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Preparation and characterization of porous Si3N4-bonded SiC ceramics and morphology change mechanism of Si3N4 whiskers
Ceramics International 45 (2019) 5922–5926
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Preparation and characterization of porous Si3N4-bonded SiC ceramics and morphology change mechanism of Si3N4 whiskers
T
⁎
Changlian Chen , Xin Liang, Maya Luo, Shicong Zhou, Jiayou Ji, Zhiliang Huang, Man Xu School of Material Science and Engineering, Wuhan Institute of Technology, Wuhan 430073, People's Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Porous Si3N4-bonded SiC ceramic Si3N4 whiskers Morphology Porosity Bending strength
Porous Si3N4-bonded SiC ceramics with high porosity were prepared by the reaction-sintering method. In this process, Si3N4 was synthesized by the nitridation of silicon powder. The X-ray diffraction (XRD) indicated that the main phases of the porous Si3N4-bonded SiC ceramics were SiC, α-Si3N4, and β-Si3N4, respectively. The contents of β-Si3N4 were increased following the sintering temperature. The morphology of Si3N4 whiskers was investigated by scanning electron microscope (SEM), which was shown that the needle-like (low sinteringtemperature) and rod-like (higher sintering-temperature) whiskers were formed, respectively. From low to high synthesized temperature, the highest porosity of the porous Si3N4 bonded SiC ceramic was up to 46.7%, and the bending strength was ~11.6 MPa. The α-Si3N4 whiskers were derived from the reaction between N2 and Si powders, the growth mechanism was proved by Vapor–Solid (VS). Meanwhile, the growth mechanism of β-Si3N4 was in accordance with Vapor–Solid–Liquid (VSL) growth mechanism. With the increase of sintering temperature, Si powders were melted to liquid silicon and the α-Si3N4 was dissolved into the liquid then the β-Si3N4 was precipitated successfully.
1. Introduction Si3N4 and SiC are both primary functional ceramic materials. Si3N4bonded SiC composite materials have attracted extensive attention due to their high strength, high hardness, and excellent corrosion resistance [1]. Moreover, the porous Si3N4-bonded SiC ceramics, which have macroscopic pores, exhibited some unique performances, such as acid and alkali resistance, high permeability, and narrow pore size distribution [2,3]. Based on the above good features, the porous Si3N4bonded SiC ceramics have been a new kind of materials to deal with the environmental issue, including industrial wastewater, hot corrosive gases, and precision filtration separation. Meanwhile, as a porous ceramic support, the porous Si3N4-bonded SiC ceramic also can filter the suspended solids, colloid substances, and microbes, or absorb them into interlaced holes [4–6]. In addition to the advantages mentioned above, however, Si and C have a high self-diffusion coefficient, which results in the sintering temperature is needed above 2100 °C to prepare porous SiC ceramics via an in situ reaction bonding technique [7]. Therefore, the widelyused method to promote the sintering of the SiC ceramics is to develop a special sintering process or rely on the second phase [8]. In the Si3N4bonded SiC composite materials, Si3N4 is one of the non-oxide bonds of SiC ceramics as a second phase. Si3N4 usually exists in α or β crystal ⁎
form, in which the α-Si3N4 is granule and the β-Si3N4 is needle-like structure [9]. Previous studies indicated that the α-Si3N4 was expected to transform into β-Si3N4 at approximate 1327 ℃ through the “dissolution–precipitation” process in the liquid [10–12]. For porous ceramics, however, there is a direct contradictory between porosity and strength. Namely, with the increase of the porosity, the strength decreases with exponential distribution [13]. In order to enhance the strength of the porous SiC ceramics without reducing the porosity, increasing the bonding degree of the SiC particle is a promising way. Compared with other reinforcement methods, ceramic whiskers are easier to prepare and control [14]. Furthermore, whisker exhibits toughening and reinforcing effect on ceramic matrixes as a second phase [15]. Nevertheless, the morphology and arrangement of Si3N4 whiskers have a significant effect on the hardness and strength because it is difficult to obtain homogeneous dispersion and distribution whisker [16]. In the whisker-reinforced ceramic matrix composites, the mechanical properties are enhanced by the reinforcing effect from the whisker-oriented alignment [17]. In this study, Si3N4 was synthesized via direct nitridation of silicon powder to bond SiC particles in the reactive sintering process. In order to reduce the sintering temperature, Y2O3 was doped into the raw materials as a sintering additive. The morphology changes and growth mechanism of Si3N4 whiskers, as well as the effect of whiskers
Corresponding author. E-mail address: [email protected] (C. Chen).
