- Email: [email protected]
High strength silica-based ceramics material for investment casting applications: Effects of adding nanosized alumina coatings
Ceramics International xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
High strength silica-based ceramics material for investment casting applications: Effects of adding nanosized alumina coatings Xiao Chena, Chunyang Liub, Wenlong Zhengb, Jiqing Hanb, Li Zhangb,∗, Chunming Liua a b
School of Materials Science and Engineering, Northeastern University, Shenyang, 110819, Liaoning, PR China School of Metallurgy, Northeastern University, Shenyang, 110819, Liaoning, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Nanosized alumina coating Silica-based ceramic core Cristobalite Flexural strength
A nanosized alumina coating was synthesized on the surface of fused silica particles by electrostatic attraction. The effects of the coated fused silica particles on the cristobalite crystallization behavior, microstructure evolution, and flexural strength of silica-based ceramic cores were investigated. X-ray diffraction (XRD) was used to characterize phase transformations in the specimens, and the results indicated that the formed nanosized alumina coatings could retard cristobalite formation by inducing compressive stress on the fused silica particle surface. A mullite phase was also found due to the reaction of the nanosized alumina coating and the surface of the fused silica when the sintering temperature was increased to 1300 °C. Analysis using scanning electron microscopy equipped with energy dispersive spectrometry (SEM/EDS) suggested that alumina nanoparticles in the coated layer dispersed into a liquid phase and formed a barrier layer to impede the movement of the liquid phase, preventing the pore-filling process and increasing the open porosity of the ceramic specimens. Flexural strengths at room temperature were tested, indicating that increases in the sintering temperature of the specimens without coated fused silica powders had little effect on flexural strength. However, the flexural strength of the specimens with coated fused silica powders increased with increases in sintering temperature. The improvement in flexural strength was related to the reinforcement by sintering necks between particles and the improvement in the strength of the coated fused silica powder.
1. Introduction Silica-based ceramics have been wildly utilized for high-temperature special structural materials and are used especially in investment metal casting to form the intricate internal cooling passages of superalloy gas turbine blades [1–4] due to their low thermal expansion coefficients (0.55 × 10−6/K between 25 and 1000 °C), excellent chemical inertness against molten metal and easy removability by a process that is not harmful to the cast [5,6]. To produce acceptable inner surface finishes and wall thicknesses for the blades, the properties of the silica-based ceramic cores must be balanced carefully, such as their mechanical strength, dimensional conformity and good gas permeability during casting. However, one of the crucial obstacles associated with engineering applications is obtaining optimal flexural strength for silica-based ceramic cores to endure the severe circumstances during casting, because the flexural strength of fused silica for the core material does not exceed 6 MPa without the use of sintering aids or additives [7]. Generally, to achieve high flexural strengths for silica-based ceramic
∗
cores, the fabrication process should be carried out at temperatures under 1300 °C. This is because higher sintering temperatures lead to the crystallization of fused silica to cristobalite, which induces shrinkage and microcracking in silica-based ceramic materials. Microcracks formed by volume contraction during the β-cristobalite to α-cristobalite phase transformation during cooling decrease the flexural strength of silica-based ceramic cores [8,9]. By using additives, the cristobalite crystallization and flexural strength of silica-based ceramics can be controlled. A number of additives have been used to improve the flexural strength of silica-based ceramic cores, including alumina [19], cristobalite [10], zircon [11–13], and silicon carbide [14]. Kim [15] used mullitization to consume the cristobalite on the silica surface and inhibit the generation of cracks. In their study, the specimens were presintered at a lower temperature (< 1200 °C) for 2 h, then immersed in colloidal alumina and heated at a higher temperature (> 1300 °C) for 2 h. The infiltrated alumina reacted with cristobalite on the silica surface to form mullite. The lower total cristobalite content on the silica particle surfaces and therefore the formation of fewer microcracks led to an enhancement of flexural strength.
Corresponding author. E-mail address: [email protected] (L. Zhang).
https://doi.org/10.1016/j.ceramint.2019.08.248 Received 16 July 2019; Received in revised form 19 August 2019; Accepted 26 August 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Xiao Chen, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.08.248
Ceramics International xxx (xxxx) xxx–xxx
X. Chen, et al.
content, while the finer fused silica content and zircon content were constant at 18 wt% and 10 wt%, respectively. The prepared bar specimens were then sintered at 1200, 1250, 1300, and 1350 °C for 2 h. To provide good support and prevent deformations or cracks in the specimens during sintering, industrial alumina powders with a purity of 99.6 wt% and average particle size of approximately 91.9 μm were used as a powder bed.
On the other hand, depending on the fracture mechanism, the improved flexural strength of silica-based ceramics can be attributed to either the transgranular fracture mode or to the intergranular fracture mode. For these two fracture mechanisms, the improvement in powder strength and the reinforcement in sintering necks play an important role for the flexural strength of silica-based ceramic cores. For example, Kazemi [11] suggested that an improvement in the strength of the raw material particles is conducive to the intergranular fracture mode. In their study, the strength increased slightly with increasing zircon content in silica-based ceramic cores, and the intergranular fracture behavior was found in the zircon particles. The inhibiting effect of zircon particles on the crack growth was mainly due to the fact that the strength of zircon particles was higher than that of fused silica. In addition, Xu [16] suggested that the resintering process reinforced the sintering necks between particles and changed the crack propagation from the sintering necks between particles to entire large particles. They demonstrated that the transition from the intergranular to transgranular fracture mode increased the strength of silica-based ceramic cores. Hence, high flexural strength silica-based ceramic cores may be created by decreasing the total cristobalite content, improving the raw material particle strength and reinforcing the sintering necks between particles. Therefore, to further improve the flexural strength of silicabased ceramic cores, a new processing route is explored in this study. First, a nanosized alumina coating was synthesized on the surface of fused silica particles by electrostatic attraction to develop a “core-shell” structure in the fused silica particles. Then, the silica-based ceramic cores with different contents of coated fused silica powders were sintered at 1200, 1250, 1300 and 1350 °C. The effects of the coated fused silica powders on the cristobalite crystallization, microstructure evolution, flexural strength and fracture mechanism of the silica-based ceramic cores was investigated.
2.3. Characterization The phase and morphology of nanosized alumina-coated fused silica powders and four kinds of sintered ceramic cores were analyzed by Xray powder diffraction using Ni-filtered Cu Kα radiation (XRD, Smart Lab 9 kW, Rigaku, Japan) and by scanning electron microscopy (SEM/ EDS, Quanta250FEG, FEI, America), respectively. To investigate the effect of nanosized alumina coatings on the cristobalite crystallization of the fused silica particles, the crystal quantitative analysis of the specimens was determined using the reference intensity ratio (RIR) method [18]. To study the effect of nanosized alumina-coated fused silica powders on the fracture mechanisms of silica-based ceramic cores, the sintered specimens were measured with a three-point bending test with a span distance of 30 mm and a loading rate of 5 N/s on a universal testing machine (UTM, AG-XPlus, Shimadzu, Japan). The open porosities of all specimens were also determined by the Archimedes method, using distilled water as an immersion fluid. The results of each report averaged three tests. 3. Results and discussion 3.1. Characterization of nanosized alumina-coated fused silica powders The XRD patterns, the SEM micrographs of the as-received fused silica powders and the nanosized alumina-coated fused silica powders, and the element mapping by EDS are shown in Fig. 1. There was no diffraction peak in the XRD pattern of as-received fused silica powders, see Fig. 1(a), suggesting that the as-received fused silica is amorphous. Compared with the XRD pattern of the as-received fused silica powders, the diffraction peaks corresponding to the Al2O3 phase in the coated fused silica powders can be clearly identified. Fig. 1(b) and (c) are the SEM micrographs of the as-received fused silica powder and the coated fused silica powder, respectively. As shown in Fig. 1(b), as-received fused silica powder presents a clean surface, and the grain shape is polygonal. From Fig. 1(c) shows that the coated fused silica powder presents a rough surface. The sharp edges of the fused silica powders become dulled and rounded after coating, indicating that the alumina nanoparticles might have adhered to the surface of the fused silica powders. EDS further reveals the existence of alumina nanoparticles on the fused silica powder surfaces, as shown in Fig. 1(d) and (e).
