Interactions of gadolinium hafnate and zirconate with silicate melts

Abstract

AbstractRare earth hafnates and zirconates are candidate materials for thermal and environmental barrier coatings (T/EBC) to protect gas turbine engine components from various environmental threats, including molten silicates derived from ingested mineral debris. This article examines the reactions of Gd2Hf2O7 (GHO) and Gd2Zr2O7 (GZO) to exemplary acidic and basic silicate melts. Exposure experiments at 1400°C reveal that both materials react to form mixed layers of apatite and fluorite. These layers largely hinder melt penetration of grain boundaries in GZO for exposures up to 4 h. However, extensive intergranular melt penetration occurs into GHO below the reaction layer within 1 h for the acidic melt and within 4 h for the basic melt. Shorter exposures (1–5 min) of Gd‐lean versions of the two compounds, viz. Gd0.2Hf0.8O1.9 and Gd0.2Zr0.8O1.9, are used to probe differences in the dissolution and diffusion rates. While both oxides form fluorite, the HfO2‐based one reacts more slowly than that based on ZrO2. Analysis of composition profiles across the solid/melt boundary reveals that Hf4+ diffuses more slowly than Zr4+ and that the hafnate dissolves more slowly than the zirconate; in both cases dissolution is diffusion‐controlled. The implications for the efficacy of reactive crystallization as a silicate mitigation strategy are discussed.