- Email: [email protected]
Preparation of Y-SiAlON rod-like crystals and whiskers by combustion synthesis
Materials Letters 59 (2005) 3955 – 3958 www.elsevier.com/locate/matlet
Preparation of Y-SiAlON rod-like crystals and whiskers by combustion synthesis Guanghua Liu a,b,*, Kexin Chen a, Heping Zhou a, C. Pereira b, S. Quaresma b, J.M.F. Ferreira b a
Department of Materials Science and Engineering, State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University, 100084, Beijing, P. R. China b Department of Ceramics and Glass Engineering, University of Aveiro, CICECO, 3810-193, Aveiro, Portugal Received 11 June 2005; accepted 16 July 2005 Available online 3 August 2005
Abstract By combustion synthesis, Y (a + h)-SiAlON ceramic powders consisting of rod-like crystals were prepared. Effect of starting composition on the phase assemblage of combustion products was studied. It was found that with the increase of oxygen content, the phase assemblage of products varied from single-phase a-SiAlON to h-SiAlON gradually. Novel h-SiAlON whiskers were also observed in the combustion products. From the whisker morphology, it was suggested that both VLS and VS mechanisms operated here. It was also noticed that the whiskers could nucleate either on the basal face or on the side faces of as-existed prismatic rod-like crystals. D 2005 Elsevier B.V. All rights reserved. Keywords: SiAlON; Rod-like crystal; Whisker; Combustion synthesis
1. Introduction SiAlON ceramics have been identified as one of the most promising structural materials owing to their high hardness, superior wear resistance and good anticorrosion performance, which are even more pronounced at elevated temperatures. SiAlON ceramics are used widely in high temperature industrial, automotive and aerospace applications such as cutting tools, wire drawing, dies and blast nozzles [1 –3]. There are two well-known variables of SiAlON called a and h˙ corresponding to a and h-Si3N4 in crystal structure, respectively. It is generally accepted that aSiAlON usually occurs in equiaxed grains while h-SiAlON in elongated grains, which results in the fact that the fracture toughness of a-SiAlON is lower than h-SiAlON. However, some recent studies show that a-SiAlON can also develop * Corresponding author. Department of Materials Science and Engineering, State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University, 100084, Beijing, P. R. China. Tel.: +86 10 62772548; fax: +86 10 62771160. E-mail address: [email protected] (G. Liu). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.07.046
into elongated morphology by controlling the nucleation and growth process [4– 7]. These results show that the grain morphology is not an intrinsic characteristic and in fact it can be tailored by choosing appropriate compositions and applying proper techniques. Combustion synthesis, which is also known as selfpropagating high-temperature synthesis (SHS), has attracted much attention as a low-cost and productive technique to fabricate a wide range of useful materials, including many kinds of ceramic powders [8]. Because of the characteristics of high reaction temperature and fast heating rate, combustion synthesis is advantageous in purifying the phase assemblage and modifying the final grain morphology. For example, both h-Si3N4 and a-SiAlON ceramic powders consisting of rod-like crystals have been successfully fabricated by combustion synthesis [9– 11]. More recently, novel a-SiAlON whiskers with remarkably higher aspect ratios were also reported [12]. If the whiskers can be aligned properly and woven into the matrix ceramics, the toughness is expected to be further improved. This paper reports the combustion synthesis of a- and hSiAlON powders with rod-like crystals or novel whiskers.
3956
G. Liu et al. / Materials Letters 59 (2005) 3955 – 3958
Chemical Co., China) as raw materials. The raw materials were mixed by agate balls in a plastic jar for 24 h with absolute ethanol as medium. The obtained slurry was dried and sieved. Then the starting powder mixture was contained in a porous graphite crucible and placed into the combustion reaction chamber. After evacuation, the reaction chamber was inflated with high-purity N2 at a pressure value of 2 MPa. The combustion reaction was triggered by passing an electric current through a tungsten coil. The reaction temperature was measured by a W-Re3/ W-Re25 thermocouple, which was inserted into the sample center and connected with a computer system for data treatment. The phase assemblage of reaction products was identified by XRD (Cu Ka, Rigaku, Japan). The microstructure was observed by SEM (JSM-6460LV, JEOL, Japan) and EDS (INCA, Oxford Instrument) was also used to perform the element analysis.
