Pore and microstructure change induced by SiC whiskers and particles in porous TiB2–TiC–Ti3SiC2 composites

Author’s Accepted Manuscript Pore and microstructure change induced by SiC whiskers and particles in porous TiB2-TiC-Ti3SiC2 composites Hongzhi Cui, Yanfeng Zhang, Guosong Zhang, Wei Liu, Xiaojie Song, Na Wei www.elsevier.com/locate/ceri

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S0272-8842(16)00313-8 http://dx.doi.org/10.1016/j.ceramint.2016.02.052 CERI12243

To appear in: Ceramics International Received date: 21 December 2015 Revised date: 6 February 2016 Accepted date: 8 February 2016 Cite this article as: Hongzhi Cui, Yanfeng Zhang, Guosong Zhang, Wei Liu, Xiaojie Song and Na Wei, Pore and microstructure change induced by SiC whiskers and particles in porous TiB2-TiC-Ti3SiC2 composites, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.02.052 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Pore and microstructure change induced by SiC whiskers and particles in porous TiB2-TiC-Ti3SiC2 composites Hongzhi Cui, Yanfeng Zhang, Guosong Zhang, Wei Liu, Xiaojie Song, Na Wei (College of Materials Science and Engineering, Shandong University of Science and Technology, Qingdao

266590, Shandong, PR China)

Abstract:

TiB2-TiC-Ti3SiC2 porous composites were prepared through a plasma heating reaction using powder

mixtures of Ti, B4C SiC whiskers (SiCw) and SiC particles (SiCp). The effects of the SiCw and SiCp content on pore structures, phase constituents, microstructure, and crystal morphology of TiC were studied. The results show that TiC,

TiB, Ti3B4 phases are formed within the 5Ti+B4C system. With the addition of SiCw and SiCp, the TiB and Ti3B4 phases are reduced, sometimes even disappeared. Interestingly, the content of TiB2 and TiC increased, resulting in Ti3SiC2 and TiSi2 being formed. The porosity of composites increases notably with the addition of SiCw. However, with the increase of SiCp, the porosity of the composites first decreases, followed by an increase. After adding the specified amount of SiCw/SiCp, the compressive strength of composites are improved significantly. Additionally, the pore size of the composites are decreased significantly with the addition of SiCw/SiCp. During the plasma heating process, some Si atoms will diffuse into the TiC lattice, which in turn made the cubic TiC grains into hexagonal

lamellar TiC or Ti3SiC2 grains. Keyword: porous composites; TiB2-TiC; SiC whiskers; SiC particles; reactive synthesis

1.

Introduction

Due to their low densities, high melting points, high hardness, excellent wear resistance, good thermal and chemical stability, as well as superior electrical and thermal conductivity, TiB2-TiC composites have received extensive attention as a promising material with many potential

applications [1,2]. Additionally, TiB and Ti3B4 are considered excellent ceramic reinforced particles, due to their compatible physical and thermodynamic properties, high hardness[3,4] Moreover, porous TiB2-TiC composites have potential applications in high-temperature filter and electrodes of solid fuel cells. The effects of sintering temperature and particle size of reactants on the porosity and microstructure of TiB2-TiC porous composites are widely studied[5,6]. Brittleness and poor fracture toughness are inherent shortcomings of TiB2-TiC composites limit their practical applications. In order to enhance the toughness of TiB2-TiC composites, researchers have attempted to add Al[7], Ni[8], Cu[9], Fe[10] and some other metal as bonding phases which resulted in the fracture toughness being enhanced significantly. However, the hardness and strength at high temperatures were reduced remarkably, due to the low melting point of intermetallic compounds. Other studies attempted to add particles or whiskers reinforcements, such as SiC particles (SiCp)[11], SiC whiskers (SiCw)[12], and carbon nanotube (CNT)[13]. The flexure strength and fracture toughness can be improved through grain refinements, crack deflection, as well as crack pinning. The SiC whiskers reinforced with TiB2-TiC composites possess excellent properties, due to their high elasticity modulus and good tensile strength[14,15]. Recently, SiCw-reinforced ceramic composites, such as SiCw-reinforced Al2O3, ZrB2, TiB2, Ti(C, N), Si3N4 composites[16–18] have become more and more popular. Peng, et al[18] compounded TiCN ceramic composites reinforced by SiC nano-whiskers through spark plasma sintering. The Vickers hardness and flexural strength of the composites were increased significantly with the addition of 2.5 wt.% SiC nano-whiskers. In addition, SiCp is uniquely desirable, due to its excellent wear and corrosion resistance, good thermal and chemical stability, high-temperature strength, and high hardness[19]. TiC-SiC composites with different SiC volume contents were prepared via spark plasma sintering (SPS), by Cabrero et al[20].

