SiC Composites Consolidated by plasma-activated sintering

Journal Pre-proof Sintering Mechanism and Microstructure of TaC/SiC Composites Consolidated by plasma-activated sintering Jianian Hu, Guoqiang Luo, Jian Zhang, Yi Sun, Qiang Shen, Lianmeng Zhang PII:

S0272-8842(19)32766-X

DOI:

https://doi.org/10.1016/j.ceramint.2019.09.229

Reference:

CERI 22996

To appear in:

Ceramics International

Received Date: 31 August 2019 Revised Date:

19 September 2019

Accepted Date: 23 September 2019

Please cite this article as: J. Hu, G. Luo, J. Zhang, Y. Sun, Q. Shen, L. Zhang, Sintering Mechanism and Microstructure of TaC/SiC Composites Consolidated by plasma-activated sintering, Ceramics International (2019), doi: https://doi.org/10.1016/j.ceramint.2019.09.229. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier Ltd.

1

Sintering Mechanism and Microstructure of TaC/SiC

2

Composites Consolidated by Plasma-Activated Sintering

3 4

Jianian Hu , Guoqiang Luoa,*, Jian Zhanga, Yi Suna,

5

Qiang Shena and Lianmeng Zhanga

6

a

a

State Key Laboratory of Advanced Technology for Materials Synthesis and

7

Processing, Wuhan University of Technology, Wuhan 430070, China

8

* Corresponding author: Tel.: +86- 87168606; E-mail: [email protected]

9 10 11 12

Abstract TaC/SiC

composites

with

5

wt.%

SiC

addition

were

densified

by

13

plasma-activated sintering (PAS) at 1500–1800 °C for 5 min under 30 MPa. The

14

effects of plasma-activated sintering on microstructures, densification and

15

mechanical properties of the composites were investigated. The results showed that

16

TaC/SiC composites achieved a relative density more than 99% theoretical density

17

at 1600 °C. A low eutectic liquid phase generated by the oxide on the particle

18

surface was observed in the composite to realize a relatively low temperature

19

sintering densification. While the TaC particle size decreased insignificantly with

20

increasing sintering temperature,the transformation of morphology of SiC particles

21

changing from equiaxed to elongated grain was activated, accompanying with a

22

slight particle size decreasing of the SiC phase, thus promoting a relatively high 1

23

flexural strength of 550 MPa under 1800 °C. Besides, some ultra-fine 2 nm Ta2Si

24

was observed in the glassy pockets, strengthening the amorphous phase and thus

25

increasing the flexural strength.

26

Keywords: TaC matrix; SiC additive; surface oxide; PAS sintering; sintering

27

mechanism; low eutectic glassy phase

28

1. Introduction

29

Ultra-high temperature ceramic (UHTC) was the potential candidates for a new

30

generation of promising materials for applications in the aerospace, which have a

31

relatively high melting point more than 3000 °C and possess a relatively high

32

strength under high temperature [1-4], Among UHTC, TaC has the highest melting

33

point at about 4000 °C, besides, TaC-based ceramics possess better mechanical

34

properties than other UHTC. TaC ceramics have a room-temperature flexural

35

strength in the range of 400–700 MPa [5-8], compared to ZrC and other UHTC of

36

220–400 MPa [9-10]. TaC shows better chemical corrosion resistance and excellent

37

thermoelectric performance and electrical conductivity. TaC is an ideal material for

38

structures operating at very high temperatures, such as parts of space shuttles and

39

rocket nozzles.

40

However, because of its high covalent bonding characteristics and low

41

self-diffusion coefficient, TaC is hard to densify [11-13]. In the case of TaC,

42

hot-pressed micro TaC powder at 30 MPa only achieved 94% of the theoretical

43

density at 30 MPa, 2300 °C [14]. It has been found that oxide impurities of Ta2O5 in

44

the starting powders are an important cause of excessive growth of TaC grains 2

45

[15-16].

