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Synthesis and microstructural features of ZrB2–SiC-based composites by reactive spark plasma sintering and reactive hot pressing
Scripta Materialia 57 (2007) 317–320 www.elsevier.com/locate/scriptamat
Synthesis and microstructural features of ZrB2–SiC-based composites by reactive spark plasma sintering and reactive hot pressing Wen-Wen Wu,a,b Guo-Jun Zhang,a,* Yan-Mei Kan,a Pei-Ling Wang,a Kim Vanmeensel,c Jozef Vleugelsc and Omer Van der Biestc a
State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Shanghai 200050, China b Graduate School of the Chinese Academy of Sciences, Beijing 200049, China c Department of Metallurgy and Materials Engineering, Katholieke Universiteit, B-3001 Leuven, Belgium Received 13 March 2007; revised 24 April 2007; accepted 24 April 2007 Available online 25 May 2007
Four kinds of composites, ZrB2–SiC, ZrB2–SiC–ZrC, ZrB2–SiC–ZrN and ZrB2–SiC–AlN, were synthesized in situ via reactive hot pressing (RHP) and reactive spark plasma sintering (R-SPS), using Zr, Si, B4C, BN and Al as raw materials. The synthesis process plays a critical role in the microstructural features of the composites obtained. The R-SPS process can lead to a more homogeneous and finer microstructure due to its high heating rates and short holding time, while the RHP process is likely to result in coarse microstructures due to a long enough holding time for grains growth. Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Synthesis; Microstructure; Ultrahigh-temperature ceramics
Zirconium and hafnium-based ceramics (borides, carbides and nitrides) possess a number of unique properties, including extremely high melting temperature and hardness, low volatility, and high thermal and electrical conductivity [1,2]. The ultrahigh-temperature ceramics (UHTC) based on mixtures of ZrB2 or HfB2 and SiC have exhibited relatively good oxidation/ablation resistance [3–5]. Studies show that the second phase of SiC acts as a grain-growth inhibitor [6,7]. Monolithic ceramics have simply been fabricated from commercially available powders by hot pressing. Considering the poor sinterability of the strongly covalent compounds, very high hot pressing temperature (2100 °C) is needed for sintering-aids-free composites to reach high relative density. But such conditions generally induce coarsening of the final microstructure. In order to improve the sinterability of ZrB2/SiC composites, a number of sintering aids, such as Ni, Fe and Si3N4, have been used [8,9]. However, the problem is that unstable second phases that come from these sintering aids will always remain at the grain boundaries in * Corresponding author. E-mail: [email protected]
the composites, leading to the deterioration of hightemperature mechanical properties. Other sintering aids, such as AlN, HfN and ZrN, have also been used [10–12]. They form only refractory secondary phases and are helpful to the performance of the composites. Reactive hot pressing (RHP) can be used as an alternative route [3,13–15]. Through this process, composites with novel and controlled microstructures and high chemical compatibility of in situ-formed individual phases can probably be created. Moreover, in comparison with traditional hot pressing, spark plasma sintering (SPS) is a new sintering technique developed to densify ceramics. Dense composites can be obtained by SPS with a very fast heating rate, a very short holding time and a lower sintering temperature. The SPS method has also been used for preparing UHTCs [16,17]. High-strength ZrB2–SiC composite can be prepared from a mixture of Zr, Si and B4C according to reaction (1) [13] 2Zr + Si + B4 C = 2ZrB2 + SiC
ð1Þ
According to the calculation method for self-propagating high-temperature synthesis (SHS), the adiabatic
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temperature of the above reaction is Tad = 3094 K and DH°298/Cp298 = 5317 K. On the basis of reaction (1), other boride-containing ceramic composites can also be synthesized by using B4C and BN as reactants. The reactions are as follows [18]: 3Zr þ B4 C ¼ 2ZrB2 þ ZrC T ad ¼ 3328 K; DH 298 =Cp298 ¼ 5798 K
ð2Þ
3Zr þ 2BN ¼ ZrB2 þ 2ZrN T ad ¼ 3138 K; DH 298 =Cp298 ¼ 4269 K
ð3Þ
Zr þ 2Al þ 2BN ¼ ZrB2 þ 2AlN T ad ¼ 2464 K; DH 298 =Cp298 ¼ 4212 K
ð4Þ
