Podcast: Breakthrough boron-silicon coatings boost durability in extreme-heat components
Key Highlights
- Dual-layer boron-silicon coatings greatly reduce oxidation in Ti-based high-entropy alloys under extreme heat.
- High-entropy alloys show promise as next-gen materials for aerospace and defense thermal environments.
- Investment casting remains essential for producing precise, high-performance components like turbine blades.
- Improved heat-resistant coatings can extend component life and boost efficiency in high-temperature systems.
There is a line of innovation from product design to metallurgy, to production, and to product performance. For aerospace and defense systems, precision parts that operate under intense heat and high pressure are critical to performance and reliability. In this episode of Great Question: A Manufacturing Podcast, American Machinist chief editor Robert Brooks explains how researchers are proposing a new coating process that will make alternative, high-entropy alloys perform at significantly higher temperatures – which could mean greater fuel efficiency and lower maintenance costs for jet engines.
Hello, and welcome to a new installment of the Great Question podcast, presented by Endeavor Business Media’s Manufacturing Group. I’m Robert Brooks with American Machinist and Foundry Management & Technology, and from time to time I like to use an episode of this podcast to describe some technical developments that will give listeners a taste of the how complex are the activities and processes, and ideas, at work in very basic manufacturing operations, like machine shops and foundries.
Today I’m going describe a development that highlights the relationship between metallurgy and product design, and between manufacturing processes and product performance. All this is based on an article available now at foundrymag.com titled, Dual-Layer Coating Improves Heat Resistance for Ti-Alloy Parts.
I imagine some listeners will be familiar with investment casting, but a quick review will be helpful: a final product shape is formed in wax or polystyrene, and this is form is dipped into a ceramic slurry. The ceramic hardens, and then the wax structure is melted away to leave a hollow mold. That hollow mold is filled with molten metal, which solidifies, and when the ceramic shell is removed a finished part is ready. There’s more to it, but that will be sufficient for now.
Investment casting is the Mercedes-Benz among casting processes. It’s the production route for heavily designed, highly engineered products that appeal to buyers for just that reason. Investment casting foundries produce automotive turbochargers, gears, and brakes; valves, pumps, and impellers used in energy and petrochemical manufacturing;
medical implants and surgical tools.
But the most typical applications for investment castings are in aerospace and defense manufacturing – turbine blades and jet engine brackets, for example. Equally, missile systems designed to travel at very high speeds require parts that are lightweight, sturdy, and durable.
Jet engine blades are particularly important: These are high-volume, high-value cast products, and the investment casting process allows foundries to produce them in a vacuum atmosphere, which means the entire blade forms and solidifies as a single crystal structure. That gives the blades a quality known as “creep resistance”. They are structurally uniform, so they endure the extreme heat and stress that prevails in a jet engine.
One more point: turbine blades are commonly produced from nickel-based superalloys, such as Inconel, which are specifically designed for high-temperature/high-stress applications.
But - for some time aerospace engineers have been seeking materials capable of enduring even hotter operating conditions, which will promote greater engine efficiency and reduce fuel burn. Sometimes, blades are treated with a ceramic coating that is intended to increase the heat-resistance for the alloy. But even with ceramic coatings, conventional nickel-based alloys tend to soften above 1,100°C.
Now, a proposed new 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 coating technology that forms a highly stable, oxidation-resistant barrier on so-called high-entropy alloys – these alloys are refractory materials that are proposed as alternatives to nickel-based superalloys.
High-entropy alloys blend multiple metallic elements into a single structure. For jet engines, and for the Hanbat researchers, the high-entropy alloy in consideration is comprised of titanium (Ti), tantalum (Ta), niobium (Nb), molybdenum (Mo), and zirconium (Zr) – giving it a formulaic label that is rather ungainly, TiTaNbMoZr.
But each of those elements imparts particular qualities, namely 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 because it is a bio-compatible material.
The Hanbat University researchers are proposing a two-step pack cementation process that imparts a protective dual-layer coating that will hold its nanostructure even after exposure in conditions of 1,300°C. In the research study they published earlier this year, they compared standard, Silicon-only coatings with the new boron-silicon coating sequence. While untreated and silicon-coated alloys suffered heavy oxidation (and even cracking) - the new boron-silicon coating produced a stable mixture of phases that resisted degradation under extreme heat.
And the performance gap was significant. After 10 hours at 1,300°C, both the uncoated alloy and the Si-coated sample registered high mass gains, signaling rapid oxidation activity. Meanwhile, the boron-silicon coated version showed dramatically reduced mass gain and a notably low parabolic oxidation rate, indicating a durable, self-protecting oxide layer. According to the lead researcher, Prof. Joonsik Park, the test results show that the new material can withstand temperatures far above the 1,100°C limit.
The implications are important. Components routinely exposed to searing combustion environments - jet-engine parts, missile bodies, and other defense and aerospace hardware - could achieve longer lifespans and improved performance. The new coating technique also may be effective in various other high-temperature uses in energy, manufacturing, and other highly engineered applications.
“Overall, our results confirm the potential of high-entropy alloys for use in high-temperature environments and emphasize the critical role of selecting suitable coating strategies tailored to the alloy composition,” according to Prof. Park.
If you’re interested in this subject, I invite you to check out the article available now at foundrymag.com titled, Dual-Layer Coating Improves Heat Resistance for Ti-Alloy Parts
And thank you for listening to the Great Question podcast, presented by Endeavor Business Media’s Manufacturing Group.
About the Podcast
Great Question: A Manufacturing Podcast offers news and information for the people who make, store and move things and those who manage and maintain the facilities where that work gets done. Manufacturers from chemical producers to automakers to machine shops can listen for critical insights into the technologies, economic conditions and best practices that can influence how to best run facilities to reach operational excellence.
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About the Author
Robert Brooks
Robert Brooks has been a business-to-business reporter, writer, editor, and columnist for more than 20 years, specializing in the primary metal and basic manufacturing industries. His work has covered a wide range of topics, including process technology, resource development, material selection, product design, workforce development, and industrial market strategies, among others. Currently, he specializes in subjects related to metal component and product design, development, and manufacturing — including castings, forgings, machined parts, and fabrications.
Brooks is a graduate of Kenyon College (B.A. English, Political Science) and Emory University (M.A. English.)