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- Aluminosilicate glasses (1)
- Borate glasses (1)
- Boratgläser (1)
- Crack initiation (1)
- Elastic constants (1)
- Hardness (1)
- Indentation (1)
- Sub-critical crack-growth (1)
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- Thermische Analyse (1)
Bulk elastic properties, hardness and fatigue of calcium aluminosilicate (CAS) glasses in the technically relevant region of the ternary with intermediate-silica fraction were determined by combining results of Vickers indentation, ultrasonic echography and Archimedian buoyancy at room temperature. Of three studied compositional series, the first series was along the meta-aluminous join, while in the two other series the molar fraction of SiO2 was kept constant. For the first series the elastic moduli and hardness show an almost linear increase with increasing SiO2 fraction. In contrast, increasing the CaO/SiO2 ratio at constant silica content results in a characteristic change when passing the meta-aluminous join. Elastic constants and hardness were lower at the percalcic than on the peraluminous side. Empirical models which relate bulk elastic moduli to internal energy, short-range inter-atomic forces and the packing density of their oxide constituents were found to be in agreement with the experimental data for meta-aluminous glasses, while deviations between the observed and predicted trends were evident for percalcic compositions. Empirical fatigue parameters such as the probability to initiate cracks after indentation reflect the bulk mechanical properties in the CAS system when conducted in inert atmosphere, but are otherwise clearly dominated by environmental effects
In the present study we have investigated whether the effect of water on properties of borate glasses resembles that of alkali oxide. Soda-lime-borate glasses with nominal compositions of x Na2O, 10 CaO, (90-x) B2O3 (x = 5, 15 and 25 mol%) were doped with up to 8 wt.% H2O by processing glass powder + distilled water in platinum capsules in an internally heated gas pressure vessel at 1523 K and 500 MPa. The water content of hydrous glasses was determined by Karl-Fischer titration and near-infrared spectroscopy. The glass transition temperature T-g. was derived from DTA and micropenetration experiments for which the effect of water loss at the surface of the hydrous glasses was studied. Heating glass samples at 10 K min(-1) in the DTA resulted in T-g values which are close to T-12 isokom temperatures confirming the equivalence of enthalpy relaxation and viscous relaxation for borate glasses. For all three glass series it is shown that T-g strongly decreases whereas the liquid fragility strongly increases upon the addition of water. These findings reveal that H2O primarily causes breaking of B-O-B bonds rather than supporting 4-fold coordinated boron as it is well-known for alkali oxides in this concentration range. (C) 2015 Elsevier B.V. All rights reserved.