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Zirconia reinforced reaction bonded silicon carbide composites: Microstructure and mechanical properties
Int. Journal of Refractory Metals and Hard Materials 35 (2012) 257–261
Contents lists available at SciVerse ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Zirconia reinforced reaction bonded silicon carbide composites: Microstructure and mechanical properties Shuang Li ⁎, Yumin Zhang, Jiecai Han, Yufeng Zhou Center for Composite Materials, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 9 March 2012 Accepted 25 June 2012 Keywords: Reaction bonding Fracture toughness Transformation toughening Residual silicon Grain size
a b s t r a c t Nano-sized zirconia particles were dispersed in a SiC/C suspension to examine the particle influence on microstructure and mechanical properties on the resulting composite. The ZrO2/reaction bonded SiC composite was produced by a combination of slip casting and liquid silicon infiltration. By dispersing zirconia particles into the matrix, the gowth of β-SiC was inhibited and the size of residual silicon was reduced. This result led to an increase of flexural strength with reinforcement ranging from 0 to 20 wt.%. Compared with the monolithic RBSC ceramics, the fracture toughness was steadily improved, reaching the peak value of appropriate 4.6 MPa m1/2 at the zirconia fraction of 25 wt.%. The XRD analysis revealed the phase transformation occurred on the fracture surface of the composites. Thus, the transformation of zirconia and the crack deflection are considered as the main toughening mechanisms. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Reaction bonded silicon carbide (RBSC) ceramics are attractive materials for high-temperature heat exchanger, piping, light armor and optical mirror, due to their low sintering temperature, near-net shaping, full densification and low fabrication cost [1–3]. Most commonly, the RBSC ceramics are prepared by infiltration of compacted SiC/C mixtures or carbon preforms with molten silicon. The reaction between carbon and molten silicon leads to the formation of β-SiC, which interconnects the original SiC particles and forms a continuous ceramic skeleton [4]. So the typical RBSC ceramics are composed of α-SiC, β-SiC and Si phases. Due to the high brittleness and low melting temperature, the residual silicon phase is a harmful constituent for the mechanical behaviors of RBSC ceramics, especially in the hightemperature fields [5]. To date, the inherent low fracture toughness of RBSC ceramics still limits their use in the large-scale, complex shaped and lightweight components. Various investigations have focused on the improvement of mechanical properties and processing techniques of RBSC ceramics [6–8]. Aroati et al. [4] used boron carbide (B4C) as an alternative source of carbon, which could significantly reduce the silicon fraction and density of the ceramics. Esfehanian et al. [9] developed Si–Ti– MoSi2 alloyed melt to infiltrate the carbon/carbon fiber materials. The alloy-infiltrated composites showed more resistance against creep deformation and higher strengths than silicon-infiltrated composites. Fine original SiC particles are also in favor of the preparation of high strength RBSC ceramics. Toshiba Corporation [10] exploited ⁎ Corresponding author. Tel./fax: +86 451 86412236. E-mail address: [email protected] (S. Li). 0263-4368/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2012.06.013
high-strength RBSC ceramics with fine SiC particles of 1 μm and carbon of 0.3 μm. The flexural strength of the as-prepared RBSC ceramics exceeded 1000 MPa; however, the fracture strength was only 3.0 MPa m 1/2. By far, the combination of high strength and high toughness is impossible because of the intrinsic brittle fracture for RBSC ceramics. Recent developments in the preparation of nanometer powders have obviously boosted the use of nanometer powders in the ceramics toughening. It has been proved that nanometer technology is an effective approach for the strengthening and toughening of various ceramics [11,12]. Researchers have shown that the ceramic composite incorporating nanoparticles within matrix grain or at the grain boundaries possess excellent mechanical properties because of the crack deflection, inhibition of grain growth, and so on [13,14]. For the toughening effect of stress-induced phase transformation at room temperature, ZrO2 is proved to be an excellent candidate for ceramics reinforcing. As the second reinforcing phase, ZrO2 has been applied as the modification of Al2O3 [15,16], hydroxyapatite (HA) [17], cordierite (2MgO·2Al2O3·5SiO2) [18], zeolite [19] and so on. It is widely accepted that the ZrO2 dispersion into the ceramic matrix can affect the sinterability and consequently improve the mechanical properties. However, few investigations are reported on the reinforcing of RBSC ceramics with ZrO2 incorporation. This work aims to improve microstructure and mechanical properties of RBSC ceramics by introduction of nano-sized ZrO2. The ZrO2/ RBSC composites were prepared by liquid silicon infiltration (LSI) at 1500 °C in vacuum. The ZrO2 fraction ranged from 10 to 25 wt.%. The dependences of phase constitution, microstructure, as well as the mechanical properties of the ZrO2/RBSC composites on the fraction of ZrO2 were analyzed.