Vickers hardness and fracture toughness of TiC-SiC composites were improved by increasing the nano-SiCp. LI et al reported that TiC-TiB2-SiC ultrafine composites were synthesized from hybrid polymer precursors with ideally distributed phases and potential superiority of thermal performance [21]. Moreover, SiCw/SiCp reacts with Ti to form Ti3SiC2, which efficiently enhances the bonding strength between the SiCw/SiCp and the TiB2-TiC matrix. Ti3SiC2 can effectively improve the fracture toughness of ceramics due to its combining characters of both ceramics and metals, such as good mechanical ability and thermal stability like that of graphite. QIN et al[22] synthesized the Ti3SiC2-reinforced TiSi2-SiC-Ti3SiC2 composites through spark plasma sintering (SPS), using TiC, Si, and C powders as raw material. Both the fracture toughness and the bending strength of composites were significantly enhanced. The plasma beam is unique within high energetic beams with an energy density as high as a laser beam. The core temperature can potentially reach 1~1.5104 K. Plasma beams boast advantages over the traditional laser beams in that they are highly efficient, non-polluting, and have a wide range of heating temperatures[23,24]. Darabara et al produced TiB2/TiCN-reinforced metal-based wear resistant coating via the plasma transferred arc technique [25]. Due to its high efficiency, low energy consumption, high product quality, and low cost, the self-propagating high temperature synthesis (SHS) is one of the effective methods of synthesizing advanced ceramic composites [26]. Many types of composites with high melting points, such as TiC, TiB2, ZrB2 and BN, which are traditionally difficult to sinter, have been synthesized by this method[27–29]. In this study, porous TiB2-TiC-Ti3SiC2 composites were produced by plasma heating reaction from powder mixtures of Ti, B4C, SiCw and SiCp. The effects of SiCw/SiCp content on pore structure,

phases, microstructure, and compressive strength of the composites were studied. Furthermore, the formation mechanisms of lamellar TiC and Ti3SiC2 induced by the addition of SiCw/SiCp were analyzed. 2. Experimental process Ti powders (54-76 μm, purity 99.5%), B4C powders (7-10 μm), SiC whiskers (D<2.5 μm, L/D>20, purity 99%) and SiC particles (1-10 μm, purity 99%) were used as raw materials, with the molar ratio of raw materials shown in Table 1. The powders were mixed in a three-dimensional powder mixing machine for 3 hours, and then compacted into 20mm20 mm samples under 120 MPa for 5 minutes through a manual hydraulic pressure machine in a steel mold. Composites were obtained through a multi-pass scanning heating, using atmospheric pressure plasma processing equipment (DGR-5) with plasma current of 60 A, voltage of 50 V, and scanning speed of 10 mm/s. Argon gas served as a protective gas. The schematic diagram of the manufacture process of porous materials via plasma heating reactions is shown in Fig. 1. After reaction synthesis, there are no obvious changes in the shape and size of productions, when compared to the green pellets. Table 1 Molar ratio of raw materials Molar ratio NO. Ti:B4C TS0

xSiCw 0

TSw0.5

0.5 5:1

0

TSw1.0

1.0

TSw1.5

1.5

TSp0.5 TSp1.0 TSp1.5

xSiCp

0.5 5:1

0

1.0 1.5

Archimedes principle was used to measure the porosity of TiB2-TiC composites. An mercury porosimetry measurement was carried out on a Micromeritics Poresizer (AutoPore IV 9500,

