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Fabrication of mullite composites by cyclic infiltration and reaction sintering
Materials Science and Engineering A298 (2001) 179 – 186 www.elsevier.com/locate/msea
Fabrication of mullite composites by cyclic infiltration and reaction sintering Yung-Jen Lin *, Yi-Chi Chen Department of Materials Engineering, Tatung Uni6ersity, 40 Chungsan North Rd., Section 3, Taipei 10451, Taiwan, ROC Received 26 April 2000; received in revised form 30 June 2000
Abstract Mullite composites were fabricated by infiltrating porous preforms containing alumina and silica powder with a liquid mullite precursor and then, by reaction sintering of the infiltrated preforms. Cyclic infiltration to saturation could introduce 15–25 wt.% of mullite precursor into preforms with 45–55% open porosity. The increase in the green density accounted for the significant reduction in sintering shrinkage of the preforms. The addition of 20 vol.% of 3 mol% yttria – zirconia in the preforms lowered the sintering temperature and increased the sinterability of the mullite composites. The reactive formation of mullite from alumina and silica started at 1300°C and became more prominent with higher sintering temperatures. The presence of mullite powder, infiltrated mullite and 3 mol% yttria–zirconia could enhance the reactive formation of mullite. SEM observations revealed that all the mullite grains were equiaxed. The reactively formed mullite and the infiltrated mullite grains were smaller than the mullite grains derived from the mullite powder in the preforms. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Reaction sintering; Liquid infiltration; Mullite; Composites; Processing
1. Introduction Mullite exhibits good high-temperature strength, good chemical stability and low thermal expansion coefficient and has attracted considerable attention in structural applications [1 – 5]. However, the decomposition of mullite at high temperatures in an environment of low oxygen partial pressures, low fracture toughness and the difficulties in sintering to achieve dense monoliths have posed great obstacles in industrial applications [5,6]. To improve the mechanical properties (strength and/or fracture toughness) of mullite, SiC particulates, SiC whiskers and zirconia are often incorporated as the reinforcing phases. The presence of reinforcing phases further impairs the densification of mullite. Consequently, many processing routes for these mullite-based composites have been designed to enhance densification and achieve proper microstructure. Several novel routes, including sol – gel technique, viscous sintering of composite powders and reaction syn* Corresponding author. Tel.: +886-2-25866040; fax: + 886-225936897. E-mail address: [email protected] (Y.-J. Lin).
thesis, have been developed and proved to be effective in lowering the sintering temperatures [7–16]. On the other hand, liquid infiltration of ceramic preforms offers several advantages for the fabrication of ceramic composites. By infiltrating porous preforms of ceramic matrix with liquid precursors that could convert into an inorganic phase upon pyrolysis, ceramic composites of desired compositions and unique microstructure could be obtained [17–23]. Moreover, the fact that the open pores of the preforms were partly filled by the infiltrant would result in the reduction of sintering shrinkage of the composites. Therefore, it would be of considerable interest to fabricate mullite composites by a combination of infiltration and reaction sintering. By the application of both infiltration and reaction sintering, dense mullite composites with low shrinkage were expected to form at relatively low sintering temperatures (5 1500°C). In this research, we fabricated mullite, mullite/mullite, mullite/SiC and mullite/zirconia composites by infiltrating compacts of powder preforms consisting of alumina, amorphous silica and reinforcing phases with a liquid precursor of mullite. The compacts were cyclically infiltrated to saturation to maximize the contents
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of the mullite precursor. After saturation infiltration, the compacts were densified at a temperature where alumina and amorphous silica would react to form mullite (reaction sintering). The cyclic infiltration process and the phase and microstructure of the sintered composites were characterized. 2. Experiments
2.1. Preforms Powders of a-alumina, mullite and 3 mol% yttria– stabilized zirconia (denoted as 3Y – Z) and SiC were obtained from commercial suppliers and their characteristics are listed in Table 1. In order to reduce the particle size of the mullite, the mullite powder was milled in ethanol for 72 h using 3Y-TZP balls of 3 mm diameter. This milling reduced the median particle size of the mullite from 4.5 to 1.6 mm (measured by a laser diffraction particle size analyzer). Amorphous silica powder was synthesized in the laboratory using sol–gel processing of tetraethyl orthosilicate (TEOS). The details of the silica powder processing were reported elsewhere [24]. Table 1 The characteristics of starting powdersa Characteristics
Starting powders
Purity (wt.%) Average particle size (mm) BET surface area (m2 g−1)
a-Al2O3b
Mullitec
3Y–ZrO2d SiCe
99.99 0.41
98 1.64
99.9 0.17
15
–
30
97f 0.45 18
a
All data except particle sizes were supplied by the manufacturers. Baikalox CR15, Baikowski International Corp., Charlotte, NC, USA. c Ball-milled for 72 h, Johnson Matthey, Ward Hills, MA. d OZC-3YA, Osaka Cement Co. Ltd., Osaka, Japan. e A-1, Showa Denko Co. Ltd., Tokyo, Japan. f Major impurities in SiC powders — 1.4 wt.% free carbon; 0.7 wt.% SiO2; 0.01 wt.% Al; 0.04 wt.% Fe.
