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Processing and characterization of fine-grained monolithic SiC ceramic synthesized by spark plasma sintering
Materials Science and Engineering A 527 (2010) 2099–2103
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Processing and characterization of fine-grained monolithic SiC ceramic synthesized by spark plasma sintering Zhao-Hui Zhang ∗ , Fu-Chi Wang, Jie Luo, Shu-Kui Lee, Lu Wang School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China
a r t i c l e
i n f o
Article history: Received 23 September 2009 Received in revised form 19 November 2009 Accepted 14 December 2009
Keywords: Spark plasma sintering SiC Microstructure Mechanical properties
a b s t r a c t Monolithic silicon carbide (SiC) ceramic with good mechanical properties was synthesized by spark plasma sintering (SPS) technique, using granulated SiC powders. The microstructures and the mechanical properties of the ceramics sintered at different temperature were investigated. The results reveal that the SiC ceramic sintered at 1860 ◦ C has relative density of 98.5%, micro-Vickers hardness of 28.5 GPa, bending strength of 395 MPa, and fracture toughness of 4.5 MPa m1/2 . By comparison, due to the present SiC ceramic exhibits fine-grained microstructures and the granulated SiC powders was used during the sintering process, the mechanical properties of the SiC ceramic synthesized by SPS are much better than that prepared by common sintering methods. © 2009 Elsevier B.V. All rights reserved.
1. Introduction SiC ceramic has good physical, chemical and mechanical properties such as high melting point, high hardness, high Young’s modulus, good corrosion resistance, and low density [1–5]. Such properties give to SiC materials a wide application area such as advanced engineering ceramics, aerospace materials, nuclear energy processing materials, and ballistic protection materials [6,7]. Usually, common sintering techniques, such as pressureless sintering, hot pressing (HP), and hot isostatic pressing (HIP) were employed to sinter the monolithic SiC ceramic, however, due to the above-mentioned methods present high sintering temperature, long sintering and cooling time [8,9], the monolithic SiC ceramic exhibits coarse-grained microstructure [10]. As a consequence, the strength and toughness of the monolithic SiC ceramic synthesized by these sintering methods referred above is very low. Thus, the applications of SiC ceramics are rather limited. Spark plasma sintering (SPS) method is a newly developed technique that enables the compacted powder to be fully densified at a comparatively low temperature, and in very short time [11–17]. During the SPS process, a high electric-pulsed current is applied on the electrodes, and the microscopic electrical discharges in the voids between the powder particles generate plasma, causing sin-
∗ Corresponding author. Fax: +86 10 6891 3951. E-mail address: [email protected] (Z.-H. Zhang). 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.12.027
tering [18–20]. Moreover, the spark discharge effect can puncture the oxide film on the surface of the sintered particles easily and improve the crystalline boundary diffusion capacity of the sintered material. Thus, the grain growth is prohibited and the densification is accelerated by rapid heating [21–25]. Hence, fully dense ceramics with higher performance can be achieved using SPS technique at lower sintering temperature in comparison with the conventional sintering process [26–28]. However, so far there are very limited data to reveal the effect of granulation on the microstructures and mechanical properties of the monolithic SiC ceramic prepared by SPS method. Therefore, the primary goal of the present work is to synthesize fine-grained SiC ceramic with good compressive mechanical properties by SPS technique, using granulated SiC powders, and to investigate the microstructures and the mechanical properties of the ceramics sintered at different temperature. 2. Experimental procedure 2.1. Starting powder materials The SiC powders with particle size ranging from 0.5 m to 1 m were used to granulate by fluidized bed binderless granulation method [29]. The specifications and the compositions of the granulated powders are summarized in Tables 1 and 2, respectively. Fig. 1 presents the scanning electronic microscopy (SEM) observations of the powders with different magnifications, indicating that the granules are composed of many fine SiC particles, and the size of the SiC
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Table 1 Specifications of the granulated SiC powders. Average grain size (m)
Specific area (m2 /g)
Apparent density (g/cm3 )
Fluidity (s/30 g)
Green density (g/cm3 )
80–100
12–15
0.75–0.85
≤20
>1.87
Table 2 Compositions of the granulated SiC powders. Elements
Qualifications
SiC
Al
Fe
Mg
Ca
F.C
F.Si
< / 99.4%
> / 0.05%
> / 0.05%
> / 0.05%
> / 0.05%
> / 0.2%
> / 0.2%
granules is about 80 m. The performance test results reveal that the granulated SiC powders have high purity and apparent density, good fluidity and surface activity. 2.2. Sintering parameters DR. SINTER type SPS-3.20 equipment (Sumitomo Coal Mining Co. Ltd., Japan) with pulse duration of 3.3 ms was used. During heating using SPS apparatus, the temperature at the surface of the graphite die was reported to be lower than that of the specimen due to the radiation cooling. Consequently, a hole with diameter and depth of 2 mm and 10 mm, respectively, was punched at the middle of the carbon mold in order to accurately measure the temperature of the sample using an infrared thermometer. A cylindrical graphite die with outside and inside diameters of 80 mm and 40 mm, respectively, was used in a 0.1 Pa vacuum chamber. The applied initial and holding compressive pressure level was 1 MPa and 50 MPa. The sintering temperatures were selected to be 800 ◦ C, 1000 ◦ C, 1600 ◦ C, 1700 ◦ C, 1790 ◦ C, and 1860 ◦ C with a heating rate of 150 ◦ C/min, and the specimens sintered at 800 ◦ C and 1000 ◦ C is used to investigate the sintering mechanism. After soaking the powder at a desired temperature for 5 min, the applied current was reduced, the pressure was released, and the specimen was cooled down to room temperature. 2.3. Characterization tests The density of the bulk compact was measured by Archimedes method. Scanning electronic microscopy (SEM) was used to evaluate the microstructures of the sintered materials. Vickers hardness of the specimen was tested by the LM700AT micro-hardness tester. Bending strength was tested by the three
Fig. 1. SEM observations of the granulated SiC powders at different magnifications (the inset shows a magnified image of the squared area).