2. Methods 2.1. Preparation of nanosized alumina coatings To prepare the nanosized alumina-coated fused silica powders, fused silica powder (amorphous SiO2, 40.4 μm, 98.76%) and nanosized colloidal alumina (alumina oxide, 10–20 nm, 40% in H2O, colloidal dispersion) were used as raw materials. First, fused silica (1000 g) in 500 ml of deionized water was mixed with 500 ml of nanosized colloidal alumina and stirred at 500 rpm for 5 h at room temperature. Regarding fused silica, the hydroxyl groups were pushed to the outside to form a negatively charged layer in deionized water, which adsorbs positively charged alumina nanoparticles via their opposite electrical charges [17]. Then, the resulting mixture was dried and annealed at 1000 °C for 2 h. As a result, the nanosized alumina-coated fused silica powders were prepared after sieving to obtain ~200 mesh particles, because the alumina nanoparticles had already coagulated onto the fused silica surface as a result of the electrical attraction force.
3.2. Characterization of the silica-based ceramic cores 2.2. Fabrication of silica-based ceramic cores It is confirmed that a thickness nanosized alumina coating could adhere to the fused silica powder surface through electrostatic attraction based on the results presented in Fig. 1. Therefore, to know how the nanosized alumina coating affects crystallization behavior and the mechanical properties of silica-based ceramic cores, the following work was performed. Fig. 2 shows the XRD patterns of the silica-based ceramic cores with different coated fused silica powders contents sintered at different temperatures (1200, 1250, 1300 and 1350 °C). As shown in Fig. 2(a) and (b), the crystallization behavior begins at 1300 °C in specimen NA42. Compared with the XRD pattern of specimen NA-0, the cristobalite peaks intensities decrease in all specimens with coated fused silica powders.
The mixture for the fabrication of the silica-based ceramic core consisted of coarse fused silica, finer fused silica, coated fused silica and zircon. Table 1 shows the characteristics of the raw materials. The purity and specific surface area of raw materials were determined by Xray fluorescence spectroscopy (XRF) and a laser particle size analyzer (LPSA), respectively. The four kinds of silica-based ceramic core specimens, containing 0, 21, 42, and 63 wt% coated fused silica, were denoted as NA-0, NA-21, NA-42, and NA-63, respectively, and were injection molded into test bar shapes with dimensions of 120 × 10 × 4 mm3. Coarse fused silica and coated fused silica were used to form the skeleton, and increases in the coated fused silica content were accompanied by decreases in the coarse fused silica 2
Ceramics International xxx (xxxx) xxx–xxx
X. Chen, et al.
Table 1 Characteristics of the coarse fused silica, finer fused silica, coated fused silica and zircon. Powder (%)
Purity (%)
Coarse fused silica Finer fused silica Coated fused silica
98.76 99.29 80.48
Zircon
94.49
Important impurities (Type-%)
Al2O3 Al2O3 Al2O3 Fe3O4 Na2O K2O HfO2 Al2O3 Sb2O3
Specific surface area (m2/g)
PSD
1.19 0.57 19.35 0.088 0.033 0.012 1.352 0.740 0.689
D10 (μm)
D50 (μm)
D90 (μm)
6.14 2.99 4.93
40.4 16.9 29.3
91.0 48.5 70.3
0.415 0.768 0.494
1.51
12.8
45.7
1.361
intensities decrease with increasing amounts of coated fused silica powders in sintered specimens, as shown in Fig. 2(b). The cristobalite contents in the sintered specimens NA-21, NA-42, and NA-63 were 8.35, 6.27, and 4.44%, respectively. This behavior can be understood by the mullitization behavior. The diffraction peaks of the mullite phase are detected at 1300 °C, as shown in Fig. 2(a) and (b). Since cristobalite crystallization always occurs on the surfaces of fused silica particles [11], the formed cristobalite is consumed by alumina nanoparticles that have coagulated on the fused silica surface during sintering at elevated temperatures. As shown in Fig. 5, the EDX analysis data show that the mullite phase forms on the surface of fused silica at 1300 °C. That mullitization can take place at lower temperatures with nanosized alumina is in agreement with Young-Hwan Kim and Gye Seok An [15,20]. Fig. 6 shows the open porosity of the silica-based ceramic cores with different coated fused silica powder contents sintered at different temperatures (1200, 1250, 1300, and 1350 °C). The open porosity of the sintered specimens decreases with increases in sintering temperature regardless of the coated fused silica powder content. As the sintering temperature increases from 1200 °C to 1350 °C, the open porosity decreases from 34.69% to 28.84% for specimen NA-0, from 34.34% to 28.02% for specimens NA-21, from 34.60% to 30.01% for specimen NA-42, and from 34.49% to 30.35% for specimen NA-63. With an increase in sintering temperature, amorphous fused silica can easily migrate through surface diffusion; as shown in Fig. 7(a), the particles connect to each other and the pores become round during this process. When the liquid phase occurs, the volume diffusion mechanism is
In a previous study [10,19], the influence of microsized alumina addition on fused silica was investigated, indicating that alumina can affect the crystallization rate of fused silica through its solution inside amorphous silica particles. The dissolved alumina increased the number of nonbridged oxygen atoms and the diffusion mobility of SiO4 tetrahedrons, improving the crystallization tendency of fused silica. However, it seems that the nanosized alumina coating does not act in the same manner as the microsized alumina particles in fused silica. The nanosized alumina coating (see Fig. 3(a)) generates a compressive stress on the fused silica particles’ surface regions and then retards the crystallization tendency. This is because the thermal expansion coefficient of alumina is greater than that for fused silica. In addition, an auxiliary experiment was designed to simulate this mechanism, and the effect of compressive stress on the inhibition of the transformation from fused silica to cristobalite was proven. As shown in Fig. 3(b), the specimens FS and FSA are formed by dry-pressing and were sintered at 1300 °C for 2 h. After removing the alumina coating from specimen FSA (the alumina coating on the surface of the specimen had a low sintering degree and could be easily removed by mechanical means), 10 wt% zircon fine powder (as an internal standard) was added to the sintered ceramic specimens before XRD testing. The XRD pattern results are shown in Fig. 4, which shows that the cristobalite peak intensity in the specimen FSA is lower than that for the normal specimen FS without an alumina coating, indicating that the compressive stress has an inhibitory effect on the transformation from fused silica to cristobalite, as predicted. When the sintering temperature is 1300 °C, the cristobalite peak
Fig. 1. (a) XRD patterns of the as-received fused silica powders and nanosized alumina-coated fused silica powders; (b)–(c) SEM images of the as-received fused silica powder and nanosized alumina-coated fused silica powder; (e) EDS mapping of nanosized alumina-coated fused silica powders in Fig. (d). 3
Ceramics International xxx (xxxx) xxx–xxx
X. Chen, et al.
Fig. 2. (a) XRD patterns for specimen NA-42 sintered at 1200, 1250, 1300 and 1350 °C for 2 h; (b) XRD patterns for the specimens NA-0, NA-21, NA-42 and NA-63 sintered at 1300 °C for 2 h.