America). The compressive strength was tested on a universal testing machine (WDW3100). The XRD data were characterized by a step-scanning diffraction (D/MAX-YB, Cu Ka) with Cu Kα radiation. Microstructures and compositions of the samples were characterized by a field emission scanning electron microscopy (FESEM, FEI Nova Nanosem 450) equipped with an energy dispersive X-ray spectroscopy (EDS). 3 Results and discussion 3.1 Phase of reaction products analysis The XRD patterns of composites with different contents of SiCw/SiCp are shown in Fig. 2, indicating that the composites produced from 5Ti+B4C consist of TiC, TiB, Ti3B4, and small amounts of TiB2. After the addition of SiCw or SiCp, the peaks of TiB and Ti3B4 reduce, and even disappear, while the peaks of TiB 2 and TiC increase greatly, and peaks of TiSi2 and Ti3SiC2 appear. This is mainly because Ti reacts partially with SiCw/SiCp in order to form TiC, Ti3SiC2 and TiSi2 phases. The reaction consumes excessive Ti, which provides favorable conditions for Ti and B4C to react with each other and form boron rich phase of TiB 2. When the amount of SiC w/SiCp is greater than 1.0, a few of residual SiC w/SiCp are found. In addition, the peaks of TiB2 and Ti3SiC2 correspond to the PDF standard card of TiB2 (No.35-0741) and Ti3SiC2 (No.48-1826), respectively. However, when compared to PDF standard card of TiC (No. 32-1383), all diffraction peaks of TiC shift to higher angles, as shown in Fig. 2(a1) and (b1). Additionally, the peaks of TiC tend to be broader with the increase of SiCw/SiCp. According to CHEN’s study [30], this is attributed to the Si atoms, which come from the decomposition of SiCw/SiCp, diffusing into TiC lattice, and causing lattice distortion. Based on Bragg’s equation: 2dsinθ =nλ (d, θ, n and λ denote the parallel lattice planes space, Bragg angle, reflection progression, and X-ray wavelength, respectively), Si atoms doping into TiC will lead to the decrease in lattice

parameter of TiC phase, and then the Bragg diffraction angle increases to some extent. 3.2 Pore structure and Microstructure The morphologies of pores and pore walls on the fracture surface of productions are shown in Fig. 3. Obviously, pores in the production of Ti+B4C（TS0）are big and inhomogeneous in their sizes, as seen in Fig. 3(a). The fracture surfaces of those pore walls are noticeably thicker and denser. The amount of pores in composites increases significantly with the addition of SiCw or SiCp into 5Ti+B4C, and the size of pores become smaller and more uniform, as shown in Fig. 3(b) and (c). Moreover, the fracture surfaces of pore walls become granular, as shown in Fig.3 (b1) and (c1). By comparing Fig. 3 (b) and (c), it can be seen that grains of the composites with SiCp are smaller than that of SiCw, indicating that the addition of SiCp can refine the grains of the composites. The smooth and denser fracture surfaces of pore walls in the composites of TS0 (Ti+B4C), shown in Fig.3 (a1) indicated that the main fracture mode of TS0 composites are transgranular. However, the rough fracture surfaces with cubic and hexagonal granules in Fig.3 (b1) and (c1) suggest that the main fracture mode of these composites reinforced by SiCw/SiCp is intergranular. 3.3 Porosity and compressive strength of composites analysis The porosity of the TiB2-TiC-Ti3SiC2 composites containing the different compounds of SiCw/SiCp are shown in Fig.4. The porosity of composites increases gradually with the increase of SiC whiskers (SiCw). The SiC whiskers prop up the reactant powders of Ti and B4C, thus enlarging the space between the reactant powders in green pellets. As a result, the porosity of the green body increases. Correspondingly, the porosities of productions are improved. However, the porosity of composites with the addition of SiCp demonstrates a gradually decrease with the increase of SiCp content at first. When the content of SiCp reaches 1.0, the composite reaches the lowest porosity of