point bending test method, the dimension of the specimen was 3 mm × 4 mm × 35 mm with a span of 25 mm. Fracture toughness (KIC) was evaluated by a single edge-notched bending test (SENB), the dimension of the testing bar was 2 mm × 4 mm × 35 mm with a notch of 0.2 mm width and 2 mm depth. Grain morphology analysis was performed from different SEM micrographs by image analysis on no less than 300 grains. 3. Results and discussion 3.1. Relative density and microstructure characteristics The relation between the relative density of SiC ceramics and sintering temperature is plotted in Fig. 2, revealing that the sintering temperature has a great influence on relative density of the SiC ceramics. With the sintering temperature rising from 1600 ◦ C to 1860 ◦ C, the relative density of the bulk compact increases from 65.1% to 98.5%, with much of the densification occurring by the time the temperature reaches 1700 ◦ C. When the sintering temperature exceeds 1790 ◦ C, the density continues to increase, but at a slower rate. The SiC ceramic sintered at 1860 ◦ C has the highest relative density of 98.5%. By comparison, the relative density of the SiC ceramic sintered at the same temperature (1860 ◦ C) is only 80.2%, using the fine SiC powders without granulating process. Therefore, use of the granulated SiC powders can effectively accelerate the densification process. Fig. 3 shows the microstructure of the SiC ceramics sintered at different temperature. Large numbers of pores were observed on the fracture surface of the ceramic sintered at 1600 ◦ C. Correspondingly, the relative density of the ceramic sintered at this temperature is only 65.1%. The porosity of the ceramic decreases with sintering temperature rising from 1600 ◦ C to 1860 ◦ C. Few pores were detected on the fracture surface of the ceramic sintered at 1860 ◦ C, making the compact nearly have the full density.
Fig. 2. Influence of sintering temperature on relative density.
Z.-H. Zhang et al. / Materials Science and Engineering A 527 (2010) 2099–2103
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Fig. 3. Microstructure of the SiC ceramics sintered at different temperature of: (a) 1600 ◦ C, (b) 1700 ◦ C, (c) 1790 ◦ C, and (d) 1860 ◦ C.
Fig. 4. Microstructure of the SiC ceramics sintered at low temperature of: (a) 800 ◦ C and (b) 1000 ◦ C.
erties of ceramics, the SiC ceramic synthesized by SPS technique should have good mechanical properties. The SEM image of the SiC ceramics sintered at low temperatures by SPS is shown in Fig. 4. Many sintering necks can be observed on the fracture surface of the SiC ceramics sintered at 800 ◦ C and 1000 ◦ C. In the initial sintering stage, the ceramic powders are loose, however, due to the strong pulse direct current, the discharge channels are quickly formed between the contiguous ceramic particles. Subsequently, the spark discharge occurs, leading to the breakdown of the oxide film on the surface of the ceramic particles and sharp
Grain size test results reveal that the average grain size of the SiC grains change from 1.25 m to 2.05 m, with sintering temperature rising from 1600 ◦ C to 1860 ◦ C. The main reason that the SiC ceramics synthesized by SPS process have fine grains is that the sintering process is very fast, and the total sintering time and cooling time would not exceed 18 min and 20 min, respectively. Moreover, compared with other sintering techniques, the sintering temperature is reduced due to the special sintering technique. Thus, the grain growth is effectively prohibited during the SPS process. Because grain size has important influence on the mechanical prop-
Table 3 A summary of properties determined in this study and the literatures. Sample
Sintering technique
Sintering temperature (◦ C)
Relative density (%)
Vickers hardness (GPa)
SiC (our work)
SPS LPS
1860 1950
98.5 98
28.5 17.5
SiC (work from others)
Pressureless SPS
2000 1900
93.4 97.8
22.0 254
Bending strength (MPa)
Fracture toughness (MPa m1/2 )
395
4.5 3.3 4.0 3.2
2102
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Fig. 5. Influence of sintering temperature on Vickers hardness.
