Principles of the polyliquid process for thermal toughening of glass

Abstract

The coefficient of heat transfer, h, is the process parameter of greatest influence on the degree of temper, when glass type and thickness are given. High values of h are achieved by quenching with liquids. However, maximum h can only be sustained temporarily since h depends on the temperature of the solid surface, Ts. A numerical difference approximation was developed that permits computation of temperature fields and resulting temper stresses for quenching with temperature-dependent heat transfer coefficient. Calculations using systematical variations of h = f(Ts) show that temper stresses do not only depend on maximum height but also on maximum position. Therefore, the h-T characteristic of any quenching liquid chosen may not be optimal. The polyliquid process provides a simple method for adjusting h = f(Ts) by applying mixtures of a high-boiling carrier with low-boiling liquids. The effect of concentration and kind of the latter is demonstrated for 2 polyliquid model systems. Calculated temper stresses based on measured values of h=f(Ts) agree satisfactorily with measured temper stresses. Thus, measurement of h=f(Ts) is a suitable means for investigating the influence of polyliquid parameters on toughening performance. This is considerably less laborious than preparing, quenching, and testing a meaningful number of glass samples. Experimental control of an optimization carried out accordingly is demonstrated for maximum bending strength. Use of a polyliquid instead of a pure carrier enables a marked surplus in strength, particularly in case of severe surface abrasion.