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Shrinkage and flexural strength improvement of silica-based composites for ceramic cores by colloidal alumina infiltration
Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Shrinkage and flexural strength improvement of silica-based composites for ceramic cores by colloidal alumina infiltration Young-Hwan Kim a,b, Jeong-gu Yeo a,n, Sung-Churl Choi b,n a b
Advanced Materials and Devices Laboratory, Korea Institute of Energy Research, 152 Gajeong-ro, Yuseong-gu, Daejeon 34129, Republic of Korea Division of Materials Science and Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul 04763, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 5 January 2016 Received in revised form 16 February 2016 Accepted 23 February 2016
The effects of colloidal alumina infiltration on porous silica-based composites for complex designed ceramic cores were investigated. The specimens pre-sintered at 1100 °C for 2 h were immersed into colloidal alumina and were sintered at 1300 °C for 2 h. Infiltrated alumina particles were coagulated on the surface of fused silica via their opposite electrical charges in the infiltrating solution. The infiltrated alumina was reacted with the surface of fused silica, and mullite was formed thereafter. The shrinkage and microcracking induced by surface crystallization of fused silica to cristobalite was prevented by mullitization. As a result, the formation of mullite by alumina infiltration for 150 min dramatically improved the flexural strength (3.3 MPa to 9.6 MPa) and reduced the linear shrinkage (2% to 1%) of the silica-based composites. However, longer infiltration time over 150 min has no significant effects on flexural strength and linear shrinkage. & 2016 Published by Elsevier Ltd.
Keywords: Fused silica Colloidal alumina Infiltration Flexural strength Shrinkage
1. Introduction Complex designed ceramic cores fabricated by injection molding have been widely used to form internal cooling passages in hollow gas turbine blades [1–3]. During investment casting of the gas turbine blades, the ceramic core undergoes thermal stress at high temperature. Ceramic cores have been manufactured using a mixture of ceramic powders based on fused silica (SiO2) and zircon (ZrSiO4) due to their excellent high temperature properties, such as thermal shock resistance and chemical inertness against molten metal [4]. Fused silica is mainly used to produce refractory materials, as well as ceramic cores, because of its low thermal expansion coefficient (0.55 10 6 K 1 between 25 °C and 1000 °C) and excellent chemical inertness against molten metal. To provide optimal flexural strength for enduring severe circumstances during investment casting, high temperature sintering is required to manufacture the ceramic cores [5]. However, high sintering temperature (Z 1300 °C) leads to crystallization of fused silica to cristobalite, thereby inducing shrinkage and microcracking of the ceramic cores. Volume contraction during β- to α-phase transformation of cristobalite leads to shrinkage of the silica-based ceramic cores. In addition, microcracks formed by volume contraction of cristobalite decrease the flexural strength of ceramic n
Corresponding authors. E-mail addresses: [email protected] (J.-g. Yeo), [email protected] (S.-C. Choi).
cores [6,7]. Consequently, the existence of cristobalite causes dimensional inaccuracy and deteriorative strength on silica-based ceramic cores. Many studies have investigated the effect of cristobalite on mechanical properties of silica-based ceramics [2,8–10]. Huseby et al. studied the shrinkage of silica-based composites with various compositions [1]. Their report indicated that the volume contraction of cristobalite during its phase transformation leads to shrinkage on silica-based composites. In addition, Breneman et al. investigated the effect of cristobalite phase on the flexural strength of fused silica [6]. They reported that the cristobalite improved flexural strength of fused silica based ceramics at 350 °C because of the existence of cristobalite for the β-phase. However, the flexural strength at room temperature was reduced due to microcracks formed by β- to α-phase transformation of cristobalite. These results demonstrate that the cristobalite in silica-based ceramics gives rise to a decrease of flexural strength and an increase of the amount of shrinkage. However, the cristobalite necessarily occurs at high sintering temperature. Therefore, in the present study, colloidal alumina was infiltrated into a silica-based ceramic core to inhibit crystallization of fused silica for improvement of the flexural strength with low shrinkage. The infiltration technique is a convenient additional process applicable to the manufacturing of the ceramic core. Preventing the crystallization of cristobalite through convenient alumina infiltrating process ahead of sintering step definitely leads to minimization of its shrinkage and reduction of the number of microcracks [9]. The present study is aimed at improving the flexural strength and reducing the amount of
http://dx.doi.org/10.1016/j.ceramint.2016.02.137 0272-8842/& 2016 Published by Elsevier Ltd.
