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Pressureless sintering ZrB2–SiC ceramics at low temperatures
Materials Letters 63 (2009) 2035–2037
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Pressureless sintering ZrB2–SiC ceramics at low temperatures Miao Zhu, Yiguang Wang ⁎ National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, Xi'an, China 710072
a r t i c l e
i n f o
Article history: Received 27 April 2009 Accepted 18 June 2009 Available online 25 June 2009 Keywords: Ceramics Sintering
a b s t r a c t ZrB2–SiC ceramics are prepared by pressureless sintering using ZrB2 powders and liquid polycarbosilane (LPCS) precursors. The LPCS can effectively reduce the sintering temperature. The phases of the sintered ceramics are characterized by X-ray diffraction, and their morphologies are observed by scanning electron microscopy. From these results, it is learned that LPCS can provide free carbon and silicon at high temperatures. Therefore, the oxides on the ZrB2 surface can be removed by free carbon, and the densification process can be promoted by silicon. These coupled effects make it possible to pressureless sinter the ZB2–SiC ceramics at low temperatures. © 2009 Elsevier B.V. All rights reserved.
1. Introduction ZrB2 is one of the materials known as ultrahigh-temperature ceramics. It has a series of excellent properties, such as high melting point, hardness, and chemical stability [1–4]. By adding SiC to ZrB2, the resultant ZrB2–SiC ceramics have better strength and oxidation resistance than ZrB2 alone [1,2,5–8]. Although ZrB2-based ceramics exhibit those attractive properties, sintering these ceramics is difficult due to their low diffusivities and surface impurities of the starting powders [2,3,9]. In most cases, dense ZrB2–SiC ceramics are obtained by hot pressing, which is limited to simple geometric shapes. Fabrication of complex components requires expensive and timeconsuming diamond machining. However, in contrast to hot pressing, pressureless sintering would be good for fabrication of near-net shapes and reduce processing costs [2,3,8–10]. Many efforts have been made to pressureless sinter ZrB2 ceramics [3,8–18]. The sintering temperatures of ZrB2 ceramics decrease to below 2150 °C by adding additives such as MoSi2, B4C, C, ZrSi2, WC or Mo [2–4,8–16]. The densification process is enhanced by either formation of liquid phase [4,11,16] or removal of oxide impurities from the surface of ZrB2 with these additives [6,9,10,13,14]. Recently, several studies have been carried out to sinter ZrB2-based ceramics by using polymeric precursors to get dense ceramics at low temperatures [6,7]. It is found that the polymeric precursor can greatly enhance the densification process. However, the mechanism for such a process is still unclear. According to the results of composition analysis [6,7], it is believed that the residual carbon produced during the pyrolysis of polymeric precursors was the reason. It was thought that the residual carbon could remove the oxides presented on the ZrB2 particle surfaces through solid-state reactions [6,7]. However, this explanation
⁎ Corresponding author. Tel.: +86 29 88494914; fax: +86 29 88494620. E-mail address: [email protected] (Y. Wang). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.06.041
is not persuadable because the carbon additive alone cannot have such a great effect on reducing the densification temperatures. The purpose of this paper is to reveal the real reasons for polymer precursors to promote the densification process of ZrB2 ceramics. In this study, ZrB2 with liquid polycarbosilane (LPCS) were sintered at different temperatures. The microstructure and composition of the sintered ceramics were analyzed. Finally, the function of polymeric precursors in sintering process was discussed based on these results.
