Thermal strengthening of low-expansion glasses and thin-walled glass products by ultra-fast heat extraction

Abstract

Thermal strengthening remains the primary method for enhancing the practical strength of commodity glass products, however, the process is limited in terms of applicable glass thickness and coefficient of thermal expansion. The primary reasons for this limitation are the achievable heat transfer coefficient when using conventional gas cooling, and the occurrence of transient surface tension in the early stages of rapid quenching. We revisit this problem for the case of thin borosilicate glass sheet. Using liquid gallium as the cooling medium, ultra-fast heat extraction is achieved, with a heat transfer coefficient exceeding 5000 Wm−2 K−1. The low vapor pressure of gallium even at high temperatures enables preheating to a wide range of sheet entrant temperatures. We demonstrate thermal strengthening of low-expansion borosilicate glass with persistent surface compression of up to 85 MPa, and quenching to a fictive temperature of ~190 K above the glass transition temperature. Glass sheet obtained in this way exhibits notably enhanced surface defect resistance to sharp indentation. In addition to thermal strengthening, the extraordinarily high heat extraction rates achieved by liquid metal immersion enable exploitation of high-Tf glass properties beyond small and thin sample geometry.