enhanced, the sintering necks between particles become coarse and the pores gradually shrink and disappear. As shown in Fig. 7(b), a layer of liquid phase is clearly observed on the particle surfaces, which promotes the coarsening of the sintering necks between particles. When the sintering temperature continues to rise, the liquid phase fills the pores between particles, inducing the initiation of consolidation. As shown in Fig. 7(c), the pores nearly disappear, but cracks form on the silica surface due to a large amount of cristobalite formation. Therefore, the decrease in open porosity can be attributed to the surface diffusion of amorphous fused silica and the volume diffusion of the liquid phase with increasing sintering temperatures. However, coated fused silica powders have a complex influence on the open porosity of silica-based ceramic cores. When the coated fused silica content increases to 21 wt%, the open porosity of a specimen sintered at 1300 °C decreases from 31.43% to 30.55%. As shown in Fig. 7(d), the single small particles are not observed in specimen NA-21 since they are bonded together by the sintering necks to form complete larger particles, and the pores between these particles are filled by the liquid phase. Fig. 8 is the EDS results of liquid phase (marked as 1) and silica particle (marked as 2) regions of specimen NA-21 sintered at 1300 °C. In addition to large amounts of Si and O, some elements including 4.96 wt% Al and 1.48 wt% Na can also be detected in the liquid phase. That is, the presence of Al and Na accelerates the formation of the liquid phase, and the increased liquid phase fills the pores, reducing the open porosity of sintered specimens. The increased liquid phase is related to the formation of a glass phase (fused silica) and low melting liquid phase. In silica systems, the impurities or oxide mineralizers can affect the production of the glass phase by changing the structure of the amorphous fused silica. It has been reported that the dissolved alumina can act as a modifier ([AlO6]) in the absence of alkaline oxides (Na2O/Al2O3 < 1), which can break the silicon oxygen bonds and increase the number of non-bridge oxygen atoms, consequently reducing the connection degree of the network structure of fused silica [21]. Therefore, the presence of Al2O3 and Na2O accelerates the softening of amorphous fused silica, turning it into a liquid glass phase during sintering. In addition, with the increase of sintering temperature, both Na2O and Al2O3 can react with SiO2 to form the low melting liquid phase (for example, Na2O·2SiO2, Na2O·Al2O3·SiO2, and 3Al2O3·2SiO2), which increases the liquid content in sintered specimens. When the coated fused silica content increases from 21 wt% to 63 wt %, the open porosity of specimens sintered at 1300 °C increases from 30.55% to 32.87%. This is related to the “core-shell” structure of coated fused silica particles, which inhibits the coarsening of the sintering necks. It is known that the diffusion activation energy of alumina is higher than that of fused silica, so it is not easy for sintering necks to
Fig. 3. Schematic diagram: (a) the effect of alumina nanoparticles coating on fused silica powder; (b) a supplementary experiment.
Fig. 4. XRD patterns of supplementary experiment specimens sintered at 1300 °C for 2 h. 4
Ceramics International xxx (xxxx) xxx–xxx
X. Chen, et al.
Fig. 5. SEM and EDS analysis results of specimen NA-63 sintered at 1300 °C.
process.
3.3. Mechanical properties Fig. 10 shows the flexural strength of silica-based ceramic cores with different coated fused silica powder contents sintered at different temperatures (1200, 1250, 1300, and 1350 °C). The flexural strength of specimen NA-0 is 7.83 MPa for 1200 °C, 8.04 MPa for 1250 °C, 8.98 MPa for 1300 °C, and 8.01 MPa for 1350 °C. It is found that the sintering temperature for specimens without coated fused silica powders has little effect on flexural strength. The XRD patterns of specimen NA-0 sintered at different temperatures are shown in Fig. 11(a); the cristobalite content is 20.66% for 1200 °C, 22.10% for 1250 °C, 32.71% for 1300 °C, and 47.41% for 1350 °C. With increases in sintering temperature, the cristobalite content increases sharply. The cracks in the fused silica particles are clearly visible due to the volume contraction of the cristobalite grains, as shown in Fig. 11(b) and (c). The higher the total cristobalite content, the more microcracks are formed. This is important evidence of the decreased flexural strength for specimen NA0 when the sintering temperature increases to 1350 °C. In contrast, the flexural strength for the specimens fabricated with coated fused silica powders can be effectively improved by increasing the sintering temperature. For example, specimens NA-21, NA-42, and NA-63 exhibited flexural strengths of 14.85 MPa, 14.94 MPa, and 15.88 MPa when sintered at 1350 °C, respectively, which are greater than for other specimens with conventional additives [11,15,19,22]. The high flexural strengths can be attributed to the reinforcement of the sintering necks with the increase in sintering temperature. To gain an understanding of the fracture mechanism, the fracture morphologies of the specimens sintered at different temperatures were examined. As shown in Fig. 12, when the sintering temperature increases from
Fig. 6. Open porosity of the silica-based ceramic core specimens.
form between coated fused silica particles through surface diffusion. As shown in Fig. 7(e) and (f), the particle size decreases significantly, and the fine particles do not adhere to the leading particles to form complete large particles in specimens NA-42 and NA-63. Additionally, nanosized alumina coatings form a barrier layer that impedes the movement of the liquid phase between the coated fused silica particles, preventing the pore filling process. As shown in Fig. 9(a), some alumina nanoparticles dispersed in the liquid phase and then moved with it to fill the pores. Therefore, the alumina nanoparticles are mainly concentrated around the pores between particles, as shown in Fig. 9(b). With an increase in alumina nanoparticles, the viscosity of the liquid phase increases. These alumina nanoparticles form a barrier layer to prevent the movement of the liquid phase, inhibiting the pore filling
Fig. 7. Microstructures of the specimens after being sintered at different temperatures: NA-0 sintered at (a) 1200 °C; (b) 1250 °C; (c) 1300 °C. Specimens sintered at 1300 °C (d) NA-21; (e) NA-42; (f) NA-63. 5
Ceramics International xxx (xxxx) xxx–xxx
X. Chen, et al.
Fig. 8. EDS results of liquid phase (marked as 1) and silica particle (marked as 2) region of specimen NA-21 sintered at 1300 °C.
Fig. 9. EDS mapping of the polished surface of sample NA-21 sintered at 1300 °C for 2 h.
fracture mechanism changes from intergranular to transgranular with increasing sintering temperatures. It can be inferred that the sintering necks produced at high temperatures play an important role in the mechanical properties of ceramic specimens. However, with the increase in coated fused silica powders, specimens NA-42 and NA-63 depict mixed fracture behavior consisting of both intergranular and transgranular fracture surfaces. This change in fracture mode does not decrease the strength of the ceramic specimens, and this can be attributed to the reinforcement of the coated fused silica particles. This is because both the decrease in cracks and the formation of mullite caused by the nanosized alumina coatings are beneficial for the improved strength of coated fused silica powders, which can resist crack growth. Therefore, an increase in coated fused silica content accompanied with a decrease in fused silica content, results in a slight increase in the flexural strength of silica-based ceramic cores. Fig. 10. Flexural strengths of the sintered silica-based ceramic cores.
4. Conclusions 1200 °C to 1300 °C, the sintering necks between particles are strengthened. Therefore, the image of ceramic specimens sintered at 1200 °C shows intact particles on the fracture surfaces, while the image of specimens sintered at 1300 °C shows fractured particles. Hence, the
In this work, the influence of nanosized alumina coated fused silica particles on the cristobalite crystallization behavior, microstructure evolution, and mechanical properties of silica-based ceramic cores was investigated. The main results are summarized as follows: 6
Ceramics International xxx (xxxx) xxx–xxx
X. Chen, et al.
Fig. 11. (a) XRD patterns for specimen NA-0 sintered at different temperatures; (b) and (c) crack formation on the fused silica particle surface.
Fig. 12. The fracture morphologies of the specimens sintered at different temperatures. Specimens sintered at 1200 °C (a) NA-0; (b) NA-21; (c) NA-42; (d) NA-63. Specimens sintered at 1300 °C (e) NA-0; (f) NA-21; (g) NA-42; (h) NA-63.
