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Alumina-alumina and mullite-mullite joining by reaction sintering process
Scripta Metallurgicae t Materialia, Vol. 31, No. 8, pp. 1031-1036, 1994 Copyright©1994 ElsevierScienceLtd Printed in the USA. All rightsreserved 0956-716X/94 $6.00 + 00
Pergamon
CONFERENCESETNo. 2 A L U M I N A - A L U M I N A A N D MULLITE-MULLITE J O I N I N G BY REACTION SINTERING PROCESS R. Torrecillas CSIC- Instituto Tecnol6gico de Materiales de Asturias Parque Tecnol6gico de Asturias 33429 Llanera Spain M.A. Sainz and J.S.Moya Instituto de Ceramica y Vidrio CSIC Arganda del Rey 28500 Madrid Spain
(Received May 6, 1994) (Revised May 26, 1994) Introduction
Joining of s i m p l e ceramic s h a p e s to form complex ceramic parts can be considered as an alternative route to the difficult and expensive methods associated with fabrication of near-netshape ceramic components (1). The ability to reliably joint ceramic parts to form large systems is considered n o w a d a y s as a key technology in enhancing the use of ceramics in high temperature structural applications and in dirty environments (2 , 3). H o w e v e r , as has been pointed out elsewhere (4), the joining of ceramics is not yet a very high developed art. Most scientific and engineering efforts have been devoted to ceramic to metal joining (5). Alumina a n d mullite are two oxides ceramics largely considered for functional as well as structural applications (6 , 7). The p u r p o s e of the present investigation is to join aluminaalumina and mullite-mullite ceramics through reaction sintering with zircon. Materials and methods
a) Dense mullite blocks of =97% th. density with a total impurity content <0.3% were obtained by isostatic pressing at 200 MPa and firing at 1650~C-4h, starting from a commercial alumino-silicate gel (Siral 28M, Condea, H a m b u r g , Germany). b) Alumina blocks of 98% th. density were obtained by isostatic pressing at 200 MPa and firing at 1650°C-2h, starting from a commercial spray-dryed powder (VAW NM 9922, Germany). c) H i g h p u r i t y a l u m i n a with density =100% th. (<30 p p m total impurities, courtesy of M. Kitiyama UC-Berkeley) was used for the alumina-zircon interfacial reaction. d) T w o types of zircon p o w d e r s with different impurities content were used: (i) a commercial p o w d e r SZ1 (Quiminsa, Castell6n, Spain), with 1.2 ~m average particle size and with the following i m p u r i t y content (in wt%): A1203, 0.48; HfO2, 0.79; Fe203, 0.11; TiO2, 0.12; Na20, 0.11; K20, 0.01; CaO, 0.03 and MgO, 0.09, and (ii) a synthetic powder SZ2 (courtesy of M. Ocafia, Institute of Materials Science, CSIC, Madrid, Spain), with the following impurity content (in 1031
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REACTION SINTERING
Vol. 31, No. 8
wt%): A1203, 0.25; HfO2, 0.99; Fe203, 0.013; TiO2, 0.020; Na20, 0.024; K20, 0.008; CaO, 0.008 and MgO, 0.009; and 1.5 p.m average particle size. Zircon containing inks were obtained by mixing ethyleneglycol (30 and 60wt%) and zircon powder (SZ1, 40-70 wt%) with 1% defloculant in a agate mortar. Mullite and alumina blocks were polished on one face d o w n to 1 pm. The polished surfaces of the different mullite and alumina plates were screen printed and subsequently, symmetric plates with polished surfaces were located on the top. These sandwiches were dried at 60QC for 24 h and then fired at 16001680~C for 1.5 h. After firing, the specimens cross-section were polished, thermally etched and studied by optical microscopy (RLOM) and scanning electron microscopy (SEM). Bars of 3x4x40 m m size perpendicular to the joining interface were sawn from these blocks for mechanical tests. The tensile surface was polished to 6 p.m. Creep tests were carried out in air on 4-point bending system (36 mm outerspan and 18 mm inner span) on 3x4x40 mm bars. The creep strain was calculated from the deflection using the Hollenberg method (8). Results and discussion
Mullite-Mullite Joining Zircon and mullite are stable and solid state compatible materials. At temperatures higher than the peritectic temperature of the subsystem zircon-zirconia mullite (~1600°C), zirconia plus a liquid phase is formed. This liquid phase is not en equilibrium with mullite; therefore mullite starts to dissolve into the glassy phase moving the composition of this glassy phase along the corresponding isotherm, until the intersection between the isotherm and the mullite zirconia primary field boundary is reached (Fig. 1A).
