Pressureless sintering of boron carbide with Cr3C2 as sintering additive

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ScienceDirect Journal of the European Ceramic Society 34 (2014) 1073–1081

Pressureless sintering of boron carbide with Cr3C2 as sintering additive Xiaoguang Li a,b,∗ , Dongliang Jiang a , Jingxian Zhang a , Qingling Lin a , Zhongming Chen a , Zhengren Huang a a

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China b Graduate University of the Chinese Academy of Sciences, Beijing 100049, China Received 5 May 2013; received in revised form 12 November 2013; accepted 23 November 2013 Available online 23 December 2013

Abstract In this study, chromium carbide (Cr3 C2 ) was selected as the sintering additive for the densification of boron carbide (B4 C). Cr3 C2 can react with B4 C and form graphite and CrB2 in situ, which is considered to be effective for the sintering of B4 C composites. The sintering behavior, microstructure development and mechanical properties of B4 C composites were studied. The density of B4 C composite increased with the increase of Cr3 C2 content and sintering temperature. The formation of liquid phase could effectively improve the densification of B4 C composites. The abnormal grains began to appear at 2080 ◦ C. The bending strength could reach 440 MPa for the 25 wt% and 30 wt% Cr3 C2 samples after sintering at 2070 ◦ C. © 2013 Elsevier Ltd. All rights reserved. Keywords: Boron carbide; Liquid phase sintering; Cr3 C2

1. Introduction B4 C is a covalently bonded compound with extremely high hardness (the third hardest material known after diamond and c-BN), relatively low density (2.52 g/cm3 ) and high neutron absorption cross section. Owing to its outstanding properties, it has been used as wear resistant linings such as sandblasting nozzles, lightweight armor for individual protection, and control rods in nuclear reactors, etc.1,2 However, the application of B4 C is restricted due to the difficulty in attaining high density B4 C ceramics. Nearly full dense B4 C ceramics have been routinely produced by hot pressing (HP) or hot isostatic pressing (HIP). But the application of the HP or HIP is limited due to the simple shaped, small sized and costly product. Compared with HP and HIP, pressureless sintering method is promising to fabricate B4 C ceramics with complex shape and large size at low cost.1,2 At present, the sintering of pure B4 C to high density is difficult through pressureless sintering. Various kinds of sintering aids have been added to obtain high density product,

∗

Corresponding author at: No. 1295, Dingxi Road, Changning District, Shanghai 200050, China. Tel.: +86 21 52412167; fax: +86 21 52413122. E-mail address: [email protected] (X. Li). 0955-2219/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jeurceramsoc.2013.11.036

such as C, SiC, AlF3 , etc.1–9 These sintering aids can obviously improve the properties of B4 C ceramics, though the additives usually increase the specific density. The best known additive, carbon,2,3,10,11 is considered to enhance the densification by removing the negative species, boron oxide, on the B4 C surface at low temperature and forming the eutectic liquid phase at grain boundary at high temperature.2,11 Carbon has usually been introduced in the form of carbonaceous precursor (phenolic resin,2,10 polysaccharide,12 etc.) or carbon black.11,13 Carbon can also be provided through in situ reaction between B4 C and metal carbide additive. The densification and mechanical property can also be improved by simultaneously formed borides. Sigl et al.14 used titanium carbide (TiC) as sintering additive. The performance of B4 C was improved by in situ-formed TiB2 , but postsintering HIP was required to further enhance the densification. Other metal carbides additives also have been reported.15 However, Cr3 C2 has rarely been employed as sintering aid for pressureless sintering of B4 C.16 Due to the relatively low sintering temperature, liquid phase sintering is usually selected as an alternative route for the densification of boron carbide. High density B4 C ceramics had been prepared using Al or Al2 O3 as additives17,18 which was proposed to be able to form liquid phase and thus improve the densification through liquid phase sintering. Yamada et al. chose CrB2 as

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Fig. 1. Photographs of the B4C (a) and Cr3C2 (b) powder.

sintering aid and obtained high density B4 C,19,20,21,22 which was also attributed to liquid phase sintering in the B4 C–CrB2 system. Moreover, The Cr was found to be able to diffuse into the grains of B4 C, which may also contribute to the densification.23 In this work, the sintering of B4 C ceramics using Cr3 C2 as the sintering additive is studied. Slip casting was used to prepare green samples with homogeneous microstructure and high reliability. The sintering behavior and microstructure evolution of B4 C–Cr3 C2 composites were investigated. The liquid phase formation in the B4 C–Cr3 C2 system was also studied.