2. Experimental procedure The ZrB2 powder (0.5 mm, Beijing Mountain Technical Development Center, Beijing, China) and LPCS (Laboratory of Advanced Materials, Xia Men University, Xiamen, China) were used as the starting materials. The LPCS was the precursor of SiC ceramics, and its pyrolysis process was described elsewhere [19]. The LPCS was diluted by acetone and then mixed with ZrB2 powder uniformly. The weight ratio of ZrB2 to LPCS was about 4:1. Afterwards, the ZrB2–LPCS slurry was gradually dried by continuous stirring. The dried powder was then pressed into cylindrical pellets with a pressure of 10 MPa, followed by cold isostatic pressing at 200 MPa for 60 s. Prior to sintering, the green compacts were heated to 900 °C in flowing argon to convert the LPCS into ceramics. The samples were then sintered at 1600 °C, 1800 °C, 1900 °C and 2000 °C for 2 h, respectively. At temperatures below 1800 °C, a mild vacuum was adopted. Above 1800 °C, the furnace was backfilled with flowing argon. The bulk densities of specimens were measured by the Archimedes method. The measured bulk density was divided by the theoretical density to obtain the relative density. The theoretical density was estimated using the mixture rule. The morphologies of the sintered ceramics were observed by scanning electron microscopy (SEM, JEOL6700F, Tokyo, Japan). X-ray diffraction (XRD) was used for the phase analysis, which was carried out by using a Rigaku D/max-2400 diffractometer (Tokyo, Japan) with copper Ka radiation. Data were
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M. Zhu, Y. Wang / Materials Letters 63 (2009) 2035–2037
Fig. 1. Relative density of the ZrB2–SiC ceramics as a function of sintering temperature.
digitally recorded in a continuous scan in the range of angles (2q) from 15° to 75°. 3. Results and discussion Fig. 1 shows the relative density of the sintered specimens as a function of temperature. It can be seen that the relative density enhances with the increase in sintering temperature. At 1900 °C, the relative density reaches its maximum value. Further increasing the sintering temperature to 2000 °C, instead, the ceramic density will decrease. The fracture surfaces of the sintered samples are shown in Fig. 2. After sintering at 1600 °C, the samples have a linear shrinkage of about 17%. However, there are still a lot of pores inside (Fig. 2a). As the sintering temperature increases to 1800 °C, the porosity decreases (Fig. 2b) and the shrinkage is nearly 20%. It is also found from the fracture surfaces that pores are mainly located at the grain boundaries. As far as we know, during the sintering process, some impurities like B2O3 and the gaseous products of the reactions between oxides and
Fig. 3. XRD patterns of the sintered ZrB2–SiC ceramics pyrolyzed at 900 °C (a) and then sintered at 1600 °C (b), 1800 °C (c), 1900 °C (d), and 2000 °C (e) respectively.
reductions can evaporate out, which will lead to the pores between the grain boundaries. With the increase in the sintering temperature, the diffusion is enhanced and the grain coarsening is enlarged. The density of sintered samples thus increases. However, if the grain grows too fast, the pores will be entrapped in the samples, leading to the decrease in density. This may be the reason for the low relative density of the samples sintered at 2000 °C. The XRD patterns of the specimens sintered at different temperatures are shown in Fig. 3. As can be seen, there are two phases in the samples treated at 900 °C (Fig. 3a): ZrB2, the main phase; ZrO2, the oxide impurity on the surface of ZrB2. There are no visible signals from LPCS because its pyrolyzed products are mainly amorphous phase [19]. Whereas, at 1600 °C, new phases of ZrC and SiC appear, while the phase of ZrO2 almost vanishes (Fig. 3b). It is believed that SiC comes from LPCS, which will crystallize at temperatures over 1400 °C [19]. Meanwhile, free carbon will be formed during the crystallization
Fig. 2. Morphologies of the fracture surfaces of ZrB2–SiC ceramics sintered at 1600 °C (a), 1800 °C (b), 1900 °C (c), and 2000 °C (d).
M. Zhu, Y. Wang / Materials Letters 63 (2009) 2035–2037
2037
The residual carbon from LPCS plays an important role to remove the oxide impurities on the ZrB2 surface according to reactions (1) and (2), which is favorable for the densification process [3,6,9,13,14]. However, the ability of carbon to reduce the sintering temperature is limited [6,12–14]. The silicon is the key to lower sintering temperature and promote the densification process. The LPCS can provide silicon, which is liquid at temperatures higher than 1450 °C. The liquid silicon can promote the diffusion of materials. When silicon volatilizes, the formed vapor pressure leads to much shrinkage of the specimens, which is very favorable for sintering. As a comparison, ZrB2–SiC pellet is prepared without added LPCS. Even sintering at 1800 °C for 2 h, the pellet shows no obvious shrinkage. On the other hand, the volatilization of silicon results in a lot of pores, which are difficult to be filled by diffusion. This is the reason why the mechanical properties of pressureless sintered ZrB2–PCS ceramics are not good enough [6]. Therefore, controlling the contents of LPCS is very important. It is the next topic of our research. Fig. 4. SEM and EDS analysis of substance deposited on graphite paper wrapped the specimens during pressureless sintering.