Please cite this article as: Y.-H. Kim, et al., Shrinkage and flexural strength improvement of silica-based composites for ceramic cores by colloidal alumina infiltration, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.137i
Y.-H. Kim et al. / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
2
Fig. 1. SEM (1) and EDX mapping (2) images of (a) non-infiltrated, (b) 30 min infiltrated, and (c) 210 min infiltrated specimens after drying.
shrinkage via infiltration of colloidal alumina into silica-based composites for complex ceramic cores.
2. Material and methods Porous silica-based specimens for alumina infiltration were prepared using ceramic injection molding. The feedstocks for injection molding were composed of 63.75 wt% of fused silica (amorphous SiO2, 325 mesh, Imerys, United States), 21.25 wt% of zircon (ZrSiO4, 1 μm, Cenotec, Korea) and 15 wt% of thermoplastic binders (paraffin wax, microcrystalline wax, stearic acid and oleic acid). The ceramic powders of fused silica and zircon flour were ball milled for 6 hours at room temperature using a 6-mm diameter zirconia ball. After ball milling, the ceramic powders and thermoplastic binders were mixed at 80 °C for 6 h under vacuum condition to produce the feedstocks. The green bodies with dimensions of 6 mm 8 mm 90 mm according to ASTM C1161-13 were injection molded using a C-frame ceramic injection molding machine (CTM-CI-CF-35100HT, Cleveland Tools and Machines, United States) with a nozzle temperature of 80 °C, a flow rate of 400 cc/s and an injection pressure of 60 bar. After a slow heating step (0.2 K/min) to burn out the thermoplastic binders, the specimens were presintered at 1100 °C for 2 h with heating rate of 5 K/min using box furnace (UAF-15-27-LHE, Lenton, England). During pre-sintering, the specimens were positioned in alumina crucible with fused silica backfill powders. The measured apparent porosity and linear shrinkage of pre-sintered specimen were 30.1% and 0.6%, respectively. The pre-sintered specimens were infiltrated by colloidal alumina suspension (Aluminum oxide, 20% in H2O, colloidal dispersion, 50 nm, Alfa-aesar) with various infiltration times of 30 min to 300 min under vacuum condition. For evaporation of solvent, the infiltrated specimens were dried at 110 °C for 5 h. The dried specimen was sintered at 1300 °C for 2 h, along with pre-sintered (non-infiltrated) specimen for comparison. The dimensional change and infiltrated alumina content of
specimens were measured after sintering.Field-emission scanning electron microscopy (FE-SEM, S-4800, Hitachi, Japan) and EDX mapping (Link Pentafet, Oxford) were used to identify the microstructures and alumina infiltration of each specimen. The flexural strength of the specimens was measured through a 3-point bending test using a universal testing machine (UTM, H10SK, Hounsefield, England) with span size of 80 mm and crosshead speed of 1.0 mm/min. The crystallization of fused silica to cristobalite and the existence of alumina and mullite were analyzed using a X-ray diffractometer (XRD, D/Max-2200, Rigaku, Japan) using Cu Kα radiation (λ ¼ 1.54178 Å). In addition, EDX line analysis (Link Pentafet, Oxford) was used to determine the existence of mullite on the surface of fused silica and the residual alumina in the specimens.
3. Results and discussion It is well-known that the isoelectric point (IEP) of fused silica and alumina are pH 2 and pH 9, respectively [11–16]. In the range of pH 2 to pH 9, fused silica has a negative electrical charge, whereas alumina exhibits a positive electrical charge [17]. It is well known that the IEP of alumina particle is near pH 9.0. However, the colloidal alumina used in this study is alumina dispersed suspension, and the described pH on the specification is 4.0. In this study, the measured pH of the colloidal alumina solution was 3.92. The fused silica and alumina were attracted each other via their opposite electrical charges. Because fused silica had been long necked via pre-sintering at 1100 °C, alumina powder was coagulated onto the fused silica. The alumina powder already coagulated onto the fused silica via electrical attraction force, therefore, it can remain on the specimen after the solvent was pulled out via drying after infiltration [17]. EDX analysis data demonstrated the existence of infiltrated alumina in the specimens (Fig. 1). No aluminum peak was observed in the non-infiltrated specimen (Fig. 1 (a)). In contrast, aluminum was detected in the infiltrated specimens (Fig. 1(b) and (c)). The EDX data indicates that the alumina coagulated homogeneously in silica-based porous specimen via
Please cite this article as: Y.-H. Kim, et al., Shrinkage and flexural strength improvement of silica-based composites for ceramic cores by colloidal alumina infiltration, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.137i
Y.-H. Kim et al. / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Fig. 2. Infiltrated alumina content and apparent porosity as a function of infiltration time.
