- Email: [email protected]
polyamide12 composites
Ceramics International 45 (2019) 20803–20809
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Lightweight mullite ceramics with controlled porosity and enhanced properties prepared by SLS using mechanical mixed FAHSs/polyamide12 composites
T
Meng Lia,1, An-Nan Chena,1, Xin Linb, Jia-Min Wua,*, Shuang Chena, Li-Jin Chenga, Ying Chena, Shi-Feng Wena, Chen-Hui Lia, Yu-Sheng Shia a State Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, 430074, China b State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an, 710072, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Fly ash hollow spheres Lightweight ceramics Selective laser sintering Controlled pore size Thermal conductivity
It is difficult to fabricate lightweight ceramic parts with well pore control and high structural complexity. In this paper, lightweight mullite ceramics with controlled porosity and enhanced properties were prepared via selective laser sintering (SLS) using mechanical mixed FAHSs/PA12 (Fly ash hollow spheres/polyamide 12) composites. Crack-free ceramic green bodies were prepared through SLS using the optimized process parameters: 7.2 W in laser power, 1600 mm/s in scanning velocity and 0.11 mm in hatch spacing. The influence of PA12 content on porosity, size distribution of open pore, mechanical properties and thermal property were studied. The pores in ceramic foams consisted of the closed pores from sphere core-shell structures, the open pore channels in the middle of stacking spheres and the special gaps in the middle of spheres related to SLS. It was found that the total porosity of lightweight mullite ceramics increased slightly from 85.1 ± 0.3% to 85.2 ± 0.4% with PA12 content increasing from 10 to 15 wt%, and then increased obviously to 86.7 ± 0.5% with further increasing PA12 addition to 25 wt%. This porosity increase was mainly attributed to the open pores resulted from the PA12 addition. The PA12 were firstly filled into the interspaces between stacking spheres and when the stacking volume reached the maximum, the interspaces expanded to form the special gaps between spheres with further PA12 addition. Thus, the size distribution of open pore in lightweight FAHS ceramics increased from 31.5 to 34.5 μm gradually with PA12 content increasing from 10 to 25 wt%. Finally, low thermal conductivity of 0.06 W/(m·K) was obtained, which probably resulted from the high porosity and the special pore structures of the SLS-formed ceramic foams. The understanding of the unique pore structures and microstructures will help the pore control and strength improvement of SLS-formed ceramic foams using the ceramic hollow spheres.
1. Introduction Lightweight ceramic foams have many excellent features including low density, excellent thermal-shock resistance, low thermal conductivity and good abrasion resistance [1–3]. Due to these features, they have been widely used in applications such as thermal insulators, sound absorption materials, filters and catalyst supports [4,5]. So far, various conventional processing methods have been used to fabricate lightweight ceramics, such as polymeric sponge impregnation [6], foam-gelcasting [7], sacrificial template [8], and direct stack sintering [9]. In foam-gelcasting methods, the pore structures generated from air
are rather complex and uncontrollable. Polymeric sponge method or sacrificial template method requires organic agents acting as templates to shape porous structures, which would give rise to environmental problems. As for direct stack sintering, pores in ceramic foams are usually uneven and the shapes lack the high complexity. Therefore, there is an urgent demand for the fabrication of lightweight ceramic parts with well pore control and high structural complexity. To control the pore structures of lightweight ceramic foams, ceramic hollow spheres are used to fabricate ceramic foams since the particle diameter and distribution of spheres could be well regulated before forming. Recently, a large number of studies have concentrated on
*
Corresponding author. E-mail address: [email protected] (J.-M. Wu). 1 These authors contributed equally to this paper. https://doi.org/10.1016/j.ceramint.2019.07.067 Received 30 May 2019; Received in revised form 29 June 2019; Accepted 6 July 2019 Available online 08 July 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 45 (2019) 20803–20809
M. Li, et al.
applying cenospheres or hollow spheres to fabricate lightweight ceramic foams through conventional forming methods including gelcasting and direct stack sintering. Sun et al. [10] prepared porous silica ceramics with uniform and interconnected pores by adding SiO2 spheres with a different size distribution into the ceramic matrix. It is found that the SiO2 spheres content has a linear effect on the pore size distribution of ceramic foams. Shao et al. [11] used fly ash cenospheres to prepare silicon nitride/silicon oxynitride ceramic foams with tailored pore structures by gelcasting method, in which the cell size can be controlled by adding fly ash cenospheres with different size grades. Wang et al. [12] fabricated silica composite porous ceramics with 29%–63% in porosity and flexure strength of 7–14 MPa by gelcasting method adding fly ash cenospheres. Li et al. [13] fabricated lightweight mullite ceramics showing high porosity reaching up to 81.37% and 6.25 ± 0.91 MPa in compressive strength via sintering SiO2–Al2O3 microspheres directly. These studies have revealed that the reserved inner hollow structures of spheres can not merely achieve the high porosity but also the control of pore structure in final ceramic parts. However, there are still great challenges in the preparation of ceramic products with high structural complexity. As one of the most promising additive manufacturing methods, selective laser sintering (SLS) can manufacture components with rather high geometrical complexity that are difficult or even impossible to be achieved through conventional methods [14]. As for ceramic materials with high-melting point, the SLS technology could be classified into two categories, i.e., direct and indirect SLS. The short laser scanning time and large temperature gradient in direct SLS usually cause the huge thermal stress and thus the cracks in ceramic parts easily [15]. Therefore, the crack-free ceramic parts are generally fabricated via indirect SLS by introducing the low-melting sacrificial binder phase that can easily fuse the ceramic components together to obtain ceramic green parts [16]. Recently, various ceramic components have already been prepared through the indirect SLS method, such as Al2O3, ZrO2, SiO2, SiC [17–20]. However, these studies have mainly focused on improving the density of ceramic parts combined with the post-treatment process such as infiltration and isostatic pressing. Besides, these post-processing methods cannot be used to prepare porous ceramics with improved performance, so there is rare literature report on the method of effectively improving the mechanical performance of SLS-formed porous ceramics. In fact, the SLS process has a unique advantage over manufacturing porous parts due to the incompact deposition of powder layers and open pore channels generated from the sacrificial binder phase after burning out. In our previous work, a new approach to fabricate lightweight mullite ceramics via selective laser sintering FAHSs was proposed, and high porosity reaching to 79.9%–88.7% and improved compressive strength up to 0.2–6.7 MPa were obtained [21]. Nevertheless, there are still some issues existing that need to be solved and studied in detail. Firstly, the core parameters of SLS have not been studied, which can greatly affects the forming quality of SLS-formed products. Additionally, the effect of PA12 content on size distribution of open pore, mechanical performance and thermal property of lightweight mullite ceramics have not been evaluated. In this paper, the optimized SLS parameters of the composite powders were evaluated in depth. The influence of PA12 content on size distribution of open pore, thermal property and mechanical strength of SLS-formed FAHS lightweight ceramic foams were investigated. Finally, the pore control and strength improvement mechanism of SLS-formed ceramic foams were discussed. 2. Experimental procedure 2.1. Raw materials Commercial FAHSs (52.6 wt% SiO2, 30.9 wt% Al2O3; D50 = 75.5 μm, Jingsheng Minerals Co., Ltd.) were used as the starting ceramic powders. As shown in Fig. 1(a), the hollow spheres with
improved mechanical strength were obtained after calcining at 900 °C for 1 h. The SEM morphology of the calcined FAHSs can be seen in Fig. 1(b). The FAHSs are in a typical spherical shape with hollow cavity, which possess good flowability suitable for powder deposition during SLS. PA12 powders with mean particle diameter of 55 μm were provided by Xincheng Engineering Plastics Co., Ltd. in China. The alumina powders with purity of 99% and mean particle diameter of 0.3 μm were obtained from Almatis, Ludwigshafen, Germany. During the sintering period, it was used as a powder bed. 2.2. Ceramic foams preparation The mixed FAHSs/PA12 powders with different mass ratio were mingled mechanically through a three-dimensional blender. Fig. 1(c) depicts the SEM photograph of the mixed FAHS/PA12 composite powders with 15 wt% PA12. The mechanically mixing composite powders are uniform and the FAHSs remain spherical shapes, which would exhibit excellent flowability and formability appropriate for SLS process. After SLS procedure, ceramic green parts were fabricated, which was conducted on a SLS machine of C250. The device was provided by Wuhan Huake 3D Technology Co. Ltd. in China, and it was equipped with CO2 laser beam. It is known that as the central parameter in the SLS process, laser energy density could affect the quality of SLS-formed products, which can be calculated from the following equation (1) [22].