AI203
~-~ A3S2
is~s' SiO2
FIG. I. (A) SiO2-AI203-ZrO2 phase equilibrium diagram (9) and (B) Micrograph of the polished cross-section of zircon-mullite interface after treatment at 1680~C (ZS is zircon, Z is zirconia, MS is mullite substrate, GP is glassy phase and P is epoxy resin).
Vol. 31, No. 8
REACTIONSINTERING
1033
Figure 1B shown the cross-section of the mullite-zircon (ZS1) interface after firing at 1680~C. As can be observed the liquid phase wets the mullite substrate, and mullite is dissolved in the glassy phase just to saturation (10). This glassy phase can be used to join mullite blocks. Figure 2 shows the SEM micrograph corresponding to the cross-sections of two mullite-zirconmullite specimens fired at 1620~C and screen printed with two different contents of commercial zircon SZ1 : (A) 70% and (B) 40%. As observed in these micrographs when a high zircon content is used, higher content of glassy phase is formed. The glassy phase promotes zirconia grain coarsening (4-10 p.m in (A) and <1 p.m in (B) specimens. This fact is detrimental to the mechanical properties of the joined materials (Table I).
FIG.2. SEM micrographs corresponding to the cross-sections of mullite-zircon-mullite specimens fired at 1629K2 and screen printed with SZ1 zircon containing inks: (A) 70 wt% and (B) 40wt%. Rows s h o w s the joint interface. When small a m o u n t of glassy phase is developed as it is the case of the specimen (B) the high temperature (1200QC) strength and creep rate values are not significantly affected as can be seen in Table I and figure 3. TABLE I. Flexural Strength of Mullite and Joined Mullite Samples of(MPa) 1200~C
RT Mullite Joined Mullite
133±10
120+10 70wt%
40wt%
70wt%*
32±5
145±5
50i-_5
*Content zircon (SZ1) in the ink.
40wt%* 160±15
1034
REACTIONSINTERING
Deformation rate= 1.77 x 10.6 s.1
f
Deformation rate= 2.65 x 10-6 s-1
.r"
Conditions: 1200aC-30 M P a
2! o
Voi. 31, No. 8
JOINED MULLITE MULLITE .
•
-
,
Time (h)
FIG. 3. Deformation versus time for mullite and joined mullite specimens. Alumina-Alumina loining Because alumina and zircon are solid state incompatible compounds, they both react according to the equation: 2 (SiO2. ZrO2) + 3 A1203 --> 2 SiO2 3A1203 + 2 ZrO2
[1]
The mechanism by which this reaction takes place has been a matter of controversy between different authors (11). Moya et al. (12) suggested that the impurities present in the commercial zircon can play an important role in the formation of a liquid phase at temperatures below the eutectic temperature of the system SiO2-AI203-ZrO2. In order to clarify this important point, regarding the possible joining mechanism, the following experiment was performed: two pellets of zircon (SZ1 and SZ2) with different impurity contents were obtained by isostatic pressing at 200 MPa then located on the polished surface of a very high purity (<30ppm) alumina plate with a density =100% th., as observed in Fig. 4. The assembly was fired at 1550QC for 4 h and subsequently cross sectioned for microstructural observation.
FIG. 4. Cross section of the assembly used for the alumina-zircon interfacial reaction study.
Vol. 31, No. 8
REACTION SINTERING
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The Figs. 5(A) and 5(B) show the SEM micrographs corresponding to SZ1/A1203 and SZ2/A1203 interfaces. It can be observed that a much lower content of glassy phase was present at the interface for the purest zircon (SZ2). The amount of glassy phase also affects the average grain size of zirconia, as observed in the case of mullite-zircon-mullite sandwiches (Fig. 2). This result indicates that zircon impurities not only enhance the kinetics of the reaction [1] but also may facilitate alumina/alumina joining through the developed glassy phase. Figure 6A shows the cross section of an alumina-zircon-alumina specimen obtained by screen printing with 40% zircon (ZS1) content suspension after firing at 1600~C, 1.5 h. As can be observed in the SEM micrograph, a perfect joining is obtained.
FIG. 5. SEM micrographs corresponding to (A) SZ1/A1203 and (B) ZS2/A1203 cross sections. Bright grains are zirconia, AS alumina substrates and GP glassy phase.
'4
i FIG. 6. (A) SEM micrograph of the cross section corresponding to Alumina-zircon-alumina after firing. (B) Photograph showing the perfect joining of two alumina tubes.
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R E A C T I O N SINTERING
Vol. 31, No. 8
Figure 6B shows a photograph of the interface of twp pieces of alumina tubes joined following the previous mentioned procedure. This joining supported a vacuum <10 -5 Pa.