increase in the local temperature. Thus, the ceramic particles are activated and the atoms diffusivity of the particles is enhanced rapidly. Then the sintering necks will emerge and grow due to the combined effects of evaporation and condensation. The formation of the sintering necks at early sintering process is very useful to the densification of the ceramic powders. Therefore, the spark discharge effect is the main factor to densify the SiC powders at a relatively low temperature and short sintering time, using SPS technique. 3.2. Mechanical properties The relation between Vickers hardness (Hv1 ) of the SiC ceramics and sintering temperature is presented in Fig. 5. The plot reveals that with raising sintering temperature, the Vickers hardness increase. When sintered at 1600 ◦ C, the Vickers hardness of the ceramic is less than 5 GPa, however, with raising sintering temperature to 1700 ◦ C, the Vickers hardness reaches 20.6 GPa. After that, the Vickers hardness increases at a slow rate with increase in the sintering temperature, and the SiC ceramic sintered at 1860 ◦ C has the highest Vickers hardness of 28.5 GPa. Fig. 6 shows the influence of the sintering temperature on bending strength and fracture toughness of the sintered SiC ceramics. With raising sintering temperature, both bending strength and fracture toughness increase. However, the increasing degree of the bending strength reduces obviously. When sintered at 1600 ◦ C, the bending strength and the fracture toughness of the ceramic are only 78 MPa, 1.65 MPa m1/2 , respectively. By comparison, the ceramic
sintered at 1860 ◦ C has the highest bending strength of 395 MPa and the highest fracture toughness of 4.5 MPa m1/2 . In ceramic materials, grain size and typical flaw structures such as pore and micro-crack have obvious influence on the mechanical properties. Usually, strength increases with decreasing porosity and reducing grain size. However, the grain growth of the SiC is prohibited effectively during the SPS process by reducing sintering temperature and sintering time, due to the special sintering technique. In addition, the use of the spherical granulated SiC powders makes the spark plasma sintering process more efficient by improving the fluidity of the powders. As a result, the average grain size of SiC has no obvious change within the range of the above-mentioned sintering temperatures. Therefore, the bending strength of the ceramic is mainly determined by the relative density. Thus, rise in sintering temperature can increase bending strength remarkably due to the densification increase. Fracture toughness is a property that materials resist to the propagating of cracks. Increase of sintering temperature makes the SiC ceramic more dense, porosity decreases, hence fracture toughness increases. In addition, fine grains lead to much grain boundaries, which can impede the crack propagating by the mechanism of crack deflection, with absorbing much energy of micro-crack expansion, fracture toughness increases obviously. Table 3 presents a comparison of mechanical properties of SiC ceramic determined in this study and the literatures. Both the bending strength and the fracture toughness of the monolithic SiC ceramic synthesized by SPS process using granulated SiC powders are better than that of the SiC ceramic provided by the literatures [30–32]. Moreover, the hardness of the ceramic reaches a fairly high level. 4. Conclusions In this study, the fine-grained monolithic SiC ceramic with nearly full density was synthesized by SPS method, using granulated SiC powders. The microstructures and the mechanical properties of the SiC ceramic sintered at different temperature were investigated. The results reveal that the SiC ceramic sintered at 1860 ◦ C has a relative density of 98.5%, Vickers hardness of 28.5 GPa, bending strength of 395 MPa, and fracture toughness of 4.5 MPa m1/2 . By comparison, the mechanical properties of the SiC ceramic synthesized by SPS are much better than that prepared by common sintering method. It is suggested that the fine-grained microstructure and the use of the granulated SiC powders are the main factors to improve the mechanical properties of the SiC ceramic synthesized by SPS process. Acknowledgements The authors wish to thank Dr. X.D. Yu and Dr. L. Wang for their contributions to the investigation. And the authors also would like to express gratitude to the unknown reviewers for their constructive comments on the original manuscript. The study was supported by program for Peking excellent talents in university under grant number 20061D0503200316 and the National Defense Pre-Research Foundation of China under grant number of 9140A12050209BQ0137. References
Fig. 6. Bending strength and fracture toughness of the ceramics sintered at different temperature.
