- Email: [email protected]
Pressure sintering of Al2O3MgO mixtures under 10 kbars
56
CERAMICS
Pressure Sintering of Al&-MgO Under 10 kbars K. KODAIRA, Department
S. TERAMOTO*,
of Applied Chemistry,
1 - INTRODUCTION Spine1 (MgA1204) has a high melting point, high strength and high resistance to chemical attack’. Dense spine1 ceramics offer applications for use as high temperature materials. Sintering of AIZOS-MgO mixtures has been studied by many investigators 2V3V4. The sintering technics have been limited to normal sintering with and without additives or hotpressing. On the other hand, recently developed pressure sintering makes it possible to prepare a theoretically dense compact at relatively low temperatures for short times”‘. The effects of high pressure on sintering are also very useful for fabrication of translucent compacts. There are no studies on the sintering of Al203-MgO mixtures under high pressure. This report describes studies on pressure sintering of A1203-MgO mixtures under 10 kbars by using a pistoncylinder type apparatus. Microstructure and translucency of compacts in the system also are discussed.
2 - EXPERIMENTAL
PROCEDURE
Aluminum oxide and magnesium oxide powders were used as a starting material. AlzOx powders were prepared by thermal decomposition of aluminum sulfate (reagent grade from Kanto Kagaku) at 1 100°C for 1 hour. The powders were composed of aggregates of very fine particles of v-Al203. MgO powders with 0.1 pm in particle size were prepared by calcining magnesium hydroxide (reagent grade from Wako Junyaku) at 700% for 1 hour in air. The desired proportions of Al203 and MgO were mixed with an agate mortar for 2 - 3 hours. The mixed powders were compressed at 1OOOKglcm’ to form a pellet 7 mm in diameter and 6 mm long and prefired at 1300% for 1 hour in an electric furnace in air. The prefired pellets were enclosed in platinum foil and inserted into a piston-cylinder type vessel for high pressure experiments. Details of the cell assembly have been described previonsly ‘. Pressure sintering was conducted at 1000 to 1300% and 10 kbars for 2 to 120 minutes. The pellets were heated up to the desired temperatures within a few minutes. After being maintained for the desired duration, the specimen was slowly cooled down to room temperature with a simultaneous reduction of pressure. After taking the compacts out of the platinum foil, crystal phases of the compacts were identified by X-ray powder diffraction. Microstructure of fractured svrfaces also was observed using a scanning electron microscope. Grain size was measured from the photographs. The compact density also was determined by the water displacement method.
Vol. 10, n. 2. ,984
Mixtures
S. SHIMADA and T. MATSUSHITA
Faculty of Engineering,
Densification of A1203-YgO mixtures was performed under 10 kbars. Theoretically dense translucent ceramics were obtained at 1000 - 1300°C for 15 -60 wt% MgO and 75 - 40 wt% AlzO+ Within this composition range, a uniform grain size of - 0.5 pm was obtained with the absence of abnormal grain growth. Fragmentation and rearrangement of particles take place as operative mechanisms in the initial stage of sintering.
INTERNATIONAL,
Hokkaido
University,
Sapporo 060, Japan
3 - RESULTS AND DISCUSSION 3.1 - Sintering of spine1 Theoretically dense translucent spine1 compacts were prepared at 1000 - 1300°C and 10 kbars for 15 - 60 minutes. An example of the compact which transmits visible light is shown in Fig. 1. The compact was sintered at 1300°C for 1 hour and was polished to 0.8 mm in thickness. Details of these results on pressure sintering are summarized in Table 1. After pressurssintered at 1000% for 15 minutes, the compact completely converted to spinel, while that prepared at 1000% for 2 minutes was composed of spine1 and small amounts of unreacted A120s-MgO. An increased reaction rate was recognized throughout the course of pressure sintering of spinel. The densification rate also was observed to be rapid under pressure. For example, the relative density of the compact prepared at 1000°C for 2 minutes was about 98% of theoretical value, and a fully dense product was obtained at 1OOO°C for 15 minutes. Thus, sintering of spine1 under pressure proceeds to full density with a high densification rate, while high temperatures and long sintering times can be required to attain theoretical density by conventional hot pressing. Therefore, the densification due to pressure sintering can be explained by the following mechanisms. Fragmentation and rearrangement of particles take place in the initial stage of sintering, because the applied pressure is much higher than the rupture strength in relation to the actual pressure at the contact areas of each grain. As sintering proceeds, plastic flow becomes dominant and densification is completed within a few minutes.
