Joining of Cf / SiC Ceramic Matrix Composites : A Review

Carbon fiber-reinforced silicon carbide (Cf/SiC) ceramic matrix composites have promising engineering applications in many fields, and they are usually geometrically complex in shape and always need to join with other materials to form a certain engineering part. Up to date, various joining technologies of Cf/SiC composites are reported, including the joining of Cf/SiCCf/SiC and Cf/SiC-metal. In this paper, a systematic review of the joining of Cf/SiC composites is conducted, and the aim of this paper is to provide some reference for researchers working on this field.


Introduction
With the rapid development of high-tech in aerospace and other industry fields, the demands for new materials, which can work in extreme harsh working environment of high temperatures, are growing.e needs for better efficiency and higher thrust-to-weight ratio promote the development of advanced materials at high temperatures, such as superalloys [1][2][3], ceramics [4][5][6], composites [7][8][9][10], and so on.Among these advanced materials, ceramic matrix composites (CMCs) are drawn great attentions for their engineering applications under extreme conditions because they can maintain low density, high strength, wear resistance, oxidation resistance, thermal shock resistance, corrosion resistance, and some other functions together [11].
Carbon fiber-reinforced silicon carbide (C f /SiC) ceramic matrix composites, one of the most famous CMCs, are becoming the most promising candidates for hightemperature structural applications (as illustrated in Figure 1), such as sharp leading edges, nose cones, aeronautic jet engines, thermal protection systems for reusable atmosphere reentry vehicles [12,13], as well as optical components [14] and nuclear fusion/fission reactors [15,16], owing to their relatively low density (∼2 g/cm 3 ), high thermal conductivity (∼67 W/(m•K)), high strength (300-800 MPa) [17][18][19], low coefficient of thermal expansion (CTE, 3.0-3.1 × 10 −6 •K −1 ), especially good stability and excellent oxidation and creep resistance at elevated temperatures [13,[21][22][23]25].In particular, C f /SiC composites have shown significant improvements in fracture toughness and thermal shock resistance.ese improvements in mechanical properties are dependent on the specific properties of the carbon fiber and the silicon carbide.According to the type of carbon fiber, it can be divided into 1D C f /SiC, 2D C f /SiC, 2.5D C f /SiC, and 3D C f /SiC and applied in different fields.
For aerospace applications, as reported by NASA, the X-37B and X-38 aircrafts employed a large number of C f /SiC composites in their nose cone [25], leading edge wing and engine components [11,26,27,29].For nuclear applications, C f /SiC composites are used as the cladding materials in pressurized water reactors and flow channel insert materials in thermonuclear fusion reactors [29,30,32].In most cases, typically, C f /SiC composite components are usually geometrically complex in shape and always need to join with other materials to form a certain engineering part.However, unfortunately, due to their poor machinability and toughness, C f /SiC composites lack good processing performance like metal material and thus cannot be processed into complex-shaped components by forging, extrusion molding, and other traditional methods.It is very difficult to produce large-size C f /SiC composite components with complex shapes, resulting in they must be joined with themselves or other materials by appropriate joining technologies [32][33][34]37].
Up to date, various joining technologies of C f /SiC composites are reported, including the joining of C f /SiC-C f /SiC and C f /SiC-metal.Table 1 lists commonly used joining technologies, such as direct bonding of C f /SiC-C f /SiC, indirect bonding of C f /SiC-C f /SiC, brazing of C f /SiCmetal, di usion bonding of C f /SiC-metal, online liquid inltration of C f /SiC-metal, ultrasonic-assisted joining of C f /SiC-metal, and electric-assisted joining of C f /SiC-metal.
To the best knowledge of the authors, however, there has been no systematic summary of the joining of C f/ SiC composites.erefore, we herein conduct a systematic review of the joining of C f /SiC composites, and the aim of this paper is to provide some reference for researchers working on this eld.

