Fabrication And Properties Of Machinable Fluorophlogopite Glass-Ceramics

Glass-ceramics are polycrystalline materials made by controlling crystallization of glasses. Compared with the matrix glasses, their mechanical and thermal properties are improved significantly. Especially, machinable glass-ceramics can be machined using the conventional metal tooling processes, which is a major progress in materials technology.Machinable glass-ceramics were fabricated based on the SiO₂-MgO-Al₂O₃-K₂O-Na₂O-F system by sintering. In the glass system, part of SiO₂ and Al₂O₃ were replaced by B₂O₃ and ZnO to decrease melting temperature of raw materials. The best composition of B₂O₃ and ZnO was determined through melting temperature test. The relationship of the microstructure, sintering process and properties was investigated using DSC, XRD and SEM. Besides, the effect of ZrO₂ on the microstructure and properties of machinable glass-ceramics based on the SiO₂-MgO-Al₂O₃-K₂O-Na₂O-F system was studied and the reinforcement mechanism of ZrO₂ in machinable glass-ceramics was also discussed. The results show that:1. B₂O₃ and ZnO were effective to decrease the melting temperature of raw materials. When the content of B₂O₃ was 9%, the melting temperature was reduced from 1400℃to 1200℃.2. By analyzing the results of density, three point bending strength, hardness of specimensunder various sintering process, the optimized sintering processes are as follows:Specimen 4a(SiO₂39.9%, Al₂O₃17.2%, MgO19.5%, F6.3%, Na₂O2.3%, K₂O5.8%, B₂O₃9.0%)the optimized sintering process is determined to be 820℃×2h+950℃×4h;Specimen 4b (SiO₂39.9%, Al₂O₃17.2%, MgO19.5%, F6.3%, Na₂O2.0%, K₂O5.8%, ZnO6.0%)the optimized sintering process is determined to be 880℃2h+1100℃×4h;Specimen 4c(SiO₂39.9%, Al₂O₃17.2%, MgO19.5%, F 6.3%, Na₂O2.3%, K₂O5.8%, B₂O₃9.0%, ZnO6.0%)the optimized sintering process is determined to be 790℃×2h+1050℃×4h. 3. The final crystals are mainly fluorophlogopite of Specimen 4a, 4b, 4c under different sintering temperature and a little Magnesium aluminate spinel, Zinc magnesium silicate and forsterite were also formed. Fluorophlogopite formed of Specimen 4a and 4b under optimized sintering process with high interlocked microstructure and high aspect ratio, which ensure good machinability and mechanical properties.4.The solubility of ZrO₂ in MgO-Al₂O₃-SiO₂-B₂O₃-K₂O-Na₂O-F system glass-ceramics is between 7%-8%.5. By analyzing the results of density, three point bending strength, hardness and fracture toughness of specimens under various sintering process, the optimized sintering processes of 3Z,5Z,7Z was 800℃×2h+1000℃×4h and the optimized content of ZrO₂ was 5%.6.Glass-ceramics strengthened by ZrO₂ have smaller grain size and higher strength. The bend -ing strength and fracture toughness of the glass-ceramics rose with the increase of ZrO₂. ZrO₂ has no effect on the crystal phase and microstructure of the glass-ceramics. The bending strength of 5Z was up to 267Mpa from 234.6Mpa and fracture toughness was up to 2.72 from 2.15MPa·m^1/2.