Microstructure, Mechanical Properties, and Reinforcement Mechanism of Second-Phase Reinforced TiC-Based Composites: A Review

: TiC ceramics have the characteristics of high melting point and density, and titanium reserves on earth are extremely large; therefore, TiC ceramics are considered ultra-high temperature materials with great research value. However, the development of TiC-based ultra-high temperature composites has been seriously hindered by their poor mechanical properties. At present, improvement of the mechanical properties of TiC is mainly accomplished by adding a second phase. In this paper, the research status of modiﬁed elements-, nitrides-, and metal-reinforced TiC matrix composites is presented. The microstructure, phase composition, and toughening mechanism of TiC matrix composites reinforced by a second phase are described. The inﬂuence of the reaction products on the matrix during the toughening process is also discussed.


Introduction
Carbide ceramics are considered the most promising thermal structural components in the aerospace field because of their excellent properties. Ti reserves in the earth crust are large, and TiC has significant advantages in production preparation and performance. TiC ceramics have an ultra-high melting point (3140 • C) [1] and low density (4.94 g/cm 3 ), as shown in Figure 1 [2], and other advantages, as shown in Table 1. TiC ceramics have been extensively used in high-strength machining as wear-resistant materials [3]. TiC ceramics are also used as mechanical seals and aeroengine bearings [4]. Moreover, TiC ceramics have been selected as inert matrix fuel (IMF) of the fourth-generation nuclear reactor, the gas-cooled fast reactor (GFR), because TiC has a small neutron absorption area and a high radiation resistance. TiC ceramics have low a self-diffusion coefficient (6.98 cm 2 /s) and contain strong covalent bonds, which decrease the toughness of pure TiC ceramics. Simply increasing the sintering temperature to solve the above problems will lead to excessive grain growth and reduce the mechanical properties of the material [5][6][7][8][9]. In order to overcome these limitations, metals, nitrides, and silicides are often used as reinforcements or dopants for titanium carbide ceramics. Babapoor et al. [10] studied the densification behavior and mechanical properties of monolithic TiC samples by the SPS method; the highest relative density (99.4%) and hardness (25.7 GPa) of TiC samples were obtained when the experimental temperature was 1900 • C. Namini et al. [11] reported the effect of nano WC content on the mechanical properties of TiC ceramics, at an experimental sintering temperature of 1900 • C, a pressure of 40 MPa, and a sintering time of 7 min. TiC-3 wt.% WC ceramics achieved the best experimental results, as the relative density of the composites reached 99%. Although there is no chemical interaction between WC and TiC, WC is high soluble in TiC, (Ti, W) C solid solutions, and the bending strength (620 MPa) and  The preparation technologies of TiC-based ceramics mainly include the infiltration method, the self-propagating high temperature synthesis method, and the powder metallurgy method [13]. The infiltration method is a preparation process in which the melt spontaneously enters the granular porous preform with the help of infiltration without an external force. This method has the advantages of simple technology, low cost, no complex mechanical alloying process, and allows a large-scale production of products [14]. However, the surface of metal Ti can be easily oxidized to TiO 2 in the powder state, which hinders the contact between Ti and C. A series of complex interfacial reactions will occur during the penetration of additives and impermeability inhibitors, forming crack sources or stress concentration sources and reducing the strength and toughness of the materials. The properties of TiC composites are affected by sintering methods, sintering temperature, and sintering additives. Spark plasma sintering (SPS) uses plasma generated by a pulse current to heat a sample, and the sample is submitted to high pressure in the pressurized mold. The SPS method has the advantages of fast heating speed, uniform heating, and high sintering efficiency. The samples prepared by the SPS process can achieve a fine and uniform microstructure and a high relative density. Self-propagating high temperature synthesis (SHS) is a process of synthesizing materials by using the self-heating and self-conduction of reactants. When the mixed powder of Ti and C is ignited, it will automatically spread to the unreacted zone until the reaction is complete. The combustion in SHS synthesis is characterized by a high reaction temperature and is rapid, but the temperature gradient in the synthesis process is large, and a defect concentration phase and a metastable phase are very likely to appear in the product [15]. Therefore, the SHS pressure method, SHS sintering method, SHS dynamic pressure method, and SHS extrusion method are often used to improve the compactness of TiC-based ceramics. The basic processes of powder metallurgy include mixing the original powder Ti and C, pressing the green blocks, sintering the green blocks, and subsequently treating the products; the powder metallurgy can ensure the correctness and uniformity of material composition, and the final size of the blank does not need subsequent machining, which can greatly reduce the production cost [16].
