In-situ TEM crystallization of silicate-glass films on Al<inf>2</inf>O<inf>3</inf>

Silicate-glass films were grown by pulsed-laser deposition on single-crystal, basal-plane Al2O3 substrates which had been pre-thinned to electron transparency. Experiments were then performed at high temperature in the TEM, which allowed direct observation and video-recording of the crystallization process. The epitactic growth of pseudo-orthorhombic anorthite (CaAl2Si2O8) and grossular (Ca3Al2Si3O12) on the basal Al2O3 surface was observed, emphasizing the role of the substrate as a "seed" for glass devitrification. The grossular crystals nucleated and grew at random on the substrate surface. In contrast, the anorthite crystals showed preferential nucleation at defect sites in the as-deposited glass film. Nucleation was followed by lateral growth along the substrate surface radiating away from the defect sites. Rotational variants for the silicate crystals were found in both cases, owing to the three-fold symmetry of the substrate. The simultaneous growth of these rotational variants on the substrate surface caused the formation of high-symmetry grain boundaries within the silicate film. In the anorthite film, these grain boundaries were often faceted parallel to low-index planes in one or both grains. © 1997 Acta Metallurgica Inc.