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S. Li et al. / Int. Journal of Refractory Metals and Hard Materials 35 (2012) 257–261 Table 1 Composition design of the ZrO2/RBSC composites. Composition SiC/C + 10 SiC/C + 15 SiC/C + 20 SiC/C + 25
wt.% wt.% wt.% wt.%
Design ZrO2 ZrO2 ZrO2 ZrO2
SC10Z SC15Z SC20Z SC25Z
size of 50 nm were used as reinforcement. The as-received ZrO2 powders were pre-dispersed in ultrasonoscope with PEG-6000 (polyethylene glycol, Qingdao In Blue Chemical Co., Ltd., China) as dispersant. Two commercial SiC powders (purity 98.5%, Huanyu, Zibo, China) of 10 μm, 60 μm were chosen. The carbon black (Baohua Carbon Black Co. Ltd., China) was selected as carbon resource. The volume ratio of coarse SiC: fine SiC: C = 10:5:1. Four mixture series were prepared from the starting materials of ZrO2, SiC and C, as listed in Table 1. All powders were attrition milled using SiC grinding media for 12 h. After mixing, the slurries were poured into the plaster mold to obtain green body. The green bodies were then dried in the ambient temperature for 72 h. Finally, the specimens were sintered by liquid silicon infiltration at 1500 °C for 120 min. The silicon powders were placed on the upper surface of the specimens during the sintering. 2.2 . Characterization
Fig. 1. Fracture morphology of the ZrO2/RBSC composite with 15 wt.% ZrO2 addition.
2 . Experimental procedure 2.1 . Preparation Commercial ZrO2 powders (TZ-3Y, 99.9% purity, Shanghai St-nano Science and Technology Co., Ltd., Shanghai, China) with an average
Bulk density of the specimens was measured by Archimedes' method. The samples were cut and ground into appropriate 3 mm × 4 mm × 36 mm in size for the three points bending strength test. The flexural strength was obtained at ambient temperature using an electron-mechanical universal material testing machine (5500R(1186), Instron, UK) with a cross-head speed of 0.5 mm/ min, and a span of 30 mm. The fracture toughness was evaluated using a single edge notched beam (SENB) test with a crosshead speed of 0.05 mm/min and a span of 16 mm. The tested bars were in dimension of 2 mm × 4 mm × 22 mm with a notch of 2 mm in depth and 0.02 mm in width. Each specimen was ground and polished with diamond slurries down to a 1 μm finish. Morphologies of the fracture surface were examined by scanning electron microscope (SEM, Sirion 200, FEI, USA) combined with an energy dispersive spectroscopy (EDS, DX-4, EDAX, USA). The composition of the specimens was analyzed by X-ray diffraction (XRD, Dmax-rb, Rigaku, Japan) using Cu Kα radiation. The scanning rate was
Fig. 2. Distribution of ZrO2 particles in the composites with reinforcement ranging from 10 to 25 wt.% (SEM, BSE): (a) SC10Z, (b) SC15Z, (c) SC20Z, (d) SC25Z.
S. Li et al. / Int. Journal of Refractory Metals and Hard Materials 35 (2012) 257–261
Fig. 3. Growth of β-SiC particles around the ZrO2 particles during the LSI.
2°min − 1 and the scanning angles ranged from 20° to 75° with a sampling width of 0.02°. The volume fraction of m-ZrO2 was calculated by the following [20]: Vm ¼
Xm ¼
1:311X m 1 þ 0:311X m − I m ð111Þ þ Im 11 1 − Im ð111Þ þ I m 1 1 1 þ I t ð111Þ
ðaÞ
ðbÞ
where Xm denoted the integrated intensity ratio of m-ZrO2, Im and It were the peak intensities of the m-ZrO2 and t-ZrO2, respectively. 3 . Results and discussion 3.1 . Microstructure and bulk density Fracture surface of the ZrO2/RBSC composite is shown in Fig. 1. During the LSI, the carbon reacted with the molten silicon to form β-SiC, accompanied by a volume expansion of about 80–150%; the residual pores were filled with molten silicon. Thus, the microstructure shows a high densification. From the image, the formed β-SiC particles of 3–5 μm were extracted from the matrix, so the fracture of the matrix is main intergranular mode. The distribution of ZrO2 powders in the composites is revealed in Fig. 2. In the BSE images, the white phase represents the ZrO2 powders with size of 1–4 μm. Although the trends of aggregating, the particles were homogeneously dispersed into the matrix. In addition, the introduction of fine ZrO2 particles effectively restrained the growth of the β-SiC particles and reduced the size of residual silicon, which became even more significant for the case of SC25Z. The size decrease of residual silicon is an effective approach for the strengthening of RBSC ceramics [21].