32.8%. Additionally, the porosity of composites with SiC whiskers is much higher than that with SiC particles at the same content. This is mainly contributed to the cluster and lower density of SiCw than that of SiCp, which makes green bodies with SiCw has higher porosity than the one with SiCp. Pore size distributions of the specimens with different content of SiCw/SiCp were also examined, and shown in Fig. 5. It can be observed that with the increasing of SiCw/SiCp, the pore size distributions of the specimen are modified. Micron sized pores with 50-300 μm are formed in the specimen without SiCw/SiCp. The pore size of composites decrease significantly and become more uniform with the increase of SiCw/SiCp. When the content of SiCw/SiCp rises to 1.0, the mean pore size shrinks to 5-15 μm. After adding a certain amount of SiCw/SiCp, the compressive strength of composites improves significantly. However, when the content of SiCw/SiCp is above 1.0, the compressive strength of composites will decrease. Moreover, the composites with SiCp have higher compressive strength than the one with SiCw at the same content. When the content of SiCp increases from 0 to 1.0, the compressive strength will increase from 53.9 MPa to 99.9 MPa, as shown in Fig6. Therefore, when the molar content of SiCw is less than 1.0, both the porosity and mechanical strength of composites are improved with the addition of SiCw. 3.4. Effects of SiCw/SiCp on pore structures and microstructures Microstructure and EDS composition analysis of the composites are shown in Fig. 7. Notably, many dendrites are distributed between some ellipse or irregular particles on the pore surfaces of TS0 in Fig. 7(a). The EDS analysis show that the dendritic crystals marked by ‘A’ consist of Ti and C. However, some regular fine particles distribute on the surfaces of pore walls spirally after adding SiCw/SiCp, as shown in Fig. 7(b) and (c). Moreover, complete spiral staircases appear in the sample

of TSw1.0, and EDS analysis of ‘B’ and ‘C’ show that these grains are mainly composed of Ti, C, and small amounts Si. As the plasma heating can reach a temperature of 1~1.5104 K, the green body is heated through a scanning plasma beam shown in Fig. 1, and Ti then reacts instantly with B4C to form TiC, TiB, Ti3B4 and TiB2. The reaction produces a mass of heat which makes Ti particles melt and infiltrate into B4C particles, which accelerates the formation of TiC, TiB, Ti3B4, and TiB2 phases. TiC will nuclear first and grow into dendrites during the rapid cooling process due to the higher melting point of TiC[31]. After that, TiB, Ti3B4 and TiB2 begin to form, and typically grow into regular shapes of cuboid and hexagon, showing in Fig 3(a1) and (b1). Moreover, as TiC particles have excellent plasticity at high temperature[32], they can be pushed into the gaps by the growing grains of borides. The TiC dendrites grow up along the fastest heat dissipating direction and form single-armed or multi-armed dendritic crystals, as the cooling process progresses, depicted in Fig. 7(a). Finally, these grains construct the walls of porous composites together. Fig. 8(a) indicates that some layered particles with triangle and dendrite shapes are found in the sample of TSw1.0. Additionally, laminar particles with spiral morphologies appear in the composites of TSp1.0, shown in Fig. 8(b). In addition, some layered particles with hexagon holes exist in composites of TSp1.5, which grow directionally along the walls of the hole, shown in Fig. 8(c). The EDS analysis of point A, B and C shows that all the points mainly include Ti, C, and small amounts of Si. These structures should become Ti3SiC2 particles through combining the XRD analysis. The Ti and C atoms of TiC crystals are arranged in a hexagon network on the crystal face of {111} as shown in Fig. 9(a) and (b), which is similar to their arrangements on {0001} crystal face of

