Spark plasma sintered ZrO2-SiO2 glass ceramics and Si3N4 bioceramics

Sammanfattning: This thesis focuses on elaboration and characterization of two types of bioceramics: one is ZrO2-SiO2 nanocrystalline glass ceramic (NCGC) for dental application. The goal is to develop new ZrO2-SiO2 NCGCs with a combination of high strength and high translucency; the other is biodegradable Si3N4 ceramics for spinal fusion. This project aims to improve the osteointergration property of Si3N4 ceramics. Translucent glass ceramics typically suffer from impaired mechanical properties, compared to full-ceramics. We presented a method of obtaining ZrO2-SiO2 NCGCs, with a microstructure of monocrystalline ZrO2 nanoparticles (NPs), embedded in an amorphous SiO2 matrix. Raw powders containing different ZrO2 contents were prepared by the sol-gel method, followed by the spark plasma sintering (SPS). The NCGC with a composition of 35%ZrO2-65%SiO2 (molar ratio, 35Zr) was transparent. Tetragonal ZrO2 NPs were spherical with a diameter of 20–40 nm. The average flexural strength of 35Zr NCGC was 234 MPa. To improve the flexural strength, NCGCs with compositions of 45%ZrO2-55%SiO2 (45Zr), 55%ZrO2-45%SiO2 (55Zr), 65%ZrO2-35%SiO2 (65Zr) were also elaborated. All NCGCs showed high translucency. The flexural strength of the NCGCs significantly increased with the increase of ZrO2 content, achieving as high as 1014 MPa for 65Zr NCGC.  ZrO2 NPs in 65Zr NCGC were ellipsoidal and had a core-shell structure with a thin Zr/Si interfacial layer as the shell. Some of the ZrO2 NPs were connected and formed ZrO2 nanofibers. Moreover, the ZrO2 nanofibers were orderly stacked in short-range to form the 3D nano-architecture. The high flexural strength of the 65Zr NCGC mainly originates from synergistic strengthening effects of the thin Zr/Si interfacial layer and 3D stacked nano-architecture. Regarding biodegradable Si3N4 bioceramics, we used a ternary sintering additive of SrO, MgO and SiO2.   The mechanical properties of the developed Si3N4 bioceramics were comparable to those of traditional Si3N4 ceramics. Sr2+, Mg2+, and Si4+ ions released from the intergranular glass phase after immersion in solution, indicating that the developed Si3N4 bioceramics showed certain biodegradable ability. These ions enhanced the proliferation and differentiation of preosteoblasts. Meanwhile, the ionic dissolution products did not show any toxic effects to the development or physiology of zebrafish embryos.