- Email: [email protected]
Vitrification and devitrification of MgO during sintering of Si3N4–MgO–CeO2 ceramics
Materials Chemistry and Physics 57 (1998) 178±181
Materials Science Communication
Vitri®cation and devitri®cation of MgO during sintering of Si3N4±MgO±CeO2 ceramics a b
Yang Haitaoa,*, Yang Guotaob, Yuan Runzhanga State Key Laboratory for Hybrid Materials, Wuhan University of Technology, Wuhan 430070, China Department of Materials Science, Central South University of Technology, Changsha 410083, China Received 13 March 1998; received in revised form 29 June 1998; accepted 30 June 1998
Abstract This paper deals with the vitri®cation and devitri®cation of MgO during sintering of Si3N4±MgO±CeO2 ceramics. At the initial stage of sintering (1450±15508C), MgO±CeO2 reacted with free silica to form suf®cient liquid phase to densify Si3N4 into high strength ceramics; at ®nal stage (1600±18008C), the crystallization of MgO reduced the amount of glassy phase which was harmful to the high-temperature properties. MgO±CeO2 is an excellent sintering aid for Si3N4. # 1998 Elsevier Science S.A. All rights reserved. Keywords: Vitri®cation; Devetri®cation; Silicon nitride
1. Introduction
2. Experimental procedure
Silicon nitride ceramics shows great potential for wide applications. Sintering is a cost-effective way to produce silicon nitride ceramics. However, it is dif®cult to densify silicon nitride due to its covalent nature of bonding. Metal oxides such as MgO [1,2], Al2O3 [3,4], and rare-earth oxides [5±7] have been found to be effective sintering aids for Si3N4. Silicon nitride is also limited in its high-temperature properties due to glassy phases formed at the grain boundaries as a result of processing with the sintering aids. Two main ways have been studied to address this problem: (1) the crystallization of the glassy phase by postsintering heat treatment [8]; (2) the formation of a solid solution, i.e. sialon [9]. However, the stresses resulting from the postsintering heat treatment process always decreases the roomtemperature as well as the high-temperature strength and monophase Si3N4 solid solution ceramic materials exhibit a low fracture toughness. This paper discusses the vitri®cation and devetri®cation of MgO during the sintering of Si3N4± MgO±CeO2 ceramics. The present work shows that MgO± CeO2 is an excellent sintering aid, which leads to high strength and a less glassy phase for the sintered Si3N4± MgO±CeO2 ceramics.
The selected (Si3N4 1 5 wt.% MgO 2 5 wt.% CeO2 3) composition was mixed and milled in anhydrous alcohol for 24 h with WC±6% Co cemented carbide medium. The powder mixtures were dry-pressed into bars in a steel die at 120 Mpa. The compacts were embedded in a Si3N4 50 wt.% BN composite powder bed in a molybdenum crucible and pressureless sintered in N2 atmosphere. Phase identi®cation was made by X-ray diffraction using Cu Ka radiation. The microstructures were observed by H800 TEM. 3. Results and discussion Fig. 1 is the XRD pattern of the green compact of Si3N4 contained 5 wt.% MgO 5 wt.% CeO2. Before sintering, a-Si3N4, b-Si3N4, MgO, CeO2, WC existed in the green compact. About 4 wt.% WC was introduced by milling process. Fig. 2 shows the XRD pattern of the sintered body heated at 18008C for 60 min. Most of the a-Si3N4 has transformed into b-Si3N4, CeO2 has disappeared, but MgO still remains. 1
*Corresponding author. Fax: +86-27-878-79-468; E-mail: sklwut@ public.wh.hb.cn
Purchased from Zhuzhou Cemented Carbide Works. Purchased from Tianjing Chemicals. Tianjing 130020 China. 3 Purchased from Hunan Rare Earth Institute, Changsha 41000 China. 2
0254-0584/98/$ ± see front matter # 1998 Elsevier Science S.A. All rights reserved. PII: S0254-0584(98)00196-5
Y. Haitao et al. / Materials Chemistry and Physics 57 (1998) 178±181
179
Fig. 1. XRD pattern of the Si3N4 5%MgO 5%CeO2 green compact (*) a-Si3N4, (~) b-Si3N4, (*) MgO, (~) CeO2, (&) WC.