3
Fig. 3. XRD patterns of the sintered specimens as a variation of infiltration time (●: zircon, ○: cristobalite, □: alumina, ■: mullite).
infiltration, exclusive of the fractured large fused silica particles (black area). In addition, the result shows that the increase of the alumina content depends on the infiltration time. The content of alumina (qth) on silica-based porous composites while the pores were filled completely with the colloidal alumina is calculated using the following equation [17].
⎡ qth=⎢1+ρc ⎣
(
100 − P P
)( ) ( 1 1−w
1 ρa
+
100 − ti ρw ti
)⎤⎦⎥
−1
ρc is the pre-sintered silica-based composite density (2.51 g/cm3), P is the apparent porosity of the pre-sintered specimen (30.1%), w is the weight loss of the dried colloidal alumina between 110 °C and 1100 °C (w ¼0.3), ti is the weight percent of alumina in the colloidal suspension (20 wt%), and ρa and ρw are the density of alumina (3.95 g/cm3) and liquid (1.00 g/cm3), respectively. The weight loss of the dried colloidal alumina was measured using a thermogravity/differential thermal analyzer (TG/DTA, SDT 2960 Simultaneous, TA instruments, United States). The dispersant was completely eliminated at 500 °C and weight of infiltrated specimen did not change at a temperature range of 500 °C to 1300 °C. The calculated content of infiltrated alumina using the above equation is 2.54%. The measured content of alumina and the apparent porosity of specimens are shown in Fig. 2. The alumina content in the specimen continually increased with infiltration time. The alumina content reached near the calculated value (2.54%) in the specimen infiltrated for 150 min. The apparent porosity reduces gradually with an increase of the infiltration time. The change rate of alumina content and the apparent porosity start decreasing after 210 min. The increasing concentration of alumina in the specimen observed in the saturated state at 150 min shows that most of the fused silica was coagulated with alumina. Therefore, the infiltrated alumina after 150 min of infiltration time appeared to be coagulated alumina that is already attached with silica. As shown in Fig. 3, X-ray diffraction analysis revealed the phase transformation of fused silica on silica-based composites for alumina infiltration after sintering at 1300 °C. All specimens contain zircon (JCPDS 6-266) and cristobalite (JCPDS 39-1425) phases. The cristobalite is formed by crystallization of fused silica at a temperature above 1300 °C [1,2]. Due to a low content of alumina (0.7%), the XRD pattern of the specimens infiltrated for 30 and 90 min contain only zircon and cristobalite. Alternatively, the respective mullite peaks (JCPDS 15-0776) are found in the specimens
Fig. 4. Flexural strength (s3pt) and linear shrinkage (ΔL/L0) as a function of infiltration time.
infiltrated above 150 min because the fine alumina particles coagulated on fused silica were transformed into mullite during sintering at 1300 °C, according to the following reaction [18–20]: 3Al2O3 þ 2SiO2-3Al2O3∙2SiO2 (Al6Si2O13) In particular, the specimen infiltrated for 210 min shows the existence of alumina (JCPDS 46-1212) as well as mullite. This result reveals the existence of unreacted excessive alumina in the specimen. Fig. 4 shows the flexural strength and the linear shrinkage of the specimens. The existence of cristobalite has a decisive effect on the shrinkage and flexural strength of silica-based composites because of its phase transformation [6,21,22]. As cristobalite changes from β- to α-phase, it contracts 5 vol%; accordingly, microcracks are formed [23]. The volume contraction during the phase change of cristobalite accompanied with microcracking caused a reduction of the flexural strength of the silica-based ceramics [24]. Therefore, compared with infiltrated specimens, the lower flexural strength and higher amount of shrinkage of noninfiltrated specimen (3.3 MPa and 2%, respectively) are caused by volume contraction via phase transformation of cristobalite. The formation of mullite via alumina infiltration on the silicabased composites leads to flexural strength enhancement and
Please cite this article as: Y.-H. Kim, et al., Shrinkage and flexural strength improvement of silica-based composites for ceramic cores by colloidal alumina infiltration, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.137i
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Table 1 The peak ratio of cristobalite to zircon of specimens with infiltration time. Infiltration time
Cristobalite/Zircon peak ratio
0 min (non-infiltrated) 30 min 90 min 150 min 210 min
1.081 0.880 0.692 0.501 0.491
reduction of the amount of linear shrinkage. As shown in Fig. 4, the flexural strength is gradually improved up to 10 MPa until 150 min of infiltration time. It is considered that the flexural strength is enhanced by formation of mullite (i.e., mullitization). However, no significant changes are revealed on the flexural strength of the specimen infiltrated above 210 min. This result shows the mullitization of fused silica surface specifically affect the flexural strength. The mullitization by alumina infiltration can prevent crystallization to cristobalite and the resulting generation of microcracks. In this study, the infiltrated alumina on the surface of fused silica became mullite instead of undergoing surface crystallization to produce cristobalite. Table 1 shows the main peak ratio of cristobalite (21.68°, corresponding to (110) plane) to the zircon (26.86°, corresponding to (200) plane) in the XRD patterns of the specimens. It is possible to assume that the relative degree of crystallization of fused silica in specimens by comparing the main peak intensity between cristobalite and zircon because all specimens have same content of zircon [7,9]. In addition, zircon does not react with fused silica or alumina at temperatures under 1600 °C [25,26]. As shown in Table 1, the relative content of cristobalite reduces with the increase of infiltrating time (or alumina content). The flexural strength (Fig. 4) and crystallization rate of cristobalite (Table 1) are consistent with the behavior that mullitization via alumina coagulated on fused silica inhibits surface crystallization, thereby increasing the flexural strength of silica-based ceramics. In addition, the change of the crystallization rate reduces relative to
the specimen infiltrated for 150 min, following the trend of the alumina content. This result is related to the content of infiltrated alumina in the specimens because the excessive alumina does not affect the mullitization. Changes of the shrinkage with infiltrating time showed exactly the opposite tendency with that of the flexural strength. The shrinkage of a specimen is decreased in half with the infiltrating time of 150 min. However, the shrinkage does not change when the specimen has excessive residual alumina; nor does the flexural strength. Consequently, mullitization on the surface of fused silica via alumina infiltration leads to flexural strength enhancement and shrinkage restraint due to inhibition of the crystallization of fused silica. Fig. 5 shows that the mullitization effect of reducing microcracks is accompanied by phase transformation of cristobalite. Many microcracks are shown in the microstructure of the noninfiltrated specimen (Fig. 5(a)). In spite of the alumina infiltration, the specimen infiltrated for 30 min still exhibits microcracks on fused silica (Fig. 5(b)), whereas the specimens infiltrated for over 150 min do not indicate microcracks (Fig. 5(c) and (d)). In addition, the existence of alumina (or mullite) particles on fused silica is revealed on the specimens infiltrated for greater than 150 min. These results demonstrate the inhibition effect of microcrack generation by infiltration of alumina in silica-based composites. Based on high resolution SEM and EDX measurements of the specimens, it was established that the mullitization occurred on surface of silica. Fig. 6 shows the microstructures of the specimen infiltrated for 150 min. Although the alumina was not revealed in the XRD patterns of the specimen, the excessive unreacted alumina is found in its microstructures. It is considered that the content of residual alumina is too low to detect using the X-ray diffractometer. As shown in EDX result, three zones (alumina, mullite and silica) are developed between coagulated alumina and fused silica, as shown in the SEM image of Fig. 6. The EDX analysis data shows the mullite was formed on the surface of fused silica due to the reaction between coagulated alumina and fused silica. In addition, excessive alumina formed in the alumina zone. These
Fig. 5. The SEM images of the sintered specimens with various infiltration time; (a) 0 min (non-infiltrated), (b) 30 min, (c) 150 min, and (d) 210 min.
Please cite this article as: Y.-H. Kim, et al., Shrinkage and flexural strength improvement of silica-based composites for ceramic cores by colloidal alumina infiltration, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.137i
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Fig. 6. SEM and EDX analysis results of the specimen infiltrated for 210 min.
results demonstrate that the alumina infiltration leads to mullitization on the surface of fused silica instead of the crystallization to cristobalite that is accompanied by microcracks. Consequently, the enhanced flexural strength and reduced shrinkage were obtained due to mullitization on the surface of fused silica.
4. Conclusions The presence of cristobalite reduces the mechanical properties, such as flexural strength and linear shrinkage, due to the volume contraction during its β- to α-phase transformation. Therefore, in the present study, the colloidal alumina was infiltrated on a presintered specimen to prevent the surface crystallization of fused silica to cristobalite. The infiltrated particles were coagulated in the specimens due to the opposite electrical charges between alumina and fused silica. The alumina infiltration improved the flexural strength and shrinkage of the silica–zircon composites because mullitization reduced the crystallization of fused silica to cristobalite. Due to inhibition of the crystallization to cristobalite, the improvement of the mechanical properties (i.e., the enhancement of flexural strength and the reduction of the linear shrinkage) was revealed with increase of the infiltration time up to 150 min. These significant mechanical properties are considered to rely on the cristobalite content in the specimens. Consequently, the mullitization via alumina infiltration improved the mechanical properties of silica–zircon composites. However, in the case of infiltration time of over 210 min, the strength and the shrinkage did not change. The excessive alumina had no significant effects on mechanical properties of silica–zircon composites because it did not influenced on inhibition of crystallization of fused silica to cristobalite.
