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The influence of the molar ratio of Al2O3 to Y2O3 on sintering behavior and the mechanical properties of a SiC–Al2O3–Y2O3 ceramic composite
Materials Science and Engineering A 486 (2008) 262–266
The influence of the molar ratio of Al2O3 to Y2O3 on sintering behavior and the mechanical properties of a SiC–Al2O3–Y2O3 ceramic composite N. Zhang a,∗ , H.Q. Ru b , Q.K. Cai a , X.D. Sun b a
b
Scientific Research Center, Shenyang University, Shenyang 110044, China School of Materials and Metallurgy, Northeastern University, Shenyang 110004, China
Received 16 May 2007; received in revised form 31 August 2007; accepted 25 September 2007
Abstract The influence of the molar ratio of Al2 O3 to Y2 O3 (i.e. MAl2 O3 /MY2 O3 ) on sintering densification, microstructure and the mechanical properties of a SiC–Al2 O3 –Y2 O3 ceramic composite were studied. It was shown that the optimal value of MAl2 O3 /MY2 O3 was 3/2, not 5/3, which is customarily considered the optimal molar ratio for the formation of YAG (Y3 Al5 O12 ) phase. When MAl2 O3 /MY2 O3 is 5/3, materials existed in two phases of YAG and very little YAM phases. The sintering mechanism of the solid phase occurred at 1850 ◦ C. When MAl2 O3 /MY2 O3 was 3/2, materials existed in the two phases YAG (Y3 Al5 O12 ) and YAM (Y4 Al2 O9 ). The formation of the low melting point eutectic liquid phase (YAG + YAM) increased sintering densification. Flexure strength, hardness and relative density were all higher. © 2007 Elsevier B.V. All rights reserved. Keywords: Ceramic composite; MAl2 O3 /MY2 O3 ; Microstructure; Mechanical properties; Sintering temperature
1. Introduction SiC is very hard, very strong, very durable, resistant to high temperature and chemical attack, and has the potential for extensive application in modern industry. However, pure SiC is a ceramic material [1] and sintering densification requires high temperature and pressure. By using a suitable sintering additive, the sintering temperature can be reduced and a dense ceramic material is produced. Since 1990, some scientists have decreased the sintering temperature by adding Al2 O3 and Y2 O3 to form a YAG liquid phase. Using pressureless sintering technology, they obtained a dense SiC ceramic composite [2–4]. It was generally thought that the optimal molar ratio of Al2 O3 to Y2 O3 (i.e. MAl2 O3 /MY2 O3 ) was 5/3 for formation of the YAG liquid phase by the reaction 5Al2 O3 + 3Y2 O3 → 2Y3 Al5 O12 [5–14]. However, we found that the optimal value of MAl2 O3 /MY2 O3 for sintering densification is not 5/3 but 3/2. As a result, the material entered a two-phase area of YAG and YAM. The formation of the low melting point eutectic liquid phase (YAG + YAM) increased sintering densification. There has been some research into the effects of different proportions of Al2 O3 and Y2 O3 as
∗
Corresponding author. Tel.: +86 24 62266946; fax: +86 24 62266018. E-mail address: [email protected] (N. Zhang).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.09.052