Ti3SiC2[33,34]. However, the crystal lattice distances are changed and the atoms rearrange with Si atoms diffusing into TiC lattice, because the diameter of the Si atoms differ from the C atoms. The rearrangement reduces the energy and the growth speed of the TiC face {110} as well as promotes the structure of TiC to change from face centered cubic to hexagonal. Subsequently, the hexagonal Ti3SiC2 platelets were formed and shown in Fig. 9(d). Finally, the platelets grow into Ti3SiC2 particles layer by layer with apex angles aligned, as shown in Fig. 9(e). Notably, the regular fine Ti3SiC2 particles are formed specifically under the condition of sufficient Si source, which grows in a spiral arrangement on TiC grains as seen in Fig. 8(b). In addition, it has been reported that the impurities in TiC crystals induce high density of defects in TiC grains, such as B, Al and Si, [35,36]. Defects tend to form on the {111} face of TiC when some Si atoms diffuse into TiC crystals. These defects can potentially cluster and serve as propitious nucleation places of Ti3SiC2 with the doping of Si. Finally, these TiC and Ti3SiC2 grains grow directionally up along the defects in order to form the layered grains with hexagonal holes, as shown in Fig.8(c) and Fig. 9(f). In all the porous products with SiCw, long rods or fibers with the diameter of 1-2 μm which are corresponding to the size of SiCw, stretch in the pores, as shown in Fig.10 (a), (b) and (c). Moreover, Fig. 10(a) and (b) show that some of the fibers are hollow, and covered with many small regular particles. It can be concluded that compositions of point A are largely Ti, C, and a small amount of Si, while point B mainly contains Ti and C, in accordance to the EDS analysis of point A in Fig. 10(a) and point B in Fig. 10(b). By combining these findings with the XRD results, one can postulate that the particles on the inner surface of fibers should be Ti3SiC2, and the irregular particles on the outer surface should be TiC.

After adding SiCw/SiCp into Ti+B4C system, the Ti is melted and will infiltrate into the intervals between SiCw/SiCp particles and surround surfaces of particles. Furthermore, SiC will decompose into Si and C, which can then react with Ti to form TiC, TiSi2, and even Ti3SiC2.The reaction between Ti and SiC is harder than the reaction between Ti and B4C[27,37]. Therefore, a majority of the melted Ti will react with B4C initially to form TiC dendrite, TiB, Ti3B4, and TiB2 regular grains as described above, when the systems is heated by plasma. Then, the residual Ti will surround the surfaces of SiCp or SiCw, and react with them to from the outermost layer. The formation of metastable MAX phase of Ti3SiC2 have much higher priority than that of TiC and TiSi2 during the reaction of Ti with SiC, due to the quick cooling process of plasma heating[38]. Most importantly, the SiC whiskers, which are covered with TiC and Ti3SiC2 layers, can make the crack deflect when the crack propagates to these long rods or fibers, as marked by the arrows in Fig. 10(b). Some of these whiskers are covered by tiny particles which combine the pore walls through bridge-connecting, and are pointed out by the arrows in Fig. 10(a). This is beneficial in increasing the compressive strength of the composites. Fig. 11 shows the crack propagation path of composites with different content of SiCp. The crack propagating is straight with a smooth fracture surface, and a small particles pullout when there is no addition of SiCp or SiCw, as indicated by the arrows in Fig. 11(a). The crack propagating path of composites become zigzag when they meet the tiny particles, which are reinforced with SiCp. The fracture surface is covered with small convex particles and pits which are caused by the particles’ pulling out, as shown by the arrows in Fig. 11 (a) and (b). This indicates that the SiCp reinforces the TiB2-TiC-Ti3SiC2 composites via the toughening mechanisms of crack deflection, particles refining, and pulling out.

