Effect of Partial Substitution of K2O for Na2O on Sintering, Crystallization and Mechanical Properties of SiO2-CaO-K2O-Na2O-CaF2 Glass-Ceramics

In this work, the effects of partial substitution of K2O for Na2O on the sintering and crystallization of a SiO2-CaO-K2O-Na2O-CaF2 glass have been studied. This glass composition was close to the glass-forming composition of fluorocanasite miner al. The glass-ceramics were made via sintering with concurrent crystallization of glass-powder compacts. The samples were characterized by hot stage microscopy, differential thermal analysis, X-ray diffraction and scanning electron microscopy. CaF2 precipitated as a primary crystal in all studied glasses and crystallization was predominant between 625 degrees and 1000 degrees C . All glasses showed the best sinterability at 1000 degrees C. By increasing the K2O content from 7 to 13 mol% (at the expense of Na2O), the crystallization peaks shifted to higher temperatures and the main crystalline phases at the best sintering temperature were fluorocanasite, cuspidine, xonotlite and wollastonite. Vickers microhardness, 3-point bending strength, fracture toughness and thermal expansion coefficient were estimated to identify the optimum composition. Measurements revealed that the addition of 10 mol% K2O to the base glass improved the bending strength from 110 +/- 10 to 137 +/- 7 MPa. The maximum achieved fracture toughness was 2.6 +/- 0.1 MPa.m(1/2).