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Subsequently, it is believed that the ZrO2 introduction can improve the flexural strength of the RBSC ceramics. During the LSI, the β-SiC particles develop due to the siliconization of carbon. Often, the formed β-SiC particles attach on the staring α-SiC and grow around it. This leads to the overgrowth of the SiC particles in the local regions, and thus a decrease of the composites' mechanical strength. From Fig. 3, the formed SiC particles developed on the surface of the ZrO2 aggregation during the LSI. This is conductive to the reinforcing of the RBSC composites for two reasons. One is the restraint of the overgrowth of the SiC particles, which is mentioned above; the other is the formation of compressive pressure between the reinforcement and the matrix. As the toughening of ZrO2 is a stress induced phase transformation toughening from tetragonal to monoclinic phase, the growth of β-SiC around the ZrO2 can amplify the compressive stress that is applied to the reinforcement during the crack propagation. The EDS analysis shows that the ZrO2 and SiC phases coexisted in the composites (Fig. 4). This implies a combination of C and ZrO2 in the green body. During the LSI, the C was transformed into SiC rapidly, ascribing to the high reactivity of Si–C reaction. From the literatures, the ZrO2 may be reduced into ZrC in vacuum at high temperature 2200 °C [22,23]. On the basis of this reduction reaction, the ZrC phase is speculated to exist in the ZrO2/RBSC composites. Such speculation will be further discussed by XRD analysis. The variation of bulk density of the composites is plotted in Fig. 5. The bulk density decreases initially, and then is enhanced steadily with the ZrO2 content increase. The small particle size and high density of ZrO2 are responsible for this enhancement. The fine ZrO2 particles reduce the porosity of the ZrO2/RBSC green body, so decrease the residual silicon of low density after LSI. Compared with the initial specimens SC10Z and SC15Z, the enhancing effect of ZrO2 on bulk density is reduced at higher content. It is considered that densification of SC20Z and SC25Z is hindered by the high fraction of ZrO2. Moreover, the bulk density is lower than the theoretical value, which implies the existence of residual silicon in the composite.
3.2 . Mechanical properties Flexural strength and fracture toughness of the composites as a function of the ZrO2 fraction are plotted in Fig. 6. The flexural strength of monolithic RBSC ceramics is only 260 MPa. With ZrO2 increasing, the flexural strength is steadily improved, reaching the maximum value of 355 MPa in the case of SC25Z. This improvement is ascribed to the change of residual silicon in the composite. From the literatures [3,24], due to the high brittleness, the residual silicon is the main deleterious phase for the flexural strength of RBSC ceramics. The ZrO2 particles reduce the porosity of the green body because of the short particle size, and thus a decrease of the residual silicon in the sintered body. Besides, ZrO2 particles inhibit the growth of β-SiC grains and
Fig. 4. Elemental composition of the ZrO2 particles in the ZrO2/RBSC composite.
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S. Li et al. / Int. Journal of Refractory Metals and Hard Materials 35 (2012) 257–261
Fig. 5. Evolution of the bulk density as a function of the ZrO2 fraction.
reduce the size of residual silicon (Fig. 2), which is also responsible for the improvement of flexural strength. With regard to the fracture toughness, the increase of ZrO2 plays a beneficial role in the RBSC composites. At reinforcement fraction of 10 wt.%, the fracture toughness is 3.4 MPa m 1/2, which nearly equals to the monolithic RBSC ceramics. With ZrO2 increasing further, the fracture toughness increases and reaches the peak value of appropriate 4.6 MPa m 1/2 in the case of SC25Z. This value is a remarkable improvement compared to the monolithic RBSC ceramics. The phasetransformation toughening of ZrO2 is considered to be responsible for the improvement. So the compositions of both the polished surface and the fracture surface were identified by XRD analysis. Fig. 7 shows the XRD patterns of the fracture surface and the polished surface in the specimens SC15Z and SC25Z. On the fracture surface (Fig. 7(a)), both m-ZrO2 and t-ZrO2 are detected. However, the peak intensity of m-ZrO2 in SC25Z is stronger than that in SC15Z, which implies a high faction of m-ZrO2 in the SC25Z. Moreover, a stronger peak density of silicon comes out in SC25Z, which implies the increase of residual silicon in the composite. This increase can explain the slower increase of bulk density in Fig. 5. The patterns of polished surface in Fig. 7(b) are smoother than that of the fracture surface. Similarly, the stronger peak density of m-ZrO2 is observed in SC25Z. The volume fraction of m-ZrO2 on the polished surface and the fracture surface is listed in Table 2. It reveals that the fraction of m-ZrO2 on the fracture surface is higher than that on the polished surface. This increase resulted from the transformation of t-ZrO2 →m-ZrO2 during the flexural strength test. Such transformation brings about toughening for the ZrO2/RBSC composite. The ZrC phase is not observed in the XRD patterns. So the ZrO2 was not reduced by C during the sintering. Due to the high reactivity between carbon and molten silicon, the siliconization reaction occurred
Fig. 7. XRD patterns of the composite; (a) and (b) corresponding to the fracture surface and polished surface, respectively.
