Latest research progress of SiCp/Al composite for electronic packaging

3.2.1 PM method

PM is a mature solid-state preparation process for composite materials, mainly for the preparation of discontinuously reinforced MMCs, which allows arbitrary adjustment of the volume fraction of the reinforcement phase (up to 70%), more precisely controlling the composition ratio, and adjustment of the particle size of the reinforcement phase to the nanometer range. The composite materials are obtained by mixing the matrix powders and the reinforcement phase powders by mechanical means such as ball milling, and then cold pressing and heating to the solid-liquid two-phase region. Powder mixing, cold pressing, and sintering are the basic steps of PM, and are also the main aspects that affect the microstructures and mechanical properties of the materials [31,32,33]. The sintering temperature is generally chosen to be lower than the solid-phase line of the matrix alloy, which can effectively mitigate the harmful interfacial reactions between the reinforcement and the matrix, and the composite materials have good mechanical properties [34]. Figure 2 shows the main flow diagram of PM [33].

Figure 2

Process flow chart of SiC/Al composites prepared by PM.

Fan et al. [35,36] successfully prepared SiCp/Al composites by PM. The macro-homogeneity of the mixed powders is easy to achieve, but the micro-homogeneity is poor. Moreover, there is magnesium enrichment in the micro-area, and point-like MgAl₂O₄ in the micro-area. The obtained materials are not compact enough. Erdemir et al. [37] prepared 30–60 vol% SiC/Al composites by PM. In addition to Al and SiC main phases, there are also Al₄C₃, CuAl₂, and CuMgAl₂ compound phases in the composites. The results show that these intermetallic compounds play an important role in improving the mechanical properties of the composites. Zhang et al. [38,39] also prepared 55–65 vol% SiCp/6061Al-based composite materials by PM under different temperatures and holding times, the interface reaction between SiC and Al intensified and produced brittle phase Al₄C₃ and MgAl₂O₄ and MgO interface products, and the nucleation and growth of Al₄C₃ were controlled by the sintering temperature and time.

3.2.2 MA method

MA is a technology developed from PM, different alloy powders are milled by high energy ball milling. The particles are repeatedly deformed by intense collision and extrusion between the powders and the grinding balls for a long time. The density increases sharply, and the grain and sub-structure are refined continuously. Then, the alloy powders are cold-pressed and sintered to obtain the required composite materials [40]. The main factors affecting MA are the types, morphology, and properties of powders, the power of ball milling, the ratio of grinding balls, the time of ball milling, etc. Besides the advantages of PM, it has the following characteristics: fine and homogeneous structure, excellent structure stability, and ability to produce metastable metal materials with unique properties [41]. Figure 3 is the main flow chart of MA. Cai et al. [42], Gou et al. [43], and Wu et al. [44] prepared SiC particle reinforced Al matrix composites by MA. The hardness, tensile strength, and yield strength of the composites increased with the increase in SiC content, while the elongation decreased. The composites exhibit mixed ductile fracture and brittle fracture.

Figure 3

Process flow chart of SiC/Al composites prepared by MA.

3.2.3 Squeeze casting method (SQCM)

SCM is considered to be one of the most effective methods in the preparation of SiCp/Al composites. The reinforcement phases are added into the high-speed stirring Al alloy liquid. After the SiC particles are wetted and dispersed into Al alloy liquid, the mixed slurry is poured into the preheated mold and pressed by squeeze casting. The infiltration and solidification process can be completed in a few minutes, and the process stability is good. But the volume fraction of the reinforcement phase in the prepared composites is limited and not suitable for the preparation of EPMs with high volume fraction SiCp/Al composites [45,46]. The schematic diagram of squeeze casting device is shown in Figure 4 [47]. Qu et al. [47], Cui et al. [48], and Zhang et al. [49] prepared SiCp/Al composites by SCM. It is found that the hot extrusion deformation can greatly improve the strength, modulus, and plasticity of the composites. The main reasons for the strength improvement of SiCp/Al composites by hot extrusion deformation are that alumina on the surface of original aluminum particles is dispersed from reticular to micro-granular, SiC particles are more uniform, the density of composites is increased, SiC-Al interface bonding is improved, dislocation density in matrix is increased, and SiC particles are oriented to a certain extent.

Figure 4

The schematic diagram of squeeze casting device for SiCp/Al composites.