TiC is a kind of ceramic material with strong covalent bonds and high melting point and can be densified only when sintered at more than 2000 • C by the traditional sintering method, but a longer sintering time will increase its production cost. A single TiC material will also limit its development in engineering application because of its intrinsic brittleness. In this study, several typical research examples of TiC matrix composites were analyzed, and the effects of additives, experimental temperature, and microstructure on the mechanical properties of TiC matrix cermets are summarized. The purpose of this work is to provide some valuable information for researchers in this field and relevant references for further research on TiC-based cermets.

Application of Titanium Carbide and Its Composites
TiC ceramics have excellent physical and chemical properties [17], which have broad application prospects in energy, aerospace, machining, and other fields [18]. The application of TiC in rocket engine tail nozzles is shown in Figure 2a. Modern aeroengines are vulnerable to a high ablation environment, as ablation will lead to the expansion of the nozzle throat area and the decrease of pressure during operation. Moshfegh et al. [19] evaluated the applicability of TiC in an aeroengine tail nozzle. The results showed that the thermal slip between micron solid particles and the gaseous carrier phase was reduced, the transfer of momentum and energy between the two phases was reduced, and the performance of the nozzle was improved. Kim et al. [20] prepared a C/C-HfC/TiC composite nozzle by using the VPS technology. The linear ablation rate of the composite throat of C/C-HfC/TiC was lower than that of the composite throat of C/C. This showed that TiC ceramics have good ablation resistance, which is of great significance for the service life and stability of aeroengines. TiC ceramics have high hardness, high strength, and low density, which can effectively reduce the velocity of projectiles and consume their kinetic energy. Therefore, TiC ceramics have also been used as composite ceramic armor in recent years, as shown in Figure 2b. Ceradyne and Cercom in the U.S.A. used TiC/TiB 2 ceramic sheets as panels and applied them to M21FV modified combat vehicles with FRP bodies. The bulletproof ability of the ceramic panels was better than that of Al 2 O 3 ; moreover, the addition of TiC improved the anti-riot ability and reduced the collapse of the armor [21]. TiC ceramics have also been often used in the civil field. TiC-HMS composites were successfully prepared by the Hunan Institute of metallurgical materials using a new preparation technology that evenly distributes TiC ceramic particles in a high-manganese steel matrix. The toughness of TiC-HMS was much higher than that of WC-Co cemented carbide, and its service life was 5-10 times that of traditional industrial high-manganese steel. This new composite material can also be used to obtain large-size one-time molded parts, which have been applied in engineering fields such as oil drill bits, as shown in Figure 2c. TiC ceramics have good corrosion resistance, low density, high elastic modulus, and are less affected by thermal expansion and cold shrinkage; therefore, TiC is widely used in the field of cutting tools and bearings ( Figure 2c). TiC-Ni-based ceramics can be traced back to 1929. They were originally used as a substitute material for the WC-Co alloy, mainly in the field of cutting. The hardness of TiC-WC ceramic material prepared by Cheng et al. [22] through SPS was 28.2 GPa, and the fracture toughness was 6.3 MPa·m 1/2 . This material is more suitable for preparing cutting tools. TiC-TiB 2 composites prepared by Wang et al. [23] through vacuum hot-pressing sintering technology have extremely high flexural strength and fracture toughness, reaching 921.2 MPa and 7.18 MPa·m 1/2 , respectively; they can meet the requirements of high temperature resistance and corrosion resistance. Due to the oil-free self-lubricating characteristics of ceramic materials, TiC ceramic bearings can also be used in the field of vacuum bearings. TiC ceramics also play an important role in the medical field, because TiC has excellent stability and wear resistance. TiC is also considered a candidate material in the field of total hip arthroplasty [24]. According to research, the friction coefficient of TiC ceramics is 37.3% lower than that of traditional 45 steel, and the wear amount after 40 H is only 0.76% of that of 45 steel; moreover, the wear rate of TiC ceramics is only 61.5% of that of cobalt chromium molybdenum alloy bone. Therefore, it was proved that TiC ceramic joints re superior to the metal alloy joints currently used.

