| Corrosion and associated degradation of silicon nitride (Si3N 4) remains an obstacle to its utilization in high-temperature turbines. This research consists of three interrelated components: comparative and comprehensive study of the oxidation behavior of three types of Si3N4 (turbine-grade Kyocera SN 282 sintered with Lu2O3, Toshiba TSN 03-NH with Y2O3/Al2O3, and Norton NBD 200 with MgO) in dry O2; investigation of the role of Na and Cs in the corrosion resistance of SN 282; and exploration of the mixed alkali effect in improving the corrosion resistance of SN Si3N 4.;SN 282, TSN 03-NH, and NBD 200 all follow a parabolic oxidation rate law when exposed in dry O2 at 930°C--1200°C, indicating a diffusion-limited oxidation process. The oxidation rate decreases in order of NBD 200, TSN 03-NH, and SN 282 due to increased crystallinity and refractoriness of the intergranular phases. The activation energies for oxidation of NBD 200, TSN 03-NH, and SN 282 are 259, 205, 107 KJ/mol, respectively. Inward diffusion of molecular oxygen O2 through the oxide layer is the rate-controlling step for oxidation of SN 282. In contrast, outward diffusion of additive cations in the underlying Si3N4 is the rate-limiting mechanism for oxidation of TSN 03-NH and NBD 200.;Na and Cs, separately introduced in vapor form of the corresponding nitrates, accelerate the corrosion of SN 282 due to their fluxing effect on the oxide, leading to rapid diffusion of O2. The adverse effect on the corrosion resistance of SN 282 is more pronounced in Cs than in Na that can be attributed to the larger negative Gibbs free energy for dissolution of SiO2 in Cs silicate than in Na silicate.;Co-introduction of Na and Cs via vapor mixtures of their nitrates at a fixed alkali concentration but varying Na/Cs ratios revealed the presence of the mixed alkali effect in corrosion for the first time, i.e., the addition of a dissimilar alkali species reduces the overall detrimental effect on the corrosion resistance of SN 282. For instance, the degree of corrosion of SN 282 in O2-alkali gas environment decreases in order of 100% Cs, 100% Na, 70% Na+30% Cs, and 50% Na+50% Cs (all at 300 ppm). The strain energy associated with the co-existence of alkali cations of different sizes in the oxide is the most likely factor contributing to the reduced rate of diffusion of individual alkali cations as well as O2. Mixed alkali effect thus offers a potentially exciting means of retarding alkali-accelerated corrosion of Si3N4 in specific and silica-forming structural materials in general. |
