Protective behaviour against high temperature oxidation of silicon nitride films synthesized by ion-beam-assisted deposition

Abstract

Silicon nitride films were synthesized on the surface of nickel-based alloys by concurrent electron beam evaporation of silicon and bombardment with 25 keV nitrogen ions (ion-beam-assisted deposition, IBAD). Analyses of X-ray photoelectron and IR spectra reveal that stoichiometric Si 3N 4 films are formed by IBAD. Transmission electron microscopy observations show that IBAD Si 3N 4 films normally have amorphous structure; selected area diffraction, however, reveals some crystallinity. An analysis of Auger electron spectroscopy profiles shows that the Si 3N 4 film consists of a silicon-enriched layer on the outer surface, an Si 3N 4 layer with uniform composition, and a mixed layer at the film-substrate interface. Isothermal oxidation tests were carried out on the uncoated nickel-based alloy specimens and those coated with films of thicknesses up to 1 μm at 1173 K, 1373 K in air for 8–20 h. The oxidation mode initially follows an approximately linear trend and later approaches a parabolic trend. The films of Si 3N 4 decrease the thermal oxidation rates considerably. X-ray diffraction analysis of the oxidation products shows that there exist five phases,. Cr 2O 3, SiO 2, NiCr 2O 4, Al 2O 3 and Ni 3Si.