Effects of the Added Phase on Microstructure and Mechanical Properties
TiC has excellent physical and chemical properties, so it has also attracted the attention of many scholars. However, brittleness limits its application in practical engineering. The introduction of a second phase to improve the properties of TiC has attracted research interest. This paper mainly introduces the effects of modified elements, nitrides, and metals on TiC matrix composites, as shown in Table 2. A solid solution forms easily in the process of nitride sintering with TiC, and the toughness of the matrix is greatly improved in the process of solid solution strengthening. Liquid phase strengthening is activated in the sintering process of metal and matrix, and the pores of the matrix can be effectively filled by the liquid phase, thus improving the relative density of the composites. The metal elements and metal oxides in the matrix can be effectively eliminated by the modified elements Si and C, increasing the contact between Ti and C, strengthening the sintering process, and maintaining the grain size of the matrix at a low level through dispersion strengthening.

Effect of Si on TiC-Based Composites
Ti will remain in the body during the preparation of TiC [25,26]. Xue et al. [27] added the Si element to the matrix and prepared TiC-Ti 3 SiC 2 composites by hot-pressure sintering at 1500 • C. The reactions between these substances are as follows: The schematic of the sintering process and microstructure evolution is shown in Figure 3. When the experimental temperature reaches 1300 • C, Ti-Si eutectic appears in the reactant [28,29]. This plays a positive role in the rearrangement and densification of the matrix. The Ti-Si liquid phase reacts with TiC in the matrix to form Ti 3 SiC 2 , according to Zener pinning theory, and the growth of TiC grains is limited by Ti 3 SiC 2 , which promotes the refinement of the TiC grains.  Figure 4 shows the mechanical properties of TiC-based ceramics with different Ti 3 SiC 2 content. It can be seen that the flexural strength of TiC-Ti 3 SiC 2 ceramics increases first and then decreases with the increase of Ti 3 SiC 2 content; the maximum flexural strength (1003 MPa) is observed at the TiC-Ti 3 SiC 2 content of 20 vol.%. The improvement of the flexural strength was attributed not only to the finer grain size, but also to the mismatch of thermal expansion coefficient between TiC and TiC-Ti 3 SiC 2 (7.4 × 10 −6 / • C and 9.7 × 10 −6 / • C) [30]. With the increase of Si content, the amount of Ti 3 SiC 2 also increased, and the flexural strength showed a downward trend. This could be due to the high-volume fraction of Ti 3 SiC 2 with large grain size and the self-contact between Ti 3 SiC 2 grains. Si was oxidized at high temperature, and SiO 2 was coated on the surface of other grains, which reduced the adhesion of the grains [31]. This was also a reason for the decrease of flexural strength. The fracture toughness increased with the increase of Ti 3 SiC 2 content, which could be predicted in advance, because more Ti 3 SiC 2 particles in the matrix hindered crack propagation or caused crack deflection [32]. The existence of Ti 3 SiC 2 decreased the Young's modulus of the matrix, which could be due to the fact that Ti 3 SiC 2 itself is a brittle phase and cannot improve the Young's modulus of the matrix. Tang et al. [33] also reached a similar conclusion.

