Temperature dependence of vickers hardness for TeO2-based and soda-lime silicate glasses

Abstract

The temperature dependence of Vickers hardness (H) of TeO2-based glasses such as 15Na2O · 15ZnO · 70TeO2 as host glasses for rare-earth doped optical amplifier was measured from room temperature to the glass transition region. The following features were found; (1) crack formation under the indenter easily occurs, the Vickers hardness at room temperature being around 3 GPa; (2) the temperature coefficient of the hardness, dH/dT, is around -4.5 × 10-3 GPa · K-1; (3) the relative change of hardness with temperature is closely related to thermal expansion coefficient, and hardness sharply decreases in the glass transition region. It was demonstrated that TeO2-based glasses are fragile even from the point of view of mechanical properties. The reason for poor mechanical performance in TeO2-based glasses is their weak bond strength and open atomic packing structures. The hardness (H=4.8 GPa) of a transparent glass-ceramic (with nano-crystalline particles) of 15K2O · 15Nb2O5 · 70TeO2 showing second harmonic generation is much larger than that of the precursor glass (H=3.3 GPa), meaning that the improvement in the fragile character of TeO2-based glasses is due to crystallization. A soda-lime silicate glass of 14Na2O · 13CaO · 73SiO2 has a value dH/dT= -6.5 × 10-3 GPa · K-1, and in contrast to TeO2-based glasses, its hardness decrease in the glass transition region is gradual, and a value H ≈ 2 GPa is obtained even at the glass transition temperature.