FIGURE 1 - Translucent spine1compact prepared at 1300°C for 1 hour. Present address: Central Laboratory, Pfizer Quigley Co. Ltd., Toda-Shi, 335 Japan. ??
PRESSURE
SINTERING
TABLE 1
OF Ah04gO
MIXTURES
UNDER
57
10 KSARS
- Results on pressure sintering of spine1 Relative Appearance density (O/O)
Temperature (“C) 1000
Duration (minutes) 2
98
1000 1000 1100 1100
15
100
80
100
5
AZ
100
60 60
100 100 100
1100 1200 1300
opaque Vans Vans opaque Vans Vans Vans trans
Grain size (pm)
Crystal pases by X-ray analysis MgAl204,
0.2 0.2 0.2 0.3 0.3 0.4 0.5 0.5
MgO, Al203
MgAl204 MgAl204 MgAl204,
MgO, Al203
MgAlz04 MgAlz04 MgAlz04 MgAlzO4
trans = translucent TABLE 2 - &?~lts
on preeeW@ &nterkrg of
Composition MgO (wt%)
A1203 (wt%) 70 65 60 50 40 30 20 10
; 40 zl ;: 90
t&@-w1204
Crystal phases by
mimm
at
1200%
Appearance
MgAlzO MgO
MgAl204,
MgO
MgAlz04, MgO b+!@l204,
MgO
MgAl204,
MgO
MgO, MgAl204 MgO, MM1204
Grain size (u)
x-ray analysis MgAl204,
for 1 hour.
trans trans trans trans trans less trans opaque white
:.z 0:5
Appearance
Grain
8::
:.5 5
trans = translucent TABLE3
-
~~si~~ofA1203~1204mixtu~at1~°Cfor
bdtson
Crystal phases by
Composition Al203 (wt%)
MgO (wt%)
73 75 77.5
27 25 22.5 20 15 10 5
K 90 95
1 hour.
size (u)
x-ray analysis MgAl204 MgAlt04 MgAl204, Al203 MgAl204, Al203 MgAl204, Al203 Al203, MgAl204 Al203. MgAl204
Vans opaque white white opaque Vans
Vans = translucent
A scanning electron microphotograph of a fractured surface of the compact prepared at 1200°C for 1 hour is shown in Fig. 2. Each grain in the figure was smooth and the grain distrib-
8:: large large large 0.5 0.5
3.2 - Sintering mixtures
of MgO-MgA1204
Phase equilibrium investigations in the system MgO-A1203 have been conducted by Alper et a/ *. Stoichiometric spine1 contains 28.35 wt% MgO and 71.88 wt% AIzO~. At high temperatures, spine1 is able to contain excess MgO in solid solution, but the solubility limits decrease with decreasing temperature. The limiting temperature is about 1500°C. Results on the sintering of MgOMgA1204 mixtures at 1200°C for 1 hour are listed in Table 2. X-ray diffraction analyses of the compacts after pressure sintering showed two phases of spine1 and periclase except for the composition of 30 wt% MgO and 70 wt% A1203. As shown in Fig. 3, compacts with a composition of 30-60 wt% MgO and 70 - 40 wt% A1203 were translucent. The translucency decreased with increasing MgO content up to 70 wt%. The compact with 80 wt% MgO was opaque, and that with 90 wt% MgO was white in color. Grains of fractured surface of the compacts with 30 - 60 wt% MgO showed a uniform distribution like that shown in Fig. 2. The grain size became larger with increasing MgO content. The compacts with 80 - 90 wt% MgO were composed of irregular grains. On the sintering of magnesia-rich MgA1204 mixtures, compacts with a fine grain structure were translucent, while irregular grains caused a reduction of translucency.