sintering additives [15–17], but the components are all located in the two-phase area of YAG and Al2 O3 in the equilibrium phase diagram of Al2 O3 –Y2 O3 [18]. Because more Al2 O3 volatilized easily, the bury-sintering method was adopted. Under the liquid phase sintering conditions, the quality of the surface finish declined, and machining costs increased. The result of changing the proportions of sintering additives Al2 O3 and Y2 O3 should be investigated in the two-phase area of YAG and YAM in order to avoid the Al2 O3 phase volatilizing. The work described here was based on earlier work with sintering technology at 1850 ◦ C for 30 min [19]. The influence of the molar ratio of Al2 O3 to Y2 O3 on sintering behavior and the mechanical properties of a SiC–Al2 O3 –Y2 O3 ceramic composite were investigated. 2. Experimental procedure The raw materials were SiC powder (average particle size 0.5 m and mass percent 99.8 wt.%), Y2 O3 powder (average particle size 1 m and mass percent 99 wt.%) and Al2 O3 powder (average particle size 1 m and mass percent 99 wt.%). The values of MAl2 O3 /MY2 O3 tested were 3/2, 5/3, 7/4 and 2/1. The quantity of Al2 O3 and Y2 O3 powder accounted for 10 wt.% of the total mass. Weighed amounts of SiC, Al2 O3 and Y2 O3 powders were put into a nylon ball-milling kettle with high-purity Al2 O3 balls and
N. Zhang et al. / Materials Science and Engineering A 486 (2008) 262–266
alcohol, and milled for 48 h. The mass ratio of the Al2 O3 balls to the raw material was 5/1. The milled mixed powder was air-dried at 90 ◦ C, sieved, molded at an isotonic cold pressure of 200 MPa and sintered in a graphite vacuum tube furnace. Argon gas was used as a protective atmosphere. An infrared optical pyrometer was used to measure the sintering temperature; the heating rate was 15 ◦ C/min, with pressureless sintering conditions of 1850 ◦ C for 30 min. A D/max-RB X-ray diffraction instrument was used to analyze the phase composition of the material. A Japanese Shimadzu EPM-810Q scanning electronic microscope was used to analyze the microstructure of the material. A Vickers hardness tester and electronic universal testing machine were used to analyze the hardness and flexure of materials, respectively. 3. Results and discussion 3.1. The influence of MAl2 O3 /MY2 O3 on sintering densification Fig. 1 shows the relationship between MAl2 O3 /MY2 O3 and relative density under sintering conditions of 1850 ◦ C for 30 min. Fig. 1 shows that the influence of the value of MAl2 O3 /MY2 O3 on relative density was very complex. Relative density and densification were highest when MAl2 O3 /MY2 O3 was 3/2, and lowest when MAl2 O3 /MY2 O3 was 5/3. When MAl2 O3 /MY2 O3 ratio increased to 7/4, relative density increased; but began to decrease again when MAl2 O3 /MY2 O3 was raised to 2/1. Fig. 2 shows the complex relationship between MAl2 O3 /MY2 O3 and sintering weight loss at 1850 ◦ C for 30 min. The sintering weight loss was lowest when MAl2 O3 /MY2 O3 was 3/2, and highest when MAl2 O3 /MY2 O3 was 5/3. The sintering weight loss decreased when MAl2 O3 /MY2 O3 was raised from 5/3 to 7/4, but increased when MAl2 O3 /MY2 O3 was raised from 7/4 to 2/1. Fig. 3 shows the equilibrium phase diagram of Al2 O3 –Y2 O3 [18], which included five compounds: Al2 O3 , Y2 O3 , YAG (Y3 Al5 O12 ) with 5/3 of MAl2 O3 /MY2 O3 , YAM (Y4 Al2 O9 ) with 1/2 of MAl2 O3 /MY2 O3 and YAP (YAlO3 ) with 1/1 of MAl2 O3 /MY2 O3 . When MAl2 O3 /MY2 O3 was 3/2, the material was in the two-phase area of YAG and YAM. A low melting point eutectic liquid phase (YAG + YAM) was formed at 1850 ◦ C.
Fig. 1. Relationship between MAl2 O3 /MY2 O3 and relative density.
263
Fig. 2. Relationship between weight loss and MAl2 O3 /MY2 O3 .