Therefore, the compressive strength of porous composites was improved significantly with adding either SiCw or SiCp, owing to the whiskers' bridge-connecting and particles' synergistically toughening of crack deflecting, refining, and pulling out in the pore walls, respectively. The schematic diagram (Fig. 12) shows reaction process between Ti, B4C and SiCw, micro structure and pore structure evolution process. The raw powders stacking within the green body is shown in Fig. 12(a). The Gibbs free energy (ΔGθ) of the reaction of Ti and B4C is much lower than that of Ti and SiC[27,37], indicating that the Ti will preferentially react with B4C particles to form TiC and TiB2 phases, releasing a large amount of heat energy. In addition, during the fast heating and cooling process of plasma, Ti will melt and adsorb on the surfaces of B4C and SiCw because of the good wettability between Ti and B4C/SiCw and the capillary force, as Fig. 12(b). The TiC and TiB2 phases quickly grow to build the walls of porous composites. Meanwhile, the remained melting Ti reacts with SiCw from the outmost surface to form TiC, Ti3SiC2 particles and Ti-Si melt, as Fig. 12(c). With the further reaction of Ti and SiCw, a layer of TiC and Ti3SiC2 grains, which serve as a shell, will be quickly formed on the surfaces of SiCw. This makes it difficult for the Si atoms to diffuse outside. In this way, the Si atoms can only aggregate in the inner shell and diffuse into TiC grains to form Ti3SiC2 particles. Some hollow tubes with a shell of TiC and Ti3SiC2 layers are created along with the continuously decomposing of SiCw, particularly in those SiC whiskers with large diameters, see Fig. 12(d). Concurrently, theTi3SiC2 grains develop and become increasingly regular, presenting a kind of typical lamellar structure of MAX phases, as Fig. 8(b), 8(c) and 12(d). In addition, due to the decomposition of SiC whiskers, some Si, and C atoms are dissolved into Ti melt, which are unavoidable. Thus, a part of Ti3SiC2 can directly form from the Ti-Si-C melt, and nuclear and grow on the surface of SiC whiskers or TiC particles, as shown in Fig. 10(b) and Fig.

12(d). 4. Conclusion (1) Porous TiB2-TiC-Ti3SiC2 composites were obtained from Ti, B4C, SiCw and SiCp through the plasma heating in-situ reaction process. When there is no addition of SiCw or SiCp into the Ti+B4C, the main resultants were TiC, TiB, Ti3B4, and small amounts of TiB2. With the increase of SiCw and SiCp content, TiB and Ti3B4 reduce and even disappear, and a few TiSi2 and Ti3SiC2 are formed. (2) Ti reacts with SiC whiskers to form a shell of regular TiC grains on the surfaces of SiC whiskers. Certain amounts of Si atoms from the decomposition of SiCw/p, which diffuses into TiC grains. With the increase of Si content, cubic TiC grains will turn into hexagonal lamellar TiC grains or layered Ti3SiC2 grains, due to the doping of Si into TiC. (3) With the increase of SiC whiskers content, the porosity of composites increases gradually. However, with the increase of the SiC particles content, the porosity of composites decreases at first, but subsequently increases when the amount of SiCp is beyond x=1.0. Additionally, the pore size of the composites were decreased significantly with the addition of SiCw/SiCp. Moreover, the introduction of SiC whiskers and particles induces obviously grain refining and crack deflecting effects in TiB2-TiC-Ti3SiC2 composites which promotes the compressive strength of porous composites significantly. Acknowledgments The authors would like to acknowledge the Natural Science Foundation of China (No.51272141), Taishan Scholars Project of Shandong (No. TS20110828), National High Technology Research and Development Program of China (863 Plan, 2015AA034404), and Science

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Combustion wave

Plasam beam Ar gas Porous composites

Fig. 1 Schematic diagram of the manufacture process of porous materials via plasma heating reaction

● ▲

●▲

▲―

TiC

●―

★― Ti3SiC2

TiB2

△

■―

SiC

(a1)

― TiSi2 ○ ― Ti B 3 4

◆― TiB

●

△

Intensity

TSw1.5

△

■

▲ ■●

●

▲

●

▲

●

●

●

★

TSw1.0

Intensity

(a)

△

○

◆ ○ ○

◆

○

◆

30

40

50

60

2θ/(°)

(b)

●

70

TiC

★― Ti3SiC2

TiB2

△

35

36

37

38

39

(105)

(008)

40

41

2θ/(°)

42

43

(b1)

― TiSi2

○―

SiC ◆― TiB

Ti3SiC2

90

●―

(104)

TiC

80

(200)

◆

▲―

■―

▲

●

◆

(111)

(110) (220)

(002)

(101)

(100)

(001)

20

◆

(112)

◆◆

○

(111) (200)

TiC TiB2

◆

○

(102)

○

◆

(111) (311) (200) (222) (201)

TS0

○

Ti3B4

●

△

★★★

20

30

40

△

◆ ○

50

60

2θ/(°)

70

(200)

Ti3SiC2

90

35

36

37

38

39

(105)