Fig. 3. XRD pattern of the Si3N4 5% MgO 5% CeO2 ceramics sintered at 14508C for 60 min (*) a-Si3N4, (~) b-Si3N4, (*) MgO, (~) CeO2, (&) WC.
Fig. 2. XRD pattern of the Si3N4 5% MgO 5% CeO2 ceramics sintered at 18008C for 60 min (*) a-Si3N4, (~) b-Si3N4, (*) MgO, (~) CeO2, (&) WC.
Fig. 4. XRD pattern of the Si3N4 5% MgO 5% CeO2 ceramics sintered at 15008C for 60 min (*) a-Si3N4, (~) b-Si3N4, (*) MgO, (~) CeO2, (&) WC.
According to MgO±SiO2 phase diagram, there is an eutectic point at 15438C. So it seems impossible that MgO did not react with SiO2 at 18008C. In order to solve this mystery, we designed a series of experiments. The sintering temperatures were 14508C, 15008C, 15508C, and 16008C, respectively. Fig. 3 is the XRD pattern for the specimen sintered at 14508C for 60 min. a-Si3N4, b-Si3N4, WC were found only, and neither MgO nor CeO2 was found. Fig. 4 is the XRD pattern for the specimen sintered at 15008C for 60 min. It is similar to Fig. 3. Both MgO and CeO2 have disappeared. The XRD results for the specimens sintered at 1450± 15008C suggest that both MgO and CeO2 have reacted with
free silica at 1450±15008C to form a liquid phase and is then transformed into a glassy phase. This explains why they cannot be identi®ed by X-ray diffraction. In fact, the volume shrinkage for the specimen sintered at 14508C was about 36%, this indicated that a lot of liquid formation occurred [10]. Fig. 5 is the XRD pattern for the specimen sintered at 15508C. CeO2 is absent. But MgO, which was absent formerly was found again. The XRD pattern for the specimen sintered at 16008C is similar to that at 15508C (Fig. 6). There were no traces of CeO2, but MgO was found. The ®nal conclusion could be made according to the above analysis.
180
Y. Haitao et al. / Materials Chemistry and Physics 57 (1998) 178±181
Fig. 5. XRD pattern of the Si3N4 5% MgO 5% CeO2 ceramics sintered at 15508C for 60 min (*) a-Si3N4, (~) b-Si3N4, (*) MgO, (~) CeO2, (&) WC.
Fig. 6. XRD pattern of the Si3N4 5% MgO 5% CeO2 ceramics sintered at 16008C for 60 min (*) a-Si3N4, (~) b-Si3N4, (*) MgO, (~) CeO2, (&) WC.
For the Si3N4±MgO±CeO2 system, MgO±CeO2 would react with free silica to form a liquid phase at a sintering temperature of 1450±15008C and is then transformed into a glassy phase after cooling down; above 15508C, MgO would crystallize during sintering. CeO2 was in a glassy phase which contained hardly any MgO. This particular phenomenon is of great value for the sintering of Si3N4. At the initial stage of sintering (1450± 15508C), MgO±CeO2 reacted with free silica to form suf®cient liquid phase to densify Si3N4 into high strength ceramics (the pressureless sintered Si3N4±MgO±CeO2 ceramics in the present study achieved a relative density of 98.5% and a strength of 950 Mpa) [10]; at ®nal stage (1600± 18008C), the crystallization of MgO reduced the amount of
Fig. 7. Typical microstructure of the Si3N4 5% MgO 5% CeO2 ceramics sintered at 18008C for 60 min (*) a-Si3N4, (~) b-Si3N4, (*) MgO, (~) CeO2, (&) WC.
glassy phase which was harmful to the high-temperature properties. Thus, MgO±CeO2 can be considered as an excellent sintering aid for Si3N4. In fact, M.J. Hoffmann has found the crystallization of the additives could already have occurred during sintering, especially for compositions directly on a tie line between a secondary phase and Si3N4. In this case, the volume fraction of the liquid phase is strongly reduced and complete densi®cation is impossible [11]. Fig. 7 is a typical microstructure of the silicon nitride sintered with 5% MgO 5% CeO2 at 18008C for 60 min. The glassy phase remained between the matrix phase at triple points and even along the Si3N4 interfaces. The glassy phase can be con®rmed directly by its electron diffraction which appears to be an ambiguous circle due to the absence of the Bragg diffraction. The result of the EDAX analysis of the glassy phase (Fig. 8) shows that Si, Ce are rich in the
Fig. 8. EDAX analysis on glassy phase in Si3N4 5% MgO 5% CeO2 ceramics sintered at 18008C for 60 min.