According to the lever principle, the material contained lower melting point eutectic liquid phase with 57% of the total sintering additive, which contributed to the glide and rearrangement of grain organization. Under the action of capillary force, sintering densification rose rapidly; as a result, the relative density was very high, and weight loss was low. Fig. 4(a) shows the X-ray diffraction patterns with a MAl2 O3 /MY2 O3 ratio of 3/2, which indicate that the material was composed of three kinds of phases with SiC, YAG and YAM, confirming the experimental result described above. When MAl2 O3 /MY2 O3 was 5/3, the material was in the single phase of YAG (in Fig. 3), where the sintering temperature (1850 ◦ C) was lower than the melting point of YAG (1930 ◦ C). The capillary force prevented sintering densification of the material. The solid sintering additive did not contribute to the glide and rearrangement of grain organization. Therefore, densification was very slow, and the sintering weight loss was high, appearing as diffraction peaks of SiC, YAG and YAM in Fig. 4(b). Sintering densification was slow, and the Al2 O3 phase volatilized easily (Al2 O3 + SiC → Al2 O(g) + SiO(g) + CO
Fig. 3. Equilibrium phase diagram of Al2 O3 –Y2 O3 [18]: (a) 1.5, (b) 1.67 and (c) 1.75.
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Fig. 5. Relationship between flexure strength and hardness with different values of MAl2 O3 /MY2 O3 .
decreased simultaneously. The Al2 O3 diffraction peak did not appear, only the SiC and YAG diffraction peaks were found (Fig. 4(c)).
Fig. 4. X-ray diffraction patterns of the materials with different values of MAl2 O3 /MY2 O3 : (a)1.5, (b) 1.67 and (c) 1.75.
[20,21]). Therefore, the total amount of sintering additive was decreased. The YAM diffraction peak appeared because of the volatilization of Al2 O3 . The material went through the YAM and YAG two-phase area, but the content of the YAM phase was very low. Densification was not easy because the material was seldom in the liquid phase. When MAl2 O3 /MY2 O3 was as high as 7/4 or 2/1, the material was in the YAG and liquid phase two-phase area at 1850 ◦ C (Fig. 3). Sintering densification rose again because of the liquid phase and capillary force. According to the lever principle, with MAl2 O3 /MY2 O3 as high as 7/4 to 2/1, the quantity of Al2 O3 increased from 43% to 47.5% of the total sintering additive. However, the liquid phase began to appear at 1760 ◦ C. Volatilization of Al2 O3 had occurred before sintering densification at 1850 ◦ C. Weight loss rose and the sintering densification
3.2. The influence of MAl2 O3 /MY2 O3 on mechanical properties Fig. 5 shows the influence of the value of MAl2 O3 /MY2 O3 on flexure strength and Vickers hardness of the SiC–Al2 O3 –Y2 O3 ceramic composite material. When MAl2 O3 /MY2 O3 was 3/2, material existed in low melting point eutectic liquid phase area of (YAG + YAM), which contributed to the guild and rearrangement of grain particles. Therefore, material had been compacted rapidly before Al2 O3 phase was volatilized. Weight loss was the lowest, and relative density, flexure strength and Vickers hardness were all highest. When MAl2 O3 /MY2 O3 rose to 5/3, material main was in YAG solid phase area, sintering densification process go along slowly. The Al2 O3 phase had been generated to volatilize before material was in sintering densification. Weight loss was the highest, and relative density, flexure strength and Vickers hardness were the lowest. When MAl2 O3 /MY2 O3 rose to 7/4, weight loss was less but relative density and flexure strength increased. When MAl2 O3 /MY2 O3 rose from 7/4 to 2/1, flexure strength and Vickers hardness
Fig. 6. Microstructure of the materials with different values of MAl2 O3 /MY2 O3 : (a) 1.67 and (b) 1.5.