Self-Joining of C f /SiC Composites
In some conditions, in order to obtain large size and complex-shaped C f /SiC composite components, it is necessary that C f /SiC composites should be joined with C f /SiC composites themselves, named as "self-joining."ere have been many reports about the self-joining of C f /SiC composites in the last decades, usually including direct bonding and indirect bonding method.
2.1.Direct Bonding.Direct bonding method is a self-joining of C f /SiC composites by solid-phase di usion without any other materials (Figure 2(a)).As reported in previous papers, the main procedure of direct bonding usually includes three procedures: (1) plastic deformation, (2) di usion, and (3) creep.Plastic deformation occurs on the interface because of heat and pressure; di usion includes surface di usion, bulk di usion, grain boundary di usion, and interfacial di usion to achieve C f /SiC bonding.Creep refers to the permanent movement or deformation of metal.However, the bonding strength of the directly joined C f /SiC composite is usually very low because a strong bonding of C f /SiC composite is di cult to obtain without any other transition phases and because the di usion between C f /SiC composites is not easy owing to the strong covalent bond and the poor deformation ability of the SiC in the composites.Rizzo et al. [48] reported that a CVD-SiC coated C f /SiC composite was directly joined to its counterparts using spark plasma sintering (SPS) technology.e results showed that the cracks in the CVD-SiC coating were visible among the interface and propagated from the SiC coating through the joint area (as is shown in Figure 3), due to the CTE mismatch between SiC coating and C f /SiC substrate (as is shown in Table 2), and the apparent shear strength was as low as 5.6 MPa.erefore, direct bonding method is merely used owing to the low bonding strength.However, it is still very promising for direct bonding method of C f /SiC composites, especially for extreme applications where it demands to avoid a second material.

Indirect Bonding.
It is well known that it is very di cult to form di usion between C f /SiC composites owing to the strong covalent bond and the poor deformation ability of the SiC in C f /SiC composites, thus resulting in a weak bonding strength of direct bonding joint (Figure 2(b)).

Metal Fillers.
is method means that C f /SiC composites are bonded with C f /SiC composites using metal llers, such as pure metal or alloys.Table 3 lists some typical reports on the self-joining of C f /SiC composites using metal llers.Liu et al. [30] reported the C f /SiC composites joined by ternary Ag-35.25 wt% Cu-1.75 wt% Ti and demonstrated that the mechanical strength decreased with the increase in temperature owing to the softening of ller.
e exural strength decreased to 46% and 26% at 300 and 500 °C compared with that at room temperature, respectively.Stefano et al. [48] fabricated C f /SiC-Ti-C f /SiC sandwich by SPS and used pure Ti foils as ller.ey also found that a Ti-Si-C-based phase (Ti 3 SiC 2 , as is shown in Figure 4) was the main reaction product, usually induced to strength decrease.
High-temperature metal llers, such as Ni and its alloys, are reported and found to greatly improve the hightemperature resistance of the joint [61,62].Cheng [51,52,55] developed a novel joining process to join the 2D/3D C f /SiC composites.Porous C f /SiC composites were fabricated through chemical vapor in ltration (CVI) process, and Ni alloy was used to join the C f /SiC composites together.Figure 5 shows the diagram of this joining process.Because the Ni alloy had a favorable wettability with C f /SiC composites, melted Ni alloy easily in ltrated into the pores among C f /SiC composites.Hence, the contact surface between Ni alloy and C f /SiC composites matrix was greatly increased, thereby improved the bonding strength.Besides, Ni alloy had a higher melting point; hence, the joint was expected to be used at high temperatures (>1000 °C).
Table 4 lists some typical reports on the self-joining of C f /SiC composites with high-temperature llers (Ni alloy).As a nontraditional joining method, the self-joining process using Ni alloy is usually carried out during composite preparation procedure, and the damage is minimal.And after the joining process, an afterward CVD process is conducted, which not only densify the porous composites but also provides antioxidation coating for the matrix and the joint.
MAX phase ceramics are reported to exhibit not only high-temperature performance, thermal shock resistance,   Advances in Materials Science and Engineering and wear resistance but also a good plastic deformation capacity.Among various MAX phase ceramics, Ti 3 SiC 2 presents a suitable wettability and CTE toward C f /SiC composites matrix (as is shown in Table 2) and is thus believed to be a promising candidate for the self-joining of C f /SiC composites [16,55,56].Dong et al. [55] used Ti 3 SiC 2 as the nonmetal ller to join C f /SiC composite together through hot pressing.e shear strength of the joint was reported as high as 110.4 MPa (56.7% of the C f /SiC composite matrix).Chemical reactions took place at the interface between Ti 3 SiC 2 and C f /SiC, and residual thermal stress was investigated.e phase compositions of the fracture surfaces for the C f /SiC joints joined at various temperatures were also analyzed by XRD (as is shown in Figure 6).In addition, the fracture behavior of joining interface and brazing application was explored in previous articles [9,65,66,69].Interfacial reactions can a ect the formation of a joint from the onset of bonding through the development of equilibrated microstructure and to the optimization of the mechanical properties.It has been demonstrated that an adequate joining interface could lead to improvements of the composite wettability by C f /SiC [39].
Besides, ceramic precursors are also used as nonmetal llers for the self-joining of C f /SiC composites.e ceramic precursor is transformed into amorphous ceramic at a certain temperature, and the composition and structure of the precursor are similar to those of the composite matrix.At the same time, the pyrolysis products are directly bonded with the composite matrix by chemical bonds.e thermodynamic properties of the joining layer obtained by this method are similar to those of the matrix [64].And it has good compatibility with the composite matrix.erefore,    4 Advances in Materials Science and Engineering the joint exhibits good mechanical strength.Previous reports showed that C f /SiC composite was joined using Si-O-C ceramic precursor as filler [57].
Si resin is transformed into Si-O-C ceramic at low temperature; the Si-O-C ceramics infiltrating into the substrate improve the filler contact with the substrate closely and increase the connection area.Moreover, the Si-O-C ceramics infiltrating into the pits can form tiny "pins," thus increasing the shear strength of the joints.Gianchandani et al. [58] reported that a MoSi 2 /Si composite obtained in situ by reaction of silicon and molybdenum at 1450 °C in Ar flow is proposed as pressure-less joining material for C f /SiC composites.
To sum up, we can know that the application of nonmetal fillers method due to the phase consistency of joining material and matrix was similar, which not only avoid the CTE mismatch between the joining material and the matrix (CTE of typical materials is shown in Table 2) but also inhibit the adverse reactions of interface.It will be a very promising method in the future.