Y. Haitao et al. / Materials Chemistry and Physics 57 (1998) 178±181
181
4. Conclusions (a) For the Si3N4±MgO±CeO2 system, the MgO±CeO2 would react with free silica to form liquid phase at the sintering temperature of 1450±15508C and then transformed into a glassy phase after cooling down; above 15508C, MgO would crystallize during the sintering. Only CeO2 was found in the glassy phase which contained hardly any MgO. (b) MgO±CeO2 is an excellent sintering aid for Si3N4. At the initial stage of sintering (1450±15508C), MgO±CeO2 reacted with free silica to form suf®cient liquid phase to densify Si3N4 into high strength ceramics; at ®nal stage (1600±18008C), the crystallization of MgO reduced the glassy phase which was harmful to the high-temperature properties. Fig. 9. Crystallized MgO particles dispersed in Si3N4 grains.
Acknowledgements This work was supported by China National postdoctoral Foundation. References
Fig. 10. EDAX analysis on MgO particles dispersed in Si3N4 grains.
glassy phase but Mg is very poor (Mg: 0.64 at.%, Si: 65.34 at.%, Ce: 9.23 at.%, Cu: 21.31 at.% W: 3.48 at.%, Cu was introduced by the specimen holder). This reveals that after sintering at 18008C, the main composition of the glassy phase in the sintered Si3N4 MgO CeO2 ceramics was cerium silicate and contained hardly any MgO. The result is consistent with the X-ray diffraction patterns. We have not observed large crystallized MgO in TEM. We only observed some very small particles dispersed in Si3N4 grains (Fig. 9). These particles are too small to be identi®ed by its electron diffraction. EDAX shows these small particles probably to be the crystallized MgO (Fig. 10) (Mg: 3.84 at.%, Si: 73.58 at.%, Ce; 1.32 at.%, Cu: 14.13 at.%, W: 6.38 at.%).
[1] G. Ziegler, J. Heinrich, G. Wotting, Relationships between processing, microstructure and properties of dense and raction-bonded silicon nitride, J. Mater. Sci. 22 (1987) 3041±3086. [2] A.J. Pyzik, D.F. Carroll, Technology of self-reinforced silicon nitride, Ann. Rev. Mater. Sci. 24 (1994) 189±212. [3] Y. Goto, G. Thomas, Phase transformation and microstructural changes of Si3N4 during sintering, J. Mater. Sci. 30 (1995) 2194± 2200. [4] S.Y. Yoon, T. Akatsu, E. Uasuda, The microstructural and creep deformation of hot-pressed Si3N4 with different amounts of sintering aids, J. Mater. Res. 11 (1996) 120±126. [5] J.T. Smith, C.L. Quakenbush, Phase effects in Si3N4 containing Y2O3 and CeO2 : I strength, Am. Ceram. Soc. Bull. 59 (1980) 529±532. [6] C.M. Wang, X. Pan, M.J. Hoffmann, Grain boundary films in rareearth-glass-based silicon nitride, J. Am. Ceram. Soc. 79 (1996) 788± 792. [7] W.A. Sanders, D.M. Mieskowski, Strength and microstructures of sintered Si3N4 with rare-earth oxide additions, J. Am. Ceram. Soc. 64 (1985) 304±309. [8] M.K. Cinibulk, G. Thomas, S.M. Johnson, Grain-boundary-phase crystallization and strength of silicon nitride sintered with a Y Si Al O N glass, J. Am. Ceram. Soc. 73 (1990) 1606±1612. [9] M.B. Trigy, K.H. Jack, Solubility of aluminum in silicon oxynitride, J. Mater. Soc. Lett. 6 (1987) 407±408. [10] H.T. Yang, G.T. Yang, R.Z. Yuan, The role of MgO±CeO2 in densification of Si3N4, J. Wuhan University Technol. 11 (1996) 1±7. [11] M.J. Hoffmann, High-temperature properties of Si3N4 ceramics, MRS Bull. 2 (1995) 28±32.
