Investigation On The Processes And Mechanisms Of Phase Transformation At R₂O-CaO-SiO₂ System Glass-ceramics

The R2O-CaO-SiO2-F glass-ceramics with high values of flexural strength and fracture toughness can be produced by continuously rolling process for the high-grade decorative materials. The researches on RaO-CaO-SiO2-F glass-ceramics at home and abroad were focused on the biologic application with canasite and apatite as main crystal phase. However, there were not any researches on the decorative R2O-CaO-SiO2-F glass-ceramics. The reasonable composition for rolling process and the mechanism of phase transformation should be investigated.Therefore, the relationship of basic composition and the structure of parent glass and the effect on the phase transformation were investigated in this dissertation. The reasonable composition, corresponding heat-treatments and the control methods of phase transformation were developed.We can obtain some conclusion according to the experiments.(1) The viscosity and melting temperature of R2O-CaO-SiO2-F system glass were low because the addition of fluorine and the content of K2O, Na2O and CaO were high. The forming temperatures were about 1100℃, which could be suitable for rolling process. However, the structure of glass melts was complicate according to the investigation of quenched specimens in the water. The phase separation occurred in the melts at high temperature. The region of phase separation was isolated island and presented layer structure. The CaF2 phase was precipitated in these phase separation region according to Raman spectra.(2) The annealing temperature of R2O-CaO-SiO2-F system glass was very important for the crystallization of glass-ceramics. The CaF2 would be precipitated in the specimens as crystal nucleus although the specimens were all transparent at this time. The reasonable annealing temperature was 400℃-450℃,(3) The phase separation was occurred and the calcium fluoride was precipitated at nucleation temperature in the R2O-CaO-SiO2-F system glass-ceramics. The phase separation and nucleation were accelerated when the heat-treated temperature was increased. The crystallinity was increased at higher temperature. However, the crystal phase was changed from the xonotlite to the canasite and the content of calcium fluoride was decreased.(4) The main crystalline phase was granular CaF2 and cylindrical cuspidate and the crystallinity were 15%with addition of 5 wt%K2O. The O/Si was increased from 2.56 to 2.64 and the viscosity and the transformation spot temperature were decreased when the addition of K2O increased from 6 wt%to 8 wt%. Moreover, the calcium fluoride were precipitated at 646℃and the temperature of maximum crystallization peak were increased. The crystalline phases were transformed to bulk radial a-canasite and CaF2 and the crystallinity were up to 40%when the addition of K2O increased from 6wt%to 8wt%.(5) The Al3+ions were four-coordination and could strengthen the glass network when the addition of Al2O3 was increased from 2.5 wt%to 4.5 wt%. The transference of the ions especially for alkali cations was restricted at high temperature, which altered the crystallization. The mian crystal phase was a-canasite and CaF2 when the addition of alumina was 2.5 wt%. However, the crystal phases were transformed to CaF2 and xonotlite when the addition of alumina was 3.5 wt%and 4 wt%. When the addition of alumina increased to 4.5 wt%, the a-canasite and CaF2 were precipitated.(6) The main crystalline phases of R20-CaO-SiO2-F system glass-ceramics transformed from canasite to xonotlite and wollastonite when the addition of CaO increased from 10.8 wt%to 16.8 wt%. The rate of crystallization of the specimens and the size of crystalline grain was increased when the addition of CaO increased, which resulted in the disordered microstructure and low flexure strength. The bending strength of specimens was decreased from 85 MPa to 72 MPa.(7) The canasite and a-canasite crystalline phase was promoted when the F content was increased from 3.4 wt%to 6.4 wt%. With the appropriate fluorine content, the flexure strength of sample was enhanced. However, the excess fluorine content could speed up the rate of crystallization, which resulted in coarse crystallite and low flexure strength. The bending strength of specimens was decreased from 86 MPa to 66 MPa.(8) The main crystal phase of R20-CaO-SiO2-F glass-ceramics treated by two-step scheme was xonotlite and a-canasite. The crystallinity of the specimens treated by two-step scheme was higher than that treated by one-step scheme. The main crystal phase of specimens directly treated at 700℃for an hour was xonotlite. The phase transformed from xonotlite to canasite by prolonged the heat-treated times. At this time, the crystal size of specimens treated by one-step scheme was larger than that treated by two-step; however, the microstructure of those specimens was similar. The reasonable heat-treatment of glass-ceramics was nucleated at 450℃-550℃and crystallized at 750℃-850℃.