in several minutes. Besides, the formed SiC layer limited the diffusion of carbon during LSI. On the basis of the above discussion, it is believed the reduction reaction of ZrO2 can be ignored during the LSI. For the ceramics toughened by ZrO2, crack deflection is considered as an important toughening mechanism. The path of crack propagation in the SC20Z is presented in Fig. 8. As the second phase, the ZrO2 hinders the straight propagation of the crack, and thus degrades the fracture energy. 4 . Conclusions The ZrO2 particles were introduced into SiC/C mixture to strengthen and toughen the RBSC ceramics. The present work studied the effect of ZrO2 introduction on the microstructure and mechanical proprieties of ZrO2/RBSC composites. The following results were obtained: (1) The nano-sized ZrO2 particles reduced the porosity of the ZrO2/ RBSC green body because of the small particle size. So the bulk density of the composites was significantly enhanced, resulting from the decrease of residual silicon. Table 2 Volume fraction of m-ZrO2 on the polished surface and fracture surfaces in the ZrO2/ RBSC composites.
Fig. 6. Evolution of flexural strength and fracture toughness as a function of the ZrO2 fraction.
Specimen
m-ZrO2 in fracture surface (vol.%)
m-ZrO2 in polished surface (vol.%)
Transformability of t-ZrO2 (vol.%)
SC15Z SC25Z
40.9 76.6
21.6 45.8
19.3 30.8
S. Li et al. / Int. Journal of Refractory Metals and Hard Materials 35 (2012) 257–261
Fig. 8. Crack propagation in the composite with 20 wt.% ZrO2 addition.
(2) By dispersing the ZrO2 particles into the RBSC matrix, the overgrowth of β-SiC was significantly suppressed and the size of residual silicon was reduced. This was beneficial to the flexural strength. The co-existence of ZrO2 and β-SiC was detected by EDS. The formation and growth of β-SiC around the ZrO2 particle were in favor of the fracture resistance when the crack propagating. (3) Both flexural strength and fracture toughness of the ZrO2/RBSC composites were elevated with the ZrO2 addition in the range of 10–25 wt.%. The transformation of ZrO2 and the crack deflection were considered as the main mechanisms of fracture toughness improving. Acknowledgments This study was financially supported by "the Fundamental Research Funds for the Central Universities” (Grant no.HIT.KLOF.2010024) of China and the Program for New Century Excellent Talents in University (NCET-10-0069). References [1] Martínez Fernández J, Muñoz A, de Arellano López AR, Valera Feria FM, Domínguez-Rodríguez A, Singh M. Microstructure–mechanical properties correlation in siliconized silicon carbide ceramics. Acta Mater 2003;51:3259–75. [2] Naslain R. Design, preparation and properties of non-oxide CMCs for application in engines and nuclear reactors: an overview. Compos Sci Technol 2004;64:155–70.
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[3] Mallick D, Kayal TK, Ghosh J, Chakrabarti OP, Biswas S, Maiti HS. Development of multi-phase B–Si–C ceramic composite by reaction sintering. Ceram Int 2009;35: 1667–9. [4] Aroati S, Cafri M, Dilman H, Dariel MP, Frage N. Preparation of reaction bonded silicon carbide (RBSC) using boron carbide as an alternative source of carbon. J Eur Ceram Soc 2011;31:841–5. [5] Xia HY, Wang JP, Jin HY, Shi ZQ, Qiao GJ. Fabrication and properties of reaction-formed SiC by infiltrating molten Si into mesocarbon microbeadsbased carbon preform. Mat Sci Eng A-Struct 2010;528:283–7. [6] Patel M, Subrahmanyam J, Prasad VVB, Goyal R. Processing and characterization of B4C–SiC–Si–TiB2 composites. Mat Sci Eng A-Struct 2010;527:4109–12. [7] Margiotta JC, Zhang D, Nagle DC. Microstructural evolution during silicon carbide (SiC) formation by liquid silicon infiltration using optical microscopy. Int J Refract Met Hard Mater 2010;28:191–7. [8] Calderon NR, Martínez-Escandell M, Narciso J, Rodríguez-Reinoso F. The combined effect of porosity and reactivity of the carbon preforms on the properties of SiC produced by reactive infiltration with liquid Si. Carbon 2009;47:2200–10. [9] Esfehanian M, Guenster J, Heinrich JG, Horvath J, Koch D, Grathwohl G. High-temperature mechanical behavior of carbon–silicide–carbide composites developed by alloyed melt infiltration. J Eur Ceram Soc 2008;28:1267–74. [10] Suyama S, Kameda T, Itoh Y. Development of high-strength reaction-sintered silicon carbide. Diamond Relat Mater 2003;12:1201–4. [11] Viswanathan V, Laha T, Balani K, Agarwal A, Seal S. Challenges and advances in nanocomposite processing techniques. Mater Sci Eng R 2006;54:121–285. [12] Casellas D, Nagl MM, Llanes L, Anglada M. Microstructural coarsening of zirconiatoughened alumina composites. J Am Ceram Soc 2005;88:1958–63. [13] Casellas D, Feder A, Llanes L, Anglada M. Fracture toughness and mechanical strength of Y-TZP/PSZ ceramics. Scr Mater 2001;45:213–20. [14] Inagaki M, Kaneko K, Nishizawa T. Nanocarbons—recent research in Japan. Carbon 2004;42:1401–17. [15] Maneshian MH, Banerjee MK. Effect of sintering on structure and mechanical properties of alumina-15 vol.