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Eingeladener Vortrag
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Water has a strong impact on the viscosity and the transition temperature of oxide glasses. It acts as a fluxing agent, reducing viscosity particularly in the glass transition range, and by that flattening the viscosity-temperature curve and decreasing the fragility of hydrous glass melts. This general behavior is found for silicate and borate as well as in borosilicate compositions. However, there is still a lack of data with respect to hydrous phosphate glasses. Here, we study the influence of structural water on the viscosity of Li2O-MgO-(Al2O3-)P2O5 glasses. The glasses were synthesized in an internally heated pressure vessel (IHPV) at a pressure of 0.5 GPa. The selected pressure allows for the synthesis of glass specimens with water contents of several wt.%. The viscosity measurements were carried out in the glass transition range, i.e. log η ≈ 10-13 (η in Pa s), using a sphere penetration viscometer. The measured data agree with glass transition temperatures measured by differential thermo analysis. First results further suggest that the fragility of the analyzed phosphates decreases with increasing water content.
185 cracks were initiated in a soda-lime-silica glass by Vickers indentation and grown under controlled dry nitrogen atmosphere. Direct observation of subcritical crack growth (SCCG) upon indentation was performed using a video camera with an image acquisition rate of 3 ms. The results show that the validity of K(c) relations used to determine the indentation fracture toughness from a single imprint is questionable. It seems that correlation of crack length to stress intensity necessitate the use of statistical significant data of multiple crack events.