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Fig. 7. fracture structure of materials with different values of MAl2 O3 /MY2 O3 : (a) 1.5, (b) 1.67, (c) 1.75 and (d) 2.0.
declined gradually, because Al2 O3 volatilized at the high temperature. 3.3. The influence of the molar ratio of Al2 O3 to Y2 O3 on microstructure Fig. 6 shows the SEM microstructure of the SiC–Al2 O3 – Y2 O3 ceramic composite material produced by sintering at 1850 ◦ C for 30 min with different values of MAl2 O3 /MY2 O3 . Fig. 6(a) and (b) shows that pore size and porosity were affected greatly by different values of MAl2 O3 /MY2 O3 . When MAl2 O3 /MY2 O3 was 5/3, sintering densification occurred very slowly, pore size and porosity were very larger because of Al2 O3 phase volatilization, and the microstructure was so loose that it included some micro-crack in (a). Thus, flexure strength, hardness and relative density were all lower. When MAl2 O3 /MY2 O3 was 3/2, sintering densification had occurred rapidly before Al2 O3 phase was volatilized, so that the microstructure was very dense and no micro-crack existed in (b). The porosity and pore size were all lower, and material densification was higher. Thus, flexure strength, hardness and relative density were all higher as well. Fig. 7 shows the fracture texture of the SiC–Al2 O3 –Y2 O3 ceramic composite material after sintering at 1850 ◦ C for 30 min with different values of MAl2 O3 /MY2 O3 . No variation of grain size was visible. All samples had inter-crystalline cracks. The grain structure and fracture pattern were not affected by differences in the value of MAl2 O3 /MY2 O3 .
4. Conclusions The influence of the value of MAl2 O3 /MY2 O3 on the sintering behavior and mechanical properties of a SiC–Al2 O3 –Y2 O3 ceramic composite were investigated. The main results obtained are summarized as follows: (i) When the value of MAl2 O3 /MY2 O3 was 3/2 at 1850 ◦ C for 30 min, material just existed in low melting point eutectic liquid phase area of (YAG + YAM), which contributed to the glide and rearrangement of grain particle. Sintering densification had occurred rapidly before Al2 O3 phase volatilized. The microstructures of materials were so dense that no micro-crack existed. As a result, flexure strength, hardness, and relative density were all highest. (ii) When the value of MAl2 O3 /MY2 O3 was 5/3 at 1850 ◦ C for 30 min, material entered into two-phase area of YAG and very little YAM, with very little liquid phase and no capillary force. Thus, sintering densification occurred slowly. Due to volatilization of Al2 O3 phase, the microstructure was so loose that it included some micro-crack. As a result, flexure strength, hardness and relative density were all lowest. (iii) When the value of MAl2 O3 /MY2 O3 was 7/4 or 2/1, flexure strength, hardness and relative density were between those corresponding to 5/3 and 3/2 of MAl2 O3 /MY2 O3 mentioned above. The optimal molar ratio of Al2 O3 to Y2 O3 was 3/2, not 5/3.
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Acknowledgements This work was supported by the National Natural Science Foundation of China (nos. 50372041 and 50672060) and the Liaoning Province Nature Science Fund (no. 20052002). References [1] P.A. Lessin, A.W. Erickson, D.C. Kunerth, J. Mater. Sci. 36 (2001) 1389–1394. [2] M.A. Mulla, V.D. Krstic, J. Am. Ceram. Soc. 70 (1991) 439–443. [3] L.S. Sigl, H.J. Kleebe, J. Am. Ceram. Soc. 76 (1993) 776–777. [4] S.K. Lee, Y.C. Kim, C.H. Kim, J. Mater. Sci. 29 (1994) 526–532. [5] A.K. Samanta, K.K. Dhargupta, S. Ghatak, Ceram. Int. 27 (2001) 123–133. [6] J.Y. Kim, Y.W. Kim, Commun. Am. Ceram. Soc. 82 (1999) 441–444. [7] H.W. Xu, T. Bhatia, A. Swarnima, J. Am. Ceram. Soc. 84 (2001) 1578–1584. [8] P. Nitin Padture, J. Am. Ceram. Soc. 77 (1994) 519–525.