TiC

80

○

◆

(008)

◆

(104)

◆

(111)

◆◆

○

(112)

◆

(002)

(100)

(001)

TiB2

●

○ ◆

(111) (200)

○

TiC

●

◆ ○

○◆

▲

(102) (111) (311) (200) (222) (201)

TS0

◆

▲ ●

■

TSp1.0 ○

●

■

Intensity

▲ ●

△

●

(110) (220)

TSp1.5

(101)

Intensity

▲

40

2θ/(°)

Fig. 2 XRD patterns of composites with different content of SiC w/ SiCp (a)/(a1) Samples with different content of SiCw; (b)/(b1) Samples with different content of SiCp

41

42

43

Fig. 3 FESEM micrographs of fracture surfaces for the composites with different content of SiC w / SiCp. (a)/(a1) TS0 (x=0), (b)/(b1) TSw1.5 (x(SiCw)=1.5), (c)/(c1) TSp1.5 (x(SiCp)=1.5) 70

Porosity (%)

60

Added different content of SiCw Added different content of SiCp

50 40 30 20 10 0

0.0

0.5 1.0 Content of SiCw/SiCp

1.5

Fig. 4 Porosity of composites with different content of SiC w / SiCp

0.5 0.4

0.6 3

0.6

0.7

dV/dlogD (cm /g)

TS0 TSp0.5 TSw0.5 TSp1.0 TSw1.0 TSp1.5 TSw1.5

0.7

0.5 0.4 0.3 0.2 0.1

0.3

0.0 0

0.2

2

4

6

8 10 12 14 16 18 20 22 24

PoreDiameter Size (μm) Pore (μm)

0.1 0.0 0

50

100

150 200 250 Pore Size (μm)

300

350

400

Compressive strength/(MPa)

Fig.5 Pore size distribution of the composites with different content of SiCw/SiCp

Added different content of SiCw Added different content of SiCp

120 100 80 60 40 20 0

0.0

0.5

1.0

1.5

Content of SiCw/SiCp

Fig. 6 Compressive strength of composites with different content of SiC w/SiCp

Fig.7 Morphologies of pore surface in the composites with different content of SiCw/SiCp and EDS analysis. (a) TS0(x=0); (b) TSp1.0(x(SiCp)=1.0); (c)TSw1.0(x(SiCw)=1.0); (d)/(e)/(f) EDS analysis of point A, B, and C,

respectively

Fig.8 Morphologies of TiC dentrite and laminar Ti3SiC2 and EDS analysis (a) TiC dentrites (x(SiCw)=1.0); (b) Ti3SiC2 laminar (x(SiCp)=1.0); (c) TiC laminar (x(SiCp)=1.5); (d)/(e)/(f) EDS analysis of point A, B, and C, respectively

(e) —Ti —C (a)

—Ti

—C (b)

(d) (c) (f)

Fig. 9 Schematic illustration of the evolution processes of layered Ti3SiC2 from TiC hexagonal platelet

Fig.10 Microstructure of the TSw1.0samples and EDS analysis. (a) / (b) /(c) TSw1.0 (x(SiCw)=1.0); (a1)/(b1)

/(c1)EDS analysis of point A, B and C respectively

Fig.11 Crack propagation path of composites with different content of SiCp (x(SiCp)=1.0)

(a) TS0 (x=0), (b) TSp1.0

(b)

(a) B4C

pole Ti

TiC

Ti(l)

B4C

SiCw

TiB2 Ti

B4C

(c)

pore wall

(d)

pore wall

Ti3SiC2 TiC hole SiCw

TiC Ti-Si(l)

pole

TiB2

Ti3SiC2 Ti3SiC2

pole

Ti3SiC2

Fig. 12 Schematic diagram of reaction mechanism between Ti, B 4C and SiCw, micro structure and pore structure evolution process (a) Raw powders stack in the green body; (b) Ti melt and react with B4C to form TiC and TiB2 phases; (c) The walls of porous composites are built and TiC and Ti3SiC2 phases are formed by the reaction between Ti and SiCw; (d) TiC and Ti3SiC2 crystals grow up and hollow tubular holes are formed