Time evolution of microstructure and mechanical properties of transparent mullite glass ceramics

Abstract Transparent mullite glass ceramics have attracted considerable attention for engineering applications due to their unique combination of excellent mechanical and optical transparency. In this study, a series of mullite glass ceramics was prepared by heating the 30Al 2 O 3 ·70SiO 2 glass at the crystallization peak temperature for varying durations (up to 40 h). The hardness exhibited a sharp increase from 7.30 to 9.30 GPa within the initial 2 h. It then increased only slightly to 9.87 GPa after 30 h, before declining marginally to 9.75 GPa after 40 h. Similar trends were observed for the elastic, shear, and bulk moduli. Correspondingly, the crystallinity increased rapidly to 60% within the first 2 h, followed by a small rise of only 5% to 65% after 40 h of heat treatment. However, prolonged heat treatment led to a continuous drop in crack initiation resistance ( C R ) from 12.30 N for the as‐prepared sample to only 1.92 N for the sample heat‐treated at the crystallization peak temperature (i.e., 993°C) for 40 h. According to the morphological analysis from field emission‐scanning electron microscopy, this phenomenon is attributed to the formation of microcracks in these mullite glass ceramics due to nanocrystal coarsening or large interfacial stress originating from the thermal expansion mismatch. These findings provide fundamental insights into designing transparent ceramics with tailored mechanical properties through controlled crystallization.