Joining of C f /SiC Composites to Metals
In order to obtain large size and complex-shaped components, the joining of C f /SiC composites to metals such as Ti [40], Nb [42,68,69], Ni [70], and TiAl alloys [30,46,71] is necessary.Due to the differences in physical, chemical and mechanical properties between C f /SiC composites and metals, there are several problems for the joining of C f /SiC composites to metals: firstly, the chemical bonds of C f /SiC composites are ionic bond and covalent bond and the valence state is stable, whereas metals mostly are metal bond and therefore it is difficult to wet the surface of C f /SiC composites by metal [40].Secondly, the CTE mismatch between metals and C f /SiC composites is very large, which will produce residual stress at the joint interface; hence, cracks, pores, and other defects exist after cooling [41,72,73].At last, a variety of chemical reactions occur in the interface, resulting in brittle compounds with high hardness, which usually is the reason for the brittle fracture of the joint during working [30].
At present, there are many technologies solving the above problems during the joining process.Brazing and diffusion bonding are the most commonly used methods.In addition, online liquid infiltration joining, ultrasonic-assisted joining, and electric-assisted field joining are also reported.

Brazing.
Brazing is one of the earliest and most commonly used methods for joining CMCs to metals   Advances in Materials Science and Engineering (Figure 7(a)).It is divided into two kinds as follows: (1) metallizing the C f /SiC composite surface and then brazing with ordinary brazing filler metals, usually known as indirect brazing, and (2) wetting CMCs surface directly using active metal, known as reactive brazing.Compared with indirect brazing, the scopes of application of reactive brazing are more extensive.Usually, metals and alloys with lower melting points are selected as the brazing fillers, and then the joint is heated to a certain temperature, which is higher than the melting point of brazing filler, and then brazing is conducted [74].

