Effect of microstructure of a phase separated sodium-borosilicate glass on mechanical properties

A glass with the mol% composition 60 SiO2.37 B2O3.30 Na2O was thermally treated at temperatures in the range from 520 to 680 degrees C for 1-80 h. This led to liquid/liquid phase separation and a droplet phase enriched in Na2O and B2O3 was formed, while the matrix phase was enriched in SiO2. The phase separated glass showed a droplet size distribution which after thermal treatment at comparatively low temperatures showed a lognormal distribution, while higher temperatures resulted in a Brailsford and Wynblatt distribution, which should be expected for Ostwald ripening. The glasses showed two glass transition temperatures and the two phases possessed two different coefficients of thermal expansion. Since the droplet phase has a higher coefficient of thermal expansion, around the droplet radial tensile stresses are formed, while the tangential stresses are compressive. While the Young's Modulus was hardly affected by the phase separation, the hardness was notably smaller for phase separated glasses. The 4-point bending strength increased with the temperature of thermal treatment from 40 MPa for the not treated glass to 115 MPa for a glass thermally treated at 640 degrees C for 20 h. The stresses formed during cooling were estimated from the elastic properties, the coefficients of thermal expansion of the two glassy phases formed, and the glass transition temperature of the matrix phase as a function of the temperature of thermal treatment. The stresses increase with the temperature of thermal treatment up to a maximum at 620 degrees C and then decreases rapidly. This is in rough agreement with the 4-point bending strength high stresses lead to high strength.