9 (RT), 21.6 (800 °C)	[66]
C/C-Ni-based superalloy	NiCrSiBFe Amorphous	1170/60	Shear: 35 (RT), 15 (800 °C), 9 (1000 °C)	[67]

clearly [57,58,59,60,61,62,63,64,65,66,67]. As shown in Figure 4A, Guo L. et al. [60] used the Ni-Ti powder to fabricate a thin intermediate layer diffusion connection between C/C composites and the GH3128 high-temperature alloy, with a welding temperature of 1050~1250 °C, pressurization of 8~20 MPa, and a holding time of 60 min. The results show that for the group of C/C composite surfaces without treatment, the shear strength of the joints was almost 0. For the group of C/C composite surfaces coated with the SiC coating, the shear strength of the joints was 23 Mpa at a heating temperature of 1170 °C. The authors found that coating SiC on the C/C surface can improve the wettability of Ti, Ni, and other elements on the C/C surface, on the one hand, and alleviate the thermal stress due to the different thermal expansion coefficients of C/C composites and the GH3128 high-temperature alloy on the other hand.

Tian et al. [34] brazed C/C composites and GH99 high-temperature alloys by adding TiH₂ to BNi₂ powder brazing material. The welding parameters were 1170 °C, vacuum < 10⁻³ Pa, and a holding time of 60 min. Brazing was performed using brazing materials containing TiH₂ at 1%, 3%, and 8%, and it was found that the best joint performance was obtained with 3% TiH₂ in the brazing material. The shear strength of the joints was 40 MPa at room temperature and 19 MPa and 10 MPa at 800 °C and 1000 °C, which showed a significant increase in strength compared to the pure BNi₂ brazing material. The authors found that Ti in TiH₂ can promote the diffusion of C/C composites into the brazing seam, produce diffuse MC particles, reduce the mismatch between the C/C composite matrix and the brazing seam, and then relieve the residual stresses.

However, excessive TiH₂ will produce a large amount of flaky TiC in the brazing seam and reduce the plastic deformation capacity of the joint. Zhang Xin et al. [60] used BNi5 to braze material vacuum brazing C/C composites with the GH3044 high-temperature alloy, with a welding temperature of 1160 °C, vacuum < 10⁻² Pa, and a holding time of 30 min. They obtained joints with a shear strength of 38.5 MPa at room temperature. As shown in Figure 4B, Shen Yuanxun et al. [68] concluded that the Al₂O₃ interlayer can effectively prevent the diffusion and reaction of Ni and Ti and reduce the residual stresses in the joints. Guo et al. [69] have reported the brazing of C/C composites and the DD3 alloy using the AgCr reactive brazing material. The maximum value of the C/C-AgCr-DD3 joint shear strength reaches 27 MPa at 1040 °C by adopting the Ag-10 wt% Cr braze. The joint shear strength shows a trend of first increasing and then decreasing along with the Cr content in the braze and the brazing temperature. The joint fracture analysis indicates the failure along the C/C substrate, mainly attributed to significant residual stress. These mechanisms are deduced from the TEM results of these interfaces shown in Figure 4C. Junliang Xue et al. [70] used different nickel-based fillers to match the high-temperature properties of the C/C composites and the In738LC superalloy. Experimental findings showed that the maximum shear strength of 28 MPa was obtained when the joint was brazed at 1100 °C. The brazed joints fractured at the C/C composite side close to the isothermal solidification zone (ISZ), which was related to the residual stress concentration at the C/C composite interface.

4. Summary

This paper summarized the current status of brazing C/C composites with metallic materials, introduced the problems faced by brazing C/C composites with metallic materials, and sorted out the related research progress. The problem of the poor wettability of metallic materials on the surface of C/C composites has been largely solved, and the thermal stress problem can be alleviated by a suitable brazing system design. However, a series of practical problems, such as low joint strength and insufficient reliability, still limit the large-scale practical application of brazing technology for C/C composites and metal materials. For the surface and nanoscale interface bonding of C/C composites and metallic materials, it has been concluded that the microstructure and cracks play a key role in the welding process of the two types of materials, and how to accurately control the surface and nanoscale interface puts forward a new direction for our future research.

To obtain high-quality joints, it is crucial to control the reaction during the brazing process to ensure that high-strength joint tissue is obtained [71,72,73]. Possible solutions are proposed as follows:

(1)

Surface treatment of C/C composites, such as coating, sintering, perforating, depositing SiC, etc., can improve the wettability of the base and brazing materials, and form a transition layer at the interface of the joint to prevent uneven thermal expansion and cracking.

(2)

The addition of some reinforcing phases, such as active elements and CNTs, to the brazing material can promote the diffusion of C/C composites into the brazing seam, produce diffuse particles, reduce the mismatch between the matrix and brazing seam of C/C composites, and relieve the residual stress.

(3)

A transition layer can be sandwiched between the base material and the intermediate layer to alleviate the difference in the degree of thermal expansion between the base material and the brazed seam.

(4)

New brazing materials can be developed, with promising applications, such as Fe-based brazing materials and high-entropy alloy brazing materials.