Materials Science Communication
Vitri®cation and devitri®cation of MgO during sintering of Si3N4±MgO±CeO2 ceramics a b
Yang Haitaoa,*, Yang Guotaob, Yuan Runzhanga State Key Laboratory for Hybrid Materials, Wuhan University of Technology, Wuhan 430070, China Department of Materials Science, Central South University of Technology, Changsha 410083, China Received 13 March 1998; received in revised form 29 June 1998; accepted 30 June 1998
Abstract This paper deals with the vitri®cation and devitri®cation of MgO during sintering of Si3N4±MgO±CeO2 ceramics. At the initial stage of sintering (1450±15508C), MgO±CeO2 reacted with free silica to form suf®cient liquid phase to densify Si3N4 into high strength ceramics; at ®nal stage (1600±18008C), the crystallization of MgO reduced the amount of glassy phase which was harmful to the high-temperature properties. MgO±CeO2 is an excellent sintering aid for Si3N4. # 1998 Elsevier Science S.A. All rights reserved. Keywords: Vitri®cation; Devetri®cation; Silicon nitride
1. Introduction
2. Experimental procedure
Silicon nitride ceramics shows great potential for wide applications. Sintering is a cost-effective way to produce silicon nitride ceramics. However, it is dif®cult to densify silicon nitride due to its covalent nature of bonding. Metal oxides such as MgO [1,2], Al2O3 [3,4], and rare-earth oxides [5±7] have been found to be effective sintering aids for Si3N4. Silicon nitride is also limited in its high-temperature properties due to glassy phases formed at the grain boundaries as a result of processing with the sintering aids. Two main ways have been studied to address this problem: (1) the crystallization of the glassy phase by postsintering heat treatment [8]; (2) the formation of a solid solution, i.e. sialon [9]. However, the stresses resulting from the postsintering heat treatment process always decreases the roomtemperature as well as the high-temperature strength and monophase Si3N4 solid solution ceramic materials exhibit a low fracture toughness. This paper discusses the vitri®cation and devetri®cation of MgO during the sintering of Si3N4± MgO±CeO2 ceramics. The present work shows that MgO± CeO2 is an excellent sintering aid, which leads to high strength and a less glassy phase for the sintered Si3N4± MgO±CeO2 ceramics.
The selected (Si3N4 1 5 wt.% MgO 2 5 wt.% CeO2 3) composition was mixed and milled in anhydrous alcohol for 24 h with WC±6% Co cemented carbide medium. The powder mixtures were dry-pressed into bars in a steel die at 120 Mpa. The compacts were embedded in a Si3N4 50 wt.% BN composite powder bed in a molybdenum crucible and pressureless sintered in N2 atmosphere. Phase identi®cation was made by X-ray diffraction using Cu Ka radiation. The microstructures were observed by H800 TEM. 3. Results and discussion Fig. 1 is the XRD pattern of the green compact of Si3N4 contained 5 wt.% MgO 5 wt.% CeO2. Before sintering, a-Si3N4, b-Si3N4, MgO, CeO2, WC existed in the green compact. About 4 wt.% WC was introduced by milling process. Fig. 2 shows the XRD pattern of the sintered body heated at 18008C for 60 min. Most of the a-Si3N4 has transformed into b-Si3N4, CeO2 has disappeared, but MgO still remains. 1
*Corresponding author. Fax: +86-27-878-79-468; E-mail: sklwut@ public.wh.hb.cn
Purchased from Zhuzhou Cemented Carbide Works. Purchased from Tianjing Chemicals. Tianjing 130020 China. 3 Purchased from Hunan Rare Earth Institute, Changsha 41000 China. 2
0254-0584/98/$ ± see front matter # 1998 Elsevier Science S.A. All rights reserved. PII: S0254-0584(98)00196-5
Y. Haitao et al. / Materials Chemistry and Physics 57 (1998) 178±181
179
Fig. 1. XRD pattern of the Si3N4 5%MgO 5%CeO2 green compact (*) a-Si3N4, (~) b-Si3N4, (*) MgO, (~) CeO2, (&) WC.
