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Mechanical properties of nano-grain SiO2 glass prepared by spark plasma sintering
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Mechanical properties of nano-grain SiO2 glass prepared by spark plasma sintering Zhenhua He a,b , Hirokazu Katsui b,∗ , Takashi Goto b a Key Laboratory of Fiber Optic Sensing Technology and Information Processing, Ministry of Education, Wuhan University of Technology, Wuhan 430070, PR China b Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
a r t i c l e
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Article history: Received 21 April 2016 Received in revised form 20 September 2016 Accepted 21 September 2016 Available online xxx Keywords: SiC-coated SiO2 glass powder Rotary chemical vapor deposition Spark plasma sintering High hardness High fracture toughness
a b s t r a c t Silicon carbide (SiC) layers were deposited on silica (SiO2 ) glass powder by rotary chemical vapor deposition (RCVD) to form SiO2 glass (core)/SiC (shell) powder; this powder was consolidated by spark plasma sintering (SPS). SiO2 glass powder with a particle size of 250 nm was coated with 5–10-nm-thick SiC layers. The resultant SiO2 glass (core)/SiC (shell) powder was consolidated to form a nano-grain SiO2 glass composite at a relative density above 90% by SPS in the sintering temperature range of 1573–1823 K. The Vickers hardness and fracture toughness of the SiO2 glass composite at 1723 K were found to be 14.2 GPa and 5.4 MPa m1/2 , respectively. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Silica (SiO2 ), one of the most common oxide materials, has excellent insulation, high chemical stability, low thermal-expansion coefficient and low thermal conductivity [1,2]. Therefore, SiO2 glass is used to produce optical fibres, chemical containers, windows and crucibles. Although SiO2 glass is usually fabricated by meltingsolidification, Zhang et al. prepared SiO2 glass by spark plasma sintering (SPS) at 1573 K to form a dense and transparent compact material with a hardness of 7.5 GPa and fracture toughness of 0.6 MPa m1/2 [3]. The refinement of the microstructure with the addition of secondary phases may reinforce the mechanical properties of SiO2 . Ning et al. consolidated SiO2 powder with 5vol% carbon nanotubes by hot-pressing and reported a bending strength and fracture toughness of 85 MPa and 2.0 MPa m1/2 , respectively [4]. Although SPS enables the consolidation of many materials in a short time, thus preventing the grain growth, SiO2 glass has no grains with brittle uniform microstructure. This characteristic prevents further application of SiO2 for structural usage. The sintering of core (glass)/shell (glass or crystalline) powder results in the formation of a fine grain microstructure even if the matrix material is glass (or amorphous). Rotary chemical
∗ Corresponding author. E-mail address: [email protected] (H. Katsui).
vapor deposition (RCVD) enables the coating of glass or crystalline materials on powder surfaces with particle sizes below several microns [5–9]. In this study, we prepared SiO2 glass (core)/SiC (shell) powder by using RCVD. SiC is a promising second phase to reinforce SiO2 -based materials due to its high strength, oxidation resistance and good compatibility with SiO2 . The SiC shell would effectively prevent the grain growth of the SiO2 matrix, thereby enabling the formation of a nano-grained SiO2 glass body. The SiO2 glass (core)/SiC (shell) powder was consolidated by SPS. The crystal phases, microstructure, and mechanical properties of the nano-grain SiO2 glass composite were investigated.
2. Experimental SiO2 glass powder (250 nm in diameter, Admatechs Ltd., Japan) was coated with SiC by RCVD using C6 H18 Si2 (hexamethyldisilane: HMDS) as a precursor, forming SiO2 glass (core)/SiC shell powder. The details of the RCVD apparatus for SiC coating are presented elsewhere [7]. The precursor was evaporated at 298 K and carried into a reaction chamber by Ar gas at a flow rate of 8.33 × 10−7 m3 s−1 . SiO2 glass powders were stirred and suspended in a reactor chamber rotating at 45 rpm. The reaction chamber was heated at 998 K. The total pressure in the chamber was maintained at 400 Pa. The deposition time was 7.2 ks. The resultant SiO2 glass (core)/SiC (shell) powder was consolidated by SPS (SPS-210LX, Fujidenpakoki, Japan) and poured into
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Fig. 1. XRD patterns of (a) monolithic SiO2 powder and (b) SiC-coated SiO2 glass powder.