[1] A. Maitre, A. Vande Put, J.P. Laval, S. Valette, G. Trolliard, J. Eur. Ceram. Soc. 28 (2008) 1881–1890. [2] M. Singh, J.A. Salem, J. Eur. Ceram. Soc. 22 (2002) 2709–2717. [3] Q.W. Huang, L.H. Zhu, Mater. Lett. 59 (2005) 1732–1735. [4] J. Sanchez-Gonzalez, A.L. Ortiz, F. Guiberteau, J. Eur. Ceram. Soc. 27 (2007) 3935–3939.
Z.-H. Zhang et al. / Materials Science and Engineering A 527 (2010) 2099–2103 [5] F.M. Varela-Feria, J. Martinez-Fernandez, A.R. Arellano-Lopez, M. Singh, J. Eur. Ceram. Soc. 22 (2002) 2719–2725. [6] M.F. Zawrah, M. El-Gazery, Mater. Chem. Phys. 106 (2007) 330–337. [7] P. Forquin, C. Denoual, C.E. Cottenot, F. Hild, Mech. Mater. 35 (2003) 987–1002. [8] M. Castillo-Rodriguez, A. Munoz, J. Eur. Ceram. Soc. 26 (2006) 2397–2405. [9] J. Ihle, M. Herrmann, J. Adler, J. Eur. Ceram. Soc. 25 (2005) 997–1003. [10] D.I. Cheong, J. Kim, S.L. Kang, J. Eur. Ceram. Soc. 22 (2005) 1321–1327. [11] Z.H. Zhang, F.C. Wang, S.K. Li, Y. Liu, J.W. Cheng, Y. Liang, Mater. Sci. Eng. A 523 (2009) 134–138. [12] R. Orru, R. Licheri, A.M. Locci, A. Cincotti, G. Cao, Mater. Sci. Eng. R 63 (2009) 127–287. [13] Z.H. Zhang, F.C. Wang, L. Wang, S.K. Li, Mater. Lett. 62 (2008) 3987–3990. [14] Z.H. Zhang, F.C. Wang, S.K. Li, M.W. Shen, S. Osamu, Mater. Charact. 59 (2008) 329–333. [15] U. Anselmi-Tamburini, J.E. Garay, Z.A. Munir, Scr. Mater. 54 (2006) 823–828. [16] W. Chen, U. Anselmi-Tamburini, J.E. Garay, J.R. Groza, Z.A. Munir, Mater. Sci. Eng. A 394 (2005) 132–138. [17] V. Trombini, E.M.J.A. Pallone, U. Anselmi-Tamburini, Z.A. Munir, R. Tomasi, Mater. Sci. Eng. A 501 (2009) 26–29. [18] C.Q. Hong, X.H. Zhang, W.J. Li, J.C. Han, S.H. Meng, Mater. Sci. Eng. A 498 (2008) 437–441.
2103
[19] P. Angerer, J. Wosik, E. Neubauer, L.G. Yu, G.E. Nauer, K.A. Khor, Int. J. Refract. Met. Hard. Mater. 27 (2009) 105–110. [20] P. Angerer, W. Artner, E. Neubauer, L.G. Yu, K.A. Khor, Int. J. Refract. Met. Hard. Mater. 26 (2008) 312–317. [21] Z.H. Zhang, F.C. Wang, L. Wang, Mater. Sci. Eng. A 476 (2008) 201–205. [22] C. Shearwood, Y.Q. Fu, L. Yu, K.A. Khor, Scr. Mater. 52 (2005) 455–460. [23] G.S. Kim, D.H. Shin, Y.I. Seo, Y.D. Kim, Mater. Charact. 59 (2008) 1201–1205. [24] R. Kumar, K.H. Prakash, Acta Mater. 53 (2005) 2327–2335. [25] K.H. Kim, K.B. Shim, Mater. Charact. 50 (2003) 31–37. [26] M. Eriksson, D. Salamon, M. Nygren, Z.J. Shen, Mater. Sci. Eng. A 475 (2008) 101–104. [27] F.C. Wang, Z.H. Zhang, J. Luo, C.C. Huang, S.K. Lee, Compos. Sci. Technol. 69 (2009) 2682–2687. [28] A. Bellosi, F. Monteverde, D. Sciti, Int. J. Appl. Ceram. Technol. 3 (2006) 32–40. [29] N. Kazuo, H. Masayuki, Powder Technol. 130 (2003) 199–202. [30] A.L. Ortiz, A. Munoz-Bernabe, O. Borrero-Lopez, A. Dominguez-Rodriguez, F. Guiberteau, N.P. Padture, J. Eur. Ceram. Soc. 24 (2004) 3245–3249. [31] L. Sea-Hoon, T. Hidehiko, K. Yutaka, J. Eur. Ceram. Soc. 29 (2009) 2087–2095. [32] T. Zhang, Z.Q. Zhang, J.X. Zhang, D.L. Jiang, Q.L. Lin, Mater. Sci. Eng. A 443 (2007) 257–261.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Processing and characterization of fine-grained monolithic SiC ceramic synthesized by spark plasma sintering Zhao-Hui Zhang ∗ , Fu-Chi Wang, Jie Luo, Shu-Kui Lee, Lu Wang School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China
a r t i c l e
i n f o
Article history: Received 23 September 2009 Received in revised form 19 November 2009 Accepted 14 December 2009
Keywords: Spark plasma sintering SiC Microstructure Mechanical properties
a b s t r a c t Monolithic silicon carbide (SiC) ceramic with good mechanical properties was synthesized by spark plasma sintering (SPS) technique, using granulated SiC powders. The microstructures and the mechanical properties of the ceramics sintered at different temperature were investigated. The results reveal that the SiC ceramic sintered at 1860 ◦ C has relative density of 98.5%, micro-Vickers hardness of 28.5 GPa, bending strength of 395 MPa, and fracture toughness of 4.5 MPa m1/2 . By comparison, due to the present SiC ceramic exhibits fine-grained microstructures and the granulated SiC powders was used during the sintering process, the mechanical properties of the SiC ceramic synthesized by SPS are much better than that prepared by common sintering methods. © 2009 Elsevier B.V. All rights reserved.