Low-Temperature Fillers.
Low-temperature filler is a kind of metal with low melting point, such as Ag and Cu, which can form brazing filler at lower temperature to realize the joining of metal.Due to the low joining temperature, the damage is low.Brazing method is simple and convenient; however, the brazing filler is mainly active metal elements, so it is necessary to protect the active metal elements from oxidation.Once the active element is oxidized, it is difficult to react with C f /SiC composites and to form a reliable joint; consequently, the joint strength is low.erefore, brazing method is generally carried out in vacuum conditions or inert protective gases [39,75].Feng et al. [72,76] investigated the microstructural evolution and joint strength of between TiAl alloys and C f /SiC composite via vacuum brazing using Ag-Cu and Ag-Cu-Ti fillers.e diffusion of Al and Ti from TiAl to the matrix had an important effect on the structure and strength of joints.When active element Ti diffused into C f /SiC composite, the formation of AlCu 2 Ti and Ag solid solution was detected with the dissolved Ti and Al; moreover, Ti 5 Si 3 phase and TiC also formed adjacent to the composite (as is shown in Figure 8).
e maximum shear strength achieved 85 MPa with the thickness of TiC layer of 4-5 μm. e fracture of the joint went through the TiC layer adjacent to its interface with the Ag solid solution and TiC bond layer.
As is shown in Table 6, Ag-Cu, Ag-Cu-Ti, and others are low-temperature fillers (900 °C) and have low yield strength and good deformation ability, which is helpful to alleviate the residual stress of the joint, thus increasing the shear strength of the joints.

3.1.2.
High-Temperature Fillers.Ag-Cu-Ti alloys have good plastic deformation behaviors (as is shown in Table 7); nevertheless, they always have low melting points and can only be used in low-temperature environments (<800 °C).Once the temperature increased, the strength of the joint drops sharply.erefore, it is necessary to develop suitable high-temperature brazing filler for high-temperature conditions.
Huang et al. [46] joined C f /SiC composite to TC 4 alloy using (Ti-Zr-Cu-Ni) and W powder as brazing fillers.Ti and Zr elements reacted with C, Cu, and Ni in the interlayer.As elements diffused to each other, a reaction layer was formed between the C f /SiC composite and TC 4 alloy.e brazing parameters had a significant effect on the interfacial reaction between C f /SiC composite and joining material, which affected the shear strength of the joints.A continuous reaction layer adjacent to C f /SiC composite and a diffusion layer near TC 4 alloy can be clearly observed (Figures 9 and 10).e addition of appropriate W powder helped to relieve residual stress and improved the strength of the joints.e shear strength of the joint was 166 MPa and 96 MPa at room temperature and 800 °C, respectively.erefore, the joint can be used under high temperature.
However, the effect of W powder on the residual stress was small and the residual stress was still high.Ti-Zr-Cu-Ni alloy and pure Ti metals were used as joining materials [30]; the molten Ti-Zr-Cu-Ni reacted with solid Ti in the liquidsolid reaction to form an in situ alloy.
e effects of Ti contents on the strength of joints were explored.With the increase in the Ti content, more tearing ridges appeared in the fracture surfaces, which indicated that the fracture possessed more plasticity.When the Ti content reached up to 40%, the shear strength of the joint reached up to 283 MPa, which was 79% higher than using Ti-Zr-Cu-Ni alone.e main reason was that the metal Ti had better plasticity, and the proper addition was beneficial for improving the interfacial reaction between C f /SiC composite and Ti-6Al-4V alloy.
ere are many research studies using brazing method for joining C f /SiC to metals as listed in Table 7. e lowexpansion material (W), the soft metal (Ni), and the hightemperature metal (Mo) as the reinforcing phase are added into the brazing filler, so that the CTE of the brazing filler is  Advances in Materials Science and Engineering reduced and the residual stress of the joint is facilitated.However, there are still some shortcomings for brazing process, such as the interface reaction is intense, to produce brittle compounds, which requires the appropriate adjustment of brazing filler and process parameters.More importantly, avoiding bad excessive interface reaction and accessing to excellent mechanical properties of joints are essential.Advances in Materials Science and Engineering metals are mismatch, and it is easy to induce high residual stress.Due to sharp structural transition near the interface and the lack of a buffer layer to relax the stress, the residual stress is high enough to lead to a lower joint strength.Simply, diffusion bonding method is a solid-state bonding process, which has been demonstrated as a viable method to overcome the problems encountered in welding.ere are many reports on the diffusion bonding C f /SiC composites to metals.In order to join 3D/2D C f /SiC composite to Nb alloy, Xiong et al. [41,42] used Ti-Cu foil as the joining material to join C f /SiC composite to Nb alloy through a two-stage joining process: solid-phase diffusion bonding and transient liquid-phase diffusion bonding.It was found that the Ti-Cu liquid eutectic alloy was formed by the reaction of Ti and Cu, not only infiltrated into open pores and microcracks as a nail but also reacted with ceramic coating.e remaining Cu was deformed by own plastic deformation and released the residual stress.In addition, the liquid layer formed by interlayer in the TLP-DB process had good wettability to C f /SiC composite and can infiltrate into C f /SiC composite matrix and encapsulated C f between the interlayer and C f /SiC interface region.ese processes were very beneficial for the mechanical strength of the joint.e shear strength of the joint between 2D C f /SiC composite and Nb alloy was 14.1 MPa, and the shear strength of the joint between 3D C f /SiC composite and Nb alloy reached up to 34.1 MPa.To our best knowledge, there were mainly two factors leading to a low shear strength of the joint between 2D C f /SiC composite and Nb alloy: the CTE mismatch between 2D C f /SiC composite and Nb alloy was larger compared with 3D C f /SiC composite and Nb, resulting in a large residual stress, and the fiber direction among 2D C f /SiC composite was parallel to the joining interface, whereas the fiber direction among 3D C f /SiC composite was perpendicular to the joining interface.When the fiber was perpendicular to the joining interface, "nail effect" formed between reaction layer and C f and shared more load than other regions in fracture test (as is shown in Figure 11).