3.2.4 Stir casting method (STCM)

STCM, also known as melting-casting method, is the technology of mixing particles reinforced with liquid or semi-solid alloy by mechanical stirring device, and then casting into ingot. It is one of the most commonly used methods to prepare particles-reinforced aluminum matrix composites, which can be divided into liquid phase method and semi-solid method. The liquid phase method is to use stir to make the liquid metal form a whirlpool. When the base metal is completely liquid phase, the particles are gradually put into the whirlpool, the particles are involved in the melt and dispersed, then poured into the metal mold to make composite materials [28,50,51,52]. Semi-solid casting is a method in which metal is near the melting point and the reinforcement phases are added to the metal liquid which contained a certain component of solid particles. Through stirring, the reinforcement phases collided with the metal alloy liquid phase and entered the metal melt [28,50,53]. The device schematic diagram of stirring casting method is shown in Figure 5 [50,54]. SiCp/Al composites are prepared by STCM, SiC particles are evenly distributed in the matrix alloy, but most of them are distributed along the grain boundary. Internally, the second phase of grain boundary is coarse and discontinuous. The structure of the composite is refined, the coarse phase of the grain boundary is fragmented to a certain extent. The homogeneity of SiC particles distribution in the matrix alloys and the strength and elongation of SiC particles are significantly improved [54,55].

Figure 5

The schematic diagram of stir casting device for SiCp/Al composites.

3.2.5 Centrifugal casting method (CCM)

CCM is one of the most favorable new materials preparation technologies for development of high-quality SiCp/Al composites, which use centrifugal force as external pressure. There are two main processes at present. One process is to prepare SiCp into porous preforms, then pour molten aluminum alloy into the high-speed rotating preforms, infiltrated into the holes of preforms by centrifugal force, and solidified to obtain SiCp/Al composites [32,50]. Another process is to add SiC particles into the melt of high-speed stirred aluminum alloy. After the SiC particles are fully wetted and evenly dispersed, the melt is poured into the metal model and formed by CCM [45,46]. At present, there is little research on the preparation of SiCp/Al composites by centrifugal casting at home and abroad, but this method may become one of the most effective technologies for the development of high-quality SiCp reinforced aluminum matrix composites. It is most suitable for the manufacture of wear-resistant high-speed rolls, guide wheels, and corrosion-resistant SiCp/Al composite pipes and sleeves. Centrifugal casting eliminated high-pressure equipment and reduces equipment investment and production costs, but its special device determines the shape and size of the product, so it is difficult to promote the use of composite materials in industry [45,50]. The schematic diagram of centrifugal casting device is shown in Figure 6 [50].

Figure 6

The schematic diagram of centrifugal casting device for SiCp/Al composites.

3.2.6 Spray deposition method (SDM)

SDM is a new solidification technology for the preparation of MMCs developed by Osprey Company in the 1980s. Aluminum alloys are melted in crucible, then dispersed into tiny droplets by high-speed gas after high-pressure atomization. SiC particles are sprayed into the liquid metal when it flows out. The two-phase mixed atomized liquid is cooled on the substrate, then the SiCp/Al composites are made. The liquid metal can be directly atomized and deposited into billets with a certain shape, uniform distribution of reinforcement phase particles, no serious interface reaction, rapid solidification of matrix structure, fine equiaxed crystal morphology, low requirement for wettability of interface, high yield, and easy preparation of large parts. Therefore, compared with casting and PM methods, this method has a higher cost performance ratio and attracts great attention of scholars [27,31,53]. The schematic diagram of spray deposition device is shown in Figure 7 [56]. Sansoucy et al. [57] prepared SiCp/Al-12 Si composites with 33 and 50% SiC volume fraction by SDM, the porosity of the composites is less than 1%, and the distribution of SiC in the aluminum matrix is uniform. The micro-hardness of the coatings is significantly improved, but the bonding strength between the coatings and the matrix decreased slightly with the increase in SiC. Therefore, it is also not suitable for the preparation of high-volume fraction SiCp/Al composites.

Figure 7

The schematic diagram of spray deposition device for SiCp/Al composites.