Alumina and amorphous silica were mixed and drypressed to prepare the mullite preforms. The weight ratio of alumina to silica was kept at 75:25 in this research (the alumina to silica ratio of stoichiometric mullite is 71.8:28.2 by weight). Previous reports showed that yttria could modify the composition of silicate glass and was an effective sintering additive in the reaction sintering of mullite [24,25]. Thus, yttrium nitrate was added during the mixing of powders in such a way that, after pyrolysis, the yttria would be 3.2 wt.% of the amount of the amorphous silica. Preforms of mullite/mullite, mullite/zirconia and mullite/SiC composites were prepared by mixing alumina and silica (with the alumina, silica and yttria ratio the same as that for mullite preforms) with milled mullite powder, 3Y–Z and SiC powders, respectively, and dry pressing. The mullite/mullite samples consisted of 60 vol.% of milled mullite powder and 40 vol.% of reactively synthesized mullite (from alumina and silica), assuming that the reaction from alumina and silica was completed after sintering. In mullite/zirconia and mullite/SiC composites, 20 vol.% of 3Y–Z and SiC were added into mullite/mullite matrix. The designations and compositions of the composites are listed in Table 2. The powder mixing was accomplished by ball milling of the powders in ethanol (with 1 wt.% of polyvinyl alcohol) in polyethylene bottles with 3Y-TZP balls (3 mm in diameter) for 20 h. The combined volume of the slurry and milling balls was 80% of the volume of polyethylene bottles. After mixing, the slurries were dried and ground to pass through a 100-mesh sieve. The dry pressing of powder mixtures was performed uniaxially using a WC mold at 50 MPa. The discshaped pellets weighed about 1 g and were 15 mm in diameter and 3 mm in thickness. To attain moderate strength for subsequent infiltration and pyrolysis, all the preforms were heated at 1200°C for 2 h in static air.
b
Table 2 Sample designation and composition of the preforms Sample designation
AS MAS ZMAS CMAS
Composition of the preforms (vol.%) Al2O3+SiO2a
Mullite
3Y–ZrO2
100 40 32 32
0 60 48 48
0 0 20 0
SiC 0 0 0 20
a Yttrium nitrate was added as a sintering additive. Weight ratio of alumina and silica was 3:1.
2.2. Liquid infiltrant The liquid infiltrant was an ethanol (95% purity, Taiwan Tobacco Monopoly Co., Taipei, Taiwan) solution of aluminum nitrate (Al(NO3)3 · 9H2O, Wako Pure Chemicals Industries Ltd., Osaka, Japan) and tetraethyl orthosilicate (TEOS, Fluka Chemie AG, Buchs, Switzerland). The proportion of aluminum nitrate and TEOS was such that, after pyrolysis, the infiltrant would convert into mullite precursor with alumina and silica weight ratio equal to 75:25 (same as the alumina:silica ratio in the preforms). The concentration of aluminum nitrate in the solution was made as high as possible to maximize the yield of the mullite precursor from the liquid infiltrant. This was accomplished by dissolving 52.7 g of aluminum nitrate in 90 ml of ethanol and then mixed with 10 ml of TEOS.
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2.5. Characterizations
Fig. 1. Accumulated weight increases versus the numbers of infiltration cycles in AS, MAS, CMAS and ZMAS preforms.
The phase evolution of precursor and sintered samples was studied by X-ray diffractometry (XRD; D-5000, Siemens, Karlsruhe, Germany). Density and open porosity of the sintered samples were measured by Archimede’s principle using deionized water as the medium. The volume shrinkage of the samples after sintering was calculated by the differences in the diameter and thickness of the sample before and after sintering. Microstructures of the sintered samples were observed in a scanning electron microscope (SEM) on cross-sectional surfaces after polishing and thermal etching. The etched surfaces were sputter-coated with Au–Pd.
3. Results and discussions The infiltrant solution was kept sealed during the infiltration process to reduce the evaporation of ethanol. Each batch of solution was repeatedly used for about 2 weeks. During this period of time, the viscosity of the solution slightly increased with time (due to the inevitable loss of ethanol). The viscosity at the shear rate of 200 s − 1 changed from 13.5 to 18.8 mPa s.
2.3. Infiltration Infiltration was carried out by immersing the preforms in the liquid infiltrant solution in laboratory environment for 24 h. Then, the infiltrated preforms were dried and pyrolyzed at 700°C for 2 h with a heating rate of 3°C min − 1 to convert the infiltrant into mullite precursor. The preforms were repeatedly infiltrated, dried and pyrolyzed till the weights of the infiltrated preforms no longer increased. The weights of the preforms were measured with an electronic balance ( 90.0001 g) after each infiltration – drying–pyrolysis cycle (or infiltration cycle, for short). The difference of the weight of the preform before infiltration and the weight after pyrolysis was the accumulated amount of mullite precursor introduced by cyclic infiltration process.
2.4. Sintering After the cyclic infiltration, the samples were sintered in a tube furnace at temperatures up to 1500°C for 2 h with an intermediate holding at 1200°C for 1 h. Both the heating and cooling rates were 5°C min − 1. The samples were sintered in static air except the samples containing SiC, which were sintered in flowing Ar (0.4 l min − 1) and with SiC packing powder to reduce the oxidation of SiC.