a 10-mm-inner-diameter graphite die. This graphite die was covered with carbon wool, which acted as a thermal insulator. The heating rate was 1.7 K s−1 and the soaking time was 0.6 ks. The sintering temperature ranged from 1073 to 1873 K. After the soaking, the application of heating current was ceased and the temperature rapidly decreased. The temperature was measured by an optical pyrometer focused on a hole (2 mm diameter and 5 mm depth) in the graphite die. The loading pressure was 100 MPa. The crystal phases of the SiO2 glass (core)/SiC (shell) powder and sintered SiO2 glass composites were identified by X-ray diffraction (XRD; CuK␣, Rigaku, Japan). The microstructures were observed by scanning electron microscopy (SEM; S-3100H, Hitachi, Japan and JSM-7500F, JEOL, Japan) and transmission electron microscopy (TEM, EM-002B, TOPCON, Japan). The values of SiC content (CSiC , mass%) of the samples were calculated from the mass gain by RCVD. Relative densities of the sintered bodies of the SiO2 glass composites were measured following the Archimedes’ method using distilled water, where the theoretical densities of SiC and SiO2 were 3.21 Mg m−3 [10] and 2.20 Mg m−3 [11], respectively. The Vickers hardness (Hv ) and fracture toughness (KIC ) were determined at room temperature using a Vickers microhardness tester (HM-221, Mitutoyo, Japan) at load P of 0.98 and 9.8 N. The hardness and toughness values were determined at 10 points. The fracture toughness of KIC (MPa m1/2 ) was calculated by Eq. (1) [12,13]: KIC = 1.173 × 10−6 ×
P c 3/2
(1)
where c (m) is the average half length of the cracks formed around the corners of the indentations.
Fig. 2. SEM images at (a) low and (b) high magnifications and (c) a TEM bright-field image of SiC-coated SiO2 glass powder.
3. Results and discussion Fig. 1 shows the XRD patterns of the monolithic SiO2 glass powder (a) and SiC-coated SiO2 glass powder prepared by RCVD (b). A broad peak at around 2 = 22.0◦ was attributed to the glass structure of SiO2 . The small reflection peaks at 2 = 35.6◦ and 60.0◦ were assigned to 111 and 220, respectively, of SiC (-type) of the SiCcoated SiO2 glass powder. No reflection peaks from impurities, e.g., carbon, were identified in the SiC-coated SiO2 glass powder. The microstructures of SiC-coated SiO2 glass powder are depicted in Fig. 2. The SiO2 glass powder particles were spherical, as shown in the SEM image in Fig. 2(a). In the image at high magnification (Fig. 2(b)) and the TEM bright-field image (Fig. 2(c)), a SiC layer of
a few nanometers thickness (5–10 nm) and particles 10–30 nm in diameter formed on the SiO2 glass powder surface. Although some SiC particles formed on the SiO2 glass powder, the SiO2 glass powder was completely coated by the SiC layer. Hereafter, the powder is denoted as SiO2 glass (core)/SiC (shell) powder. The SiC content in the powder was estimated as 20 mass% according to the mass gain during RCVD. The SiO2 glass (core)/SiC (shell) powder was consolidated by SPS; the effect of sintering temperature on XRD patterns of the sintered bodies at 1073–1873 K is shown in Fig. 3. At sintering temperatures below 1473 K, the XRD patterns had a broad peak at approximately 2 = 22.0◦ , attributed to amorphous SiO2 , and reflec-
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Fig. 4. Effect of sintering temperature on the relative density of the SiO2 glass composite. Fig. 3. Effect of sintering temperature on the XRD patterns of the SiO2 glass composites: (a) SiC-coated SiO2 glass powder, (b) 1073 K, (c) 1273 K, (d) 1473 K, (e) 1673 K, (f) 1773 K and (g) 1873 K.