3. Results and discussion
Fig. 1. XRD patterns of the combustion products: (a) SN12; (b) SN18; (c) SN22.
The effect of starting composition on the phase assemblage of combustion products and the formation mechanism of whiskers are discussed.
Fig. 1 shows XRD patterns of the combustion products for SN12, SN18, and SN22. It can be seen that with different starting compositions, the final phase assemblages of combustion products are remarkably different. In the sample SN12, single-phase aSiAlON powder has been prepared, while in the sample SN22 the predominant phase is h-SiAlON, and in the sample SN18, the combustion product is a composite of a- and h-SiAlON. The XRD patterns of the samples SN16 and SN22, which are not shown here, are basically similar to those of SN12 and SN22. These results are basically as expected, for with the increase of oxygen content (nvalue) the starting composition point moves from the inner part of a-SiAlON phase area to the multi-phase area. AlN polytypoids, which are usual intermediate phases accompanied with the formation of a-SiAlON, are not found here in all samples. This may be attributed to the fast heating and cooling rates in combustion synthesis and short reaction period compared with general reaction sintering. Ignoring the small amount of residual silicon powder, the relative phase contents of a-SiAlON and h-SiAlON are semiquantitatively calculated on the basis of XRD results, according to
2. Experimental In the general formula of Ym / 3 Si12 (m + n)Alm + n On N16 n , five compositions were investigated, where m = 1.2 was fixed and n varied from 1.2 to 1.6, 1.8, 2.0, and 2.2. According to different n values, the samples were briefly designated as SN12, SN16, SN18, SN20, and SN22, respectively. Starting compositions were fulfilled using Y2O 3 (99.9%, General Research Institute for Nonferrous Metals, China), Si (99.0%, Fushun Al Factory, China), Al (99.5%, Gaizhou Al Co., China), a-Si3N4 (Fangda High-Technology Ceramics Co., China), and SiO2 (A.R., Beijing
Fig. 2. Effect of the starting composition on final phase assemblage of combustion products.
G. Liu et al. / Materials Letters 59 (2005) 3955 – 3958
3957
Fig. 3. Microstructure of the combustion products: (a) SN12, a-SiAlON; (b) SN16, 97% a-SiAlON + 3% h-SiAlON; (c) SN20, 16% a-SiAlON + 84% hSiAlON; (d) SN22, 5% a-SiAlON + 95% h-SiAlON.
the method described in reference [13]. The calculated results are shown in Fig. 2. It is clear that, with the increase of n-value from 1.2 to 2.2, the relative content of a-SiAlON gradually decreases and h-SiAlON increases. This change in phase content shows the important influence of starting composition on final phase assemblage of the combustion products.
Fig. 3 shows the typical micrographs of the combustion products. In the sample SN12, the grain growth is incomplete and no rod-like crystals are found. But in the sample SN16, many rod-like crystals are visible. This difference in grain morphology for the two samples indicates that more liquid phase resulting from high oxygen content is beneficial to the development of rod-like a-
Fig. 4. Whiskers obtained in the combustion product from SN22: (a) general morphology; (b) wool-like whiskers; (c) comb-like morphology, the inset EDS result is for the large rod-like crystal; (d) arrays of straight whiskers.
3958
G. Liu et al. / Materials Letters 59 (2005) 3955 – 3958
Additionally, the new whiskers can nucleate and grow on the basal face of the hexagonal prismatic crystals, forming a whisker array, as shown in Fig. 4 (d). In this nucleation mode, it is noticed that the new-formed straight whiskers have the same orientation with the substrate prismatic crystals, i.e. their length directions are parallel.
4. Conclusion
Fig. 5. Temperature history of the sample SN22 during combustion synthesis.