Diffusion
ese results demonstrated that the direction of fiber was directly related to the interface structure of the joint, which In addition, the reactions between joining material and composite matrix have been recently recognized as critical factors for determining the strength of the joint.C f /SiC composite and Ti-6Al-4V alloy were joined by Ban et al. [79] with the mixed powder of Cu, Ti, and graphite under vacuum environment.In situ synthetic TiC that reduced the thermal stress significantly was synthesized by interdiffusing of C element in the graphite particle and Ti element in the liquid bonding layer (as is shown in Figure 12).e positive effect of TiC on joint strength was also described in other papers [56,72,80].Table 8 summarizes the data of diffusion bonded joining.e utility model has the advantages of high strength, stable joint quality, and good corrosion resistance, especially for the joining of C f /SiC composites and metals for high-temperature and corrosion-resistance application.

Online Liquid Infiltration
Joining.Online liquid infiltration joining is a novel technology, which is applied to the joining of fiber-reinforced ceramic matrix composites.C f /SiC composites are usually porous both for CVI and PIP processing.An online liquid infiltration joining method that is suitable for the composites was reported.e porosity of C f /SiC composites was controlled and then the compact process was carried out after the joining has finished, which reduced the damnification of joints as much as possible.e wettability between the joining material and C f /SiC composite was improved; moreover, the joining material could be melted and infiltrated into the C f /SiC matrix, which increased the joining area and reinforced the joint strength [81,82].In addition, a root-like morphology was formed in C f /SiC composite substrate, which could greatly enhance the reliability of joint [83].
e only paper that attempts to join C f /SiC composite to metal via online liquid infiltration joining was presented in 2004 [84].e authors joined 2D/3D C f /SiC composites to Nb with Ni-based filler by the online liquid infiltration joining method (as shown in Figure 13).e joint between 2D C f /SiC composite and Nb was failure and separated during the cooling.However, the favorable joint between 3D C f /SiC composite and Nb was obtained.Approaches such as reactive brazed [68,69] and diffusion bonding [41,42] have also been successfully used to join C f /SiC composites to Nb alloy.Unfortunately, the bonding processes above were usually conducted after the preparation of the composite matrix, which damaged the strength of the matrix.Online liquid infiltration joining, which is completed in the preparation process, is different from the above methods.Afterward, chemical vapor deposition (CVD) process not only complete the preparation of materials but also can provide antioxidation coating for the matrix and the joint, reflects the joining, preparation, and processing integration [51,85].
3.4.Ultrasonic-Assisted Joining.Ultrasonic-assisted joining is employed to join aluminum alloy structural parts at first.Afterward, ultrasonic is used for copper and alloy, gradually widely used in CMCs and metals, as shown in Figure 14 [86].Since ultrasound exists as an energy form, it produces some unique ultrasonic effects when it propagates in the medium.
e ultrasonic-assisted joining utilizes ultrasonic vibrations to interact the contact area of the CMCs with the metal.e ultrasonic effect causes the liquid joining material to spread on the surface of the matrix and form a joint with the metal [87].In 1990s, ultrasonicassisted joining technology facilitated the wetting of materials with poor wetting properties such as ceramics, glass, and stainless steel [88][89][90]94].
e liquid-connecting materials spread and moisten, through the ultrasonic wave effect that from the vibrations of ultrasonic, the surface of the CMCs and metal to achieve good connection.Moreover, it is worth mentioning that ultrasonic-assisted joining technology can improve the wettability of connection materials on the surface of matrixes such as ceramics, glass, and stainless steel.erefore, this technology has been widely applied in many fields.
