Influence of Al2O3 content on mechanical properties of silica-based ceramic cores prepared by stereolithography

Silica ceramic cores have played an important part in the manufacture of hollow blades due to their excellent chemical stability and moderate high-temperature mechanical properties. In this study, silica-based ceramics were prepared with Al2O3 addition by stereolithography, and the influence of Al2O3 content on mechanical properties of the silica-based ceramics was investigated. The Al2O3 in silica-based ceramics can improve the mechanical properties by playing a role as a seed for the crystallization of fused silica into cristobalite. As a result, with the increase of Al2O3 content, the linear shrinkage of the silica-based ceramics first decreased and then increased, while the room-temperature flexural strength and the high-temperature flexural strength first increased and then decreased. As the Al2O3 content increased to 1.0 vol%, the linear shrinkage was reduced to 1.64% because of the blocked viscous flow caused by Al2O3. Meanwhile, the room-temperature flexural strength and the high-temperature flexural strength were improved to 20.38 and 21.43 MPa with 1.0 vol% Al2O3, respectively, due to the increased α-cristobalite and β-cristobalite content. Therefore, using the optimal content of Al2O3 in silica-based ceramics can provide excellent mechanical properties, which are suitable for the application of ceramic cores in the manufacturing of hollow blades.


Introduction 
Silica-based ceramic cores with excellent thermal stability which will limit the application and development of silica ceramic cores [6][7][8]. In order to deal with these problems relevant to the forming of complicated ceramic parts, a new advanced manufacturing technology-3D printing technology has been developed [9][10][11][12]. Compared with other ceramic 3D printing technologies, stereolithography possesses higher forming precision and better surface quality. The technology can use the laser beam to cure ceramic slurry layer by layer and obtain ceramic parts with complex structure by debinding and sintering process [13][14][15][16][17].
The forming process, sintering process, and raw materials have a great influence on the microstructure and mechanical properties of 3D-printed silica ceramics, whereas cracks often appear in the sintering stage because of shrinkage and phase transformation. Li et al. [18] investigated the effects of sintering temperatures on the mechanical properties of 3D-printed silica ceramic cores based on stereolithography. The room-temperature flexural strength of silica ceramic cores reached a maximum value of 12.1 MPa at the sintering temperature of 1300 ℃. Kotz et al. [19] realized the significance of raw materials for preparing high-quality fused silica glass. They could create arbitrary macroand microstructures from photocurable silica nanocomposites, thereby broadening the selection of 3D printing materials. Cai et al. [20] pointed out the influence of solid loading and mixing methods on silica glass fabricated by digital light processing (DLP). Their experimental data showed that the viscosity of slurry could be reduced effectively by the multi-step mixing method compared with the one-step mixing method. Liu et al. [21] noticed the efficiency problem of fabricating silica glass by stereolithography, thus developing a stereolithography system using top-down method and obtaining the glassware with similar properties to fused silica.
Although stereolithography provides a practical technical way for the efficient and high-quality manufacturing of silica ceramic parts [22,23], there exist some research difficulties. Firstly, the forming mode of layer-by-layer curing can easily cause larger linear shrinkage [24], which will not only affect the mechanical properties of ceramic parts, but also limit the wide application of ceramic products [25,26]. Secondly, 3D-printed silica ceramics show lower flexural strength. In addition, the high-temperature strength and room-temperature strength of 3D-printed silica ceramics are rarely investigated at the same time, which may have a strong influence on the performance of ceramic cores. At present, the reinforcements in silica ceramics, such as cristobalite [27] or silicon carbide [4], can improve the mechanical properties effectively by promoting the development of cristobalite. Furthermore, Al 2 O 3 has a fairly high flexural strength of 472 MPa [28] and thus contributes to enhancing the flexural strength of silica ceramics. Al 2 O 3 can be also very helpful in reducing the shrinkage of silica ceramics by promoting the crystallization of the fused silica [29]. Therefore, the Al 2 O 3 is promising to be regarded as a powerful reinforcement in silica ceramic cores.