Q= P/(V·L)
(1) 2
where Q is the laser energy density (J/mm ), P is the laser power (W), V is the scanning velocity (mm/s) and L is the hatch spacing (mm). In order to analyze the influence of these processing parameters on forming property of SLS-formed green parts, an orthogonal test (see Table 1) was designed in the present experiment. Laser power (6.0, 6.6, 7.6 W), scanning velocity (1600, 1800, 2000 mm/s), hatch spacing (110, 130, 150 μm) were chosen to execute the laser scanning tests. Layer thickness of 130 μm was used to prepare green samples in each parameter combination. The optimized parameter combination was chosen to prepare green parts with high geometrical complexity after a formability analysis. Subsequently, in order to remove the binder, the green ceramic parts were kept at 650 °C for 2 h to burn the PA12. Finally rose the temperature from 650 °C to 1350 °C holding for another 3 h to densify ceramic green parts. 2.3. Characterization The morphology of the FAHSs after calcination and the fracture surfaces of the ceramic foams were captured by a scanning electron microscope (JSM-7600 F, JEOL, Japan). Under the scan rate of 5°/min, the phase of lightweight ceramics were analyzed by a X-ray diffractometer (XRD-7000s, Shimadzu, Japan), using a Cu tube whose λ is 1.5406 Å at 40 kV and 30 mA. The radial error(s) of SLS-formed green parts was calculated from Equation (2)
ϕ s = ⎜⎛1 − 1 ⎟⎞ × 100% ϕ 0⎠ ⎝
(2)
where ϕ1 and ϕ0 are the true radius and designed radius of green parts, respectively. For compression tests, column ceramics after sintering with around 15.0 mm in diameter and 15–18 mm in height were prepared and tested on an AG-100KN mechanical testing machine (Zwick/ Roell, Germany). The measurements were performed under condition of 0.5 mm/min in crosshead loading rate with 5 specimens for each group. The size distribution measurement of open pore in ceramic foams was characterized using an automatic mercury pressure meter (AutoPore IV 9500, Micromeritics Instrument, China). As for the total porosity and bulk density of the lightweight ceramics, they were measured by the Archimedes' method. The thermal conductivity of FAHS
20804
Ceramics International 45 (2019) 20803–20809
M. Li, et al.
Fig. 1. Schematic diagram of preparation of ceramic foams by SLS: (a) photograph and (b) SEM of calcined FAHSs; (c) SEM of FAHSs/PA12 composites with 15 wt% PA12; (d) SEM and (e) photograph of SLS-formed honeycomb ceramic green body. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) Table 1 Levels of laser scanning parameters of SLS. No.
Factor
Level 1
Level 2
Level 3
1 2 3
Laser power (W) Scanning speed (mm/s) Scanning space (μm)
6.0 1600 110
6.6 1800 130
7.6 2000 150
ceramic foams was counted according to Equation (3)
λ = ρ × α × cm
(3)
where ρ is the bulk density, α is the thermal diffusion coefficient and cm is specific heat capacity of FAHS ceramics. α of porous mullite ceramics was measured through a laser thermal conductivity analyzer (NETZSCH-Gerätebau GmbH, Germany). cm of porous mullite ceramics was evaluated by a differential scanning calorimeter (Diamond, PerkinElmer Instruments, China).
Fig. 2. Effect of laser parameters in compressive strength and radial error of SLS-formed ceramics.
3. Results and discussion The effect of laser parameters on compressive strength and radial error of SLS-formed ceramics is shown in Fig. 2. It is found that the radial error decreases gradually with increasing the laser power, while increases with increasing the hatch spacing and scanning velocity. It should be noted that the average radius error hardly exceeds 5%, which ensures the dimensional accuracy of the SLS process. The compressive strength has the opposite trend of the radial error. When laser power increases from 6.6 to 7.2 w, the radius error keeps in a low level with a slight decrease and there is an obvious increase of compressive strength. So 7.2 W in laser power was chosen. When increase the scanning velocity from 1600 to 1800 mm/s, the radius error obviously increases and the compressive strength keeps in a high level with a slight drop. Therefore, 1600 mm/s in scanning velocity was selected. With hatch spacing changing from 0.11 to 0.13 mm, both the compressive strength and the radius error become worse obviously. Hence,0.11 mm in hatch spacing was determined. As a result, the SLS processing parameter combination of 7.2 W in laser power, 1600 mm/s in scanning velocity and 0.11 mm in hatch spacing is selected for the following SLS experiment as its samples show the relatively low radius error and high compressive strength.
Fig. 1(d) shows the photograph of the SLS-formed honeycomb ceramic green part using 15 wt% PA12, which shows a high geometrical complexity. It should be noted that the intersecting pore channels in the SLS-formed honeycomb ceramics are rather difficult to be obtained through the conventional forming methods. Fig. 1(e) shows the SEM morphology of ceramic green samples, in which the ceramic particles still maintain spherical with inner hollow structures after laser scanning and large quantities of pores exist between FAHSs. The FAHSs are fused together through bonding necks formed by fusing PA12 after laser irradiation. Some agglomerated PA12 particles can be observed due to the incompletely laser sintering, and this may enlarge the gaps between spheres since the PA12 will be burned out in the final treatment. The SEM morphology of lightweight FAHS ceramics sintered at 1350 °C with varying PA12 contents are exhibited in Fig. 3(a-d). It is obvious that through the sintering necks generated at elevated temperature, individual hollow spheres are connected together, which form the shape of ceramic foams. There are two kinds of pore structures in ceramic foams including the closed inner pores from core-shell structure of FAHSs and the open pore channels between FAHSs that consist of stacking pores (pores between stacking spheres) and forming pores
20805
Ceramics International 45 (2019) 20803–20809
M. Li, et al.
Fig. 3. SEM of lightweight FAHS ceramics sintered at 1350 °C with varying PA12 contents (a) 10 wt%, (b) 15 wt%, (c) 20 wt% and (d) 25 wt%, and the insert show the fracture details; (e) SEM images of mullite grains on FAHS shell and XRD patterns of lightweight FAHS ceramics sintered at 1350 °C with 15 wt% PA12 content; (f) SEM images of sintering necks between the FAHSs.
Fig. 4. Size distribution of open pores in lightweight FAHS ceramics with varying PA12 contents.
(special gaps resulted from excessive PA12 content) related to SLS. It should be noted that the stacking pores would change into the forming pores with increasing PA12 content, which could increase the total porosity of ceramic foams. Since the XRD patterns and microstructure
of FAHS ceramics sintered at the same temperature with different PA12 contents are very similar, a typical example with 15 wt% PA12 is displayed in Fig. 3(e). As it can be seen, there are plenty of typical needlelike mullite grains forming in the shell of FAHSs. The elongated mullite grains exhibit about 0.1–0.2 μm in diameter and 1–2 μm in length distributing uniformly and orienting randomly. Furthermore, the peak of mainly mullite phase and a small amount of cristobalite phase are detected in the XRD patterns. It is reported that alkalies and alkaline earth elements in fly ash and the drastic thermal variations on forming process promote the production of cristobalite phase during heat treatments ranging 1200–1400 °C [23]. Fig. 3(f) shows morphology of sintering necks between the FAHSs. Dense sphere shell wall can be observed that could act as the skeleton increasing the mechanical properties of FAHS ceramics. In this case, the strength of sintering necks are rather strong since the cracks generate across the dense FAHS shells instead of sintering necks. The fracture surface is smooth and shows a typical lamellar structure, suggesting a brittle fracture mode, and the microstructures of the shell wall of FAHSs are very homogenous [24]. Besides, few small pores can be observed in the shell wall, which also make up a part of the porosity of ceramic foams. In addition to that, the size distribution of interspaces between spheres, i.e., the open pores in sintering lightweight ceramics with varying PA12 contents is plotted in Fig. 4. It is obvious that all lines show a unimodal peak and the width of the peaks are small which reveals a concentrated size distribution of open pore. The average open pore size first increases slightly from 31.5 to 31.8 μm with the increase
20806
Ceramics International 45 (2019) 20803–20809
M. Li, et al.
Fig. 5. The schematic diagram of the trend in size distribution of open pore in lightweight FAHS ceramics with varying PA12 contents.