It can be seen that the reactions are highly exothermic and satisfy the thermodynamic conditions for selfsustaining combustion process (Tad P 1800 K and DH°298/Cp298 > 2000 K). Through the combination of reaction (1) with the other three reactions, three kinds of ceramics, ZrB2–SiC–ZrC, ZrB2–SiC–ZrN and ZrB2– SiC–AlN can be formed in the same way as the ZrB2– SiC composite. The ZrC, ZrN, AlN phases formed ‘‘in situ’’ may have some positive impacts on the control of microstructures and the improvement of sinterability. In the present work, these four kinds of ceramics are prepared by both RHP and reactive SPS (R-SPS), using zirconium, silicon, B4C, BN and aluminum as starting powders. The manufacturing process, mechanical properties and microstructures of the composites processed by RHP and R-SPS are reported, and the effect of ZrC, ZrN and AlN on sintering behavior are also studied and compared. Four different ceramic systems, ZrB2–SiC (ZS), ZrB2–SiC–ZrC (ZSC), ZrB2–SiC–ZrN (ZSN) and ZrB2–SiC–AlN (ZSA), were produced. The calculated phase compositions according to the expected reactions are listed in Table 1. ZSC, ZSN and ZSA were all designed on the basis of ZS, with the same amount (5 vol.%) of the third phases (ZrC, ZrN and AlN), and with the amount of ZrB2 and SiC differing only slightly. Based on the densities of ZrB2(6.09), SiC (3.21), ZrC
Table 1. Specimen designation, reaction and the calculated phase composition Reactions
Calculated composition (vol.%)
ZS
2Zr + Si + B4C ! 2ZrB2 + SiC
ZrB2:74.85 SiC:25.15
ZSC
(2 + x)Zr + (1 x)Si + B4C ! 2ZrB2 + (1 x)SiC + xZrC X = 0.15628
ZrB2:73.96 SiC:21.04 ZrC:5
ZSN
(1 + x)Zr + (0.5–0.25x)Si + (0.5–0.25x)B4 C + xBN ! ZrB2 + (0.5–0.25x)SiC + xZrN X = 0.08968
ZrB2:71.86 SiC:23.14 ZrN:5
ZSA
Zr + (0.5–0.25x)Si + (0.5–0.25x)B4C + xBN + xAl ! ZrB2 + (0.5–0.25x)SiC + xAlN X = 0.10236
ZrB2:71.98 SiC:23.02 AlN:5
(6.44), ZrN (7.32) and AlN (3.26), the calculated theoretical densities (g cm3) of ZS, ZSC, ZSN and ZSA according to the rule of mixtures and the expected phase compositions are 5.37, 5.50, 5.49, and 5.29, respectively. The starting powders were Zr (purity 95.82%, impurities include Ti 2.34, Hf 0.52, Fe 0.24, W 0.08, Cr 0.06, Ni 0.03, O 1.09, particle size < 25 lm; Guoyao Chemicals Co. Ltd., Shanghai, China), Si (purity > 99%, particle size < 50 lm; Yinfeng Silcon Co. Ltd., Jinan, China), B4C (purity 99%, particle size about 2 lm; Jingangzuan Boron Carbide Co., Ltd, Mudanjiang, China), BN (Sanxing Ceramic Materials Co. Ltd., Gongyi, China) and Al (Huabang Fine Powder Materials, Co. Ltd., Liuyang, China). The stoichiometric powders were mixed for 24 h in a polyethylene jars using ethanol and zirconia balls, and then dried by rotary evaporation. The mixture was placed in a graphite die with a BN coating. In the RHP process, the compacts were heated at a slow heating rate (10 °C min1) to 1800 °C and then held for 60 min under a pressure of 20 MPa in an argon atmosphere. The application of pressure was initiated at 1550 °C. The obtained disk had dimensions of 20 mm diameter and 5 mm thickness. The R-SPS process was carried out at a designated temperature for 5 min under 50 MPa in vacuum with a heating rate of 100 °C min1. The gas pressure in the chamber was kept at about 4 mbar, and the gas phase generated during reactions owing to the oxide impurities and the melting of the metal were pumped out immediately. Different sintering temperatures (1750 and 1800 °C) were employed in order to investigate the sintering behavior. Unlike the usual case for temperature measurement, in which the optical pyrometer focuses on the outer die wall surface, in the SPS equipment (FCT-Systeme, Gewerbepark 11, 96528 Rauenstein, Germany) which we used, the sample’s temperature was measured by focusing an optical pyrometer on the bottom of the upper graphite punch. In the former case, the as-received temperature is actually the temperature of the die, which is lower than the actual temperature of the sample as measured by the latter. Research has shown that the difference is about 200 °C at a temperature of 1500 °C [19]. After removing the surface layer from the obtained disk by grinding, the bulk density was measured using the Archimedes method. The dense sample was milled to a fine powder, then the phase composition was determined by X-ray diffractometry (XRD) using Cu Ka radiation with Si as an internal standard. After this, the disk was ground with SiC abrasives and then polished using a diamond paste of 1 lm. Scanning electron microscopy (SEM) was performed to observe the microstructures of the composites. The samples’ densities calculated according to the supposed reactions are listed in Table 2. The XRD patterns of R-SPS composites of ZS, ZSC, ZSN and ZSA are shown in Figure 1. The XRD of the RHP composites are the same, so they are not presented here. In the R-SPS case, as the sintering temperature increased from 1750 to 1800 °C, the density of all the composites increased as well. This indicates that temperature is a very important factor and a sintering temperature as high as 1800 °C is needed.