3.3 - Sintering of A1203-MgAi204 mixtures
ution was uniform without abnormal grain growth. The grain size was 0.2 pm at 1000°C for 1 hour and 0.5 cm at 1300°C for 1 hour, while the size in the prefired pellet was about 0.1 pm. The rate of grain growth was not particularly remarkable during the course of pressure sintering.
Phase equilibrium in the system A1203 - Mg A1204 has been investigated by Roy et EI/~. A spine1 solid solution region extends to about 93 wt% Al203 and 7 wt% MgO at high temperature, but the solubility limits decrease rapidly with decreasing temperature. For example, alumina disolves about 5 wt% (the composition is 76.6 wt% A1203 and 33.4 wt% MgO) into the spine1 lattice at 1200X, the sintering temperature in
FIGURE 2 - Fractured surface of a spine1 compact prepared at 1200°C for 1 hour.
FIGURE 3 - Developement of translucency in MgO-MgA1204 compacts. Each figures indicate MgO contents (wt%) in the MgO-Al203 system.
58
the present experiments. The results of x-ray powder diffraction analysis indicated that compositions with 73 - 75 wt% Al203 result in a single phase spine1 solid solution, while other compositions result in a spine1 solid solution plus alumina. The relation between appearance and composition of the compacts prepared at 1200°C for 2 hours is summarized in Table 3. Within the composition range of spine1 solid solution (71.66 - 76.6 wt% Al203 at 1200°C), compacts showed good translucency, which decreased with increasing alumina contents. Opaque compacts were obtained at 77.5 - 90 wt% A1203 and 22.5 - 10 wt% MgO. Translucent compact again resulted at a high alumina content (95 wt%). The microstructure of fractured surfaces of the translucent compacts was that formed by very smooth grains of about 0.5 pm. Abnormal grain growth was recognized in the composition of 77.5 - 85 wt% A120S and 22.5 - 15 wto/o MgO, in which opaque compacts were produced.
4 - SUMMARY On sintering of A1203-MgO mixtures under 10 kbars, translucent compacts with theoretical density were prepared from the composition of 25 - 60 wt% MgO and 75 - 40 wto/o Al203. Densification due to pressure sintering proceeded to full density within a few minutes by fragmentation and rear-
K.KODAIRA,
S. TERAMOTO.
S. SHIMADA
and T. MATSUSHITA
rangement of particles, and plastic flow. A composition in the system was found to have an interesting influence on translucency and microstructure of the compacts. Within the composition range showing translucency, grains were uniformly distributed and abnormal grain growth was not found. MgO or Al203 rich-MgAlz04 mixtures led to discontinuous grain growth during sintering and resulted in opaque compacts.
REFERENCES 1. P. HING, J. Mat. Sci. ll(1978) 1919. 2. R.J. BRATTON, J. Amer. Ceram. Sot. 54(1971) 141. 3. J.T. BAILEY and R. RUSSEL, JR., Ceram. Bull. 68 (1971) 493. 4. J.T. BAILEY and R. RUSSEL, JR., Trans. Brit. Ceram. Sot. 66 (1969) 159.
5. K. ATAKATORI, 6. 7. 8. 9.
M. SHIMADA and M. KOIZUMI, J. Ceram Sot. Jap. 69 (1981) 197. K. KODAIRA and m. KOIZUMI, Mat. Res. Bull. 6 (1971) 261. K. KODAIRA, Memo. Fat. Eng. Hokkaido Univ. 14 (1975) 107. A.M. ALPER, R.N. MCNALLY, P.H. RIBBE and R.C. DOMAN, J. Amer. Ceram. Sot. 45 (1962) 263. D.M. ROY. R. ROY and E.F. OSBORN, Amer J. Sci. 251 (1953) 337.
