Why jet engine makers are rediscovering materials abandoned 50 years ago

For decades, the aerospace industry has relied heavily on yttria-stabilised zirconia to ensure its jet engines do not overheat. In the 1970s, Nasa researcher Stephan Stecura tested different mixtures and found that a formulation containing roughly 7% yttria by weight was unusually durable to the wear and tear that planes go through. What became known as 7YSZ blended the best of both worlds thanks to its stable crystalline structure – it was highly resistant to repeated heating and cooling, and had the kind of manufacturing properties that make it so useful for plane design.

“Seven [7SYZ] was an experimental discovery, and people said ‘Look at this material; it's working so much better than the others,’” said Sanjay Sampath, a distinguished professor at Stony Brook University in New York state, who has studied thermal spray coatings for more than 30 years. Yttria and zirconia had similar vapour pressures, making them relatively straightforward to process together, while they were also comparatively inexpensive for what they offered.

Also known as yttrium oxide, yttria has been widely used in the intervening decades, but the industry now faces a problem: China dominates the processing of rare earths, and export restrictions have hit the supply of yttria.

Faced with shortages of a vital material that keeps jet engines from overheating – and as a result, planes in the sky – researchers are leafing through decades-old experiments in search of an answer. The National Research Council of Canada is working with industry partners to investigate whether ceramic coatings developed in the 1970s and 1980s could replace some of those used in modern turbines. European supplier Oerlikon Metco, meanwhile, is developing coatings that use magnesium and calcium oxides rather than rare-earth ingredients.

The problem is that magnesium- and calcium-stabilised zirconia are not as strong as 7YSZ. Modern turbines operate in punishing heat and need thick thermal barrier coatings that insulate parts from the hot gases that pass through an engine during flight. The industry moved on from magnesium and calcium-based components for a reason.

One possible workaround could be to lower the temperatures within an engine – but that would reduce performance, unless better efficiencies could be found in design and engineering. Turning back the clock on materials selection could threaten to reduce fuel efficiency and increase emissions – two things a cash-strapped and carefully regulated aerospace industry will want to avoid.

Engineers are therefore looking for alternative materials to offer a solution. Any promising candidates would require a long lead time to reach production, however, needing to be turned into a powder, processed into a coating, put through long-duration heat testing, then assessed in engines and approved for use. “My guess is even if I magically found a material, it’ll be five years,” Sampath said. He doubts that more research funding could speed the process up much more. Tests designed to show how a material responds to heat over time have to run for long enough to provide meaningful evidence, particularly in such a safety-conscious industry as aviation.

Any alternative material would also need to be produced at scale. Sampath estimates that the gas turbine industry, including power generation as well as aviation, uses about 1,360 tonnes of yttrium-stabilised zirconia a year. Replacing it would need suppliers and coating shops around the world to change how they worked.

That’s why Sampath’s money is on a different potential solution – finding new sources of yttrium and rebuilding the capacity to refine it. Like many rare earths, the mineral is not only found in China. But, Sampath argued, concerns about its environmental impact and the labour involved meant that many countries stopped exploiting their reserves, leaving China to dominate. “It’s just that we in the western world decided not to process these things,” he said.

Rebuilding supply chains could be a more practical solution than reinventing the coatings themselves, but that does not mean the search for alternatives is futile. Modern manufacturing techniques and better testing equipment could help engineers get more out of materials that were discarded decades ago. Whether those improvements will be enough to make them viable replacements is another question. “We have better tools, but more data and more knowledge doesn’T mean a better product,” said Sampath. For anyone claiming to have found a quick fix for the industry’s problems, he has a warning: “I would be very wary of it.”

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