In this work, silica-based ceramics added to various Al 2 O 3 contents via stereolithography were fabricated. The influence of Al 2 O 3 content on the mechanical properties of silica-based ceramics was investigated, and the room-temperature and the high-temperature mechanical properties of the as-obtained silica-based ceramics were both examined.

2 Sample preparation
The solid loading of each ceramic slurry was set to 45 vol%. To match with the solid loading, the Al 2 O 3 contents of the ceramic powders were 0, 0.5, 1.0, 1.5, and 2.0 vol%. When weighing Al 2 O 3 in the preparation of the slurry, the corresponding mass was converted according to the density of 4.0 g/cm 3 provided by the powder manufacturer, and then the composite powders were ball-milled in vacuum. The ceramic slurries were obtained by milling the mixture about 30 min at room temperature. Table 1 shows the viscosity of the slurries with different contents of Al 2 O 3 at the same shear rate of 50 s −1 . As a result, all the slurries had good liquidity that the viscosity of them was less than 10 Pa·s, which is suitable for forming by stereolithography [30,31]. After obtaining the slurries, the green bodies were formed by a 3D printer (Wuhan Intelligent Laser Technology Co., Ltd., China). And the sizes of the ceramic green bodies used to test flexural strength were 60 mm × 5 mm × 3.6 mm (L × W × H). In this 3D printing mode, the z-axis displacement distance of the 3D printer for the green bodies was set to 3.6 mm. With the process parameters optimized, the laser power was 0.75 W, the scanning speed was 3000 mm/s, and the thickness of each layer was 50 m.
A thermogravimetric analyzer (TGA; PerkinElmer Instrument (Shanghai) Co., Ltd., China) was used to analyze the powder ground from green bodies. The thermogravimetric curve shown in Fig. 2(a) was obtained, and the optimal debinding and sintering process could be determined from the curve analysis. As shown in Fig. 2(b), the resin could be completely removed by holding at 600 ℃ for 2 h. Sintered at 1200 ℃ for about 6 h, silica-based ceramic samples could achieve a higher degree of densification.

3 Characterization
The particle size distribution was measured by a laser  diffraction particle size analyzer (Mastersizer 3000, UK). The phase transformations of ceramic samples were detected by an X-ray diffractometer (XRD-6100, Shimadzu, Japan). Meanwhile, scanning electron microscopy (SEM) images were observed by field-emission SEM (Quanta650 FEG, FEI, USA). The bulk density and apparent porosity of the silica-based ceramics were measured by the Archimedes principle [32]. To measure the flexural strength of the silica-based ceramic samples at room temperature, an electronic universal testing machine (E1000, ITW Group Instron Inc., USA) was used. The span was adjusted to 30 mm, and the loading rate was set to 0.5 mm/min. Five samples fabricated with each Al 2 O 3 content were tested, and the average value was taken as the reference value of the room-temperature flexural strength. Moreover, the high-temperature strength was measured with a heating rate of 5 ℃/min up to 1550 ℃, using a ceramic flexural strength tester (WDW-20, Shenyang Foundry Research Institute Co., Ltd., China). Test methods were performed consistent with the standard HB5352. . The formula adopted to calculate the flexural strength (σ) is as follows: where F is the load when the test sample is broken, L is the span between the two fulcrums, b is the width of the test sample, and h is the thickness of the test sample.