https://doi.org/10.1016/j.ceramint.2018.12.060 Received 23 November 2018; Received in revised form 4 December 2018; Accepted 8 December 2018 Available online 10 December 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 45 (2019) 5922–5926
C. Chen et al.
YSiO2N existed in the sintered samples, and the supposed reactions have been listed as follows:
morphology on the mechanical properties for porous Si3N4, bonded SiC ceramics were investigated.
S i (s) + SiO2(S ) = 2SiO (g)
(1)
6S iO (s)+4N2(g ) = 2S i3N4(s) + 3O2(g )
(2)
2Y2 O3(s) + S iO2(s) + Si3 N4(s) = 4YS iO2 N(s)
(3)
2. Material and methods The raw materials used in this work are commercial SiC powder (purity > 98.5%; d50 = 20.9 µm; Lifeng Silicon Carbide Micro-powder Co. Ltd., Henan, China), Si powder (purity > 99%; d50 = 20.3 µm), and Y2O3 (purity > 99.99%; Aladdin chemistry Co. Ltd., Shanghai, China), respectively. The weight ratio of SiC, Si, and Y2O3 is 81:15:4. These powders were mixed with ethyl alcohol in an agate mortar for 6 h. After vacuum dried at 90 °C for 12 h, certain binder (PEG) was added into the mixed powder, then 3.5 g uniformly-mixed powder was uniaxially pressed into a circular steel die (diameter is 25 mm) under the 24 MPa. The green samples were pre-sintered at 270 °C in the air and kept for 1 h in order to get rid of PEG. Under a nitrogen atmosphere, the pre-sintered compacts were further sintered at 1400 °C, 1440 °C, 1480 °C and 1520 °C, respectively for 3 h. The phase composition of the sintered specimens was determined by X-ray diffractometry (XRD, D/MAX-IIIB, Rigaku, Osaka, Japan) with CuKα radiation. The microstructure and morphology of porous Si3N4-bonded SiC ceramics were observed by scanning electron microscope (SEM, JEOL JSM5610LV, Tokyo, Japan). The porosity of the samples was measured by the hydrostatic weighing method which combined with the Archimedes method. The bending strength was determined using three-point loading with 30 mm span (WDW50 material testing machine, Jinan, China). The pore size distribution was measured by capillary flow porometer (Porolux 500, Germany).
Fig. 2 presented the SEM micrographs of the fractured surfaces of the samples sintered at various sintering temperatures. The SEM images illustrated the existence of various morphologies of the as-synthesized Si3N4 crystal and the pore structures of the specimens. According to the SEM micrographs, the sintering temperature had an obvious effect on the morphology of the Si3N4 whiskers and resulted from indifferent pore structures of the sintered samples. When the sintering temperature was between 1400 and 1440 °C, the needle-like Si3N4 whiskers grew in the intergranular pores radially. With the sintering temperature up to 1480 °C, most of Si3N4 presented granular morphology and attached on the SiC particles while the rest of Si3N4 grew thick and short. While the sintering temperature reached to 1520 °C, the Si3N4 grains became rodlike shape and gathered on the surface of SiC particles, and it was obvious that the specimens had a lot of intergranular pores which were not plugged by rod-like whiskers. 3.2. Porosity and bending strength The trend of porosity of the different sintered samples was displayed in Fig. 3. The porosity gradually increased with the increase of sintering temperature. At 1520 °C, the porosity reached the maximum value of 46.7%, which can be explained by the fact of the morphology changes of Si3N4 whiskers. The Si3N4 whiskers were prepared by a direct nitridation of silicon among the SiC grains, and the SiC grains did not obey the principle of close-packed structure, which led to the porosity of SiC matrix more than 25.95%. Combining with Fig. 2, the needle-like Si3N4 whiskers were gradually transformed into rod-like shape with the increase of sintering temperature. Simultaneously, the intergranular pores were no longer filled with Si3N4 whiskers, which helped the SiC matrix to build a high whole hollow structure. Along with the sintering temperature rising, the bending strength was dropped from 19.8 MPa to 14.3 MPa as shown in Fig. 4. Previous researches [18] have demonstrated that the bending strength was decreased according to the exponential relationship with porosity increase when the pore size was greater than the critical crash size of the material. The empirical formula is listed as follow:
