Ceramic Matrix Composite Seals Proving Reliable for Jet Engine Nozzles

  • Published
  • By Heyward Burnette
  • Materials and Manufacturing Directorate
WRIGHT-PATTERSON AIR FORCE BASE, Ohio --  AFRL engineers are working with industry partners to test the use of ceramic matrix composite (CMC) materials as divergent seals in F100-PW-229 gas turbine engine exhaust nozzles. Geared towards increasing part life, improving reliability, and reducing operating and maintenance costs, the research is focusing on SEPCARBINOX® A500, an advanced self-sealing CMC manufactured by French company Snecma Propulsion Solide (SPS). Thus far, the seals have performed extremely well, and a life-cycle cost analysis is under way to determine whether they should be introduced into the fleet as preferred spares. 

CMCs are excellent candidates for replacing the nickel-based superalloys currently used in exhaust nozzle parts, primarily due to their capacity to withstand the high temperatures and severe operational environment for much longer periods of time with minimal changes in structural behavior.

In examining the feasibility of using the A500 seals on the divergent section of the exhaust nozzles, AFRL researchers are addressing a number of key Air Force issues--one of which involves the performance comparison of CMC parts in flight and during engine ground testing. SPS has developed a novel CMC that uses carbon fibers in a sequentially layered carbide matrix produced via chemical vapor infiltration. Because this resultant matrix is self-sealing, it helps protect the carbon fibers from oxidation. The fibers are woven in a multidimensional, ply-to-ply angle interlock pattern to reduce the chance of delamination.

Researchers completed extensive ground tests simulating the part's full, 2,000-flight-hour life cycle on one CMC seal. Measurements indicating the retained tensile strength of the ground-tested seal verified the excellent mechanical durability of the CMC hardware. The part was in "like new" condition, with no measurable degradation. The successful results obtained from ground testing prompted the start of a field service evaluation for assessing the seal hardware on operational aircraft. Researchers subsequently removed and evaluated an A500 seal having 350 flight hours. The seal underwent visual inspection, as well as thermography-based damage checkout. Despite its 17-month service life and associated flight hours, the seal showed no signs of surface erosion, wear, or degradation. The seal's appearance was essentially identical to its as-produced condition. Tension tests conducted on this same seal demonstrated that its properties of retained tensile strength were equal to or higher than stored database values delineating this performance measure.