Acknowledgments
(1) The alumina nanoparticles adhere to the surface of fused silica particles by electrostatic attraction. The nanosized alumina coating formed can be introduced as a shell structure to retard cristobalite formation by inducing compressive stress on the surfaces of fused silica particles. Therefore, the total cristobalite content decreases with the addition of coated fused silica particles to silica-based ceramic cores. (2) The presence of Al and Na elements in nanosized alumina coatings provides a liquid phase during sintering. When the content of coated fused silica particles is low (≤21 wt%), this liquid flows into the pores between particles, decreasing the open porosity. Therefore, when the coated fused silica content increases to 21 wt %, the open porosity of specimens sintered at 1300 °C decreases from 31.43% to 30.55%. With a further increase in coated fused silica particle content (≤63 wt%), some alumina nanoparticles in the coated layer disperse into the liquid phase, increasing the viscosity of the liquid phase and hindering the process of pore filling. Hence, when the coated fused silica content further increases to 63 wt%, the open porosity of specimens sintered at 1300 °C increases from 30.55% to 32.87%. (3) The increase in sintering temperature is beneficial to the fracture changes from intergranular to transgranular fractures due to the reinforcement of the sintering necks between particles. When the sintering temperature increases from 1200 °C to 1350 °C, the specimens without coated fused silica powders showed little change in flexural strength. However, the flexural strength of specimen NA-63 increased from 7.67 MPa to 15.88 MPa, and the addition of coated fused silica particles caused the fracture mode to be more intergranular. This can be attributed to the reinforcement by coated fused silica particles. Both the decrease in cracks and the formation of mullite caused by the nanosized alumina coatings are beneficial for the improvement of coated fused silica powders' strength, which can aid in resisting crack growth.
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. References [1] S. Pattnaik, D.B. Karunakar, P.K. Jha, Developments in investment casting process—a review, J. Mater. Process. Technol. 212 (2012) 2332–2348 https://doi.org/ 10.1016/j.jmatprotec.2012.06.003. [2] C.J. Bae, D. Kim, J.W. Halloran, Mechanical and kinetic studies on the refractory fused silica of integrally cored ceramic mold fabricated by additive manufacturing, J. Eur. Ceram. Soc. 39 (2019) 618–623 https://doi.org/10.1016/j.jeurceramsoc. 2018.09.013. [3] M. Gromada, A. Świeca, M. Kostecki, A. Olszyna, R. Cygan, Ceramic cores for turbine blades via injection moulding, J. Mater. Process. Technol. 220 (2015) 107–112 https://doi.org/10.1016/j.jmatprotec.2015.01.010. [4] K. Li, W. Jiang, S. Wang, J. Xiao, L. Lou, Effect of specimen thickness on the creep deformation of a silica-based ceramic core material, J. Alloy. Comp. 763 (2018) 781–790 https://doi.org/10.1016/j.jallcom.2018.05.253. [5] E.P. Kruglov, G.K. Kochetova, Improvement of a technological process for ceramic core removal out of internal cavities of aircraft GTE turbine blade castings, Russ. Aeronaut. 50 (2007) 227–229 https://doi.org/10.3103/s1068799807020201. [6] B. Kühn, R. Schadrack, Thermal expansion of synthetic fused silica as a function of OH content and fictive temperature, J. Non-Cryst. Solids 355 (2009) 323–326 https://doi.org/10.1016/j.jnoncrysol.2008.11.005. [7] E.H. Kim, G.H. Cho, Y.S. Yoo, S.M. Seo, Y.G. Jung, Development of a new process in high functioning ceramic core without shape deformation, Ceram. Int. 39 (2013) 9041–9045 https://doi.org/10.1016/j.ceramint.2013.04.107. [8] W. Wisniewski, S. Berndt, M. Müller, C. Rüssel, Stress induced texture formation in surface crystallized SiO2 glass, CrystEngComm 15 (2013) 2392–2400 https://doi. org/10.1039/c3ce26843h. [9] R. Pascova, G. Avdeev, I. Gutzow, I. Penkov, F.P. Ludwing, J.W.P. Schmelzer, Refractory alkali-free cristobalite glass-ceramics: activated reaction sinter-crystallization synthesis and properties, Int. J. Appl. Glass Sci. 3 (2012) 75–87 https://doi. org/10.1111/j.2041-1294.2011.00072.x. [10] R.C. Breneman, J.W. Halloran, V. Sglavo, Effect of cristobalite on the strength of sintered fused silica above and below the cristobalite transformation, J. Am. Ceram. Soc. 98 (2015) 1611–1617 https://doi.org/10.1111/jace.13505. [11] A. Kazemi, M.A. Faghihi-Sani, M.J. Nayyeri, M. Mohammadi, M. Hajfathalian, Effect of zircon content on chemical and mechanical behavior of silica-based ceramic cores, Ceram. Int. 40 (2014) 1093–1098 https://doi.org/10.1016/j. ceramint.2013.06.108.
7
Ceramics International xxx (xxxx) xxx–xxx
X. Chen, et al.
[17] Z. Jiang, J. Wang, P. Wang, S. Yang, Z. Zou, Y. Wu, Preparation core/shell-type microparticles consisting of cBN cores aluminum coating via composite method, J. Alloy. Comp. 733 (2019) 234–238 https://doi.org/10.1016/j.jallcom.2018.09.117. [18] R.L. Snyder, The use of reference intensity ratios in X-Ray quantitative analysis, Powder Diffr. 7 (1992) 186–193 https://doi.org/10.1017/s0885715600018686. [19] J.J. Liang, Q.H. Lin, X. Zhang, T. Jin, Y.Z. Zhou, X.F. Sun, B.G. Choi, I.S. Kim, J.H. Do, C.Y. Jo, Effects of alumina on cristobalite crystallization and properties of silica-based ceramic cores, J. Mater. Sci. Technol. 33 (2017) 90–95 https://doi.org/ 10.1016/j.jmst.2016.02.012. [20] G.S. An, S.W. Choi, T.G. Kim, J.R. Shin, Y.I. Kim, S.C. Choi, Y.G. Jung, Aminofunctionalization of colloidal alumina particles for enhancement of the infiltration behavior in a silica-based ceramic core, Ceram. Int. 43 (2017) 157–161 https://doi. org/10.1016/j.ceramint.2016.09.127. [21] W.D. Kingery, Introduction to Ceramics, J Wiley, New York, 1975. [22] A. Kazemi, M.A. Faghihi-Sani, H.R. Alizadeh, Investigation on cristobalite crystallization in silica-based ceramic cores for investment casting, J. Eur. Ceram. Soc. 33 (2013) 3397–3402 https://doi.org/10.1016/j.jeurceramsoc.2013.06.025.
[12] L.Y. Wang, M.H. Hon, The effects of zircon addition on the crystallization of fused silica, J. Ceram. Soc. Jpn. 102 (1994) 517–521 https://doi.org/10.2109/jcersj.102. 517. [13] P.J. Wilson, S. Blackburn, R.W. Greenwood, B. Prajapti, K. Smalley, The role of zircon particle size distribution, surface area and contamination on the properties of silica–zircon ceramic materials, J. Eur. Ceram. Soc. 31 (2011) 1849–1855 https:// doi.org/10.1016/j.jeurceramsoc.2011.03.005. [14] Y.H. Kim, J.G. Yeo, J.S. Lee, S.C. Choi, Influence of silicon carbide as a mineralizer on mechanical and thermal properties of silica-based ceramic cores, Ceram. Int. 42 (2016) 14738–14742 https://doi.org/10.1016/j.ceramint.2016.06.100. [15] Y.H. Kim, J.G. Yeo, S.C. Choi, Shrinkage and flexural strength improvement of silica-based composites for ceramic cores by colloidal alumina infiltration, Ceram. Int. 42 (2016) 8878–8883 https://doi.org/10.1016/j.ceramint.2016.02.137. [16] Z.L. Xu, J.W. Zhong, X.L. Su, Q.Y. Xu, B.C. Liu, Experimental study on mechanical properties of silica-based ceramic core for directional solidification of single crystal superalloy, Ceram. Int. 44 (2018) 394–401 https://doi.org/10.1016/j.ceramint. 2017.09.189.
