Study On Machinable Glass-Ceramics

Machinable ceramics has the ability to machine parts to required size precision and shape by traditional tools. Recently, the applications in high-tech industry need more and more advanced ceramics with machinable performances, so it is very important to develop machinable ceramics with high strength. Mica glass-ceramics absorbed more attention due to the excellent machinability and electricity.In this thesis, the effects of different oxides on the melting and properties of glass were investigated. The results show that MgO contributes to the weakening of silica network and reduce the stability of glass structure. The increase of reheating temperature and MgO content are beneficial for the separation of phlogopite crystals, and cause an higher aspect ratio of the phlogopite phase, which improves the machinability of the glass-ceramics. Excellent machinability is obtained when MgO content is 31.4mol% at the processing temperature of 1100℃for 2h. Al2O3 results in the increase of melting temperature, transition temperature and crystallization temperature of glasses, and the decrease of crystallization tendency. Different reheating schedules can obtain phlogopite with different shapes. Al2O3 content is beneficial for the separation of spinel. K2O contributes to the formation of mica units [AlSiO3], which is beneficial for the separation of phologopite.Uniform design is an useful method for the design of glass compositions and can reduce the experimental works compared with traditional methods. The application of grey theory to the analysis of experimental results is helpful for identifying qualitatively the major and minor factors for the formation of mica.Effects of nucleating agents ZrO2, TiO2 and P2O5 on the crystallization of SiO2-Al2O3-ZrO2 glasses were investigated. The results show that ZrO2 is not an effective nucleating agent in SiO2-Al2O3-ZrO2 system, while TiO2 is effective for the sepatation of spinel, and P2O5 facilitates the solubility of ZrO2 in the glass then precipitation in glass-ceramics. The specimen contains the crystalline phases of spinel, anorthite and tetragonal zirconia. When the main crystal phase is tetragonal zirconia, the bending strength of glass-ceramics increased much (up to 120MPa) than others.Phologopite-mullite glass-ceramics were prepared by melting method. The results show that the increase of mullite content reduces the crystallization tendency of glass, and is beneficial for the separation of cordierite and the formation of fine microstructure. Microhardness and machinability are related closely to its microstructure. The microhardness decreases remarkably with the increase of crystallization and the formation of interlocking structure. The microhardness of the glass-ceramics is slightly increased and then decreased with the addition of mullite. When the content of mullite is 15wt%, the glass-ceramics shows good machinability after reheated at 1200℃for 2h.The glass powder with 40wt% mica composition was mixed with the powder containing different mole ratio of Al2O3/SiO2, and then the mixtures were sintered. The crystallized mica-composition has not obvious effect on the separated crystal phases, but mica-composition glass powder promotes the sintering process. A glass-ceramics with high relative density and bending strength (214MPa) was obtained by sintering at 1200℃.The addition of nucleating agents ZrO₂, La₂O₃, CeO₂, Yb₂O₃, and V₂O₅ promotes the bulk crystallization of the glasses. The effecting order to crystallization tendency is ZrO₂ > V₂O₅ > Yb₂O₃ > CeO₂ > La₂O₃. The higher the cationic field strength is, the smaller the active energy of basic glass will be. When cationic field strength is larger than 8, the active energy is essentialy no change. The additions of nucleating agents restrain the separation of cordierite, which is beneficial for the improvment of machinability. The addition of ZrO2 contributes to the formation of fine microstrucutre and higher mechanical properties. The content of rare earth oxides La2O3 and CeO2 should be less than 1wt% and 2wt%, respectively.