Scale adhesion, sulfur content, and TBC failure on single crystal superalloys

摘要

The scale adhesion of single crystal superalloys was studied as a function of sulfur content. Cyclic oxidation tests of as-received PWA 1480 (6 ppmw S) at 1100 °C resulted in severe scale spallation and produced weight losses of >30 mg/cm~2 after 500 1-h cycles. However, samples desulfurized by hydrogen annealing (<0.1 ppmw S) produced adherent scales and weight gains of only ～1 mg/cm~2 after 1000 1-hr cycles. Similar data, obtained as a function of intermediate sulfur contents, were used to construct an adhesion map, ultimately suggesting a critical sulfur content equivalent to ～1 monolayer of total interfacial segregation potential. Rene'N5, desulfurized by hydrogen annealing (<0.01 ppmw S), produced adherent scales and weight gains of ～1 mg/cm~2 after oxidation at 1150 °C for 1000 1-h cycles as compared to a loss of 40 mg/cm~2 for as-received samples. Hydrogen annealing also improved scale adhesion for alloys doped with Y (15-100 ppmw), without removing sulfur, but perhaps due to decarburization. Similar overall compositional effects were exhibited in water immersion-induced spallation tests. A critical Y/S ratio for adhesion was indicated to be ～1:1 atomic, or 3:1 by weight. PWA 1484 (1.2 ppmw S) was also improved by hydrogen annealing (0.01 ppmw S) or melt desulfurization (0.25 ppmw S). The cyclic furnace life at 1100 °C for 250 μm thick PS-8YSZ TBC's of PWA 1484, without a bond coat, was improved accordingly from ～20 h to 600-800 h. Life was further extended to >2000 h by geometric modifications that avert standard edge delamination and buckling failure modes.