[9] Z.R. Huang, C. Zhao, S.H. Tan, J. Inorg. Mater. 14 (1999) 726–731. [10] J.W. Wang, H. Yang, X.Z. Guo, J. Refract. Mater. 39 (2005) 192–195. [11] Y.W. Kim, J.Y. Kim, S.H. Rhee, D.Y. Kim, J. Eur. Ceram. Soc. 20 (2000) 945–949. [12] O. Borrero-L´opez, A.L. Ortiz, F. Guiberteau, P. Padture, J. Eur. Ceram. Soc. 27 (2007) 2521–2527. [13] L. Gao, H.Z. Wang, H. Kawaoka, T. Sekino, K. Niihara, J. Eur. Ceram. Soc. 22 (2002) 785–789. [14] L.S. Sigl, J. Eur. Ceram. Soc. 23 (2003) 1115–1122. [15] S.H. Tan, Z.M. Chen, D.L. Jiang, J. Chin. Ceram. Soc. 26 (1998) 192–197. [16] J.H. She, Mater. Chem. Phys. 59 (1999) 139–142. [17] R. Huang, H. Gu, J.X. Zhang, D.L. Jiang, Acta Mater. 53 (2005) 2521– 2529. [18] M. Ernes, H. Levin, F. Mcmuedie, Phase Diagrams for Ceramics [M], American Ceramics Society, Columbus, 1975. [19] N. Zhang, H.Q. RU, Q.K. CAI, J. Northeast Univ. 23 (2002) 667–670. [20] T. Grande, H. Sommerset, E. Hagen, J. Am. Ceram. Soc. 80 (1997) 1047–1052. [21] N. Zhang, H.Q. Ru, X.D. Sun, J. Rare Earth 23 (2005) 132–136.
The influence of the molar ratio of Al2O3 to Y2O3 on sintering behavior and the mechanical properties of a SiC–Al2O3–Y2O3 ceramic composite N. Zhang a,∗ , H.Q. Ru b , Q.K. Cai a , X.D. Sun b a
b
Scientific Research Center, Shenyang University, Shenyang 110044, China School of Materials and Metallurgy, Northeastern University, Shenyang 110004, China
Received 16 May 2007; received in revised form 31 August 2007; accepted 25 September 2007
Abstract The influence of the molar ratio of Al2 O3 to Y2 O3 (i.e. MAl2 O3 /MY2 O3 ) on sintering densification, microstructure and the mechanical properties of a SiC–Al2 O3 –Y2 O3 ceramic composite were studied. It was shown that the optimal value of MAl2 O3 /MY2 O3 was 3/2, not 5/3, which is customarily considered the optimal molar ratio for the formation of YAG (Y3 Al5 O12 ) phase. When MAl2 O3 /MY2 O3 is 5/3, materials existed in two phases of YAG and very little YAM phases. The sintering mechanism of the solid phase occurred at 1850 ◦ C. When MAl2 O3 /MY2 O3 was 3/2, materials existed in the two phases YAG (Y3 Al5 O12 ) and YAM (Y4 Al2 O9 ). The formation of the low melting point eutectic liquid phase (YAG + YAM) increased sintering densification. Flexure strength, hardness and relative density were all higher. © 2007 Elsevier B.V. All rights reserved. Keywords: Ceramic composite; MAl2 O3 /MY2 O3 ; Microstructure; Mechanical properties; Sintering temperature
1. Introduction SiC is very hard, very strong, very durable, resistant to high temperature and chemical attack, and has the potential for extensive application in modern industry. However, pure SiC is a ceramic material [1] and sintering densification requires high temperature and pressure. By using a suitable sintering additive, the sintering temperature can be reduced and a dense ceramic material is produced. Since 1990, some scientists have decreased the sintering temperature by adding Al2 O3 and Y2 O3 to form a YAG liquid phase. Using pressureless sintering technology, they obtained a dense SiC ceramic composite [2–4]. It was generally thought that the optimal molar ratio of Al2 O3 to Y2 O3 (i.e. MAl2 O3 /MY2 O3 ) was 5/3 for formation of the YAG liquid phase by the reaction 5Al2 O3 + 3Y2 O3 → 2Y3 Al5 O12 [5–14]. However, we found that the optimal value of MAl2 O3 /MY2 O3 for sintering densification is not 5/3 but 3/2. As a result, the material entered a two-phase area of YAG and YAM. The formation of the low melting point eutectic liquid phase (YAG + YAM) increased sintering densification. There has been some research into the effects of different proportions of Al2 O3 and Y2 O3 as
∗