2. Experimental procedure B4 C powder (Dalian Jinma Group, China) with average particle size of 374 nm and specific surface area of 12.10 m2 /g and Cr3 C2 (Zhuzhou SanLi Carbide Material Co., LTD., China) powder with the particle size in the range of 1.0–1.5 ␮m were used as starting materials. Fig. 1 shows the morphology of the B4 C and Cr3 C2 powders. The B4 C particles are plate-like while the Cr3 C2 particles are more spherical. The addition content of Cr3 C2 was in the range of 5–30 wt% (based on the weight of B4 C). In order to prepare well dispersed slurries, the B4 C powder was firstly treated using acid solutions. The purity of the B4 C powder could reach above 99% after treatment. The as-treated B4 C and Cr3 C2 powder were dispersed and mixed in aqueous media using TMAH (Tetramethyl ammonium hydroxide, Analytical, Sinopharm Chemical Reagent Co., Ltd., China) as the dispersant and ball milled to achieve 50 vol.% slurries using SiC as milling media. After milling, the slurries were cast into plaster mold. The solidified green samples were removed from the mold and dried at 100 ◦ C for 12 h. The as-dried green samples were then calcinated at 900 ◦ C for 1 h in vacuum. Then, the sintering was conducted at the temperatures of 1200 ◦ C, 2030 ◦ C, 2050 ◦ C, 2070 ◦ C, 2080 ◦ C, 2100 ◦ C and 2150 ◦ C respectively for 1 h with the heating rate of 10 ◦ C/min in flow argon atmosphere. In order to study the shrinkage rates of samples with and without Cr3 C2 as additives, the shrinkages of B4 C and B4 C + 10 wt% Cr3 C2 were recorded using a Thermo-OpticalMeasurement Automatic system (Tomac, Fraunhofer Institute for Silicate Research, Germany). The Tomac system was also

used to collect the video pictures of the sample to verify the liquid phase formation during sintering. After sintering, the density of B4 C sample was measured using Archimedes’s method. Phase components were identified using X-ray diffraction (XRD, D/max 2550 V, Rigaku, Japan). The microstructure of the sample was observed using a field emission scanning electron microscopy (FESEM, JSM-6700F, Hitachi, Japan) with an energy dispersive X-ray spectrometer (EDS, INCA, Oxford instruments, Britain) for chemical analysis. The chemical analysis was also carried out using an electron probe micro-analyzer (EPMA, JXA-8100, JEOL, Japan). The three-point flexural strength of the sintered sample was measured using a material testing system (Mold 5566, Instron Corp., UK) with the span width of 30 mm and the crosshead speed of 0.5 mm/min. 3. Result and discussion 3.1. Reactions and components of composites 2B2 O3 + 6C = B4 C + 6CO

(1)

1.5B4 C + Cr3 C2 = 3CrB2 + 3.5C

(2)

XRD patterns show that the as-treated B4 C powder is mainly composed of B4 C, with a small amount of B2 O3 and graphite

Fig. 2. XRD pattern of the as-treated B4 C powder.

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Fig. 3. Calculated Gibbs free energy of reaction (2) as a function of temperature.

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Fig. 5. Shrinkage rates of B4 C and B4 C + 10 wt% Cr3 C2 after sintering at 2150 ◦ C for 1 h.

B4 C powder surface was estimated to be 1.7 wt% based on the oxygen content measurement (1.2 wt%), assuming that all the oxygen in the B4 C powder was in the form of B2 O3 . According to reaction (1), 0.8 wt% of carbon (corresponding to 3.7 wt% Cr3 C2 ) was required for completely removing the oxide. Table 1 shows that the carbon provided by 5 wt% Cr3 C2 is enough to remove the oxide completely. Within the limit of XRD analysis, B2 O3 in the B4 C powder was not detected (Fig. 4), which indicated that the B2 O3 may be partially or totally removed by the original and in situ-formed carbon. Thus, the B4 C composites with the varied contents of Cr3 C2 are composed of B4 C, CrB2 and graphite at the sintering temperature 1200 ◦ C or above. Fig. 4. XRD pattern of the B4 C with 10 wt% Cr3 C2 as sintering aid after sintering at 1200 ◦ C.

