ZENG Xiaofeng ^{1, 2, 3}, LIU Yuandong ², LI Yongquan ¹, PENG Bo ¹,

LI Changquan ³, GAO Pengzhao ³, XIAO Hanning ³

(1. Hunan Province Silicon Nitride Composite Materials Engineering Technology Research Center, Hengyang Kaixin Special Materials Technology Co., Ltd., Hengyang 421000, Hunan, China; 2. College of Resource Processing and Biology,Central South University, Changsha 410083, Hunan, China; 3. College of Materials Science and Engineering,Hunan University, Changsha 410082, Hunan, China)

Extended abstract:[Background and purposes] Silicon nitride (Si₃N₄) ceramics own excellent mechanical properties, while their low biological activity and difficulty in forming complex shaped products limit their application in bone tissue replacement materials. Therefore, the development of Si₃N₄ ceramic with excellent mechanical properties and biological activity through the compositions and molding processing optimization is a prerequisite for such applications. Fused Deposition Modeling technology possesses the advantages of easy precise control of the size of complex shaped ceramic products, together with simple process, low cost and easy industrialization. However, there are few reports on the use of this technology to prepare complex shaped Si₃N₄ ceramic materials. At the same time, the addition of calcium oxide (CaO) could significantly enhance the biological activity of Si₃N₄ ceramics, but seldom literature reported on the mechanical properties of the materials after addition.[Methods] In this paper, Fused deposition modeling technology (FDM) combined with pressure sintering method as used to prepare Si₃N₄-CaO bioceramics. Influence of CaO on composition, microstructure, mechanical properties and biological activity of the bioceramic materials were studied by using thermal analyzer (TG-DSC), universal material testing machine, scanning electron microscope (SEM), X-ray diffraction (XRD), as well as cell toxicity testing, antibacterial evaluation and cell adhesion and morphology observation. Finally, biological activity of the obtained ceramics was systematically explored, while the intrinsic reasons for the excellent biological activity of the ceramics were ultimately explained.[Results](1) XRD patterns of the Si₃N₄ ceramics with different contents of CaO were basically consistent, being mainly β phase, indicating that the addition of CaO did not change the crystal structure of the ceramics. At the same time, no diffraction peaks of  Y₂O₃, CeO₂, MgO and added CaO were observed, due probably to the liquid-phase sintering mechanism of Si₃N₄ ceramics;

(2) SEM results showed that the obtained Si₃N₄ ceramic without CaO was mainly composed of columnar grains with similar size and shape, with clear water angles and relatively few grain boundaries. The bioceramics with 3 wt.% and 5 wt.% CaO began to show obvious grain boundary bonding phases, with significant differences in grain shape and size, transitioning from cylindrical grains to long plate-like ones. When the content of CaO further was increased to 8 wt.% and 13 wt.%, the microstructure of the bioceramics underwent significant changes. The shape of Si₃N₄ grains was irregular and there were a large number of grain boundary bonding phases in between the grains. Especially in the Si₃N₄-CaO bioceramics with 13% CaO, the morphology of Si₃N₄ grains was difficult to identify, while glass phase was present at the grain boundaries.(3) With increasing content of CaO, mechanical properties of the Si₃N₄-CaO bioceramics decreased, while maintaining at acceptable level. The ceramics with 8 wt.% CaO had flexural strength of 448 MPa, fracture toughness of 5.15 MPam^1/2, Vickers hardness of 13.0 GPa and compressive strength of 2134 MPa, which were significantly higher than those of the traditional biomedical materials. The addition of an appropriate amount of CaO could enriched the phases at the grain boundary, while the mechanical properties of the Si₃N₄-CaO bioceramics decreased, due to the increase in porosity.(4) Compared with traditional biomedical materials and single-phase Si₃N₄ ceramics, the Si₃N₄-CaO bioceramics exhibited an excellent in vitro biological activity and antibacterial performance, when the content of CaO was 8 wt.%. The value of OD₄₅₀ increased by 0.54d⁻¹ and the antibacterial rate reached 72.5% (E. coli) and 73.0% (S. aureus), indicating an active cell activity on the surface of the bioceramics, due to the increase in the content Ca element and porosity on the surface.[Conclusions] Si₃N₄-CaO bioceramics with excellent mechanical strength, antibacterial properties and high biocompatibility were prepared by using the fused deposition modeling technology combined with gas pressure sintering method. The whole preparation route is simple, cost-effective and easy to achieve precise control of complex shape product dimensions, showing high possibility for industrialization.

Key words:Si₃N₄-CaO bioceramic materials; 3D printing fused deposition molding; mechanical properties; biological activity