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

: 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.


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/cm 3 ), 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 Crystals 2023, 13, 1444 2 of 13 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 hightemperature 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.

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].

Improving the Wettability
The behavior of a non-reactive liquid wetting a solid can be described by the classical Young's equation [4]: where σ sv , σ sl , and σ lv denote the surface tension between a solid and gas, solid and liquid, and gas and liquid.The angle of θ 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.
Table 1.Correspondence between the wetting angle and the wettability.
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 Crystals 2023, 13, 1444 3 of 13 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 Cr 3 C 2 coatings on and inside C/C preforms.The wetting angle of molten copper with C/C decreased from 140 • to 60 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].

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 : Crystals 2023, 13, 1444 In Equations ( 2) and ( 3), ∆α 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, σ 0.2 is the yield limit of the material, ω is the coefficient of work hardening of the material, and ∆α 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.

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.
In Equations ( 2) and ( 3), Δα 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, σ . is the yield limit of the material, ω is the coefficient of work hardening of the material, and Δα , 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 Tf and Rf.Moreover, the thickness of the connecting layer should be as small as possible under the premise of ensuring sufficient physical contact.

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.

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, Ti 3 Al alloy), stainless steel, Nb, W, TZM alloy, and so on (as shown in Figure 2).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).

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 [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.

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 [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, Crystals 2023, 13, 1444 6 of 13 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 x Cr 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.
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 GexCry 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.

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 clearly [47][48][49][50][51][52][53][54][55].M.

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 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 titaniumbased 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 x Cu y and Ti x Ni 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/Cu 51 Zr 14 /Ti 2 (Cu,Ni) + Ti(Cu,Ni) + TiCu + Cu 2 TiZr/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).
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/Ti 2 Cu/TiCu/Ti 3 Cu 4 /TiCu 4 /TiC + TiCu 2 + Ag(s.s) + Cu(s.s)/Ti 3 Cu 4 /TiCu 4 /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.

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 hightemperature alloys, which are shown in Table 4 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.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.

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 Febased brazing materials and high-entropy alloy brazing materials.

Figure 1 .
Figure 1.Strategies for improving the brazing performance of C/C composites with metals [35-37].Figure 1. Strategies for improving the brazing performance of C/C composites with metals [35-37].

Figure 1 .
Figure 1.Strategies for improving the brazing performance of C/C composites with metals [35-37].Figure 1. Strategies for improving the brazing performance of C/C composites with metals [35-37].

Figure 3 .
Figure 3. (A) Graphite sample holder for the preparation of the joints.Cross-section of C/C/Cu brazed at 1008 °C for 10 min [41].(B): (a) Macroscopic appearance of the brazed C/C composite/AgCuTi/Cu joints.There was no difference on the outside surface between straight and conical interface brazed joints.(b,c) OM views of straight and conical interface joints, respectively.Insets show that the interface for the straight interface joint is flat, while the conical interface shows a zigzag structure due to the conical design[42].(C) Experimental setup for the CFC/pure copper/Cu-CrZr one-step brazing process[46].

Figure 3 .
Figure 3. (A) Graphite sample holder for the preparation of the joints.Cross-section of C/C/Cu brazed at 1008 • C for 10 min [41].(B): (a) Macroscopic appearance of the brazed C/C composite/AgCuTi/Cu joints.There was no difference on the outside surface between straight and conical interface brazed joints.(b,c) OM views of straight and conical interface joints, respectively.Insets show that the interface for the straight interface joint is flat, while the conical interface shows a zigzag structure due to the conical design[42].(C) Experimental setup for the CFC/pure copper/CuCrZr one-step brazing process[46].

Figure 4 .
Figure 4. (A) SEM image of the C/C and GH3128 joint at 1500 °C [65].(B) SEM micrograph of the: (a) C/C-Al2O3 joint in mode 2, (b) C/C-braze interface, (c) braze/Al2O3 interface, and (d) Al2O3superalloy joint [68].(C) TEM results of the interface: (a) HADDF image, elemental mapping of (b) Cr and (c) C, (d) corresponding bright-field image, received SAED patterns in (e) region 1 and (f) region 2, and (g) HRTEM observation of region 3 [69].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 hightemperature 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

Figure 4 .
Figure 4. (A) SEM image of the C/C and GH3128 joint at 1500 • C [65].(B) SEM micrograph of the: (a) C/C-Al 2 O 3 joint in mode 2, (b) C/C-braze interface, (c) braze/Al 2 O 3 interface, and (d) Al 2 O 3 -superalloy joint [68].(C) TEM results of the interface: (a) HADDF image, elemental mapping of (b) Cr and (c) C, (d) corresponding bright-field image, received SAED patterns in (e) region 1 and (f) region 2, and (g) HRTEM observation of region 3 [69].Tian et al.[34]  brazed C/C composites and GH99 high-temperature alloys by adding TiH 2 to BNi 2 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 TiH 2 at 1%, 3%, and 8%, and it was found that the best joint performance was obtained with 3% TiH 2 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 BNi 2 brazing material.The authors found that Ti in TiH 2 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 TiH 2 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 Figure4B, Shen Yuanxun et al.[68] concluded that the Al 2 O 3 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 • , indicating a significant improvement in the wettability.Wenyan Zhou et al. [28] fabricated Mo 2 C 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 Mo 2 C layer that uniformly covered the inner surface of the C/C preforms was obtained.The formation of the Mo 2 C layer shows a transformation from the non-wettable interface of Cu/C into the wettable interfaces Mo 2 C/C and Cu/Mo 2 C, respectively.Good interface bonding was observed between Mo 2 C/C and Cu/Mo 2 C.(2) An appropriate amount of the active metal element is added to the brazing material

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

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

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

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