46

Relative fine TaC powders with a particle size less than 2 µm were

47

consolidated at 2300 °C without pressure, reaching a relative density of 97.5%

48

without the use of additives [31], the small particle size of TaC powder turned out

49

to be an important factor for preparing a dense TaC ceramic. Reducing additives

50

such as C and B4C were added into a TaC ceramic to promote sintering [17-18].

51

While TaC ceramic was introduced by 0.36 wt.% B4C as a sintering agent, the

52

relative density was increased from 89.1% to 98% at 2200 °C [19]. In the case of

53

B4C, the reaction between TaC and B4C particles generated TaB2 phase and graphite

54

C, physically immobilizing the TaC particles, thus avoiding the residual pores from

55

being trapped. SiC was also added to UHTC to enhance sintering and improve

56

mechanical properties. In addition to conventionally pinning grain boundaries and

57

refining the matrix grains, SiC also provides an oxide layer (SiO2) to react with the

58

matrix, generating a low eutectic mixture to enhance the consolidation [20-21,30].

59

Conventional sintering methods, such as pressureless sintering and hot-pressing

60

sintering, performed on UHTC need a very high sintering temperature with a

61

relatively high sintering time, which enhances grain growth but diminishes the

62

mechanical properties [31,32]. The literature has shown that spark plasma sintering

63

(SPS) can facilitate the densifying of UHTC. In the SPS process, the graphite mold

64

with the powder significantly increases heating rate via a strong current. Under the

65

heating rate of 100°C per min, the rapid heating speed prevents grain growth before

66

reaching the holding temperature, which is beneficial to ultrahigh-temperature 3

67

technology [22-23]. In addition to the advantages of SPS, plasma activated

68

sintering (PAS) is very helpful in reducing the negative effects of oxide layers on

69

sintering densification by providing a 30 seconds with supplied pulsed current before

70

sintering. In the PAS sintering process, by generating the discharge between

71

different particles, particle surface adsorbent was eliminated and the oxide layer was

72

broken down, thus contributing to sintering; in addition, the process produced the

73

composite at a lower temperature in a relatively short period of time [24-25].

74

The comparison of fracture strength of TaC-SiC system composites by several

75

methods was listed in table 1, hot pressing and cold isostatically pressing/hot pressing

76

method was difficult to density the TaC/SiC composites, thus TaSi2 and MoSi2 was

77

introduced into the system as sintering aid. The fracture strength of TaC/15 vol.%SiC

78

with 10 vol. % TaSi2 reached as high as 506 MPa by conventional hot pressing

79

sintering [33, 34]. However, the fracture strength of the TaC/ 20 vol.%SiC composites

80

without sintering aid sintered by SPS reached 682 MPa [21]. Fast current assisted

81

sintering is beneficial to the densification and fracture strength of TaC/SiC composites.

82

However, the use of PAS for the densification of TaC has not received much

83

attention. In our study, PAS was used to produce a TaC/5 wt.% SiC composite.

84

Sintering was carried out at 30 MPa while keeping the temperature lower than

85

1800 °C. It was found that using 5 wt.% SiC as an agent could produce the composite

86

with a relative density of 99.3% by PAS at 1600 °C for 5 min, by generating a low

87

eutectic mixture to enhance the densification. The flexural strength of the composite

88

of 550 MPa was obtained under 1800 °C due to the transformation of the equiaxed 4

89

SiC phase to the plate-like SiC phase and the enhancement of the low eutectic

90

mixture with the ultrafine Ta2Si phase. The effects of PAS on microstructures,

91

densification and mechanical properties of the composites were investigated.