% zirconia nanocomposite compacts. J Alloys Compd 2010;493:613–8. [16] Bartolomé JF, De Aza AH, Martín A, Pastor JY, Llorca J, Torrecillas R, et al. Alumina/ zirconia micro/nanocomposites: a new material for biomedical applications with superior sliding wear resistance. J Am Ceram Soc 2007;90:3177–84. [17] Erkmen ZE, Genç Y, Oktar FN. Microstructural and mechanical properties of hydroxyapatite–zirconia composites. J Am Ceram Soc 2007;90:2885–92. [18] Costa Oliveira FA, Cruz Fernandes J. Mechanical and thermal behaviour of cordierite–zirconia composites. Ceram Int 2002;28:79–91. [19] Wang Z, Al-Daous MA, Kiesel ER, Li F, Stein A. Design and synthesis of 3D ordered macroporous ZrO2/Zeolite nanocomposites. Microporous Mesoporous Mater 2009;120:351–8. [20] Kim JK, Jo YJ, Han SC, Lee HY, Lee JM. The effect of surface roughness on SiC by wet chemical etching. J Korean Inst Met Mater 2009;47:748–53. [21] Wilhelm M, Wruss W. Influence of annealing on the mechanical properties of SiC-Si composites with sub-micron SiC microstructures. J Eur Ceram Soc 2000;20:1205–13. [22] Zhou GH, Wang SW, Guo JK, Zhang Z. The preparation and mechanical properties of the unidirectional carbon fiber reinforced zirconia composite. J Eur Ceram Soc 2008;28:787–92. [23] Sharif MA, Sueyoshi H. Preparation and properties of C/SiC/ZrO2 porous composites by hot isostatic pressing the pyrolyzed preforms. Ceram Int 2009;35:349–58. [24] Chakrabarti OP, Das PK, Mukerji J. Growth of SiC particles in reaction sintered SiC. Mater Chem Phys 2001;67:199–202.
Contents lists available at SciVerse ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Zirconia reinforced reaction bonded silicon carbide composites: Microstructure and mechanical properties Shuang Li ⁎, Yumin Zhang, Jiecai Han, Yufeng Zhou Center for Composite Materials, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 9 March 2012 Accepted 25 June 2012 Keywords: Reaction bonding Fracture toughness Transformation toughening Residual silicon Grain size
a b s t r a c t Nano-sized zirconia particles were dispersed in a SiC/C suspension to examine the particle influence on microstructure and mechanical properties on the resulting composite. The ZrO2/reaction bonded SiC composite was produced by a combination of slip casting and liquid silicon infiltration. By dispersing zirconia particles into the matrix, the gowth of β-SiC was inhibited and the size of residual silicon was reduced. This result led to an increase of flexural strength with reinforcement ranging from 0 to 20 wt.%. Compared with the monolithic RBSC ceramics, the fracture toughness was steadily improved, reaching the peak value of appropriate 4.6 MPa m1/2 at the zirconia fraction of 25 wt.%. The XRD analysis revealed the phase transformation occurred on the fracture surface of the composites. Thus, the transformation of zirconia and the crack deflection are considered as the main toughening mechanisms. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Reaction bonded silicon carbide (RBSC) ceramics are attractive materials for high-temperature heat exchanger, piping, light armor and optical mirror, due to their low sintering temperature, near-net shaping, full densification and low fabrication cost [1–3]. Most commonly, the RBSC ceramics are prepared by infiltration of compacted SiC/C mixtures or carbon preforms with molten silicon. The reaction between carbon and molten silicon leads to the formation of β-SiC, which interconnects the original SiC particles and forms a continuous ceramic skeleton [4]. So the typical RBSC ceramics are composed of α-SiC, β-SiC and Si phases. Due to the high brittleness and low melting temperature, the residual silicon phase is a harmful constituent for the mechanical behaviors of RBSC ceramics, especially in the hightemperature fields [5]. To date, the inherent low fracture toughness of RBSC ceramics still limits their use in the large-scale, complex shaped and lightweight components. Various investigations have focused on the improvement of mechanical properties and processing techniques of RBSC ceramics [6–8]. Aroati et al. [4] used boron carbide (B4C) as an alternative source of carbon, which could significantly reduce the silicon fraction and density of the ceramics. Esfehanian et al. [9] developed Si–Ti– MoSi2 alloyed melt to infiltrate the carbon/carbon fiber materials. The alloy-infiltrated composites showed more resistance against creep deformation and higher strengths than silicon-infiltrated composites. Fine original SiC particles are also in favor of the preparation of high strength RBSC ceramics. Toshiba Corporation [10] exploited ⁎ Corresponding author. Tel./fax: +86 451 86412236. E-mail address: [email protected] (S. Li). 0263-4368/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2012.06.013