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Table 2. Density and open porosity of the samples processed by RHP and R-SPS Theoretical density (g/cm3)
ZS
ZSC
ZSN
ZSA
5.37
5.50
5.49
5.29
RHP 1800 °C
Archimedes density (g/cm3) Relative density (%) Open porosity (%)
5.19 96.6 2.4
5.42 98.5 0.7
5.24 95.5 1.5
5.17 97.8 0.17
R-SPS 1750 °C
Archimedes density (g/cm3) Relative density (%) Open porosity (%)
4.87 90.7 2.0
5.14 93.5 3.4
5.20 94.8 3.1
5.10 96.5 0.5
R-SPS 1800 °C
Archimedes density (g/cm3) Relative density (%) Open porosity (%)
5.19 96.6 2.4
5.45 99.1 0.5
5.57 101.5 0.6
5.32 100.6 1.1
Figure 1. XRD patterns of composites of ZS, ZSC, ZSN, ZSA produced by R-SPS.
In the ZS and ZSC systems, according to the XRD patterns shown in Figure 1, phases of the obtained composites agree with those predicted from the reactions, namely ZrB2, SiC, and ZrB2, SiC, ZrC, respectively. The bulk density of the composites sintered by RHP and R-SPS at 1800 °C had no apparent difference. Although reactants of the two systems are similar except for the small difference in mixture composition, the relative density of ZSC is higher than that of ZS, which is free of ZrC, suggesting that the formation of a small amount of ZrC can improve the composites’ sinterability in both the RHP and R-SPS conditions. However, it is not the same case in the ZSN and ZSA systems. According to the XRD patterns, the phases existing in the RHP-ZSN and the RHP-ZSA are ZrB2, SiC and a solid solution of Zr(C, N) instead of ZrN and AlN. As we know, the lattice parameters of the IVb group carbonitrides obey a linear function between the nitrides and the corresponding carbides [20]. ˚ for The lattice parameters were 4.6930 and 4.5776 A ˚ ZrC and ZrN respectively, but 4.6593 and 4.6471 A for the formed Zr(C, N) in the RHP-ZSN and RHPZSA, respectively (silicon was used as an internal standard). Moreover, a detailed study of the XRD patterns showed that the intensity ratio of the strongest line of Zr(C, N) to ZrB2 ðI ZrðC; NÞ;ð1 1 1Þ =I ZrB2 ;ð1 0 1Þ Þ was 6.22 and 12.01, respectively, for RHP-ZSN and R-SPS-ZSN. The former value was consistent with the expected phase composition according to the reactions of ZSN and ZSA, but the latter is much higher. That means that SPS-ZSN results in more Zr(C, N) with a high density in view of the high density of ZrC and ZrN, and accordingly less SiC with a relatively low density than HPZSN. That is why the density of both R-SPS-ZSN and R-SPS-ZSA sintered at 1800 °C is much higher than that
of RHP-ZSN and RHP-ZSA, and even higher than the theoretical density. Our former study showed that ZrC acts as an intermediate phase during sintering of the ZrB2–SiC–ZrC composite [21]. It can be deduced that the Zr(C, N) in the current work acted in the same way as ZrC did. Unlike the HP process, the R-SPS process had very high heating rate and short holding time. The intermediate Zr(C, N) cannot transfer to ZrB2 and SiC completely, so the reaction proceeds far away from the equilibrium state, and the content of Zr(C, N) will be higher in the R-SPS composites. In addition, since this did not happen in the ZS and ZSC systems, the formed Zr(C, N) solid solution may have less reactivity than ZrC and thus occur at a reduced comversion rate. Furthermore, comparison between the relative densities of RHP-ZSN and RHP-ZS processed at 1800 °C showed that the formation of a small amount of Zr(C, N) did not have much positive effect on the sinterability of the composites. This may also be ascribed to the lower reactivity of Zr(C, N). The same thing happened in the ZSA system, where Zr(C, N) was also formed as the third phase. In the ZSA system, the above-mentioned ratio is 3.52 and 5.05 for RHP-ZSA and R-SPS-ZSA respectively. It is noteworthy that the expected AlN did not appear. One possible explanation is that as the melting point of Al is only 660 °C, it would volatilize before the reaction took place. Another reason is that a solid solution could form between AlN and SiC; this also applies to the ZSA system, where less Zr(C, N) formed in the ZSN system. The microstructures of the obtained ZSC composites processed by RHP and R-SPS are shown in Figure 2a, c and b, d, respectively. Here we chose ZSC as the typical system to investigate the microstructural feature, as it is obtained from the general raw materials Zr, Si and B4C which were used in all the systems, and the amount of BN and Al employed in ZSN and ZSA is relatively small. It is obvious that the distributions of the in situformed ZrB2, SiC, ZrC phases in the R-SPS composite are more homogeneous than those in the RHP composite at different microscale levels. Under the RHP condition with a slow heating rate, the reaction is considered to have taken place in steps. The raw powders reacted with each other locally, and the reaction was controlled by the diffusion process. Accordingly, the coarse particles of the starting powders resulted in an inhomogeneous microstructure, as discussed in detail earlier [21]. As for R-SPS, the heating rate of 100 °C min1 is one
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were synthesized in situ via RHP and R-SPS. Instead of forming ZrN and AlN according to the supposed reactions, Zr(C, N) was formed in the last two composite systems. The R-SPS process tends to form a more homogeneous and finer microstructure because of its high heating rate and short holding time, while the RHP process is likely to form coarse microstructures due to a holding time long enough for grain-growth to proceed. The short holding time of the SPS process also gives the advantage of the densification of the materials.
