High temperature performance of multilayer (Gd0.2Dy0.2Ho0.2Tm0.2Yb0.2)4Hf3O12 thermal/environmental barrier coatings at 1450°C.

A novel thermal/environmental barrier coating (T/EBC) composed of a Si-15 mol% HfO 2 bond coat, a Yb 2 SiO 5 -50mol% Yb 2 Si 2 O 7 intermediate layer, and a (Gd 0.2 Dy 0.2 Ho 0.2 Tm 0.2 Yb 0.2 ) 4 Hf 3 O 12 (15SH/50YbMDS/(5RE) 4 Hf 3 O 12 ) top layer was fabricated by air plasma spraying (APS). Its oxidation and corrosion behavior in air/water vapor environments at 1450 °C were systematically studied. In air environments exposure, the coating underwent sintering, characterized by the grain growth and surface roughening of (5RE) 4 Hf 3 O 12 layer. With prolonged oxidation, the micro-cracks become discontinuous and even heal, dominated by the multipoint contacts mechanism. Concurrently, the 15SH bond coat undergoes oxidation, especially at the edge regions. Under water vapor environments, edge regions degraded via (5RE) 4 Hf 3 O 12 layer dissolution into non-stoichiometric garnet (RE 3 Al 5-x Si x O 12 ) and oxygen-deficient hafnates (5RE) 4 Hf 3-x O 12-2x ), driven by synergistic Si(OH)₄ volatilization and Al(OH) 3 attack, while the corrosion of central region was limited. Compared to its oxidized counterpart, the 15SH bond coat underwent more severe oxidation under corrosive environments. The incorporation of HfO 2 partially improves the service performance of the 15SH bond coat by reacting with SiO 2 to form HfSiO 4 . These findings provide essential insights into the design of high‑entropy hafnate‑based T/EBCs with enhanced durability in both dry and humid high‑temperature environments.