Why Jet Engine Makers Are Going Back to Materials They Abandoned 50 Years Ago

The turbine gas industry is facing a problem: for many years jet engine makers have used a ceramic coating named yttria-stabilised zirconia, or 7YSZ, to protect turbine blades from high temperatures. It functions well, but the yttrium this coating relies on is a rare earth element and China controls most of the world’s supply.

Currently, as export controls tighten and prices rise, researchers are revisiting ceramic compositions from the 1970s and 1980s that the industry abandoned when 7YSZ proved better. The issue is whether today’s engineering techniques can bring those older materials up to an acceptable standard.

The coating that keeps engines running

The metal alloys from which gas turbine blades operate at temperatures that would cause them to melt are kept from melting thanks to a thin thermal barrier coating, and for the last four decades the usual choice of coating has been 7YSZ, which is a mixture of zirconium dioxide stabilised with about 7% yttria by weight.

Photo by Haci on Unsplash

Sanjay Sampath, a distinguished professor at Stony Brook University in New York and someone who has for over 30 years studied thermal spray coatings, states that the gas turbine industry - encompassing both aviation and power generation - uses about 1,360 tonnes of yttrium-stabilised zirconia each year^[1].

7YSZ came to be the accepted standard thanks to a researcher called Stephan Stecura from NASA, who in the 1970s examined different mixtures of yttria and zirconia and discovered that the one with a 7% composition offered the best balance of thermal stability, resistance to repeated heating and cooling, and ease of manufacturing. The fact that yttria and zirconia have similar vapour pressures means that they process together smoothly, which is important when spraying molten ceramic powder onto a spinning turbine blade.

Sampath stated that seven had been an experimental discovery^[2] and that people then said, “Look at this material.”

The supply chain squeeze

China has a dominant position in the processing of rare earth elements; although there are yttrium deposits elsewhere in the world, the western countries largely gave up processing them many decades ago due to environmental concerns and the high labour costs.

“That is because in the Western world^[3] we chose not to deal with these issues,” Sampath said.

Geoffrey Gertz, who is a senior fellow at the Center for a New American Security and was a official at the White House National Security Council, stated it bluntly: the industry is “years, not months, away from ending our dependence on China for rare earths^[4].”

The timeline is important; if there were a sudden disruption in the supply of yttrium, engine manufacturers would encounter a real production bottleneck since thermal barrier coatings are essential for running a modern jet engine and there is at present no drop-in replacement for 7YSZ.

What the alternatives look like

The Canadian National Research Council is collaborating with industry partners in order to look again at the ceramic coatings which were developed and tested in the 1970s and 1980s before the industry decided on 7YSZ.

Oerlikon Metco^[5], a company which produces thermal coatings in Europe, is now working on alternatives using magnesium oxide and calcium oxide rather than rare earth elements. The materials in question are plentiful and inexpensive, but there is a drawback: coatings stabilised with magnesium or calcium are mechanically weaker than 7YSZ. In fact, that is the very reason the industry abandoned them initially.

Rogerio Lima, who is a senior research officer with the NRC, thinks that modern tools might be able to narrow the gap. “We could improve those materials^[6],” he said, referring to the advances in characterisation instruments, computational modelling, and manufacturing control which did not exist 40 years ago.

Sampath is not so optimistic regarding the timeline. The new coating material has to be turned into a powder, then processed into a coating, undergo long-duration heat testing, and afterwards be evaluated and approved before it can be used in real engines. The certification process alone takes several years.

Sampath guessed that even if he somehow came across a material it would still take five years^[7], and he believes that additional funding wouldn’t greatly accelerate the process. “We have better tools^[8], but having more data and more knowledge doesn’t mean that we’ll have a better product.”

What this means for you

Materials science is at the heart of almost every major engineering challenge, from jet engines to batteries to medical implants. The thermal barrier coating problem is a perfect example of how supply chain politics can creaIf someone is interested in the challenge of getting materials to work at their physical limits, they should consider taking degree courses in materials science, metallurgy, or mechanical engineering with a focus on materials. The universities at Cambridge, Sheffield, Manchester, and Swansea all have solid research groups in this field. Rolls-Royce, which is located in Derby, is one of the largest manufacturers of gas turbines globally and offers graduate and apprenticeship programmes in materials engineering. The problem with the supply of rare earth elements is not going to disappear, and for years to come there will be a demand for engineers who specialise in coatings, ceramics, and high-temperature materials.iNext week we’ll be featuring more engineering stories from around the UK and from other parts of the world. If you’ve enjoyed this article, why not share it with a friend who may be interested in engineering? be curious about engineering.