(Fig. 2). B4 C powder was fabricated by the carbothermic method as reaction (1) shows.1 It was possible that a little amount of C and B2 O3 remained after the reaction. Since B4 C can be easily oxidized in air and water,2 B2 O3 may also originate from the re-oxidation of B4 C exposed in humid air during handling and processing. Reaction (2) was expected to occur during sintering, which was similar with B4 C with TiC as sintering aid.14 The thermodynamic calculation showed that reaction (2) was favorable in the temperature range of 0–2000 ◦ C (Fig. 3). B4 C, CrB2 and graphite were identified in the sample of B4 C–10 wt% Cr3 C2 after sintering at 1200 ◦ C (Fig. 4), which was in agreement with the calculation in Fig. 3. Based on reaction (2), the yields of graphite and CrB2 with varied contents of Cr3 C2 were calculated, Table 1. It has been accepted that B2 O3 on B4C particles surface will hinder the densification by facilitating particles coarsening at low temperatures.2 The content of B2 O3 on the Table 1 Calculated carbon and CrB2 content based on reaction (2) at the different amount of Cr3C2 (base on B4 C). Before sintering

Cr3C2 (wt%)

5

After sintering

Carbon (wt%) CrB2 (vol.%)

1.1 3.0

10 2.1 5.8

20

25

30

3.9 11.2

4.7 13.7

5.4 16.1

3.2. Densification Fig. 5 shows the shrinkage rates of B4 C and B4 C + 10 wt% Cr3 C2 after sintering at 2150 ◦ C for 1 h (with 1 h soaking at 2000 ◦ C). It can be seen that the shrinkage of B4 C + 10 wt% Cr3 C2 began at 1750 ◦ C, lower than that (1830 ◦ C) of the pure B4 C sample. The densification process is reflected by the shrinkage. The densification of pure B4 C began at 1800 ◦ C in the literature,2 which is close to our measurement (1830 ◦ C). It was reported that the liquid phase in the B4 C–CrB2 –C system could be formed at 1875 ◦ C,24 which may contribute to the densification of B4 C composites as Fig. 5 shows. Meanwhile, the graphite has been generated at 1200 ◦ C. The removal of the surface B2 O3 layer by the graphite after reaction (1) might also take effect on the densification other than the contribution of liquid phase. From Fig. 5, it can be seen that the shrinkage rate (about 5.5%) of B4 C + 10 wt% Cr3 C2 was higher than that (1%) of pure B4 C. The result indicates that Cr3 C2 can effectively improve the densification of B4 C. Tables 1 and 2 show the contents of B4 C, CrB2 and graphite and the theoretical densities of B4 C with the varied contents of Cr3 C2 as sintering aids after sintering. It was apparent that the contents of CrB2 and graphite and the density all increased with the increase of Cr3 C2 content. As shown in Fig. 6, the relative density of B4 C composites increased with the increase of Cr3 C2 content, suggesting that the in situ formed CrB2 and C can help to enhance the densification of B4 C. At the low Cr3C2 content

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Table 2 Calculated theoretical densities of the B4 C ceramics and the volume content of B4 C phase with the different content of Cr3 C2 after sintering. Before sintering

Cr3 C2 (wt%)

After sintering

B4 C (vol.%) Density (g/cm3 )