Conclusions The following conclusions can be drawn: 1. Reaction sintering of zircon-mullite and zircon-alumina has proved to be a simple and suitable route to join mullite-mullite and alumina-alumina blocks. 2. The detrimental effect of large amount af glassy phase on the mechanical properties (of, c) of mullite-mullite joined specimens has been pointed out. The amount of liquid phase developed at the interface is related to the concentration of the zircon in the starting ink. 3. The effect of zircon impurities on the glassy phase developed at zircon/alumina interface has also been clearly determined.
Acknowledgement The authors thank C. Grande for experimental assistance. Ths work was suported by CICYT Spain.
References (1) S. L. Swartz, B.S. Majumdar, A. Skidmore and B. C. Mutsuddy, Materials Letters, vol. 7, 407410 (1989) (2) A. J. Moorhead, Adv. Ceram. Mater., 2 (2) 159-166 (1987). (3) ASM Intemacional, Engineered Materials Handbook, Vol. 4, Ceramics and Glass, 478-481 (1991). (4) H. P. Kirchner, J. C. Conway JR. and A. E. Segall, J. Am. Ceram. Soc., 70 (2) 104-109 (1987). (5) A. P. Tomsia and R. E. Loehman, Glass and Ceramic Joints, in "Characterization of ceramics", Ed. R. E. Loehman, Manning Publications Co., 221-228 (1993). (6) E. D6rre and H. Hfibner, "Alumina, Processing, Properties and Applications", SpringerVerlag, 1984 (7) S. Somiya, R. F. Davis and J. A. Pask (Eds), Ceramic Transactions, Vol. 6, "Mullite and Mullite Matrix Composites". American Ceramic Society, Westerville, OH, 1990. (8) G. W. Hollenberg, G. R. Terwilliger and R. S. Gordon, J. Am. Ceram. Soc. 54, 196 (1971). (9) P. Pena, Ph. D. Thesis. Universidad Complutense de Madrid, Spain, 1979. (10) M. A. Sainz, R. Torrecillas and J. S. Moya, J. Am. Ceram. Soc., 76 (7) 1869-72 (1993). (11) J. S. Wallace, G. Petzow, & N. Claussen, Advances in Ceramics, Vol. 12, 1984, pp. 436-42. (12) J. S. Moya, R. Moreno, J. Requena, R. Torrecillas and G. Fantozzi, J. European Ceram. Soc., 7 (1991), 27-30.
Pergamon
CONFERENCESETNo. 2 A L U M I N A - A L U M I N A A N D MULLITE-MULLITE J O I N I N G BY REACTION SINTERING PROCESS R. Torrecillas CSIC- Instituto Tecnol6gico de Materiales de Asturias Parque Tecnol6gico de Asturias 33429 Llanera Spain M.A. Sainz and J.S.Moya Instituto de Ceramica y Vidrio CSIC Arganda del Rey 28500 Madrid Spain
(Received May 6, 1994) (Revised May 26, 1994) Introduction
Joining of s i m p l e ceramic s h a p e s to form complex ceramic parts can be considered as an alternative route to the difficult and expensive methods associated with fabrication of near-netshape ceramic components (1). The ability to reliably joint ceramic parts to form large systems is considered n o w a d a y s as a key technology in enhancing the use of ceramics in high temperature structural applications and in dirty environments (2 , 3). H o w e v e r , as has been pointed out elsewhere (4), the joining of ceramics is not yet a very high developed art. Most scientific and engineering efforts have been devoted to ceramic to metal joining (5). Alumina a n d mullite are two oxides ceramics largely considered for functional as well as structural applications (6 , 7). The p u r p o s e of the present investigation is to join aluminaalumina and mullite-mullite ceramics through reaction sintering with zircon. Materials and methods
a) Dense mullite blocks of =97% th. density with a total impurity content <0.3% were obtained by isostatic pressing at 200 MPa and firing at 1650~C-4h, starting from a commercial alumino-silicate gel (Siral 28M, Condea, H a m b u r g , Germany). b) Alumina blocks of 98% th. density were obtained by isostatic pressing at 200 MPa and firing at 1650°C-2h, starting from a commercial spray-dryed powder (VAW NM 9922, Germany). c) H i g h p u r i t y a l u m i n a with density =100% th. (<30 p p m total impurities, courtesy of M. Kitiyama UC-Berkeley) was used for the alumina-zircon interfacial reaction. d) T w o types of zircon p o w d e r s with different impurities content were used: (i) a commercial p o w d e r SZ1 (Quiminsa, Castell6n, Spain), with 1.2 ~m average particle size and with the following i m p u r i t y content (in wt%): A1203, 0.48; HfO2, 0.79; Fe203, 0.11; TiO2, 0.12; Na20, 0.11; K20, 0.01; CaO, 0.03 and MgO, 0.09, and (ii) a synthetic powder SZ2 (courtesy of M. Ocafia, Institute of Materials Science, CSIC, Madrid, Spain), with the following impurity content (in 1031
