article posted 22 March 2016
Joachim Deubener is Professor of Glass Science and Engineering at Clausthal University of Technology and Chair of TC 07 (Crystallization and glass-ceramics)
Glass stability vs. glass forming ability – A crystal nucleation study
Joachim Deubener* & Susanne Krüger
Institute of Non-Metallic Materials, Clausthal University of Technology, Germany
The persistence of undercooled melts and their stability against crystallization during forming and cooling processes are the foundation of glass industry. With respect to the underlying kinetics of the liquid-crystal transformation it is frequently distinguished between the terms: "Glass stability" (GS) and "glass forming ability" (GFA). The former qualifies difficulties in crystallization if heated from below glass transition temperature while the latter describes the same behaviour if cooled from above liquidus temperature. This concept is tested for a model glass (lithium disilicate) including isochronal experiments using a constant heating/cooling rate and isothermal experiments where dwelling followed rapid annealing or fast quenching. It is shown that different crystallization mechanisms are present in lithium disilicate glasses, i.e. homogeneous and heterogeneous nucleation. GFA is dominated by heterogeneous nucleation, while GS is dominated by homogeneous nucleation. The results can help to explain why previous attempts to calculate critical cooling rates from homogeneous nucleation failed by orders of magnitude. In order to improve predictions of GFA from theory data on heterogeneous nucleation at surfaces of these glasses or in contact with other materials have to be included in the calculation, which are not readily available for most glass forming melts.