Figure 2. BEIs of a polished surface of RHP-ZSC ((a) and (c)) and RSPS-ZSC ((b) and (d)) sintered at 1800 °C. The gray phase is ZrB2, the dark phase is SiC and the white phase is ZrC.
Figure 3. BEIs of a cross-section of R-SPS-ZS sintered at 1800 °C. The gray phase is ZrB2 and the dark phase is SiC.
order of magnitude higher than that of the RHP process; the compact could be ignited based on the SHS thermodynamic consideration. Because the melting points of Zr and Si are as low as 1852 and 1414 °C, they would melt during the combustion when outside mechanical pressure was applied and the melting Zr and Si would be redistributed in a more homogeneous state, resulting in a uniform microstructure in the composite. Figure 3 shows the backscattered electron images (BEIs) of a cross-section of R-SPS-ZS sintered at 1800 °C. The existence of disturbances indicates that very drastic SHS reactions have taken place and a liquid phase existed during the reaction. In addition, according to the pictures shown in Figure 2a and b, the particle size in the specimens subjected to R-SPS (<5 lm) was finer than those subjected to RHP (5–10 lm). This is also related to the mechanism mentioned above, that the R-SPS process prefers to form more uniform microstructures, with the homogeneous distribution of SiC as a grain-growth inhibitor reducing the growth of ZrB2. In addition, in the R-SPS process there was not enough time for graingrowth, as the holding time was only several minutes, while in the RHP process the long holding time (1 h) gave the opportunity for grain-growth. In summary, four kinds of composites, ZrB2–SiC, ZrB2–SiC–ZrC, ZrB2–SiC–ZrN, and ZrB2–SiC–AlN,
Financial support from the Chinese Academy of Sciences under the Program for Recruiting Outstanding Overseas Chinese (Hundred Talents Program), the National Natural Science Foundation of China (No. 50632070 and No. 50602048), the State Key Laboratory of High Performance Ceramics and Superfine Microstructures of Shanghai Institute of Ceramics and the Research Fund K.U. Leuven: Flanders-China bilateral project BIL/04/13 & BIL/04/14 is gratefully acknowledged. [1] K. Upadhya, J.M. Yang, W.P. Hoffman, Am. Ceram. Soc. Bull. 58 (1997) 51. [2] E. Wuchina, M. Opeka, S. Causey, K. Buesking, J. Spain, A. Cull, J. Routbort, F. Huitierrez-Mora, J. Mater. Sci. 39 (2004) 5939. [3] M.M. Opeka, I.G. Talmy, E.J. Wuchina, J.A. Zaykoski, S.J. Causey, J. Eur. Ceram. Soc. 19 (1999) 2405. [4] W.C. Tripp, H.H. Davis, H.C. Graham, J. Eur. Ceram. Soc. 52 (1973) 612. [5] M. Gasch, D. Ellerby, E. Irby, S. Beckman, M. Gusman, S. Johnson, J. Mater. Sci. 39 (2004) 5925. [6] A.L. Chamberlain, W.G. Fahrenholtz, G.E. Hilmas, D.T. Ellerby, J. Am. Ceram. Soc. 87 (2004) 1170. [7] F. Monteverde, S. Guicciardi, A. Bellesi, Mat. Sci. Eng. A 346 (2003) 310. [8] J.J. Melendez-Martines, A. Dominguez-Rodriguez, F. Monteverde, C. Melandri, G. Portu, J. Eur. Cer. Soc. 22 (2002) 2543. [9] F. Monteverde, A. Bellesi, Scripta Mater. 46 (2002) 223. [10] F. Monteverde, A. Bellesi, Adv. Eng. Mater. 5 (2003) 508. [11] F. Monteverde, A. Bellesi, J. Mater. Res. 19 (2004) 3576. [12] F. Monteverde, A. Bellesi, Solid State Sci. 7 (2005) 622. [13] G.J. Zhang, Z.Y. Deng, N. Kondo, J.F. Yang, T. Ohji, J. Am. Ceram. Soc. 83 (2000) 2330. [14] Y.D. Blum, H.J. Kleebe, J. Mater. Res. 39 (2004) 6023. [15] F. Monteverde, Compos. Sci. Technol. 65 (2005) 1869. [16] V. Medri, F. Monteverde, Adv. Eng. Mater. 7 (2005) 159. [17] A. Bellesi, F. Monteverde, D. Sciti, Int. J. Appl. Technol. 3 (2006) 32. [18] G.J. Zhang, M. Ando, J.F. Yang, T. Ohji, S. Kanzaki, J. Eur. Ceram. Soc. 24 (2004) 171. [19] K. Vanmeensel, A. Laptev, J. Hennicke, J. Vleugels, O. Van der Biest, Acta Mater. 53 (2005) 4379. [20] W. Lengauer, S. Binder, K. Aigner, P. Ettmayer, A. Guillou, J. Debuigne, G. Groboth, J. Alloy. Compd. 217 (1995) 137. [21] W.W. Wu, G.J. Zhang, Y.M. Kan, P.L. Wang, J. Am. Ceram. Soc. 89 (2006) 2967.