3. Results and discussion 3.1. Phase and microstructure The XRD patterns of the samples sintered at different temperatures were illustrated in Fig. 1. It was observed that the specimens were mainly composed of α- and β-phase SiC, α- and β-phase Si3N4 and minor YSiO2N. The diffraction peaks of α-Si3N4 and β- Si3N4 were detected obviously without Si peak, which demonstrated that the silicon was entirely azotizing in the process of sintering. Si3N4 existed only as α crystal form for the samples sintered at 1400 °C, while both α-Si3N4 and β-Si3N4 were detected for the samples sintered at 1440 °C. Furthermore, the number of diffraction peaks of β-Si3N4 increased as enhancing sintering temperature. The observations were highly in accordance with the previous researches [11–13], which reported that the α-Si3N4 was transformed into β-Si3N4 with the increase of sintering temperature. Owing to a little amount of silicon dioxide mixed in the raw materials,
σf = σ0 exp(−nP )
(4)
where the σf is bending strength, P is porosity [14]. Thus, there is a linear relationship between ln σf and P, and the linear relevant fitting result was shown in Fig. 5. The linear relevant fitting result was Y = 6.7771–0.0876X. Based on the fitting curve, the bending strength of Si3N4 bonded SiC dense ceramics was theoretically up to 877 MPa according to the Y-intercept value. The experimental results also confirmed the relationship between bending strength and porosity. Hence, the morphology changes of Si3N4 whiskers were affected not only by porosity but also by bending strength. The experimental results revealed that needle-like whisker contributed more to the bending strength than rod-like whisker owing to the bridging of the needle-like Si3N4 whiskers. The Si3N4-bonded SiC ceramics with high porosity were prepared, of which the pore diameter distribution was also an important factor for the application in adsorbent filtering fields. Fig. 6 exhibited the pore diameter flow distribution of porous Si3N4-bonded SiC ceramics sintered at various temperatures. The pore diameter distribution of the sintered samples was relatively narrow and concentrated in 0.5–2 µm, and the maximum percent flow of the samples was almost at the range of 18–23%, which demonstrated that the sintering temperature had less
Fig. 1. XRD patterns of the samples sintered at different temperatures. 5923
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Fig. 2. SEM micrographs of the fractured surfaces of the samples sintered at various temperatures. (A) 1400 °C, (B)1440 °C, (C) 1480 °C, (D)1520 °C.
Fig. 3. Effect of different sintering temperatures on the porosity of porous Si3N4 bonded SiC ceramics.
Fig. 4. Bending strength of porous Si3N4 bonded SiC ceramics sintered at various temperatures.
influence on the pore diameter distribution. The contribution to the filter flow by different pore diameter was shown in Fig. 7. Although, the pore diameter distribution of the samples distributed in 0.5–2 µm range, the real contribution to the filter flow was the pore diameter for 0.75–1.8 µm. Compared with the results in Fig. 3, the porosity of samples achieved maximum when sintered at 1520 °C, which may benefit from the large filter flow of the pore diameter for 0.8–1.8 µm. Although the inner pore structure of the SiC matrix was very complex, large porosity was conductive to produce high percent flow value. In addition, the percent flow of the porous Si3N4 bonded SiC ceramics were also impacted by the pore-throat configuration. Three typical cases of the pore-throat configuration were displayed in Fig. 8. Besides example (A), the percent flow of the porous ceramics was impaired by
the other two pore-throat configurations due to the pore throat or pore was so narrow and small that the testing gas cannot go through the samples smoothly. 3.3. The Growth mechanism of Si3N4 crystal with various morphologies Up to now, Vapor–Solid (VS) and Vapor–Liquid–Solid (VLS) mechanisms are two widely accepted Si3N4 whisker growth mechanisms. The obvious distinction between two growth mechanisms is that a spherical liquid droplet existed on the whisker tip under VLS mechanism owing to a few catalysts like Fe, which provided liquid phase for whisker impurity nucleation. At the beginning of the growth of silicon nitride whisker, the prime whisker was α-Si3N4 and the growth 5924
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Fig. 8. Three pore structures of porous ceramics, (A) the pore throat radius is nearly the same to pore size, (B) the pore throat radius is smaller than pore size, (C) the pore throat radius is much bigger than the pore size.