3.2.7 Pressure infiltration method (PIM)

PIM is used to make the reinforcement phases into preforms with corresponding shape, then putting them into metal pressing mold, pouring metal liquid, and infiltrating into the gaps of preforms under liquid pressure or gas pressure. The required MMCs can be obtained after solidification. This method can eliminate the interference of wettability, reactivity, and specific gravity difference between reinforcement phases and liquid metal [58]. MMCs prepared by PIM have the advantages of large adjustable range of reinforcement content, uniform distribution, and good performance. But the process equipment is extremely complex and the interface reaction is easy to occur due to improper process. The schematic diagram of PIM for preparing SiCp/Al composites is shown in Figure 8 [59]. Candan and Bilgic [60] prepared Al-Si/60 vol% SiC composites by PIM. The results showed that the porosity of the composites decreased with the increase in Si content in the matrix, and the addition of Si effectively inhibited the formation of Al₄C₃ in the composites, but when the content of Si exceeded 1%, the strength and wear resistance of the composites decreased. The corrosion behavior of the composites is affected by the addition of Mg in the matrix alloys. Maleki et al. [61] prepared 67–75 wt% SiC/6061Al composites by pressure-assisted infiltration, the composite containing 75 wt% SiC exhibited the highest compressive strength as well as the maximum hardness. Zhu et al. [62] also prepared the oxidized-SiC_3D/Al composites by vacuum-pressure infiltration technique, the results suggested an improvement in interfacial structure between Al and the oxidized SiC preforms due to the formation of a continuous interfacial reaction layer with a non-uniform thickness.

Figure 8

The schematic diagram of pressure infiltration device for SiCp/Al composites.

3.2.8 Pressureless infiltration method (PLIM)

PLIM is one of the most effective methods to produce MMCs, also known as Lanxide process, which is developed by Lanxide Company in the 1980s. SiC particles are bonded and sintered into preforms with binder. Aluminum alloy ingots are placed on the SiC preforms and the temperature is maintained to be above the melting point of the alloy. The Al alloy liquid is infiltrated into the SiC preforms by the action of self-weight pressure, surface pressure, and capillary force of the metal melt. Among them, the realization of infiltration process mainly needs two conditions: one is that Al alloy contains at least 1 wt% Mg, preferably 3 wt%; the other is that infiltration process needs to be carried out in nitrogen atmosphere. Compared with PIM, this method has the advantages of no pressure effect, easy selection of infiltration mold, simple equipment needed, easy operation, low production cost, large and complex components can be manufactured, and mass production can be easily realized [31,32,56]. The schematic diagram of the device for preparing SiCp/Al composites by pressureless infiltration is shown in Figure 9.

Figure 9

The schematic diagram of PLIM for SiCp/Al composites.

Liu et al. [63,64] prepared SiC preforms with different pore contents by oxidation bonding method, using micron SiC and graphite powder as raw materials, and then infiltrated ZL101 alloy solution into the preforms by PLIM. SiCp/Al composites with compact structure and uniform distribution of reinforcement phases are obtained. Cui et al. [65] fabricated 55 vol% SiCp/Al composites by PLIM, the interfacial product Al₄C₃ only existed near the infiltration interface between Al alloy and SiC, and the content decreased gradually along the infiltration direction, Al₄C₃ was hydrolyzed in a wet environment, and its macro performance was material pulverization.

3.2.9 Spark plasma sintering (SPS)

SPS is a new generation of sintering method developed in recent years, which combined the advantages of activated sintering and hot-pressing sintering. The sample is heated by high pulse current, which made the heating rate of the sample up to 600 K·min⁻¹. In addition, pressure can be exerted on the mold head during sintering process to realize low temperature and fast sintering. With the advantages of uniform heating, fast heating speed, low sintering temperature, short time, high production efficiency, fine and uniform product structure and controllable, can maintain the natural state of raw materials, high density, and good mechanical properties [27,66]. The schematic diagram of the device for preparing SiCp/Al composites by SPS is shown in Figure 10 [27,67]. Yin et al. [68] prepared SiCp/Al composites with 70% SiCp volume fraction by SPS. SiCp and Al powders are mixed by high energy ball milling according to the designed proportion. Then, SiCp and Al powders are loaded into graphite mold by SPS. The CTE is only 6.8 × 10⁻⁶ K⁻¹ and the TC is 195 W·(m·K)⁻¹. Kang et al. [69] investigated the effects of sintering pressure, sintering temperature and holding time on the mechanical properties of 50 vol% SiCp/2024Al composites by orthogonal experimental design prepared by SPS, the optimized process conditions were to sinter at 550°C for 5 min under 40 MPa, which resulted in a composite material with a density of 99.7% and good interface bonding with a comparatively high-bending strength of 766.65 MPa.