Synthesis and microstructural features of ZrB2–SiC-based composites by reactive spark plasma sintering and reactive hot pressing Wen-Wen Wu,a,b Guo-Jun Zhang,a,* Yan-Mei Kan,a Pei-Ling Wang,a Kim Vanmeensel,c Jozef Vleugelsc and Omer Van der Biestc a
State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Shanghai 200050, China b Graduate School of the Chinese Academy of Sciences, Beijing 200049, China c Department of Metallurgy and Materials Engineering, Katholieke Universiteit, B-3001 Leuven, Belgium Received 13 March 2007; revised 24 April 2007; accepted 24 April 2007 Available online 25 May 2007
Four kinds of composites, ZrB2–SiC, ZrB2–SiC–ZrC, ZrB2–SiC–ZrN and ZrB2–SiC–AlN, were synthesized in situ via reactive hot pressing (RHP) and reactive spark plasma sintering (R-SPS), using Zr, Si, B4C, BN and Al as raw materials. The synthesis process plays a critical role in the microstructural features of the composites obtained. The R-SPS process can lead to a more homogeneous and finer microstructure due to its high heating rates and short holding time, while the RHP process is likely to result in coarse microstructures due to a long enough holding time for grains growth. Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Synthesis; Microstructure; Ultrahigh-temperature ceramics
Zirconium and hafnium-based ceramics (borides, carbides and nitrides) possess a number of unique properties, including extremely high melting temperature and hardness, low volatility, and high thermal and electrical conductivity [1,2]. The ultrahigh-temperature ceramics (UHTC) based on mixtures of ZrB2 or HfB2 and SiC have exhibited relatively good oxidation/ablation resistance [3–5]. Studies show that the second phase of SiC acts as a grain-growth inhibitor [6,7]. Monolithic ceramics have simply been fabricated from commercially available powders by hot pressing. Considering the poor sinterability of the strongly covalent compounds, very high hot pressing temperature (2100 °C) is needed for sintering-aids-free composites to reach high relative density. But such conditions generally induce coarsening of the final microstructure. In order to improve the sinterability of ZrB2/SiC composites, a number of sintering aids, such as Ni, Fe and Si3N4, have been used [8,9]. However, the problem is that unstable second phases that come from these sintering aids will always remain at the grain boundaries in * Corresponding author. E-mail: [email protected]
the composites, leading to the deterioration of hightemperature mechanical properties. Other sintering aids, such as AlN, HfN and ZrN, have also been used [10–12]. They form only refractory secondary phases and are helpful to the performance of the composites. Reactive hot pressing (RHP) can be used as an alternative route [3,13–15]. Through this process, composites with novel and controlled microstructures and high chemical compatibility of in situ-formed individual phases can probably be created. Moreover, in comparison with traditional hot pressing, spark plasma sintering (SPS) is a new sintering technique developed to densify ceramics. Dense composites can be obtained by SPS with a very fast heating rate, a very short holding time and a lower sintering temperature. The SPS method has also been used for preparing UHTCs [16,17]. High-strength ZrB2–SiC composite can be prepared from a mixture of Zr, Si and B4C according to reaction (1) [13] 2Zr + Si + B4 C = 2ZrB2 + SiC
ð1Þ
According to the calculation method for self-propagating high-temperature synthesis (SHS), the adiabatic
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temperature of the above reaction is Tad = 3094 K and DH°298/Cp298 = 5317 K. On the basis of reaction (1), other boride-containing ceramic composites can also be synthesized by using B4C and BN as reactants. The reactions are as follows [18]: 3Zr þ B4 C ¼ 2ZrB2 þ ZrC T ad ¼ 3328 K; DH 298 =Cp298 ¼ 5798 K
ð2Þ
3Zr þ 2BN ¼ ZrB2 þ 2ZrN T ad ¼ 3138 K; DH 298 =Cp298 ¼ 4269 K
ð3Þ
Zr þ 2Al þ 2BN ¼ ZrB2 þ 2AlN T ad ¼ 2464 K; DH 298 =Cp298 ¼ 4212 K
ð4Þ
It can be seen that the reactions are highly exothermic and satisfy the thermodynamic conditions for selfsustaining combustion process (Tad P 1800 K and DH°298/Cp298 > 2000 K). Through the combination of reaction (1) with the other three reactions, three kinds of ceramics, ZrB2–SiC–ZrC, ZrB2–SiC–ZrN and ZrB2– SiC–AlN can be formed in the same way as the ZrB2– SiC composite. The ZrC, ZrN, AlN phases formed ‘‘in situ’’ may have some positive impacts on the control of microstructures and the improvement of sinterability. In the present work, these four kinds of ceramics are prepared by both RHP and reactive SPS (R-SPS), using zirconium, silicon, B4C, BN and aluminum as starting powders. The manufacturing process, mechanical properties and microstructures of the composites processed by RHP and R-SPS are reported, and the effect of ZrC, ZrN and AlN on sintering behavior are also studied and compared. Four different ceramic systems, ZrB2–SiC (ZS), ZrB2–SiC–ZrC (ZSC), ZrB2–SiC–ZrN (ZSN) and ZrB2–SiC–AlN (ZSA), were produced. The calculated phase compositions according to the expected reactions are listed in Table 1. ZSC, ZSN and ZSA were all designed on the basis of ZS, with the same amount (5 vol.%) of the third phases (ZrC, ZrN and AlN), and with the amount of ZrB2 and SiC differing only slightly. Based on the densities of ZrB2(6.09), SiC (3.21), ZrC