8
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
High strength silica-based ceramics material for investment casting applications: Effects of adding nanosized alumina coatings Xiao Chena, Chunyang Liub, Wenlong Zhengb, Jiqing Hanb, Li Zhangb,∗, Chunming Liua a b
School of Materials Science and Engineering, Northeastern University, Shenyang, 110819, Liaoning, PR China School of Metallurgy, Northeastern University, Shenyang, 110819, Liaoning, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Nanosized alumina coating Silica-based ceramic core Cristobalite Flexural strength
A nanosized alumina coating was synthesized on the surface of fused silica particles by electrostatic attraction. The effects of the coated fused silica particles on the cristobalite crystallization behavior, microstructure evolution, and flexural strength of silica-based ceramic cores were investigated. X-ray diffraction (XRD) was used to characterize phase transformations in the specimens, and the results indicated that the formed nanosized alumina coatings could retard cristobalite formation by inducing compressive stress on the fused silica particle surface. A mullite phase was also found due to the reaction of the nanosized alumina coating and the surface of the fused silica when the sintering temperature was increased to 1300 °C. Analysis using scanning electron microscopy equipped with energy dispersive spectrometry (SEM/EDS) suggested that alumina nanoparticles in the coated layer dispersed into a liquid phase and formed a barrier layer to impede the movement of the liquid phase, preventing the pore-filling process and increasing the open porosity of the ceramic specimens. Flexural strengths at room temperature were tested, indicating that increases in the sintering temperature of the specimens without coated fused silica powders had little effect on flexural strength. However, the flexural strength of the specimens with coated fused silica powders increased with increases in sintering temperature. The improvement in flexural strength was related to the reinforcement by sintering necks between particles and the improvement in the strength of the coated fused silica powder.
1. Introduction Silica-based ceramics have been wildly utilized for high-temperature special structural materials and are used especially in investment metal casting to form the intricate internal cooling passages of superalloy gas turbine blades [1–4] due to their low thermal expansion coefficients (0.55 × 10−6/K between 25 and 1000 °C), excellent chemical inertness against molten metal and easy removability by a process that is not harmful to the cast [5,6]. To produce acceptable inner surface finishes and wall thicknesses for the blades, the properties of the silica-based ceramic cores must be balanced carefully, such as their mechanical strength, dimensional conformity and good gas permeability during casting. However, one of the crucial obstacles associated with engineering applications is obtaining optimal flexural strength for silica-based ceramic cores to endure the severe circumstances during casting, because the flexural strength of fused silica for the core material does not exceed 6 MPa without the use of sintering aids or additives [7]. Generally, to achieve high flexural strengths for silica-based ceramic
∗
cores, the fabrication process should be carried out at temperatures under 1300 °C. This is because higher sintering temperatures lead to the crystallization of fused silica to cristobalite, which induces shrinkage and microcracking in silica-based ceramic materials. Microcracks formed by volume contraction during the β-cristobalite to α-cristobalite phase transformation during cooling decrease the flexural strength of silica-based ceramic cores [8,9]. By using additives, the cristobalite crystallization and flexural strength of silica-based ceramics can be controlled. A number of additives have been used to improve the flexural strength of silica-based ceramic cores, including alumina [19], cristobalite [10], zircon [11–13], and silicon carbide [14]. Kim [15] used mullitization to consume the cristobalite on the silica surface and inhibit the generation of cracks. In their study, the specimens were presintered at a lower temperature (< 1200 °C) for 2 h, then immersed in colloidal alumina and heated at a higher temperature (> 1300 °C) for 2 h. The infiltrated alumina reacted with cristobalite on the silica surface to form mullite. The lower total cristobalite content on the silica particle surfaces and therefore the formation of fewer microcracks led to an enhancement of flexural strength.
Corresponding author. E-mail address: [email protected] (L. Zhang).
https://doi.org/10.1016/j.ceramint.2019.08.248 Received 16 July 2019; Received in revised form 19 August 2019; Accepted 26 August 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Xiao Chen, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.08.248
Ceramics International xxx (xxxx) xxx–xxx
X. Chen, et al.
content, while the finer fused silica content and zircon content were constant at 18 wt% and 10 wt%, respectively. The prepared bar specimens were then sintered at 1200, 1250, 1300, and 1350 °C for 2 h. To provide good support and prevent deformations or cracks in the specimens during sintering, industrial alumina powders with a purity of 99.6 wt% and average particle size of approximately 91.9 μm were used as a powder bed.
On the other hand, depending on the fracture mechanism, the improved flexural strength of silica-based ceramics can be attributed to either the transgranular fracture mode or to the intergranular fracture mode. For these two fracture mechanisms, the improvement in powder strength and the reinforcement in sintering necks play an important role for the flexural strength of silica-based ceramic cores. For example, Kazemi [11] suggested that an improvement in the strength of the raw material particles is conducive to the intergranular fracture mode. In their study, the strength increased slightly with increasing zircon content in silica-based ceramic cores, and the intergranular fracture behavior was found in the zircon particles. The inhibiting effect of zircon particles on the crack growth was mainly due to the fact that the strength of zircon particles was higher than that of fused silica. In addition, Xu [16] suggested that the resintering process reinforced the sintering necks between particles and changed the crack propagation from the sintering necks between particles to entire large particles. They demonstrated that the transition from the intergranular to transgranular fracture mode increased the strength of silica-based ceramic cores. Hence, high flexural strength silica-based ceramic cores may be created by decreasing the total cristobalite content, improving the raw material particle strength and reinforcing the sintering necks between particles. Therefore, to further improve the flexural strength of silicabased ceramic cores, a new processing route is explored in this study. First, a nanosized alumina coating was synthesized on the surface of fused silica particles by electrostatic attraction to develop a “core-shell” structure in the fused silica particles. Then, the silica-based ceramic cores with different contents of coated fused silica powders were sintered at 1200, 1250, 1300 and 1350 °C. The effects of the coated fused silica powders on the cristobalite crystallization, microstructure evolution, flexural strength and fracture mechanism of the silica-based ceramic cores was investigated.
2.3. Characterization The phase and morphology of nanosized alumina-coated fused silica powders and four kinds of sintered ceramic cores were analyzed by Xray powder diffraction using Ni-filtered Cu Kα radiation (XRD, Smart Lab 9 kW, Rigaku, Japan) and by scanning electron microscopy (SEM/ EDS, Quanta250FEG, FEI, America), respectively. To investigate the effect of nanosized alumina coatings on the cristobalite crystallization of the fused silica particles, the crystal quantitative analysis of the specimens was determined using the reference intensity ratio (RIR) method [18]. To study the effect of nanosized alumina-coated fused silica powders on the fracture mechanisms of silica-based ceramic cores, the sintered specimens were measured with a three-point bending test with a span distance of 30 mm and a loading rate of 5 N/s on a universal testing machine (UTM, AG-XPlus, Shimadzu, Japan). The open porosities of all specimens were also determined by the Archimedes method, using distilled water as an immersion fluid. The results of each report averaged three tests. 3. Results and discussion 3.1. Characterization of nanosized alumina-coated fused silica powders The XRD patterns, the SEM micrographs of the as-received fused silica powders and the nanosized alumina-coated fused silica powders, and the element mapping by EDS are shown in Fig. 1. There was no diffraction peak in the XRD pattern of as-received fused silica powders, see Fig. 1(a), suggesting that the as-received fused silica is amorphous. Compared with the XRD pattern of the as-received fused silica powders, the diffraction peaks corresponding to the Al2O3 phase in the coated fused silica powders can be clearly identified. Fig. 1(b) and (c) are the SEM micrographs of the as-received fused silica powder and the coated fused silica powder, respectively. As shown in Fig. 1(b), as-received fused silica powder presents a clean surface, and the grain shape is polygonal. From Fig. 1(c) shows that the coated fused silica powder presents a rough surface. The sharp edges of the fused silica powders become dulled and rounded after coating, indicating that the alumina nanoparticles might have adhered to the surface of the fused silica powders. EDS further reveals the existence of alumina nanoparticles on the fused silica powder surfaces, as shown in Fig. 1(d) and (e).