Fig. 6. Porosity and bulk density of lightweight mullite ceramics fabricated with varying PA12 contents.
Fig. 7. (a) Compressive strength of lightweight mullite ceramics fabricated via SLS process sintered at 1350 °C with varying PA12 contents; (b) Compressive strength varies with deformation of lightweight FAHS ceramics with 15 wt% PA12 and photograph of the test samples.
Fig. 8. Thermal conductivity versus temperature for mullite ceramic foams prepared with different PA12 contents.
of PA12 content from 10 to 15 wt% and then obviously increases from 31.8 to 34.5 μm with further increase to 25 wt%. The schematic diagram of the trend in size distribution of open pore in lightweight FAHS ceramics with varying PA12 contents is exhibited in Fig. 5. The open pores in ceramic foams formed using low PA12 contents are small and hardly increase due to the insufficient filling of PA12 into the stacking volume between FAHS (shown in Fig. 5(a)). When PA12 is less than 15 wt%, any further increases in PA12 content will fill the stacking volume making up the stacking pores, i.e., the interspaces between stacking spheres, without open pores increasing. When PA12 is above 15 wt%, the open pores enlarge due to the excessive PA12 (shown in Fig. 5(b)), which will not only fulfill the stacking volume but also expand the stacking pores into the forming pores, i.e., the special gaps between spheres related to SLS. Therefore, the stacking spaces between FAHSs play a vital role in open pore size at a small amount of PA12 (< 15 wt%), while the addition of PA12 acts at high contents (> 15 wt %). Consequently, as can be seen in Fig. 6, the bulk density of lightweight ceramics decreases from 0.45 to 0.40 g cm−3. Meanwhile, the total porosity of lightweight ceramics first increases slightly from 85.1 ± 0.3% to 85.2 ± 0.4% and then obviously increases to 86.7 ± 0.5% with increasing PA12 content, which is coincident with the tendency of open pore size and distribution (shown in Fig. 4). Due to the closed pores of FAHSs and the special pore structures related to SLS, the obtained total porosity is much higher than porosity of ceramic foams fabricated by conventional methods, for example, the gel-casting method which achieves a porosity typically ranging from 60 to 70% [25–27]. The compressive properties of ceramic foams are illustrated in Fig. 7(a). As can be observed that the compressive strength first decreases slightly from 2.08 ± 0.10 to 2.02 ± 0.24 MPa, and after that it obviously declines to 1.40 ± 0.20 MPa due to the increasing total porosity. What is noteworthy is that the compressive strength of SLSformed ceramic foams is as high as that of ceramic foams with relative lower porosity prepared through other methods [28–30]. Fig. 7(b) depicts the curve of compressive strength varies with deformation of lightweight FAHS ceramics with 15 wt% PA12, and the inner cylinders are photograph of the test samples. The compression procedure includes two major grades. In the first place, the compressive stress increases elastically and then reaches to the first peak. Afterwards, sectional fracture of spheres in core-shell structure takes place on the most fragile part like in Fig. 3(f) with further pressing, leading to a fast fall in compressive stress. Finally, densification and compaction occur in the
20807
Ceramics International 45 (2019) 20803–20809
M. Li, et al.
Table 2 Properties of lightweight ceramic foams prepared by different methods. Methods
Porosity (%)
Compressive strength (MPa)
Thermal conductivity (W/(m·K))
References
Gel-casting molding Slurry-filtration and heat-treating Foaming and starch consolidation Foam-gelcasting Freeze-cast sol-gel SLS
75 82.3 79.7 86.3 79.4 83.4 78.1 85.1
1.00 1.21 – 1.02 9.00 3.80 1.36 2.08
0.17 0.083 0.20 0.09 0.21 0.17 0.064 0.18
[27] [28] [29] [30] [35] [36] [37] Present work
weakest part and ceramic part regains the capability to sustain loading, leading to the fluctuation of the compressive strength curve. It has the same tendency with brittle materials, in which a population of small cracks extends stably until they intersect with each other and gives final failure [31]. Fig. 8 depicts the curve of thermal conductivity versus temperature of lightweight ceramics fabricated with varying PA12 contents. The thermal conductivity of all ceramic samples increases gradually when measurement temperature increases from 25 to 450 °C, and it should be noted that in no case the thermal conductivity exceeds 0.4 W/(m·K). This relatively low thermal conductivity probably resulted from the high porosity and the special pore structures of the SLS-formed ceramic foams. Additionally, the thermal conductivity of lightweight mullite ceramics decreases with increasing PA12 content from 10 to 25 wt%. As is well known, the thermal conductivity results from different factors, including conduction through the solid substance and the gas possessed in the pores, convection, and radiation [32]. Generally, the radiation depends on the temperature while the conduction is relevant to the gas contained in the pores which mainly depends on the porosity of materials [33,34]. In this case, with the same sintering temperature thus the same phase, the thermal conductivity of lightweight mullite ceramics increases mainly due to the improved heat radiation when the measurement temperature increases from 25 to 450 °C. While the thermal conductivity decreases mainly attributed to deteriorated heat conduction with increased total porosity for materials with PA12 content increasing from 10 to 25 wt%. In Table 2, the comparison on the thermal conductivity, porosity and compressive strength of SLS-formed and conventional methodsformed lightweight ceramics is demonstrated. When compared with ceramic foams fabricated by other methods, the lightweight ceramics prepared by SLS exhibits relative high porosity and compressive strength while thermal conductivity is low, which is probably due to the special pore structures of products related to the FAHSs and SLS technique as schematically illustrated in Fig. 5. It should also be noted that SLS technique can fabricate ceramic products with rather complex shapes, for instance, the honeycomb ceramics with cross pore channels (shown in Fig. 1(e)), which are rather difficult and challenging to be fabricated by conventional methods. 4. Conclusion In this paper, lightweight mullite ceramics with complicated structures were successfully fabricated via SLS technology employing FAHSs as raw materials. The mixed FAHSs/PA12 powders were mingled mechanically, and after that the SLS-formed green parts were prepared using optimized parameters of 7.2 W in laser power, 1600 mm in scanning velocity and 0.11 mm in hatch spacing. With PA12 content increased from 10 to 25 wt%, the open pore size distribution first increased slightly from 31.5 to 31.8 μm and then obviously increased to 34.5 μm due to the open pores change from stacking pores to forming pores after PA12 content excessing maximum of the stacking volume of FAHS. Consequently, the total porosity firstly increased from 85.1 ± 0.3% to 85.2 ± 0.4% and then obviously increased to
86.7 ± 0.5% and the compressive strength correspondingly decreased from 2.08 ± 0.10 to 1.40 ± 0.20 MPa. The thermal conductivity of the lightweight ceramics was also relatively low which could be as low as 0.06 W/(m·K) due to the special pore structures formed from the FAHSs and SLS technique. In summary, this paper proposes a simple and feasible method to dramatically enhance the mechanical strength of lightweight mullite ceramics via SLS without any post-processing techniques, which also lay the root for the manufacture of lightweight ceramic foams with controlled pores and complex shapes. Acknowledgments The work presented in the current article was supported by National Natural Science Foundation of China (51605177), Fundamental Research Funds for the Central Universities (2018KFYYXJJ030), Hubei Provincial Natural Science Foundation of China (2018CFB484), fund of the State Key Laboratory of Solidification Processing in NWPU (SKLSP201829), and Open Project of State Key Laboratory of New Ceramics and Fine Processing of Tsinghua University (KF201810). The authors are grateful for the Analysis and Testing Center of Huazhong University of Science and Technology for XRD and SEM tests. References [1] L. Yuan, B.Y. Ma, Q. Zhu, X.D. Zhang, H. Zhang, J.K. Yu, Preparation and properties of mullite-bonded porous fibrous mullite ceramics by an epoxy resin gel-casting process, Ceram. Int. 43 (2017) 5478–5483. [2] B. Zhang, H.M. Huang, X.L. Lu, X.L. Xu, J. Yao, Fabrication and properties of SiC porous ceramics using a polyurethane preparation process, Ceram. Int. 44 (2018) 16589–16593. [3] X.M. Ren, B.Y. Ma, Y.R. Zhang, et al., Effects of sintering temperature and V2O5 additive on the properties of SiC-Al2O3 ceramic foams, J. Alloy. Comp. 732 (2018) 716–724. [4] J.H. She, T. Ohji, Fabrication and characterization of highly porous mullite ceramics, Mater. Chem. Phys. 80 (2003) 610–614. [5] L. Han, X.G. Deng, F.L. Li, L. Huang, Y.T. Pei, L.H. Dong, S.S. Li, Q.L. Jia, H.J. Zhang, S.W. Zhang, Preparation of high strength porous mullite ceramics via combined foam-gelcasting and microwave heating, Ceram. Int. 41 (2018) 14728–14733. [6] I. Sopyan, J. Kaur, Preparation and characterization of porous hydroxyapatite through polymeric sponge method, Ceram. Int. 35 (2009) 3161–3168. [7] X.G. Deng, S.L. Ran, L. Han, H. Zhang, S.T. Ge, S.W. Zhang, Foam-gelcasting preparation of high-strength self-reinforced porous mullite ceramics, J. Eur. Ceram. Soc. 37 (2017) 4059–4066. [8] A. Dey, N. Kayal, M.D.M. Innocentini, O. Chakrabarti, Investigation on sacrificial pore former removal and mullite binder phase transformation in powder formulations used for preparation of oxide bonded porous SiC ceramics, Ceram. Int. 43 (2017) 9416–9423. [9] W.L. Huo, X.Y. Zhang, Y.G. Chen, Y.J. Lu, J.J. Liu, S. Yan, J.M. Wu, J.L. Yang, Novel lightweight mullite ceramics with high porosity and strength using only fly ash hollow spheres as raw material, J. Eur. Ceram. Soc. 38 (2018) 2035–2042. [10] Z. Sun, J. Fan, F. Yuan, Three-dimensional porous silica ceramics with tailored uniform pores: prepared by inactive spheres, J. Eur. Ceram. Soc. 35 (2015) 3559–3566. [11] Y.F. Shao, D.C. Jia, Y. Zhou, B.Y. Liu, Novel method for fabrication of silicon nitride/silicon oxynitride composite ceramic foams using fly ash cenosphere as a pore-forming agent, J. Am. Ceram. Soc. 91 (2008) 5. [12] C. Wang, J.C. Liu, H.Y. Du, A. Guo, Effect of fly ash cenospheres on the microstructure and properties of silica-based composites, Ceram. Int. 38 (2012) 4395–4400. [13] N. Li, X.Y. Zhang, Y.N. Qu, Y.N. Qu, J. Xu, N. Ma, W.L. Huo, J.L. Long, A simple and efficient way to prepare porous mullite matrix ceramics via directly sintering SiO2-