Figure 10

The schematic diagram of SPS device for SiCp/Al composites.

In addition to the above processes, some new composite material preparation processes have also been developed, such as 3D printing combined with pressureless infiltration or pressure infiltration, in-situ synthesis technique, and plasma spraying method. Unlike conventional composites in which SiC particles are diffusely distributed in the Al matrix phase, in the three-dimensional interpenetrating network structure SiC/Al composites, the SiC and aluminum alloy phases are continuous in three-dimensional space and interpenetrate to form a network structure. The prerequisite and key to preparing the material is the network structure SiC preform. 3D printing technology, due to its strong forming ability of complex structures and wide material applicability, is becoming an emerging hot spot in the preparation of composite material preforms and the development of new composite materials. Simultaneously, the three-dimensional interpenetrating network structure SiC/Al composite material can be obtained by pressureless infiltration of Al–Mg alloy under nitrogen atmosphere [70,71]. The in situ synthesis technique is to select Si powder as the Si source and phenolic resin as C source to strengthen the metal matrix by generating SiC reinforced phase in situ within the metal matrix through the chemical reaction between elements and compounds under certain conditions. The SiC/Al composite prepared by this process has the characteristics of good wettability and easy control of interface reaction, and also provides an important raw material for the preparation of high-performance MMCs by traditional casting process, which is a potential composite powder preparation process with high application value. However, the research of in situ synthesis technique SiC/Al composites is in the initial stage due to the limitation of traditional high-temperature synthesis of SiC [72]. The plasma spraying method uses a compressed electric arc as the heat source, and the plasma is excited to produce a high temperature plasma arc. The high temperature melts the matrix and then sprays onto the surface of the substrate as a high-speed jet after being compressed by the orifice at high pressure, cooled, and deposited to obtain the target material. The advantages are fast molding speed, short cycle time, and high bonding strength of coating materials, but the spraying equipment is expensive, and the sprayed composite materials have a large number of pores and residual stresses, which limits further applications and development [73].

As an EPM, in order to obtain the CTE matching the chip materials, the content of reinforcement is usually high and the selection of the preparation process is particularly important. There are various preparation processes for SiC/Al composites, each with its own advantages and disadvantages, the distribution of SiC particles in the composites and the interfacial bonding between SiC and Al are the main factors affecting their final properties of the composites. The CTE of SiC/Al composites decreases with the increase in the volume fraction of SiC particles in the reinforcement phase and gradually approaches the CTE of SiC itself. Therefore, when using this material as EPMs, the volume fraction of SiC particles is usually high in order to match the CTE of the composite material with the chip material to be encapsulated. Among the many preparation methods for composites, those suitable for EPMs focused on PM, MA, PIM, PLIM, SPS, and other new techniques.

3.3.1 Interfacial chemical reaction

In the preparation of SiCp/Al composites, due to the different Al alloys used and the different treatment processes of SiC particles, many possible interfacial chemical reactions may occur as shown in Table 2 [74,75].

3.3.2 Interface reaction process and model

There are three types of interfaces in the SiC/Al composites. The first is the Al/Al interface, due to the low melting point of Al powder, the sintering temperature of the composites is generally higher than the melting point of the Al powder, which basically turns into a liquid phase during the sintering process and then solidifies during the cooling process. The second type is the interface between SiC particles, the melting point of the SiC ceramic phase is above 2,700°C, and the sintering temperature of the composites is well below this temperature point, so the interface between the SiC particles does not basically change. The third type is the interface between Al and SiC, the reaction process at this type of interface is the most complex and directly affects the performance of the composites, and scientific researchers have concentrated on the interfacial reaction in the composites. In recent years, there has been a further understanding of the interfacial reaction process, and direct and reaction bonding mechanisms have been proposed for the interfacial bonding between SiC/Al.