e joining of SiC and Ti-6Al-4V alloy via ultrasonicassisted joining was conducted by Chen et al. [91,92].SiC Advances in Materials Science and Engineering was employed by a joining material with an Al-12Si alloy at low temperature (620 °C), and the shear strength of the joint was 84-94 MPa.In their study, the oxide layer of the matrix was broken by the ultrasonic, and the joining material can form a good interface between SiC and Ti-6Al-4V alloy.However, cracks were observed in SiC material and the propagation direction was parallel to joint.e main reason was that the nonuniform shrinkage of material at the joint and residual stress, which leads to crack formation in the SiC substrate, was produced during the cooling process.ey obtained an integrated joint when using the novel joining  10 Advances in Materials Science and Engineering material in their study.SiC and Ti-6Al-4V alloy were joined with AlSnSiZnMg mixed metal, which reduced the joining temperature and the residual stress of the joint, inhibiting the occurrence of cracks and other defects.Unfortunately, the shear strength of the joint was not improved (77.8 MPa).
On the other hand, ultrasonic-assisted joining technology can also be used to join oxide ceramics to metals.Naka et al. [88] joined Al 2 O 3 to Cu with Zn, Zn-5Al, and Zn 100−x (Al 0.6 + Cu 0.4 ) x (x � 0-30) as the joining materials.It was shown that with the time and joining temperature increased, the shear strength of the joint with Zn-6A1-4Cu filler was improved and reached ∼62 MPa.In the above literatures, some of them were reported that the ultrasound was beneficial to improve the wettability of Al 2 O 3 and metals.
e mechanism of ultrasonic effect on the joining process can be summarized as follows: (1) the macroscopic bubbles between the filled metal and the ceramic were removed by the ultrasonic cavitation; (2) the C f /SiC substrate surface was subjected to high-speed impact of atoms under ultrasonic vibration; (3) the ultrasonic vibration and friction between the joining material and metal.

Electric-Assisted Field Joining.
Although diffusion bonding is widely used to join CMCs and metals, generally, it requires high temperatures, high pressure, vacuum or inert atmosphere, and long joining time [93,94,98].Electricassisted field joining is an effective way to solve these above problems, as shown in Figure 15.Since the joining between CMCs and metal was realized by chemical reaction, interfacial structure formed by reaction determines the mechanical properties of the joint.Better joint can be obtained using the electric-assisted field method.
e interface between CMCs and metal were polarized under electrostatic field.On the one hand, it promotes atomic migration and vacancy diffusion.On the other hand, it accelerates the interface reaction, which reduces the joining temperature, the pressure, and the residual stress.Moreover, the interface reaction is easy to control, and joining time is very short [96][97][98]101].
Initially, the electric-assisted field joining is mainly employed for joining ceramics to metals [100].e interface composition and mechanical properties of joints between SiC and Ti were investigated by Wang et al. [98] in the electric field.It was shown that the external electric field reduced the joining temperature and time and improved the shear strength.It is important that the external electric field can improve the diffusion rate of interface atoms.Moreover, it promoted the interface reaction and improved the joining efficiency.
Owing to its simplicity and efficiency, electric-assisted field joining became a useful method employed for joining C f /C composites [101] and C f /SiC composites [48].C f /C composites were firstly joined by combining electric fieldassisted sintering technology and using a Ti 3 SiC 2 tape film as the interlayer [101].In their work, the interdiffusion speed between the interlayer and the metal was accelerated by an electric field and the joining time was only 12 min.To our knowledge, the Ti 3 SiC 2 exhibited pseudoplastic at 1300 °C or higher [102,103].erefore, Ti 3 SiC 2 infiltrated into the composite matrix and a "nail" that clamps the matrix was observed, as shown in Figure 16, which improved shear strength of the joint.In the joining process, two key factors affected the strength of the joint: (1) the interdiffusion between the joining material and the matrix was promoted by electric field and (2) Ti 3 SiC 2 showed good plastic deformation ability in the electric field.
Atoms spread to the interface under electric field.It is necessary to pass through the potential through the gap, the original position occupied by their own formed a new space.
e energy of the atoms across the barrier was provided by the electric field.At the same time, the atoms across the barrier potential energy are reduced by the electric field.Combined with these two effects, the diffusion activation of atoms can be greatly reduced, thereby increasing the diffusion rate of solute atoms and obtaining a uniform structure [104].erefore, the electric-assisted fields joining method has drawn great attention and is expected to become an important way for the joining of C f /SiC composites in the future.