tion peaks at 2 = 35.6◦ and 60.0◦ , assigned as 111 and 220 SiC, respectively. With increasing sintering temperature, the sharp peak appeared at 2 = 22.0◦ in the temperature range of 1473–1723 K, indicating crystallization of SiO2 glass from amorphous to cristobalite. Upon further increasing the temperature to 1873 K, the sintered body comprised SiO2 glass and SiC (-type). At temper-
atures higher than the melting point of cristobalite, the SiO2 could be melted and quenched to be SiO2 glass at the high cooling rate in the SPS procedure. The melting point of cristobalite is 1996 K. The electrical current could pass through the SiC shell at high temperatures during the SPS sintering and the sample in a graphite die might be heated to temperature higher than the monitored temperature by using the optical pyrometer focused on a hole in the graphite die. Fig. 4 depicts the effect of sintering temperature on the relative density of the SiO2 glass composite. As the sintering temperature
Fig. 5. SEM images of the SiO2 glass composites sintered at 1073 (a), 1473 (b), 1573 (c), 1723 (d), 1773 (e) and 1873 K (f).
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Fig. 6. TEM bright-field images of the SiO2 glass composite sintered at 1723 K. (b) The high-magnification image of (a) and the corresponding SEAD pattern.
was increased, the SiO2 glass composite was significantly densified at 1500 K, with the relative density reaching 90%, whereas the density decreased to 80% at 1873 K due to the reaction between SiO2 and SiC [5]. Fig. 5 shows the microstructures of the SiO2 glass composite in the sintering temperature range of 1073–1873 K. At 1073 K (Fig. 5(a)), the powder was not densified and the small particulate shape remained, as the relative density was low (Fig. 4). At 1473 K (Fig. 5(b)), the powder was densified, although a significant number of pores were observed. In the sintering temperature range of 1573–1773 K (Fig. 5(c–e)), the SiO2 glass composite had a dense morphology with a few pores. As the sintering temperature was further increased to 1873 K (Fig. 5(f)), the number of pores increased and the polished surface became rough; as a result, the relative density decreased. Fig. 6 depicts TEM bright-field images and a selected area electron diffraction (SAED) pattern of the SiO2 glass composite sintered at 1723 K. In Fig. 6(a), SiO2 grains with size 100–500 nm were separated by 20–30-nm-thick intergranular layers. Because the starting powder size was 250 nm, the grain growth was not significant during the sintering at 1723 K. Fig. 6(b) depicts the high magnification image of the intergranular layers and the corresponding SAED pattern. The SiC lattice patterns with the spacing of ∼0.25 nm in the size of several nanometers were observed, e.g., as designated with region (i). This observation corresponded to the polycrystalline Debye rings in the SAED pattern, as shown in Fig. 6(b). In addition to the SiC lattice patterns, lattice images with the spacing of ∼0.34 nm were found in region (ii) in Fig. 6(b), indicating the presence of graphite in intergranular layers. Thus, the intergranular layers were nanocrystalline composites of SiC and graphite. Prior to SPS sintering, particulate SiC layers were deposited by RCVD using HMDS at 998 K, which would result in a carbon-rich composition. The carbon-rich SiC layers resulted in the formation of intergranular layers comprised of nanocrystalline SiC and graphite during SPS sintering. Fig. 7 depicts the effect of sintering temperature on the Hv of the SiO2 glass composites. The Hv values gradually increased with the sintering temperature increased from 1073 to 1473 K, and significantly increased at approximately 1600 K, corresponding to the densification as shown in Fig. 4. In the sintering temperature range of 1623–1823 K, the Hv values of the SiO2 glass composite reached 14 GPa. This value is almost twice that of monolithic SiO2 glass prepared by SPS [3]. Kurkjian et al. reported that the Hv values of the typical glass materials of soda-lime silica glass, fused silica and single-crystal ␣-quartz were 5.5, 7–9 and 12.2 GPa, respectively, at room temperature [14,15]. The effect of sintering temperatures on KIC of the SiO2 glass composite is shown in Fig. 8. The KIC value of the SiO2 glass composite increased with sintering temperature, reaching a maximum value of 5.4 MPa m1/2 at 1723 K. This value
Fig. 7. Effect of sintering temperature on HV of the SiO2 glass composite.