3.1. Mullite precursor The infiltration solution was formulated to form mullite after pyrolysis. The phase evolution of the dried infiltrant (mullite precursor) has been reported elsewhere [26]. In short, the mullite precursor started to crystallize into Al–Si spinel and mullite at about 972°C. Then the spinel converted into mullite when the precursor was heated at 1200°C. Hence, after the infiltrated preforms were dried and pyrolyzed at 700°C, the infiltrant deposited in the pores of the preforms was amorphous mullite precursor.
3.2. Infiltration Fig. 1 is a plot of the accumulated weight increases versus the numbers of infiltration–drying–pyrolysis cycles in AS, MAS, CMAS and ZMAS preforms. This figure showed that the accumulated infiltrated amounts increased with each infiltration cycle and eventually attained saturation. The saturated amounts differed and appeared to be dependent on the initial open porosity of these preforms. Fig. 2 shows the variation of the saturated infiltrated amounts with the initial open porosities of the preforms. It shows that the maximum amounts of infiltrant that could be introduced decreased with the decreasing of the initial open porosity of the preforms. The extrapolation of the fitting line in Fig. 2 intersected the abscissa at 30%. This implied that the liquid infiltrant (liquid mullite precursor) used in this study would be ineffective for preforms with less than 30 vol.% of open porosity. This critical open porosity was about the same as that in our studies of infiltration of porous zirconia preforms [26].
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3.3. Densification and phase e6olutions Since the samples underwent reactive sintering, densification and phase evolution would occur simultaneously as the temperature was raised. Table 3 compared the bulk densities, open porosities and volume shrinkage of the uninfiltrated and fully infiltrated preforms after they were sintered at 1500°C for 2 h. From this table it was noted that uninfiltrated samples except the CMAS could be densified to have open porosities less than 2%. But when the preforms were fully infiltrated, the densification deteriorated more or less. After sintering, the open porosities of the fully infiltrated samples were larger than those of their uninfiltrated counterparts (especially evident for sample MAS). The infiltrated mullite precursor would crystallize during sintering process and would retard the densification. Nevertheless, the samples containing 3Y–Z (sample ZMAS) could still be densified to have 98% of theoretical density even after they were fully infiltrated. On the other hand, infiltration could significantly reduce the sintering shrinkage. The volume shrinkage after sintering decreased by 10 – 15 vol.% when the preforms were fully infiltrated. The reduction of sintering shrinkage arose from two effects. Firstly, the infiltration process filled the open pores of the preforms with mullite precursor and increased the green density of the infiltrated preforms. Secondly, the fully infiltrated preforms were more difficult to densify. Of these two effects, the increase in green density was the main reason for the reduction in sintering shrinkage since the open porosity only increased slightly in the fully infiltrated preforms (especially true for the ZMAS samples). As the sample containing 20 vol.% of SiC (CMAS) could hardly be densified, our attention and characterizations were focused on the samples of other compositions (i.e. AS, MAS and ZMAS).
Fig. 2. Saturated infiltrated amounts versus the initial open porosities of the preforms.
Fig. 3a–c show XRD patterns of fully infiltrated AS, MAS and ZMAS samples after they were sintered between 1300 and 1500°C for 2 h. In Fig. 3a, the reaction of alumina and silica to form mullite is clearly demonstrated in the AS samples. Mullitization already occurred at 1300°C and became more pronounced as the sintering temperature increased. Although yttrium was added as a sintering aid, it did not form a distinct phase. However, XRD patterns revealed that large amounts of 3Y–Z were present in the AS samples, which, apparently, were derived from milling balls. The presence of yttrium (from yttrium nitrate addition and from 3Y–Z milling contamination) in the AS sample might account for the better sintering in this study as compared with the sintering of AS samples in our previous studies (the open porosity of AS samples was 1.3% in this study and was 15% in AS samples without yttrium [24]). Nevertheless, the reaction to form mullite was not complete since much alumina still remained after sintering at 1500°C for 2 h (XRD patterns in Fig. 3a). The XRD patterns of MAS and ZMAS samples (Fig. 3b and c) were generally similar to those of AS samples (Fig. 3a). However, several differences were noted. First, unreacted alumina peaks were much weaker in Fig. 3b and c than in Fig. 3a. This indicated that the reaction of alumina and silica in MAS and ZMAS samples was nearly complete. Second, significant silicate phases (cristobalite in MAS and zircon in ZMAS) formed after the samples were sintered at 1300°C for 2 h. These silicate phases became less as the sintering temperature increased. The MAS and ZMAS samples only consisted of mullite and zirconia after they were sintered at 1500°C for 2 h. The phase contents of uninfiltrated samples after they were sintered at 1500°C for 2 h were revealed in the XRD patterns in Fig. 4. These XRD patterns were similar to those of their fully infiltrated counterparts (shown in Fig. 3a–c) except that more unreacted alumina was left in the uninfiltrated samples. It appeared that the introduction of mullite precursor also enhanced the mullite formation from alumina and silica. Although the phase evolutions of fully infiltrated MAS and ZMAS samples were comparable, the densification behaviors were different. Fig. 5a and b show the bulk density and open porosity of fully infiltrated AS, MAS and ZMAS preforms after sintering at various temperatures for 2 h. Since the phases of the sample changed with sintering temperatures, the bulk density of each sample was affected not only by densification, but also by the changes of phase compositions. Hence, the sintering behavior of the samples could be evaluated better by the change of open porosities. From Fig. 5a and b, it was noted that while AS and MAS samples densified notably only after sintering at temperatures] 1450°C, ZMAS already showed signifi-
a
1.75 1.69 2.16 1.72
54.2 47.8 43.4 46.8
3.64 3.01 3.69 1.83
1.3 0.5 0.2 45.6
Significant weight loss (]8 wt.%) after sintering at 1500°C/2 h in flowing argon.