92

2. Experiment and procedure

93

We used commercial-SiC powder (mainly -6H polymorph SiC, Aladdin,

94

Shanghai, China) and TaC (cubic TaC Aladdin, Shanghai, China) as raw materials

95

with diameters of 0.5-1 µm and 3-5 µm, respectively. The purities of TaC raw

96

powder were more than 99% and the main impurities were Ca and F. The purity of

97

the SiC was 99.5 wt%, and the main impurities were SiO2 and free carbon. TaC

98

and 5wt.% SiC powder mixture was dry-blended in a ZrO2 cylinder with ZrO2

99

balls as grinding media for 24 h. A batch of the mixed powder was put into 20 mm

100

inner diameter graphite mold. A power supply offered a DC pulse current

101

discharge (20 V, 100 A) to activate the particle surface in 30 s, and then raise the

102

temperature to the target temperature at a rate of 120 °C/min. A 30 MPa uniaxial

103

pressure was performed on the diameter graphite mold at the target temperature for

104

300 s, finally reduce the temperature to the target temperature at a rate of

105

60 °C/min, approximately.

106

For the microstructure, polished surfaces and the fracture morphology of the

107

materials were characterized by field-emission scanning electron microscopy

108

(FESEM, Quanta-250). For the detailed microstructure, JEM-2100F STEM was

109

performed on the samples to get the high-resolution transmission electron

110

microscopy (HRTEM) images of the materials. X-ray diffraction patterns were 5

111

obtained on the machine (XRD, Rigaku Ultima3) using Cu Kα radiation. The

112

flexural strength of the composite was characterized by the bending method on a

113

mechanical test system (Instron-5966).

114

3. Results and discussion

115

3.1 powder characterization

116

The TEM images of the TaC and SiC mixture after ball milling are shown in

117

Figure 1. The particle surface of the TaC and SiC particles were both surrounded

118

by a 2-3 nm coating, the coating materials was determined as Ta2O5 and SiO2 in

119

the related literature [15,16,30,35], respectively.

120

3.2 Phase identification

121

The XRD pattern of the TaC/SiC composite sintered at different

122

temperatures from 1500 °C to 1800 °C is shown in Figure 2. The main phase

123

composition is cubic TaC, corresponding to the starting ingredients. Because the

124

content of the SiC phase was small (5 wt.% SiC), 6H-SiC could hardly be

125

discerned in the XRD pattern of the composite (Figure 2). No other crystal phase

126

was shown in the X-ray diffraction pattern. As the sintering temperature

127

increased, the XRD peak of the TaC phase in different materials moved to a

128

lower position, indicating a systematic increase in the TaC lattice [26]. Therefore,

129

TaC/SiC composites can be consolidated by PAS without changing main

130

chemical composition.

131

3.3 Density

132

The relative density of the composites is listed in Table 2. The relative density 6

133

of the composites sintered under different temperatures was relatively high. The

134

composites have relative density more than 99% of the theoretical density when the

135

temperature was above1600 °C. The high densification of TaC/SiC composites was

136

partly explained PAS sintering system. TaC grains growing were prevented by the

137

fast heating rate of 120°C/min and sintering time of 5min, consequently alleviated

138

the pore entrapment. The high density can also be realized by the formation of a

139

liquid phase during sintering, which was observed in the microstructure in Figure 4

140

3.4 Microstructure

141

SEM images of the TaC/SiC composite microstructure at different sintering

142

temperatures are shown in Figure 3. The comparison of grey and black is TaC and SiC

143

phases, respectively. The secondary phase SiC was uniformly distributed in the TaC

144

matrix. As the sintering temperature increases, the grain size of the TaC phase

145

decreases. Given the temperatures of 1500 °C and 1800 °C, the average particle size

146

of the TaC and SiC were 2.8 µm and 1.7 µm, separately, compared to that of 4.7 µm

147

in pure TaC bulk without adding SiC produced by PAS sintered at 1800 °C for 5

148

minutes, which demonstrates the significant effect of small SiC particles on the

149

physical pinning of TaC particles. Due to the SiC particles pinning the grain

150

boundaries between TaC particles. Therefore, preventing excessive growth of TaC

151

particles and eliminating residual porosity [21]. The SiC particle size also changed

152

with increasing sintering temperature. The phenomenon was completely unanticipated,

153

however, it is demonstrated by comparing the results in Figures 3a and 3d. From the