5 95.7 2.59

10

20

25

30

91.7 2.67

84.1 2.80

80.5 2.86

77.0 2.93

(<20 wt%), the relative density was lower than 90% in the temperature range of 2030–2150 ◦ C. At the higher Cr3 C2 content (>20 wt%), the relative density was higher than 90%. The highest density of about 95% could be reached for the B4 C–30 wt% Cr3 C2 sample sintered at 2100 ◦ C and 2150 ◦ C respectively, or for the B4 C–25 wt% Cr3 C2 sample sintered at 2150 ◦ C. The result shows that at least 20 wt% of Cr3 C2 is required to obtain the relative density above 90% at the sintering temperature above 2030 ◦ C. Fig. 7 shows the microstructure of B4 C with the different contents of Cr3 C2 after sintering at 2030 ◦ C. It showed that the second phase (light color) was homogeneously distributed in the composites (Fig. 7). However, the second phase was in irregular shape, Fig. 7(e), which was quite different from that of the original Cr3 C2 powder, Fig. 1(b). The shape change of second phase may be caused by the formation of liquid phase. As reported in literature, the liquid phase formed at 2015 ◦ C in the B4 C–CrB2 system.20 The liquid phase has been present at 2000 ◦ C, which will be discussed in the following section. In literatures, the change of the grain shape was also attributed to the presence of liquid phase during sintering.19–22 CrB2 , B4 C and graphite were identified in the XRD pattern of the B4 C–30 wt% Cr3 C2 samples. In the back scattering micrograph, the second phase, CrB2 , containing higher atomic number element, Cr, was in light color, while the B4 C containing the lower atomic number elements, B and C, exhibited dark color. A lot of pores still existed in the samples, and abnormal grain growth did not take place. The microstructures of B4 C with the different contents of Cr3 C2 sintered at 2050 ◦ C and 2070 ◦ C were similar to those sintered at 2030 ◦ C. Fig. 8(a) and (b) show the microstructures of B4 C–30 wt% Cr3 C2 after sintering at 2050 ◦ C and 2070 ◦ C, respectively.

Fig. 6. Relative densities of B4 C with the different Cr3 C2 contents (5 wt%, 10 wt%, 20 wt%, 25 wt% and 30 wt%) as a function of sintering temperature.

The abnormal grain growth of B4 C with the different contents of Cr3 C2 began to appear at 2080 ◦ C, Fig. 9. The dark regions marked by red curves in Fig. 9(a) and (b) were B4 C grains. The large dark grains in Fig. 9(c) and (d) were also ascribed to B4 C. For the samples with the low Cr3 C2 content (5 wt% and 10 wt%), a lot of pores were observed inside the grains and at the grain boundaries, and the second phase distributed discretely. At the higher Cr3 C2 content, the porosity decreased greatly and the second phase formed interconnected microstructure. The reduced porosity is in good agreement with the result in Fig. 6. Since the liquid phase can be formed at 2000 ◦ C, the abnormal grain growth and the densification process can be related to liquid phase sintering. At the Cr3 C2 contents of 5 wt% and 10 wt%, liquid phase was not enough to improve the densification of B4 C composites. While at the higher Cr3 C2 contents, the adequate liquid phase formed interconnected network and wetted B4 C particle well to improve the densification. The diffusion between B4 C and CrB2 was also detected at 2050 ◦ C, which will be shown in the following part. The diffusion may also contribute to the densification and grain coarsening. The microstructures of the samples sintered at 2100 ◦ C and 2150 ◦ C are similar, and Fig. 6 displays the density improvement of the samples from 2100 ◦ C to 2150 ◦ C was not significant. Fig. 10 shows the typical microstructures of B4 C with the different contents of Cr3 C2 after sintering at 2150 ◦ C. The abnormal grains were clearly observed in all the samples. The grain size increased considerably, compared with the samples sintered at 2080 ◦ C (Fig. 9). The porosity was obviously reduced with the increase of Cr3 C2 after sintering at 2150 ◦ C, especially for the samples with the Cr3 C2 content increasing from 10 wt% to 20 wt%. The result demonstrates that high Cr3 C2 content can help to reduce the porosity, and to enhance the densification. 3.3. Chemical and phase analysis Fig. 11 shows the EPMA characterization of B4 C–30 wt% Cr3 C2 after sintering at 2050 ◦ C. The unindexed peak belonged to Au (Fig. 11), since Au was sprayed on the surface of the sample before testing. Cr was detected in the dark phase, B4 C, and C was observed in the phase of CrB2 (light color). This might suggest the diffusion between B4 C and CrB2 phase took place. In literatures, Cr was also reported to be able to diffuse into B4 C during sintering,23 and carbon solubility in CrB2 –B4 C and CrB2 –C system was also observed. However, the content of Cr in B4 C and C in CrB2 might be low at 2050 ◦ C, because no obvious peak shift in XRD pattern was evidenced (Supporting Information (1)). The limited diffusion of Cr and C, in certain extent, may contribute to the densification of B4 C. Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jeurceramsoc. 2013.11.036. Fig. 12 shows the microstructure and chemical analysis of B4 C–30 wt% Cr3 C2 after sintering at 2150 ◦ C. The light phase should be CrB2 as described in the above, in which the dark region 1 located (Fig. 12(a)). Chemical analysis of region 1 showed that B, C and Cr all can be detected, which evidently resulted from the dissolution of B4 C into the CrB2 phase. Region

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Fig. 7. Micrographs of B4 C with 5 wt% (a), 10 wt% (b), 20 wt% (c), 25 wt% (d) and 30 wt% (e) Cr3 C2 as sintering aids after sintering at 2030 ◦ C.