1. Introduction SiC ceramic has good physical, chemical and mechanical properties such as high melting point, high hardness, high Young’s modulus, good corrosion resistance, and low density [1–5]. Such properties give to SiC materials a wide application area such as advanced engineering ceramics, aerospace materials, nuclear energy processing materials, and ballistic protection materials [6,7]. Usually, common sintering techniques, such as pressureless sintering, hot pressing (HP), and hot isostatic pressing (HIP) were employed to sinter the monolithic SiC ceramic, however, due to the above-mentioned methods present high sintering temperature, long sintering and cooling time [8,9], the monolithic SiC ceramic exhibits coarse-grained microstructure [10]. As a consequence, the strength and toughness of the monolithic SiC ceramic synthesized by these sintering methods referred above is very low. Thus, the applications of SiC ceramics are rather limited. Spark plasma sintering (SPS) method is a newly developed technique that enables the compacted powder to be fully densified at a comparatively low temperature, and in very short time [11–17]. During the SPS process, a high electric-pulsed current is applied on the electrodes, and the microscopic electrical discharges in the voids between the powder particles generate plasma, causing sin-
∗ Corresponding author. Fax: +86 10 6891 3951. E-mail address: [email protected] (Z.-H. Zhang). 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.12.027
tering [18–20]. Moreover, the spark discharge effect can puncture the oxide film on the surface of the sintered particles easily and improve the crystalline boundary diffusion capacity of the sintered material. Thus, the grain growth is prohibited and the densification is accelerated by rapid heating [21–25]. Hence, fully dense ceramics with higher performance can be achieved using SPS technique at lower sintering temperature in comparison with the conventional sintering process [26–28]. However, so far there are very limited data to reveal the effect of granulation on the microstructures and mechanical properties of the monolithic SiC ceramic prepared by SPS method. Therefore, the primary goal of the present work is to synthesize fine-grained SiC ceramic with good compressive mechanical properties by SPS technique, using granulated SiC powders, and to investigate the microstructures and the mechanical properties of the ceramics sintered at different temperature. 2. Experimental procedure 2.1. Starting powder materials The SiC powders with particle size ranging from 0.5 m to 1 m were used to granulate by fluidized bed binderless granulation method [29]. The specifications and the compositions of the granulated powders are summarized in Tables 1 and 2, respectively. Fig. 1 presents the scanning electronic microscopy (SEM) observations of the powders with different magnifications, indicating that the granules are composed of many fine SiC particles, and the size of the SiC
2100
Z.-H. Zhang et al. / Materials Science and Engineering A 527 (2010) 2099–2103
Table 1 Specifications of the granulated SiC powders. Average grain size (m)
Specific area (m2 /g)
Apparent density (g/cm3 )
Fluidity (s/30 g)
Green density (g/cm3 )
80–100
12–15
0.75–0.85
≤20
>1.87
Table 2 Compositions of the granulated SiC powders. Elements
Qualifications
SiC
Al
Fe
Mg
Ca
F.C
F.Si
< / 99.4%
> / 0.05%
> / 0.05%
> / 0.05%
> / 0.05%
> / 0.2%
> / 0.2%
granules is about 80 m. The performance test results reveal that the granulated SiC powders have high purity and apparent density, good fluidity and surface activity. 2.2. Sintering parameters DR. SINTER type SPS-3.20 equipment (Sumitomo Coal Mining Co. Ltd., Japan) with pulse duration of 3.3 ms was used. During heating using SPS apparatus, the temperature at the surface of the graphite die was reported to be lower than that of the specimen due to the radiation cooling. Consequently, a hole with diameter and depth of 2 mm and 10 mm, respectively, was punched at the middle of the carbon mold in order to accurately measure the temperature of the sample using an infrared thermometer. A cylindrical graphite die with outside and inside diameters of 80 mm and 40 mm, respectively, was used in a 0.1 Pa vacuum chamber. The applied initial and holding compressive pressure level was 1 MPa and 50 MPa. The sintering temperatures were selected to be 800 ◦ C, 1000 ◦ C, 1600 ◦ C, 1700 ◦ C, 1790 ◦ C, and 1860 ◦ C with a heating rate of 150 ◦ C/min, and the specimens sintered at 800 ◦ C and 1000 ◦ C is used to investigate the sintering mechanism. After soaking the powder at a desired temperature for 5 min, the applied current was reduced, the pressure was released, and the specimen was cooled down to room temperature. 2.3. Characterization tests The density of the bulk compact was measured by Archimedes method. Scanning electronic microscopy (SEM) was used to evaluate the microstructures of the sintered materials. Vickers hardness of the specimen was tested by the LM700AT micro-hardness tester. Bending strength was tested by the three
Fig. 1. SEM observations of the granulated SiC powders at different magnifications (the inset shows a magnified image of the squared area).