Corresponding author. Tel.: +86 24 62266946; fax: +86 24 62266018. E-mail address: [email protected] (N. Zhang).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.09.052
sintering additives [15–17], but the components are all located in the two-phase area of YAG and Al2 O3 in the equilibrium phase diagram of Al2 O3 –Y2 O3 [18]. Because more Al2 O3 volatilized easily, the bury-sintering method was adopted. Under the liquid phase sintering conditions, the quality of the surface finish declined, and machining costs increased. The result of changing the proportions of sintering additives Al2 O3 and Y2 O3 should be investigated in the two-phase area of YAG and YAM in order to avoid the Al2 O3 phase volatilizing. The work described here was based on earlier work with sintering technology at 1850 ◦ C for 30 min [19]. The influence of the molar ratio of Al2 O3 to Y2 O3 on sintering behavior and the mechanical properties of a SiC–Al2 O3 –Y2 O3 ceramic composite were investigated. 2. Experimental procedure The raw materials were SiC powder (average particle size 0.5 m and mass percent 99.8 wt.%), Y2 O3 powder (average particle size 1 m and mass percent 99 wt.%) and Al2 O3 powder (average particle size 1 m and mass percent 99 wt.%). The values of MAl2 O3 /MY2 O3 tested were 3/2, 5/3, 7/4 and 2/1. The quantity of Al2 O3 and Y2 O3 powder accounted for 10 wt.% of the total mass. Weighed amounts of SiC, Al2 O3 and Y2 O3 powders were put into a nylon ball-milling kettle with high-purity Al2 O3 balls and
N. Zhang et al. / Materials Science and Engineering A 486 (2008) 262–266
alcohol, and milled for 48 h. The mass ratio of the Al2 O3 balls to the raw material was 5/1. The milled mixed powder was air-dried at 90 ◦ C, sieved, molded at an isotonic cold pressure of 200 MPa and sintered in a graphite vacuum tube furnace. Argon gas was used as a protective atmosphere. An infrared optical pyrometer was used to measure the sintering temperature; the heating rate was 15 ◦ C/min, with pressureless sintering conditions of 1850 ◦ C for 30 min. A D/max-RB X-ray diffraction instrument was used to analyze the phase composition of the material. A Japanese Shimadzu EPM-810Q scanning electronic microscope was used to analyze the microstructure of the material. A Vickers hardness tester and electronic universal testing machine were used to analyze the hardness and flexure of materials, respectively. 3. Results and discussion 3.1. The influence of MAl2 O3 /MY2 O3 on sintering densification Fig. 1 shows the relationship between MAl2 O3 /MY2 O3 and relative density under sintering conditions of 1850 ◦ C for 30 min. Fig. 1 shows that the influence of the value of MAl2 O3 /MY2 O3 on relative density was very complex. Relative density and densification were highest when MAl2 O3 /MY2 O3 was 3/2, and lowest when MAl2 O3 /MY2 O3 was 5/3. When MAl2 O3 /MY2 O3 ratio increased to 7/4, relative density increased; but began to decrease again when MAl2 O3 /MY2 O3 was raised to 2/1. Fig. 2 shows the complex relationship between MAl2 O3 /MY2 O3 and sintering weight loss at 1850 ◦ C for 30 min. The sintering weight loss was lowest when MAl2 O3 /MY2 O3 was 3/2, and highest when MAl2 O3 /MY2 O3 was 5/3. The sintering weight loss decreased when MAl2 O3 /MY2 O3 was raised from 5/3 to 7/4, but increased when MAl2 O3 /MY2 O3 was raised from 7/4 to 2/1. Fig. 3 shows the equilibrium phase diagram of Al2 O3 –Y2 O3 [18], which included five compounds: Al2 O3 , Y2 O3 , YAG (Y3 Al5 O12 ) with 5/3 of MAl2 O3 /MY2 O3 , YAM (Y4 Al2 O9 ) with 1/2 of MAl2 O3 /MY2 O3 and YAP (YAlO3 ) with 1/1 of MAl2 O3 /MY2 O3 . When MAl2 O3 /MY2 O3 was 3/2, the material was in the two-phase area of YAG and YAM. A low melting point eutectic liquid phase (YAG + YAM) was formed at 1850 ◦ C.