Acknowledgments This work was supported by the Power Generation and Electricity Delivery Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry and Energy,
Republic of Korea (20141020102460).
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Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Shrinkage and flexural strength improvement of silica-based composites for ceramic cores by colloidal alumina infiltration Young-Hwan Kim a,b, Jeong-gu Yeo a,n, Sung-Churl Choi b,n a b
Advanced Materials and Devices Laboratory, Korea Institute of Energy Research, 152 Gajeong-ro, Yuseong-gu, Daejeon 34129, Republic of Korea Division of Materials Science and Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul 04763, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 5 January 2016 Received in revised form 16 February 2016 Accepted 23 February 2016
The effects of colloidal alumina infiltration on porous silica-based composites for complex designed ceramic cores were investigated. The specimens pre-sintered at 1100 °C for 2 h were immersed into colloidal alumina and were sintered at 1300 °C for 2 h. Infiltrated alumina particles were coagulated on the surface of fused silica via their opposite electrical charges in the infiltrating solution. The infiltrated alumina was reacted with the surface of fused silica, and mullite was formed thereafter. The shrinkage and microcracking induced by surface crystallization of fused silica to cristobalite was prevented by mullitization. As a result, the formation of mullite by alumina infiltration for 150 min dramatically improved the flexural strength (3.3 MPa to 9.6 MPa) and reduced the linear shrinkage (2% to 1%) of the silica-based composites. However, longer infiltration time over 150 min has no significant effects on flexural strength and linear shrinkage. & 2016 Published by Elsevier Ltd.
Keywords: Fused silica Colloidal alumina Infiltration Flexural strength Shrinkage
1. Introduction Complex designed ceramic cores fabricated by injection molding have been widely used to form internal cooling passages in hollow gas turbine blades [1–3]. During investment casting of the gas turbine blades, the ceramic core undergoes thermal stress at high temperature. Ceramic cores have been manufactured using a mixture of ceramic powders based on fused silica (SiO2) and zircon (ZrSiO4) due to their excellent high temperature properties, such as thermal shock resistance and chemical inertness against molten metal [4]. Fused silica is mainly used to produce refractory materials, as well as ceramic cores, because of its low thermal expansion coefficient (0.55 10 6 K 1 between 25 °C and 1000 °C) and excellent chemical inertness against molten metal. To provide optimal flexural strength for enduring severe circumstances during investment casting, high temperature sintering is required to manufacture the ceramic cores [5]. However, high sintering temperature (Z 1300 °C) leads to crystallization of fused silica to cristobalite, thereby inducing shrinkage and microcracking of the ceramic cores. Volume contraction during β- to α-phase transformation of cristobalite leads to shrinkage of the silica-based ceramic cores. In addition, microcracks formed by volume contraction of cristobalite decrease the flexural strength of ceramic n
Corresponding authors. E-mail addresses: [email protected] (J.-g. Yeo), [email protected] (S.-C. Choi).
cores [6,7]. Consequently, the existence of cristobalite causes dimensional inaccuracy and deteriorative strength on silica-based ceramic cores. Many studies have investigated the effect of cristobalite on mechanical properties of silica-based ceramics [2,8–10]. Huseby et al. studied the shrinkage of silica-based composites with various compositions [1]. Their report indicated that the volume contraction of cristobalite during its phase transformation leads to shrinkage on silica-based composites. In addition, Breneman et al. investigated the effect of cristobalite phase on the flexural strength of fused silica [6]. They reported that the cristobalite improved flexural strength of fused silica based ceramics at 350 °C because of the existence of cristobalite for the β-phase. However, the flexural strength at room temperature was reduced due to microcracks formed by β- to α-phase transformation of cristobalite. These results demonstrate that the cristobalite in silica-based ceramics gives rise to a decrease of flexural strength and an increase of the amount of shrinkage. However, the cristobalite necessarily occurs at high sintering temperature. Therefore, in the present study, colloidal alumina was infiltrated into a silica-based ceramic core to inhibit crystallization of fused silica for improvement of the flexural strength with low shrinkage. The infiltration technique is a convenient additional process applicable to the manufacturing of the ceramic core. Preventing the crystallization of cristobalite through convenient alumina infiltrating process ahead of sintering step definitely leads to minimization of its shrinkage and reduction of the number of microcracks [9]. The present study is aimed at improving the flexural strength and reducing the amount of
http://dx.doi.org/10.1016/j.ceramint.2016.02.137 0272-8842/& 2016 Published by Elsevier Ltd.
Please cite this article as: Y.-H. Kim, et al., Shrinkage and flexural strength improvement of silica-based composites for ceramic cores by colloidal alumina infiltration, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.137i
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2
Fig. 1. SEM (1) and EDX mapping (2) images of (a) non-infiltrated, (b) 30 min infiltrated, and (c) 210 min infiltrated specimens after drying.
shrinkage via infiltration of colloidal alumina into silica-based composites for complex ceramic cores.