Effect of the C Elements on TiC-Based Composites
The strength of ceramic composites mainly depends on the chemical composition and microstructure of the materials. It has been reported that it is difficult to obtain TiC materials at low sintering temperature without proper additives [34]. Ti has high reactivity [35], and the initial TiC powder was covered with a titanium dioxide layer. The direct contact between pure TiC particles was hindered by the presence of this oxide, so the densification of the material was prevented [36,37]. The EDS diagram of a TiC sample is shown in Figure 5a-d. It can be seen that all elements (Ti, C, O) were evenly distributed and contact with each other, which contribute to the formation of TiO 2 . Nguyen et al. [38] added 5 wt.% graphite powder to a TiC matrix; composites with relative density close to 100% were prepared at a lower temperature (1900 • C), and the relative density of the TiC composites without additives was only 95%. This was due to the reaction between carbon black and TiO 2 on the surface of TiC particles. TiO 2 was eliminated, and the sintering and densification processes were carried out smoothly. The cross-section diagrams of TiC and TiC-5 wt.% carbon black samples are shown in Figure 5e-g; the results of residual porosity and relative density were consistent. The addition of carbon black exerted a constructive effect on the sintering of TiC. Most of the pores were filled with fine carbon black, grain growth was significantly inhibited by 5 wt.% carbon black, and a fine structure was prepared; the mechanism is shown in Figure 6. The grain size of TiC materials without carbon black as mainly 15-20 µm. The grain size of the TiC composites with carbon black was mainly 10 µm, as shown in Figure 5h. The Vickers hardness of the TiC-5 wt.% C composites decreased to 2071 (HV 0.1 kg ) [39], because C itself is a relatively soft phase. The addition of carbon black had a negative effect on the improvement of the composites' hardness. The flexural strength of the TiC-5 wt.% C material was increased from 550 to 660 MPa; the relative density and grain size were the main factors improving the flexural strength of TiC. This conclusion was also confirmed in the study of Fattahi et al. [40].

Effect of Nitride on TiC-Based Composites
Nguyen et al. [44] added 5 wt.% ZrN to a TiC matrix and prepared new composites at 1900 • C by the SPS method. The relative density of the TiC-5 wt.% ZrN composites decreased by about 2% compared with that without TiC. Firstly, ZrO 2 present in the purchased raw materials and the oxide layer on the starting material appeared harmful to the sintering performance [45]. In addition, complex chemical reactions took place in the matrix during sintering, and gas was produced during the reaction, which might be intercepted in the matrix at high temperature [46]. These were the reasons for the reduction of the sintering density ( Figure 7a shows the residual porosity). The hardness and flexural strength of the TiC-5 wt.% ZrN composites decreased by 15.6% and 11.9%, respectively, as shown in Figure 8. This was because the addition of ZrN made the matrix produce more free carbon, and the hardness of the carbon phase was low [47]. Therefore, the existence of a carbon phase had a negative impact on Vickers hardness. The decrease of flexural strength was due to the fact that the consolidation stage was not completely carried out in the ZrNreinforced specimens, extensive carbon-rich regions formed between the TiC particles, and crack propagation occurred in the interface region. The addition of reinforcement changed the fracture mode from transgranular to intergranular. As shown in Figure 7b, the preferred location for crack propagation was the grain boundary. Therefore, the flexural strength was negatively affected by this phenomenon. On this basis, the TiC-5 wt.% ZrN-5 wt.