Summary
With the rapid development of high-tech in aerospace and other industry fields, carbon fiber-reinforced silicon carbide (C f /SiC) ceramic matrix composites, one of the most famous CMCs, are becoming the most promising candidates for high-temperature structural applications.In most cases, typically, it is very difficult to produce large-size C f /SiC composite components with complex shapes, resulting in that they must be joined with themselves or other materials by appropriate joining technologies.At present, various joining technologies of C f /SiC composites are reported, including the joining of C f /SiC-C f /SiC and C f /SiC-metal, such as direct bonding of C f /SiC-C f /SiC, indirect bonding of C f /SiC-C f /SiC, brazing of C f /SiC-metal, diffusion bonding of C f /SiC-metal, online liquid infiltration of C f /SiC-metal, ultrasonic-assisted joining of C f /SiC-metal, and electricassisted joining of C f /SiC-metal.

Advances in Materials Science and Engineering
To the best knowledge of the authors, however, there has been no systematic summary of the joining of C f/ SiC composites.In this paper, a systematic review of the joining of C f /SiC composites is conducted, and the aim of this paper is to provide some reference for researchers working on this field.pp.827-834, 1999.
[18] K. Jian, Z. H. Chen, Q. S. Ma, and W. W. Zheng, "Effects of pyrolysis processes on the microstructures and mechanical properties of C f /SiC composites using polycarbosilane," 12 Advances in Materials Science and Engineering

Figure 1 :
Figure 1: Various engineering applications of C f /SiC composites.

Figure 2 :
Figure 2: e diagram of (a) direct bonding and (b) indirect bonding.

Figure 3 :
Figure 3: Scanning electron microscopy of polished cross sections of C f /SiC specimens joined by SPS: direct bonding [48].

Figure 4 :
Figure 4: (a) Backscattered electron images of polished cross sections and (b) micro-XRD on the fracture surfaces of C f /SiC joined by SPS with Ti foil [48].

Figure 5 :
Figure 5: e diagram of self-joining using Ni alloy.

Figure 6 :
Figure 6: (a) e backscattered electron images and (b) XRD patterns of the fracture surfaces of the C f /SiC joints [55].

Figure 7 :
Figure 7: e schematic diagram of (a) brazing method and (b) diffusion bonding method.

Figure 8 :
Figure 8: (a) Microstructures and (b) XRD patterns of the joint from C/SiC composites to TiAl brazed at 900 °C for 10 min [72].

Figure 9 :Figure 10 :
Figure 9: BSE images of the joint: (a) micrograph of the joint; (b) interface between C f /SiC composite and interlayer [46].
Figure 15: e diagram of the experimental equipment under electric field.

Table 1 :
Commonly used joining methods of C f /SiC composites.

Table 3 :
Self-joining of C f /SiC composites using metal ller (SS shear strength).

Table 5 :
Self-joining of C f /SiC composites using inorganic filler (BS � bend strength).

Table 4 :
Self-joining of C f /SiC composites with high-temperature fillers.

Table 6 :
Brazing of C f /SiC composites to metals with low-temperature fillers.

Table 7 :
Brazing of C f /SiC composites to metals with high-temperature fillers.

Table 8 :
Diffusion bonding of C f /SiC composites to metals.