In situ observations of the amorphous-to-gamma transformation in ion-implanted Al2O3

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Transmission electron microscopy (TEM) was used in an investigation of the recrystallization behavior of amorphous surface layers on single crystal [0001] α-Al2O3. Conventional imaging and diffraction techniques were used to characterize the microstructures produced by annealing bulk specimens in the temperature range from 800°C to 1200°C. It was found that the amorphous Al2O3 first transforms to γ-Al2O3, a cubic transitional form, which in turn transforms to α-Al2O3. The γ to α transformation begins at the original crystalline-amorphous interface and proceeds toward the specimen surface. By measuring the position of the γ-α interface as a function of annealing temperature and time it was possible to determine the temperature-dependent velocity of the γ-α transformation front. Assuming a thermally activated process, the data are consistent with an activation energy of 3.6 eV for the transformation. However, no amorphous material remained, after even the shortest anneals (15 min) at the lowest temperature (800°C). These results indicate that the kinetics of the amorphous to γ transformation are faster than those of the γ to α transformation. Therefore a series of in-situ experiments was carried out to investigate the transformation of amorphous Al2O3 to γ-Al2O3. Amorphous Al2O3 surface layers, free of implanted impurities, were produced by implantation at −185°C with 2 × 1016 Al/cm2 at 90 keV and 3 × 1016 O/cm2 at 55 keV. Cross-sectioned specimens were prepared following established techniques.2 The specimens were annealed in-situ in a Philips CM12 operating at 120 kV with a Gatan Model 628 single tilt heating holder. Progress of the transformations was monitored with a Gatan model 622 video system.