1032
REACTION SINTERING
Vol. 31, No. 8
wt%): A1203, 0.25; HfO2, 0.99; Fe203, 0.013; TiO2, 0.020; Na20, 0.024; K20, 0.008; CaO, 0.008 and MgO, 0.009; and 1.5 p.m average particle size. Zircon containing inks were obtained by mixing ethyleneglycol (30 and 60wt%) and zircon powder (SZ1, 40-70 wt%) with 1% defloculant in a agate mortar. Mullite and alumina blocks were polished on one face d o w n to 1 pm. The polished surfaces of the different mullite and alumina plates were screen printed and subsequently, symmetric plates with polished surfaces were located on the top. These sandwiches were dried at 60QC for 24 h and then fired at 16001680~C for 1.5 h. After firing, the specimens cross-section were polished, thermally etched and studied by optical microscopy (RLOM) and scanning electron microscopy (SEM). Bars of 3x4x40 m m size perpendicular to the joining interface were sawn from these blocks for mechanical tests. The tensile surface was polished to 6 p.m. Creep tests were carried out in air on 4-point bending system (36 mm outerspan and 18 mm inner span) on 3x4x40 mm bars. The creep strain was calculated from the deflection using the Hollenberg method (8). Results and discussion
Mullite-Mullite Joining Zircon and mullite are stable and solid state compatible materials. At temperatures higher than the peritectic temperature of the subsystem zircon-zirconia mullite (~1600°C), zirconia plus a liquid phase is formed. This liquid phase is not en equilibrium with mullite; therefore mullite starts to dissolve into the glassy phase moving the composition of this glassy phase along the corresponding isotherm, until the intersection between the isotherm and the mullite zirconia primary field boundary is reached (Fig. 1A).
AI203
~-~ A3S2
is~s' SiO2
FIG. I. (A) SiO2-AI203-ZrO2 phase equilibrium diagram (9) and (B) Micrograph of the polished cross-section of zircon-mullite interface after treatment at 1680~C (ZS is zircon, Z is zirconia, MS is mullite substrate, GP is glassy phase and P is epoxy resin).
Vol. 31, No. 8
REACTIONSINTERING
1033
Figure 1B shown the cross-section of the mullite-zircon (ZS1) interface after firing at 1680~C. As can be observed the liquid phase wets the mullite substrate, and mullite is dissolved in the glassy phase just to saturation (10). This glassy phase can be used to join mullite blocks. Figure 2 shows the SEM micrograph corresponding to the cross-sections of two mullite-zirconmullite specimens fired at 1620~C and screen printed with two different contents of commercial zircon SZ1 : (A) 70% and (B) 40%. As observed in these micrographs when a high zircon content is used, higher content of glassy phase is formed. The glassy phase promotes zirconia grain coarsening (4-10 p.m in (A) and <1 p.m in (B) specimens. This fact is detrimental to the mechanical properties of the joined materials (Table I).
FIG.2. SEM micrographs corresponding to the cross-sections of mullite-zircon-mullite specimens fired at 1629K2 and screen printed with SZ1 zircon containing inks: (A) 70 wt% and (B) 40wt%. Rows s h o w s the joint interface. When small a m o u n t of glassy phase is developed as it is the case of the specimen (B) the high temperature (1200QC) strength and creep rate values are not significantly affected as can be seen in Table I and figure 3. TABLE I. Flexural Strength of Mullite and Joined Mullite Samples of(MPa) 1200~C
RT Mullite Joined Mullite
133±10
120+10 70wt%
40wt%
70wt%*
32±5
145±5
50i-_5
*Content zircon (SZ1) in the ink.
40wt%* 160±15
1034
REACTIONSINTERING
Deformation rate= 1.77 x 10.6 s.1
f
Deformation rate= 2.65 x 10-6 s-1
.r"
Conditions: 1200aC-30 M P a
2! o
Voi. 31, No. 8
JOINED MULLITE MULLITE .
