FELTMETAL™ Abradable Turbine Seal Materials: Structure and Property Responses to Blade Rub and Oxidation

Abradable seals are a common technology employed to improve turbine engine performance by reducing the gap between blades and the casing in compressor and turbine stages. Because operating conditions and material requirements vary significantly as a function of turbine stage and turbine design, many abradable compositions have been developed. Generally, all abradable seal materials employ voids or pores in the microstructure as a designed-defect to mitigate thermal and kinetic energy from blade rub. Some also have added non-metallic phases to provide other means of maintaining a seal that is sacrificial to the blade tip. This paper describes the microstructures of specific fiber metal abradable materials known as FELTMETAL™, as well as the distinctly different abradable microstructures of honeycomb material and thermal spray coating. The effect of the porosity and metallic surface area in each type of microstructure on seal abradability and seal oxidation is discussed. In addition, the effect of oxidation by aging at elevated temperatures on strength, ductility, and abradability for several FELTMETAL™ materials is also discussed. It is proposed that the pore structure aids in mitigating blade contact energy primarily by internal deformation of the metal matrix surfaces and that abradability is improved by formation of protective metal oxide films that reduce ductility without significant loss of strength.