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Novel rod-like yttrium α-sialon crystalline powders prepared by combustion synthesis
Materials Chemistry and Physics 75 (2002) 252–255
Novel rod-like yttrium ␣-sialon crystalline powders prepared by combustion synthesis Kexin Chen a,b,∗ , M.E.F.L. Costa b , Heping Zhou a , J.M.F. Ferreira b b
a Department of Material Science and Engineering, Tsinghua University, Beijing 100084, PR China Department of Ceramics and Glass Engineering, UIMC, University of Aveiro, 3810-193 Aveiro, Portugal
Abstract Novel rod-like yttrium ␣-sialon crystalline powders were fabricated by combustion synthesis process. The experimental results show that the combustion temperature, the rate of the Si3 N4 conversion, the nitrogen pressure and the composition of the additives prominently affect the phase formation and microstructures of the resulting ␣-sialon powders. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Combustion synthesis; Yttrium ␣-sialon; Rod-like crystalline powders
1. Introduction
2. Experimental
There are two well-known sialon phases, called ␣ and . It is generally accepted that -sialon grains tend to develop in elongated shapes, while ␣-sialon usually exhibits equi-axed grains. This results in the fact that the toughness of ␣-sialon is lower than that of -sialon, even though the former is harder than the latter one [1,2]. Recently, unexpected evidence of the formation of elongated ␣-sialon grains were found, which means that fabrication of a new ␣-sialon ceramics, superimposing the high toughness of -sialon and the high hardness of ␣-sialon could be realised [3–5]. So far to date, most of the works have been focused on in situ growing rod-like ␣-sialon crystals during densification of polycrystalline compacts [3–5]. Most of the efforts were trying to increase the amount of rod-like ␣-sialon crystals in the final product. However, a higher sintering temperature and/or a longer sintering time are required to achieve a fully dense body. If the rod-like ␣-sialon crystalline particles could be obtained, dense and high tough ␣-sialon ceramics would be realised by using them as reinforcing agents without greatly changing the normal sintering process. In this paper, yttrium-stabilised rod-like ␣-sialon crystalline powders were prepared by combustion synthesis.
The compositions studied here in the so-called yttrium ␣-sialon plane, which is defined as Ym/3 Si12−(m+n) Alm+n On N16−n . The value of m = n = 1.2 were selected with the purpose of studying the effects of reaction parameters on phase formation and microstructure of the final product. Since the combustion temperatures are usually much higher than the melting point of Al (660 ◦ C) and Si (1410 ◦ C), these metal powders with low melting point will melt and agglomerate, preventing the nitrogen gas infiltrating into the sample. Therefore, some non-melting material powders, such as silicon nitride, aluminium nitride or ␣-sialon, were required to improve the gas infiltration. In the present experiments, the ratio of the low melting point materials (Al + Si) was fixed at about 51%. Initial powder mixtures with compositions listed in Table 1 were prepared by ball milling in the ethanol for 8 h. After drying, the powder mixtures were put into a porous crucible with a diameter of 25 mm and a height of 40 mm, which was then placed into a high-pressure chamber as illustrated in the previous paper [6]. Evacuation was performed up to a vacuum of 10−4 MPa. The mixed powders were then ignited at required nitrogen pressure values. The combustion reaction temperature was recorded through acquisition data system. The crystalline phase formation and microstructure of the products were identified and observed by XRD and SEM, respectively.
∗ Corresponding author. Present address: Department of Material Science and Engineering, Tsinghua University, Beijing 100084, PR China. Tel.: +86-10-62772548; fax: +86-10-62771160. E-mail address: [email protected] (K. Chen).
0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 2 ) 0 0 0 7 1 - 8
K. Chen et al. / Materials Chemistry and Physics 75 (2002) 252–255
253
Table 1 Compositions of the raw material mixtures Sample name
Y1212-1 Y1212-2 Y1212-3 Y1212-4 Y1212-5
Composition (wt.%) Y 2 O3
AlN
Si
Al
␣-Si3 N4
-Si3 N4
SiO2
NH4 F
␣-Sialon
10.04 9.65 10.04 8.69 6.67
0 0 0 0 0
37.33 35.90 37.33 50.08 38.43
14.40 13.85 14.40 12.46 9.56
34.22 32.90 0 0 0
0 0 34.22 0 0
4.01 3.85 4.01 3.46 2.66
0 3.85 0 2.23 24.99
0 0 0 23.08 17.71
3. Results 3.1. Effects of reaction parameters on the phase formation 3.1.1. Effects of nitrogen pressure and NH4 F addition Fig. 1 shows the phase formation in the product from sample Y1212-1 under different nitrogen pressure values. It can be seen that the peak strength of ␣-sialon decreased, while the peak strength of -sialon increased, with the increasing nitrogen pressure. The residual Si in the final product maintained almost the same level as increasing nitrogen pressure. An amount of 3.85 wt.% of NH4 F was added to the sample Y1212-1, designated it as Y1212-2. The relative amount of ␣- and -sialons changed greatly as Fig. 2 shows. Compared with sample Y1212-1, the peak strength of ␣-sialon decreased observably, while that of -sialon increased. Moreover, the residual Si was reduced further in sample Y1212-2 than that in sample Y1212-1. As increasing nitrogen pressure, the trend of change in ␣- and -sialon and Si peak strengths keep similar with sample Y1212-1.