Direct bonding mechanism: There are two different opinions about the direct bonding mechanism between SiC and Al. One is that there is an existing diffusion at the interface, the other is that there is no diffusion layer at the interface. High strength bonding is obtained because there is a certain orientation relationship between SiC and Al [75]. In the diffusion bonding mechanism, Romero and Arsenault [76] found that Al diffuses through the interface into the SiC in the composites, and the diffusion distance is much higher than the calculated value. This phenomenon is mainly produced by the dissolution of SiC in the molten Al, forming a jagged surface and subsequently the molten Al infiltrates into the SiC gaps, and no Al diffusion into SiC was found. Almost all the studies have concluded that diffusion bonding between SiC and Al leads to high matrix/reinforcement bond strength. Electron transfer occurs between the matrix and reinforced phase surfaces, forming chemical bonding between interfacial atoms. The strength of interfacial chemical bonding depends on the type of bond and the number of bonds per unit area, and the strength of SiC–Al interfacial bonding in SiC/Al composites was found to be 1.5 times stronger than the strength of Al atomic bonding in the Al matrix. In the orientation binding mechanism, majority of scholars believe that there are certain orientation relationships between SiC and Al to obtain high strength binding. Geng and Yao [77] suggested that five orientation relationships exist between SiC and 6061Al. About 70% of the Al/SiC interface belongs to the semi-coherent interface with orientation relationship, and it is found in the experiment that the interface between SiC and Al is well bonded, no reaction layer exists at the interface, neither C and Si diffuse into the matrix through the interface, nor does Al diffuse into SiC through the interface. From the perspective of metal crystallization theory, this crystal orientation relationship is formed because the interface between SiC and Al is a semi-coherent interface, and this tightly matched atomic bonding leads to a high bond strength at the composite interface.

Reaction bonding mechanism: The products of interfacial reactions have a significant influence on the macroscopic properties of SiCp/Al composites. There are three main types of interfaces, namely, products of interphase, elemental segregation, and precipitation phases, and direct atomic bonding of the reinforcement to the matrix. Slight interfacial reactions can effectively improve the infiltration and bonding of the alloy matrix with the reinforcement, which is beneficial to the performance of the composite. However, severe interfacial reactions can cause damage to the reinforcement and the formation of brittle phases, which is detrimental to the overall performance of the composite [78]. Kang et al. [69] prepared the 50 vol% SiCp/2024Al composites by SPS, the interfaces are shown in Figure 11, without the presence of the brittle phases MgAl₂O₄ and Al₄C₃ at the interface, some needle-like precipitates were observed in the Al matrix and good interfacial bonding was obtained between SiCp and the Al matrix.

Figure 11

TEM images of 50% SiCp/2024Al composites: (a) low magnification image and (b) high-resolution TEM image of the interface between the SiC and Al matrix.

When unprotected SiC is added to high-temperature Al solution, it is easy to react with Al solution. Most scholars believe that Al/SiC composites are easy to form a thin layer of Al₄C₃. From Figure 12, it can been seen that Al₄C₃ [79] can be formed when SiC dissolved in molten Al and reacts with Al liquid. However, some scholars believed that the reaction can occur more quickly when the temperature was over the isotherm (580°C) in practice. After the contact between SiC particles and Al alloy melt, SiC starts to dissolve at the SiC–Al interface. Because of the anisotropy of the interfacial energy, there is a preferred choice of dissolution, in order to reduce the overall interface energy of the composite, the dissolution of the SiC particle surface occurs preferentially at the high energy location, and the corresponding steps are formed on the particle surface. Therefore, only the low-energy and low-index interfaces of SiC particles remain for bonding with the Al matrix. It has been suggested that Al₄C₃ is distributed at the Al–SiC interface in a layered form, but in the actual study Al₄C₃ is present in the molten metal Al at the SiC–Al interface in the form of discontinuous needles, prisms, rods, and blocks, and the SiC particles are trapped in the Al₄C₃ grains as shown in Figures 13–15 [80,81,82].

Figure 12

Al-Si-C phase diagram in the experimental component area.

Figure 13

SEM micrograph of needle and block Al₄C₃ particles in the SiC/Al composites: (a) detailed morphologies of Al4C3 and (b) Magnified view of (a).

Figure 14

Microstructure of 10 wt% SiC/Al composite prepared by SPS: (a, b) matrix area, (c) Al₄C₃ carbide and Al matrix as well as (d) diffraction patterns and EDS spectrum from chemical composition analysis from Al₄C₃ area.

Figure 15

TEM micrograph of needle and block Al₄C₃ particles in the SiC/Al composites: (a) Lower magnification; (b) Higher magnification.

