A novel coating offers potential as a heat shield for high-alloy materials, to improve the performance and longevity of high-temperature aerospace components. A team of researchers at Hanbat National University in South Korea introduced a sequential boron-silicon (B-Si) coating technology that forms a highly stable, oxidation-resistant barrier on TiTaNbMoZr alloys – refractory high-entropy materials that are proposed as alternatives to nickel-based superalloys.
The alloys are comprised of titanium (Ti), tantalum (Ta), niobium (Nb), molybdenum (Mo), and zirconium (Zr), which bring a range of qualities for lightness and strength, corrosion resistance, ductility, and high melt points. Investment casting and graphite-mold casting foundries also use TiTaNbMoZr to form medical devices and surgical implants due to biocompatibility.
For decades, aerospace engineers have been seeking materials capable of enduring hotter operating conditions, where higher temperatures promote greater engine efficiency and reduced fuel burn. But even with ceramic coatings, conventional Ni-based alloys are limited by their tendency to soften above roughly 1100 °C.
High-entropy alloys (HEAs), which blend multiple metallic elements into a single robust structure, have emerged as a potential alternative to address that limitation. Now, the Hanbat researchers’ two-step pack cementation process proposes a protective dual-layer coating that will hold its nanostructure even after exposure to an intense 1300 °C.
In a research study they published earlier this year, they compared traditional Si-only coatings with the new B-Si sequence. While untreated and Si-coated alloys suffered heavy oxidation (and even cracking triggered by the transformation of Zr-rich silicides)- the B-Si coating produced a stable mix of XB₂, XSi₂, and X₅SiB₂ phases that resisted degradation under extreme heat.