20808
Ceramics International 45 (2019) 20803–20809
M. Li, et al.
Al2O3 microspheres, J. Eur. Ceram. Soc. 36 (2016) 2807–2812. [14] A.N. Chen, J.M. Wu, K. Liu, J.Y. Chen, H. Xiao, P. Chen, C.H. Li, Y.S. Shi, Highperformance ceramic parts with complex shape prepared by selective laser sintering: a review, Adv. Appl. Ceram. 117 (2018) 100–117. [15] K. Shahzad, J. Decker, J.P. Kruth, J. Vleugels, Additive manufacturing of alumina parts by indirect selective laser sintering and post processing, J. Mater. Process. Tech. 213 (2013) 1484–1494. [16] K. Subramanian, N. Vail, J. Barlow, H. Marcus, Selective laser sintering of alumina with polymer binders, Rapid Prototyp. J. 1 (2017) 24–35. [17] S.M. Nazemosadat, E. Foroozmehr, M. Badrossamay, Preparation of alumina/ polystyrene core-shell composite powder via phase inversion process for indirect selective laser sintering applications, Ceram. Int. 44 (2018) 596–604. [18] F. Chen, J.M. Wu, H.Q. Wu, Y. Chen, C.H. Li, Y.S. Shi, Microstructure and mechanical properties of 3Y-TZP dental ceramics fabricated by selective laser sintering combined with cold isostatic pressing, Int. J. Lightweight Mater. Manuf. 1 (2018) 239–245. [19] S. Chang, L.Q. Li, L. Lu, J.Y.H. Fuh, Selective Laser sintering of porous silica enabled by carbon additive, Materials 10 (2017) 1313. [20] L.Z. Jin, K. Zhang, T.T. Xu, T. Zhang, S. Cheng, The fabrication and mechanical properties of SiC/SiC composites prepared by SLS combined with PIP, Ceram. Int. 44 (2018) 20992–20999. [21] A.N. Chen, M. Li, J. Xu, C.H. Lou, J.M. Wu, L.J. Cheng, Y.S. Shi, C.H. Li, Highporosity lightweight mullite ceramics prepared by selective laser sintering using fly ash hollow spheres as raw materials, J. Eur. Ceram. Soc. 38 (2018) 4553–4559. [22] Y. Khalil, A. Kowalski, N. Hopkinson, Influence of energy density on flexural properties of laser-sintered UHMWPE, Addit. Manuf. 10 (2016) 67–75. [23] M.Y.A. Mollah, S. Promreuk, R. Schennach, D.L. Cocke, R. Güler, Cristobalite formation from thermal treatment of Texas lignite fly ash, Fuel 78 (1999) 1277–1282. [24] J. Zhang, Z. Wang, P. Lin, H. Yuan, Z. Zhou, S. Jiang, Effect of vacuum annealing on the characteristics of plasma sprayed Al2O3-13wt.%TiO2 Coatings, J. Therm. Spray Techn. 21 (2012) 782–791. [25] R. Liu, Y. Li, C.A. Wang, S. Tie, Fabrication of porous alumina–zirconia ceramics by gel-casting and infiltration methods, Mater. Des. 63 (2014) 1–5.
[26] Z.L. Lu, J.W. Cao, S.Z. Bai, M.Y. Wang, D.C. Li, Microstructure and mechanical properties of TiAl-based composites prepared by Stereolithography and gelcasting technologies, J. Alloy. Comp. 633 (2015) 280–287. [27] J.M. Wu, X.Y. Zhang, J.L. Yang, Novel porous Si3N4 ceramics prepared by aqueous gelcasting using Si3N4 poly-hollow microspheres as pore-forming agent, J. Eur. Ceram. Soc. 34 (2014) 1089–1096. [28] X. Dong, G. Sui, Z. Yun, M. Wang, A. Guo, J. Zhang, J. Liu, Effect of temperature on the mechanical behavior of mullite fibrous ceramics with a 3D skeleton structure prepared by molding method, Mater. Des. 90 (2016) 942–948. [29] S. Liu, J. Liu, H. Du, F. Hou, S. Ren, H. Geng, Hierarchical mullite structures and their heat-insulation and compression-resilience properties, Ceram. Int. 40 (2014) 5611–5617. [30] L. Gong, Y. Wang, X. Cheng, R. Zhang, H. Zhang, Porous mullite ceramics with low thermal conductivity prepared by foaming and starch consolidation, J. Porous Mat. 21 (2014) 15–21. [31] M.F. Ashby, C.G. Sammis, The damage mechanics of brittle solids in compression, Pure Appl. Geophys. 133 (1990) 489–521. [32] K.E. Pappacena, K.T. Faber, H. Wang, W.D. Porter, Thermal conductivity of porous silicon carbide derived from wood precursors, J. Am. Ceram. Soc. 90 (2007) 8. [33] T. Shimizu, K. Matsuura, H. Furue, K. Matsuzak, Thermal conductivity of high porosity alumina refractory bricks made by a slurry gelation and foaming method, J. Eur. Ceram. Soc. 33 (2013) 3429–3435. [34] L.L. Gong, Y.H. Wang, X.D. Cheng, R.F. Zhang, H.P. Zhang, Thermal conductivity of highly porous mullite materials, Int. J. Heat. Mass. Tran. 67 (2013) 253–259. [35] X.G. Deng, J.K. Wang, H.J. Zhang, J.H. Liu, W.G. Zhao, Z. Huang, S.W. Zhang, Effects of firing temperature on the microstructures and properties of porous mullite ceramics prepared by foam-gelcasting, Adv. Appl. Ceram. 115 (2016) 204–209. [36] Z. Wang, P. Feng, X. Wang, P. Geng, F. Akhtar, H. Zhang, Fabrication and properties of freeze-cast mullite foams derived from coal-series kaolin, Ceram. Int. 42 (2016) 12414–12421. [37] R. Zhang, X. Hou, C. Ye, B. Wang, Enhanced mechanical and thermal properties of anisotropic fibrous porous mulliteezirconia composites produced using sol-gel impregnation, J. Alloy. Comp. 699 (2017) 511–516.