Fig. 5. Linear fitting of ln σf and porosity of porous Si3N4 bonded SiC ceramics.
Fig. 9. Schematic illustration for the proposed growth mechanism of Si3N4 whisker.
Fig. 6. Pore diameter flow distribution of porous Si3N4 bonded SiC ceramics sintered at various temperatures.
Fig. 10. Schematic illustration for the morphology transformation process of Si3N4 whisker.
between SiC grains. The high saturation of Si vapors promoted the small crystal nuclei to be aggregated to form Si3N4 grains. Then Si vapor, SiO and nitrogen gas continuously reacted on Si3N4 grain. Si3N4 was polar molecular crystal because of the hexagonal structure, the interfacial energy of C-axis is lower than A-axis so that the needle-like whisker was grown up along the C-axis direction [19]. From Fig. 1, it has been proved that high temperature promoted αSi3N4 change to β phase, and in general, β-Si3N4 existed as rod-like shape. The change from needle-like whiskers to rod-like whiskers may be related to the crystal growth process, as shown in Fig. 10. Park, [20] has found that liquid silicon was a suitable liquid to solve Si3N4 and assisted the transformation from α phase to β phase. Gibbs [21] presented a definition of the equilibrium shape of the crystal, which indicated that the crystal thermodynamically had a minimum surface
Fig. 7. The filter flow in different pore diameter of porous Si3N4 bonded SiC ceramics sintered at various temperatures.
mechanism was VS mechanism. With the increase of sintering temperature, the α-Si3N4 was changed into β-Si3N4, the main whisker growth mechanism of β-Si3N4 was became to VLS mechanism. The formation of α-Si3N4 whiskers go through a nucleation and the growth process, as shown in Fig. 9. With the temperature increasing, a little of silicon was converted to vapor, and a part of silicon was reacted with silicon dioxide to form SiO gas as expressed by Eq. (2). The rapid nucleation process of the reaction occurred between Si and nitrogen with lots of small crystal nuclei of Si3N4, which adhered to the interface 5925
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energy at given constant volume. When the system temperature was higher than 1410 °C, the solid silicon was melted to be fused silicon, needle-like whisker was gradually dissolved in the liquid silicon. Meanwhile, high temperature accelerated the diffusion rate of Si, N2, and SiO, which deposited on the dissolved Si3N4 whiskers, then the rodlike Si3N4 whiskers grew up because of the surface tension effect and the interfacial energy of needle-like Si3N4 whisker which was wetted by fused silicon.
[2] [3]
[4]
[5]
4. Conclusions
[6]
(1) Porous Si3N4-bonded SiC ceramics were prepared via reactive sintering under the nitrogen atmosphere. The porosity of the sintered samples was increased from 43.16% to 46.68% with the increase of sintering temperatures. On the contrary, the variation of the bending strength of the sintered samples showed a reverse rule, which decreased from 19.8 MPa to 14.3 MPa. The porous Si3N4bonded SiC ceramics possessed a relatively narrow aperture size distribution of 0.5–2 µm, and the 0.75–1.8 µm pore contributed the filter flow of the sintered samples sintered at the various sintering temperature. (2) The sintering temperature controlled the diffusion rate and concentration of the reactant gas. Following the temperature increasing, The morphology of Si3N4 whisker was gradually transformed from needle-like to rod-like. The needle-like Si3N4 whisker which lapped among the SiC particles improved the strength of the porous ceramics. However, the rod-like Si3N4 whiskers were beneficial to the porosity. (3) The growth mechanism of α-Si3N4 was VS mechanism, and the growth mechanism of β-Si3N4 obeyed VLS mechanism. Taking the effect of screw dislocation and interfacial energy into account, the whisker finally grew along the long axis direction. Furthermore, the minimum surface energy promoted the needle-like Si3N4 whisker which was dissolved in liquid silicon to grow thick and short, then needle-like was gradually transformed into rod-like Si3N4 whiskers.
[7]
[8] [9]
[10]
[11] [12]
[13] [14]
[15]
[16]
[17]
[18]
Acknowledgments
[19]
This work was supported by the Natural Science Foundation of Hubei Province (2014CFB796), the Science & Technology Pillar Program of Hubei Province (2015BAA105) and the National Natural Science Foundation of China (21305105).