AS MAS ZMAS CMASa
Open porosity (%)
Bulk density (g cm−3)
Bulk density (g cm−3)
Open porosity (%)
Uninfiltrated preform after sintering
Preform (uninfiltrated)
51.8 43.4 41.7 11.8
Sintering shrinkage (vol.%)
3.57 2.90 3.68 –
Bulk density (g cm−3)
2.1 7.0 0.5 –
Open porosity (%)
Fully infiltrated preform after sintering
Table 3 Changes of bulk density, open porosity and volume shrinkage of the uninfiltrated and fully infiltrated preforms after sintering at 1500°C for 2 h
35.6 27.5 31.6 –
Sintering shrinkage (vol.%)
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Fig. 3. XRD patterns of fully infiltrated samples after they were sintered between 1300 and 1500°C for 2 h. (a) AS; (b) MAS; (c) ZMAS; , mullite; , alumina; , tetragonal zirconia; , monoclinic zirconia; , zircon; c, cristobalite.
cant densification at temperatures] 1350°C and reached final densification after sintering at 1450°C for 2 h. Therefore, the addition of 20 vol.% 3Y – Z significantly improved the sintering. This was consistent with previous reports on the advantages of 3Y – Z in the reaction synthesis of mullite and mullite composites [24,27].
3.4. Microstructure Fig. 6a–c show SEM micrographs of cross-sectional views of fully infiltrated AS, MAS and ZMAS samples after sintering at 1500°C for 2 h. These micrographs were taken from regions near the outer surfaces of the preforms. The reactively formed mullite and the
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infiltrated mullite in AS samples were generally equiaxed and small in size (51 mm) after sintering (shown in Fig. 6a). Fig. 6a also revealed alumina and zirconia grains, which were darker and brighter grains in the micrograph, respectively. In MAS samples (Fig. 6b), the microstructure was primarily equiaxed mullite grains of various grain sizes. The smaller grains corresponded to the reactively formed mullite or infiltrated mullite while the larger grains ( ]3 mm) were derived from the mullite particles in the preforms (i.e. those added during the preparation of the preforms). In ZMAS samples, zirconia grains were scattered in a matrix of mullite and were located in the boundaries or multiple junctions of mullite grains.
4. Conclusions In infiltrating mullite preforms with a liquid mullite precursor, the maximum amounts of the mullite precursor that could be infiltrated decreased with the decrease of initial open porosities of the preforms. For preforms with about 45– 55% open porosity, the saturated infiltrated amounts ranged from 15 to 25 wt.%. Although the infiltrated mullite in the fully infiltrated samples compensated for 10 – 15 vol.% of sintering shrinkage, the infiltration deteriorated the densification of the samples slightly. The addition of 20 vol.% yttria–zirconia in the mullite preforms significantly improved the sintering, lowering the temperature of significant densification from 1450 to 1350°C.
Fig. 5. Variation of (a) bulk density and (b) open porosity of fully infiltrated AS, MAS and ZMAS preforms after sintering at various temperatures for 2 h.
Fig. 4. XRD patterns of uninfiltrated AS, MAS and ZMAS samples after they were sintered at 1500°C for 2 h. , Mullite; , alumina; , tetragonal zirconia; , monoclinic zirconia; , zircon; c, cristobalite.
Reactive formation of mullite from alumina and amorphous silica started at 1300°C and became more prominent with higher sintering temperatures. The presence of mullite powder, infiltrated mullite and 3 mol% yttria–zirconia could enhance the reactive formation of mullite. All the mullite grains were equiaxed. The reactively formed mullite and the infiltrated mullite grains were smaller than the mullite grains derived from the mullite powders in the preforms. The zirconia grains were located in the boundaries or multiple junctions of mullite grains.
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Fig. 6. SEM micrographs of cross-sectional views of fully infiltrated samples after sintering at 1500°C for 2 h. (a) AS; (b) MAS; (c) ZMAS.
Acknowledgements This work was financially supported by Tatung University under Grant Nos. B88-1709-01 and B87-170901.