4. Summary process of LPCS. Therefore, the following reactions may happen to reduce ZrO2 and to form ZrC [12–14]. ZrO2 þ B2 O3 ðlÞ þ 5C ¼ ZrB2 þ 5COðgÞ
ð1Þ
ZrO2 þ 3C ¼ ZrC þ 2COðgÞ
ð2Þ
After sintering at 1800 °C, the samples have only the phases of ZrB2 and ZrC (Fig. 3c). The ZrC contents are calculated to be about 3 wt.%. However, SiC phase is not detected in the samples. Previous study indicated that the following reaction will be favorable at temperatures above 1750 °C [20]. SiC þ ZrO2 ¼ ZrC þ SiOðgÞ þ COðgÞ
ð3Þ
Reaction (3) contributes to the vanishing SiC at high temperatures. However, due to the limitation of ZrO2 contamination, the SiC could not be totally consumed by the above reaction. As known, the ZrO2 is about 9 wt.% in the starting ZrB2 powders, and the LPCS could provide about 20 mol% SiC in the final composites according to the calculation based on LPCS pyrolysis behaviors [19]. Even though all of the ZrO2 were consumed, there still was more than 10 mol% SiC. Therefore, there are other reasons to cause the absence of SiC in the specimens. During sintering ZrB2–LPCS samples in vacuum, the vacuum of the furnace wildly increase in the temperature range of 1440–1700 °C. It is believed that some substances evaporate out. Moreover, a layer of loose faint yellow substances is found on the graphite papers that wrapped the specimens during vacuum sintering. SEM result (Fig. 4) indicates that these substances are finely crystallized. According to the analysis of EDS (Fig. 4), the compositions of the crystalline are silicon and carbon with an atomic ratio close to 1:1, which demonstrates that the substance is SiC crystalline. Subsequently, it is thought that silicon will evaporate out during sintering other than the reactions (1)–(3) to cause weight loss and SiC vanishing. Silicon is believed to originate from the pyrolysis of LPCS. It is generally thought that the structure of polymer precursors will be rearranged during pyrolysis [21]. The silicon domain and carbon domain do exist during the rearranged process. At temperatures higher than its melting point (1410 °C), the silicon will easily evaporate out to deteriorate the vacuum.
ZrB2–SiC were pressureless sintered at low temperatures using ZrB2 and LPCS. LPCS can provide free carbon and silicon at high temperatures. Free carbon can remove the oxides on the ZrB2 surface, and silicon can promote the densification process. These coupling effects make it possible to pressureless sinter ZrB2-based ceramics at low temperatures. This methodology can also be used to sinter other ultrahigh-temperature ceramics such as HfC, TaC, or HfB2.
Acknowledgement This work is financially supported by the Chinese Natural Science Foundation (Grant # 90176023).