2. Material and methods Porous silica-based specimens for alumina infiltration were prepared using ceramic injection molding. The feedstocks for injection molding were composed of 63.75 wt% of fused silica (amorphous SiO2, 325 mesh, Imerys, United States), 21.25 wt% of zircon (ZrSiO4, 1 μm, Cenotec, Korea) and 15 wt% of thermoplastic binders (paraffin wax, microcrystalline wax, stearic acid and oleic acid). The ceramic powders of fused silica and zircon flour were ball milled for 6 hours at room temperature using a 6-mm diameter zirconia ball. After ball milling, the ceramic powders and thermoplastic binders were mixed at 80 °C for 6 h under vacuum condition to produce the feedstocks. The green bodies with dimensions of 6 mm 8 mm 90 mm according to ASTM C1161-13 were injection molded using a C-frame ceramic injection molding machine (CTM-CI-CF-35100HT, Cleveland Tools and Machines, United States) with a nozzle temperature of 80 °C, a flow rate of 400 cc/s and an injection pressure of 60 bar. After a slow heating step (0.2 K/min) to burn out the thermoplastic binders, the specimens were presintered at 1100 °C for 2 h with heating rate of 5 K/min using box furnace (UAF-15-27-LHE, Lenton, England). During pre-sintering, the specimens were positioned in alumina crucible with fused silica backfill powders. The measured apparent porosity and linear shrinkage of pre-sintered specimen were 30.1% and 0.6%, respectively. The pre-sintered specimens were infiltrated by colloidal alumina suspension (Aluminum oxide, 20% in H2O, colloidal dispersion, 50 nm, Alfa-aesar) with various infiltration times of 30 min to 300 min under vacuum condition. For evaporation of solvent, the infiltrated specimens were dried at 110 °C for 5 h. The dried specimen was sintered at 1300 °C for 2 h, along with pre-sintered (non-infiltrated) specimen for comparison. The dimensional change and infiltrated alumina content of
specimens were measured after sintering.Field-emission scanning electron microscopy (FE-SEM, S-4800, Hitachi, Japan) and EDX mapping (Link Pentafet, Oxford) were used to identify the microstructures and alumina infiltration of each specimen. The flexural strength of the specimens was measured through a 3-point bending test using a universal testing machine (UTM, H10SK, Hounsefield, England) with span size of 80 mm and crosshead speed of 1.0 mm/min. The crystallization of fused silica to cristobalite and the existence of alumina and mullite were analyzed using a X-ray diffractometer (XRD, D/Max-2200, Rigaku, Japan) using Cu Kα radiation (λ ¼ 1.54178 Å). In addition, EDX line analysis (Link Pentafet, Oxford) was used to determine the existence of mullite on the surface of fused silica and the residual alumina in the specimens.
3. Results and discussion It is well-known that the isoelectric point (IEP) of fused silica and alumina are pH 2 and pH 9, respectively [11–16]. In the range of pH 2 to pH 9, fused silica has a negative electrical charge, whereas alumina exhibits a positive electrical charge [17]. It is well known that the IEP of alumina particle is near pH 9.0. However, the colloidal alumina used in this study is alumina dispersed suspension, and the described pH on the specification is 4.0. In this study, the measured pH of the colloidal alumina solution was 3.92. The fused silica and alumina were attracted each other via their opposite electrical charges. Because fused silica had been long necked via pre-sintering at 1100 °C, alumina powder was coagulated onto the fused silica. The alumina powder already coagulated onto the fused silica via electrical attraction force, therefore, it can remain on the specimen after the solvent was pulled out via drying after infiltration [17]. EDX analysis data demonstrated the existence of infiltrated alumina in the specimens (Fig. 1). No aluminum peak was observed in the non-infiltrated specimen (Fig. 1 (a)). In contrast, aluminum was detected in the infiltrated specimens (Fig. 1(b) and (c)). The EDX data indicates that the alumina coagulated homogeneously in silica-based porous specimen via
Please cite this article as: Y.-H. Kim, et al., Shrinkage and flexural strength improvement of silica-based composites for ceramic cores by colloidal alumina infiltration, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.137i
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Fig. 2. Infiltrated alumina content and apparent porosity as a function of infiltration time.
3
Fig. 3. XRD patterns of the sintered specimens as a variation of infiltration time (●: zircon, ○: cristobalite, □: alumina, ■: mullite).
infiltration, exclusive of the fractured large fused silica particles (black area). In addition, the result shows that the increase of the alumina content depends on the infiltration time. The content of alumina (qth) on silica-based porous composites while the pores were filled completely with the colloidal alumina is calculated using the following equation [17].