Fig. 8. Effect of sintering temperature on KIC of the SiO2 glass composite.
was almost nine times higher than that of monolithic SiO2 glass by SPS [3]; the typical KIC value of fused SiO2 was approximately 0.8 MPa m1/2 [16,17]. As the sintering temperature was further increased to 1873 K, the KIC values decreased. The SiO2 glass composite prepared in the sintering temperature range of 1623–1773 K
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would be associated with the crack deflections, resulting in the high KIC values. 4. Conclusions SiC layers approximately 10 nm thick were deposited on SiO2 powder to form SiO2 glass (core)/SiC (shell) by RCVD at a deposition temperature of 998 K. The powder was consolidated to dense SiO2 glass composite at relative densities greater than 90% by SPS at 1573–1823 K. The SiO2 glass composite had a small-sized microstructure consisting of 100–500 nm SiO2 glass grains bonded via inergranular layers of nanocrystalline SiC of the thickness of 5–10 nm. The SiO2 glass composite at the optimum sintering temperature of 1723 K exhibited the highest fracture toughness and hardness of 14.2 GPa and 5.4 MPa m1/2 , respectively. Acknowledgements This research was supported by Grant-in-Aid for Young Scientists (A) No. 16K14089, the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. This research was partly supported by the Fundamental Research Funds for the Central Universities (WUT: 2015IVA061) and the Doctoral Scientific Research Foundation of Wuhan University of Technology No. 40120233. References
Fig. 9. The indentation imprint (a) and crack propagation ((b) and (c)) in the SiO2 glass composite sintered at 1723 K.
exhibited significantly high hardness (Hv > 12 GPa) and toughness (KIC > 4.5 MPa m1/2 ) with dense microstructure. The nano-grains of SiO2 glass composite would resulted in the high hardness of the composite. Fig. 9 shows SEM images of the indentation imprint (a) and radial crack propagation ((b) and (c)) at an indentation load of 9.8 N in the SiO2 glass composite at 1723 K. In Fig. 8(a), the indentation imprint involved cracks at the contact point and the cone crack at the center; these features corresponded to a typical regime of indentation imprint in fused silica and silicate glasses [18,19]. In Fig. 8(b) and (c), apparent crack deflections and tiny deflections at the tip of the radial cracks were observed. The intergranular layers suppressed the grain growth of SiO2 , forming the nano-grain SiO2 glass composite. The microstructure with nano-grain SiO2 glass
[1] T. Kokubo, Bioactive glass ceramics: properties and applications, Biomaterials 12 (1991) 155–163. [2] R.K. Brow, M.L. Schmitt, A survey of energy and environmental applications of glass, J. Eur. Ceram. Soc. 29 (2009) 1193–1201. [3] J.F. Zhang, R. Tu, T. Goto, Preparation of Ni-precipitated hBN powder by rotary chemical vapor deposition and its consolidation by spark plasma sintering, J. Alloys Compd. 502 (2010) 371–375. [4] J. Ning, J. Zhang, Y. Pan, J. Guo, Fabrication and mechanical properties of SiO2 matrix composites reinforced by carbon nanotube, Mater. Sci. Eng. A 357 (2003) 392–396. [5] Z. He, H. Katsui, R. Tu, T. Goto, High-hardness and ductile mosaic SiC/SiO2 composite by spark plasma sintering, J. Am. Ceram. Soc. 97 (2014) 681–683. [6] J. Zhang, R. Tu, T. Goto, Densification of SiO2 –cBN composites by using Ni nanoparticle and SiO2 nanolayer coated cBN powder, Ceram. Int. 38 (2012) 4961–4966. [7] Z. He, R. Tu, H. Katsui, T. Goto, Synthesis of SiC/SiO2 core-shell powder by rotary chemical vapor deposition and its consolidation by spark plasma sintering, Ceram. Int. 39 (2013) 2605–2610. [8] H. Katsui, Z.H. He, T. Goto, Silicon carbide coating on diamond powder by rotary chemical vapor deposition, Key Eng. Mater. 508 (2012) 65–68. [9] Z.H. He, H. Katsui, R. Tu, T. Goto, Surface modification of silicon carbide powder with silica coating by rotary chemical vapor deposition, Key Eng. Mater. 