high-strength RBSC ceramics with fine SiC particles of 1 μm and carbon of 0.3 μm. The flexural strength of the as-prepared RBSC ceramics exceeded 1000 MPa; however, the fracture strength was only 3.0 MPa m 1/2. By far, the combination of high strength and high toughness is impossible because of the intrinsic brittle fracture for RBSC ceramics. Recent developments in the preparation of nanometer powders have obviously boosted the use of nanometer powders in the ceramics toughening. It has been proved that nanometer technology is an effective approach for the strengthening and toughening of various ceramics [11,12]. Researchers have shown that the ceramic composite incorporating nanoparticles within matrix grain or at the grain boundaries possess excellent mechanical properties because of the crack deflection, inhibition of grain growth, and so on [13,14]. For the toughening effect of stress-induced phase transformation at room temperature, ZrO2 is proved to be an excellent candidate for ceramics reinforcing. As the second reinforcing phase, ZrO2 has been applied as the modification of Al2O3 [15,16], hydroxyapatite (HA) [17], cordierite (2MgO·2Al2O3·5SiO2) [18], zeolite [19] and so on. It is widely accepted that the ZrO2 dispersion into the ceramic matrix can affect the sinterability and consequently improve the mechanical properties. However, few investigations are reported on the reinforcing of RBSC ceramics with ZrO2 incorporation. This work aims to improve microstructure and mechanical properties of RBSC ceramics by introduction of nano-sized ZrO2. The ZrO2/ RBSC composites were prepared by liquid silicon infiltration (LSI) at 1500 °C in vacuum. The ZrO2 fraction ranged from 10 to 25 wt.%. The dependences of phase constitution, microstructure, as well as the mechanical properties of the ZrO2/RBSC composites on the fraction of ZrO2 were analyzed.
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S. Li et al. / Int. Journal of Refractory Metals and Hard Materials 35 (2012) 257–261 Table 1 Composition design of the ZrO2/RBSC composites. Composition SiC/C + 10 SiC/C + 15 SiC/C + 20 SiC/C + 25
wt.% wt.% wt.% wt.%
Design ZrO2 ZrO2 ZrO2 ZrO2
SC10Z SC15Z SC20Z SC25Z
size of 50 nm were used as reinforcement. The as-received ZrO2 powders were pre-dispersed in ultrasonoscope with PEG-6000 (polyethylene glycol, Qingdao In Blue Chemical Co., Ltd., China) as dispersant. Two commercial SiC powders (purity 98.5%, Huanyu, Zibo, China) of 10 μm, 60 μm were chosen. The carbon black (Baohua Carbon Black Co. Ltd., China) was selected as carbon resource. The volume ratio of coarse SiC: fine SiC: C = 10:5:1. Four mixture series were prepared from the starting materials of ZrO2, SiC and C, as listed in Table 1. All powders were attrition milled using SiC grinding media for 12 h. After mixing, the slurries were poured into the plaster mold to obtain green body. The green bodies were then dried in the ambient temperature for 72 h. Finally, the specimens were sintered by liquid silicon infiltration at 1500 °C for 120 min. The silicon powders were placed on the upper surface of the specimens during the sintering. 2.2 . Characterization
Fig. 1. Fracture morphology of the ZrO2/RBSC composite with 15 wt.% ZrO2 addition.
2 . Experimental procedure 2.1 . Preparation Commercial ZrO2 powders (TZ-3Y, 99.9% purity, Shanghai St-nano Science and Technology Co., Ltd., Shanghai, China) with an average
Bulk density of the specimens was measured by Archimedes' method. The samples were cut and ground into appropriate 3 mm × 4 mm × 36 mm in size for the three points bending strength test. The flexural strength was obtained at ambient temperature using an electron-mechanical universal material testing machine (5500R(1186), Instron, UK) with a cross-head speed of 0.5 mm/ min, and a span of 30 mm. The fracture toughness was evaluated using a single edge notched beam (SENB) test with a crosshead speed of 0.05 mm/min and a span of 16 mm. The tested bars were in dimension of 2 mm × 4 mm × 22 mm with a notch of 2 mm in depth and 0.02 mm in width. Each specimen was ground and polished with diamond slurries down to a 1 μm finish. Morphologies of the fracture surface were examined by scanning electron microscope (SEM, Sirion 200, FEI, USA) combined with an energy dispersive spectroscopy (EDS, DX-4, EDAX, USA). The composition of the specimens was analyzed by X-ray diffraction (XRD, Dmax-rb, Rigaku, Japan) using Cu Kα radiation. The scanning rate was
Fig. 2. Distribution of ZrO2 particles in the composites with reinforcement ranging from 10 to 25 wt.% (SEM, BSE): (a) SC10Z, (b) SC15Z, (c) SC20Z, (d) SC25Z.