154

images in Figures 3a and 3d for the composites sintered at 1500 °C and 1800 °C, the 7

155

particle size of SiC phase decreases from approximately 1.0 µm to 0.5 µm. In addition

156

to the SiC grain size, the morphology also changed. For the composites sintered under

157

a temperature below 1700 °C, the morphology SiC grains are mostly equiaxed,

158

nevertheless, anisotropic morphology of SiC was demonstrated in the composites

159

sintered under 1800 °C, thus promoting a better flexural strength. The main reason

160

for the equiaxed SiC transformation into a plate-like SiC was that the SiC phase

161

partially dissolved in a glassy layer and subsequently reprecipitated with an elongated

162

direction to lower the surface energy, which was observed in the HRTEM in Figure 5.

163

A transmission electron micrograph of the composite consolidated under 1600 °C

164

and a phase chemical analysis is shown in Figure 4. SiC and TaC particles were

165

distinguished by an electron diffraction pattern, which was shown in Figures 4b and

166

4c, separately. The electron diffraction of SiC and TaC suggested that the crystal types

167

of 6H-SiC and SiC were hexagonal and cubical, respectively. A stacking fault was

168

observed in intragranular SiC. Figure 4a also shows a glassy pocket with three particle

169

junctions, connecting the surface oxide of SiC and TaC in Figure 1. The glassy pocket

170

is thought to be generated by a reaction between SiO2 and Ta2O5 forming a low

171

eutectic glass, and the phenomenon of two oxides forming a low eutectic glass was

172

observed in a ZnO and SiO2 system [30]. In Figure 4f, the HRTEM image of this area

173

demonstrates amorphous feature of the phase. Such a smooth glassy pocket exists

174

merely around SiC particles boundary. As shown in Figure 4e, there was a clean

175

interface between SiC and TaC, and no oxide on the surface of TaC and SiC was

176

observed, suggesting the effect of PAS sintering on cleaning the oxide impurities on 8

177

the starting powder. Additionally, a parallel crystal plane of TaC (1 1 1) and SiC (1

178

01 2) was observed, and the interplanar space of the TaC (1 1 1) plane and SiC (1

179

01 2) plane was 0.257 nm and 0.251 nm, respectively, suggesting a relatively low

180

lattice mismatch: 0.02=(0.257-0.251)/0.257, thus creating a strong bonding between

181

TaC and SiC. As shown in Figure 4d, the glass capsules were revealed by EDS

182

analysis. In addition to Si, C and Ta, elements of F, Ca and O, which were introduced

183

by the impurities in the starting powder of the mixture of TaC and SiC. The main

184

reason for forming the glassy phase during the sintering process was that the oxide

185

layer of the surface SiO2 and Ta2O5 have a low eutectic, thus forming a small amount

186

of liquid under PAS high-temperature sintering. The liquid (Ta-Si-C-O) flows into the

187

grain boundary and collects most of the impurities (Ca, F). After sintering, rapid PAS

188

cooling quenches the sample, with a 100 °C/min cooling rate, preventing the liquid

189

phase from crystallizing and thereby forming an amorphous layer consisting of

190

Ta-Si-C-O and the impurity on the powder surface; thus, no significant glass layer

191

between the TaC/SiC or TaC/TaC particles was observed by HRTEM, as shown in

192

Figure 4a. Similar amorphous layers are typically formed around SiC particles

193

because the primary component of the amorphous layer is SiO2 from the EDS analysis,

194

which preferentially wets SiC. The formation of a small amount of liquid glassy phase

195

during the sintering process can effectively improve the densification, and therefore,

196

in addition to the physical pinning effect on the grain boundaries between TaC/TaC

197

particles of the SiC phase, the high relative density of the obtained TaC/SiC composite

198

material is partially explained by the low eutectic glassy phase. With introducing a 9

199

liquid phase during the sintering process, the densification of the composite was

200

effectively enhanced, thus partially accounting for the high densified TaC/SiC

201

composite.