Received March 3, 1983; final test received October 7, 1983.
CERAMICS
Pressure Sintering of Al&-MgO Under 10 kbars K. KODAIRA, Department
S. TERAMOTO*,
of Applied Chemistry,
1 - INTRODUCTION Spine1 (MgA1204) has a high melting point, high strength and high resistance to chemical attack’. Dense spine1 ceramics offer applications for use as high temperature materials. Sintering of AIZOS-MgO mixtures has been studied by many investigators 2V3V4. The sintering technics have been limited to normal sintering with and without additives or hotpressing. On the other hand, recently developed pressure sintering makes it possible to prepare a theoretically dense compact at relatively low temperatures for short times”‘. The effects of high pressure on sintering are also very useful for fabrication of translucent compacts. There are no studies on the sintering of Al203-MgO mixtures under high pressure. This report describes studies on pressure sintering of A1203-MgO mixtures under 10 kbars by using a pistoncylinder type apparatus. Microstructure and translucency of compacts in the system also are discussed.
2 - EXPERIMENTAL
PROCEDURE
Aluminum oxide and magnesium oxide powders were used as a starting material. AlzOx powders were prepared by thermal decomposition of aluminum sulfate (reagent grade from Kanto Kagaku) at 1 100°C for 1 hour. The powders were composed of aggregates of very fine particles of v-Al203. MgO powders with 0.1 pm in particle size were prepared by calcining magnesium hydroxide (reagent grade from Wako Junyaku) at 700% for 1 hour in air. The desired proportions of Al203 and MgO were mixed with an agate mortar for 2 - 3 hours. The mixed powders were compressed at 1OOOKglcm’ to form a pellet 7 mm in diameter and 6 mm long and prefired at 1300% for 1 hour in an electric furnace in air. The prefired pellets were enclosed in platinum foil and inserted into a piston-cylinder type vessel for high pressure experiments. Details of the cell assembly have been described previonsly ‘. Pressure sintering was conducted at 1000 to 1300% and 10 kbars for 2 to 120 minutes. The pellets were heated up to the desired temperatures within a few minutes. After being maintained for the desired duration, the specimen was slowly cooled down to room temperature with a simultaneous reduction of pressure. After taking the compacts out of the platinum foil, crystal phases of the compacts were identified by X-ray powder diffraction. Microstructure of fractured svrfaces also was observed using a scanning electron microscope. Grain size was measured from the photographs. The compact density also was determined by the water displacement method.
Vol. 10, n. 2. ,984
Mixtures
S. SHIMADA and T. MATSUSHITA
Faculty of Engineering,
Densification of A1203-YgO mixtures was performed under 10 kbars. Theoretically dense translucent ceramics were obtained at 1000 - 1300°C for 15 -60 wt% MgO and 75 - 40 wt% AlzO+ Within this composition range, a uniform grain size of - 0.5 pm was obtained with the absence of abnormal grain growth. Fragmentation and rearrangement of particles take place as operative mechanisms in the initial stage of sintering.
INTERNATIONAL,
Hokkaido
University,
Sapporo 060, Japan
3 - RESULTS AND DISCUSSION 3.1 - Sintering of spine1 Theoretically dense translucent spine1 compacts were prepared at 1000 - 1300°C and 10 kbars for 15 - 60 minutes. An example of the compact which transmits visible light is shown in Fig. 1. The compact was sintered at 1300°C for 1 hour and was polished to 0.8 mm in thickness. Details of these results on pressure sintering are summarized in Table 1. After pressurssintered at 1000% for 15 minutes, the compact completely converted to spinel, while that prepared at 1000% for 2 minutes was composed of spine1 and small amounts of unreacted A120s-MgO. An increased reaction rate was recognized throughout the course of pressure sintering of spinel. The densification rate also was observed to be rapid under pressure. For example, the relative density of the compact prepared at 1000°C for 2 minutes was about 98% of theoretical value, and a fully dense product was obtained at 1OOO°C for 15 minutes. Thus, sintering of spine1 under pressure proceeds to full density with a high densification rate, while high temperatures and long sintering times can be required to attain theoretical density by conventional hot pressing. Therefore, the densification due to pressure sintering can be explained by the following mechanisms. Fragmentation and rearrangement of particles take place in the initial stage of sintering, because the applied pressure is much higher than the rupture strength in relation to the actual pressure at the contact areas of each grain. As sintering proceeds, plastic flow becomes dominant and densification is completed within a few minutes.