Fig. 1. Crystallite phases in the products from sample Y1212-1 under different nitrogen pressures.
Fig. 2. Crystallite phases in the products from sample Y1212-2 under different nitrogen pressures.
3.1.2. Effects of diluent kind on the phase formation Fig. 3 compares the effect of different diluents on the crystalline phases identified in the final products. It can be
Fig. 3. Crystallite phases in the products from samples Y1212-3, Y1212-4 and Y1212-5 under a fixed nitrogen pressure of 2 MPa.
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K. Chen et al. / Materials Chemistry and Physics 75 (2002) 252–255
Fig. 4. Microstructure of the final product obtained from sample Y1212-5: (A) low magnification image; (B) high magnification image.
seen that when ␣-Si3 N4 in sample Y1212-1 is exchanged for -Si3 N4 (sample Y1212-3), the amount of ␣-sialon decreased greatly, corresponding to a prominent increasing of -sialon content. However, when ␣-sialon was used as diluents almost all the products consisted of ␣-sialon (samples Y1212-4 and Y1212-5), regardless of whether NH4 F was added. 3.1.3. Characteristics of the microstructure of the final products Fig. 4 shows the microstructure of the final product obtained from sample Y1212-5. It can be seen that almost all the ␣-sialon crystals were developed into rod-like morphologies.
4. Discussion During the combustion process, the occurrence of some liquid phases may prevent nitrogen gas infiltrating into the reacting sample. This would control the untime nitrogen supply for the combustion reaction. Under these conditions, aluminium should first react with nitrogen gas to form AlN because the lower values of chemical potential and activation energy involved with increasing N2 pressure, silicon is likely to react with N2 , increasing the formation of Si3 N4 and the quick heat release at the initial combustion period. This quick heat release would lead to more liquid phase occurred since the melting of Si and the formation of transient liquid. More liquid occurred could prevent the nitrogen gas infil-
K. Chen et al. / Materials Chemistry and Physics 75 (2002) 252–255
Fig. 5. Temperature–time histories for sample Y1212-1 under different nitrogen pressures.
trating from surface to the interior, which result in a reducing nitridation of silicon. However, increasing the nitrogen pressure will enhance the driving force of nitrogen gas infiltration. The above two processes on the nitridation of silicon were countervailed. Therefore, the residual Si in the final product maintained almost at the same level as the nitrogen pressure increases. To further confirm this hypothesis, the combustion temperature–time histories were recorded, as shown in Fig. 5. It can be seen that the combustion temperature rising rate is speeded up with increasing N2 pressure. As far as the effect of NH4 F on the conversion rate of Si to Si3 N4 is concerned, decomposition of NH4 F generates N2 , NH3 and HF, thus enhancing the concentration of N2 in the reacting mixtures. Moreover, NH4 F could provide an easier route for the nitridation of silicon [7]. The residual Si was reduced more in sample Y1212-2 than that in sample Y1212-1. It is well known that there is a phase transformation from ␣-Si3 N4 to -Si3 N4 above 1400 ◦ C in the presence of a liquid phase. During combustion synthesis of Y ␣-sialon, all the combustion temperatures measured under different experimental conditions were higher than 1700 ◦ C. This means that there should be a Si3 N4 phase transformation from ␣ to . This transformation was further certified by a quenching experiment. It was found that the final product was a single phase of -Si3 N4 , while the primarily formed compound was ␣-Si3 N4 and unreacted Si. From the above analysis, the experimental results can be summarised as follows. Increasing nitrogen pressure or adding NH4 F enhances ␣-Si3 N4 to -Si3 N4 transformation when ␣-Si3 N4 cannot be consumed by forming sialon timely. The as-formed -Si3 N4 can provide an easier path for forming -sialon than for forming ␣-sialon owing to the
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homogeneous nucleation. This was certified by the sample Y1212-3 with -Si3 N4 being added as diluent, instead of ␣-Si3 N4 as in sample Y1212-1. The amount of -sialon was increased greatly as compared to Y1212-1. This was also supported by sample Y1212-4 with ␣-sialon as diluent and without any Si3 N4 in the starting mixtures. The final products of Y1212-4 and Y1212-5 were almost single-phase ␣-sialon. For the process of combustion synthesis, non-equilibrium situations are likely to occur owing to the very quick combustion process. As a consequence, some ␣-sialon crystals may nucleate and grow, while part of the raw materials still keep in the intermediate reaction paths. This delays the supply of the necessary substances for the growing of ␣-sialon crystals into a preferred spatial direction, which should possess the lowest crystalline formation energy. These characteristics of combustion synthesis led to the rod-like ␣-sialon crystals.