616 (2014) 232–236. [10] S. Baud, F. Thevenot, Microstructures and mechanical properties of liquid-phase sintered seeded silicon carbide, Mater. Chem. Phys. 67 (2001) 165–174. [11] J.F. Zhang, R. Tu, T. Goto, Densification, microstructure and mechanical properties of SiO2 –cBN composites by spark plasma sintering, Ceram. Int. 38 (2012) 351–356. [12] B.R. Lawn, E.R. Fuller, Equilibrium penny-like cracks in indentation fracture, J. Mater. Sci. 10 (1975) 2016–2024. [13] K. Tanaka, Elastic/plastic indentation hardness and indentation fracture toughness: the inclusion core model, J. Mater. Sci. 22 (1987) 1501–1508. [14] R.F. Cook, G.M. Pharr, Direct observation and analysis of indentation cracking in glasses and ceramics, J. Am. Ceram. Soc. 73 (1990) 787–817. [15] C.R. Kurkjian, G.W. Kammlott, M.M. Chaudhri, Indentation behavior of soda-lime silica glass fused silica, and single-crystal quartz at liquid nitrogen temperature, J. Am. Ceram. Soc. 78 (1995) 737–744. [16] N. Shinkai, R.C. Bradt, G.E. Rindone, Fracture toughness of fused SiO2 and float glass at elevated temperatures, J. Am. Ceram. Soc. 64 (1981) 426–430. [17] S.M. Wiederhorn, Fracture surface energy of glass, J. Am. Ceram. Soc. 52 (1969) 99–105. [18] J.T. Hagan, Cone cracks around Vickers indentations in fused silica glass, J. Mater. Sci. 14 (1979) 462–466. [19] S. Yoshida, M. Kato, A. Yokota, S. Sasaki, A. Yamada, J. Matsuoka, N. Soga, C.R. Kurkjian, Direct observation of indentation deformation and cracking of silicate glasses, J. Mater. Res. 30 (2015) 2291–2299.
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Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Mechanical properties of nano-grain SiO2 glass prepared by spark plasma sintering Zhenhua He a,b , Hirokazu Katsui b,∗ , Takashi Goto b a Key Laboratory of Fiber Optic Sensing Technology and Information Processing, Ministry of Education, Wuhan University of Technology, Wuhan 430070, PR China b Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
a r t i c l e
i n f o
Article history: Received 21 April 2016 Received in revised form 20 September 2016 Accepted 21 September 2016 Available online xxx Keywords: SiC-coated SiO2 glass powder Rotary chemical vapor deposition Spark plasma sintering High hardness High fracture toughness
a b s t r a c t Silicon carbide (SiC) layers were deposited on silica (SiO2 ) glass powder by rotary chemical vapor deposition (RCVD) to form SiO2 glass (core)/SiC (shell) powder; this powder was consolidated by spark plasma sintering (SPS). SiO2 glass powder with a particle size of 250 nm was coated with 5–10-nm-thick SiC layers. The resultant SiO2 glass (core)/SiC (shell) powder was consolidated to form a nano-grain SiO2 glass composite at a relative density above 90% by SPS in the sintering temperature range of 1573–1823 K. The Vickers hardness and fracture toughness of the SiO2 glass composite at 1723 K were found to be 14.2 GPa and 5.4 MPa m1/2 , respectively. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Silica (SiO2 ), one of the most common oxide materials, has excellent insulation, high chemical stability, low thermal-expansion coefficient and low thermal conductivity [1,2]. Therefore, SiO2 glass is used to produce optical fibres, chemical containers, windows and crucibles. Although SiO2 glass is usually fabricated by meltingsolidification, Zhang et al. prepared SiO2 glass by spark plasma sintering (SPS) at 1573 K to form a dense and transparent compact material with a hardness of 7.5 GPa and fracture toughness of 0.6 MPa m1/2 [3]. The refinement of the microstructure with the addition of secondary phases may reinforce the mechanical properties of SiO2 . Ning et al. consolidated SiO2 powder with 5vol% carbon nanotubes by hot-pressing and reported a bending strength and fracture toughness of 85 MPa and 2.0 MPa m1/2 , respectively [4]. Although SPS enables the consolidation of many materials in a short time, thus preventing the grain growth, SiO2 glass has no grains with brittle uniform microstructure. This characteristic prevents further application of SiO2 for structural usage. The sintering of core (glass)/shell (glass or crystalline) powder results in the formation of a fine grain microstructure even if the matrix material is glass (or amorphous). Rotary chemical
∗ Corresponding author. E-mail address: [email protected] (H. Katsui).