2. Methods 2.1. Preparation of nanosized alumina coatings To prepare the nanosized alumina-coated fused silica powders, fused silica powder (amorphous SiO2, 40.4 μm, 98.76%) and nanosized colloidal alumina (alumina oxide, 10–20 nm, 40% in H2O, colloidal dispersion) were used as raw materials. First, fused silica (1000 g) in 500 ml of deionized water was mixed with 500 ml of nanosized colloidal alumina and stirred at 500 rpm for 5 h at room temperature. Regarding fused silica, the hydroxyl groups were pushed to the outside to form a negatively charged layer in deionized water, which adsorbs positively charged alumina nanoparticles via their opposite electrical charges [17]. Then, the resulting mixture was dried and annealed at 1000 °C for 2 h. As a result, the nanosized alumina-coated fused silica powders were prepared after sieving to obtain ~200 mesh particles, because the alumina nanoparticles had already coagulated onto the fused silica surface as a result of the electrical attraction force.
3.2. Characterization of the silica-based ceramic cores 2.2. Fabrication of silica-based ceramic cores It is confirmed that a thickness nanosized alumina coating could adhere to the fused silica powder surface through electrostatic attraction based on the results presented in Fig. 1. Therefore, to know how the nanosized alumina coating affects crystallization behavior and the mechanical properties of silica-based ceramic cores, the following work was performed. Fig. 2 shows the XRD patterns of the silica-based ceramic cores with different coated fused silica powders contents sintered at different temperatures (1200, 1250, 1300 and 1350 °C). As shown in Fig. 2(a) and (b), the crystallization behavior begins at 1300 °C in specimen NA42. Compared with the XRD pattern of specimen NA-0, the cristobalite peaks intensities decrease in all specimens with coated fused silica powders.
The mixture for the fabrication of the silica-based ceramic core consisted of coarse fused silica, finer fused silica, coated fused silica and zircon. Table 1 shows the characteristics of the raw materials. The purity and specific surface area of raw materials were determined by Xray fluorescence spectroscopy (XRF) and a laser particle size analyzer (LPSA), respectively. The four kinds of silica-based ceramic core specimens, containing 0, 21, 42, and 63 wt% coated fused silica, were denoted as NA-0, NA-21, NA-42, and NA-63, respectively, and were injection molded into test bar shapes with dimensions of 120 × 10 × 4 mm3. Coarse fused silica and coated fused silica were used to form the skeleton, and increases in the coated fused silica content were accompanied by decreases in the coarse fused silica 2
Ceramics International xxx (xxxx) xxx–xxx
X. Chen, et al.
Table 1 Characteristics of the coarse fused silica, finer fused silica, coated fused silica and zircon. Powder (%)
Purity (%)
Coarse fused silica Finer fused silica Coated fused silica
98.76 99.29 80.48
Zircon
94.49
Important impurities (Type-%)
Al2O3 Al2O3 Al2O3 Fe3O4 Na2O K2O HfO2 Al2O3 Sb2O3
Specific surface area (m2/g)
PSD
1.19 0.57 19.35 0.088 0.033 0.012 1.352 0.740 0.689
D10 (μm)
D50 (μm)
D90 (μm)
6.14 2.99 4.93
40.4 16.9 29.3
91.0 48.5 70.3
0.415 0.768 0.494
1.51
12.8
45.7
1.361
intensities decrease with increasing amounts of coated fused silica powders in sintered specimens, as shown in Fig. 2(b). The cristobalite contents in the sintered specimens NA-21, NA-42, and NA-63 were 8.35, 6.27, and 4.44%, respectively. This behavior can be understood by the mullitization behavior. The diffraction peaks of the mullite phase are detected at 1300 °C, as shown in Fig. 2(a) and (b). Since cristobalite crystallization always occurs on the surfaces of fused silica particles [11], the formed cristobalite is consumed by alumina nanoparticles that have coagulated on the fused silica surface during sintering at elevated temperatures. As shown in Fig. 5, the EDX analysis data show that the mullite phase forms on the surface of fused silica at 1300 °C. That mullitization can take place at lower temperatures with nanosized alumina is in agreement with Young-Hwan Kim and Gye Seok An [15,20]. Fig. 6 shows the open porosity of the silica-based ceramic cores with different coated fused silica powder contents sintered at different temperatures (1200, 1250, 1300, and 1350 °C). The open porosity of the sintered specimens decreases with increases in sintering temperature regardless of the coated fused silica powder content. As the sintering temperature increases from 1200 °C to 1350 °C, the open porosity decreases from 34.69% to 28.84% for specimen NA-0, from 34.34% to 28.02% for specimens NA-21, from 34.60% to 30.01% for specimen NA-42, and from 34.49% to 30.35% for specimen NA-63. With an increase in sintering temperature, amorphous fused silica can easily migrate through surface diffusion; as shown in Fig. 7(a), the particles connect to each other and the pores become round during this process. When the liquid phase occurs, the volume diffusion mechanism is
In a previous study [10,19], the influence of microsized alumina addition on fused silica was investigated, indicating that alumina can affect the crystallization rate of fused silica through its solution inside amorphous silica particles. The dissolved alumina increased the number of nonbridged oxygen atoms and the diffusion mobility of SiO4 tetrahedrons, improving the crystallization tendency of fused silica. However, it seems that the nanosized alumina coating does not act in the same manner as the microsized alumina particles in fused silica. The nanosized alumina coating (see Fig. 3(a)) generates a compressive stress on the fused silica particles’ surface regions and then retards the crystallization tendency. This is because the thermal expansion coefficient of alumina is greater than that for fused silica. In addition, an auxiliary experiment was designed to simulate this mechanism, and the effect of compressive stress on the inhibition of the transformation from fused silica to cristobalite was proven. As shown in Fig. 3(b), the specimens FS and FSA are formed by dry-pressing and were sintered at 1300 °C for 2 h. After removing the alumina coating from specimen FSA (the alumina coating on the surface of the specimen had a low sintering degree and could be easily removed by mechanical means), 10 wt% zircon fine powder (as an internal standard) was added to the sintered ceramic specimens before XRD testing. The XRD pattern results are shown in Fig. 4, which shows that the cristobalite peak intensity in the specimen FSA is lower than that for the normal specimen FS without an alumina coating, indicating that the compressive stress has an inhibitory effect on the transformation from fused silica to cristobalite, as predicted. When the sintering temperature is 1300 °C, the cristobalite peak
Fig. 1. (a) XRD patterns of the as-received fused silica powders and nanosized alumina-coated fused silica powders; (b)–(c) SEM images of the as-received fused silica powder and nanosized alumina-coated fused silica powder; (e) EDS mapping of nanosized alumina-coated fused silica powders in Fig. (d). 3
Ceramics International xxx (xxxx) xxx–xxx
X. Chen, et al.
Fig. 2. (a) XRD patterns for specimen NA-42 sintered at 1200, 1250, 1300 and 1350 °C for 2 h; (b) XRD patterns for the specimens NA-0, NA-21, NA-42 and NA-63 sintered at 1300 °C for 2 h.