SiAlON grains. In the samples SN20 and SN22 with h-SiAlON as main phase, most grains develop into the rod-like shape. Except the rod-like crystals, novel h-SiAlON whiskers are also obtained in the sample SN22, as shown in Fig. 4. Both VLS whiskers with the typical round droplet on head and VS whiskers without the droplet are observed, proving that both mechanisms operate here. At the same time, besides the straight whiskers, some wool-like whiskers are also noticed, as shown in Fig. 4 (b). In the combustion synthesis process, once the combustion reaction is triggered, the nitridation reaction of Al and Si will produce a large amount of hear energy, which leads to a dramatic increase in temperature. Fig. 5 shows the temperature history of the sample SN22, from which it can be seen that the temperature increases quickly to higher than 1800 -C in only several seconds. Since the reaction temperature is much higher than the melting points of Al and Si particles and the formation temperature of the ternary oxide melt, the evaporation of the melt phase will occur and bring the reactant elements into the vapor phase. At the same time, some monoxides may be formed and also exist in the vapor. In this case, if a liquid droplet is formed on the surface of a certain asexisted crystal, the reactant molecules or atoms in vapor phase will dissolve into the liquid and create a supersaturated nitrogen-rich liquid phase, from which new h-SiAlON crystals can precipitate. With the continuous precipitation process, the liquid droplet is detached from the substrate surface and h-SiAlON whiskers are formed. During the growth of a VLS-formed h-SiAlON whisker, the liquid droplet may evaporate under proper conditions. At this time, the growth mechanism of the whisker can transform into VS mode and the whisker growth is fulfilled by direct deposition of the reactant molecules or atoms on the growth front. At the same time, the reactant elements can deposit on the surface of as-existed large crystals and form VS whiskers directly without the mediate VLS process. As shown in Fig. 4 (c), new whiskers can nucleate and grow on the side faces of a large rod-like h-SiAlON crystal, forming an interesting comb-like morphology.
By combustion synthesis, Y (a + h)-SiAlON powders consisting of rod-like crystals were prepared. Effect of starting composition on the phase assemblage of combustion products was studied. It was found that with the increase of oxygen content, the phase assemblage of products varied from single-phase a-SiAlON to h-SiAlON gradually. Novel h-SiAlON whiskers with high aspect ratios were also observed in the combustion products. From the whisker morphology, it was suggested that both VLS and VS mechanisms operated here. It was also noticed that the whiskers could nucleate either on the basal face or on the side faces of as-existed prismatic rod-like crystals.
Acknowledgements This work is supported by National Natural Science Foundation of China (Grant No. 50102002), and by the Foundation for Science and Technology of Portugal, Project FCT SAPIENS—Reference: POCTI/CTM/39419/2001.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
K.H. Jack, J. Mater. Sci. 11 (1976) 1135. T. Ekstrom, M. Nygren, J. Am. Ceram. Soc. 75 (1992) 259. A. Rosenflanz, Curr. Opin. Solid State Mater. Sci. 4 (1999) 453. I.W. Chen, A. Rosenflanz, Nature 389 (1997) 701. C.L. Hewett, Y.B. Cheng, B.C. Muddle, J. Am. Ceram. Soc. 81 (1998) 1781. Z.J. Shen, Z. Zhao, H. Peng, M. Nygren, Nature 417 (2002) 266. K.X. Chen, H.B. Jin, M. Oliveira, H.P. Zhou, J.M.F. Ferreira, J. Mater. Res. 16 (2001) 1928. Z.A. Munir, A.T. Umberto, Mater. Sci. Rep. 3 (1989) 277. I.G. Cano, M.A. Rodriguez, Scr. Mater. 50 (2004) 383. G.H. Liu, K.X. Chen, H.P. Zhou, J.M.F. Ferreira, J. Mater. Res. 19 (2004) 3408. R.L. Fu, K.X. Chen, X. Xu, J.M.F. Ferreira, Mater. Lett. 58 (2004) 1956. G.H. Liu, K.X. Chen, H.P. Zhou, X.S. Ning, C. Pereira, J.M.F. Ferreira, J. Mater. Res. 20 (2005) 889. C.P. Gazzara, D.R. Messier, Am. Ceram. Soc. Bull. 56 (1977) 777.