Fig. 1. Relationship between MAl2 O3 /MY2 O3 and relative density.
263
Fig. 2. Relationship between weight loss and MAl2 O3 /MY2 O3 .
According to the lever principle, the material contained lower melting point eutectic liquid phase with 57% of the total sintering additive, which contributed to the glide and rearrangement of grain organization. Under the action of capillary force, sintering densification rose rapidly; as a result, the relative density was very high, and weight loss was low. Fig. 4(a) shows the X-ray diffraction patterns with a MAl2 O3 /MY2 O3 ratio of 3/2, which indicate that the material was composed of three kinds of phases with SiC, YAG and YAM, confirming the experimental result described above. When MAl2 O3 /MY2 O3 was 5/3, the material was in the single phase of YAG (in Fig. 3), where the sintering temperature (1850 ◦ C) was lower than the melting point of YAG (1930 ◦ C). The capillary force prevented sintering densification of the material. The solid sintering additive did not contribute to the glide and rearrangement of grain organization. Therefore, densification was very slow, and the sintering weight loss was high, appearing as diffraction peaks of SiC, YAG and YAM in Fig. 4(b). Sintering densification was slow, and the Al2 O3 phase volatilized easily (Al2 O3 + SiC → Al2 O(g) + SiO(g) + CO
Fig. 3. Equilibrium phase diagram of Al2 O3 –Y2 O3 [18]: (a) 1.5, (b) 1.67 and (c) 1.75.
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Fig. 5. Relationship between flexure strength and hardness with different values of MAl2 O3 /MY2 O3 .
decreased simultaneously. The Al2 O3 diffraction peak did not appear, only the SiC and YAG diffraction peaks were found (Fig. 4(c)).
Fig. 4. X-ray diffraction patterns of the materials with different values of MAl2 O3 /MY2 O3 : (a)1.5, (b) 1.67 and (c) 1.75.
[20,21]). Therefore, the total amount of sintering additive was decreased. The YAM diffraction peak appeared because of the volatilization of Al2 O3 . The material went through the YAM and YAG two-phase area, but the content of the YAM phase was very low. Densification was not easy because the material was seldom in the liquid phase. When MAl2 O3 /MY2 O3 was as high as 7/4 or 2/1, the material was in the YAG and liquid phase two-phase area at 1850 ◦ C (Fig. 3). Sintering densification rose again because of the liquid phase and capillary force. According to the lever principle, with MAl2 O3 /MY2 O3 as high as 7/4 to 2/1, the quantity of Al2 O3 increased from 43% to 47.5% of the total sintering additive. However, the liquid phase began to appear at 1760 ◦ C. Volatilization of Al2 O3 had occurred before sintering densification at 1850 ◦ C. Weight loss rose and the sintering densification
3.2. The influence of MAl2 O3 /MY2 O3 on mechanical properties Fig. 5 shows the influence of the value of MAl2 O3 /MY2 O3 on flexure strength and Vickers hardness of the SiC–Al2 O3 –Y2 O3 ceramic composite material. When MAl2 O3 /MY2 O3 was 3/2, material existed in low melting point eutectic liquid phase area of (YAG + YAM), which contributed to the guild and rearrangement of grain particles. Therefore, material had been compacted rapidly before Al2 O3 phase was volatilized. Weight loss was the lowest, and relative density, flexure strength and Vickers hardness were all highest. When MAl2 O3 /MY2 O3 rose to 5/3, material main was in YAG solid phase area, sintering densification process go along slowly. The Al2 O3 phase had been generated to volatilize before material was in sintering densification. Weight loss was the highest, and relative density, flexure strength and Vickers hardness were the lowest. When MAl2 O3 /MY2 O3 rose to 7/4, weight loss was less but relative density and flexure strength increased. When MAl2 O3 /MY2 O3 rose from 7/4 to 2/1, flexure strength and Vickers hardness
Fig. 6. Microstructure of the materials with different values of MAl2 O3 /MY2 O3 : (a) 1.67 and (b) 1.5.