Results and discussion
The phase compositions of silica-based ceramics with various Al 2 O 3 contents are gathered in Fig. 3. All sintered ceramics contain quartz resulting from the crystallization of the fused silica. The fused silica has crystallized into quartz at the sintering temperature of 1200 ℃ [33]. On the XRD patterns, the peak of the Al 2 O 3 pattern is not very evident due to its low content. Meanwhile, the cristobalite peaks located at about 21.80° are detected in all-ceramic samples that contain  regardless of the Al 2 O 3 content. To explore the elemental composition of the sintered ceramics, the ceramic samples sintered at 1200 ℃ are observed by energy dispersive X-ray spectroscopy (EDS) and the data are obtained. As shown in Fig. 4(f), the Al and Si elements are both found in the ceramic samples containing 1.5 vol% Al 2 O 3 . Meanwhile, plenty of SiO 2 crystals with 3D network structure can be seen because the crystallization temperature of the fused silica is less than 1200 ℃ [34]. In the fracture of the silica ceramic without adding Al 2 O 3 , the grains are tightly bonded. After adding Al 2 O 3 , there are more pores in silica-based ceramics containing 0.5 and 1.0 vol% Al 2 O 3 than that without Al 2 O 3 . Thus it can be seen that Al 2 O 3 has some inhibitory effects on the binding between grains. As the Al 2 O 3 increases to 1.5 and 2.0 vol%, the ceramic grains grow coarsely, weakening the inhibitory effect to a certain extent. As shown in Fig. 5(a), with the decrease of temperature during the cooling process, β-cristobalite gradually transforms into α-cristobalite in turn, demonstrating the existence of the phase transformation in the silica-based ceramics. Meanwhile, mullite is formed because of the reaction between quartz and Al 2 O 3 at the high temperature of 1550 ℃, as presented in Eq. (2). In addition, the silica-based ceramics measured at 1550 ℃ with 1.0 vol% Al 2 O 3 generate liquid phase, as presented in Fig. 5(b), which is absent under the sintering temperature of 1200 ℃ , as suggested in Fig. 4. The liquid phase is generated by the melting silica at high temperatures, connecting interlayer independent grains [18], as well as increasing the adhesion between ceramic grains and contributing to improved mechanical properties of silica-based ceramics. Furthermore, visible cracks appear in the silica-based ceramics measured at 1550 ℃ due to the phase transformation from α-cristobalite to stable β-cristobalite during the thermal treatment process.  cristobalite, can promote the crystallization of the fused silica, but cause relatively small shrinkage. Additionally, the crystalline phases also hinder the viscous flow that is the driving force of the densification of the fused silica, which becomes an obstacle to the shrinkage [35,36]. Consequently, the addition of Al 2 O 3 degrades the linear shrinkage of the silica-based ceramics. Nevertheless, the linear shrinkage is upgraded to 2.52% with excessive Al 2 O 3 (1.5 and 2.0 vol%), which is still lower than the silica ceramics without Al 2 O 3 . When the Al 2 O 3 content increases to 2.0 vol%, the Al 2 O 3 improves the densification of the ceramics by promoting the growth of coarse grains, as shown in Fig. 4. The improvement has nullified part of the weakening effect caused by the crystalline phases, bringing about the increased linear shrinkage of the silica-based ceramics.
The bulk density and the apparent porosity results of the silica-based ceramics sintered at 1200 ℃ are shown in Fig. 6(b). When the Al 2 O 3 increases from 0 to 1.0 vol%, the fused silica is transformed into β-cristobalite with cubic structure during high-temperature sintering due to the emergence of Al 2 O 3 . Meanwhile, around 250 ℃ in the cooling course, the phase transition from β-cristobalite to α-cristobalite with tetragonal structure is realized [37,38]. The phase transition will bring a 5% reduction of volume, which is likely to cause the propagation of microcracks in silica ceramics [38,39]. Under the combined effect of phase transition and crystallization, the apparent porosity increases and the bulk density decreases. With Al 2 O 3 added to 2.0 vol%, the ceramic grains grow coarsely due to the effect of mass transfer between phases induced by Al 2 O 3 . The volume change resulting from coarse grains can improve the densification of silica-based ceramics. At this moment, the improvement of the densification overcomes the weakening effect of phase transition, which leads to the decrease of the apparent porosity and the increase of the bulk density.