Preparation of Y-SiAlON rod-like crystals and whiskers by combustion synthesis Guanghua Liu a,b,*, Kexin Chen a, Heping Zhou a, C. Pereira b, S. Quaresma b, J.M.F. Ferreira b a
Department of Materials Science and Engineering, State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University, 100084, Beijing, P. R. China b Department of Ceramics and Glass Engineering, University of Aveiro, CICECO, 3810-193, Aveiro, Portugal Received 11 June 2005; accepted 16 July 2005 Available online 3 August 2005
Abstract By combustion synthesis, Y (a + h)-SiAlON ceramic powders consisting of rod-like crystals were prepared. Effect of starting composition on the phase assemblage of combustion products was studied. It was found that with the increase of oxygen content, the phase assemblage of products varied from single-phase a-SiAlON to h-SiAlON gradually. Novel h-SiAlON whiskers were also observed in the combustion products. From the whisker morphology, it was suggested that both VLS and VS mechanisms operated here. It was also noticed that the whiskers could nucleate either on the basal face or on the side faces of as-existed prismatic rod-like crystals. D 2005 Elsevier B.V. All rights reserved. Keywords: SiAlON; Rod-like crystal; Whisker; Combustion synthesis
1. Introduction SiAlON ceramics have been identified as one of the most promising structural materials owing to their high hardness, superior wear resistance and good anticorrosion performance, which are even more pronounced at elevated temperatures. SiAlON ceramics are used widely in high temperature industrial, automotive and aerospace applications such as cutting tools, wire drawing, dies and blast nozzles [1 –3]. There are two well-known variables of SiAlON called a and h˙ corresponding to a and h-Si3N4 in crystal structure, respectively. It is generally accepted that aSiAlON usually occurs in equiaxed grains while h-SiAlON in elongated grains, which results in the fact that the fracture toughness of a-SiAlON is lower than h-SiAlON. However, some recent studies show that a-SiAlON can also develop * Corresponding author. Department of Materials Science and Engineering, State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University, 100084, Beijing, P. R. China. Tel.: +86 10 62772548; fax: +86 10 62771160. E-mail address: [email protected] (G. Liu). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.07.046
into elongated morphology by controlling the nucleation and growth process [4– 7]. These results show that the grain morphology is not an intrinsic characteristic and in fact it can be tailored by choosing appropriate compositions and applying proper techniques. Combustion synthesis, which is also known as selfpropagating high-temperature synthesis (SHS), has attracted much attention as a low-cost and productive technique to fabricate a wide range of useful materials, including many kinds of ceramic powders [8]. Because of the characteristics of high reaction temperature and fast heating rate, combustion synthesis is advantageous in purifying the phase assemblage and modifying the final grain morphology. For example, both h-Si3N4 and a-SiAlON ceramic powders consisting of rod-like crystals have been successfully fabricated by combustion synthesis [9– 11]. More recently, novel a-SiAlON whiskers with remarkably higher aspect ratios were also reported [12]. If the whiskers can be aligned properly and woven into the matrix ceramics, the toughness is expected to be further improved. This paper reports the combustion synthesis of a- and hSiAlON powders with rod-like crystals or novel whiskers.
3956
G. Liu et al. / Materials Letters 59 (2005) 3955 – 3958
Chemical Co., China) as raw materials. The raw materials were mixed by agate balls in a plastic jar for 24 h with absolute ethanol as medium. The obtained slurry was dried and sieved. Then the starting powder mixture was contained in a porous graphite crucible and placed into the combustion reaction chamber. After evacuation, the reaction chamber was inflated with high-purity N2 at a pressure value of 2 MPa. The combustion reaction was triggered by passing an electric current through a tungsten coil. The reaction temperature was measured by a W-Re3/ W-Re25 thermocouple, which was inserted into the sample center and connected with a computer system for data treatment. The phase assemblage of reaction products was identified by XRD (Cu Ka, Rigaku, Japan). The microstructure was observed by SEM (JSM-6460LV, JEOL, Japan) and EDS (INCA, Oxford Instrument) was also used to perform the element analysis.
