Phase-stability engineering of oxide scales in multicomponent (Hf,Zr,Sc)B2 coatings for durable cyclic ablation resistance

Abstract

The growing demand for reusable thermal protection systems calls for ceramic coatings that can retain protective oxide scales under repeated ultra-high-temperature ablation. However, conventional ultra-high-temperature ceramic (UHTC) coatings often fail under thermal cycling because their HfO2/ZrO2-dominated oxide scales suffer phase-transformation-induced cracking and delamination. Here, we address this limitation by incorporating Sc into multicomponent (Hf,Zr,Sc)B2 solid-solution coatings to engineer oxide-scale phase stability. The optimized coating containing 10 mol% Sc maintains scale integrity after 6 × 120 s exposures at temperatures above 2300°C and exhibits a negative linear recession rate of −0.11 μm s−1. Multiscale characterization of the resulting oxide scale reveals partial tetragonal-to-monoclinic martensitic transformation within parent tetragonal grains, producing monoclinic laths while retaining Sc-stabilized metastable tetragonal domains and thereby limiting the overall transformation-induced volumetric strain. These tetragonal domains are preferentially retained at grain boundaries and triple junctions along intergranular crack paths, a spatial distribution consistent with transformation-associated crack tolerance. These findings establish phase-stability engineering of oxide scales as an effective strategy for maintaining scale integrity and protecting high-temperature structural materials under repeated extreme thermal exposure.