Ceramic Matrix Composite seals proving reliable for jet engine nozzles

  • Published
  • By Pete Meltzer, Jr.
  • AFRL Materials & Manufacturing Directorate
WRIGHT-PATTERSON AIR FORCE BASE, Ohio --  Air Force Research engineers are working with industry partners to test ceramic matrix composite materials in F100 PW 229 gas turbine engines as exhaust nozzle divergent seals in an effort to increase part life, improve reliability, and reduce operating and maintenance costs. 

The AFRL Materials and Manufacturing Directorate research focuses on an advanced self-sealing ceramic matrix composite or CMC called SEPCARBINOX® A500, manufactured by the French company Snecma Propulsion Solide (SPS). 

In recent years, a considerable effort has been directed at evaluating CMC materials for aerospace gas turbine engines. For military applications, the focus has been on the afterburner section of the aircraft --the flameholder, augmentor liner, and the convergent and divergent segments of the exhaust nozzle--engine parts that must withstand high temperatures and rapid thermal cycles. 

Researchers in the Directorate's Metals, Ceramics, and Nondestructive Evaluation (NDE) Division are examining the potential for using CMC seals on the divergent section of the exhaust nozzles, the easily recognized flaps and seals at the rear of the nozzles used to funnel engine exhaust and control thrust. 

According to Larry P. Zawada, AFRL lead engineer for the program, the CMC research effort addresses a number of key issues important to the Air Force, including the performance of CMC parts in flight as compared to their performance during engine testing on the ground.

"CMCs are excellent candidates for replacing nickel-based superalloys, currently used in exhaust nozzles, because they can withstand the high temperatures and severe operating environment for much longer periods of time with little to no changes in structural behavior," Zawada explained. "They are demonstrating a potential for extending the operating lives of these components and reducing maintenance costs," he added.

SPS has developed a novel CMC that uses carbon fibers in a sequentially-layered carbide matrix made by chemical vapor infiltration. The matrix is self-sealing, which helps protect the carbon fibers from oxidation. "The fibers are woven in a multi-dimensional ply-to-ply angle interlock pattern to reduce the chance of delamination," Zawada said.

Extensive ground testing that simulated full part life of 2000 flight hours was completed on one CMC seal. The retained tensile strength values measured on this ground tested seal provided verification of the excellent mechanical durability of the CMC seal hardware. The part was in "like new" condition with no measurable degradation. The successful results from the ground testing led to the start of a field service evaluation to actually test the hardware on operational aircraft.

The main objectives of the field service evaluation program are "to fully qualify the new seals as flight-worthy and to certify them as preferred spares for the F100 engine," Zawada said.

Eight A500 divergent seals began flying at an operational base on operational F-16 fighter aircraft in July 2005, and 20 additional divergent seals began flying on operational F-15 aircraft at a second installation in February 2006. If everything goes as planned, these seals will be removed from the aircraft at designated flight hour intervals and sectioned for microstructural characterization and tension testing.

"The tensile strength testing is essential to determine whether there has been any strength degradation of the seals as a function of flight time," Zawada explained.

To date, one A500 seal has been removed and evaluated. This seal had 350 flight hours and had been in service for 17 months. The seal was visually inspected and checked for damage using thermography. There were no signs of surface erosion, wear, or degradation. The appearance of the seal was essentially identical to the as-produced condition. Tension tests on this seal demonstrated that the retained tensile properties were equal to or higher than the database values.

The A500 CMC divergent seals have performed extremely well thus far, and a life cycle cost analysis is underway to determine if the seals should be introduced into the fleet as preferred spares.