Study of bond coats and failure mechanisms of thermal barrier coatings

Abstract

Bond coats for thermal barrier coating (TBC) applications and the failure mechanisms of TBCs are addressed in this thesis, with a focus on i) the early stage oxidation of Pt diffusion bond coats, ii) substrate effects on TBC failure and iii) a new bond coat design. The early stage oxidation behaviours of r/r'-based NiAl bond coats with different Pt additions are investigated. Pt can slow down the theta-Al2O3 to alpha-Al2O3 transformation. High resolution phase mapping by scanning diffraction analysis shows that alpha-Al2O3 nucleation in the theta-Al2O3 scale is inhomogeneous along the coating/scale interface. Spatially resolved PLPS (photoluminescence piezospectroscopy) results show a clear correlation between the theta-Al2O3 to alpha-Al2O3 transition and the r or r' microstructure in the underlying alloy: where Pt stabilises the r' structure, the suppression of theta-Al2O3 to alpha-Al2O3 transition is observed. The slower alpha-Al2O3 to alpha-Al2O3 transition rate due to Pt addition leads to a lower compressive stress of the stable oxide scale, which contributes to the long term stability of the oxide scale. The effects of substrate composition on the lifetime of TBCs were studied by comparing two TBCs applied to a CMSX-4 and a Rene N5 single crystal superalloy substrate, respectively. Both TBCs were applied by EB-PVD on top of the Pt-diffused r/r' bond coats. Cyclic oxidation test showed that TBCs deposited on the CMSX-4 substrates exhibited an average lifetime 20% higher than that deposited on the Rene N5 substrate. The TGO thickness evolution and the roughness of the TGO/bond coat interface were comparable for the two TBCs during cyclic oxidation. To find out the mechanism for the substrate composition effect, a strain-to-fail test combined with 3D-DIC was employed to measure the bond coat/TGO interface toughness and its evolution for the two TBCs. The mode I interfacial toughness values were almost identical for the two TBCs (~ 30 J/m2) in the as-deposited state. However, it decreased much faster for the TBC with a Rene N5 substrate after oxidation. The fast decrease of interface toughness was attributed to the sulphur segregation at the TGO/bond coat interface. A new Al-enriched gamma prime-Ni3Al bond coat has been developed and its high temperature oxidation behaviour has been examined and compared with that of the conventional Pt-diffused r/r' coating and the beta-NiPtAl coating. This new gamma prime-phase coating exhibited significantly reduced Al and Pt depletion during oxidation compared to the two conventional diffusion coatings. Moreover, although the Al-enriched gamma prime-phase coating presented faster thermal grown oxide (TGO) growth than that of the beta-NiPtAl coating during isothermal oxidation, it outperformed the Îbeta-NiPtAl coating regarding the rumpling resistance during cyclic oxidation. The Al-enriched gamma prime-phase coating also exhibited superior TGO spallation resistance compared to the Pt-diffused r/r' coating. The mechanisms for the combination of good rumpling resistance and oxidation performance of this gamma prime-Ni3Al coating will be addressed.

Date of Award	1 Aug 2020
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Philip Withers (Supervisor) & Ping Xiao (Supervisor)