vapor deposition (RCVD) enables the coating of glass or crystalline materials on powder surfaces with particle sizes below several microns [5–9]. In this study, we prepared SiO2 glass (core)/SiC (shell) powder by using RCVD. SiC is a promising second phase to reinforce SiO2 -based materials due to its high strength, oxidation resistance and good compatibility with SiO2 . The SiC shell would effectively prevent the grain growth of the SiO2 matrix, thereby enabling the formation of a nano-grained SiO2 glass body. The SiO2 glass (core)/SiC (shell) powder was consolidated by SPS. The crystal phases, microstructure, and mechanical properties of the nano-grain SiO2 glass composite were investigated.
2. Experimental SiO2 glass powder (250 nm in diameter, Admatechs Ltd., Japan) was coated with SiC by RCVD using C6 H18 Si2 (hexamethyldisilane: HMDS) as a precursor, forming SiO2 glass (core)/SiC shell powder. The details of the RCVD apparatus for SiC coating are presented elsewhere [7]. The precursor was evaporated at 298 K and carried into a reaction chamber by Ar gas at a flow rate of 8.33 × 10−7 m3 s−1 . SiO2 glass powders were stirred and suspended in a reactor chamber rotating at 45 rpm. The reaction chamber was heated at 998 K. The total pressure in the chamber was maintained at 400 Pa. The deposition time was 7.2 ks. The resultant SiO2 glass (core)/SiC (shell) powder was consolidated by SPS (SPS-210LX, Fujidenpakoki, Japan) and poured into
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Fig. 1. XRD patterns of (a) monolithic SiO2 powder and (b) SiC-coated SiO2 glass powder.
a 10-mm-inner-diameter graphite die. This graphite die was covered with carbon wool, which acted as a thermal insulator. The heating rate was 1.7 K s−1 and the soaking time was 0.6 ks. The sintering temperature ranged from 1073 to 1873 K. After the soaking, the application of heating current was ceased and the temperature rapidly decreased. The temperature was measured by an optical pyrometer focused on a hole (2 mm diameter and 5 mm depth) in the graphite die. The loading pressure was 100 MPa. The crystal phases of the SiO2 glass (core)/SiC (shell) powder and sintered SiO2 glass composites were identified by X-ray diffraction (XRD; CuK␣, Rigaku, Japan). The microstructures were observed by scanning electron microscopy (SEM; S-3100H, Hitachi, Japan and JSM-7500F, JEOL, Japan) and transmission electron microscopy (TEM, EM-002B, TOPCON, Japan). The values of SiC content (CSiC , mass%) of the samples were calculated from the mass gain by RCVD. Relative densities of the sintered bodies of the SiO2 glass composites were measured following the Archimedes’ method using distilled water, where the theoretical densities of SiC and SiO2 were 3.21 Mg m−3 [10] and 2.20 Mg m−3 [11], respectively. The Vickers hardness (Hv ) and fracture toughness (KIC ) were determined at room temperature using a Vickers microhardness tester (HM-221, Mitutoyo, Japan) at load P of 0.98 and 9.8 N. The hardness and toughness values were determined at 10 points. The fracture toughness of KIC (MPa m1/2 ) was calculated by Eq. (1) [12,13]: KIC = 1.173 × 10−6 ×
P c 3/2
(1)
where c (m) is the average half length of the cracks formed around the corners of the indentations.
Fig. 2. SEM images at (a) low and (b) high magnifications and (c) a TEM bright-field image of SiC-coated SiO2 glass powder.
