Research On The Modulation Of Microstructure And Properties Of Glass-ceramics With High Strength And Low Thermal Expansion

Glass-ceramics is a kind of multiple-phases solid materials containing glassy and crystalline phases,which have both the characteristic properties of glass and ceramic material.Li₂O-Al₂O₃-Si O₂（LAS）glass-ceramics has been extensively studied by scholars.Several crystalline phases can be formed in the LAS glass-ceramics,such as,petalite（Li₂O A1₂O₃8Si O₂）,spodumene（Li₂O A1₂O₃4Si O₂）,eucryptite（Li₂O A1₂O₃2Si O₂）,and a series of solid solution phases.The properties of glass-ceramics are determined by the crystals and microstructure.Due to the ultra-low thermal expansion,excellent heat shock resistance and mechanical properties,the LAS glass-ceramics are widely used in our daily life,medical devices,as well as military equipment.To develop a glass-ceramic with desirable properties,controlling of compositions,nucleation,and crystallization of glasses is a key problem.In this paper,we studied the LAS glass-ceramics using P₂O₅+Zr O₂as nucleation agents.Various concentrations of Li₂O were added into LAS glass to decrease the melting temperature and improve the crystallization,and the effects of Li₂O concentrations on the crystallization behavior and properties were studied.With the determination of optimum concertation of Li₂O,the heat treatments of this glass were systematic studied and optimized.Finally,the different concentrations of Ti O₂were introduced into LAS glass,and their effect on the crystallization process and properties were studied.The LAS glass-ceramics with low thermal expansion coefficient(below 10×10^-7/℃)and high hardness（over 750kgf/mm²）were obtained.The results showed as follows:（1）With the increasing concentration of Li₂O from 15 to 21 mol%,the T_gdecreased from559 to 526℃,indicating the depolymerization of the glass structure.Upon the heat treatments of550℃/2h+（700-790）℃/2h,there is only cristobalite precipitated from glass with 15 mol%Li₂O.When the Li₂O content increases to 17mol%,cristobalite,Li Ai Si₄O₁₀and Li₂Si₂O₅were formed in the glass,and with the occurrence of Li₂Si₂O₅at higher heating temperature,the contents of cristobalite and Li Ai Si₄O₁₀decreased.The main crystalline phase of glass with 19 mol%Li₂O wasβ-quartz solid solution.Meanwhile,the glass with 21 mol%Li₂O showed similar crystallization behavior with the glass containing 17 mol%Li₂O.In this work,the glass with 17mol%Li₂O showed high hardness at relatively wide crystallization temperatures,and the glass-ceramic heat-treated at 550℃/2h+760℃/2h displayed the hardness of 754 kgf/mm²and CET of 9.0×10^-7℃^-1.（2）The T_gof glass with 17 mol%Li₂O was～526℃,and the glass were pre-heated at530-570℃for 2 h,which was a little higher than its T_g.The pre-heated glass showed lower crystallization peak temperature than the untreated glass,and the 550℃was determined as the optimum nucleation temperature.Upon the heat treatment of 550℃/2h+780℃/1h,the glass-ceramics with the crystalline phases of Li Al Si₄O₁₀,Si O₂,and Li₂Si₂O₅,have the optimum properties with the Vickers hardness of 752kgf/mm²and low thermal expansion coefficient of8.57×10^-7/℃^-1.（3）The crystallization process of LAS glass-ceramics:with the increasing of crystallization temperature,the quartz and petalite were precipitated first,and then the Li₂Si₂O₅formed with the decreasing of quartz.Finally,Li Al Si₃O₈precipitated from glass at higher cyrstallzation temperature at the cost of petalite.0-1.5 mol%Ti O₂were introduced into LAS glass,and the crystallization process did not change,while,the crystallized temperatures of Li₂Si₂O₅and Li Al Si₃O₈decreased with the increasing of Ti O₂addition.The glass-ceramics with0.5 mol%Ti O₂heat treated at 550℃/2h+810℃/2h exhibited the Vickers hardness of 815 kgf/mm²,and thermal expansion coefficient of～9.1×10^-7℃^-1.As the inceasing of Ti O₂additions,the hardess of glass-ceramics increased first and then decreased.Meanwhile,all the glass-ceramics at 750℃/2h showed low CET of（9.1-9.4）×10^-7℃^-1.