5. Conclusions Reaction parameters strongly affect the phase formation of combustion products. -Sialon phase is promoted by increasing nitrogen pressure or by adding NH4 F, which provides homogeneous nucleus sites for -sialon. Single-phase ␣-sialon can be obtained by using ␣-sialon as the diluent, instead of ␣-Si3 N4 . The addition of NH4 F is beneficial to the nitridation of silicon. The ␣-sialon made by combustion synthesis process exhibited rod-like crystallite morphology. Acknowledgements Chen and Costa are grateful to Fundação para Ciência e a Tecnologia of Portugal for the grants PRAXIS XXI/B/PD/20113/99 and PRAXIS XXI/BD/18065/98. Project 50102002 supported by NSFC. References [1] L. Dumitrescu, B. Sundman, J. Eur. Ceram. Soc. 15 (1995) 239. [2] C.L. Hewett, Y.B. Cheng, B.C. Muddle, J. Am. Ceram. Soc. 81 (1998) 1781. [3] I.-W. Chen, A. Rosenflanz, Nature 389 (1997) 701. [4] Z.J. Shen, T. Ekström, M. Nygren, J. Phys. D 29 (1996) 893. [5] H. Wang, Y.B. Cheng, B.C. Muddle, J. Mater. Sci. Lett. 15 (1996) 1447. [6] K.X. Chen, C.C. Ge, J.T. Li, W.B. Cao, J. Mater. Res. 14 (1999) 1944. [7] W.C. Lee, S.L. Chung, J. Mater. Res. 12 (1997) 805.
Novel rod-like yttrium ␣-sialon crystalline powders prepared by combustion synthesis Kexin Chen a,b,∗ , M.E.F.L. Costa b , Heping Zhou a , J.M.F. Ferreira b b
a Department of Material Science and Engineering, Tsinghua University, Beijing 100084, PR China Department of Ceramics and Glass Engineering, UIMC, University of Aveiro, 3810-193 Aveiro, Portugal
Abstract Novel rod-like yttrium ␣-sialon crystalline powders were fabricated by combustion synthesis process. The experimental results show that the combustion temperature, the rate of the Si3 N4 conversion, the nitrogen pressure and the composition of the additives prominently affect the phase formation and microstructures of the resulting ␣-sialon powders. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Combustion synthesis; Yttrium ␣-sialon; Rod-like crystalline powders
1. Introduction
2. Experimental
There are two well-known sialon phases, called ␣ and . It is generally accepted that -sialon grains tend to develop in elongated shapes, while ␣-sialon usually exhibits equi-axed grains. This results in the fact that the toughness of ␣-sialon is lower than that of -sialon, even though the former is harder than the latter one [1,2]. Recently, unexpected evidence of the formation of elongated ␣-sialon grains were found, which means that fabrication of a new ␣-sialon ceramics, superimposing the high toughness of -sialon and the high hardness of ␣-sialon could be realised [3–5]. So far to date, most of the works have been focused on in situ growing rod-like ␣-sialon crystals during densification of polycrystalline compacts [3–5]. Most of the efforts were trying to increase the amount of rod-like ␣-sialon crystals in the final product. However, a higher sintering temperature and/or a longer sintering time are required to achieve a fully dense body. If the rod-like ␣-sialon crystalline particles could be obtained, dense and high tough ␣-sialon ceramics would be realised by using them as reinforcing agents without greatly changing the normal sintering process. In this paper, yttrium-stabilised rod-like ␣-sialon crystalline powders were prepared by combustion synthesis.