20809
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Lightweight mullite ceramics with controlled porosity and enhanced properties prepared by SLS using mechanical mixed FAHSs/polyamide12 composites
T
Meng Lia,1, An-Nan Chena,1, Xin Linb, Jia-Min Wua,*, Shuang Chena, Li-Jin Chenga, Ying Chena, Shi-Feng Wena, Chen-Hui Lia, Yu-Sheng Shia a State Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, 430074, China b State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an, 710072, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Fly ash hollow spheres Lightweight ceramics Selective laser sintering Controlled pore size Thermal conductivity
It is difficult to fabricate lightweight ceramic parts with well pore control and high structural complexity. In this paper, lightweight mullite ceramics with controlled porosity and enhanced properties were prepared via selective laser sintering (SLS) using mechanical mixed FAHSs/PA12 (Fly ash hollow spheres/polyamide 12) composites. Crack-free ceramic green bodies were prepared through SLS using the optimized process parameters: 7.2 W in laser power, 1600 mm/s in scanning velocity and 0.11 mm in hatch spacing. The influence of PA12 content on porosity, size distribution of open pore, mechanical properties and thermal property were studied. The pores in ceramic foams consisted of the closed pores from sphere core-shell structures, the open pore channels in the middle of stacking spheres and the special gaps in the middle of spheres related to SLS. It was found that the total porosity of lightweight mullite ceramics increased slightly from 85.1 ± 0.3% to 85.2 ± 0.4% with PA12 content increasing from 10 to 15 wt%, and then increased obviously to 86.7 ± 0.5% with further increasing PA12 addition to 25 wt%. This porosity increase was mainly attributed to the open pores resulted from the PA12 addition. The PA12 were firstly filled into the interspaces between stacking spheres and when the stacking volume reached the maximum, the interspaces expanded to form the special gaps between spheres with further PA12 addition. Thus, the size distribution of open pore in lightweight FAHS ceramics increased from 31.5 to 34.5 μm gradually with PA12 content increasing from 10 to 25 wt%. Finally, low thermal conductivity of 0.06 W/(m·K) was obtained, which probably resulted from the high porosity and the special pore structures of the SLS-formed ceramic foams. The understanding of the unique pore structures and microstructures will help the pore control and strength improvement of SLS-formed ceramic foams using the ceramic hollow spheres.
1. Introduction Lightweight ceramic foams have many excellent features including low density, excellent thermal-shock resistance, low thermal conductivity and good abrasion resistance [1–3]. Due to these features, they have been widely used in applications such as thermal insulators, sound absorption materials, filters and catalyst supports [4,5]. So far, various conventional processing methods have been used to fabricate lightweight ceramics, such as polymeric sponge impregnation [6], foam-gelcasting [7], sacrificial template [8], and direct stack sintering [9]. In foam-gelcasting methods, the pore structures generated from air
are rather complex and uncontrollable. Polymeric sponge method or sacrificial template method requires organic agents acting as templates to shape porous structures, which would give rise to environmental problems. As for direct stack sintering, pores in ceramic foams are usually uneven and the shapes lack the high complexity. Therefore, there is an urgent demand for the fabrication of lightweight ceramic parts with well pore control and high structural complexity. To control the pore structures of lightweight ceramic foams, ceramic hollow spheres are used to fabricate ceramic foams since the particle diameter and distribution of spheres could be well regulated before forming. Recently, a large number of studies have concentrated on
*
Corresponding author. E-mail address: [email protected] (J.-M. Wu). 1 These authors contributed equally to this paper. https://doi.org/10.1016/j.ceramint.2019.07.067 Received 30 May 2019; Received in revised form 29 June 2019; Accepted 6 July 2019 Available online 08 July 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 45 (2019) 20803–20809
M. Li, et al.
applying cenospheres or hollow spheres to fabricate lightweight ceramic foams through conventional forming methods including gelcasting and direct stack sintering. Sun et al. [10] prepared porous silica ceramics with uniform and interconnected pores by adding SiO2 spheres with a different size distribution into the ceramic matrix. It is found that the SiO2 spheres content has a linear effect on the pore size distribution of ceramic foams. Shao et al. [11] used fly ash cenospheres to prepare silicon nitride/silicon oxynitride ceramic foams with tailored pore structures by gelcasting method, in which the cell size can be controlled by adding fly ash cenospheres with different size grades. Wang et al. [12] fabricated silica composite porous ceramics with 29%–63% in porosity and flexure strength of 7–14 MPa by gelcasting method adding fly ash cenospheres. Li et al. [13] fabricated lightweight mullite ceramics showing high porosity reaching up to 81.37% and 6.25 ± 0.91 MPa in compressive strength via sintering SiO2–Al2O3 microspheres directly. These studies have revealed that the reserved inner hollow structures of spheres can not merely achieve the high porosity but also the control of pore structure in final ceramic parts. However, there are still great challenges in the preparation of ceramic products with high structural complexity. As one of the most promising additive manufacturing methods, selective laser sintering (SLS) can manufacture components with rather high geometrical complexity that are difficult or even impossible to be achieved through conventional methods [14]. As for ceramic materials with high-melting point, the SLS technology could be classified into two categories, i.e., direct and indirect SLS. The short laser scanning time and large temperature gradient in direct SLS usually cause the huge thermal stress and thus the cracks in ceramic parts easily [15]. Therefore, the crack-free ceramic parts are generally fabricated via indirect SLS by introducing the low-melting sacrificial binder phase that can easily fuse the ceramic components together to obtain ceramic green parts [16]. Recently, various ceramic components have already been prepared through the indirect SLS method, such as Al2O3, ZrO2, SiO2, SiC [17–20]. However, these studies have mainly focused on improving the density of ceramic parts combined with the post-treatment process such as infiltration and isostatic pressing. Besides, these post-processing methods cannot be used to prepare porous ceramics with improved performance, so there is rare literature report on the method of effectively improving the mechanical performance of SLS-formed porous ceramics. In fact, the SLS process has a unique advantage over manufacturing porous parts due to the incompact deposition of powder layers and open pore channels generated from the sacrificial binder phase after burning out. In our previous work, a new approach to fabricate lightweight mullite ceramics via selective laser sintering FAHSs was proposed, and high porosity reaching to 79.9%–88.7% and improved compressive strength up to 0.2–6.7 MPa were obtained [21]. Nevertheless, there are still some issues existing that need to be solved and studied in detail. Firstly, the core parameters of SLS have not been studied, which can greatly affects the forming quality of SLS-formed products. Additionally, the effect of PA12 content on size distribution of open pore, mechanical performance and thermal property of lightweight mullite ceramics have not been evaluated. In this paper, the optimized SLS parameters of the composite powders were evaluated in depth. The influence of PA12 content on size distribution of open pore, thermal property and mechanical strength of SLS-formed FAHS lightweight ceramic foams were investigated. Finally, the pore control and strength improvement mechanism of SLS-formed ceramic foams were discussed. 2. Experimental procedure 2.1. Raw materials Commercial FAHSs (52.6 wt% SiO2, 30.9 wt% Al2O3; D50 = 75.5 μm, Jingsheng Minerals Co., Ltd.) were used as the starting ceramic powders. As shown in Fig. 1(a), the hollow spheres with
improved mechanical strength were obtained after calcining at 900 °C for 1 h. The SEM morphology of the calcined FAHSs can be seen in Fig. 1(b). The FAHSs are in a typical spherical shape with hollow cavity, which possess good flowability suitable for powder deposition during SLS. PA12 powders with mean particle diameter of 55 μm were provided by Xincheng Engineering Plastics Co., Ltd. in China. The alumina powders with purity of 99% and mean particle diameter of 0.3 μm were obtained from Almatis, Ludwigshafen, Germany. During the sintering period, it was used as a powder bed. 2.2. Ceramic foams preparation The mixed FAHSs/PA12 powders with different mass ratio were mingled mechanically through a three-dimensional blender. Fig. 1(c) depicts the SEM photograph of the mixed FAHS/PA12 composite powders with 15 wt% PA12. The mechanically mixing composite powders are uniform and the FAHSs remain spherical shapes, which would exhibit excellent flowability and formability appropriate for SLS process. After SLS procedure, ceramic green parts were fabricated, which was conducted on a SLS machine of C250. The device was provided by Wuhan Huake 3D Technology Co. Ltd. in China, and it was equipped with CO2 laser beam. It is known that as the central parameter in the SLS process, laser energy density could affect the quality of SLS-formed products, which can be calculated from the following equation (1) [22].