FIGURE 1 - Translucent spine1compact prepared at 1300°C for 1 hour. Present address: Central Laboratory, Pfizer Quigley Co. Ltd., Toda-Shi, 335 Japan. ??
PRESSURE
SINTERING
TABLE 1
OF Ah04gO
MIXTURES
UNDER
57
10 KSARS
- Results on pressure sintering of spine1 Relative Appearance density (O/O)
Temperature (“C) 1000
Duration (minutes) 2
98
1000 1000 1100 1100
15
100
80
100
5
AZ
100
60 60
100 100 100
1100 1200 1300
opaque Vans Vans opaque Vans Vans Vans trans
Grain size (pm)
Crystal pases by X-ray analysis MgAl204,
0.2 0.2 0.2 0.3 0.3 0.4 0.5 0.5
MgO, Al203
MgAl204 MgAl204 MgAl204,
MgO, Al203
MgAlz04 MgAlz04 MgAlz04 MgAlzO4
trans = translucent TABLE 2 - &?~lts
on preeeW@ &nterkrg of
Composition MgO (wt%)
A1203 (wt%) 70 65 60 50 40 30 20 10
; 40 zl ;: 90
t&@-w1204
Crystal phases by
mimm
at
1200%
Appearance
MgAlzO MgO
MgAl204,
MgO
MgAlz04, MgO b+!@l204,
MgO
MgAl204,
MgO
MgO, MgAl204 MgO, MM1204
Grain size (u)
x-ray analysis MgAl204,
for 1 hour.
trans trans trans trans trans less trans opaque white
:.z 0:5
Appearance
Grain
8::
:.5 5
trans = translucent TABLE3
-
~~si~~ofA1203~1204mixtu~at1~°Cfor
bdtson
Crystal phases by
Composition Al203 (wt%)
MgO (wt%)
73 75 77.5
27 25 22.5 20 15 10 5
K 90 95
1 hour.
size (u)
x-ray analysis MgAl204 MgAlt04 MgAl204, Al203 MgAl204, Al203 MgAl204, Al203 Al203, MgAl204 Al203. MgAl204
Vans opaque white white opaque Vans
Vans = translucent
A scanning electron microphotograph of a fractured surface of the compact prepared at 1200°C for 1 hour is shown in Fig. 2. Each grain in the figure was smooth and the grain distrib-
8:: large large large 0.5 0.5
3.2 - Sintering mixtures
of MgO-MgA1204
Phase equilibrium investigations in the system MgO-A1203 have been conducted by Alper et a/ *. Stoichiometric spine1 contains 28.35 wt% MgO and 71.88 wt% AIzO~. At high temperatures, spine1 is able to contain excess MgO in solid solution, but the solubility limits decrease with decreasing temperature. The limiting temperature is about 1500°C. Results on the sintering of MgOMgA1204 mixtures at 1200°C for 1 hour are listed in Table 2. X-ray diffraction analyses of the compacts after pressure sintering showed two phases of spine1 and periclase except for the composition of 30 wt% MgO and 70 wt% A1203. As shown in Fig. 3, compacts with a composition of 30-60 wt% MgO and 70 - 40 wt% A1203 were translucent. The translucency decreased with increasing MgO content up to 70 wt%. The compact with 80 wt% MgO was opaque, and that with 90 wt% MgO was white in color. Grains of fractured surface of the compacts with 30 - 60 wt% MgO showed a uniform distribution like that shown in Fig. 2. The grain size became larger with increasing MgO content. The compacts with 80 - 90 wt% MgO were composed of irregular grains. On the sintering of magnesia-rich MgA1204 mixtures, compacts with a fine grain structure were translucent, while irregular grains caused a reduction of translucency.