3. Results and discussion
Fig. 1. XRD patterns of the combustion products: (a) SN12; (b) SN18; (c) SN22.
The effect of starting composition on the phase assemblage of combustion products and the formation mechanism of whiskers are discussed.
Fig. 1 shows XRD patterns of the combustion products for SN12, SN18, and SN22. It can be seen that with different starting compositions, the final phase assemblages of combustion products are remarkably different. In the sample SN12, single-phase aSiAlON powder has been prepared, while in the sample SN22 the predominant phase is h-SiAlON, and in the sample SN18, the combustion product is a composite of a- and h-SiAlON. The XRD patterns of the samples SN16 and SN22, which are not shown here, are basically similar to those of SN12 and SN22. These results are basically as expected, for with the increase of oxygen content (nvalue) the starting composition point moves from the inner part of a-SiAlON phase area to the multi-phase area. AlN polytypoids, which are usual intermediate phases accompanied with the formation of a-SiAlON, are not found here in all samples. This may be attributed to the fast heating and cooling rates in combustion synthesis and short reaction period compared with general reaction sintering. Ignoring the small amount of residual silicon powder, the relative phase contents of a-SiAlON and h-SiAlON are semiquantitatively calculated on the basis of XRD results, according to
2. Experimental In the general formula of Ym / 3 Si12 (m + n)Alm + n On N16 n , five compositions were investigated, where m = 1.2 was fixed and n varied from 1.2 to 1.6, 1.8, 2.0, and 2.2. According to different n values, the samples were briefly designated as SN12, SN16, SN18, SN20, and SN22, respectively. Starting compositions were fulfilled using Y2O 3 (99.9%, General Research Institute for Nonferrous Metals, China), Si (99.0%, Fushun Al Factory, China), Al (99.5%, Gaizhou Al Co., China), a-Si3N4 (Fangda High-Technology Ceramics Co., China), and SiO2 (A.R., Beijing
Fig. 2. Effect of the starting composition on final phase assemblage of combustion products.
G. Liu et al. / Materials Letters 59 (2005) 3955 – 3958
3957
Fig. 3. Microstructure of the combustion products: (a) SN12, a-SiAlON; (b) SN16, 97% a-SiAlON + 3% h-SiAlON; (c) SN20, 16% a-SiAlON + 84% hSiAlON; (d) SN22, 5% a-SiAlON + 95% h-SiAlON.
the method described in reference [13]. The calculated results are shown in Fig. 2. It is clear that, with the increase of n-value from 1.2 to 2.2, the relative content of a-SiAlON gradually decreases and h-SiAlON increases. This change in phase content shows the important influence of starting composition on final phase assemblage of the combustion products.
Fig. 3 shows the typical micrographs of the combustion products. In the sample SN12, the grain growth is incomplete and no rod-like crystals are found. But in the sample SN16, many rod-like crystals are visible. This difference in grain morphology for the two samples indicates that more liquid phase resulting from high oxygen content is beneficial to the development of rod-like a-
Fig. 4. Whiskers obtained in the combustion product from SN22: (a) general morphology; (b) wool-like whiskers; (c) comb-like morphology, the inset EDS result is for the large rod-like crystal; (d) arrays of straight whiskers.
3958
G. Liu et al. / Materials Letters 59 (2005) 3955 – 3958
Additionally, the new whiskers can nucleate and grow on the basal face of the hexagonal prismatic crystals, forming a whisker array, as shown in Fig. 4 (d). In this nucleation mode, it is noticed that the new-formed straight whiskers have the same orientation with the substrate prismatic crystals, i.e. their length directions are parallel.
4. Conclusion
Fig. 5. Temperature history of the sample SN22 during combustion synthesis.