Q= P/(V·L)
(1) 2
where Q is the laser energy density (J/mm ), P is the laser power (W), V is the scanning velocity (mm/s) and L is the hatch spacing (mm). In order to analyze the influence of these processing parameters on forming property of SLS-formed green parts, an orthogonal test (see Table 1) was designed in the present experiment. Laser power (6.0, 6.6, 7.6 W), scanning velocity (1600, 1800, 2000 mm/s), hatch spacing (110, 130, 150 μm) were chosen to execute the laser scanning tests. Layer thickness of 130 μm was used to prepare green samples in each parameter combination. The optimized parameter combination was chosen to prepare green parts with high geometrical complexity after a formability analysis. Subsequently, in order to remove the binder, the green ceramic parts were kept at 650 °C for 2 h to burn the PA12. Finally rose the temperature from 650 °C to 1350 °C holding for another 3 h to densify ceramic green parts. 2.3. Characterization The morphology of the FAHSs after calcination and the fracture surfaces of the ceramic foams were captured by a scanning electron microscope (JSM-7600 F, JEOL, Japan). Under the scan rate of 5°/min, the phase of lightweight ceramics were analyzed by a X-ray diffractometer (XRD-7000s, Shimadzu, Japan), using a Cu tube whose λ is 1.5406 Å at 40 kV and 30 mA. The radial error(s) of SLS-formed green parts was calculated from Equation (2)
ϕ s = ⎜⎛1 − 1 ⎟⎞ × 100% ϕ 0⎠ ⎝
(2)
where ϕ1 and ϕ0 are the true radius and designed radius of green parts, respectively. For compression tests, column ceramics after sintering with around 15.0 mm in diameter and 15–18 mm in height were prepared and tested on an AG-100KN mechanical testing machine (Zwick/ Roell, Germany). The measurements were performed under condition of 0.5 mm/min in crosshead loading rate with 5 specimens for each group. The size distribution measurement of open pore in ceramic foams was characterized using an automatic mercury pressure meter (AutoPore IV 9500, Micromeritics Instrument, China). As for the total porosity and bulk density of the lightweight ceramics, they were measured by the Archimedes' method. The thermal conductivity of FAHS
20804
Ceramics International 45 (2019) 20803–20809
M. Li, et al.
Fig. 1. Schematic diagram of preparation of ceramic foams by SLS: (a) photograph and (b) SEM of calcined FAHSs; (c) SEM of FAHSs/PA12 composites with 15 wt% PA12; (d) SEM and (e) photograph of SLS-formed honeycomb ceramic green body. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) Table 1 Levels of laser scanning parameters of SLS. No.
Factor
Level 1
Level 2
Level 3
1 2 3
Laser power (W) Scanning speed (mm/s) Scanning space (μm)
6.0 1600 110
6.6 1800 130
7.6 2000 150
ceramic foams was counted according to Equation (3)
λ = ρ × α × cm
(3)
where ρ is the bulk density, α is the thermal diffusion coefficient and cm is specific heat capacity of FAHS ceramics. α of porous mullite ceramics was measured through a laser thermal conductivity analyzer (NETZSCH-Gerätebau GmbH, Germany). cm of porous mullite ceramics was evaluated by a differential scanning calorimeter (Diamond, PerkinElmer Instruments, China).
Fig. 2. Effect of laser parameters in compressive strength and radial error of SLS-formed ceramics.
3. Results and discussion The effect of laser parameters on compressive strength and radial error of SLS-formed ceramics is shown in Fig. 2. It is found that the radial error decreases gradually with increasing the laser power, while increases with increasing the hatch spacing and scanning velocity. It should be noted that the average radius error hardly exceeds 5%, which ensures the dimensional accuracy of the SLS process. The compressive strength has the opposite trend of the radial error. When laser power increases from 6.6 to 7.2 w, the radius error keeps in a low level with a slight decrease and there is an obvious increase of compressive strength. So 7.2 W in laser power was chosen. When increase the scanning velocity from 1600 to 1800 mm/s, the radius error obviously increases and the compressive strength keeps in a high level with a slight drop. Therefore, 1600 mm/s in scanning velocity was selected. With hatch spacing changing from 0.11 to 0.13 mm, both the compressive strength and the radius error become worse obviously. Hence,0.11 mm in hatch spacing was determined. As a result, the SLS processing parameter combination of 7.2 W in laser power, 1600 mm/s in scanning velocity and 0.11 mm in hatch spacing is selected for the following SLS experiment as its samples show the relatively low radius error and high compressive strength.
Fig. 1(d) shows the photograph of the SLS-formed honeycomb ceramic green part using 15 wt% PA12, which shows a high geometrical complexity. It should be noted that the intersecting pore channels in the SLS-formed honeycomb ceramics are rather difficult to be obtained through the conventional forming methods. Fig. 1(e) shows the SEM morphology of ceramic green samples, in which the ceramic particles still maintain spherical with inner hollow structures after laser scanning and large quantities of pores exist between FAHSs. The FAHSs are fused together through bonding necks formed by fusing PA12 after laser irradiation. Some agglomerated PA12 particles can be observed due to the incompletely laser sintering, and this may enlarge the gaps between spheres since the PA12 will be burned out in the final treatment. The SEM morphology of lightweight FAHS ceramics sintered at 1350 °C with varying PA12 contents are exhibited in Fig. 3(a-d). It is obvious that through the sintering necks generated at elevated temperature, individual hollow spheres are connected together, which form the shape of ceramic foams. There are two kinds of pore structures in ceramic foams including the closed inner pores from core-shell structure of FAHSs and the open pore channels between FAHSs that consist of stacking pores (pores between stacking spheres) and forming pores
20805
Ceramics International 45 (2019) 20803–20809
M. Li, et al.
Fig. 3. SEM of lightweight FAHS ceramics sintered at 1350 °C with varying PA12 contents (a) 10 wt%, (b) 15 wt%, (c) 20 wt% and (d) 25 wt%, and the insert show the fracture details; (e) SEM images of mullite grains on FAHS shell and XRD patterns of lightweight FAHS ceramics sintered at 1350 °C with 15 wt% PA12 content; (f) SEM images of sintering necks between the FAHSs.
Fig. 4. Size distribution of open pores in lightweight FAHS ceramics with varying PA12 contents.
(special gaps resulted from excessive PA12 content) related to SLS. It should be noted that the stacking pores would change into the forming pores with increasing PA12 content, which could increase the total porosity of ceramic foams. Since the XRD patterns and microstructure
of FAHS ceramics sintered at the same temperature with different PA12 contents are very similar, a typical example with 15 wt% PA12 is displayed in Fig. 3(e). As it can be seen, there are plenty of typical needlelike mullite grains forming in the shell of FAHSs. The elongated mullite grains exhibit about 0.1–0.2 μm in diameter and 1–2 μm in length distributing uniformly and orienting randomly. Furthermore, the peak of mainly mullite phase and a small amount of cristobalite phase are detected in the XRD patterns. It is reported that alkalies and alkaline earth elements in fly ash and the drastic thermal variations on forming process promote the production of cristobalite phase during heat treatments ranging 1200–1400 °C [23]. Fig. 3(f) shows morphology of sintering necks between the FAHSs. Dense sphere shell wall can be observed that could act as the skeleton increasing the mechanical properties of FAHS ceramics. In this case, the strength of sintering necks are rather strong since the cracks generate across the dense FAHS shells instead of sintering necks. The fracture surface is smooth and shows a typical lamellar structure, suggesting a brittle fracture mode, and the microstructures of the shell wall of FAHSs are very homogenous [24]. Besides, few small pores can be observed in the shell wall, which also make up a part of the porosity of ceramic foams. In addition to that, the size distribution of interspaces between spheres, i.e., the open pores in sintering lightweight ceramics with varying PA12 contents is plotted in Fig. 4. It is obvious that all lines show a unimodal peak and the width of the peaks are small which reveals a concentrated size distribution of open pore. The average open pore size first increases slightly from 31.5 to 31.8 μm with the increase
20806
Ceramics International 45 (2019) 20803–20809
M. Li, et al.
Fig. 5. The schematic diagram of the trend in size distribution of open pore in lightweight FAHS ceramics with varying PA12 contents.
Fig. 6. Porosity and bulk density of lightweight mullite ceramics fabricated with varying PA12 contents.