⎡ qth=⎢1+ρc ⎣
(
100 − P P
)( ) ( 1 1−w
1 ρa
+
100 − ti ρw ti
)⎤⎦⎥
−1
ρc is the pre-sintered silica-based composite density (2.51 g/cm3), P is the apparent porosity of the pre-sintered specimen (30.1%), w is the weight loss of the dried colloidal alumina between 110 °C and 1100 °C (w ¼0.3), ti is the weight percent of alumina in the colloidal suspension (20 wt%), and ρa and ρw are the density of alumina (3.95 g/cm3) and liquid (1.00 g/cm3), respectively. The weight loss of the dried colloidal alumina was measured using a thermogravity/differential thermal analyzer (TG/DTA, SDT 2960 Simultaneous, TA instruments, United States). The dispersant was completely eliminated at 500 °C and weight of infiltrated specimen did not change at a temperature range of 500 °C to 1300 °C. The calculated content of infiltrated alumina using the above equation is 2.54%. The measured content of alumina and the apparent porosity of specimens are shown in Fig. 2. The alumina content in the specimen continually increased with infiltration time. The alumina content reached near the calculated value (2.54%) in the specimen infiltrated for 150 min. The apparent porosity reduces gradually with an increase of the infiltration time. The change rate of alumina content and the apparent porosity start decreasing after 210 min. The increasing concentration of alumina in the specimen observed in the saturated state at 150 min shows that most of the fused silica was coagulated with alumina. Therefore, the infiltrated alumina after 150 min of infiltration time appeared to be coagulated alumina that is already attached with silica. As shown in Fig. 3, X-ray diffraction analysis revealed the phase transformation of fused silica on silica-based composites for alumina infiltration after sintering at 1300 °C. All specimens contain zircon (JCPDS 6-266) and cristobalite (JCPDS 39-1425) phases. The cristobalite is formed by crystallization of fused silica at a temperature above 1300 °C [1,2]. Due to a low content of alumina (0.7%), the XRD pattern of the specimens infiltrated for 30 and 90 min contain only zircon and cristobalite. Alternatively, the respective mullite peaks (JCPDS 15-0776) are found in the specimens
Fig. 4. Flexural strength (s3pt) and linear shrinkage (ΔL/L0) as a function of infiltration time.
infiltrated above 150 min because the fine alumina particles coagulated on fused silica were transformed into mullite during sintering at 1300 °C, according to the following reaction [18–20]: 3Al2O3 þ 2SiO2-3Al2O3∙2SiO2 (Al6Si2O13) In particular, the specimen infiltrated for 210 min shows the existence of alumina (JCPDS 46-1212) as well as mullite. This result reveals the existence of unreacted excessive alumina in the specimen. Fig. 4 shows the flexural strength and the linear shrinkage of the specimens. The existence of cristobalite has a decisive effect on the shrinkage and flexural strength of silica-based composites because of its phase transformation [6,21,22]. As cristobalite changes from β- to α-phase, it contracts 5 vol%; accordingly, microcracks are formed [23]. The volume contraction during the phase change of cristobalite accompanied with microcracking caused a reduction of the flexural strength of the silica-based ceramics [24]. Therefore, compared with infiltrated specimens, the lower flexural strength and higher amount of shrinkage of noninfiltrated specimen (3.3 MPa and 2%, respectively) are caused by volume contraction via phase transformation of cristobalite. The formation of mullite via alumina infiltration on the silicabased composites leads to flexural strength enhancement and
Please cite this article as: Y.-H. Kim, et al., Shrinkage and flexural strength improvement of silica-based composites for ceramic cores by colloidal alumina infiltration, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.137i
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Table 1 The peak ratio of cristobalite to zircon of specimens with infiltration time. Infiltration time
Cristobalite/Zircon peak ratio
0 min (non-infiltrated) 30 min 90 min 150 min 210 min
1.081 0.880 0.692 0.501 0.491
reduction of the amount of linear shrinkage. As shown in Fig. 4, the flexural strength is gradually improved up to 10 MPa until 150 min of infiltration time. It is considered that the flexural strength is enhanced by formation of mullite (i.e., mullitization). However, no significant changes are revealed on the flexural strength of the specimen infiltrated above 210 min. This result shows the mullitization of fused silica surface specifically affect the flexural strength. The mullitization by alumina infiltration can prevent crystallization to cristobalite and the resulting generation of microcracks. In this study, the infiltrated alumina on the surface of fused silica became mullite instead of undergoing surface crystallization to produce cristobalite. Table 1 shows the main peak ratio of cristobalite (21.68°, corresponding to (110) plane) to the zircon (26.86°, corresponding to (200) plane) in the XRD patterns of the specimens. It is possible to assume that the relative degree of crystallization of fused silica in specimens by comparing the main peak intensity between cristobalite and zircon because all specimens have same content of zircon [7,9]. In addition, zircon does not react with fused silica or alumina at temperatures under 1600 °C [25,26]. As shown in Table 1, the relative content of cristobalite reduces with the increase of infiltrating time (or alumina content). The flexural strength (Fig. 4) and crystallization rate of cristobalite (Table 1) are consistent with the behavior that mullitization via alumina coagulated on fused silica inhibits surface crystallization, thereby increasing the flexural strength of silica-based ceramics. In addition, the change of the crystallization rate reduces relative to