point bending test method, the dimension of the specimen was 3 mm × 4 mm × 35 mm with a span of 25 mm. Fracture toughness (KIC) was evaluated by a single edge-notched bending test (SENB), the dimension of the testing bar was 2 mm × 4 mm × 35 mm with a notch of 0.2 mm width and 2 mm depth. Grain morphology analysis was performed from different SEM micrographs by image analysis on no less than 300 grains. 3. Results and discussion 3.1. Relative density and microstructure characteristics The relation between the relative density of SiC ceramics and sintering temperature is plotted in Fig. 2, revealing that the sintering temperature has a great influence on relative density of the SiC ceramics. With the sintering temperature rising from 1600 ◦ C to 1860 ◦ C, the relative density of the bulk compact increases from 65.1% to 98.5%, with much of the densification occurring by the time the temperature reaches 1700 ◦ C. When the sintering temperature exceeds 1790 ◦ C, the density continues to increase, but at a slower rate. The SiC ceramic sintered at 1860 ◦ C has the highest relative density of 98.5%. By comparison, the relative density of the SiC ceramic sintered at the same temperature (1860 ◦ C) is only 80.2%, using the fine SiC powders without granulating process. Therefore, use of the granulated SiC powders can effectively accelerate the densification process. Fig. 3 shows the microstructure of the SiC ceramics sintered at different temperature. Large numbers of pores were observed on the fracture surface of the ceramic sintered at 1600 ◦ C. Correspondingly, the relative density of the ceramic sintered at this temperature is only 65.1%. The porosity of the ceramic decreases with sintering temperature rising from 1600 ◦ C to 1860 ◦ C. Few pores were detected on the fracture surface of the ceramic sintered at 1860 ◦ C, making the compact nearly have the full density.
Fig. 2. Influence of sintering temperature on relative density.
Z.-H. Zhang et al. / Materials Science and Engineering A 527 (2010) 2099–2103
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Fig. 3. Microstructure of the SiC ceramics sintered at different temperature of: (a) 1600 ◦ C, (b) 1700 ◦ C, (c) 1790 ◦ C, and (d) 1860 ◦ C.
Fig. 4. Microstructure of the SiC ceramics sintered at low temperature of: (a) 800 ◦ C and (b) 1000 ◦ C.
erties of ceramics, the SiC ceramic synthesized by SPS technique should have good mechanical properties. The SEM image of the SiC ceramics sintered at low temperatures by SPS is shown in Fig. 4. Many sintering necks can be observed on the fracture surface of the SiC ceramics sintered at 800 ◦ C and 1000 ◦ C. In the initial sintering stage, the ceramic powders are loose, however, due to the strong pulse direct current, the discharge channels are quickly formed between the contiguous ceramic particles. Subsequently, the spark discharge occurs, leading to the breakdown of the oxide film on the surface of the ceramic particles and sharp
Grain size test results reveal that the average grain size of the SiC grains change from 1.25 m to 2.05 m, with sintering temperature rising from 1600 ◦ C to 1860 ◦ C. The main reason that the SiC ceramics synthesized by SPS process have fine grains is that the sintering process is very fast, and the total sintering time and cooling time would not exceed 18 min and 20 min, respectively. Moreover, compared with other sintering techniques, the sintering temperature is reduced due to the special sintering technique. Thus, the grain growth is effectively prohibited during the SPS process. Because grain size has important influence on the mechanical prop-
Table 3 A summary of properties determined in this study and the literatures. Sample
Sintering technique
Sintering temperature (◦ C)
Relative density (%)
Vickers hardness (GPa)
SiC (our work)
SPS LPS
1860 1950
98.5 98
28.5 17.5
SiC (work from others)
Pressureless SPS
2000 1900
93.4 97.8
22.0 254
Bending strength (MPa)
Fracture toughness (MPa m1/2 )
395
4.5 3.3 4.0 3.2
2102
Z.-H. Zhang et al. / Materials Science and Engineering A 527 (2010) 2099–2103
Fig. 5. Influence of sintering temperature on Vickers hardness.