•
-
,
Time (h)
FIG. 3. Deformation versus time for mullite and joined mullite specimens. Alumina-Alumina loining Because alumina and zircon are solid state incompatible compounds, they both react according to the equation: 2 (SiO2. ZrO2) + 3 A1203 --> 2 SiO2 3A1203 + 2 ZrO2
[1]
The mechanism by which this reaction takes place has been a matter of controversy between different authors (11). Moya et al. (12) suggested that the impurities present in the commercial zircon can play an important role in the formation of a liquid phase at temperatures below the eutectic temperature of the system SiO2-AI203-ZrO2. In order to clarify this important point, regarding the possible joining mechanism, the following experiment was performed: two pellets of zircon (SZ1 and SZ2) with different impurity contents were obtained by isostatic pressing at 200 MPa then located on the polished surface of a very high purity (<30ppm) alumina plate with a density =100% th., as observed in Fig. 4. The assembly was fired at 1550QC for 4 h and subsequently cross sectioned for microstructural observation.
FIG. 4. Cross section of the assembly used for the alumina-zircon interfacial reaction study.
Vol. 31, No. 8
REACTION SINTERING
1035
The Figs. 5(A) and 5(B) show the SEM micrographs corresponding to SZ1/A1203 and SZ2/A1203 interfaces. It can be observed that a much lower content of glassy phase was present at the interface for the purest zircon (SZ2). The amount of glassy phase also affects the average grain size of zirconia, as observed in the case of mullite-zircon-mullite sandwiches (Fig. 2). This result indicates that zircon impurities not only enhance the kinetics of the reaction [1] but also may facilitate alumina/alumina joining through the developed glassy phase. Figure 6A shows the cross section of an alumina-zircon-alumina specimen obtained by screen printing with 40% zircon (ZS1) content suspension after firing at 1600~C, 1.5 h. As can be observed in the SEM micrograph, a perfect joining is obtained.
FIG. 5. SEM micrographs corresponding to (A) SZ1/A1203 and (B) ZS2/A1203 cross sections. Bright grains are zirconia, AS alumina substrates and GP glassy phase.
'4
i FIG. 6. (A) SEM micrograph of the cross section corresponding to Alumina-zircon-alumina after firing. (B) Photograph showing the perfect joining of two alumina tubes.
1036
R E A C T I O N SINTERING
Vol. 31, No. 8
Figure 6B shows a photograph of the interface of twp pieces of alumina tubes joined following the previous mentioned procedure. This joining supported a vacuum <10 -5 Pa.
Conclusions The following conclusions can be drawn: 1. Reaction sintering of zircon-mullite and zircon-alumina has proved to be a simple and suitable route to join mullite-mullite and alumina-alumina blocks. 2. The detrimental effect of large amount af glassy phase on the mechanical properties (of, c) of mullite-mullite joined specimens has been pointed out. The amount of liquid phase developed at the interface is related to the concentration of the zircon in the starting ink. 3. The effect of zircon impurities on the glassy phase developed at zircon/alumina interface has also been clearly determined.
Acknowledgement The authors thank C. Grande for experimental assistance. Ths work was suported by CICYT Spain.
References (1) S. L. Swartz, B.S. Majumdar, A. Skidmore and B. C. Mutsuddy, Materials Letters, vol. 7, 407410 (1989) (2) A. J. Moorhead, Adv. Ceram. Mater., 2 (2) 159-166 (1987). (3) ASM Intemacional, Engineered Materials Handbook, Vol. 4, Ceramics and Glass, 478-481 (1991). (4) H. P. Kirchner, J. C. Conway JR. and A. E. Segall, J. Am. Ceram. Soc., 70 (2) 104-109 (1987). (5) A. P. Tomsia and R. E. Loehman, Glass and Ceramic Joints, in "Characterization of ceramics", Ed. R. E. Loehman, Manning Publications Co., 221-228 (1993). (6) E. D6rre and H. Hfibner, "Alumina, Processing, Properties and Applications", SpringerVerlag, 1984 (7) S. Somiya, R. F. Davis and J. A. Pask (Eds), Ceramic Transactions, Vol. 6, "Mullite and Mullite Matrix Composites". American Ceramic Society, Westerville, OH, 1990. (8) G. W. Hollenberg, G. R. Terwilliger and R. S. Gordon, J. Am. Ceram. Soc. 54, 196 (1971). (9) P. Pena, Ph. D. Thesis. Universidad Complutense de Madrid, Spain, 1979. (10) M. A. Sainz, R. Torrecillas and J. S. Moya, J. Am. Ceram. Soc., 76 (7) 1869-72 (1993). (11) J. S. Wallace, G. Petzow, & N. Claussen, Advances in Ceramics, Vol. 12, 1984, pp. 436-42. (12) J. S. Moya, R. Moreno, J. Requena, R. Torrecillas and G. Fantozzi, J. European Ceram. Soc., 7 (1991), 27-30.