Experimental Assessment of Fiber-Reinforced Ceramics for Combustor Walls

Experimental and theoretical work concerning the application of ceramic components in small high-temperature gas turbines has been performed for several years. The significance of some nonoxide ceramic materials for gas turbines in particular is based on their excellent high-temperature properties. The application of ceramic materials allows an increase of the turbine inlet temperature resulting in higher efficiencies and a reduction of pollution emissions. The inherent brittleness of monolithic ceramic materials can be virtually reduced by reinforcement with ceramic fibers leading to a quasiductile behavior. Unfortunately, some problems arise due to oxidation of these composite materials in the presence of hot gas flow containing oxygen. At the Motoren und Turbinen Union, Munchen GmbH, comprehensive investigations including strength, oxidation, and thermal shock tests of several materials that seemed to be appropriate for combustor liner applications were undertaken. As a result, C/C, SiC/SiC, and two C/SiC composites coated with SiC, as oxidation protection, were chosen for examination in a gas turbine combustion chamber. To prove the suitability of these materials under real engine conditions, the fiber-reinforced flame tubes were installed in a small gas turbine operating under varying conditions. The loading of the flame tubes was characterized by wall temperature measurements. The materials showed different oxidation behavior when exposed to the hot gas flow. Inspection of the C/SiC composites revealed debonding of the coatings. The C/C and SiC/SiC materials withstood the tests with a maximum cumulated test duration of 90 h without damage.