[20] [21]
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Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Preparation and characterization of porous Si3N4-bonded SiC ceramics and morphology change mechanism of Si3N4 whiskers
T
⁎
Changlian Chen , Xin Liang, Maya Luo, Shicong Zhou, Jiayou Ji, Zhiliang Huang, Man Xu School of Material Science and Engineering, Wuhan Institute of Technology, Wuhan 430073, People's Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Porous Si3N4-bonded SiC ceramic Si3N4 whiskers Morphology Porosity Bending strength
Porous Si3N4-bonded SiC ceramics with high porosity were prepared by the reaction-sintering method. In this process, Si3N4 was synthesized by the nitridation of silicon powder. The X-ray diffraction (XRD) indicated that the main phases of the porous Si3N4-bonded SiC ceramics were SiC, α-Si3N4, and β-Si3N4, respectively. The contents of β-Si3N4 were increased following the sintering temperature. The morphology of Si3N4 whiskers was investigated by scanning electron microscope (SEM), which was shown that the needle-like (low sinteringtemperature) and rod-like (higher sintering-temperature) whiskers were formed, respectively. From low to high synthesized temperature, the highest porosity of the porous Si3N4 bonded SiC ceramic was up to 46.7%, and the bending strength was ~11.6 MPa. The α-Si3N4 whiskers were derived from the reaction between N2 and Si powders, the growth mechanism was proved by Vapor–Solid (VS). Meanwhile, the growth mechanism of β-Si3N4 was in accordance with Vapor–Solid–Liquid (VSL) growth mechanism. With the increase of sintering temperature, Si powders were melted to liquid silicon and the α-Si3N4 was dissolved into the liquid then the β-Si3N4 was precipitated successfully.
1. Introduction Si3N4 and SiC are both primary functional ceramic materials. Si3N4bonded SiC composite materials have attracted extensive attention due to their high strength, high hardness, and excellent corrosion resistance [1]. Moreover, the porous Si3N4-bonded SiC ceramics, which have macroscopic pores, exhibited some unique performances, such as acid and alkali resistance, high permeability, and narrow pore size distribution [2,3]. Based on the above good features, the porous Si3N4bonded SiC ceramics have been a new kind of materials to deal with the environmental issue, including industrial wastewater, hot corrosive gases, and precision filtration separation. Meanwhile, as a porous ceramic support, the porous Si3N4-bonded SiC ceramic also can filter the suspended solids, colloid substances, and microbes, or absorb them into interlaced holes [4–6]. In addition to the advantages mentioned above, however, Si and C have a high self-diffusion coefficient, which results in the sintering temperature is needed above 2100 °C to prepare porous SiC ceramics via an in situ reaction bonding technique [7]. Therefore, the widelyused method to promote the sintering of the SiC ceramics is to develop a special sintering process or rely on the second phase [8]. In the Si3N4bonded SiC composite materials, Si3N4 is one of the non-oxide bonds of SiC ceramics as a second phase. Si3N4 usually exists in α or β crystal ⁎
form, in which the α-Si3N4 is granule and the β-Si3N4 is needle-like structure [9]. Previous studies indicated that the α-Si3N4 was expected to transform into β-Si3N4 at approximate 1327 ℃ through the “dissolution–precipitation” process in the liquid [10–12]. For porous ceramics, however, there is a direct contradictory between porosity and strength. Namely, with the increase of the porosity, the strength decreases with exponential distribution [13]. In order to enhance the strength of the porous SiC ceramics without reducing the porosity, increasing the bonding degree of the SiC particle is a promising way. Compared with other reinforcement methods, ceramic whiskers are easier to prepare and control [14]. Furthermore, whisker exhibits toughening and reinforcing effect on ceramic matrixes as a second phase [15]. Nevertheless, the morphology and arrangement of Si3N4 whiskers have a significant effect on the hardness and strength because it is difficult to obtain homogeneous dispersion and distribution whisker [16]. In the whisker-reinforced ceramic matrix composites, the mechanical properties are enhanced by the reinforcing effect from the whisker-oriented alignment [17]. In this study, Si3N4 was synthesized via direct nitridation of silicon powder to bond SiC particles in the reactive sintering process. In order to reduce the sintering temperature, Y2O3 was doped into the raw materials as a sintering additive. The morphology changes and growth mechanism of Si3N4 whiskers, as well as the effect of whiskers
Corresponding author. E-mail address: [email protected] (C. Chen).
https://doi.org/10.1016/j.ceramint.2018.12.060 Received 23 November 2018; Received in revised form 4 December 2018; Accepted 8 December 2018 Available online 10 December 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 45 (2019) 5922–5926
C. Chen et al.
YSiO2N existed in the sintered samples, and the supposed reactions have been listed as follows:
morphology on the mechanical properties for porous Si3N4, bonded SiC ceramics were investigated.