References [1] Levinea SR, Opilab EJ, Halbigc MC, Kisera JD, Singh M, Salema JA. J Eur Ceram Soc 2002;22:2757–67. [2] Fahrenholtz WG, Hilmas GE, Talmy IG, Zaykoski JA. J Am Ceram Soc 2007;90(5): 1347–64. [3] Chamberlain AL, Fahrenholtz WG, Hilmas GE. J Am Ceram Soc 2006;89(2):450–6. [4] Sciti D, Monteverde F, Guicciardi S, Pezzotti G, Bellosi A. Mater Sci Eng A 2006;434:303–9. [5] Chamberlain AL, Fahrenholtz WG, Hilmas GE, Ellerby DT. J Am Ceram Soc 2004;87(6): 1170–2. [6] Zhu S, Fahrenholtz WG, Hilmas GE. Scr Mater 2008;59:123–6. [7] Zhou XJ, Zhang GJ, Li YG, Kan YM, Wang PL. Mater Lett 2007;61:960–3. [8] Yan YJ, Huang ZR, Dong SM, Jiang DL. J Am Ceram Soc 2006;89(11):3589–92. [9] Zhang SC, Hilmas GE, Fahrenholtz WG. J Am Ceram Soc 2008;91(1):26–32. [10] Zhang SC, Hilmas GE, Fahrenholtz WG. J Am Ceram Soc 2006;89(5):1544–50. [11] Sciti D, Guicciardi S, Bellosi A, pezzotti G. J Am Ceram Soc 2006;89(7):2320–2. [12] Fahrenholtz WG, Hilmas GE, Zhang SC, Zhu S. J Am Ceram Soc 2008;91(5): 1398–404. [13] Zhu S, Fahrenholtz WG, Hilmas GE, Zhang SC. Mater Sci Eng A 2007;459:167–71. [14] Zhu S, Fahrenholtz WG, Hilmas GE, Zhang SC. J Am Ceram Soc 2007;90(11): 3660–3. [15] Silvestroni L, Sciti D. Scr Mater 2007;57:165–8. [16] Guo SQ, Kagawa Y, Nishimura T, Tanaka H. Scr Mater 2008;58:579–82. [17] Zhang H, Yan YJ, Huang ZG, Liu XJ, Jiang DL. Scr Mater 2009;60:559–62. [18] Postrach S, Pötschke J. J Eur Ceram Soc 2000;20:1459–68. [19] Li HB, Zhang LT, Cheng LF, Wang YG, Yu ZJ, Huang MH, et al. J Mater Sci 2008;43: 2806–11. [20] Zhang P, Hu P, Zhang XH, Han JC, Meng SH. J Alloy Compd 2009;472:358–62. [21] Dhamne A, Xu WX, Fookes BG, Fan Y, Zhang LG, Burton S, et al. J Am Ceram Soc 2005;88(9):2415–9.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Pressureless sintering ZrB2–SiC ceramics at low temperatures Miao Zhu, Yiguang Wang ⁎ National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, Xi'an, China 710072
a r t i c l e
i n f o
Article history: Received 27 April 2009 Accepted 18 June 2009 Available online 25 June 2009 Keywords: Ceramics Sintering
a b s t r a c t ZrB2–SiC ceramics are prepared by pressureless sintering using ZrB2 powders and liquid polycarbosilane (LPCS) precursors. The LPCS can effectively reduce the sintering temperature. The phases of the sintered ceramics are characterized by X-ray diffraction, and their morphologies are observed by scanning electron microscopy. From these results, it is learned that LPCS can provide free carbon and silicon at high temperatures. Therefore, the oxides on the ZrB2 surface can be removed by free carbon, and the densification process can be promoted by silicon. These coupled effects make it possible to pressureless sinter the ZB2–SiC ceramics at low temperatures. © 2009 Elsevier B.V. All rights reserved.