3.3 - Sintering of A1203-MgAi204 mixtures
ution was uniform without abnormal grain growth. The grain size was 0.2 pm at 1000°C for 1 hour and 0.5 cm at 1300°C for 1 hour, while the size in the prefired pellet was about 0.1 pm. The rate of grain growth was not particularly remarkable during the course of pressure sintering.
Phase equilibrium in the system A1203 - Mg A1204 has been investigated by Roy et EI/~. A spine1 solid solution region extends to about 93 wt% Al203 and 7 wt% MgO at high temperature, but the solubility limits decrease rapidly with decreasing temperature. For example, alumina disolves about 5 wt% (the composition is 76.6 wt% A1203 and 33.4 wt% MgO) into the spine1 lattice at 1200X, the sintering temperature in
FIGURE 2 - Fractured surface of a spine1 compact prepared at 1200°C for 1 hour.
FIGURE 3 - Developement of translucency in MgO-MgA1204 compacts. Each figures indicate MgO contents (wt%) in the MgO-Al203 system.
58
the present experiments. The results of x-ray powder diffraction analysis indicated that compositions with 73 - 75 wt% Al203 result in a single phase spine1 solid solution, while other compositions result in a spine1 solid solution plus alumina. The relation between appearance and composition of the compacts prepared at 1200°C for 2 hours is summarized in Table 3. Within the composition range of spine1 solid solution (71.66 - 76.6 wt% Al203 at 1200°C), compacts showed good translucency, which decreased with increasing alumina contents. Opaque compacts were obtained at 77.5 - 90 wt% A1203 and 22.5 - 10 wt% MgO. Translucent compact again resulted at a high alumina content (95 wt%). The microstructure of fractured surfaces of the translucent compacts was that formed by very smooth grains of about 0.5 pm. Abnormal grain growth was recognized in the composition of 77.5 - 85 wt% A120S and 22.5 - 15 wto/o MgO, in which opaque compacts were produced.
4 - SUMMARY On sintering of A1203-MgO mixtures under 10 kbars, translucent compacts with theoretical density were prepared from the composition of 25 - 60 wt% MgO and 75 - 40 wto/o Al203. Densification due to pressure sintering proceeded to full density within a few minutes by fragmentation and rear-
K.KODAIRA,
S. TERAMOTO.
S. SHIMADA
and T. MATSUSHITA
rangement of particles, and plastic flow. A composition in the system was found to have an interesting influence on translucency and microstructure of the compacts. Within the composition range showing translucency, grains were uniformly distributed and abnormal grain growth was not found. MgO or Al203 rich-MgAlz04 mixtures led to discontinuous grain growth during sintering and resulted in opaque compacts.
REFERENCES 1. P. HING, J. Mat. Sci. ll(1978) 1919. 2. R.J. BRATTON, J. Amer. Ceram. Sot. 54(1971) 141. 3. J.T. BAILEY and R. RUSSEL, JR., Ceram. Bull. 68 (1971) 493. 4. J.T. BAILEY and R. RUSSEL, JR., Trans. Brit. Ceram. Sot. 66 (1969) 159.
5. K. ATAKATORI, 6. 7. 8. 9.
M. SHIMADA and M. KOIZUMI, J. Ceram Sot. Jap. 69 (1981) 197. K. KODAIRA and m. KOIZUMI, Mat. Res. Bull. 6 (1971) 261. K. KODAIRA, Memo. Fat. Eng. Hokkaido Univ. 14 (1975) 107. A.M. ALPER, R.N. MCNALLY, P.H. RIBBE and R.C. DOMAN, J. Amer. Ceram. Sot. 45 (1962) 263. D.M. ROY. R. ROY and E.F. OSBORN, Amer J. Sci. 251 (1953) 337.
Received March 3, 1983; final test received October 7, 1983.