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Fig. 7. fracture structure of materials with different values of MAl2 O3 /MY2 O3 : (a) 1.5, (b) 1.67, (c) 1.75 and (d) 2.0.
declined gradually, because Al2 O3 volatilized at the high temperature. 3.3. The influence of the molar ratio of Al2 O3 to Y2 O3 on microstructure Fig. 6 shows the SEM microstructure of the SiC–Al2 O3 – Y2 O3 ceramic composite material produced by sintering at 1850 ◦ C for 30 min with different values of MAl2 O3 /MY2 O3 . Fig. 6(a) and (b) shows that pore size and porosity were affected greatly by different values of MAl2 O3 /MY2 O3 . When MAl2 O3 /MY2 O3 was 5/3, sintering densification occurred very slowly, pore size and porosity were very larger because of Al2 O3 phase volatilization, and the microstructure was so loose that it included some micro-crack in (a). Thus, flexure strength, hardness and relative density were all lower. When MAl2 O3 /MY2 O3 was 3/2, sintering densification had occurred rapidly before Al2 O3 phase was volatilized, so that the microstructure was very dense and no micro-crack existed in (b). The porosity and pore size were all lower, and material densification was higher. Thus, flexure strength, hardness and relative density were all higher as well. Fig. 7 shows the fracture texture of the SiC–Al2 O3 –Y2 O3 ceramic composite material after sintering at 1850 ◦ C for 30 min with different values of MAl2 O3 /MY2 O3 . No variation of grain size was visible. All samples had inter-crystalline cracks. The grain structure and fracture pattern were not affected by differences in the value of MAl2 O3 /MY2 O3 .
4. Conclusions The influence of the value of MAl2 O3 /MY2 O3 on the sintering behavior and mechanical properties of a SiC–Al2 O3 –Y2 O3 ceramic composite were investigated. The main results obtained are summarized as follows: (i) When the value of MAl2 O3 /MY2 O3 was 3/2 at 1850 ◦ C for 30 min, material just existed in low melting point eutectic liquid phase area of (YAG + YAM), which contributed to the glide and rearrangement of grain particle. Sintering densification had occurred rapidly before Al2 O3 phase volatilized. The microstructures of materials were so dense that no micro-crack existed. As a result, flexure strength, hardness, and relative density were all highest. (ii) When the value of MAl2 O3 /MY2 O3 was 5/3 at 1850 ◦ C for 30 min, material entered into two-phase area of YAG and very little YAM, with very little liquid phase and no capillary force. Thus, sintering densification occurred slowly. Due to volatilization of Al2 O3 phase, the microstructure was so loose that it included some micro-crack. As a result, flexure strength, hardness and relative density were all lowest. (iii) When the value of MAl2 O3 /MY2 O3 was 7/4 or 2/1, flexure strength, hardness and relative density were between those corresponding to 5/3 and 3/2 of MAl2 O3 /MY2 O3 mentioned above. The optimal molar ratio of Al2 O3 to Y2 O3 was 3/2, not 5/3.
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