SiAlON grains. In the samples SN20 and SN22 with h-SiAlON as main phase, most grains develop into the rod-like shape. Except the rod-like crystals, novel h-SiAlON whiskers are also obtained in the sample SN22, as shown in Fig. 4. Both VLS whiskers with the typical round droplet on head and VS whiskers without the droplet are observed, proving that both mechanisms operate here. At the same time, besides the straight whiskers, some wool-like whiskers are also noticed, as shown in Fig. 4 (b). In the combustion synthesis process, once the combustion reaction is triggered, the nitridation reaction of Al and Si will produce a large amount of hear energy, which leads to a dramatic increase in temperature. Fig. 5 shows the temperature history of the sample SN22, from which it can be seen that the temperature increases quickly to higher than 1800 -C in only several seconds. Since the reaction temperature is much higher than the melting points of Al and Si particles and the formation temperature of the ternary oxide melt, the evaporation of the melt phase will occur and bring the reactant elements into the vapor phase. At the same time, some monoxides may be formed and also exist in the vapor. In this case, if a liquid droplet is formed on the surface of a certain asexisted crystal, the reactant molecules or atoms in vapor phase will dissolve into the liquid and create a supersaturated nitrogen-rich liquid phase, from which new h-SiAlON crystals can precipitate. With the continuous precipitation process, the liquid droplet is detached from the substrate surface and h-SiAlON whiskers are formed. During the growth of a VLS-formed h-SiAlON whisker, the liquid droplet may evaporate under proper conditions. At this time, the growth mechanism of the whisker can transform into VS mode and the whisker growth is fulfilled by direct deposition of the reactant molecules or atoms on the growth front. At the same time, the reactant elements can deposit on the surface of as-existed large crystals and form VS whiskers directly without the mediate VLS process. As shown in Fig. 4 (c), new whiskers can nucleate and grow on the side faces of a large rod-like h-SiAlON crystal, forming an interesting comb-like morphology.
By combustion synthesis, Y (a + h)-SiAlON powders consisting of rod-like crystals were prepared. Effect of starting composition on the phase assemblage of combustion products was studied. It was found that with the increase of oxygen content, the phase assemblage of products varied from single-phase a-SiAlON to h-SiAlON gradually. Novel h-SiAlON whiskers with high aspect ratios were also observed in the combustion products. From the whisker morphology, it was suggested that both VLS and VS mechanisms operated here. It was also noticed that the whiskers could nucleate either on the basal face or on the side faces of as-existed prismatic rod-like crystals.
Acknowledgements This work is supported by National Natural Science Foundation of China (Grant No. 50102002), and by the Foundation for Science and Technology of Portugal, Project FCT SAPIENS—Reference: POCTI/CTM/39419/2001.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
K.H. Jack, J. Mater. Sci. 11 (1976) 1135. T. Ekstrom, M. Nygren, J. Am. Ceram. Soc. 75 (1992) 259. A. Rosenflanz, Curr. Opin. Solid State Mater. Sci. 4 (1999) 453. I.W. Chen, A. Rosenflanz, Nature 389 (1997) 701. C.L. Hewett, Y.B. Cheng, B.C. Muddle, J. Am. Ceram. Soc. 81 (1998) 1781. Z.J. Shen, Z. Zhao, H. Peng, M. Nygren, Nature 417 (2002) 266. K.X. Chen, H.B. Jin, M. Oliveira, H.P. Zhou, J.M.F. Ferreira, J. Mater. Res. 16 (2001) 1928. Z.A. Munir, A.T. Umberto, Mater. Sci. Rep. 3 (1989) 277. I.G. Cano, M.A. Rodriguez, Scr. Mater. 50 (2004) 383. G.H. Liu, K.X. Chen, H.P. Zhou, J.M.F. Ferreira, J. Mater. Res. 19 (2004) 3408. R.L. Fu, K.X. Chen, X. Xu, J.M.F. Ferreira, Mater. Lett. 58 (2004) 1956. G.H. Liu, K.X. Chen, H.P. Zhou, X.S. Ning, C. Pereira, J.M.F. Ferreira, J. Mater. Res. 20 (2005) 889. C.P. Gazzara, D.R. Messier, Am. Ceram. Soc. Bull. 56 (1977) 777.