Effects of fine grains and sintering additives on stereolithography additive manufactured Al2O3 ceramic
Introduction
Alumina (Al2O3) ceramic is one of the most important high-performance ceramics used for engineering application, due to its excellent mechanical property, physical and chemical property, corrosion resistance as well as thermal insulation properties [[1], [2], [3], [4]]. Normally, there are three main procedures in the fabrication of complex-shaped Al2O3 ceramic products, including the mold design and fabrication, the processing of product, and the post-processing, which are complicated and potentially increase the production cycle time and waste raw materials. These drawbacks have made Al2O3 ceramic inaccessible to modern manufacturing techniques. Additive manufacturing (AM) technique is a unique manufacturing philosophy and has more advantages than the traditional casting and machining fabrication methods, such as the flexible preparation of highly complex-shaped, precise structures, and a short production cycle time [[5], [6], [7], [8], [9]].
At present, a variety of additive manufacturing techniques have been developed for the production of ceramics, such as direct ink writing (DIW) [10,11], three-dimension printing (3DP) [12], fused deposition modeling (FDM) [13], selective laser sintering (SLS) [[14], [15], [16]], and stereolithography [[17], [18], [19], [20], [21], [22], [23]], among which stereolithography based additive manufacturing technique, including stereolithography apparatus (SLA) and digital light processing (DLP), are promising and applied in various ceramics fabrication due to the advantages to fabricate the ceramics with complex geometry and high precision. Dense and defect-free Al2O3 ceramic products were fabricated via stereolithography based additive manufacturing in Wu's works [24,25]. The effects of particle size, drying process and debinding process on the microstructure and densification of Al2O3 ceramic were investigated. Shuai et al. [26] proposed a method for preparing subtle and complex structured Al2O3 ceramic lattices using stereolithography based additive manufacturing and determined the optimum processing parameters during the debinding and sintering process. Schmidt et al. [27] developed mullite ceramic structures from a photoreactive siloxane suspension containing both nano-sized and micron-sized Al2O3 particles by stereolithography based additive manufacturing. The porous components had a total porosity of 90 vol% and a compression strength of 1.8 ± 0.3 MPa. Kirihara et al. [28] also fabricated Al2O3 micro-lattice photonic crystals with a linear shrinkage and relative density of 25% and 99%, respectively, using stereolithography based additive manufacturing. In one word, it is found that the academic community has extensively explored the effects of the debinding and sintering processes, such as sintering temperature, soaking time, and sintering atmosphere, on the performance of Al2O3 ceramic by stereolithography based additive manufacturing. Li et al. [29,30] prepared various Al2O3 ceramic samples with density of 2.43 g/cm3 and 2.5 g/cm3, and flexural strength of 33.7 MPa and 26.7 MPa using stereolithography based additive manufacturing. The effects of the sintering process on the microstructure and mechanical properties were initially discussed. However, the interactions between sintering and performance (including density, microstructure, mechanical properties, and so on) during the stereolithography based additive manufacturing of Al2O3 ceramic have not been clearly analyzed yet.
It is reported that fine grains and sintered additives can significantly improve the relative density and mechanical strength of Al2O3 ceramic. Nakajima et al. [31] investigated the effects of grain size on the spinel formation kinetics and densification kinetics during sintering. The results showed that the relative density of the Al2O3 parts from 0.2 μm to 1.8 μm particles were as high as 98% and 85%, respectively. Nano-sized Al2O3 particles with average grain sizes of 4-8 nm and 50-100 nm were also used in Peng's work [32]. It was observed that full dense Al2O3 ceramic parts were sintered from 4 to 8 nm particles by microwave plasma method, but the Al2O3 ceramic parts from 50 to 100 nm particles only reached relative density of 89%. Therefore, the finer the grain, the easier the sintering and densification. The reason can be attributed to that finer grains exhibit higher surface energy and grain activity. In addition, the sinter ability of ceramic can also be significantly enhanced by adding sintering additives. Sintering additives can reduce the sintering temperature of Al2O3 ceramics and improve their density via two sintering methods, named liquid phase sintering and solid phase sintering. Pal et al. [33] reported that a 99% dense alumina (α-Al2O3) ceramic with a widely accepted dopant MgO as sintering additive. Sathiyakumar et al. [34] also described the effects of sintering additives, MnO and TiO2, on the sintering of Al2O3 ceramic, and found the Al2O3 sample doped with 0.2 wt% TiO2 reached 98% of theoretical density. Fabris et al. [35] studied the liquid phase sintering of Al2O3 ceramic with MgO-Al2O3-SiO2 glass-ceramic as sintering additives. The glass-ceramic system indicated a promising alternative to replace pure Al2O3 in reinforcement due to its good sintering behavior. As far as we know, the separate effect of fine Al2O3 particles or sintering additives on the performance of Al2O3 ceramics during stereolithography based additive manufacturing were studied, however, synergistic effects of fine grains and sintering additives have not been reported yet.
In this paper, Al2O3 ceramics were manufactured by adding fine grains and sintering additives simultaneously. Fine grains were Al2O3 ceramic powders (d50 = 1.05 μm), while sintering additives were TiO2 and MgO. Al2O3 ceramics were fabricated firstly through stereolithography based additive manufacturing, and debinding and sintering subsequently. The relative density, microstructure, mechanical properties, and physical properties of the Al2O3 ceramics were characterized. Finally, defect-free Al2O3 ceramic lattice structures with high precision and high compressive strength were manufactured.
Section snippets
Raw materials
Al2O3 coarse grain particles (3.8 g/cm3, d50 = 10.34 μm, Zhongzhou Alloy Material Co., Ltd., China) were used as the raw material to prepare Al2O3 ceramic slurry. Al2O3 fine grain particles (3.8 g/cm3, d50 = 1.05 μm, Zhongzhou Alloy Material Co., Ltd., China) were added as fine grains. Here, the Al2O3 coarse grain particles and Al2O3 fine grain particles were named as c-Al2O3 and f-Al2O3 for simplicity, respectively. TiO2 (anatase, 3.84 g/cm3, purity>98%, 60 nm, Aladdin, China) and MgO
Results and discussion
The c-Al2O3 and f-Al2O3 particles were put together and observed their morphology simultaneously, as shown in Fig. 1(a). It is found that both c-Al2O3 and f-Al2O3 particles exhibit typical sphere shapes. As declared by the manufacturer, the d50 of c-Al2O3 and f-Al2O3 particles is 10.34 μm and 1.05 μm, respectively. Before investigating the effects of fine grains and sintering additives, pristine Ac ceramic was firstly fabricated from 50 vol% c-Al2O3 slurry without any fine grains and sintering
Conclusions
In this study, Al2O3 ceramics were fabricated by stereolithography based additive manufacturing, debinding and sintering, when Al2O3 ceramic powders (d50 = 1.05 μm) were used as the fine grains, TiO2 and MgO were added as sintering additives. The results show that f-Al2O3 particles can improve the fineness and activity of the raw powders, but also significantly reduce the sintering temperature of Al2O3 ceramics. TiO2 can usually form a solid solution with Al2O3, resulting in main lattice
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
We sincerely appreciate the financial supports from the Open Project of State Key Laboratory of Explosion Science and Technology (No. QNKT19-08), the National Natural Science Foundation of China (No. 51772028), the Beijing Natural Science Foundation (No. 2182064), the Opening Project of State Key Laboratory of High Performance Ceramics and Superfine Microstructure (SKL201902SIC), and the Graduate Technology Innovation Project of Beijing Institute of Technology (No. 2019CX10020).
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