1. Introduction ZrB2 is one of the materials known as ultrahigh-temperature ceramics. It has a series of excellent properties, such as high melting point, hardness, and chemical stability [1–4]. By adding SiC to ZrB2, the resultant ZrB2–SiC ceramics have better strength and oxidation resistance than ZrB2 alone [1,2,5–8]. Although ZrB2-based ceramics exhibit those attractive properties, sintering these ceramics is difficult due to their low diffusivities and surface impurities of the starting powders [2,3,9]. In most cases, dense ZrB2–SiC ceramics are obtained by hot pressing, which is limited to simple geometric shapes. Fabrication of complex components requires expensive and timeconsuming diamond machining. However, in contrast to hot pressing, pressureless sintering would be good for fabrication of near-net shapes and reduce processing costs [2,3,8–10]. Many efforts have been made to pressureless sinter ZrB2 ceramics [3,8–18]. The sintering temperatures of ZrB2 ceramics decrease to below 2150 °C by adding additives such as MoSi2, B4C, C, ZrSi2, WC or Mo [2–4,8–16]. The densification process is enhanced by either formation of liquid phase [4,11,16] or removal of oxide impurities from the surface of ZrB2 with these additives [6,9,10,13,14]. Recently, several studies have been carried out to sinter ZrB2-based ceramics by using polymeric precursors to get dense ceramics at low temperatures [6,7]. It is found that the polymeric precursor can greatly enhance the densification process. However, the mechanism for such a process is still unclear. According to the results of composition analysis [6,7], it is believed that the residual carbon produced during the pyrolysis of polymeric precursors was the reason. It was thought that the residual carbon could remove the oxides presented on the ZrB2 particle surfaces through solid-state reactions [6,7]. However, this explanation
⁎ Corresponding author. Tel.: +86 29 88494914; fax: +86 29 88494620. E-mail address: [email protected] (Y. Wang). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.06.041
is not persuadable because the carbon additive alone cannot have such a great effect on reducing the densification temperatures. The purpose of this paper is to reveal the real reasons for polymer precursors to promote the densification process of ZrB2 ceramics. In this study, ZrB2 with liquid polycarbosilane (LPCS) were sintered at different temperatures. The microstructure and composition of the sintered ceramics were analyzed. Finally, the function of polymeric precursors in sintering process was discussed based on these results.
2. Experimental procedure The ZrB2 powder (0.5 mm, Beijing Mountain Technical Development Center, Beijing, China) and LPCS (Laboratory of Advanced Materials, Xia Men University, Xiamen, China) were used as the starting materials. The LPCS was the precursor of SiC ceramics, and its pyrolysis process was described elsewhere [19]. The LPCS was diluted by acetone and then mixed with ZrB2 powder uniformly. The weight ratio of ZrB2 to LPCS was about 4:1. Afterwards, the ZrB2–LPCS slurry was gradually dried by continuous stirring. The dried powder was then pressed into cylindrical pellets with a pressure of 10 MPa, followed by cold isostatic pressing at 200 MPa for 60 s. Prior to sintering, the green compacts were heated to 900 °C in flowing argon to convert the LPCS into ceramics. The samples were then sintered at 1600 °C, 1800 °C, 1900 °C and 2000 °C for 2 h, respectively. At temperatures below 1800 °C, a mild vacuum was adopted. Above 1800 °C, the furnace was backfilled with flowing argon. The bulk densities of specimens were measured by the Archimedes method. The measured bulk density was divided by the theoretical density to obtain the relative density. The theoretical density was estimated using the mixture rule. The morphologies of the sintered ceramics were observed by scanning electron microscopy (SEM, JEOL6700F, Tokyo, Japan). X-ray diffraction (XRD) was used for the phase analysis, which was carried out by using a Rigaku D/max-2400 diffractometer (Tokyo, Japan) with copper Ka radiation. Data were
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M. Zhu, Y. Wang / Materials Letters 63 (2009) 2035–2037
Fig. 1. Relative density of the ZrB2–SiC ceramics as a function of sintering temperature.
digitally recorded in a continuous scan in the range of angles (2q) from 15° to 75°. 3. Results and discussion Fig. 1 shows the relative density of the sintered specimens as a function of temperature. It can be seen that the relative density enhances with the increase in sintering temperature. At 1900 °C, the relative density reaches its maximum value. Further increasing the sintering temperature to 2000 °C, instead, the ceramic density will decrease. The fracture surfaces of the sintered samples are shown in Fig. 2. After sintering at 1600 °C, the samples have a linear shrinkage of about 17%. However, there are still a lot of pores inside (Fig. 2a). As the sintering temperature increases to 1800 °C, the porosity decreases (Fig. 2b) and the shrinkage is nearly 20%. It is also found from the fracture surfaces that pores are mainly located at the grain boundaries. As far as we know, during the sintering process, some impurities like B2O3 and the gaseous products of the reactions between oxides and
Fig. 3. XRD patterns of the sintered ZrB2–SiC ceramics pyrolyzed at 900 °C (a) and then sintered at 1600 °C (b), 1800 °C (c), 1900 °C (d), and 2000 °C (e) respectively.