2 located at CrB2 phase, which contained B, C, O, Al and Cr (Fig. 12(b)). The C must originate from the dissolved B4 C, and the trace amount of O and Al were from the impurities. Region 3 located at the dark matrix phase, B4 C, in which Al and Cr were observed (Fig. 12(c)). Al also originated from the impurity as the above described. The Cr must be provided by the

CrB2 . The content of Cr was much lower than those of B and C, which revealed that the diffusion of CrB2 into B4 C was not very significant. Fig. 13 shows the XRD pattern of B4 C–30 wt% Cr3 C2 after sintering at 2100 ◦ C. The enlarged XRD peaks at the 2θ ranges of 28.0–29.0◦ and 45.5–46.5◦ show that the (0 0 1) and (1 0 1)

Fig. 8. Micrographs of B4 C–30 wt% Cr3 C2 samples after sintering at 2050 ◦ C (a) and 2070 ◦ C (b).

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Fig. 9. Micrographs of B4 C with 5 wt% Cr3 C2 (a), 10 wt% Cr3 C2 (b), 20 wt% Cr3 C2 (c) and 30 wt% (d) after sintering at 2080 ◦ C.

peak positions of CrB2 obviously shift toward higher diffraction angle (Fig. 13(b) and (c)), compared with the standard spectrum of CrB2 (JCPDS 34-0369). The XRD pattern of Si (standard sample) was also measured for calibration in the same 2θ range as shown in Fig. 13(b). The peak shift toward higher diffraction angle indicates a decrease in the lattice parameter of CrB2 . The dissolution of B4 C probably took place and led to the massive diffusing of B or C into the CrB2 , and subsequently resulted in the change of the chemical composition and the lattice structure of CrB2 . The result agreed with that of Fig. 12(a), in which the dark phase, B4 C, was embedded in the light phase, CrB2 . Meanwhile, the XRD pattern of B4 C did not exhibit great difference from the standard pattern (JCPDS 65-3703) for B4 C–30 wt% Cr2 C3 samples sintered at 2100 ◦ C. As shown in Figs. 11 and 12, Cr was also detected in the B4 C. Nevertheless, the atomic size of Cr was too large to heavily diffuse into B4 C crystal lattice. Therefore, the content of Cr in B4 C may be low, which cannot be reflected on the XRD pattern.

shown in the video. Based on the video, the shrinkage of the sample was observed before 2000 ◦ C. At 2000 ◦ C, the liquid phase appeared after carefully comparing the picture of 2000 ◦ C to that of 1980 ◦ C. The picture of 2000 ◦ C showed the bottom sides of the sample were not perpendicular to the substrate as the pictures of lower temperatures displayed, which indicated liquid phase was present and began to wet the substrate. In the temperature range of 2041–2060 ◦ C, something similar to the bubble appeared at the top left side of the sample, the bottom part of sample was also melting. When the temperature reached 2101 ◦ C or above, the liquid phase could be clearly observed, and wet well the B4 C substrate. The occurrence and wetting of the liquid phase evidently mean that liquid phase formed during pressureless sintering. The liquid phase could improve the densification by enhancing mass transfer and resulted in the abnormal grain growth as discussed in the above sections. Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jeurceramsoc. 2013.11.036.

3.4. Liquid phase sintering 3.5. Flexural strength B4 C–Cr3 C2 sample was prepared for directly observing the liquid formation temperature. The composition of the B4 C–Cr3 C2 sample guaranteed that the molar ratio of B4 C/CrB2 after reaction (2) was 3/7, which was corresponded to the eutectic point in the phase diagram of B4 C–CrB2 as shown in Fig. 14. The sample was placed on a B4 C substrate and observed using the Tomac system. The heating rate was 10 ◦ C/min and the pictures of the sample were captured every 2 min. Video file (Supporting Information (2)) was obtained by collecting the pictures of the sample at different temperatures. Since there is no substantial change before 1200 ◦ C, the pictures only after 1200 ◦ C were

