Influence of Reactive Elements on Next-Generation Bond Coats in Thermal Barrier Coating Systems Using High Entropy Alloys

Increasing demands for higher operating temperatures and improved efficiency reveal the limitations of conventional bond coat materials like MCrAlX, which are susceptible to oxidation and degradation. This study investigates FeCoNiCrAlYHf and FeCoNiCrAl HEAs as alternative bond coat materials to overcome these limitations. The bond coats were applied using low-temperature spray techniques, including HVOF and HVAF, to assess the impact of spray temperature on coating microstructure and performance. All samples underwent isothermal oxidation testing at 1150°C for 200 hours to measure mass gain, both before and after vacuum heat treatment (conducted at 1050°C for 4 hours), to evaluate the behavior of the TGO layer in response to the presence of reactive elements and heat treatment. The results show that HEA bond coats, with and without reactive elements, successfully formed dense, continuous, and uniform protective Al₂O₃ layers without mixed oxide formation. In contrast, mixed oxide formation was observed in the MCrAlX benchmark sprayed with HVOF. No significant difference in TGO formation was found between HEA coatings with reactive elements, demonstrating superior oxidation resistance. Additionally, vacuum heat treatment reduced TGO growth by approximately 40%, which is considered to be a promising result.