S i (s) + SiO2(S ) = 2SiO (g)
(1)
6S iO (s)+4N2(g ) = 2S i3N4(s) + 3O2(g )
(2)
2Y2 O3(s) + S iO2(s) + Si3 N4(s) = 4YS iO2 N(s)
(3)
2. Material and methods The raw materials used in this work are commercial SiC powder (purity > 98.5%; d50 = 20.9 µm; Lifeng Silicon Carbide Micro-powder Co. Ltd., Henan, China), Si powder (purity > 99%; d50 = 20.3 µm), and Y2O3 (purity > 99.99%; Aladdin chemistry Co. Ltd., Shanghai, China), respectively. The weight ratio of SiC, Si, and Y2O3 is 81:15:4. These powders were mixed with ethyl alcohol in an agate mortar for 6 h. After vacuum dried at 90 °C for 12 h, certain binder (PEG) was added into the mixed powder, then 3.5 g uniformly-mixed powder was uniaxially pressed into a circular steel die (diameter is 25 mm) under the 24 MPa. The green samples were pre-sintered at 270 °C in the air and kept for 1 h in order to get rid of PEG. Under a nitrogen atmosphere, the pre-sintered compacts were further sintered at 1400 °C, 1440 °C, 1480 °C and 1520 °C, respectively for 3 h. The phase composition of the sintered specimens was determined by X-ray diffractometry (XRD, D/MAX-IIIB, Rigaku, Osaka, Japan) with CuKα radiation. The microstructure and morphology of porous Si3N4-bonded SiC ceramics were observed by scanning electron microscope (SEM, JEOL JSM5610LV, Tokyo, Japan). The porosity of the samples was measured by the hydrostatic weighing method which combined with the Archimedes method. The bending strength was determined using three-point loading with 30 mm span (WDW50 material testing machine, Jinan, China). The pore size distribution was measured by capillary flow porometer (Porolux 500, Germany).
Fig. 2 presented the SEM micrographs of the fractured surfaces of the samples sintered at various sintering temperatures. The SEM images illustrated the existence of various morphologies of the as-synthesized Si3N4 crystal and the pore structures of the specimens. According to the SEM micrographs, the sintering temperature had an obvious effect on the morphology of the Si3N4 whiskers and resulted from indifferent pore structures of the sintered samples. When the sintering temperature was between 1400 and 1440 °C, the needle-like Si3N4 whiskers grew in the intergranular pores radially. With the sintering temperature up to 1480 °C, most of Si3N4 presented granular morphology and attached on the SiC particles while the rest of Si3N4 grew thick and short. While the sintering temperature reached to 1520 °C, the Si3N4 grains became rodlike shape and gathered on the surface of SiC particles, and it was obvious that the specimens had a lot of intergranular pores which were not plugged by rod-like whiskers. 3.2. Porosity and bending strength The trend of porosity of the different sintered samples was displayed in Fig. 3. The porosity gradually increased with the increase of sintering temperature. At 1520 °C, the porosity reached the maximum value of 46.7%, which can be explained by the fact of the morphology changes of Si3N4 whiskers. The Si3N4 whiskers were prepared by a direct nitridation of silicon among the SiC grains, and the SiC grains did not obey the principle of close-packed structure, which led to the porosity of SiC matrix more than 25.95%. Combining with Fig. 2, the needle-like Si3N4 whiskers were gradually transformed into rod-like shape with the increase of sintering temperature. Simultaneously, the intergranular pores were no longer filled with Si3N4 whiskers, which helped the SiC matrix to build a high whole hollow structure. Along with the sintering temperature rising, the bending strength was dropped from 19.8 MPa to 14.3 MPa as shown in Fig. 4. Previous researches [18] have demonstrated that the bending strength was decreased according to the exponential relationship with porosity increase when the pore size was greater than the critical crash size of the material. The empirical formula is listed as follow:
3. Results and discussion 3.1. Phase and microstructure The XRD patterns of the samples sintered at different temperatures were illustrated in Fig. 1. It was observed that the specimens were mainly composed of α- and β-phase SiC, α- and β-phase Si3N4 and minor YSiO2N. The diffraction peaks of α-Si3N4 and β- Si3N4 were detected obviously without Si peak, which demonstrated that the silicon was entirely azotizing in the process of sintering. Si3N4 existed only as α crystal form for the samples sintered at 1400 °C, while both α-Si3N4 and β-Si3N4 were detected for the samples sintered at 1440 °C. Furthermore, the number of diffraction peaks of β-Si3N4 increased as enhancing sintering temperature. The observations were highly in accordance with the previous researches [11–13], which reported that the α-Si3N4 was transformed into β-Si3N4 with the increase of sintering temperature. Owing to a little amount of silicon dioxide mixed in the raw materials,
σf = σ0 exp(−nP )
(4)
where the σf is bending strength, P is porosity [14]. Thus, there is a linear relationship between ln σf and P, and the linear relevant fitting result was shown in Fig. 5. The linear relevant fitting result was Y = 6.7771–0.0876X. Based on the fitting curve, the bending strength of Si3N4 bonded SiC dense ceramics was theoretically up to 877 MPa according to the Y-intercept value. The experimental results also confirmed the relationship between bending strength and porosity. Hence, the morphology changes of Si3N4 whiskers were affected not only by porosity but also by bending strength. The experimental results revealed that needle-like whisker contributed more to the bending strength than rod-like whisker owing to the bridging of the needle-like Si3N4 whiskers. The Si3N4-bonded SiC ceramics with high porosity were prepared, of which the pore diameter distribution was also an important factor for the application in adsorbent filtering fields. Fig. 6 exhibited the pore diameter flow distribution of porous Si3N4-bonded SiC ceramics sintered at various temperatures. The pore diameter distribution of the sintered samples was relatively narrow and concentrated in 0.5–2 µm, and the maximum percent flow of the samples was almost at the range of 18–23%, which demonstrated that the sintering temperature had less
Fig. 1. XRD patterns of the samples sintered at different temperatures. 5923
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Fig. 2. SEM micrographs of the fractured surfaces of the samples sintered at various temperatures. (A) 1400 °C, (B)1440 °C, (C) 1480 °C, (D)1520 °C.
Fig. 3. Effect of different sintering temperatures on the porosity of porous Si3N4 bonded SiC ceramics.
Fig. 4. Bending strength of porous Si3N4 bonded SiC ceramics sintered at various temperatures.
influence on the pore diameter distribution. The contribution to the filter flow by different pore diameter was shown in Fig. 7. Although, the pore diameter distribution of the samples distributed in 0.5–2 µm range, the real contribution to the filter flow was the pore diameter for 0.75–1.8 µm. Compared with the results in Fig. 3, the porosity of samples achieved maximum when sintered at 1520 °C, which may benefit from the large filter flow of the pore diameter for 0.8–1.8 µm. Although the inner pore structure of the SiC matrix was very complex, large porosity was conductive to produce high percent flow value. In addition, the percent flow of the porous Si3N4 bonded SiC ceramics were also impacted by the pore-throat configuration. Three typical cases of the pore-throat configuration were displayed in Fig. 8. Besides example (A), the percent flow of the porous ceramics was impaired by
the other two pore-throat configurations due to the pore throat or pore was so narrow and small that the testing gas cannot go through the samples smoothly. 3.3. The Growth mechanism of Si3N4 crystal with various morphologies Up to now, Vapor–Solid (VS) and Vapor–Liquid–Solid (VLS) mechanisms are two widely accepted Si3N4 whisker growth mechanisms. The obvious distinction between two growth mechanisms is that a spherical liquid droplet existed on the whisker tip under VLS mechanism owing to a few catalysts like Fe, which provided liquid phase for whisker impurity nucleation. At the beginning of the growth of silicon nitride whisker, the prime whisker was α-Si3N4 and the growth 5924
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Fig. 8. Three pore structures of porous ceramics, (A) the pore throat radius is nearly the same to pore size, (B) the pore throat radius is smaller than pore size, (C) the pore throat radius is much bigger than the pore size.