increase in the local temperature. Thus, the ceramic particles are activated and the atoms diffusivity of the particles is enhanced rapidly. Then the sintering necks will emerge and grow due to the combined effects of evaporation and condensation. The formation of the sintering necks at early sintering process is very useful to the densification of the ceramic powders. Therefore, the spark discharge effect is the main factor to densify the SiC powders at a relatively low temperature and short sintering time, using SPS technique. 3.2. Mechanical properties The relation between Vickers hardness (Hv1 ) of the SiC ceramics and sintering temperature is presented in Fig. 5. The plot reveals that with raising sintering temperature, the Vickers hardness increase. When sintered at 1600 ◦ C, the Vickers hardness of the ceramic is less than 5 GPa, however, with raising sintering temperature to 1700 ◦ C, the Vickers hardness reaches 20.6 GPa. After that, the Vickers hardness increases at a slow rate with increase in the sintering temperature, and the SiC ceramic sintered at 1860 ◦ C has the highest Vickers hardness of 28.5 GPa. Fig. 6 shows the influence of the sintering temperature on bending strength and fracture toughness of the sintered SiC ceramics. With raising sintering temperature, both bending strength and fracture toughness increase. However, the increasing degree of the bending strength reduces obviously. When sintered at 1600 ◦ C, the bending strength and the fracture toughness of the ceramic are only 78 MPa, 1.65 MPa m1/2 , respectively. By comparison, the ceramic
sintered at 1860 ◦ C has the highest bending strength of 395 MPa and the highest fracture toughness of 4.5 MPa m1/2 . In ceramic materials, grain size and typical flaw structures such as pore and micro-crack have obvious influence on the mechanical properties. Usually, strength increases with decreasing porosity and reducing grain size. However, the grain growth of the SiC is prohibited effectively during the SPS process by reducing sintering temperature and sintering time, due to the special sintering technique. In addition, the use of the spherical granulated SiC powders makes the spark plasma sintering process more efficient by improving the fluidity of the powders. As a result, the average grain size of SiC has no obvious change within the range of the above-mentioned sintering temperatures. Therefore, the bending strength of the ceramic is mainly determined by the relative density. Thus, rise in sintering temperature can increase bending strength remarkably due to the densification increase. Fracture toughness is a property that materials resist to the propagating of cracks. Increase of sintering temperature makes the SiC ceramic more dense, porosity decreases, hence fracture toughness increases. In addition, fine grains lead to much grain boundaries, which can impede the crack propagating by the mechanism of crack deflection, with absorbing much energy of micro-crack expansion, fracture toughness increases obviously. Table 3 presents a comparison of mechanical properties of SiC ceramic determined in this study and the literatures. Both the bending strength and the fracture toughness of the monolithic SiC ceramic synthesized by SPS process using granulated SiC powders are better than that of the SiC ceramic provided by the literatures [30–32]. Moreover, the hardness of the ceramic reaches a fairly high level. 4. Conclusions In this study, the fine-grained monolithic SiC ceramic with nearly full density was synthesized by SPS method, using granulated SiC powders. The microstructures and the mechanical properties of the SiC ceramic sintered at different temperature were investigated. The results reveal that the SiC ceramic sintered at 1860 ◦ C has a relative density of 98.5%, Vickers hardness of 28.5 GPa, bending strength of 395 MPa, and fracture toughness of 4.5 MPa m1/2 . By comparison, the mechanical properties of the SiC ceramic synthesized by SPS are much better than that prepared by common sintering method. It is suggested that the fine-grained microstructure and the use of the granulated SiC powders are the main factors to improve the mechanical properties of the SiC ceramic synthesized by SPS process. Acknowledgements The authors wish to thank Dr. X.D. Yu and Dr. L. Wang for their contributions to the investigation. And the authors also would like to express gratitude to the unknown reviewers for their constructive comments on the original manuscript. The study was supported by program for Peking excellent talents in university under grant number 20061D0503200316 and the National Defense Pre-Research Foundation of China under grant number of 9140A12050209BQ0137. References
Fig. 6. Bending strength and fracture toughness of the ceramics sintered at different temperature.