Fig. 7. (a) Compressive strength of lightweight mullite ceramics fabricated via SLS process sintered at 1350 °C with varying PA12 contents; (b) Compressive strength varies with deformation of lightweight FAHS ceramics with 15 wt% PA12 and photograph of the test samples.
Fig. 8. Thermal conductivity versus temperature for mullite ceramic foams prepared with different PA12 contents.
of PA12 content from 10 to 15 wt% and then obviously increases from 31.8 to 34.5 μm with further increase to 25 wt%. The schematic diagram of the trend in size distribution of open pore in lightweight FAHS ceramics with varying PA12 contents is exhibited in Fig. 5. The open pores in ceramic foams formed using low PA12 contents are small and hardly increase due to the insufficient filling of PA12 into the stacking volume between FAHS (shown in Fig. 5(a)). When PA12 is less than 15 wt%, any further increases in PA12 content will fill the stacking volume making up the stacking pores, i.e., the interspaces between stacking spheres, without open pores increasing. When PA12 is above 15 wt%, the open pores enlarge due to the excessive PA12 (shown in Fig. 5(b)), which will not only fulfill the stacking volume but also expand the stacking pores into the forming pores, i.e., the special gaps between spheres related to SLS. Therefore, the stacking spaces between FAHSs play a vital role in open pore size at a small amount of PA12 (< 15 wt%), while the addition of PA12 acts at high contents (> 15 wt %). Consequently, as can be seen in Fig. 6, the bulk density of lightweight ceramics decreases from 0.45 to 0.40 g cm−3. Meanwhile, the total porosity of lightweight ceramics first increases slightly from 85.1 ± 0.3% to 85.2 ± 0.4% and then obviously increases to 86.7 ± 0.5% with increasing PA12 content, which is coincident with the tendency of open pore size and distribution (shown in Fig. 4). Due to the closed pores of FAHSs and the special pore structures related to SLS, the obtained total porosity is much higher than porosity of ceramic foams fabricated by conventional methods, for example, the gel-casting method which achieves a porosity typically ranging from 60 to 70% [25–27]. The compressive properties of ceramic foams are illustrated in Fig. 7(a). As can be observed that the compressive strength first decreases slightly from 2.08 ± 0.10 to 2.02 ± 0.24 MPa, and after that it obviously declines to 1.40 ± 0.20 MPa due to the increasing total porosity. What is noteworthy is that the compressive strength of SLSformed ceramic foams is as high as that of ceramic foams with relative lower porosity prepared through other methods [28–30]. Fig. 7(b) depicts the curve of compressive strength varies with deformation of lightweight FAHS ceramics with 15 wt% PA12, and the inner cylinders are photograph of the test samples. The compression procedure includes two major grades. In the first place, the compressive stress increases elastically and then reaches to the first peak. Afterwards, sectional fracture of spheres in core-shell structure takes place on the most fragile part like in Fig. 3(f) with further pressing, leading to a fast fall in compressive stress. Finally, densification and compaction occur in the
20807
Ceramics International 45 (2019) 20803–20809
M. Li, et al.
Table 2 Properties of lightweight ceramic foams prepared by different methods. Methods
Porosity (%)
Compressive strength (MPa)
Thermal conductivity (W/(m·K))
References
Gel-casting molding Slurry-filtration and heat-treating Foaming and starch consolidation Foam-gelcasting Freeze-cast sol-gel SLS
75 82.3 79.7 86.3 79.4 83.4 78.1 85.1
1.00 1.21 – 1.02 9.00 3.80 1.36 2.08
0.17 0.083 0.20 0.09 0.21 0.17 0.064 0.18
[27] [28] [29] [30] [35] [36] [37] Present work
weakest part and ceramic part regains the capability to sustain loading, leading to the fluctuation of the compressive strength curve. It has the same tendency with brittle materials, in which a population of small cracks extends stably until they intersect with each other and gives final failure [31]. Fig. 8 depicts the curve of thermal conductivity versus temperature of lightweight ceramics fabricated with varying PA12 contents. The thermal conductivity of all ceramic samples increases gradually when measurement temperature increases from 25 to 450 °C, and it should be noted that in no case the thermal conductivity exceeds 0.4 W/(m·K). This relatively low thermal conductivity probably resulted from the high porosity and the special pore structures of the SLS-formed ceramic foams. Additionally, the thermal conductivity of lightweight mullite ceramics decreases with increasing PA12 content from 10 to 25 wt%. As is well known, the thermal conductivity results from different factors, including conduction through the solid substance and the gas possessed in the pores, convection, and radiation [32]. Generally, the radiation depends on the temperature while the conduction is relevant to the gas contained in the pores which mainly depends on the porosity of materials [33,34]. In this case, with the same sintering temperature thus the same phase, the thermal conductivity of lightweight mullite ceramics increases mainly due to the improved heat radiation when the measurement temperature increases from 25 to 450 °C. While the thermal conductivity decreases mainly attributed to deteriorated heat conduction with increased total porosity for materials with PA12 content increasing from 10 to 25 wt%. In Table 2, the comparison on the thermal conductivity, porosity and compressive strength of SLS-formed and conventional methodsformed lightweight ceramics is demonstrated. When compared with ceramic foams fabricated by other methods, the lightweight ceramics prepared by SLS exhibits relative high porosity and compressive strength while thermal conductivity is low, which is probably due to the special pore structures of products related to the FAHSs and SLS technique as schematically illustrated in Fig. 5. It should also be noted that SLS technique can fabricate ceramic products with rather complex shapes, for instance, the honeycomb ceramics with cross pore channels (shown in Fig. 1(e)), which are rather difficult and challenging to be fabricated by conventional methods. 4. Conclusion In this paper, lightweight mullite ceramics with complicated structures were successfully fabricated via SLS technology employing FAHSs as raw materials. The mixed FAHSs/PA12 powders were mingled mechanically, and after that the SLS-formed green parts were prepared using optimized parameters of 7.2 W in laser power, 1600 mm in scanning velocity and 0.11 mm in hatch spacing. With PA12 content increased from 10 to 25 wt%, the open pore size distribution first increased slightly from 31.5 to 31.8 μm and then obviously increased to 34.5 μm due to the open pores change from stacking pores to forming pores after PA12 content excessing maximum of the stacking volume of FAHS. Consequently, the total porosity firstly increased from 85.1 ± 0.3% to 85.2 ± 0.4% and then obviously increased to
86.7 ± 0.5% and the compressive strength correspondingly decreased from 2.08 ± 0.10 to 1.40 ± 0.20 MPa. The thermal conductivity of the lightweight ceramics was also relatively low which could be as low as 0.06 W/(m·K) due to the special pore structures formed from the FAHSs and SLS technique. In summary, this paper proposes a simple and feasible method to dramatically enhance the mechanical strength of lightweight mullite ceramics via SLS without any post-processing techniques, which also lay the root for the manufacture of lightweight ceramic foams with controlled pores and complex shapes. Acknowledgments The work presented in the current article was supported by National Natural Science Foundation of China (51605177), Fundamental Research Funds for the Central Universities (2018KFYYXJJ030), Hubei Provincial Natural Science Foundation of China (2018CFB484), fund of the State Key Laboratory of Solidification Processing in NWPU (SKLSP201829), and Open Project of State Key Laboratory of New Ceramics and Fine Processing of Tsinghua University (KF201810). The authors are grateful for the Analysis and Testing Center of Huazhong University of Science and Technology for XRD and SEM tests. References [1] L. Yuan, B.Y. Ma, Q. Zhu, X.D. Zhang, H. Zhang, J.K. Yu, Preparation and properties of mullite-bonded porous fibrous mullite ceramics by an epoxy resin gel-casting process, Ceram. Int. 43 (2017) 5478–5483. [2] B. Zhang, H.M. Huang, X.L. Lu, X.L. Xu, J. Yao, Fabrication and properties of SiC porous ceramics using a polyurethane preparation process, Ceram. Int. 44 (2018) 16589–16593. [3] X.M. Ren, B.Y. Ma, Y.R. Zhang, et al., Effects of sintering temperature and V2O5 additive on the properties of SiC-Al2O3 ceramic foams, J. Alloy. Comp. 732 (2018) 716–724. [4] J.H. She, T. Ohji, Fabrication and characterization of highly porous mullite ceramics, Mater. Chem. Phys. 80 (2003) 610–614. [5] L. Han, X.G. Deng, F.L. Li, L. Huang, Y.T. Pei, L.H. Dong, S.S. Li, Q.L. Jia, H.J. Zhang, S.W. Zhang, Preparation of high strength porous mullite ceramics via combined foam-gelcasting and microwave heating, Ceram. Int. 41 (2018) 14728–14733. [6] I. Sopyan, J. Kaur, Preparation and characterization of porous hydroxyapatite through polymeric sponge method, Ceram. Int. 35 (2009) 3161–3168. [7] X.G. Deng, S.L. Ran, L. Han, H. Zhang, S.T. Ge, S.W. Zhang, Foam-gelcasting preparation of high-strength self-reinforced porous mullite ceramics, J. Eur. Ceram. Soc. 37 (2017) 4059–4066. [8] A. Dey, N. Kayal, M.D.M. Innocentini, O. Chakrabarti, Investigation on sacrificial pore former removal and mullite binder phase transformation in powder formulations used for preparation of oxide bonded porous SiC ceramics, Ceram. Int. 43 (2017) 9416–9423. [9] W.L. Huo, X.Y. Zhang, Y.G. Chen, Y.J. Lu, J.J. Liu, S. Yan, J.M. Wu, J.L. Yang, Novel lightweight mullite ceramics with high porosity and strength using only fly ash hollow spheres as raw material, J. Eur. Ceram. Soc. 38 (2018) 2035–2042. [10] Z. Sun, J. Fan, F. Yuan, Three-dimensional porous silica ceramics with tailored uniform pores: prepared by inactive spheres, J. Eur. Ceram. Soc. 35 (2015) 3559–3566. [11] Y.F. Shao, D.C. Jia, Y. Zhou, B.Y. Liu, Novel method for fabrication of silicon nitride/silicon oxynitride composite ceramic foams using fly ash cenosphere as a pore-forming agent, J. Am. Ceram. Soc. 91 (2008) 5. [12] C. Wang, J.C. Liu, H.Y. Du, A. Guo, Effect of fly ash cenospheres on the microstructure and properties of silica-based composites, Ceram. Int. 38 (2012) 4395–4400. [13] N. Li, X.Y. Zhang, Y.N. Qu, Y.N. Qu, J. Xu, N. Ma, W.L. Huo, J.L. Long, A simple and efficient way to prepare porous mullite matrix ceramics via directly sintering SiO2-
