Interfacial phenomena and evolution of modified aluminide bondcoatings in Thermal Barrier Coatings

Abstract

The paper presents results of microstructural investigations of TBCs on Pt- and PdPt-aluminide bondcoatings with focus on the interfacial phenomena that take place during the pre-oxidation treatment as well as further thermal cycling at 1100 °C. The Pt and Pd coatings were produced using a Physical Vapor Deposition (PVD) method while aluminizing was performed using a high activity Vapor Phase Aluminizing (VPA). The ceramic yttria stabilized zirconia (YSZ) top coating was deposited using Electron Beam Physical Vapor Deposition (EB-PVD) method. Prior to the EB-PVD process the bondcoatings were pre-oxidized at 1140 °C for 2 h in air atmosphere in order to form a stable and adherent α-Al2O3 TGO with minimum transient oxidation. Special effort has been done in order to investigate the microstructure of the Thermally Grown Oxide (TGO) formed during pre-oxidation treatment prior to YSZ deposition and the phenomena occurring at the interface between the YSZ, TGO, and the bondcoatings. Microstructure evolution of the TGOs is described and related to the conditions of the pre-oxidation treatment and the chemistry of the bondcoatings. During thermal cycling at 1100 °C the TGO grows by inward oxygen diffusion forming distinctive columnar grains which grow deeper at triple grain junctions with the bondcoating. Therefore the TGO/bondcoating interface evolves from flat to a convoluted during high temperature oxidation. Several segregation phenomena can be found in the TBCs that include i.a. the Reactive Element Effect involving the presence of Hf and Y on α-alumina grain boundary disconnections in the TGO. It was found that compared to bare René N5 superalloy the Hf segregation is hindered in the presence of modified aluminide bondcoatings.