Complex, Precision Cast Columbium Alloy Gas Turbine Engine Nozzles Coated to Resist Oxidation.

Abstract : The objectives of this program were to produce investment cast single vane nozzle segments in C129Y alloy (Nb-10Hf-10W-0.3Y) and to coat these vanes with the NS-4 (50W-20Mo-15Ti-10V) silicide coating. Initially, both cored and solid vane segments were specified but due to problems experienced in the consumable arc process, cored vanes were deleted from the program scope. Several problems developed. The electrode material supplied was low in yttrium, which might have contributed to the poor fluidity during arc melt and incomplete fill of thin walled sections. The solid nozzles and round test bars were acceptable and were used in coating processing and testing. Process specifications for both slurry spray and dip applications of the NS-4 modifier were prepared. Tensile and stress rupture tests of coated specimens displayed exceptional properties at elevated temperatures and showed superiority to cobalt-base vane alloys. Isothermal oxidation lives in excess of 5500 hours (at 760 and 1093 C) were exhibited by NS-4 coated C129Y specimens. In rig testing, under the severe thermal cycles and profiles imposed, the airfoil specimens typically survived about 100 hours.