References
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[1] I.A. Aksay, D.M. Dabbs, M. Sarikaya, J. Am. Ceram. Soc. 74 (10) (1991) 2343. [2] H. Schneider, E. Eberhard, J. Am. Ceram. Soc. 73 (7) (1990) 2073. [3] P.C. Dokko, J.A. Pask, K.S. Mazdiyashi, J. Am. Ceram. Soc. 60 (3 – 4) (1977) 150. [4] S. Somiya, Y. Hirata, Ceram. Bull. 70 (10) (1991) 1624. [5] S. Somiya, R.F. Davis, J.A. Pask (Eds.), Ceramic Transactions, vol. 6. Mullite and Mullite Matrix Composites, The American Ceramic Society, Westerville, OH, 1990. [6] J. Am. Ceram. Soc. 74 (10) (1991) 2341. [7] C. Nischik, M.M. Seibold, N.A. Travitzky, N. Claussen, J. Am. Ceram. Soc. 74 (10) (1991) 2464. [8] K. Okada, N. Otsuka, R.J. Brook, A.J. Moulson, J. Am. Ceram. Soc. 72 (12) (1989) 2369. [9] S. Rajendran, H.J. Rossell, J. Mater. Sci. 26 (21) (1991) 5815. [10] T. Teigs, P. Becher, P. Angelini, Microsturctures and properties of SiC whisker-reinforced mullite composites, in: S. Somiya, R.F. Davis, J.A. Pask (Eds.), Ceramic Transactions, vol. 6.
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Fabrication of mullite composites by cyclic infiltration and reaction sintering Yung-Jen Lin *, Yi-Chi Chen Department of Materials Engineering, Tatung Uni6ersity, 40 Chungsan North Rd., Section 3, Taipei 10451, Taiwan, ROC Received 26 April 2000; received in revised form 30 June 2000
Abstract Mullite composites were fabricated by infiltrating porous preforms containing alumina and silica powder with a liquid mullite precursor and then, by reaction sintering of the infiltrated preforms. Cyclic infiltration to saturation could introduce 15–25 wt.% of mullite precursor into preforms with 45–55% open porosity. The increase in the green density accounted for the significant reduction in sintering shrinkage of the preforms. The addition of 20 vol.% of 3 mol% yttria – zirconia in the preforms lowered the sintering temperature and increased the sinterability of the mullite composites. The reactive formation of mullite from alumina and silica started at 1300°C and became more prominent with higher sintering temperatures. The presence of mullite powder, infiltrated mullite and 3 mol% yttria–zirconia could enhance the reactive formation of mullite. SEM observations revealed that all the mullite grains were equiaxed. The reactively formed mullite and the infiltrated mullite grains were smaller than the mullite grains derived from the mullite powder in the preforms. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Reaction sintering; Liquid infiltration; Mullite; Composites; Processing
1. Introduction Mullite exhibits good high-temperature strength, good chemical stability and low thermal expansion coefficient and has attracted considerable attention in structural applications [1 – 5]. However, the decomposition of mullite at high temperatures in an environment of low oxygen partial pressures, low fracture toughness and the difficulties in sintering to achieve dense monoliths have posed great obstacles in industrial applications [5,6]. To improve the mechanical properties (strength and/or fracture toughness) of mullite, SiC particulates, SiC whiskers and zirconia are often incorporated as the reinforcing phases. The presence of reinforcing phases further impairs the densification of mullite. Consequently, many processing routes for these mullite-based composites have been designed to enhance densification and achieve proper microstructure. Several novel routes, including sol – gel technique, viscous sintering of composite powders and reaction syn* Corresponding author. Tel.: +886-2-25866040; fax: + 886-225936897. E-mail address: [email protected] (Y.-J. Lin).
thesis, have been developed and proved to be effective in lowering the sintering temperatures [7–16]. On the other hand, liquid infiltration of ceramic preforms offers several advantages for the fabrication of ceramic composites. By infiltrating porous preforms of ceramic matrix with liquid precursors that could convert into an inorganic phase upon pyrolysis, ceramic composites of desired compositions and unique microstructure could be obtained [17–23]. Moreover, the fact that the open pores of the preforms were partly filled by the infiltrant would result in the reduction of sintering shrinkage of the composites. Therefore, it would be of considerable interest to fabricate mullite composites by a combination of infiltration and reaction sintering. By the application of both infiltration and reaction sintering, dense mullite composites with low shrinkage were expected to form at relatively low sintering temperatures (5 1500°C). In this research, we fabricated mullite, mullite/mullite, mullite/SiC and mullite/zirconia composites by infiltrating compacts of powder preforms consisting of alumina, amorphous silica and reinforcing phases with a liquid precursor of mullite. The compacts were cyclically infiltrated to saturation to maximize the contents
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of the mullite precursor. After saturation infiltration, the compacts were densified at a temperature where alumina and amorphous silica would react to form mullite (reaction sintering). The cyclic infiltration process and the phase and microstructure of the sintered composites were characterized. 2. Experiments
2.1. Preforms Powders of a-alumina, mullite and 3 mol% yttria– stabilized zirconia (denoted as 3Y – Z) and SiC were obtained from commercial suppliers and their characteristics are listed in Table 1. In order to reduce the particle size of the mullite, the mullite powder was milled in ethanol for 72 h using 3Y-TZP balls of 3 mm diameter. This milling reduced the median particle size of the mullite from 4.5 to 1.6 mm (measured by a laser diffraction particle size analyzer). Amorphous silica powder was synthesized in the laboratory using sol–gel processing of tetraethyl orthosilicate (TEOS). The details of the silica powder processing were reported elsewhere [24]. Table 1 The characteristics of starting powdersa Characteristics
Starting powders
Purity (wt.%) Average particle size (mm) BET surface area (m2 g−1)
a-Al2O3b
Mullitec
3Y–ZrO2d SiCe
99.99 0.41
98 1.64
99.9 0.17
15
–
30
97f 0.45 18
a
All data except particle sizes were supplied by the manufacturers. Baikalox CR15, Baikowski International Corp., Charlotte, NC, USA. c Ball-milled for 72 h, Johnson Matthey, Ward Hills, MA. d OZC-3YA, Osaka Cement Co. Ltd., Osaka, Japan. e A-1, Showa Denko Co. Ltd., Tokyo, Japan. f Major impurities in SiC powders — 1.4 wt.% free carbon; 0.7 wt.% SiO2; 0.01 wt.% Al; 0.04 wt.% Fe.