Table 1. Specimen designation, reaction and the calculated phase composition Reactions
Calculated composition (vol.%)
ZS
2Zr + Si + B4C ! 2ZrB2 + SiC
ZrB2:74.85 SiC:25.15
ZSC
(2 + x)Zr + (1 x)Si + B4C ! 2ZrB2 + (1 x)SiC + xZrC X = 0.15628
ZrB2:73.96 SiC:21.04 ZrC:5
ZSN
(1 + x)Zr + (0.5–0.25x)Si + (0.5–0.25x)B4 C + xBN ! ZrB2 + (0.5–0.25x)SiC + xZrN X = 0.08968
ZrB2:71.86 SiC:23.14 ZrN:5
ZSA
Zr + (0.5–0.25x)Si + (0.5–0.25x)B4C + xBN + xAl ! ZrB2 + (0.5–0.25x)SiC + xAlN X = 0.10236
ZrB2:71.98 SiC:23.02 AlN:5
(6.44), ZrN (7.32) and AlN (3.26), the calculated theoretical densities (g cm3) of ZS, ZSC, ZSN and ZSA according to the rule of mixtures and the expected phase compositions are 5.37, 5.50, 5.49, and 5.29, respectively. The starting powders were Zr (purity 95.82%, impurities include Ti 2.34, Hf 0.52, Fe 0.24, W 0.08, Cr 0.06, Ni 0.03, O 1.09, particle size < 25 lm; Guoyao Chemicals Co. Ltd., Shanghai, China), Si (purity > 99%, particle size < 50 lm; Yinfeng Silcon Co. Ltd., Jinan, China), B4C (purity 99%, particle size about 2 lm; Jingangzuan Boron Carbide Co., Ltd, Mudanjiang, China), BN (Sanxing Ceramic Materials Co. Ltd., Gongyi, China) and Al (Huabang Fine Powder Materials, Co. Ltd., Liuyang, China). The stoichiometric powders were mixed for 24 h in a polyethylene jars using ethanol and zirconia balls, and then dried by rotary evaporation. The mixture was placed in a graphite die with a BN coating. In the RHP process, the compacts were heated at a slow heating rate (10 °C min1) to 1800 °C and then held for 60 min under a pressure of 20 MPa in an argon atmosphere. The application of pressure was initiated at 1550 °C. The obtained disk had dimensions of 20 mm diameter and 5 mm thickness. The R-SPS process was carried out at a designated temperature for 5 min under 50 MPa in vacuum with a heating rate of 100 °C min1. The gas pressure in the chamber was kept at about 4 mbar, and the gas phase generated during reactions owing to the oxide impurities and the melting of the metal were pumped out immediately. Different sintering temperatures (1750 and 1800 °C) were employed in order to investigate the sintering behavior. Unlike the usual case for temperature measurement, in which the optical pyrometer focuses on the outer die wall surface, in the SPS equipment (FCT-Systeme, Gewerbepark 11, 96528 Rauenstein, Germany) which we used, the sample’s temperature was measured by focusing an optical pyrometer on the bottom of the upper graphite punch. In the former case, the as-received temperature is actually the temperature of the die, which is lower than the actual temperature of the sample as measured by the latter. Research has shown that the difference is about 200 °C at a temperature of 1500 °C [19]. After removing the surface layer from the obtained disk by grinding, the bulk density was measured using the Archimedes method. The dense sample was milled to a fine powder, then the phase composition was determined by X-ray diffractometry (XRD) using Cu Ka radiation with Si as an internal standard. After this, the disk was ground with SiC abrasives and then polished using a diamond paste of 1 lm. Scanning electron microscopy (SEM) was performed to observe the microstructures of the composites. The samples’ densities calculated according to the supposed reactions are listed in Table 2. The XRD patterns of R-SPS composites of ZS, ZSC, ZSN and ZSA are shown in Figure 1. The XRD of the RHP composites are the same, so they are not presented here. In the R-SPS case, as the sintering temperature increased from 1750 to 1800 °C, the density of all the composites increased as well. This indicates that temperature is a very important factor and a sintering temperature as high as 1800 °C is needed.
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Table 2. Density and open porosity of the samples processed by RHP and R-SPS Theoretical density (g/cm3)
ZS
ZSC
ZSN
ZSA
5.37
5.50
5.49
5.29
RHP 1800 °C
Archimedes density (g/cm3) Relative density (%) Open porosity (%)
5.19 96.6 2.4
5.42 98.5 0.7
5.24 95.5 1.5
5.17 97.8 0.17
R-SPS 1750 °C
Archimedes density (g/cm3) Relative density (%) Open porosity (%)
4.87 90.7 2.0
5.14 93.5 3.4
5.20 94.8 3.1
5.10 96.5 0.5
R-SPS 1800 °C
Archimedes density (g/cm3) Relative density (%) Open porosity (%)
5.19 96.6 2.4
5.45 99.1 0.5
5.57 101.5 0.6
5.32 100.6 1.1
Figure 1. XRD patterns of composites of ZS, ZSC, ZSN, ZSA produced by R-SPS.
