Investigating CMAS corrosion in ultrahigh temperature Hf ₆ Ta ₂ O ₁₇ ‐based thermal barrier coatings: A mechanism by which corrosion products hinder corrosion resistance

Abstract Hf 6 Ta 2 O 17 ceramic has outstanding thermal and mechanical properties, along with an extremely high phase transition temperature (2517 K), making it a promising candidate for next‐generation thermal barrier coatings (TBCs). High‐temperature corrosion of TBCs by calcium‐magnesium‐alumino‐silicates (CMAS) is a major failure mode. To understand why Hf 6 Ta 2 O 17 ceramic resists CMAS corrosion well at high temperatures, the CMAS/Hf 6 Ta 2 O 17 (010) system was analyzed via density functional theory to study CMAS – induced corrosion processes. Results show that above 1523 K, the diffusion coefficient of atoms in CMAS exceeds that in Hf 6 Ta 2 O 17 (010). Also, there is significant mutual diffusion between (Ca, Si) and (Hf, Ta) with a low diffusion activation energy. The interaction between CMAS and Hf 6 Ta 2 O 17 has a low reaction energy, enabling the quick formation of dense corrosion products (CaTa 2 O 6 and HfSiO 4 ) at the interface. These products have high phase stability and fast formation rates, remaining intact when in contact with residual CMAS. The interface‐formed corrosion products greatly reduce the CMAS diffusion coefficient into Hf 6 Ta 2 O 17 ; for instance, calcium, the fastest‐diffusing element, has its diffusion coefficient reduced by over five times. These mechanisms effectively limit CMAS corrosion of Hf 6 Ta 2 O 17 , enhancing its potential as an ultrahigh temperature TBCs for the next generation.