Compared to a single metal, the matrix alloys enhance the wettability of the composite and facilitate the full contact between SiC particles and molten Al. Therefore, in the preparation of SiCp/Al composite materials, Al matrix alloys gradually replaces pure Al. However, the presence of alloying elements is an important source of elemental segregation and precipitation phases at the interface of SiCp/Al composites. Zhang et al. [38] obtained the 55% SiCp/6061Al composite by PM method, the interface of the composite is the “tie” between the Al matrix and the SiC particles, and the state of the interfacial bonding directly affects the properties of the composite. As can be seen in Figure 16, with the increase in sintering temperature and holding time, the interface of the composite changes from a flat and smooth SiC–Al interface to a slight interfacial reaction to form MgAl₂O₄ and MgO, and finally A1₄C₃.

Figure 16

Interfacial reaction of 55% SiCp/Al composite under different sintering processes. (a) 630°C – 12 h, (b) 630°C – 24 h; (c) 650°C – 12 h, (d) 650°C – 24 h, (e) 670°C – 12 h, and (f) 670°C – 24 h.

3.3.3 Influencing factors of interfacial reaction

Although the dissolution reaction of SiC particles also existed in Al–Mg matrix composites, because of the existence of Mg in the alloy, the surface energy of the alloy solution was reduced, and spinel was easily formed on the surface of SiC particles. A large number of nano-sized MgO or MgAl₂O₄ particles are formed on the SiC/Al interface, it will lead to a significant increase in the content of Si and a strong interface bonding between the reinforcement phases and the matrix [83]. The reaction at the SiC–Al/Mg interface in the high-temperature state is complicated when the Al alloy matrix containing Mg elements is selected. It is generally believed that the reaction at the interface is preferred to generate nano MgO, and whether the reaction process continues depends on the content of Mg elements in the Al alloy and whether the generated nano MgO is dense at the interface [84]. When the Mg content in the Al alloy is insufficient, the generated nano MgO layer at the interface is not dense enough, SiO₂ on the surface of SiC particles will continue to react with the generated MgO and Al elements to form MgAl₂O₄ crystals, while the Mg content is sufficient to generate a dense MgO layer, which can better protect the inner SiC layer from continuing to react with the Al liquid, the generated MgO and MgAl₂O₄ at the interface are the products of high-temperature thermally stable interfacial reactions as shown in Figure 17 [85].

Figure 17

The TEM and HRTEM image of MgAl₂O₄ and Mg₂Si crystals on the SiC/Al composites interface.

At 900–1,000°C, when the molecule concentration of Si is less than 10%, Al₄C₃ is stable, and when the content of Si is more than 18%, SiC is stable [86]. For Al–Si matrix composites, the dissolution reaction of SiC will be inhibited due to the high content of Si, so there was no Al₄C₃ formation. During cooling, the Si element preferentially adsorbs at the low-index and low-energy crystalline surface of SiC particles and then starts nucleation and growth to form Si phase [87]. The nucleation and growth of Si element at the interface can control the occurrence of interface reaction at the interface and improve the mechanical properties of the composites, but at the same time, it will also produce a certain lattice distortion and increase the stress, thus reducing the mechanical properties of the composites. Meanwhile, although the dissolution reaction of SiC ions also exists in the Al–Mg matrix composites, the original Al–Mg binary system in the melt at the interfacial front has changed into Al–Mg–Si ternary system, so Mg₂Si is generated, and the phase first attaches to certain favorable crystalline surfaces on the surface of SiC ions instead of nucleating spontaneously to grow. Wang et al. [24] used mold-forming and PLIM to prepare the 3D-67 vol% SiC/Al-15Si-10Mg composites which is shown in Figure 18, the use of Al-15Si-10Mg alloy makes it possible to achieve full pressureless melt penetration into non-oxidized SiC powder and preform, thus avoiding the detrimental effect of the SiO₂ surface layer on the TC and mechanical strength of the composite. Brittle Al₄C₃ phase was not found in the composite produced.

Figure 18

TEM and SEM images of the 3D-SiC/Al-15Si-10Mg IPC: (a) mapping for Si, Al, Mg and C elements in the interface region; (b) SAED spectrum of Al, SiC and Si; (c) HRTEM images of the interface region; (d) and (f) before T6 treatment; (e) and (g) after T6 treatment.

