Microstructural and mechanical characterization of a hybrid oxide CMC combustor liner after 25,000-hour engine test

A hybrid oxide ceramic matrix composite (CMC) outer combustor liner was tested in a Solar Turbines Incorporated Centaur® 50S engine between 2003 and 2006, accumulating >25,000 hours of field exposure. The hybrid CMC liner, which was ∼76 cm in diameter, had an alumina matrix with a Nextel 720 fiber-reinforcement (A/N720). The CMC, produced by ATK-COI Ceramics, Inc., was coated with a ceramic insulation layer known as FGI (Friable Graded Insulation) developed by Siemens Energy Incorporated. Post-test microstructural and mechanical evaluation was conducted on the field-exposed liner at Oak Ridge National Laboratory (ORNL) to determine the types of surface and structural damage that occurred to the combustor liner during engine exposure to elevated temperatures (>1200°C), thermal cycling (stop-start cycles), and combustion gases (especially water vapor). In this study, numerous sections were cut from the liner for mechanical and microstructural characterization that exhibited varying amounts of FGI and/or CMC degradation. In this way, damage accumulation was assessed (1) within the CMC and FGI layers, both on the gas-path surface and below the surface and (2) as a function of liner position (fore-to-aft) in the engine. The amount and type of damage observed was directly related to the starting CMC and FGI microstructures. The tensile strength of the hybrid liner after field exposure was found to be 19 MPa. The FGI layer remained well bonded to the CMC and the fracture surface of the CMC exhibited scissor-like features, which is typical of composites with ±45° fiber architecture. The stress acting on the CMC at failure was 53 MPa.