Engineers are always trying to squeeze more power out of engines, and those in the military have added incentive. Military researchers looking to increase the power and thrust of a commercial compact jet engine turned to ceramics to meet the increased turbine inlet temperature that prevented them from using metals. Using finite element analysis and 3D printing, they showed that a turbine rotor made of silicon nitride could handle the higher temperatures and still provide increased power and thrust.

“We wanted to take something that was a small-scale engine that was already fairly power-dense to begin with and try to make it more power dense,” said U.S. Air Force Lt. Col. Brian Bohan. “There’s a big gap in the commercial sector for this scale. We were trying to come up with ways to make an engine as powerful as some of the more expensive military engines, but at a more cost-effective price point.”

Bohan, formerly on the faculty at the U.S. Air Force Institute of Technology, was advisor and a co-author to Bryan T. Leicht, the lead author of “Manufacturing a Ceramic Turbine Rotor for a Compact Jet Engine,” published in the August 2023 issue of ASME’s Journal of Turbomachinery.

Bohan said the work began with the idea of bolstering engines for small unmanned aircraft, or drones, that would be powerful enough to keep up with a regular jet engine and cheap enough to make it cost effective. But the work soon concentrated on a more generic purpose.

“There are hobby-class engines at the small scale, and then you have military engines that are designed for small scale,” he said. “But they are $70,000 to $80,000 apiece. If you’re trying to design an engine for somewhere in between those levels, you really have no options. That led to ‘How do we get more power out of these commercial hobby-class engines, and how do we get something better out of the small scale?’”

The researchers worked with a JetCat P400, which at $12,500 was a more affordable test object. But achieving the higher power density required increasing the turbine inlet temperature, which would exceed the maximum temperature limitations of its metallic turbine rotor. The authors also noted that traditional blade cooling methods could not be used because of its small size.

So they turned to ceramics, which can withstand higher temperatures while maintaining strength. A drop-in replacement ceramic turbine would allow temperatures to rise from 765˚ C in the stock engine to 1,200˚ C and improve thrust and power by a factor of 1.44, noted the authors.


3D-printed rotor made from alumina.

The preferred material is silicon nitride, which the researchers determined could work through the use of finite element analysis. The problem is that the material is still being developed for use in additive manufacturing. For a test, the researchers 3D printed an alumina rotor using digital light projection. Bohan believes the results can be duplicated with silicon nitride when it becomes available.

“There’s not a ton on ceramic 3D printing out there,” he said. “It is still pretty new. We started the process with cast ceramics, looking at different mold materials. We exhausted every possible option with molding and casting, mainly due to the complexity of the part. We could make a mold, but we could not get the part to dry successfully because we have very thin blades and a very thick hub. The two dry at very different rates.”

Their work improved when they obtained access to an ADMATEC Admaflex 300 DLP printer. Although engineers at Oak Ridge National Laboratory are working with silicon nitride for additive manufacturing, its current unavailability forced them to use an alumina resin for proof of concept. Both materials have a similar density and maximum service temperature, but silicon nitride outperforms in strength and fracture toughness through its unique interlocking grain structure, according to the authors. It provides greater resistance to failure from thermal shock and/or mechanical loads.


Combined thermal and centrifugal stress plot of the alumina turbine rotor design under idle power conditions within a jet engine.

Bohan said there were still some obstacles to overcome with the 3D printer due to limitations of the printer and the thickness required. After the part is printed it is put into a tank of water to dissolve out binding materials. If the thickness is too great, water cannot penetrate far enough into the part.

“We had to do some design variations to get to a final net shape that both met our requirements, as well as the thickness requirements for manufacturing to fully dissolve the binder,” Bohan said. “Just coming up with the balance of both of those was tricky.”

Bohan said much of the work on printing ceramics for aviation is focused on large-scale applications such as single turbine blades for large engines. That would be difficult for smaller engines simply because of their size.

For now, he said the team is encouraged that they were able to make a part that looks correct. “I wasn’t sure that we were going to get to that point,” he said. “It is in the wrong material, but the process of using the printer is identical between different materials. We may have to run the oven at different temperatures for different materials, but the actual manufacturing process should be the same, or fairly close.”

John Kosowatz is senior editor.