reductions can evaporate out, which will lead to the pores between the grain boundaries. With the increase in the sintering temperature, the diffusion is enhanced and the grain coarsening is enlarged. The density of sintered samples thus increases. However, if the grain grows too fast, the pores will be entrapped in the samples, leading to the decrease in density. This may be the reason for the low relative density of the samples sintered at 2000 °C. The XRD patterns of the specimens sintered at different temperatures are shown in Fig. 3. As can be seen, there are two phases in the samples treated at 900 °C (Fig. 3a): ZrB2, the main phase; ZrO2, the oxide impurity on the surface of ZrB2. There are no visible signals from LPCS because its pyrolyzed products are mainly amorphous phase [19]. Whereas, at 1600 °C, new phases of ZrC and SiC appear, while the phase of ZrO2 almost vanishes (Fig. 3b). It is believed that SiC comes from LPCS, which will crystallize at temperatures over 1400 °C [19]. Meanwhile, free carbon will be formed during the crystallization
Fig. 2. Morphologies of the fracture surfaces of ZrB2–SiC ceramics sintered at 1600 °C (a), 1800 °C (b), 1900 °C (c), and 2000 °C (d).
M. Zhu, Y. Wang / Materials Letters 63 (2009) 2035–2037
2037
The residual carbon from LPCS plays an important role to remove the oxide impurities on the ZrB2 surface according to reactions (1) and (2), which is favorable for the densification process [3,6,9,13,14]. However, the ability of carbon to reduce the sintering temperature is limited [6,12–14]. The silicon is the key to lower sintering temperature and promote the densification process. The LPCS can provide silicon, which is liquid at temperatures higher than 1450 °C. The liquid silicon can promote the diffusion of materials. When silicon volatilizes, the formed vapor pressure leads to much shrinkage of the specimens, which is very favorable for sintering. As a comparison, ZrB2–SiC pellet is prepared without added LPCS. Even sintering at 1800 °C for 2 h, the pellet shows no obvious shrinkage. On the other hand, the volatilization of silicon results in a lot of pores, which are difficult to be filled by diffusion. This is the reason why the mechanical properties of pressureless sintered ZrB2–PCS ceramics are not good enough [6]. Therefore, controlling the contents of LPCS is very important. It is the next topic of our research. Fig. 4. SEM and EDS analysis of substance deposited on graphite paper wrapped the specimens during pressureless sintering.
4. Summary process of LPCS. Therefore, the following reactions may happen to reduce ZrO2 and to form ZrC [12–14]. ZrO2 þ B2 O3 ðlÞ þ 5C ¼ ZrB2 þ 5COðgÞ
ð1Þ
ZrO2 þ 3C ¼ ZrC þ 2COðgÞ
ð2Þ
After sintering at 1800 °C, the samples have only the phases of ZrB2 and ZrC (Fig. 3c). The ZrC contents are calculated to be about 3 wt.%. However, SiC phase is not detected in the samples. Previous study indicated that the following reaction will be favorable at temperatures above 1750 °C [20]. SiC þ ZrO2 ¼ ZrC þ SiOðgÞ þ COðgÞ
ð3Þ
Reaction (3) contributes to the vanishing SiC at high temperatures. However, due to the limitation of ZrO2 contamination, the SiC could not be totally consumed by the above reaction. As known, the ZrO2 is about 9 wt.% in the starting ZrB2 powders, and the LPCS could provide about 20 mol% SiC in the final composites according to the calculation based on LPCS pyrolysis behaviors [19]. Even though all of the ZrO2 were consumed, there still was more than 10 mol% SiC. Therefore, there are other reasons to cause the absence of SiC in the specimens. During sintering ZrB2–LPCS samples in vacuum, the vacuum of the furnace wildly increase in the temperature range of 1440–1700 °C. It is believed that some substances evaporate out. Moreover, a layer of loose faint yellow substances is found on the graphite papers that wrapped the specimens during vacuum sintering. SEM result (Fig. 4) indicates that these substances are finely crystallized. According to the analysis of EDS (Fig. 4), the compositions of the crystalline are silicon and carbon with an atomic ratio close to 1:1, which demonstrates that the substance is SiC crystalline. Subsequently, it is thought that silicon will evaporate out during sintering other than the reactions (1)–(3) to cause weight loss and SiC vanishing. Silicon is believed to originate from the pyrolysis of LPCS. It is generally thought that the structure of polymer precursors will be rearranged during pyrolysis [21]. The silicon domain and carbon domain do exist during the rearranged process. At temperatures higher than its melting point (1410 °C), the silicon will easily evaporate out to deteriorate the vacuum.
