Coated Columbium for Gas Turbine Engine Application

High temperature strength and high thermal conductivity make columbium an attractive candidate for gas turbine engine components despite poor oxidation resistance relative to superalloys. The creep strengths of existing columbium alloys are adequate for burner and turbine vane applications up to metal temperatures of 2200-2400 F with considerably reduced cooling. The high conductivity and ductility of columbium along with low modulus and low thermal expansion coefficient produce very high resistance to thermal fatigue. In addition, the silicide coating technology has advanced to the point that multi-hundred hour lives at 2200-2400 F for coated columbium components are now attainable. The fabricability of air cooled turbine vanes from welded B-66 columbium alloy sheet has been demonstrated, including the coatability of air cooling passages with a slurry silicide. These vanes have run in a turbine development engine for 100 hr of steady state and cyclic operation at metal temperatures in excess of 2000 F. Coated columbium sheet alloys are also being evaluated for burner application with encouraging results. For example, thermal fatigue lives beyond 14,000 cycles to 2400 F have been measured, compared to less than 3000 cycles to 1800 F for nickel-base alloys including TD nickel. A film-cooled burner liner of FS-85 columbium alloy sheet material coated with the Sylvania SiCrFe slurry coating has been fabricated and will be rig- and engine-tested. The applicability of columbium to turbine vanes has recently been enhanced by the demonstration of an investment cast columbium vane shape.