Superalloys resist wear at nearly forge-level heat using new process

Finding lubricants that work at exceptionally high temperatures challenges researchers and industries alike. Recently, a Virginia Tech team may have uncovered a promising candidate by happenstance: transition metal spinel oxides formed on nickel-chromium-based superalloys.

Unlike common lubricants that break down under high heat, spinel oxide maintains lubrication up to 700 degrees Celsius (1,292 degrees Fahrenheit) - that's nearly as hot as a metal forge. Enabling metallic materials to withstand hotter temperatures could ignite a new wave of metals manufacturing for industries like aerospace and nuclear energy, which demand innovations in equipment that can withstand extremely high heat. Sparked to find solutions to this critical need, the researchers were funded by multiple grants from the U.S. National Science Foundation and published their results in Nature Communications.

Spinels and spinel structured oxide belong to a group of semi-precious gemstones sometimes found alongside rubies in rare rocks. The researchers found that the mineral also holds a rare quality: the ability to self-lubricate under heat stress and friction. But there's a catch. It only appears to do so under certain circumstances, and only when paired with a certain superalloy thus far.

Demand for metal parts that resist rigorous wear at extremely high temperatures is rising in many industries. Solid lubricants such as thin layers of molybdenum disulfide and graphite on metal surfaces can forestall this wear in some examples. However, none withstand temperatures greater than 600 degrees Celsius (1,112 degrees Fahrenheit) in tests - and not without corrosion.

By comparison, the researchers demonstrated a process by which an additively manufactured sample of a nickel- and chromium-based "superalloy" called Inconel 718 is lubricated by spinel at temperatures exceeding 600 degrees Celsius. Using a novel approach, the team heat-treated its surface before exposing it to these temperatures. The superalloy formed lubricating spinel-based oxides and did not thicken or lose friction tolerance.

The researchers note that it could be the unique structure of spinel itself helping it outcompete similar oxides as a lubricant.

Says Jonathan Madison, program director in the NSF Division of Materials Research, "This work underscores the beautiful complexity that is material science. The structure, properties and performance of materials are not static, they are deeply dynamic and heavily contextual. They are influenced by their environment, their history and in this case what they are next to and what they rub against."

"It is discoveries like this that possess the potential to revolutionize industry, advance technology and ultimately change the world."