202

When the temperature is further increased, as seen from the TEM microstructure

203

images of the TaC/SiC composite sintered under 1800 °C in Figure 5, we can observe

204

that there are very small nanoparticles with a diameter of 2 nm generated in the

205

amorphous layer with a crystal plane space of 0.257 nm, corresponding with the plane

206

space of Ta2Si (0 0 2); combined with the EDS analysis of this area, this suggests that

207

the nanoparticle was Ta2Si. The main reason is that some Ta and Si diffuses into the

208

glassy liquid during sintering and subsequently reprecipitates into nanoparticles with a

209

diameter of 2 nm in the cooling process. In addition, from the morphology change of

210

the SiC, we hypothesize that the elongation of SiC grains also depends on the liquid

211

generated in the sintering process with increasing sintering temperature. 6H-SiC grain

212

partially dissolved in the liquid and then reprecipitated on the direction to elongation,

213

thus reducing the surface energy of SiC and forming a plate-like SiC. This

214

transformation from equiaxed morphology to elongated morphology was found in the

215

SiC/ZrB2 ceramic composite prepared at elevated temperatures, but the reason for the

216

conversion is not mentioned in reference [27]. In PAS sintering, a high energy current

217

encouraged reverse vacancy jumping from the grain boundaries to the exterior of the

218

grains, thus reducing the net vacancy jump frequency [28,29], therefore, enhancing

219

the diffusion of TaC and SiC and contributing to generating a plate-like SiC phase and

220

Ta2Si phase. In any case, pore elimination benefits from the spread of species. 10

221

To examine the microstructure, we can use a schematic diagram to explain the

222

sintering mechanism of the PAS consolidating the TaC/SiC composite in Figure 6. In

223

the initial configuration, TaC particles contact with the SiC additive. Then, the

224

reaction between the oxide layer on the surface of the TaC and SiC was activated,

225

forming the liquid Ta-Si–O–C phase and removing the surface impurities such as F

226

and Ca, the reaction was illustrated as equation (1), this reaction leveled the

227

densification mechanism. Further increasing the sintering temperature, a certain

228

amount of Ta and Si dissolved in the liquid and reprecipitated into nanoparticles with

229

a diameter of 2 nm Ta2Si, as illustrated in Figure 6c and Figure 6d, which was

230

described by equation (2), thus strengthening the glassy area and promoting a better

231

mechanical property.

232

Eutectic Ta2O5 + SiO2  → Ta − Si − C − O(liqiud ) ( SiC / TaC )

233

dissolving (Ta − Si −C − O ) TaC + SiC  → Ta2 Si reprecipitating

234

(2)

235

3.5 Fracture strength

236

Table 2 summarizes the flexural strength of the composite at room temperature. with

237

the sintering temperature increasing, the flexural strength of the material increases.

238

The minimum and maximum flexural strength of the composite sintered at 1500 °C

239

and 1800 °C are 465 MPa and 550 MPa, respectively. The promoted flexural strength

240

demonstrates the enhanced effect of the SiC additive on the TaC ceramic matrix. The

241

fracture morphology is shown in Figure 7. No significant defects were observed at the

242

origin of the crack, and the high uniformity of the microstructure and the dispersion of

(1)

11

243

SiC in the microstructure were verified, thereby increasing the strength of the material.

244

The fracture surface was dominated by interparticle breaks. A small amount of

245

intragranular fracture also occurred.

246

4. Conclusions

247

(1) A high densified TaC/SiC composite with 5 wt.% SiC as an addition agent was

248

prepared by PAS at temperatures from 1500 °C to 1800 °C under 30 MPa for 5

249

minutes, and the density reached 99.3% at 1600 °C.

250

(2) The sintering mechanism of the PAS sintered composites activated the oxide

251

layer on the surface of the TaC and SiC, forming a liquid Si–O–Ta–C phase and

252

cleaning the surface of impurities, further contributing to the densification.