Fig. 5. Linear fitting of ln σf and porosity of porous Si3N4 bonded SiC ceramics.
Fig. 9. Schematic illustration for the proposed growth mechanism of Si3N4 whisker.
Fig. 6. Pore diameter flow distribution of porous Si3N4 bonded SiC ceramics sintered at various temperatures.
Fig. 10. Schematic illustration for the morphology transformation process of Si3N4 whisker.
between SiC grains. The high saturation of Si vapors promoted the small crystal nuclei to be aggregated to form Si3N4 grains. Then Si vapor, SiO and nitrogen gas continuously reacted on Si3N4 grain. Si3N4 was polar molecular crystal because of the hexagonal structure, the interfacial energy of C-axis is lower than A-axis so that the needle-like whisker was grown up along the C-axis direction [19]. From Fig. 1, it has been proved that high temperature promoted αSi3N4 change to β phase, and in general, β-Si3N4 existed as rod-like shape. The change from needle-like whiskers to rod-like whiskers may be related to the crystal growth process, as shown in Fig. 10. Park, [20] has found that liquid silicon was a suitable liquid to solve Si3N4 and assisted the transformation from α phase to β phase. Gibbs [21] presented a definition of the equilibrium shape of the crystal, which indicated that the crystal thermodynamically had a minimum surface
Fig. 7. The filter flow in different pore diameter of porous Si3N4 bonded SiC ceramics sintered at various temperatures.
mechanism was VS mechanism. With the increase of sintering temperature, the α-Si3N4 was changed into β-Si3N4, the main whisker growth mechanism of β-Si3N4 was became to VLS mechanism. The formation of α-Si3N4 whiskers go through a nucleation and the growth process, as shown in Fig. 9. With the temperature increasing, a little of silicon was converted to vapor, and a part of silicon was reacted with silicon dioxide to form SiO gas as expressed by Eq. (2). The rapid nucleation process of the reaction occurred between Si and nitrogen with lots of small crystal nuclei of Si3N4, which adhered to the interface 5925
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energy at given constant volume. When the system temperature was higher than 1410 °C, the solid silicon was melted to be fused silicon, needle-like whisker was gradually dissolved in the liquid silicon. Meanwhile, high temperature accelerated the diffusion rate of Si, N2, and SiO, which deposited on the dissolved Si3N4 whiskers, then the rodlike Si3N4 whiskers grew up because of the surface tension effect and the interfacial energy of needle-like Si3N4 whisker which was wetted by fused silicon.
[2] [3]
[4]
[5]
4. Conclusions
[6]
(1) Porous Si3N4-bonded SiC ceramics were prepared via reactive sintering under the nitrogen atmosphere. The porosity of the sintered samples was increased from 43.16% to 46.68% with the increase of sintering temperatures. On the contrary, the variation of the bending strength of the sintered samples showed a reverse rule, which decreased from 19.8 MPa to 14.3 MPa. The porous Si3N4bonded SiC ceramics possessed a relatively narrow aperture size distribution of 0.5–2 µm, and the 0.75–1.8 µm pore contributed the filter flow of the sintered samples sintered at the various sintering temperature. (2) The sintering temperature controlled the diffusion rate and concentration of the reactant gas. Following the temperature increasing, The morphology of Si3N4 whisker was gradually transformed from needle-like to rod-like. The needle-like Si3N4 whisker which lapped among the SiC particles improved the strength of the porous ceramics. However, the rod-like Si3N4 whiskers were beneficial to the porosity. (3) The growth mechanism of α-Si3N4 was VS mechanism, and the growth mechanism of β-Si3N4 obeyed VLS mechanism. Taking the effect of screw dislocation and interfacial energy into account, the whisker finally grew along the long axis direction. Furthermore, the minimum surface energy promoted the needle-like Si3N4 whisker which was dissolved in liquid silicon to grow thick and short, then needle-like was gradually transformed into rod-like Si3N4 whiskers.
[7]
[8] [9]
[10]
[11] [12]
[13] [14]
[15]
[16]
[17]
[18]
Acknowledgments
[19]
This work was supported by the Natural Science Foundation of Hubei Province (2014CFB796), the Science & Technology Pillar Program of Hubei Province (2015BAA105) and the National Natural Science Foundation of China (21305105).
[20] [21]
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