Microstructure of thermally grown and deposited alumina films probed with positrons

Aluminum oxide films used for corrosion protection of iron and nickel alumi nides were generated by substrate oxidation as well as plasma and physical vapor depositions. The films grown by oxidation were crystalline. The other s were amorphous. Defect structures of the films were studied by positron s pectroscopy techniques. Lifetimes of the positrons, and Doppler broadening of the gamma photons generated by their annihilation, were measured as func tions of the energies with which they were injected. In this manner, densit ies and sizes of the defects were determined as functions of depths from th e outer surfaces of the films. Alumina films generated by oxidation had hig h densities of open volume defects, mainly consisting of a few aggregated v acancies. In the outer regions of the films the structures of the defects d id not depend on substrate compositions. Positron lifetime measurements, an d the S and W parameters extracted from Doppler broadening spectra, showed uniform distributions of defects in the crystalline Al2O3 films grown on ni ckel aluminide substrates, but these data indicated intermediate layers of higher defect contents at the film/substrate interfaces of oxides grown on iron aluminide substrates. Amorphous films generated by plasma and physical vapor deposition had much larger open volume defects, which caused the ave rage lifetimes of the injected positrons to be significantly longer. The pl asma deposited film exhibited a high density of large cavities. [S0163-1829 (99)09409-6].