20808
Ceramics International 45 (2019) 20803–20809
M. Li, et al.
Al2O3 microspheres, J. Eur. Ceram. Soc. 36 (2016) 2807–2812. [14] A.N. Chen, J.M. Wu, K. Liu, J.Y. Chen, H. Xiao, P. Chen, C.H. Li, Y.S. Shi, Highperformance ceramic parts with complex shape prepared by selective laser sintering: a review, Adv. Appl. Ceram. 117 (2018) 100–117. [15] K. Shahzad, J. Decker, J.P. Kruth, J. Vleugels, Additive manufacturing of alumina parts by indirect selective laser sintering and post processing, J. Mater. Process. Tech. 213 (2013) 1484–1494. [16] K. Subramanian, N. Vail, J. Barlow, H. Marcus, Selective laser sintering of alumina with polymer binders, Rapid Prototyp. J. 1 (2017) 24–35. [17] S.M. Nazemosadat, E. Foroozmehr, M. Badrossamay, Preparation of alumina/ polystyrene core-shell composite powder via phase inversion process for indirect selective laser sintering applications, Ceram. Int. 44 (2018) 596–604. [18] F. Chen, J.M. Wu, H.Q. Wu, Y. Chen, C.H. Li, Y.S. Shi, Microstructure and mechanical properties of 3Y-TZP dental ceramics fabricated by selective laser sintering combined with cold isostatic pressing, Int. J. Lightweight Mater. Manuf. 1 (2018) 239–245. [19] S. Chang, L.Q. Li, L. Lu, J.Y.H. Fuh, Selective Laser sintering of porous silica enabled by carbon additive, Materials 10 (2017) 1313. [20] L.Z. Jin, K. Zhang, T.T. Xu, T. Zhang, S. Cheng, The fabrication and mechanical properties of SiC/SiC composites prepared by SLS combined with PIP, Ceram. Int. 44 (2018) 20992–20999. [21] A.N. Chen, M. Li, J. Xu, C.H. Lou, J.M. Wu, L.J. Cheng, Y.S. Shi, C.H. Li, Highporosity lightweight mullite ceramics prepared by selective laser sintering using fly ash hollow spheres as raw materials, J. Eur. Ceram. Soc. 38 (2018) 4553–4559. [22] Y. Khalil, A. Kowalski, N. Hopkinson, Influence of energy density on flexural properties of laser-sintered UHMWPE, Addit. Manuf. 10 (2016) 67–75. [23] M.Y.A. Mollah, S. Promreuk, R. Schennach, D.L. Cocke, R. Güler, Cristobalite formation from thermal treatment of Texas lignite fly ash, Fuel 78 (1999) 1277–1282. [24] J. Zhang, Z. Wang, P. Lin, H. Yuan, Z. Zhou, S. Jiang, Effect of vacuum annealing on the characteristics of plasma sprayed Al2O3-13wt.%TiO2 Coatings, J. Therm. Spray Techn. 21 (2012) 782–791. [25] R. Liu, Y. Li, C.A. Wang, S. Tie, Fabrication of porous alumina–zirconia ceramics by gel-casting and infiltration methods, Mater. Des. 63 (2014) 1–5.
[26] Z.L. Lu, J.W. Cao, S.Z. Bai, M.Y. Wang, D.C. Li, Microstructure and mechanical properties of TiAl-based composites prepared by Stereolithography and gelcasting technologies, J. Alloy. Comp. 633 (2015) 280–287. [27] J.M. Wu, X.Y. Zhang, J.L. Yang, Novel porous Si3N4 ceramics prepared by aqueous gelcasting using Si3N4 poly-hollow microspheres as pore-forming agent, J. Eur. Ceram. Soc. 34 (2014) 1089–1096. [28] X. Dong, G. Sui, Z. Yun, M. Wang, A. Guo, J. Zhang, J. Liu, Effect of temperature on the mechanical behavior of mullite fibrous ceramics with a 3D skeleton structure prepared by molding method, Mater. Des. 90 (2016) 942–948. [29] S. Liu, J. Liu, H. Du, F. Hou, S. Ren, H. Geng, Hierarchical mullite structures and their heat-insulation and compression-resilience properties, Ceram. Int. 40 (2014) 5611–5617. [30] L. Gong, Y. Wang, X. Cheng, R. Zhang, H. Zhang, Porous mullite ceramics with low thermal conductivity prepared by foaming and starch consolidation, J. Porous Mat. 21 (2014) 15–21. [31] M.F. Ashby, C.G. Sammis, The damage mechanics of brittle solids in compression, Pure Appl. Geophys. 133 (1990) 489–521. [32] K.E. Pappacena, K.T. Faber, H. Wang, W.D. Porter, Thermal conductivity of porous silicon carbide derived from wood precursors, J. Am. Ceram. Soc. 90 (2007) 8. [33] T. Shimizu, K. Matsuura, H. Furue, K. Matsuzak, Thermal conductivity of high porosity alumina refractory bricks made by a slurry gelation and foaming method, J. Eur. Ceram. Soc. 33 (2013) 3429–3435. [34] L.L. Gong, Y.H. Wang, X.D. Cheng, R.F. Zhang, H.P. Zhang, Thermal conductivity of highly porous mullite materials, Int. J. Heat. Mass. Tran. 67 (2013) 253–259. [35] X.G. Deng, J.K. Wang, H.J. Zhang, J.H. Liu, W.G. Zhao, Z. Huang, S.W. Zhang, Effects of firing temperature on the microstructures and properties of porous mullite ceramics prepared by foam-gelcasting, Adv. Appl. Ceram. 115 (2016) 204–209. [36] Z. Wang, P. Feng, X. Wang, P. Geng, F. Akhtar, H. Zhang, Fabrication and properties of freeze-cast mullite foams derived from coal-series kaolin, Ceram. Int. 42 (2016) 12414–12421. [37] R. Zhang, X. Hou, C. Ye, B. Wang, Enhanced mechanical and thermal properties of anisotropic fibrous porous mulliteezirconia composites produced using sol-gel impregnation, J. Alloy. Comp. 699 (2017) 511–516.
20809