In the ZS and ZSC systems, according to the XRD patterns shown in Figure 1, phases of the obtained composites agree with those predicted from the reactions, namely ZrB2, SiC, and ZrB2, SiC, ZrC, respectively. The bulk density of the composites sintered by RHP and R-SPS at 1800 °C had no apparent difference. Although reactants of the two systems are similar except for the small difference in mixture composition, the relative density of ZSC is higher than that of ZS, which is free of ZrC, suggesting that the formation of a small amount of ZrC can improve the composites’ sinterability in both the RHP and R-SPS conditions. However, it is not the same case in the ZSN and ZSA systems. According to the XRD patterns, the phases existing in the RHP-ZSN and the RHP-ZSA are ZrB2, SiC and a solid solution of Zr(C, N) instead of ZrN and AlN. As we know, the lattice parameters of the IVb group carbonitrides obey a linear function between the nitrides and the corresponding carbides [20]. ˚ for The lattice parameters were 4.6930 and 4.5776 A ˚ ZrC and ZrN respectively, but 4.6593 and 4.6471 A for the formed Zr(C, N) in the RHP-ZSN and RHPZSA, respectively (silicon was used as an internal standard). Moreover, a detailed study of the XRD patterns showed that the intensity ratio of the strongest line of Zr(C, N) to ZrB2 ðI ZrðC; NÞ;ð1 1 1Þ =I ZrB2 ;ð1 0 1Þ Þ was 6.22 and 12.01, respectively, for RHP-ZSN and R-SPS-ZSN. The former value was consistent with the expected phase composition according to the reactions of ZSN and ZSA, but the latter is much higher. That means that SPS-ZSN results in more Zr(C, N) with a high density in view of the high density of ZrC and ZrN, and accordingly less SiC with a relatively low density than HPZSN. That is why the density of both R-SPS-ZSN and R-SPS-ZSA sintered at 1800 °C is much higher than that
of RHP-ZSN and RHP-ZSA, and even higher than the theoretical density. Our former study showed that ZrC acts as an intermediate phase during sintering of the ZrB2–SiC–ZrC composite [21]. It can be deduced that the Zr(C, N) in the current work acted in the same way as ZrC did. Unlike the HP process, the R-SPS process had very high heating rate and short holding time. The intermediate Zr(C, N) cannot transfer to ZrB2 and SiC completely, so the reaction proceeds far away from the equilibrium state, and the content of Zr(C, N) will be higher in the R-SPS composites. In addition, since this did not happen in the ZS and ZSC systems, the formed Zr(C, N) solid solution may have less reactivity than ZrC and thus occur at a reduced comversion rate. Furthermore, comparison between the relative densities of RHP-ZSN and RHP-ZS processed at 1800 °C showed that the formation of a small amount of Zr(C, N) did not have much positive effect on the sinterability of the composites. This may also be ascribed to the lower reactivity of Zr(C, N). The same thing happened in the ZSA system, where Zr(C, N) was also formed as the third phase. In the ZSA system, the above-mentioned ratio is 3.52 and 5.05 for RHP-ZSA and R-SPS-ZSA respectively. It is noteworthy that the expected AlN did not appear. One possible explanation is that as the melting point of Al is only 660 °C, it would volatilize before the reaction took place. Another reason is that a solid solution could form between AlN and SiC; this also applies to the ZSA system, where less Zr(C, N) formed in the ZSN system. The microstructures of the obtained ZSC composites processed by RHP and R-SPS are shown in Figure 2a, c and b, d, respectively. Here we chose ZSC as the typical system to investigate the microstructural feature, as it is obtained from the general raw materials Zr, Si and B4C which were used in all the systems, and the amount of BN and Al employed in ZSN and ZSA is relatively small. It is obvious that the distributions of the in situformed ZrB2, SiC, ZrC phases in the R-SPS composite are more homogeneous than those in the RHP composite at different microscale levels. Under the RHP condition with a slow heating rate, the reaction is considered to have taken place in steps. The raw powders reacted with each other locally, and the reaction was controlled by the diffusion process. Accordingly, the coarse particles of the starting powders resulted in an inhomogeneous microstructure, as discussed in detail earlier [21]. As for R-SPS, the heating rate of 100 °C min1 is one
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were synthesized in situ via RHP and R-SPS. Instead of forming ZrN and AlN according to the supposed reactions, Zr(C, N) was formed in the last two composite systems. The R-SPS process tends to form a more homogeneous and finer microstructure because of its high heating rate and short holding time, while the RHP process is likely to form coarse microstructures due to a holding time long enough for grain-growth to proceed. The short holding time of the SPS process also gives the advantage of the densification of the materials.