ZrB2–SiC were pressureless sintered at low temperatures using ZrB2 and LPCS. LPCS can provide free carbon and silicon at high temperatures. Free carbon can remove the oxides on the ZrB2 surface, and silicon can promote the densification process. These coupling effects make it possible to pressureless sinter ZrB2-based ceramics at low temperatures. This methodology can also be used to sinter other ultrahigh-temperature ceramics such as HfC, TaC, or HfB2.
Acknowledgement This work is financially supported by the Chinese Natural Science Foundation (Grant # 90176023).
References [1] Levinea SR, Opilab EJ, Halbigc MC, Kisera JD, Singh M, Salema JA. J Eur Ceram Soc 2002;22:2757–67. [2] Fahrenholtz WG, Hilmas GE, Talmy IG, Zaykoski JA. J Am Ceram Soc 2007;90(5): 1347–64. [3] Chamberlain AL, Fahrenholtz WG, Hilmas GE. J Am Ceram Soc 2006;89(2):450–6. [4] Sciti D, Monteverde F, Guicciardi S, Pezzotti G, Bellosi A. Mater Sci Eng A 2006;434:303–9. [5] Chamberlain AL, Fahrenholtz WG, Hilmas GE, Ellerby DT. J Am Ceram Soc 2004;87(6): 1170–2. [6] Zhu S, Fahrenholtz WG, Hilmas GE. Scr Mater 2008;59:123–6. [7] Zhou XJ, Zhang GJ, Li YG, Kan YM, Wang PL. Mater Lett 2007;61:960–3. [8] Yan YJ, Huang ZR, Dong SM, Jiang DL. J Am Ceram Soc 2006;89(11):3589–92. [9] Zhang SC, Hilmas GE, Fahrenholtz WG. J Am Ceram Soc 2008;91(1):26–32. [10] Zhang SC, Hilmas GE, Fahrenholtz WG. J Am Ceram Soc 2006;89(5):1544–50. [11] Sciti D, Guicciardi S, Bellosi A, pezzotti G. J Am Ceram Soc 2006;89(7):2320–2. [12] Fahrenholtz WG, Hilmas GE, Zhang SC, Zhu S. J Am Ceram Soc 2008;91(5): 1398–404. [13] Zhu S, Fahrenholtz WG, Hilmas GE, Zhang SC. Mater Sci Eng A 2007;459:167–71. [14] Zhu S, Fahrenholtz WG, Hilmas GE, Zhang SC. J Am Ceram Soc 2007;90(11): 3660–3. [15] Silvestroni L, Sciti D. Scr Mater 2007;57:165–8. [16] Guo SQ, Kagawa Y, Nishimura T, Tanaka H. Scr Mater 2008;58:579–82. [17] Zhang H, Yan YJ, Huang ZG, Liu XJ, Jiang DL. Scr Mater 2009;60:559–62. [18] Postrach S, Pötschke J. J Eur Ceram Soc 2000;20:1459–68. [19] Li HB, Zhang LT, Cheng LF, Wang YG, Yu ZJ, Huang MH, et al. J Mater Sci 2008;43: 2806–11. [20] Zhang P, Hu P, Zhang XH, Han JC, Meng SH. J Alloy Compd 2009;472:358–62. [21] Dhamne A, Xu WX, Fookes BG, Fan Y, Zhang LG, Burton S, et al. J Am Ceram Soc 2005;88(9):2415–9.