The flexural strength of the silica-based ceramics prepared from green bodies with various contents of Al 2 O 3 printed by the 3D printer is presented in Fig. 7. It is indicated that the flexural strength of the ceramics all shows an opposite tendency to the linear shrinkage. The room-temperature flexural strength of the ceramic samples is improved from 11.56 to 20.38 MPa as the Al 2 O 3 increases to 1.0 vol%. Compared with the amorphous silica, cristobalite has a higher strength. Therefore, the crystallization of the fused silica into cristobalite can improve the room-temperature flexural strength of the silica ceramics [40,41]. However, the room-temperature flexural strength of the samples decreases when the Al 2 O 3 increases to 1.5 and 2.0 vol%. Although cristobalite provides silica-based ceramics with better strength, excessive cristobalite may weaken the trend in turn. The flexural strength of the fused silica ceramics will decrease when the generation of cristobalite is more than 30% [40]. In this study, it seems easier for the fused silica added Al 2 O 3 to crystallize into α-cristobalite. As a result, the room-temperature flexural strength is enhanced as the Al 2 O 3 increases to 1.0 vol%. But excessive Al 2 O 3 (1.5 and 2.0 vol%) reduces the internal binding force by accelerating the nucleation of the crystallization process, thereby degrading the flexural strength. Furthermore, both the flexural strength and the apparent porosity of the silica-based ceramics increase as the Al 2 O 3 increases to 1.0 vol%. Although the volume reduction brought by the phase transition is disadvantageous to the strength, the enhancement of cristobalite induced by Al 2 O 3 is much stronger than the weakening effect. Thus, the flexural strength of the silica-based ceramics is also improved with the increase of the apparent porosity. As for the high-temperature flexural strength, it is improved from 9.39 to 21.43 MPa as the Al 2 O 3 increases to 1.0 vol%, but excessive Al 2 O 3 (1.5 and 2.0 vol%) reduces the strength. Obviously, the trend is similar to the flexural strength at room temperature. What's more, there are some different degrees of improvement compared with the room-temperature flexural strength under the same content of Al 2 O 3 . The surface of the fused silica grains will spontaneously separate stable β-cristobalite crystals out at high temperature [42,43], thus improving the high-temperature strength of silica-based ceramics. However, when the content of β-cristobalite is too high, the volume effect caused by the secondary crystal transformation of cristobalite produces more microcracks inside the silica ceramics, reducing the high-temperature flexural strength. Arguably, the β-cristobalite generated under the high temperature of 1550 ℃ is highly responsible for improving the high-temperature flexural strength of silica-based ceramics due to its high strength.

Conclusions
In this study, the silica-based green bodies were fabricated via stereolithography, and the influence of Al 2 O 3 content on mechanical properties and microstructures of the as-sintered silica-based ceramics was systematically investigated. With the increase of Al 2 O 3 content, the linear shrinkage of the sintered ceramics first decreased and then increased, while the room-temperature flexural strength and the high-temperature flexural strength first increased and then decreased. With Al 2 O 3 added to 1.0 vol%, the linear shrinkage reached 1.64% due to the blocked viscous flow. Nevertheless, the excessive Al 2 O 3 (1.5 and 2.0 vol%) could increase the linear shrinkage by promoting the growth of coarse grains which can improve the densification of silica-based ceramics. The Al 2 O 3 in the silica-based ceramics served as a seed for the crystallization of the fused silica into cristobalite, thereby improving the mechanical properties. Therefore, the room-temperature flexural strength was enhanced to 20.38 MPa and the high-temperature flexural strength was improved to 21.43 MPa, as the Al 2 O 3 content increased to 1.0 vol%. However, a large amount of cristobalite generated by the high content of Al 2 O 3 could easily reduce the internal binding force and cause the propagation of microcracks inside the silica-based ceramics, thus degrading the roomtemperature flexural strength and the high-temperature flexural strength. All in all, the results suggest that using the optimal content of Al 2 O 3 provides optimum mechanical properties, which are appropriate for the wide application of silica-based ceramic cores and contribute to the manufacturing of hollow blades.