Induced crystallization porosity and properties of sintereds diopside and wollastonite glass-ceramics

摘要

A diopside (CaO centre dot MgO centre dot 2SiO_2) and wollastonite (CaO centre dot SiO_2) glass-ceramics, forming approx 60 percent crystal phase and approx 40 percent albite-like (Na_2O centre dot Al_2O_3 centre dot 6SiO_2) residual glass were studied. Two other diopside glass-ceramics with higher Na_2O and Al_2O_3 content were also investigated. The phase formation was estimated by DTA, XRD and pycnometry, while the densification process was evaluated by porosity variations, dilatometry and SEM observations. The bending strength, Young modulus and coefficient of thermal expansion of the glass-ceramics were also measured and discussed. The compositions were characterized by a good sinter-ability and surface crystallization and, due to the different volume variation of diopside and wollastonite during the crystallization, diverse amounts of intragranular crystallization induced porosity, P_(CR), were formed in the glass-ceramics. Notwithstanding the higher porosity, the diopside glass-ceramics have better mechanical properties than wollastonite one. At the same time, despite of the different residual glass compositions and the diverse coefficients of thermal expansion, all diopside materials have comparable bending strength and Young modulus. The obtained diopside sintered glass-ceramics are characterized by significant porosity of 10-12 vol. percent, high amount of residual glass (about 40 percent), large initial particles (< 75 mu m) and elevate crystal size (15-25 mu m). Nevertheless high bending strength of 130 MPa and Young modulus of 100 GPa were obtained after only 1 min sinter-crystallization at 900 deg C. The good mechanical properties were related to the P_(CR) formation.