| Corrosion of silicon nitride (Si{dollar}sb3{dollar}N{dollar}sb4{dollar}) and associated degradation processes can severely limit the performance and reliability of advanced engine and structural systems employing it as a key component. The corrosion resistance of {dollar}rm Sisb3Nsb4{dollar} is adversely affected by sodium, a reactive species commonly present in many service environments. Despite a number of studies on the sodium-accelerated corrosion, few attempts have been made to reduce the adverse effects of sodium on the corrosion resistance of {dollar}rm Sisb3Nsb4{dollar}.; This work aimed to investigate the detailed role of aluminum surface alloying in minimizing the detrimental effect of sodium on the corrosion behavior of {dollar}rm Sisb3Nsb4{dollar}. Ion implantation was used as an alloying tool and pure hot-isostatically-pressed {dollar}rm Sisb3Nsb4{dollar} as a base material. Surface regions ({dollar}sim{dollar}200nm) of highly polished {dollar}rm Sisb3Nsb4{dollar} platelets were implanted with aluminum at multi-energies and multi-doses to achieve a uniform concentration distribution of 1, 5, and 10 at.%. Unimplanted and implanted {dollar}rm Sisb3Nsb4{dollar} samples were exposed in atmospheric pressure oxygen enriched with 100 and 220 ppm sodium nitrate vapor at 900{dollar}spcirc{dollar}-1100{dollar}spcirc{dollar}C for 0.5 to 8 hours. Kinetics of corrosion were evaluated using profilometry in conjunction with etch patterning. The morphological, structural, and chemical characteristics of the corrosion layers were studied using various analytical techniques which include x-ray diffraction, secondary electron microscopy, atomic absorption analysis, Raman spectroscopy, and secondary ion mass spectroscopy.; This investigation has shown that, under the influence of sodium, corrosion of unimplanted {dollar}rm Sisb3Nsb4{dollar} follows a rapid and linear kinetic law. The corrosion layers are non-protective and rough. They also exhibit a high degree of morphological and phase instability, which can be attributed to increased thermodynamic and kinetic tendency towards the formation of low eutectic products, phase separation, and devitrification in the corrosion layer. Aluminum surface alloying, however, can bring about an apparent reversal of these sodium-induced changes. Principally, the corrosion layers formed are protective, smooth, and morphologically stable. Corrosion of aluminum-implanted {dollar}rm Sisb3Nsb4{dollar}, in the presence of sodium, follows a favorable parabolic kinetic law. More than ten-fold an increase in the corrosion resistance is achievable, depending upon the alloying concentration, sodium content, and temperature.; It has been revealed, experimentally and theoretically, that for a given sodium content in the gas phase, an optimum aluminum alloying concentration exists which will minimize the detrimental effect of sodium on the corrosion resistance of {dollar}rm Sisb3Nsb4{dollar}. A simple numerical approach has been developed to correlate the aluminum and sodium concentration parameters. The knowledge established in this investigation will serve an important guideline for the enhancement of corrosion resistance of {dollar}rm Sisb3Nsb4{dollar} in specific, and silicon-based materials in general, not only by ion implantation, but also by other surface alloying techniques such as chemical vapor co-deposition. |