S. Li et al. / Int. Journal of Refractory Metals and Hard Materials 35 (2012) 257–261
Fig. 3. Growth of β-SiC particles around the ZrO2 particles during the LSI.
2°min − 1 and the scanning angles ranged from 20° to 75° with a sampling width of 0.02°. The volume fraction of m-ZrO2 was calculated by the following [20]: Vm ¼
Xm ¼
1:311X m 1 þ 0:311X m − I m ð111Þ þ Im 11 1 − Im ð111Þ þ I m 1 1 1 þ I t ð111Þ
ðaÞ
ðbÞ
where Xm denoted the integrated intensity ratio of m-ZrO2, Im and It were the peak intensities of the m-ZrO2 and t-ZrO2, respectively. 3 . Results and discussion 3.1 . Microstructure and bulk density Fracture surface of the ZrO2/RBSC composite is shown in Fig. 1. During the LSI, the carbon reacted with the molten silicon to form β-SiC, accompanied by a volume expansion of about 80–150%; the residual pores were filled with molten silicon. Thus, the microstructure shows a high densification. From the image, the formed β-SiC particles of 3–5 μm were extracted from the matrix, so the fracture of the matrix is main intergranular mode. The distribution of ZrO2 powders in the composites is revealed in Fig. 2. In the BSE images, the white phase represents the ZrO2 powders with size of 1–4 μm. Although the trends of aggregating, the particles were homogeneously dispersed into the matrix. In addition, the introduction of fine ZrO2 particles effectively restrained the growth of the β-SiC particles and reduced the size of residual silicon, which became even more significant for the case of SC25Z. The size decrease of residual silicon is an effective approach for the strengthening of RBSC ceramics [21].
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Subsequently, it is believed that the ZrO2 introduction can improve the flexural strength of the RBSC ceramics. During the LSI, the β-SiC particles develop due to the siliconization of carbon. Often, the formed β-SiC particles attach on the staring α-SiC and grow around it. This leads to the overgrowth of the SiC particles in the local regions, and thus a decrease of the composites' mechanical strength. From Fig. 3, the formed SiC particles developed on the surface of the ZrO2 aggregation during the LSI. This is conductive to the reinforcing of the RBSC composites for two reasons. One is the restraint of the overgrowth of the SiC particles, which is mentioned above; the other is the formation of compressive pressure between the reinforcement and the matrix. As the toughening of ZrO2 is a stress induced phase transformation toughening from tetragonal to monoclinic phase, the growth of β-SiC around the ZrO2 can amplify the compressive stress that is applied to the reinforcement during the crack propagation. The EDS analysis shows that the ZrO2 and SiC phases coexisted in the composites (Fig. 4). This implies a combination of C and ZrO2 in the green body. During the LSI, the C was transformed into SiC rapidly, ascribing to the high reactivity of Si–C reaction. From the literatures, the ZrO2 may be reduced into ZrC in vacuum at high temperature 2200 °C [22,23]. On the basis of this reduction reaction, the ZrC phase is speculated to exist in the ZrO2/RBSC composites. Such speculation will be further discussed by XRD analysis. The variation of bulk density of the composites is plotted in Fig. 5. The bulk density decreases initially, and then is enhanced steadily with the ZrO2 content increase. The small particle size and high density of ZrO2 are responsible for this enhancement. The fine ZrO2 particles reduce the porosity of the ZrO2/RBSC green body, so decrease the residual silicon of low density after LSI. Compared with the initial specimens SC10Z and SC15Z, the enhancing effect of ZrO2 on bulk density is reduced at higher content. It is considered that densification of SC20Z and SC25Z is hindered by the high fraction of ZrO2. Moreover, the bulk density is lower than the theoretical value, which implies the existence of residual silicon in the composite.
3.2 . Mechanical properties Flexural strength and fracture toughness of the composites as a function of the ZrO2 fraction are plotted in Fig. 6. The flexural strength of monolithic RBSC ceramics is only 260 MPa. With ZrO2 increasing, the flexural strength is steadily improved, reaching the maximum value of 355 MPa in the case of SC25Z. This improvement is ascribed to the change of residual silicon in the composite. From the literatures [3,24], due to the high brittleness, the residual silicon is the main deleterious phase for the flexural strength of RBSC ceramics. The ZrO2 particles reduce the porosity of the green body because of the short particle size, and thus a decrease of the residual silicon in the sintered body. Besides, ZrO2 particles inhibit the growth of β-SiC grains and
Fig. 4. Elemental composition of the ZrO2 particles in the ZrO2/RBSC composite.