[1] A. Maitre, A. Vande Put, J.P. Laval, S. Valette, G. Trolliard, J. Eur. Ceram. Soc. 28 (2008) 1881–1890. [2] M. Singh, J.A. Salem, J. Eur. Ceram. Soc. 22 (2002) 2709–2717. [3] Q.W. Huang, L.H. Zhu, Mater. Lett. 59 (2005) 1732–1735. [4] J. Sanchez-Gonzalez, A.L. Ortiz, F. Guiberteau, J. Eur. Ceram. Soc. 27 (2007) 3935–3939.
Z.-H. Zhang et al. / Materials Science and Engineering A 527 (2010) 2099–2103 [5] F.M. Varela-Feria, J. Martinez-Fernandez, A.R. Arellano-Lopez, M. Singh, J. Eur. Ceram. Soc. 22 (2002) 2719–2725. [6] M.F. Zawrah, M. El-Gazery, Mater. Chem. Phys. 106 (2007) 330–337. [7] P. Forquin, C. Denoual, C.E. Cottenot, F. Hild, Mech. Mater. 35 (2003) 987–1002. [8] M. Castillo-Rodriguez, A. Munoz, J. Eur. Ceram. Soc. 26 (2006) 2397–2405. [9] J. Ihle, M. Herrmann, J. Adler, J. Eur. Ceram. Soc. 25 (2005) 997–1003. [10] D.I. Cheong, J. Kim, S.L. Kang, J. Eur. Ceram. Soc. 22 (2005) 1321–1327. [11] Z.H. Zhang, F.C. Wang, S.K. Li, Y. Liu, J.W. Cheng, Y. Liang, Mater. Sci. Eng. A 523 (2009) 134–138. [12] R. Orru, R. Licheri, A.M. Locci, A. Cincotti, G. Cao, Mater. Sci. Eng. R 63 (2009) 127–287. [13] Z.H. Zhang, F.C. Wang, L. Wang, S.K. Li, Mater. Lett. 62 (2008) 3987–3990. [14] Z.H. Zhang, F.C. Wang, S.K. Li, M.W. Shen, S. Osamu, Mater. Charact. 59 (2008) 329–333. [15] U. Anselmi-Tamburini, J.E. Garay, Z.A. Munir, Scr. Mater. 54 (2006) 823–828. [16] W. Chen, U. Anselmi-Tamburini, J.E. Garay, J.R. Groza, Z.A. Munir, Mater. Sci. Eng. A 394 (2005) 132–138. [17] V. Trombini, E.M.J.A. Pallone, U. Anselmi-Tamburini, Z.A. Munir, R. Tomasi, Mater. Sci. Eng. A 501 (2009) 26–29. [18] C.Q. Hong, X.H. Zhang, W.J. Li, J.C. Han, S.H. Meng, Mater. Sci. Eng. A 498 (2008) 437–441.
2103
[19] P. Angerer, J. Wosik, E. Neubauer, L.G. Yu, G.E. Nauer, K.A. Khor, Int. J. Refract. Met. Hard. Mater. 27 (2009) 105–110. [20] P. Angerer, W. Artner, E. Neubauer, L.G. Yu, K.A. Khor, Int. J. Refract. Met. Hard. Mater. 26 (2008) 312–317. [21] Z.H. Zhang, F.C. Wang, L. Wang, Mater. Sci. Eng. A 476 (2008) 201–205. [22] C. Shearwood, Y.Q. Fu, L. Yu, K.A. Khor, Scr. Mater. 52 (2005) 455–460. [23] G.S. Kim, D.H. Shin, Y.I. Seo, Y.D. Kim, Mater. Charact. 59 (2008) 1201–1205. [24] R. Kumar, K.H. Prakash, Acta Mater. 53 (2005) 2327–2335. [25] K.H. Kim, K.B. Shim, Mater. Charact. 50 (2003) 31–37. [26] M. Eriksson, D. Salamon, M. Nygren, Z.J. Shen, Mater. Sci. Eng. A 475 (2008) 101–104. [27] F.C. Wang, Z.H. Zhang, J. Luo, C.C. Huang, S.K. Lee, Compos. Sci. Technol. 69 (2009) 2682–2687. [28] A. Bellosi, F. Monteverde, D. Sciti, Int. J. Appl. Ceram. Technol. 3 (2006) 32–40. [29] N. Kazuo, H. Masayuki, Powder Technol. 130 (2003) 199–202. [30] A.L. Ortiz, A. Munoz-Bernabe, O. Borrero-Lopez, A. Dominguez-Rodriguez, F. Guiberteau, N.P. Padture, J. Eur. Ceram. Soc. 24 (2004) 3245–3249. [31] L. Sea-Hoon, T. Hidehiko, K. Yutaka, J. Eur. Ceram. Soc. 29 (2009) 2087–2095. [32] T. Zhang, Z.Q. Zhang, J.X. Zhang, D.L. Jiang, Q.L. Lin, Mater. Sci. Eng. A 443 (2007) 257–261.