The compositions studied here in the so-called yttrium ␣-sialon plane, which is defined as Ym/3 Si12−(m+n) Alm+n On N16−n . The value of m = n = 1.2 were selected with the purpose of studying the effects of reaction parameters on phase formation and microstructure of the final product. Since the combustion temperatures are usually much higher than the melting point of Al (660 ◦ C) and Si (1410 ◦ C), these metal powders with low melting point will melt and agglomerate, preventing the nitrogen gas infiltrating into the sample. Therefore, some non-melting material powders, such as silicon nitride, aluminium nitride or ␣-sialon, were required to improve the gas infiltration. In the present experiments, the ratio of the low melting point materials (Al + Si) was fixed at about 51%. Initial powder mixtures with compositions listed in Table 1 were prepared by ball milling in the ethanol for 8 h. After drying, the powder mixtures were put into a porous crucible with a diameter of 25 mm and a height of 40 mm, which was then placed into a high-pressure chamber as illustrated in the previous paper [6]. Evacuation was performed up to a vacuum of 10−4 MPa. The mixed powders were then ignited at required nitrogen pressure values. The combustion reaction temperature was recorded through acquisition data system. The crystalline phase formation and microstructure of the products were identified and observed by XRD and SEM, respectively.
∗ Corresponding author. Present address: Department of Material Science and Engineering, Tsinghua University, Beijing 100084, PR China. Tel.: +86-10-62772548; fax: +86-10-62771160. E-mail address: [email protected] (K. Chen).
0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 2 ) 0 0 0 7 1 - 8
K. Chen et al. / Materials Chemistry and Physics 75 (2002) 252–255
253
Table 1 Compositions of the raw material mixtures Sample name
Y1212-1 Y1212-2 Y1212-3 Y1212-4 Y1212-5
Composition (wt.%) Y 2 O3
AlN
Si
Al
␣-Si3 N4
-Si3 N4
SiO2
NH4 F
␣-Sialon
10.04 9.65 10.04 8.69 6.67
0 0 0 0 0
37.33 35.90 37.33 50.08 38.43
14.40 13.85 14.40 12.46 9.56
34.22 32.90 0 0 0
0 0 34.22 0 0
4.01 3.85 4.01 3.46 2.66
0 3.85 0 2.23 24.99
0 0 0 23.08 17.71
3. Results 3.1. Effects of reaction parameters on the phase formation 3.1.1. Effects of nitrogen pressure and NH4 F addition Fig. 1 shows the phase formation in the product from sample Y1212-1 under different nitrogen pressure values. It can be seen that the peak strength of ␣-sialon decreased, while the peak strength of -sialon increased, with the increasing nitrogen pressure. The residual Si in the final product maintained almost the same level as increasing nitrogen pressure. An amount of 3.85 wt.% of NH4 F was added to the sample Y1212-1, designated it as Y1212-2. The relative amount of ␣- and -sialons changed greatly as Fig. 2 shows. Compared with sample Y1212-1, the peak strength of ␣-sialon decreased observably, while that of -sialon increased. Moreover, the residual Si was reduced further in sample Y1212-2 than that in sample Y1212-1. As increasing nitrogen pressure, the trend of change in ␣- and -sialon and Si peak strengths keep similar with sample Y1212-1.
Fig. 1. Crystallite phases in the products from sample Y1212-1 under different nitrogen pressures.
Fig. 2. Crystallite phases in the products from sample Y1212-2 under different nitrogen pressures.
3.1.2. Effects of diluent kind on the phase formation Fig. 3 compares the effect of different diluents on the crystalline phases identified in the final products. It can be
Fig. 3. Crystallite phases in the products from samples Y1212-3, Y1212-4 and Y1212-5 under a fixed nitrogen pressure of 2 MPa.
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K. Chen et al. / Materials Chemistry and Physics 75 (2002) 252–255
Fig. 4. Microstructure of the final product obtained from sample Y1212-5: (A) low magnification image; (B) high magnification image.
seen that when ␣-Si3 N4 in sample Y1212-1 is exchanged for -Si3 N4 (sample Y1212-3), the amount of ␣-sialon decreased greatly, corresponding to a prominent increasing of -sialon content. However, when ␣-sialon was used as diluents almost all the products consisted of ␣-sialon (samples Y1212-4 and Y1212-5), regardless of whether NH4 F was added. 3.1.3. Characteristics of the microstructure of the final products Fig. 4 shows the microstructure of the final product obtained from sample Y1212-5. It can be seen that almost all the ␣-sialon crystals were developed into rod-like morphologies.