Heating treatment of SiC particles before compounding can remove harmful adsorbed materials on the one hand, and form SiO₂ films on the surface of SiC particles that can improve the wettability of SiC/Al on the other hand. Thermodynamically, SiC is easy to be oxidized, and its initial oxidation temperature is 800–850°C. The relationship between the volume fraction and thickness of SiO₂ and the retention time of high-temperature oxidation treatment is parabolic. When heated above 900°C, the oxidation rate of SiC particles is accelerated and the SiO₂ surface film starts to thicken significantly. The dense continuous amorphous SiO₂ layer, which can effectively prevent the erosion of SiC by Al at high temperature, will have a significant impact on the interfacial chemical reaction, wettability, and interfacial structure. When the Mg content in the aluminum solution is high, MgO will be formed, whereas when the Mg content is low, MgAl₂O₄ will be formed preferentially. The formation of dense MgO or MgAl₂O₄ particles on the surface of SiC particles through interfacial reactions can effectively protect the formation of Al₄C₃ [88]. Zhu et al. [62] used vacuum-PIM to prepare the SiC/Al composite with heat treatment of SiC particles as shown in Figure 19. A continuous interfacial reaction layer with a non-uniform thickness is formed and the interfacial structure between the Al and oxidized SiC preforms is improved. The layer mainly consisted of the Al₂O₃ phase with non-oriented flakes and approximately equiaxed particles. However, the Al₂O₃ phase did not form a dense enough layer to provide complete protection from the effects of direct contact with Al and SiC.

Figure 19

TEM micrographs of Al₂O₃ lamellae in the SiC/Al composites: (a) precipitated in the Al phase; (b) precipitated in the interfacial reaction layer; (c) and (d) HRTEM micrographs of the area enclosed by the square in panels (a) and (b) for composite S6; (e) Al₂O₃ particles in composite S6; (f–i) EDS spectra of the points highlighted by stars in panel (e); (j) TEM micrographs of the interfacial structure; (k) EDS line-scan image of the region shown in panel (j) for composite S9.

The degree of chemical reaction between SiC and Al is obviously affected by temperature. The higher the temperature is, the more intense the reaction of 3 SiC + 4 Al → Al 4 C 3 + 3 Si will be [89]. The interfacial structure of composite slurry is obviously different with different cooling rates [90]. The cooling rate under different sintering processes has a certain influence on the interfacial reaction. During the rapid cooling process after PM sintering, the interface shows direct mechanical bonding between α(Al) phase and SiC, and the interface is smooth and straight without interfacial reaction products and intermediate transition phases. When stir casting is cooled with the furnace, the interface of the composite is basically a direct bonding between eutectic tissue and SiC particles.

Therefore, we usually consider adding an appropriate amount of alloy elements to the Al matrix, pre oxidizing the surface of SiC particles, and choosing a suitable sintering process to minimize the sintering temperature, which can effectively improve the interfacial wettability and avoid the occurrence of harmful interfacial reactions during the sintering process.

3.3.4 Effective ways to control interfacial reaction

The interfacial reaction can enhance the interfacial wettability between SiC particles and Al matrix. But in the preparation process, the excessive interfacial reaction made the interface fragile, reduced the properties of composites, and Al₄C₃ is a brittle phase, which can react with water vapor in the air [41,91]. In order to control harmful interfacial chemical reactions, a lot of studies have been carried out at home and abroad.

Matrix alloy: Added Si element or selected Si-containing alloy as the matrix material, such as A356, A357, A359, etc. The interfacial reaction of 3 SiC + 4 Al → Al 4 C 3 + 3 Si can be inhibited by Si in matrix alloys. Added active elements to the matrix (such as Li, Cu, Ti, Zr, P, etc.) can effectively reduce surface tension and improve wettability. Surface treatment of SiC particles: K₂ZrF₆, Ni, Cu, Ag, and Cr can be deposited using processes such as electroplating and chemical plating. pre oxidation treatment of SiC particles or coated with TiO₂ through the sol-gel method, laser modification is applied to the surface of SiC particles, and treatment of SiC particles with salts such as Na₂CO₃ [92], the surface of SiC particles is pretreated by other methods, such as PVD, CVD and electroplating [93]. Process selection: The degree of interfacial reaction mainly depended on the preparation method and process parameters. Control the process parameters, such as temperature, pressure, solidification, or cooling rate, can inhibit the dynamic conditions of harmful interface reaction [94].