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Fig. 5. Evolution of the bulk density as a function of the ZrO2 fraction.
reduce the size of residual silicon (Fig. 2), which is also responsible for the improvement of flexural strength. With regard to the fracture toughness, the increase of ZrO2 plays a beneficial role in the RBSC composites. At reinforcement fraction of 10 wt.%, the fracture toughness is 3.4 MPa m 1/2, which nearly equals to the monolithic RBSC ceramics. With ZrO2 increasing further, the fracture toughness increases and reaches the peak value of appropriate 4.6 MPa m 1/2 in the case of SC25Z. This value is a remarkable improvement compared to the monolithic RBSC ceramics. The phasetransformation toughening of ZrO2 is considered to be responsible for the improvement. So the compositions of both the polished surface and the fracture surface were identified by XRD analysis. Fig. 7 shows the XRD patterns of the fracture surface and the polished surface in the specimens SC15Z and SC25Z. On the fracture surface (Fig. 7(a)), both m-ZrO2 and t-ZrO2 are detected. However, the peak intensity of m-ZrO2 in SC25Z is stronger than that in SC15Z, which implies a high faction of m-ZrO2 in the SC25Z. Moreover, a stronger peak density of silicon comes out in SC25Z, which implies the increase of residual silicon in the composite. This increase can explain the slower increase of bulk density in Fig. 5. The patterns of polished surface in Fig. 7(b) are smoother than that of the fracture surface. Similarly, the stronger peak density of m-ZrO2 is observed in SC25Z. The volume fraction of m-ZrO2 on the polished surface and the fracture surface is listed in Table 2. It reveals that the fraction of m-ZrO2 on the fracture surface is higher than that on the polished surface. This increase resulted from the transformation of t-ZrO2 →m-ZrO2 during the flexural strength test. Such transformation brings about toughening for the ZrO2/RBSC composite. The ZrC phase is not observed in the XRD patterns. So the ZrO2 was not reduced by C during the sintering. Due to the high reactivity between carbon and molten silicon, the siliconization reaction occurred
Fig. 7. XRD patterns of the composite; (a) and (b) corresponding to the fracture surface and polished surface, respectively.
in several minutes. Besides, the formed SiC layer limited the diffusion of carbon during LSI. On the basis of the above discussion, it is believed the reduction reaction of ZrO2 can be ignored during the LSI. For the ceramics toughened by ZrO2, crack deflection is considered as an important toughening mechanism. The path of crack propagation in the SC20Z is presented in Fig. 8. As the second phase, the ZrO2 hinders the straight propagation of the crack, and thus degrades the fracture energy. 4 . Conclusions The ZrO2 particles were introduced into SiC/C mixture to strengthen and toughen the RBSC ceramics. The present work studied the effect of ZrO2 introduction on the microstructure and mechanical proprieties of ZrO2/RBSC composites. The following results were obtained: (1) The nano-sized ZrO2 particles reduced the porosity of the ZrO2/ RBSC green body because of the small particle size. So the bulk density of the composites was significantly enhanced, resulting from the decrease of residual silicon. Table 2 Volume fraction of m-ZrO2 on the polished surface and fracture surfaces in the ZrO2/ RBSC composites.
Fig. 6. Evolution of flexural strength and fracture toughness as a function of the ZrO2 fraction.
Specimen
m-ZrO2 in fracture surface (vol.%)
m-ZrO2 in polished surface (vol.%)
Transformability of t-ZrO2 (vol.%)
SC15Z SC25Z
40.9 76.6
21.6 45.8
19.3 30.8
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Fig. 8. Crack propagation in the composite with 20 wt.% ZrO2 addition.
(2) By dispersing the ZrO2 particles into the RBSC matrix, the overgrowth of β-SiC was significantly suppressed and the size of residual silicon was reduced. This was beneficial to the flexural strength. The co-existence of ZrO2 and β-SiC was detected by EDS. The formation and growth of β-SiC around the ZrO2 particle were in favor of the fracture resistance when the crack propagating. (3) Both flexural strength and fracture toughness of the ZrO2/RBSC composites were elevated with the ZrO2 addition in the range of 10–25 wt.%. The transformation of ZrO2 and the crack deflection were considered as the main mechanisms of fracture toughness improving. Acknowledgments This study was financially supported by "the Fundamental Research Funds for the Central Universities” (Grant no.HIT.KLOF.2010024) of China and the Program for New Century Excellent Talents in University (NCET-10-0069). References [1] Martínez Fernández J, Muñoz A, de Arellano López AR, Valera Feria FM, Domínguez-Rodríguez A, Singh M. Microstructure–mechanical properties correlation in siliconized silicon carbide ceramics. Acta Mater 2003;51:3259–75. [2] Naslain R. Design, preparation and properties of non-oxide CMCs for application in engines and nuclear reactors: an overview. Compos Sci Technol 2004;64:155–70.
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