MICROSTRUCTURAL EVOLUTION AND PROPERTIES OF NANOCRYSTALLINE ALUMINA MADE BY REACTIVE SPUTTERING DEPOSITION

Nanocrystalline aluminum oxide films have been synthesized by r.f. reactive sputtering deposition, and their structure and properties have been studied by grazing angle X-ray diffraction, nanoindentation and transmission electron microscopy. The as-deposited films contained nanocrystalline gamma-Al2O3 embedded in an amorphous Al2O3 matrix. Annealing of the films between 800 and 1200-degrees-C resulted in crystallization of the amorphous phase into gamma-Al2O3 and various degrees of gamma --> alpha-phase transformation. Prolonged anneals of the films at 800-degrees-C for 24 h gave rise to the development of structural texture in gamma-Al2O3 grains. No appreciable grain growth of gamma-Al2O3 was detected in the films annealed at temperatures below 1200-degrees-C. Annealing of the films at 1200-degrees-C for 2 h resulted in the formation of explosively grown, single-crystal alpha-Al2O3 grains in a highly textured, polycrystalline gamma-Al2O3 matrix. The microstructure of gamma-Al2O3 exhibited an intriguing layered characteristic. These layers were aligned consistently throughout the films; most of them were oriented at 90-degrees to each other. The layered microstructure is believed to be characteristic of an intermediate state during gamma --> alpha-transformation, which contributes to the concurrent explosive grain growth of alpha-Al2O3. The nucleation of alpha-Al2O3 is suggested to occur along the {220} crystallographic planes of gamma-Al2O3. Preliminary micromechanical properties of as-deposited nanocrystalline Al2O3 films are reported.