3. Results and discussion Fig. 1 shows the XRD patterns of the monolithic SiO2 glass powder (a) and SiC-coated SiO2 glass powder prepared by RCVD (b). A broad peak at around 2 = 22.0◦ was attributed to the glass structure of SiO2 . The small reflection peaks at 2 = 35.6◦ and 60.0◦ were assigned to 111 and 220, respectively, of SiC (-type) of the SiCcoated SiO2 glass powder. No reflection peaks from impurities, e.g., carbon, were identified in the SiC-coated SiO2 glass powder. The microstructures of SiC-coated SiO2 glass powder are depicted in Fig. 2. The SiO2 glass powder particles were spherical, as shown in the SEM image in Fig. 2(a). In the image at high magnification (Fig. 2(b)) and the TEM bright-field image (Fig. 2(c)), a SiC layer of
a few nanometers thickness (5–10 nm) and particles 10–30 nm in diameter formed on the SiO2 glass powder surface. Although some SiC particles formed on the SiO2 glass powder, the SiO2 glass powder was completely coated by the SiC layer. Hereafter, the powder is denoted as SiO2 glass (core)/SiC (shell) powder. The SiC content in the powder was estimated as 20 mass% according to the mass gain during RCVD. The SiO2 glass (core)/SiC (shell) powder was consolidated by SPS; the effect of sintering temperature on XRD patterns of the sintered bodies at 1073–1873 K is shown in Fig. 3. At sintering temperatures below 1473 K, the XRD patterns had a broad peak at approximately 2 = 22.0◦ , attributed to amorphous SiO2 , and reflec-
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Fig. 4. Effect of sintering temperature on the relative density of the SiO2 glass composite. Fig. 3. Effect of sintering temperature on the XRD patterns of the SiO2 glass composites: (a) SiC-coated SiO2 glass powder, (b) 1073 K, (c) 1273 K, (d) 1473 K, (e) 1673 K, (f) 1773 K and (g) 1873 K.
tion peaks at 2 = 35.6◦ and 60.0◦ , assigned as 111 and 220 SiC, respectively. With increasing sintering temperature, the sharp peak appeared at 2 = 22.0◦ in the temperature range of 1473–1723 K, indicating crystallization of SiO2 glass from amorphous to cristobalite. Upon further increasing the temperature to 1873 K, the sintered body comprised SiO2 glass and SiC (-type). At temper-
atures higher than the melting point of cristobalite, the SiO2 could be melted and quenched to be SiO2 glass at the high cooling rate in the SPS procedure. The melting point of cristobalite is 1996 K. The electrical current could pass through the SiC shell at high temperatures during the SPS sintering and the sample in a graphite die might be heated to temperature higher than the monitored temperature by using the optical pyrometer focused on a hole in the graphite die. Fig. 4 depicts the effect of sintering temperature on the relative density of the SiO2 glass composite. As the sintering temperature
Fig. 5. SEM images of the SiO2 glass composites sintered at 1073 (a), 1473 (b), 1573 (c), 1723 (d), 1773 (e) and 1873 K (f).
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Fig. 6. TEM bright-field images of the SiO2 glass composite sintered at 1723 K. (b) The high-magnification image of (a) and the corresponding SEAD pattern.
was increased, the SiO2 glass composite was significantly densified at 1500 K, with the relative density reaching 90%, whereas the density decreased to 80% at 1873 K due to the reaction between SiO2 and SiC [5]. Fig. 5 shows the microstructures of the SiO2 glass composite in the sintering temperature range of 1073–1873 K. At 1073 K (Fig. 5(a)), the powder was not densified and the small particulate shape remained, as the relative density was low (Fig. 4). At 1473 K (Fig. 5(b)), the powder was densified, although a significant number of pores were observed. In the sintering temperature range of 1573–1773 K (Fig. 5(c–e)), the SiO2 glass composite had a dense morphology with a few pores. As the sintering temperature was further increased to 1873 K (Fig. 5(f)), the number of pores increased and the polished surface became rough; as a result, the relative density decreased. Fig. 6 depicts TEM bright-field images and a selected area electron diffraction (SAED) pattern of the SiO2 glass composite sintered at 1723 K. In Fig. 6(a), SiO2 grains with size 100–500 nm were separated by 20–30-nm-thick intergranular layers. Because the starting powder size was 250 nm, the grain growth was not significant during the sintering at 1723 K. Fig. 6(b) depicts the high magnification image of the intergranular