% C composites were prepared by Nguyen et al. [48]. With the introduction of graphite, the relative density of the composites increased significantly, consistent with the principle of TiC-5 wt.% graphite reinforcement mentioned above [49,50]. The introduction of graphite compensated for the harmful effects of ZrN to a great extent. However, the addition of a softer graphite phase still did not improve the hardness of the composites, but the flexural strength of the TiC-5 wt.% ZrN-5 wt.% C composite reached an amazing 741 MPa, because the ZrN and graphite additives had a good synergistic effect on the flexural strength of TiC. A (Ti, Zr)(C, N) solid solution formed in the TiC-5 wt.% ZrN-5 wt.% C composites, as shown in Figure 7c. The formation of a solid solution made it difficult for dislocations to move [51,52], and this contributed to the improvement of the flexural strength of TiC-5 wt.% ZrN-5 wt.% C compared with TiC-5 wt.% ZrN.  The strength of composites is affected by the properties of the second phase itself. Pazhouhanfa et al. [53] added a harder material, Si 3 N 4 , obtaining TiC-5 wt.% Si 3 N 4 composites using the same method (SPS, 1900 • C), but the mechanical properties of the matrix were not positively affected by the Si 3 N 4 phase, and the flexural strength was even 43.4% of that of pure TiC. This phenomenon was caused by the complex reaction between Si 3 N 4 and the TiC matrix during hot pressing, as gases were produced in these reactions, as shown in Figure 9a; Equations (1)-(3) describe this process. The TiC-5 wt.% Si 3 N 4 composites showed high porosity. At the same time, the free C element was consumed by Si 3 N 4 [54], and the residual TiO 2 in the microstructure promoted crack propagation along the grain boundary [55]. A large number of pores were also found in the TiC-5 wt.% BN composites prepared by Shaddel et al. [56]. The gas produced by the reaction was also the dominant factor leading to the increase in porosity, as shown in Figure 9b. The reactions supporting this process were mainly (4)-(7). 6TiC + 2Si 3 N 4 = 6SiC + 6TiN + N 2 (g) (1) TiN + 2C = 2TiC + N 2 (g) (2) Si 3 N 4 + 3C = 3SiC + 2N 2 (g) 2TiC + 3BN = 2TiB + 2C + N 2 (g) (4) 2TiC + 3BN + TiO 2 = 3TiB + 2CO (g) + 1.5N 2 (g) (5) 2TiC + 6BN + TiO 2 = 3TiB 2 + 2CO (g) + 3N 2 (g) TiC + 2BN = TiB 2 + C + N 2 (g) (7) Pazhouhanfar et al. [57] prepared TiC-5 wt.% TiN composites at 1900 • C by the SPS technology. It was reported that a Ti (C 1-x N x ) continuous solid solution can be prepared because TiN and TiC have a similar structure [58]. The production of a Ti (C, N) solid solution made the matrix produced more free C, and the relative density of the matrix was positively affected; consequently, the relative density of the TiC-5 wt.% TiN composites reached 97.1%. The hardness of the Ti (C, N) solid solution was about 60%-80% of the theoretical hardness of a single crystal [59]. Therefore, the formation of a solid solution had a certain destructive effect on the hardness of the matrix [60]. As presented in Figure 7e, the grain size of the TiC-5 wt.% TiN composites was small, which indicated that the addition of TiN hinders excessive grain growth. TiC-5 wt.% AlN composites were prepared by Fattahi et al. [61] using the SPS technology at 1900 • C. This material presented a high relative density. Meanwhile, the flexural strength (688 MPa) of the TiC-5.wt.% AlN composites was greatly improved. Cao et al. [62] pointed out that a Ti 3 Al phase formed at high temperature when Ti and Al elements were adjacent to each other. This intermetallic compound had a low melting point, and could lead to the activation of a liquid phase sintering mechanism during the SPS process, so that a large number of pores were eliminated (as shown in Figure 7f). This was the main reason for the improvement of the flexural strength. Vickers hardness still showed a downward trend, which may be due to the lower hardness of the Ti 3 Al and AlN phases [63,64].