Figure 2. BEIs of a polished surface of RHP-ZSC ((a) and (c)) and RSPS-ZSC ((b) and (d)) sintered at 1800 °C. The gray phase is ZrB2, the dark phase is SiC and the white phase is ZrC.
Figure 3. BEIs of a cross-section of R-SPS-ZS sintered at 1800 °C. The gray phase is ZrB2 and the dark phase is SiC.
order of magnitude higher than that of the RHP process; the compact could be ignited based on the SHS thermodynamic consideration. Because the melting points of Zr and Si are as low as 1852 and 1414 °C, they would melt during the combustion when outside mechanical pressure was applied and the melting Zr and Si would be redistributed in a more homogeneous state, resulting in a uniform microstructure in the composite. Figure 3 shows the backscattered electron images (BEIs) of a cross-section of R-SPS-ZS sintered at 1800 °C. The existence of disturbances indicates that very drastic SHS reactions have taken place and a liquid phase existed during the reaction. In addition, according to the pictures shown in Figure 2a and b, the particle size in the specimens subjected to R-SPS (<5 lm) was finer than those subjected to RHP (5–10 lm). This is also related to the mechanism mentioned above, that the R-SPS process prefers to form more uniform microstructures, with the homogeneous distribution of SiC as a grain-growth inhibitor reducing the growth of ZrB2. In addition, in the R-SPS process there was not enough time for graingrowth, as the holding time was only several minutes, while in the RHP process the long holding time (1 h) gave the opportunity for grain-growth. In summary, four kinds of composites, ZrB2–SiC, ZrB2–SiC–ZrC, ZrB2–SiC–ZrN, and ZrB2–SiC–AlN,
Financial support from the Chinese Academy of Sciences under the Program for Recruiting Outstanding Overseas Chinese (Hundred Talents Program), the National Natural Science Foundation of China (No. 50632070 and No. 50602048), the State Key Laboratory of High Performance Ceramics and Superfine Microstructures of Shanghai Institute of Ceramics and the Research Fund K.U. Leuven: Flanders-China bilateral project BIL/04/13 & BIL/04/14 is gratefully acknowledged. [1] K. Upadhya, J.M. Yang, W.P. Hoffman, Am. Ceram. Soc. Bull. 58 (1997) 51. [2] E. Wuchina, M. Opeka, S. Causey, K. Buesking, J. Spain, A. Cull, J. Routbort, F. Huitierrez-Mora, J. Mater. Sci. 39 (2004) 5939. [3] M.M. Opeka, I.G. Talmy, E.J. Wuchina, J.A. Zaykoski, S.J. Causey, J. Eur. Ceram. Soc. 19 (1999) 2405. [4] W.C. Tripp, H.H. Davis, H.C. Graham, J. Eur. Ceram. Soc. 52 (1973) 612. [5] M. Gasch, D. Ellerby, E. Irby, S. Beckman, M. Gusman, S. Johnson, J. Mater. Sci. 39 (2004) 5925. [6] A.L. Chamberlain, W.G. Fahrenholtz, G.E. Hilmas, D.T. Ellerby, J. Am. Ceram. Soc. 87 (2004) 1170. [7] F. Monteverde, S. Guicciardi, A. Bellesi, Mat. Sci. Eng. A 346 (2003) 310. [8] J.J. Melendez-Martines, A. Dominguez-Rodriguez, F. Monteverde, C. Melandri, G. Portu, J. Eur. Cer. Soc. 22 (2002) 2543. [9] F. Monteverde, A. Bellesi, Scripta Mater. 46 (2002) 223. [10] F. Monteverde, A. Bellesi, Adv. Eng. Mater. 5 (2003) 508. [11] F. Monteverde, A. Bellesi, J. Mater. Res. 19 (2004) 3576. [12] F. Monteverde, A. Bellesi, Solid State Sci. 7 (2005) 622. [13] G.J. Zhang, Z.Y. Deng, N. Kondo, J.F. Yang, T. Ohji, J. Am. Ceram. Soc. 83 (2000) 2330. [14] Y.D. Blum, H.J. Kleebe, J. Mater. Res. 39 (2004) 6023. [15] F. Monteverde, Compos. Sci. Technol. 65 (2005) 1869. [16] V. Medri, F. Monteverde, Adv. Eng. Mater. 7 (2005) 159. [17] A. Bellesi, F. Monteverde, D. Sciti, Int. J. Appl. Technol. 3 (2006) 32. [18] G.J. Zhang, M. Ando, J.F. Yang, T. Ohji, S. Kanzaki, J. Eur. Ceram. Soc. 24 (2004) 171. [19] K. Vanmeensel, A. Laptev, J. Hennicke, J. Vleugels, O. Van der Biest, Acta Mater. 53 (2005) 4379. [20] W. Lengauer, S. Binder, K. Aigner, P. Ettmayer, A. Guillou, J. Debuigne, G. Groboth, J. Alloy. Compd. 217 (1995) 137. [21] W.W. Wu, G.J. Zhang, Y.M. Kan, P.L. Wang, J. Am. Ceram. Soc. 89 (2006) 2967.