the specimen infiltrated for 150 min, following the trend of the alumina content. This result is related to the content of infiltrated alumina in the specimens because the excessive alumina does not affect the mullitization. Changes of the shrinkage with infiltrating time showed exactly the opposite tendency with that of the flexural strength. The shrinkage of a specimen is decreased in half with the infiltrating time of 150 min. However, the shrinkage does not change when the specimen has excessive residual alumina; nor does the flexural strength. Consequently, mullitization on the surface of fused silica via alumina infiltration leads to flexural strength enhancement and shrinkage restraint due to inhibition of the crystallization of fused silica. Fig. 5 shows that the mullitization effect of reducing microcracks is accompanied by phase transformation of cristobalite. Many microcracks are shown in the microstructure of the noninfiltrated specimen (Fig. 5(a)). In spite of the alumina infiltration, the specimen infiltrated for 30 min still exhibits microcracks on fused silica (Fig. 5(b)), whereas the specimens infiltrated for over 150 min do not indicate microcracks (Fig. 5(c) and (d)). In addition, the existence of alumina (or mullite) particles on fused silica is revealed on the specimens infiltrated for greater than 150 min. These results demonstrate the inhibition effect of microcrack generation by infiltration of alumina in silica-based composites. Based on high resolution SEM and EDX measurements of the specimens, it was established that the mullitization occurred on surface of silica. Fig. 6 shows the microstructures of the specimen infiltrated for 150 min. Although the alumina was not revealed in the XRD patterns of the specimen, the excessive unreacted alumina is found in its microstructures. It is considered that the content of residual alumina is too low to detect using the X-ray diffractometer. As shown in EDX result, three zones (alumina, mullite and silica) are developed between coagulated alumina and fused silica, as shown in the SEM image of Fig. 6. The EDX analysis data shows the mullite was formed on the surface of fused silica due to the reaction between coagulated alumina and fused silica. In addition, excessive alumina formed in the alumina zone. These
Fig. 5. The SEM images of the sintered specimens with various infiltration time; (a) 0 min (non-infiltrated), (b) 30 min, (c) 150 min, and (d) 210 min.
Please cite this article as: Y.-H. Kim, et al., Shrinkage and flexural strength improvement of silica-based composites for ceramic cores by colloidal alumina infiltration, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.137i
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Fig. 6. SEM and EDX analysis results of the specimen infiltrated for 210 min.
results demonstrate that the alumina infiltration leads to mullitization on the surface of fused silica instead of the crystallization to cristobalite that is accompanied by microcracks. Consequently, the enhanced flexural strength and reduced shrinkage were obtained due to mullitization on the surface of fused silica.
4. Conclusions The presence of cristobalite reduces the mechanical properties, such as flexural strength and linear shrinkage, due to the volume contraction during its β- to α-phase transformation. Therefore, in the present study, the colloidal alumina was infiltrated on a presintered specimen to prevent the surface crystallization of fused silica to cristobalite. The infiltrated particles were coagulated in the specimens due to the opposite electrical charges between alumina and fused silica. The alumina infiltration improved the flexural strength and shrinkage of the silica–zircon composites because mullitization reduced the crystallization of fused silica to cristobalite. Due to inhibition of the crystallization to cristobalite, the improvement of the mechanical properties (i.e., the enhancement of flexural strength and the reduction of the linear shrinkage) was revealed with increase of the infiltration time up to 150 min. These significant mechanical properties are considered to rely on the cristobalite content in the specimens. Consequently, the mullitization via alumina infiltration improved the mechanical properties of silica–zircon composites. However, in the case of infiltration time of over 210 min, the strength and the shrinkage did not change. The excessive alumina had no significant effects on mechanical properties of silica–zircon composites because it did not influenced on inhibition of crystallization of fused silica to cristobalite.
Acknowledgments This work was supported by the Power Generation and Electricity Delivery Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry and Energy,
Republic of Korea (20141020102460).
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