Alumina and amorphous silica were mixed and drypressed to prepare the mullite preforms. The weight ratio of alumina to silica was kept at 75:25 in this research (the alumina to silica ratio of stoichiometric mullite is 71.8:28.2 by weight). Previous reports showed that yttria could modify the composition of silicate glass and was an effective sintering additive in the reaction sintering of mullite [24,25]. Thus, yttrium nitrate was added during the mixing of powders in such a way that, after pyrolysis, the yttria would be 3.2 wt.% of the amount of the amorphous silica. Preforms of mullite/mullite, mullite/zirconia and mullite/SiC composites were prepared by mixing alumina and silica (with the alumina, silica and yttria ratio the same as that for mullite preforms) with milled mullite powder, 3Y–Z and SiC powders, respectively, and dry pressing. The mullite/mullite samples consisted of 60 vol.% of milled mullite powder and 40 vol.% of reactively synthesized mullite (from alumina and silica), assuming that the reaction from alumina and silica was completed after sintering. In mullite/zirconia and mullite/SiC composites, 20 vol.% of 3Y–Z and SiC were added into mullite/mullite matrix. The designations and compositions of the composites are listed in Table 2. The powder mixing was accomplished by ball milling of the powders in ethanol (with 1 wt.% of polyvinyl alcohol) in polyethylene bottles with 3Y-TZP balls (3 mm in diameter) for 20 h. The combined volume of the slurry and milling balls was 80% of the volume of polyethylene bottles. After mixing, the slurries were dried and ground to pass through a 100-mesh sieve. The dry pressing of powder mixtures was performed uniaxially using a WC mold at 50 MPa. The discshaped pellets weighed about 1 g and were 15 mm in diameter and 3 mm in thickness. To attain moderate strength for subsequent infiltration and pyrolysis, all the preforms were heated at 1200°C for 2 h in static air.
b
Table 2 Sample designation and composition of the preforms Sample designation
AS MAS ZMAS CMAS
Composition of the preforms (vol.%) Al2O3+SiO2a
Mullite
3Y–ZrO2
100 40 32 32
0 60 48 48
0 0 20 0
SiC 0 0 0 20
a Yttrium nitrate was added as a sintering additive. Weight ratio of alumina and silica was 3:1.
2.2. Liquid infiltrant The liquid infiltrant was an ethanol (95% purity, Taiwan Tobacco Monopoly Co., Taipei, Taiwan) solution of aluminum nitrate (Al(NO3)3 · 9H2O, Wako Pure Chemicals Industries Ltd., Osaka, Japan) and tetraethyl orthosilicate (TEOS, Fluka Chemie AG, Buchs, Switzerland). The proportion of aluminum nitrate and TEOS was such that, after pyrolysis, the infiltrant would convert into mullite precursor with alumina and silica weight ratio equal to 75:25 (same as the alumina:silica ratio in the preforms). The concentration of aluminum nitrate in the solution was made as high as possible to maximize the yield of the mullite precursor from the liquid infiltrant. This was accomplished by dissolving 52.7 g of aluminum nitrate in 90 ml of ethanol and then mixed with 10 ml of TEOS.
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2.5. Characterizations
Fig. 1. Accumulated weight increases versus the numbers of infiltration cycles in AS, MAS, CMAS and ZMAS preforms.
The phase evolution of precursor and sintered samples was studied by X-ray diffractometry (XRD; D-5000, Siemens, Karlsruhe, Germany). Density and open porosity of the sintered samples were measured by Archimede’s principle using deionized water as the medium. The volume shrinkage of the samples after sintering was calculated by the differences in the diameter and thickness of the sample before and after sintering. Microstructures of the sintered samples were observed in a scanning electron microscope (SEM) on cross-sectional surfaces after polishing and thermal etching. The etched surfaces were sputter-coated with Au–Pd.
3. Results and discussions The infiltrant solution was kept sealed during the infiltration process to reduce the evaporation of ethanol. Each batch of solution was repeatedly used for about 2 weeks. During this period of time, the viscosity of the solution slightly increased with time (due to the inevitable loss of ethanol). The viscosity at the shear rate of 200 s − 1 changed from 13.5 to 18.8 mPa s.
2.3. Infiltration Infiltration was carried out by immersing the preforms in the liquid infiltrant solution in laboratory environment for 24 h. Then, the infiltrated preforms were dried and pyrolyzed at 700°C for 2 h with a heating rate of 3°C min − 1 to convert the infiltrant into mullite precursor. The preforms were repeatedly infiltrated, dried and pyrolyzed till the weights of the infiltrated preforms no longer increased. The weights of the preforms were measured with an electronic balance ( 90.0001 g) after each infiltration – drying–pyrolysis cycle (or infiltration cycle, for short). The difference of the weight of the preform before infiltration and the weight after pyrolysis was the accumulated amount of mullite precursor introduced by cyclic infiltration process.
