Interactions of gadolinium hafnate and zirconate with silicate melts

Abstract Rare 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 Gd 2 Hf 2 O 7 (GHO) and Gd 2 Zr 2 O 7 (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 . Gd 0.2 Hf 0.8 O 1.9 and Gd 0.2 Zr 0.8 O 1.9 , are used to probe differences in the dissolution and diffusion rates. While both oxides form fluorite, the HfO 2 ‐based one reacts more slowly than that based on ZrO 2 . Analysis of composition profiles across the solid/melt boundary reveals that Hf 4+ diffuses more slowly than Zr 4+ 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.