layers and the corresponding SAED pattern. The SiC lattice patterns with the spacing of ∼0.25 nm in the size of several nanometers were observed, e.g., as designated with region (i). This observation corresponded to the polycrystalline Debye rings in the SAED pattern, as shown in Fig. 6(b). In addition to the SiC lattice patterns, lattice images with the spacing of ∼0.34 nm were found in region (ii) in Fig. 6(b), indicating the presence of graphite in intergranular layers. Thus, the intergranular layers were nanocrystalline composites of SiC and graphite. Prior to SPS sintering, particulate SiC layers were deposited by RCVD using HMDS at 998 K, which would result in a carbon-rich composition. The carbon-rich SiC layers resulted in the formation of intergranular layers comprised of nanocrystalline SiC and graphite during SPS sintering. Fig. 7 depicts the effect of sintering temperature on the Hv of the SiO2 glass composites. The Hv values gradually increased with the sintering temperature increased from 1073 to 1473 K, and significantly increased at approximately 1600 K, corresponding to the densification as shown in Fig. 4. In the sintering temperature range of 1623–1823 K, the Hv values of the SiO2 glass composite reached 14 GPa. This value is almost twice that of monolithic SiO2 glass prepared by SPS [3]. Kurkjian et al. reported that the Hv values of the typical glass materials of soda-lime silica glass, fused silica and single-crystal ␣-quartz were 5.5, 7–9 and 12.2 GPa, respectively, at room temperature [14,15]. The effect of sintering temperatures on KIC of the SiO2 glass composite is shown in Fig. 8. The KIC value of the SiO2 glass composite increased with sintering temperature, reaching a maximum value of 5.4 MPa m1/2 at 1723 K. This value
Fig. 7. Effect of sintering temperature on HV of the SiO2 glass composite.
Fig. 8. Effect of sintering temperature on KIC of the SiO2 glass composite.
was almost nine times higher than that of monolithic SiO2 glass by SPS [3]; the typical KIC value of fused SiO2 was approximately 0.8 MPa m1/2 [16,17]. As the sintering temperature was further increased to 1873 K, the KIC values decreased. The SiO2 glass composite prepared in the sintering temperature range of 1623–1773 K
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would be associated with the crack deflections, resulting in the high KIC values. 4. Conclusions SiC layers approximately 10 nm thick were deposited on SiO2 powder to form SiO2 glass (core)/SiC (shell) by RCVD at a deposition temperature of 998 K. The powder was consolidated to dense SiO2 glass composite at relative densities greater than 90% by SPS at 1573–1823 K. The SiO2 glass composite had a small-sized microstructure consisting of 100–500 nm SiO2 glass grains bonded via inergranular layers of nanocrystalline SiC of the thickness of 5–10 nm. The SiO2 glass composite at the optimum sintering temperature of 1723 K exhibited the highest fracture toughness and hardness of 14.2 GPa and 5.4 MPa m1/2 , respectively. Acknowledgements This research was supported by Grant-in-Aid for Young Scientists (A) No. 16K14089, the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. This research was partly supported by the Fundamental Research Funds for the Central Universities (WUT: 2015IVA061) and the Doctoral Scientific Research Foundation of Wuhan University of Technology No. 40120233. References
Fig. 9. The indentation imprint (a) and crack propagation ((b) and (c)) in the SiO2 glass composite sintered at 1723 K.
exhibited significantly high hardness (Hv > 12 GPa) and toughness (KIC > 4.5 MPa m1/2 ) with dense microstructure. The nano-grains of SiO2 glass composite would resulted in the high hardness of the composite. Fig. 9 shows SEM images of the indentation imprint (a) and radial crack propagation ((b) and (c)) at an indentation load of 9.8 N in the SiO2 glass composite at 1723 K. In Fig. 8(a), the indentation imprint involved cracks at the contact point and the cone crack at the center; these features corresponded to a typical regime of indentation imprint in fused silica and silicate glasses [18,19]. In Fig. 8(b) and (c), apparent crack deflections and tiny deflections at the tip of the radial cracks were observed. The intergranular layers suppressed the grain growth of SiO2 , forming the nano-grain SiO2 glass composite. The microstructure with nano-grain SiO2 glass
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