253

(3) The reason for the plate-like growth of SiC was that SiC dissolved in the liquid

254

phase in the sintering process and grew along the elongation direction to reduce the

255

surface energy. Some nanoscaled Ta2Si was observed and was thought to be

256

generated by an increasing amount of Ta and Si dissolving in the liquid and

257

reprecipitating into nanoscaled Ta2Si particles, promoting a flexural strength of

258

550 MPa.

259

Acknowledgements

260

This work was supported by the National Natural Science Foundation of China

261

under Grant Nos. 51202175 and 51521001; the 111 Project under Grant No.

262

B13035; and the Joint Fund under Grant No. 6141A02022255.

263

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of laminated HfC–SiC ceramics under oxyacetylene torch, Corros. Sci. 93 (2015)

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172-179.

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densification of ZrB2-based ultra high temperature ceramics by reactive hot pressing,

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and oxidation behavior of TaC-and HfC-based materials containing short SiC fiber,

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challenging case of TaC-based composites. Composites Part B, 72 (2015) 10–20.

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interfaces: Silicate adlayers on hexagonal (0001) and (0001̄) SiC surfaces. Appl. phy.

355

let. 74 (1994) 1084-1086.

356 357

Figure description:

358

Figure 1. (a) SEM morphology of the TaC and SiC mixture after ball milling, (b)

359

oxide impurities on the SiC surface, (c) oxide impurities on the TaC surface

360

Figure 2. XRD patterns of the TaC/SiC composites sintered under different

361

temperatures

362

Figure 3. SEM images of the microstructures of the TaC/SiC composites sintered

363

under different temperatures. (a) 1500 °C; (b) 1600 °C; (c) 1700 °C; (d) 1800 °C

364

Figure 4. (a) TEM micrographs of the TaC/SiC composite sintered at 1600 °C. (b), (c)

365

The selected area electron diffraction pattern of the TaC and the SiC, respectively. (d)

366

EDS analysis of the glassy phase. (e), (f) HRTEM image of the interface between

367

TaC/SiC and the SiC/glassy phase.

368

Figure 5. (a) TEM micrographs of the TaC/SiC composite sintered at 1800 °C. (b)

369

selected area HRTEM images of triangular grain boundary betwen TaC, SiC and

370

glassy phase. (c) selected area electron diffraction pattern of SiC (d), (e) the inverse

371

FFT images of the selected area and interplanar spacing measurement of this area,

372

respectively.

373

Figure 6. Schematic diagram of the sintering mechanism of the TaC/SiC composite

374

Figure 7. Fracture SEM images of the microstructures of the TaC/SiC composites

375

sintered under different temperatures. (a) 1500 °C; (b) 1600 °C; (c) 1700 °C; (d)

376

1800 °C 17

377

Table description:

378

Table 1 The comparison of fracture strength of TaC-SiC system composites by several

379

methods

380

Table 2 Sintering condition, relative density, and flexural strength of the

381

investigated materials

18

Table 1 Comparison of fracture strength of TaC/SiC system composites by several methods Sintered sample

Additive volume fraction (vol%)

Fracture strength

Hot pressing[33]

TaC - 10 TaSi2 - 15 SiC fibers

432MPa

Hot pressing[33]

TaC - 10 TaSi2 - 15 SiC

506 MPa

TaC - 10 MoSi2 - 15 SiC

348 MPa

TaC - 20 SiC

682 MPa

TaC - 17.78 SiC

550 MPa

Cold isostatically pressing/Hot pressing[34] SPS [21] PAS(this work)

Table 2 Sintering condition, relative density, and flexural strength of the investigated materials Sintering Condition

Relative Density (%)

Flexural Strength σ (MPa)

1500

，5 min

98.4

465

1600

，5 min

99.3

493

1700

，5 min

99.5

526

1800

，5 min

99.8

550

We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of the manuscript entitled “Sintering Mechanism and Microstructure of TaC/SiC Composites Consolidated by Plasma-Activated Sintering”.