Microstructural evolution and properties of nanocrystalline alumina made by reactive sputtering deposition

Abstract

Nanocrystalline aluminum oxide films have been synthesized by r.f. reactivesputtering 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 γ−Al 2O 3 embedded in an amorphous Al 2O 3 matrix. Annealing of the films between 800 and 1200°C resulted in crystallization of the amorphousAl 2O 3 matrix. various degrees of γ→α phase transformation. Prolonged anneals of the films at 800 °C for 24h gave arise to the development of structural texture in γ−Al 2O 3 grains. No appreciable grain growth of γ−Al 2O 3 was detected in the films annealed at temperatures below 1200°C. Annealing of the films at 1200°C for 2 h resulted in the formation of explosively grown, single-crytsal α−Al 2O 2 grains in a highly textured, polycrystalline γ−Al 2O 3 matrix. The microstructure of γ−Al 2 O 3 exhibited an intriguing layered characteristic. These layers were aligned consistently throughout the films; most of them were oriented at 90° to each other. The layered microstructure is believed to be the characteristic of an intermediate state during γ→α transformation, which contributes to the concurrent explosive grain growth of α−Al 2O 3. The nucleation of γ-Al 2O 3 is suggested to occur along the {220} crystallographic planes of γ−Al 2O 3.Preliminary micromechanical properties of as-deposited nanocrystalline Al 2O 3 films are reported.