Improved oxidation resistance of silicon nitride by aluminum implantation:I, kinetics and oxide characteristics

Hot-isostatically-pressed, additive-free Si3N4 ceramics were implanted with aluminum at multi-energies and multidoses to achieve uniform implant conce ntrations at 1, 5, and 10 at. % to a depth of about 200 nm. The oxidation b ehavior of unimplanted and aluminum-implanted Si3N4 samples was investigate d in 1 atm flowing oxygen entrained with 100 and 220 ppm NaNO3 vapor at 900 -1100 degrees C. Unimplanted Si3N4 exhibits a rapid, linear oxidation rate with an apparent activation energy of about 70 kJ/mol, independent of the s odium content in the gas phase. Oxides formed on the unimplanted samples ar e rough and are populated with cracks and pores. In contrast, aluminum-impl anted Si3N4 shows a significantly reduced, parabolic oxidation rate with ap parent activation energies in the range of 90-140 kJ/mol, depending on the sodium content as well as the implant concentration. The oxides formed on t he implanted samples are glassy and mostly free from surface flaws. The alt eration of the oxidation kinetics and mechanism of Si3N4 in a sodium-contai ning environment by aluminum implantation is a consequence of the effective modification of the properties of the sodium silicates through aluminum in corporation.