Effect of Metals on the TiC-Based Composites
A large number of studies have shown that metal elements can significantly improve the mechanical properties of TiC ceramics. Among these metal elements, Al is a relatively active metal [65,66]. With different process parameters and Al content, there some intermetallic compounds were found in the Ti-Al-C system, in addition to TiC and Al. TiC-40 vol.% Al composites were studied by Krasnowski et al. [67]. When the sintering temperature was 1000 • C, the hardness value of the sample was 13.28 GPa. With the increase of temperature, the hardness value of the sample at 1200 • C was 10.22 GPa, because the reaction products at 1000 • C and 1200 • C were TiC-Al 3 Ti-Al and TiC-Al 3 Ti, respectively. The intermetallic compound Al 3 Ti was beneficial to improving the hardness of the matrix. Similar conclusions were also obtained in the Ti-Ni-C system by Vasudevan et al. [68]. The hardness of the samples was increased by intermetallic compounds such as NiTi, Ni 3 Ti, and NiTi 2 . On this basis, the Ti-M-C-Ni (M = Mo, W and Ta) quaternary system was explored by Chen's team [69]. The alloying elements W, Mo, and Ta dissolved in the liquid phase because of a large amount of heat released by MSR [70,71]; with the solidification of the liquid phase, these alloy elements formed an ultrafine (Ti, M) C solid solution by diffusion [72]. The schematic of Ti-MC-Ni preparation is shown in Figure 10. For the (Ti, M) C solid solution with specific orientation and distribution, cracks are difficult to deflect and can only continue to propagate according to the original propagation direction. In other words, (Ti, M) C form small bridges on both sides of the crack, connecting the two sides together. As shown in Figure 11a, whiskers bridge the cracks and apply closing stress to the surface of the cracks, which prevents the cracks from propagating; thus, they play a toughening role. The addition of Ta made the thermal hardness of the composite reach 14.2 ± 0.2 MPa, and the addition of W and Mo played an important role in eliminating pores and refining the grain size. This made the fracture toughness of the samples reach 12.45 MPa m 1/2 and 11.74 MPa m 1/2 , respectively. The crack growth diagrams of (Ti, W)C-Ni and (Ti, Mo)C-Ni are shown in Figure 12. When the crack propagates to the vicinity of (Ti, W) C-Ni, because of the high modulus of (Ti, W) C-Ni, it cannot easily pass through (Ti, W) C-Ni and propagate around (Ti, W) C-Ni. The propagation path of the crack increases, so more energy is consumed in the process of crack propagation, which makes it difficult for the crack to continue to propagate. The mechanism of crack deflection is shown in Figure 11b.

Summary and Outlook
The addition of the modified element Si produced Ti 3 SiC 2 in the matrix, the grain growth was inhibited because of the pinning effect of Ti 3 SiC 2 , and the matrix density was increased by Si in a certain range. However, with the increase of the Si content, the flexural strength and Young's modulus decreased. This is because Ti 3 SiC 2 itself is a brittle phase, and the grain size of Ti 3 SiC 2 is large. In addition, due to the accelerated oxidation of Si at high temperature, TiC-based matrix composites containing Si cannot be used at high temperature. The mechanism of coupling and interaction between process parameters was not ideal; therefore, how to establish the mapping relationship between preparation process, organizational structure, and application performance is a key scientific problem to be solved. TiO 2 produced during sintering can be effectively eliminated by the modified element C. The disappearance of surface oxide can promote the contact between Ti and C, the C/Ti ratio of the TiCx material changed, and the densification process was carried out more smoothly. The pores of the matrix were effectively filled by C, so as to obtain a finer organizational structure. Research on the growth and evolution law of the non-quantitative specific compound TiCx and its influence on the mechanical properties is relatively limited and is should be developed in the future.
The addition of Si 3 N 4 , BN, and ZrN could not enhance the relative density and mechanical properties of the materials, because these compounds were involved a complex chemical reaction with TiC. The gas produced in the reaction process led to the increase of porosity. The flexural strength of the TiC-ZrN-C material reached 741 MPa, because ZrN and C had good synergy, and a (Ti, Zr) (C, N) solid solution formed in the composite. The grain growth was hindered by the addition of TiN; samples with higher relative density were prepared. However, Ti (C, N) was produced in the reaction process forming a brittle phase; so, the mechanical properties of the TiC-TiN composites were not improved. With the addition of AlN, the flexural strength and relative density of the matrix were greatly improved, because the liquid phase sintering mechanism was activated in the SPS process; however, Ti 3 Al in the product could significantly decrease the hardness of the matrix. The selection and design of additives is an effective way to improve the performance of the existing system. We can obtain the expected function of composites by designing new multielement cooperation systems. The low temperature properties of TiC matrix composites containing metal additives were greatly improved. In the sintering process, Al, Ni, W, Mo, Ta contribute to a liquid phase strengthening. The metal elements diffused uniformly in the matrix, the grain growth was restrained, and the relative density was increased. A solid solution and an intermetallic compound formed during sintering. Therefore, the fracture toughness and hardness of the composites were also greatly improved. Expanding the effective temperature range of this mechanism and identifying new toughening mechanisms of phase transformation will be the key to solve the problem of high-temperature toughening.