Fig. 3. XRD pattern of the Si3N4 5% MgO 5% CeO2 ceramics sintered at 14508C for 60 min (*) a-Si3N4, (~) b-Si3N4, (*) MgO, (~) CeO2, (&) WC.
Fig. 2. XRD pattern of the Si3N4 5% MgO 5% CeO2 ceramics sintered at 18008C for 60 min (*) a-Si3N4, (~) b-Si3N4, (*) MgO, (~) CeO2, (&) WC.
Fig. 4. XRD pattern of the Si3N4 5% MgO 5% CeO2 ceramics sintered at 15008C for 60 min (*) a-Si3N4, (~) b-Si3N4, (*) MgO, (~) CeO2, (&) WC.
According to MgO±SiO2 phase diagram, there is an eutectic point at 15438C. So it seems impossible that MgO did not react with SiO2 at 18008C. In order to solve this mystery, we designed a series of experiments. The sintering temperatures were 14508C, 15008C, 15508C, and 16008C, respectively. Fig. 3 is the XRD pattern for the specimen sintered at 14508C for 60 min. a-Si3N4, b-Si3N4, WC were found only, and neither MgO nor CeO2 was found. Fig. 4 is the XRD pattern for the specimen sintered at 15008C for 60 min. It is similar to Fig. 3. Both MgO and CeO2 have disappeared. The XRD results for the specimens sintered at 1450± 15008C suggest that both MgO and CeO2 have reacted with
free silica at 1450±15008C to form a liquid phase and is then transformed into a glassy phase. This explains why they cannot be identi®ed by X-ray diffraction. In fact, the volume shrinkage for the specimen sintered at 14508C was about 36%, this indicated that a lot of liquid formation occurred [10]. Fig. 5 is the XRD pattern for the specimen sintered at 15508C. CeO2 is absent. But MgO, which was absent formerly was found again. The XRD pattern for the specimen sintered at 16008C is similar to that at 15508C (Fig. 6). There were no traces of CeO2, but MgO was found. The ®nal conclusion could be made according to the above analysis.
180
Y. Haitao et al. / Materials Chemistry and Physics 57 (1998) 178±181
Fig. 5. XRD pattern of the Si3N4 5% MgO 5% CeO2 ceramics sintered at 15508C for 60 min (*) a-Si3N4, (~) b-Si3N4, (*) MgO, (~) CeO2, (&) WC.
Fig. 6. XRD pattern of the Si3N4 5% MgO 5% CeO2 ceramics sintered at 16008C for 60 min (*) a-Si3N4, (~) b-Si3N4, (*) MgO, (~) CeO2, (&) WC.
For the Si3N4±MgO±CeO2 system, MgO±CeO2 would react with free silica to form a liquid phase at a sintering temperature of 1450±15008C and is then transformed into a glassy phase after cooling down; above 15508C, MgO would crystallize during sintering. CeO2 was in a glassy phase which contained hardly any MgO. This particular phenomenon is of great value for the sintering of Si3N4. At the initial stage of sintering (1450± 15508C), MgO±CeO2 reacted with free silica to form suf®cient liquid phase to densify Si3N4 into high strength ceramics (the pressureless sintered Si3N4±MgO±CeO2 ceramics in the present study achieved a relative density of 98.5% and a strength of 950 Mpa) [10]; at ®nal stage (1600± 18008C), the crystallization of MgO reduced the amount of
Fig. 7. Typical microstructure of the Si3N4 5% MgO 5% CeO2 ceramics sintered at 18008C for 60 min (*) a-Si3N4, (~) b-Si3N4, (*) MgO, (~) CeO2, (&) WC.
glassy phase which was harmful to the high-temperature properties. Thus, MgO±CeO2 can be considered as an excellent sintering aid for Si3N4. In fact, M.J. Hoffmann has found the crystallization of the additives could already have occurred during sintering, especially for compositions directly on a tie line between a secondary phase and Si3N4. In this case, the volume fraction of the liquid phase is strongly reduced and complete densi®cation is impossible [11]. Fig. 7 is a typical microstructure of the silicon nitride sintered with 5% MgO 5% CeO2 at 18008C for 60 min. The glassy phase remained between the matrix phase at triple points and even along the Si3N4 interfaces. The glassy phase can be con®rmed directly by its electron diffraction which appears to be an ambiguous circle due to the absence of the Bragg diffraction. The result of the EDAX analysis of the glassy phase (Fig. 8) shows that Si, Ce are rich in the
Fig. 8. EDAX analysis on glassy phase in Si3N4 5% MgO 5% CeO2 ceramics sintered at 18008C for 60 min.
