Mixed Alkali Effect for Mitigation of Sodium-Accelerated Corrosion of Silicon Nitride Ceramics

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0102340DuThis project is proposed to study the mixed alkali effect (MAE) in the corrosion behavior of Kyocera's SN282, a turbine-grade Si3N4 ceramic, and chemically vapor deposited Si3N4 in oxygen environments containing single alkali (sodium, cesium) and mixed alkali (sodium and cesium) elements. It aims to reveal the potential and establish the understanding of MAE in high-temperature corrosion retardation of Si3N4 ceramics. Questions that will be answered include: (1) Will the incorporation of Na and Cs in the oxide layer on Si3N4 be mutually retarded compared to a single alkali (Na or Cs) situation, and if so, what is the quantitative correlation? (2) How will the corrosion kinetics (rate and activation energy) and oxide characteristics (composition, phase, morphology, and structure) compare under mixed alkali and single alkali conditions? (3) Is MAE in corrosion a function of alkali ratio (e.g., [Na]/[Cs]) in the oxide and in the environment? (4) Do corrosion minima exist at some alkali composition(s)? (5) How will MAE affect the corrosion mechanism? And (6) What are the effects, if any, of the additive and impurity cations in SN282 on (1)-(5)? The objectives of the project will be achieved by evaluating the corrosion kinetics of Si3N4 in O2-NaNO3, O2-CsNO3, and O2-NaNO3-CsNO3 gas mixtures of selected compositions over a broad temperature range and by characterizing the oxide layers using various analytical tools. %%%The susceptibility of silica-forming ceramics to hot corrosion by sodium salts remains a severe roadblock to the exploitation of ceramic benefits in advanced gas turbine engines and other structural applications. A critical need exists to search for means to mitigate the corrosion problem in order for the realization of their projected applications. This project aims to explore the mixed alkali effect (MAE) in silicate glasses for corrosion retardation of silicon nitride ceramics in sodium-containing environments. MAE refers to the pronounced, non-linear changes in many properties of a glass when a dissimilar alkali oxide is added. The premise of the proposal rests on two known facts. First, sodium-accelerated corrosion of silicon nitride results from the dissolution of the oxidation layer through sodium incorporation. Second, MAE lowers the alkali activity and reduces alkali diffusivity in silicates by 2 to 6 orders of magnitude. This project represents the first known attempt to use the MAE concept to address sodium-accelerated corrosion of silica-forming ceramics. It will yield rich kinetic and analytical information on and insights into MAE in high-temperature corrosion of Si3N4. The knowledge generated will provide the design and user communities with the scientific basis for practical exploitation of MAE for corrosion retardation and durability improvement of silica-forming ceramics for advanced turbine engines and other demanding applications where they would otherwise be unsuitable.