4. Discussion During the combustion process, the occurrence of some liquid phases may prevent nitrogen gas infiltrating into the reacting sample. This would control the untime nitrogen supply for the combustion reaction. Under these conditions, aluminium should first react with nitrogen gas to form AlN because the lower values of chemical potential and activation energy involved with increasing N2 pressure, silicon is likely to react with N2 , increasing the formation of Si3 N4 and the quick heat release at the initial combustion period. This quick heat release would lead to more liquid phase occurred since the melting of Si and the formation of transient liquid. More liquid occurred could prevent the nitrogen gas infil-
K. Chen et al. / Materials Chemistry and Physics 75 (2002) 252–255
Fig. 5. Temperature–time histories for sample Y1212-1 under different nitrogen pressures.
trating from surface to the interior, which result in a reducing nitridation of silicon. However, increasing the nitrogen pressure will enhance the driving force of nitrogen gas infiltration. The above two processes on the nitridation of silicon were countervailed. Therefore, the residual Si in the final product maintained almost at the same level as the nitrogen pressure increases. To further confirm this hypothesis, the combustion temperature–time histories were recorded, as shown in Fig. 5. It can be seen that the combustion temperature rising rate is speeded up with increasing N2 pressure. As far as the effect of NH4 F on the conversion rate of Si to Si3 N4 is concerned, decomposition of NH4 F generates N2 , NH3 and HF, thus enhancing the concentration of N2 in the reacting mixtures. Moreover, NH4 F could provide an easier route for the nitridation of silicon [7]. The residual Si was reduced more in sample Y1212-2 than that in sample Y1212-1. It is well known that there is a phase transformation from ␣-Si3 N4 to -Si3 N4 above 1400 ◦ C in the presence of a liquid phase. During combustion synthesis of Y ␣-sialon, all the combustion temperatures measured under different experimental conditions were higher than 1700 ◦ C. This means that there should be a Si3 N4 phase transformation from ␣ to . This transformation was further certified by a quenching experiment. It was found that the final product was a single phase of -Si3 N4 , while the primarily formed compound was ␣-Si3 N4 and unreacted Si. From the above analysis, the experimental results can be summarised as follows. Increasing nitrogen pressure or adding NH4 F enhances ␣-Si3 N4 to -Si3 N4 transformation when ␣-Si3 N4 cannot be consumed by forming sialon timely. The as-formed -Si3 N4 can provide an easier path for forming -sialon than for forming ␣-sialon owing to the
255
homogeneous nucleation. This was certified by the sample Y1212-3 with -Si3 N4 being added as diluent, instead of ␣-Si3 N4 as in sample Y1212-1. The amount of -sialon was increased greatly as compared to Y1212-1. This was also supported by sample Y1212-4 with ␣-sialon as diluent and without any Si3 N4 in the starting mixtures. The final products of Y1212-4 and Y1212-5 were almost single-phase ␣-sialon. For the process of combustion synthesis, non-equilibrium situations are likely to occur owing to the very quick combustion process. As a consequence, some ␣-sialon crystals may nucleate and grow, while part of the raw materials still keep in the intermediate reaction paths. This delays the supply of the necessary substances for the growing of ␣-sialon crystals into a preferred spatial direction, which should possess the lowest crystalline formation energy. These characteristics of combustion synthesis led to the rod-like ␣-sialon crystals.
5. Conclusions Reaction parameters strongly affect the phase formation of combustion products. -Sialon phase is promoted by increasing nitrogen pressure or by adding NH4 F, which provides homogeneous nucleus sites for -sialon. Single-phase ␣-sialon can be obtained by using ␣-sialon as the diluent, instead of ␣-Si3 N4 . The addition of NH4 F is beneficial to the nitridation of silicon. The ␣-sialon made by combustion synthesis process exhibited rod-like crystallite morphology. Acknowledgements Chen and Costa are grateful to Fundação para Ciência e a Tecnologia of Portugal for the grants PRAXIS XXI/B/PD/20113/99 and PRAXIS XXI/BD/18065/98. Project 50102002 supported by NSFC. References [1] L. Dumitrescu, B. Sundman, J. Eur. Ceram. Soc. 15 (1995) 239. [2] C.L. Hewett, Y.B. Cheng, B.C. Muddle, J. Am. Ceram. Soc. 81 (1998) 1781. [3] I.-W. Chen, A. Rosenflanz, Nature 389 (1997) 701. [4] Z.J. Shen, T. Ekström, M. Nygren, J. Phys. D 29 (1996) 893. [5] H. Wang, Y.B. Cheng, B.C. Muddle, J. Mater. Sci. Lett. 15 (1996) 1447. [6] K.X. Chen, C.C. Ge, J.T. Li, W.B. Cao, J. Mater. Res. 14 (1999) 1944. [7] W.C. Lee, S.L. Chung, J. Mater. Res. 12 (1997) 805.