enhanced, the sintering necks between particles become coarse and the pores gradually shrink and disappear. As shown in Fig. 7(b), a layer of liquid phase is clearly observed on the particle surfaces, which promotes the coarsening of the sintering necks between particles. When the sintering temperature continues to rise, the liquid phase fills the pores between particles, inducing the initiation of consolidation. As shown in Fig. 7(c), the pores nearly disappear, but cracks form on the silica surface due to a large amount of cristobalite formation. Therefore, the decrease in open porosity can be attributed to the surface diffusion of amorphous fused silica and the volume diffusion of the liquid phase with increasing sintering temperatures. However, coated fused silica powders have a complex influence on the open porosity of silica-based ceramic cores. When the coated fused silica content increases to 21 wt%, the open porosity of a specimen sintered at 1300 °C decreases from 31.43% to 30.55%. As shown in Fig. 7(d), the single small particles are not observed in specimen NA-21 since they are bonded together by the sintering necks to form complete larger particles, and the pores between these particles are filled by the liquid phase. Fig. 8 is the EDS results of liquid phase (marked as 1) and silica particle (marked as 2) regions of specimen NA-21 sintered at 1300 °C. In addition to large amounts of Si and O, some elements including 4.96 wt% Al and 1.48 wt% Na can also be detected in the liquid phase. That is, the presence of Al and Na accelerates the formation of the liquid phase, and the increased liquid phase fills the pores, reducing the open porosity of sintered specimens. The increased liquid phase is related to the formation of a glass phase (fused silica) and low melting liquid phase. In silica systems, the impurities or oxide mineralizers can affect the production of the glass phase by changing the structure of the amorphous fused silica. It has been reported that the dissolved alumina can act as a modifier ([AlO6]) in the absence of alkaline oxides (Na2O/Al2O3 < 1), which can break the silicon oxygen bonds and increase the number of non-bridge oxygen atoms, consequently reducing the connection degree of the network structure of fused silica [21]. Therefore, the presence of Al2O3 and Na2O accelerates the softening of amorphous fused silica, turning it into a liquid glass phase during sintering. In addition, with the increase of sintering temperature, both Na2O and Al2O3 can react with SiO2 to form the low melting liquid phase (for example, Na2O·2SiO2, Na2O·Al2O3·SiO2, and 3Al2O3·2SiO2), which increases the liquid content in sintered specimens. When the coated fused silica content increases from 21 wt% to 63 wt %, the open porosity of specimens sintered at 1300 °C increases from 30.55% to 32.87%. This is related to the “core-shell” structure of coated fused silica particles, which inhibits the coarsening of the sintering necks. It is known that the diffusion activation energy of alumina is higher than that of fused silica, so it is not easy for sintering necks to
Fig. 3. Schematic diagram: (a) the effect of alumina nanoparticles coating on fused silica powder; (b) a supplementary experiment.
Fig. 4. XRD patterns of supplementary experiment specimens sintered at 1300 °C for 2 h. 4
Ceramics International xxx (xxxx) xxx–xxx
X. Chen, et al.
Fig. 5. SEM and EDS analysis results of specimen NA-63 sintered at 1300 °C.
process.
3.3. Mechanical properties Fig. 10 shows the flexural strength of silica-based ceramic cores with different coated fused silica powder contents sintered at different temperatures (1200, 1250, 1300, and 1350 °C). The flexural strength of specimen NA-0 is 7.83 MPa for 1200 °C, 8.04 MPa for 1250 °C, 8.98 MPa for 1300 °C, and 8.01 MPa for 1350 °C. It is found that the sintering temperature for specimens without coated fused silica powders has little effect on flexural strength. The XRD patterns of specimen NA-0 sintered at different temperatures are shown in Fig. 11(a); the cristobalite content is 20.66% for 1200 °C, 22.10% for 1250 °C, 32.71% for 1300 °C, and 47.41% for 1350 °C. With increases in sintering temperature, the cristobalite content increases sharply. The cracks in the fused silica particles are clearly visible due to the volume contraction of the cristobalite grains, as shown in Fig. 11(b) and (c). The higher the total cristobalite content, the more microcracks are formed. This is important evidence of the decreased flexural strength for specimen NA0 when the sintering temperature increases to 1350 °C. In contrast, the flexural strength for the specimens fabricated with coated fused silica powders can be effectively improved by increasing the sintering temperature. For example, specimens NA-21, NA-42, and NA-63 exhibited flexural strengths of 14.85 MPa, 14.94 MPa, and 15.88 MPa when sintered at 1350 °C, respectively, which are greater than for other specimens with conventional additives [11,15,19,22]. The high flexural strengths can be attributed to the reinforcement of the sintering necks with the increase in sintering temperature. To gain an understanding of the fracture mechanism, the fracture morphologies of the specimens sintered at different temperatures were examined. As shown in Fig. 12, when the sintering temperature increases from
Fig. 6. Open porosity of the silica-based ceramic core specimens.
form between coated fused silica particles through surface diffusion. As shown in Fig. 7(e) and (f), the particle size decreases significantly, and the fine particles do not adhere to the leading particles to form complete large particles in specimens NA-42 and NA-63. Additionally, nanosized alumina coatings form a barrier layer that impedes the movement of the liquid phase between the coated fused silica particles, preventing the pore filling process. As shown in Fig. 9(a), some alumina nanoparticles dispersed in the liquid phase and then moved with it to fill the pores. Therefore, the alumina nanoparticles are mainly concentrated around the pores between particles, as shown in Fig. 9(b). With an increase in alumina nanoparticles, the viscosity of the liquid phase increases. These alumina nanoparticles form a barrier layer to prevent the movement of the liquid phase, inhibiting the pore filling
Fig. 7. Microstructures of the specimens after being sintered at different temperatures: NA-0 sintered at (a) 1200 °C; (b) 1250 °C; (c) 1300 °C. Specimens sintered at 1300 °C (d) NA-21; (e) NA-42; (f) NA-63. 5
Ceramics International xxx (xxxx) xxx–xxx
X. Chen, et al.
Fig. 8. EDS results of liquid phase (marked as 1) and silica particle (marked as 2) region of specimen NA-21 sintered at 1300 °C.
Fig. 9. EDS mapping of the polished surface of sample NA-21 sintered at 1300 °C for 2 h.
fracture mechanism changes from intergranular to transgranular with increasing sintering temperatures. It can be inferred that the sintering necks produced at high temperatures play an important role in the mechanical properties of ceramic specimens. However, with the increase in coated fused silica powders, specimens NA-42 and NA-63 depict mixed fracture behavior consisting of both intergranular and transgranular fracture surfaces. This change in fracture mode does not decrease the strength of the ceramic specimens, and this can be attributed to the reinforcement of the coated fused silica particles. This is because both the decrease in cracks and the formation of mullite caused by the nanosized alumina coatings are beneficial for the improved strength of coated fused silica powders, which can resist crack growth. Therefore, an increase in coated fused silica content accompanied with a decrease in fused silica content, results in a slight increase in the flexural strength of silica-based ceramic cores. Fig. 10. Flexural strengths of the sintered silica-based ceramic cores.
4. Conclusions 1200 °C to 1300 °C, the sintering necks between particles are strengthened. Therefore, the image of ceramic specimens sintered at 1200 °C shows intact particles on the fracture surfaces, while the image of specimens sintered at 1300 °C shows fractured particles. Hence, the
In this work, the influence of nanosized alumina coated fused silica particles on the cristobalite crystallization behavior, microstructure evolution, and mechanical properties of silica-based ceramic cores was investigated. The main results are summarized as follows: 6
Ceramics International xxx (xxxx) xxx–xxx
X. Chen, et al.
Fig. 11. (a) XRD patterns for specimen NA-0 sintered at different temperatures; (b) and (c) crack formation on the fused silica particle surface.
Fig. 12. The fracture morphologies of the specimens sintered at different temperatures. Specimens sintered at 1200 °C (a) NA-0; (b) NA-21; (c) NA-42; (d) NA-63. Specimens sintered at 1300 °C (e) NA-0; (f) NA-21; (g) NA-42; (h) NA-63.