2.4. Sintering After the cyclic infiltration, the samples were sintered in a tube furnace at temperatures up to 1500°C for 2 h with an intermediate holding at 1200°C for 1 h. Both the heating and cooling rates were 5°C min − 1. The samples were sintered in static air except the samples containing SiC, which were sintered in flowing Ar (0.4 l min − 1) and with SiC packing powder to reduce the oxidation of SiC.
3.1. Mullite precursor The infiltration solution was formulated to form mullite after pyrolysis. The phase evolution of the dried infiltrant (mullite precursor) has been reported elsewhere [26]. In short, the mullite precursor started to crystallize into Al–Si spinel and mullite at about 972°C. Then the spinel converted into mullite when the precursor was heated at 1200°C. Hence, after the infiltrated preforms were dried and pyrolyzed at 700°C, the infiltrant deposited in the pores of the preforms was amorphous mullite precursor.
3.2. Infiltration Fig. 1 is a plot of the accumulated weight increases versus the numbers of infiltration–drying–pyrolysis cycles in AS, MAS, CMAS and ZMAS preforms. This figure showed that the accumulated infiltrated amounts increased with each infiltration cycle and eventually attained saturation. The saturated amounts differed and appeared to be dependent on the initial open porosity of these preforms. Fig. 2 shows the variation of the saturated infiltrated amounts with the initial open porosities of the preforms. It shows that the maximum amounts of infiltrant that could be introduced decreased with the decreasing of the initial open porosity of the preforms. The extrapolation of the fitting line in Fig. 2 intersected the abscissa at 30%. This implied that the liquid infiltrant (liquid mullite precursor) used in this study would be ineffective for preforms with less than 30 vol.% of open porosity. This critical open porosity was about the same as that in our studies of infiltration of porous zirconia preforms [26].
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3.3. Densification and phase e6olutions Since the samples underwent reactive sintering, densification and phase evolution would occur simultaneously as the temperature was raised. Table 3 compared the bulk densities, open porosities and volume shrinkage of the uninfiltrated and fully infiltrated preforms after they were sintered at 1500°C for 2 h. From this table it was noted that uninfiltrated samples except the CMAS could be densified to have open porosities less than 2%. But when the preforms were fully infiltrated, the densification deteriorated more or less. After sintering, the open porosities of the fully infiltrated samples were larger than those of their uninfiltrated counterparts (especially evident for sample MAS). The infiltrated mullite precursor would crystallize during sintering process and would retard the densification. Nevertheless, the samples containing 3Y–Z (sample ZMAS) could still be densified to have 98% of theoretical density even after they were fully infiltrated. On the other hand, infiltration could significantly reduce the sintering shrinkage. The volume shrinkage after sintering decreased by 10 – 15 vol.% when the preforms were fully infiltrated. The reduction of sintering shrinkage arose from two effects. Firstly, the infiltration process filled the open pores of the preforms with mullite precursor and increased the green density of the infiltrated preforms. Secondly, the fully infiltrated preforms were more difficult to densify. Of these two effects, the increase in green density was the main reason for the reduction in sintering shrinkage since the open porosity only increased slightly in the fully infiltrated preforms (especially true for the ZMAS samples). As the sample containing 20 vol.% of SiC (CMAS) could hardly be densified, our attention and characterizations were focused on the samples of other compositions (i.e. AS, MAS and ZMAS).
Fig. 2. Saturated infiltrated amounts versus the initial open porosities of the preforms.
Fig. 3a–c show XRD patterns of fully infiltrated AS, MAS and ZMAS samples after they were sintered between 1300 and 1500°C for 2 h. In Fig. 3a, the reaction of alumina and silica to form mullite is clearly demonstrated in the AS samples. Mullitization already occurred at 1300°C and became more pronounced as the sintering temperature increased. Although yttrium was added as a sintering aid, it did not form a distinct phase. However, XRD patterns revealed that large amounts of 3Y–Z were present in the AS samples, which, apparently, were derived from milling balls. The presence of yttrium (from yttrium nitrate addition and from 3Y–Z milling contamination) in the AS sample might account for the better sintering in this study as compared with the sintering of AS samples in our previous studies (the open porosity of AS samples was 1.3% in this study and was 15% in AS samples without yttrium [24]). Nevertheless, the reaction to form mullite was not complete since much alumina still remained after sintering at 1500°C for 2 h (XRD patterns in Fig. 3a). The XRD patterns of MAS and ZMAS samples (Fig. 3b and c) were generally similar to those of AS samples (Fig. 3a). However, several differences were noted. First, unreacted alumina peaks were much weaker in Fig. 3b and c than in Fig. 3a. This indicated that the reaction of alumina and silica in MAS and ZMAS samples was nearly complete. Second, significant silicate phases (cristobalite in MAS and zircon in ZMAS) formed after the samples were sintered at 1300°C for 2 h. These silicate phases became less as the sintering temperature increased. The MAS and ZMAS samples only consisted of mullite and zirconia after they were sintered at 1500°C for 2 h. The phase contents of uninfiltrated samples after they were sintered at 1500°C for 2 h were revealed in the XRD patterns in Fig. 4. These XRD patterns were similar to those of their fully infiltrated counterparts (shown in Fig. 3a–c) except that more unreacted alumina was left in the uninfiltrated samples. It appeared that the introduction of mullite precursor also enhanced the mullite formation from alumina and silica. Although the phase evolutions of fully infiltrated MAS and ZMAS samples were comparable, the densification behaviors were different. Fig. 5a and b show the bulk density and open porosity of fully infiltrated AS, MAS and ZMAS preforms after sintering at various temperatures for 2 h. Since the phases of the sample changed with sintering temperatures, the bulk density of each sample was affected not only by densification, but also by the changes of phase compositions. Hence, the sintering behavior of the samples could be evaluated better by the change of open porosities. From Fig. 5a and b, it was noted that while AS and MAS samples densified notably only after sintering at temperatures] 1450°C, ZMAS already showed signifi-
a
1.75 1.69 2.16 1.72
54.2 47.8 43.4 46.8
3.64 3.01 3.69 1.83
1.3 0.5 0.2 45.6
Significant weight loss (]8 wt.%) after sintering at 1500°C/2 h in flowing argon.