Y. Haitao et al. / Materials Chemistry and Physics 57 (1998) 178±181
181
4. Conclusions (a) For the Si3N4±MgO±CeO2 system, the MgO±CeO2 would react with free silica to form liquid phase at the sintering temperature of 1450±15508C and then transformed into a glassy phase after cooling down; above 15508C, MgO would crystallize during the sintering. Only CeO2 was found in the glassy phase which contained hardly any MgO. (b) MgO±CeO2 is an excellent sintering aid for Si3N4. At the initial stage of sintering (1450±15508C), MgO±CeO2 reacted with free silica to form suf®cient liquid phase to densify Si3N4 into high strength ceramics; at ®nal stage (1600±18008C), the crystallization of MgO reduced the glassy phase which was harmful to the high-temperature properties. Fig. 9. Crystallized MgO particles dispersed in Si3N4 grains.
Acknowledgements This work was supported by China National postdoctoral Foundation. References
Fig. 10. EDAX analysis on MgO particles dispersed in Si3N4 grains.
glassy phase but Mg is very poor (Mg: 0.64 at.%, Si: 65.34 at.%, Ce: 9.23 at.%, Cu: 21.31 at.% W: 3.48 at.%, Cu was introduced by the specimen holder). This reveals that after sintering at 18008C, the main composition of the glassy phase in the sintered Si3N4 MgO CeO2 ceramics was cerium silicate and contained hardly any MgO. The result is consistent with the X-ray diffraction patterns. We have not observed large crystallized MgO in TEM. We only observed some very small particles dispersed in Si3N4 grains (Fig. 9). These particles are too small to be identi®ed by its electron diffraction. EDAX shows these small particles probably to be the crystallized MgO (Fig. 10) (Mg: 3.84 at.%, Si: 73.58 at.%, Ce; 1.32 at.%, Cu: 14.13 at.%, W: 6.38 at.%).
[1] G. Ziegler, J. Heinrich, G. Wotting, Relationships between processing, microstructure and properties of dense and raction-bonded silicon nitride, J. Mater. Sci. 22 (1987) 3041±3086. [2] A.J. Pyzik, D.F. Carroll, Technology of self-reinforced silicon nitride, Ann. Rev. Mater. Sci. 24 (1994) 189±212. [3] Y. Goto, G. Thomas, Phase transformation and microstructural changes of Si3N4 during sintering, J. Mater. Sci. 30 (1995) 2194± 2200. [4] S.Y. Yoon, T. Akatsu, E. Uasuda, The microstructural and creep deformation of hot-pressed Si3N4 with different amounts of sintering aids, J. Mater. Res. 11 (1996) 120±126. [5] J.T. Smith, C.L. Quakenbush, Phase effects in Si3N4 containing Y2O3 and CeO2 : I strength, Am. Ceram. Soc. Bull. 59 (1980) 529±532. [6] C.M. Wang, X. Pan, M.J. Hoffmann, Grain boundary films in rareearth-glass-based silicon nitride, J. Am. Ceram. Soc. 79 (1996) 788± 792. [7] W.A. Sanders, D.M. Mieskowski, Strength and microstructures of sintered Si3N4 with rare-earth oxide additions, J. Am. Ceram. Soc. 64 (1985) 304±309. [8] M.K. Cinibulk, G. Thomas, S.M. Johnson, Grain-boundary-phase crystallization and strength of silicon nitride sintered with a Y Si Al O N glass, J. Am. Ceram. Soc. 73 (1990) 1606±1612. [9] M.B. Trigy, K.H. Jack, Solubility of aluminum in silicon oxynitride, J. Mater. Soc. Lett. 6 (1987) 407±408. [10] H.T. Yang, G.T. Yang, R.Z. Yuan, The role of MgO±CeO2 in densification of Si3N4, J. Wuhan University Technol. 11 (1996) 1±7. [11] M.J. Hoffmann, High-temperature properties of Si3N4 ceramics, MRS Bull. 2 (1995) 28±32.