Acknowledgments
(1) The alumina nanoparticles adhere to the surface of fused silica particles by electrostatic attraction. The nanosized alumina coating formed can be introduced as a shell structure to retard cristobalite formation by inducing compressive stress on the surfaces of fused silica particles. Therefore, the total cristobalite content decreases with the addition of coated fused silica particles to silica-based ceramic cores. (2) The presence of Al and Na elements in nanosized alumina coatings provides a liquid phase during sintering. When the content of coated fused silica particles is low (≤21 wt%), this liquid flows into the pores between particles, decreasing the open porosity. Therefore, when the coated fused silica content increases to 21 wt %, the open porosity of specimens sintered at 1300 °C decreases from 31.43% to 30.55%. With a further increase in coated fused silica particle content (≤63 wt%), some alumina nanoparticles in the coated layer disperse into the liquid phase, increasing the viscosity of the liquid phase and hindering the process of pore filling. Hence, when the coated fused silica content further increases to 63 wt%, the open porosity of specimens sintered at 1300 °C increases from 30.55% to 32.87%. (3) The increase in sintering temperature is beneficial to the fracture changes from intergranular to transgranular fractures due to the reinforcement of the sintering necks between particles. When the sintering temperature increases from 1200 °C to 1350 °C, the specimens without coated fused silica powders showed little change in flexural strength. However, the flexural strength of specimen NA-63 increased from 7.67 MPa to 15.88 MPa, and the addition of coated fused silica particles caused the fracture mode to be more intergranular. This can be attributed to the reinforcement by coated fused silica particles. Both the decrease in cracks and the formation of mullite caused by the nanosized alumina coatings are beneficial for the improvement of coated fused silica powders' strength, which can aid in resisting crack growth.
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. References [1] S. Pattnaik, D.B. Karunakar, P.K. Jha, Developments in investment casting process—a review, J. Mater. Process. Technol. 212 (2012) 2332–2348 https://doi.org/ 10.1016/j.jmatprotec.2012.06.003. [2] C.J. Bae, D. Kim, J.W. Halloran, Mechanical and kinetic studies on the refractory fused silica of integrally cored ceramic mold fabricated by additive manufacturing, J. Eur. Ceram. Soc. 39 (2019) 618–623 https://doi.org/10.1016/j.jeurceramsoc. 2018.09.013. [3] M. Gromada, A. Świeca, M. Kostecki, A. Olszyna, R. Cygan, Ceramic cores for turbine blades via injection moulding, J. Mater. Process. Technol. 220 (2015) 107–112 https://doi.org/10.1016/j.jmatprotec.2015.01.010. [4] K. Li, W. Jiang, S. Wang, J. Xiao, L. Lou, Effect of specimen thickness on the creep deformation of a silica-based ceramic core material, J. Alloy. Comp. 763 (2018) 781–790 https://doi.org/10.1016/j.jallcom.2018.05.253. [5] E.P. Kruglov, G.K. Kochetova, Improvement of a technological process for ceramic core removal out of internal cavities of aircraft GTE turbine blade castings, Russ. Aeronaut. 50 (2007) 227–229 https://doi.org/10.3103/s1068799807020201. [6] B. Kühn, R. Schadrack, Thermal expansion of synthetic fused silica as a function of OH content and fictive temperature, J. Non-Cryst. Solids 355 (2009) 323–326 https://doi.org/10.1016/j.jnoncrysol.2008.11.005. [7] E.H. Kim, G.H. Cho, Y.S. Yoo, S.M. Seo, Y.G. Jung, Development of a new process in high functioning ceramic core without shape deformation, Ceram. Int. 39 (2013) 9041–9045 https://doi.org/10.1016/j.ceramint.2013.04.107. [8] W. Wisniewski, S. Berndt, M. Müller, C. Rüssel, Stress induced texture formation in surface crystallized SiO2 glass, CrystEngComm 15 (2013) 2392–2400 https://doi. org/10.1039/c3ce26843h. [9] R. Pascova, G. Avdeev, I. Gutzow, I. Penkov, F.P. Ludwing, J.W.P. Schmelzer, Refractory alkali-free cristobalite glass-ceramics: activated reaction sinter-crystallization synthesis and properties, Int. J. Appl. Glass Sci. 3 (2012) 75–87 https://doi. org/10.1111/j.2041-1294.2011.00072.x. [10] R.C. Breneman, J.W. Halloran, V. Sglavo, Effect of cristobalite on the strength of sintered fused silica above and below the cristobalite transformation, J. Am. Ceram. Soc. 98 (2015) 1611–1617 https://doi.org/10.1111/jace.13505. [11] A. Kazemi, M.A. Faghihi-Sani, M.J. Nayyeri, M. Mohammadi, M. Hajfathalian, Effect of zircon content on chemical and mechanical behavior of silica-based ceramic cores, Ceram. Int. 40 (2014) 1093–1098 https://doi.org/10.1016/j. ceramint.2013.06.108.
7
Ceramics International xxx (xxxx) xxx–xxx
X. Chen, et al.
[17] Z. Jiang, J. Wang, P. Wang, S. Yang, Z. Zou, Y. Wu, Preparation core/shell-type microparticles consisting of cBN cores aluminum coating via composite method, J. Alloy. Comp. 733 (2019) 234–238 https://doi.org/10.1016/j.jallcom.2018.09.117. [18] R.L. Snyder, The use of reference intensity ratios in X-Ray quantitative analysis, Powder Diffr. 7 (1992) 186–193 https://doi.org/10.1017/s0885715600018686. [19] J.J. Liang, Q.H. Lin, X. Zhang, T. Jin, Y.Z. Zhou, X.F. Sun, B.G. Choi, I.S. Kim, J.H. Do, C.Y. Jo, Effects of alumina on cristobalite crystallization and properties of silica-based ceramic cores, J. Mater. Sci. Technol. 33 (2017) 90–95 https://doi.org/ 10.1016/j.jmst.2016.02.012. [20] G.S. An, S.W. Choi, T.G. Kim, J.R. Shin, Y.I. Kim, S.C. Choi, Y.G. Jung, Aminofunctionalization of colloidal alumina particles for enhancement of the infiltration behavior in a silica-based ceramic core, Ceram. Int. 43 (2017) 157–161 https://doi. org/10.1016/j.ceramint.2016.09.127. [21] W.D. Kingery, Introduction to Ceramics, J Wiley, New York, 1975. [22] A. Kazemi, M.A. Faghihi-Sani, H.R. Alizadeh, Investigation on cristobalite crystallization in silica-based ceramic cores for investment casting, J. Eur. Ceram. Soc. 33 (2013) 3397–3402 https://doi.org/10.1016/j.jeurceramsoc.2013.06.025.
[12] L.Y. Wang, M.H. Hon, The effects of zircon addition on the crystallization of fused silica, J. Ceram. Soc. Jpn. 102 (1994) 517–521 https://doi.org/10.2109/jcersj.102. 517. [13] P.J. Wilson, S. Blackburn, R.W. Greenwood, B. Prajapti, K. Smalley, The role of zircon particle size distribution, surface area and contamination on the properties of silica–zircon ceramic materials, J. Eur. Ceram. Soc. 31 (2011) 1849–1855 https:// doi.org/10.1016/j.jeurceramsoc.2011.03.005. [14] Y.H. Kim, J.G. Yeo, J.S. Lee, S.C. Choi, Influence of silicon carbide as a mineralizer on mechanical and thermal properties of silica-based ceramic cores, Ceram. Int. 42 (2016) 14738–14742 https://doi.org/10.1016/j.ceramint.2016.06.100. [15] Y.H. Kim, J.G. Yeo, S.C. Choi, Shrinkage and flexural strength improvement of silica-based composites for ceramic cores by colloidal alumina infiltration, Ceram. Int. 42 (2016) 8878–8883 https://doi.org/10.1016/j.ceramint.2016.02.137. [16] Z.L. Xu, J.W. Zhong, X.L. Su, Q.Y. Xu, B.C. Liu, Experimental study on mechanical properties of silica-based ceramic core for directional solidification of single crystal superalloy, Ceram. Int. 44 (2018) 394–401 https://doi.org/10.1016/j.ceramint. 2017.09.189.
8