The flexural strength of boron carbide is shown in Fig. 15. Before 2080 ◦ C, the flexural strengths of the samples increased with the increase of the sintering temperature and Cr3 C2 content, especially for the samples with 20 wt% and 30 wt% Cr3 C2 . This was related to the density increase as shown in Fig. 6, which showed the similar trend with the increase in temperature and Cr3 C2 content. The highest strength reached ∼440 MPa for B4 C samples with 30 wt% Cr3 C2 sintered at 2050 ◦ C and 2070 ◦ C. After 2070 ◦ C, the flexural strength decreased sharply and then became almost constant. It was interesting that the

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Fig. 10. Micrographs of B4 C with 5 wt% (a), 10 wt% (b), 20 wt% (c), 25 wt% (d) and 30 wt% (e) Cr3C2 after sintering at 2150 ◦ C.

Fig. 11. EPMA analysis of the dark region (a) and light region (b) in the B4 C–30 wt% Cr3 C2 sample after sintering at 2050 ◦ C.

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Fig. 12. Chemical analysis of the region 1 (a) region 2 (b), and the region 3 (c) in the B4 C–30 wt% Cr3 C2 sample after sintering at 2150 ◦ C.

Fig. 13. XRD pattern of B4 C + 30 wt% Cr3 C2 sintered at 2100 ◦ C (a), and the magnified XRD pattern (b) at the 2θ range of 28.0–29.0◦ , and the magnified XRD pattern (c) at the 2θ range of 45.5–46.5◦ .

flexural strength of the 5 wt% Cr3 C2 was higher than the samples with higher Cr3 C2 content sintered at 2100 ◦ C and 2150 ◦ C. As shown previously, the abnormal grains began to appear at 2080 ◦ C and the grain size increased with the increase of temperature. Thus, the low strength may be due to the presence of abnormal grains.

Fig. 15. Flexural strengths of B4 C with the different Cr3 C2 contents (5 wt%, 10 wt%, 20 wt%, 25 wt%, 30 wt%) after sintering at 2030 ◦ C, 2050 ◦ C, 2070 ◦ C, 2100 ◦ C, 2150 ◦ C.

4. Conclusion

Fig. 14. The phase diagram of the B4 C–CrB2 system.20

The density of B4 C composites increased with the increase of Cr3 C2 content and sintering temperature. The improvement in the densification of B4 C ceramics was related to the formation of liquid phase. At 2100 ◦ C and 2150 ◦ C, the density of boron carbide composite can reach 95% when Cr3 C2 was above 25%. The optimum sintering temperature may be 2070 ◦ C before abnormal

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grain growth occurred. The bending strength can reach 440 MPa of B4 C–30%Cr3 C2 with the density of 93%. Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 50990301, 51072210), Shanghai Science and Technology Committee and the State Key Laboratory of High Performance Ceramics and Superfine Microstructures. References 1. Thevenot F. Boron carbide – a comprehension review. J Eur Ceram Soc 1990;6:205–25. 2. Lee H, Speyer RF. Pressureless sintering of boron carbide. J Am Ceram Soc 2003;86:1468–73. 3. Dole S, Prochazaka S. Densification and microstructure development in boron carbide. In: Proceedings of the 9th annual conference on composites and advanced ceramic materials: ceramic engineering and science proceedings, vol. 6. 1985. p. 1151–60. 4. Bougion M, Thevenot F. Pressureless sintering of boron carbide with an addition of polycarbosilane. J Mater Sci 1987;22:109–14. 5. Lange R, Munir Z, Holt J. Sintering kinetics of pure and doped boron carbide. In: 5th international conference on sintering and related phenomena. 1979. 6. Prochazka S. Dense sintered boron carbide containing beryllium carbide. US Patent 4005235, 1977. 7. Golstein A, Yeshurun Y, Goldenberg A. B4 C/metal boride composites derived from B4 C/metal oxide mixtures. J Eur Ceram Soc 2007;27: 697–700. 8. Zakhariev Z, Radev D. Properties of polycrystalline boron carbide sintered in the presence of W2 B5 without pressing. J Mater Sci Lett 1988;7: 695–6. 9. Goldstein A, Geffen Y, Goldenberg A. Boron carbide–zirconium boride in situ composites by the reactive pressureless sintering of boron carbide–zirconia mixtures. J Am Ceram Soc 2001;84:642–4.

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