AS MAS ZMAS CMASa
Open porosity (%)
Bulk density (g cm−3)
Bulk density (g cm−3)
Open porosity (%)
Uninfiltrated preform after sintering
Preform (uninfiltrated)
51.8 43.4 41.7 11.8
Sintering shrinkage (vol.%)
3.57 2.90 3.68 –
Bulk density (g cm−3)
2.1 7.0 0.5 –
Open porosity (%)
Fully infiltrated preform after sintering
Table 3 Changes of bulk density, open porosity and volume shrinkage of the uninfiltrated and fully infiltrated preforms after sintering at 1500°C for 2 h
35.6 27.5 31.6 –
Sintering shrinkage (vol.%)
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Fig. 3. XRD patterns of fully infiltrated samples after they were sintered between 1300 and 1500°C for 2 h. (a) AS; (b) MAS; (c) ZMAS; , mullite; , alumina; , tetragonal zirconia; , monoclinic zirconia; , zircon; c, cristobalite.
cant densification at temperatures] 1350°C and reached final densification after sintering at 1450°C for 2 h. Therefore, the addition of 20 vol.% 3Y – Z significantly improved the sintering. This was consistent with previous reports on the advantages of 3Y – Z in the reaction synthesis of mullite and mullite composites [24,27].
3.4. Microstructure Fig. 6a–c show SEM micrographs of cross-sectional views of fully infiltrated AS, MAS and ZMAS samples after sintering at 1500°C for 2 h. These micrographs were taken from regions near the outer surfaces of the preforms. The reactively formed mullite and the
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infiltrated mullite in AS samples were generally equiaxed and small in size (51 mm) after sintering (shown in Fig. 6a). Fig. 6a also revealed alumina and zirconia grains, which were darker and brighter grains in the micrograph, respectively. In MAS samples (Fig. 6b), the microstructure was primarily equiaxed mullite grains of various grain sizes. The smaller grains corresponded to the reactively formed mullite or infiltrated mullite while the larger grains ( ]3 mm) were derived from the mullite particles in the preforms (i.e. those added during the preparation of the preforms). In ZMAS samples, zirconia grains were scattered in a matrix of mullite and were located in the boundaries or multiple junctions of mullite grains.
4. Conclusions In infiltrating mullite preforms with a liquid mullite precursor, the maximum amounts of the mullite precursor that could be infiltrated decreased with the decrease of initial open porosities of the preforms. For preforms with about 45– 55% open porosity, the saturated infiltrated amounts ranged from 15 to 25 wt.%. Although the infiltrated mullite in the fully infiltrated samples compensated for 10 – 15 vol.% of sintering shrinkage, the infiltration deteriorated the densification of the samples slightly. The addition of 20 vol.% yttria–zirconia in the mullite preforms significantly improved the sintering, lowering the temperature of significant densification from 1450 to 1350°C.
Fig. 5. Variation of (a) bulk density and (b) open porosity of fully infiltrated AS, MAS and ZMAS preforms after sintering at various temperatures for 2 h.
Fig. 4. XRD patterns of uninfiltrated AS, MAS and ZMAS samples after they were sintered at 1500°C for 2 h. , Mullite; , alumina; , tetragonal zirconia; , monoclinic zirconia; , zircon; c, cristobalite.
Reactive formation of mullite from alumina and amorphous silica started at 1300°C and became more prominent with higher sintering temperatures. The presence of mullite powder, infiltrated mullite and 3 mol% yttria–zirconia could enhance the reactive formation of mullite. All the mullite grains were equiaxed. The reactively formed mullite and the infiltrated mullite grains were smaller than the mullite grains derived from the mullite powders in the preforms. The zirconia grains were located in the boundaries or multiple junctions of mullite grains.
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Fig. 6. SEM micrographs of cross-sectional views of fully infiltrated samples after sintering at 1500°C for 2 h. (a) AS; (b) MAS; (c) ZMAS.
Acknowledgements This work was financially supported by Tatung University under Grant Nos. B88-1709-01 and B87-170901.
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