Date of Award
Master of Science
Materials Science and Engineering
The formation of surface undulations (i.e. rumpling) at the bond coat/thermally grown oxide (TGO) interface has been shown to cause failure by spallation of the ceramic top coat in aero-turbine systems. Many mechanisms have been proposed concerning the cause of these surface distortions; however, there is little agreement on what may be the dominating cause of the rumpling behavior. Of these mechanisms, the reversible phase transformation from a cubic beta-NiAl structure to a face centered tetragonal (FCT) martensitic phase was of particular interest because of its ability to form surface rumpling in Pt-modified beta bulk alloys. However, the bulk alloys used in obtaining that result were simple ternary systems and not relevant to actual coating compositions as other alloying elements enter the coating due to coating/substrate interdiffusion at high temperature. In the current study, the depletion behavior of a commercial coating was studied. Compositions from the depletion path were determined and bulk alloys representing these coating compositions were prepared. The martensitic phase transformation was then characterized using DSC and XRD. The martensitic start temperature on cooling, Ms, was consistently found to be significantly lower than previously reported values (e.g. 530°C vs. 100°C). Because of the low Ms temperature, the formation of the martensitic phase was concluded to be unnecessary for the occurrence of rumpling. However, cyclic exposure treatments at low temperature (∼400°C) of bulk alloys and commercial coatings did show the detrimental effects of the phase transformation in the form of crack formation and propagation leading to eventual failure of the alloys. The current work also infers that the differences in coefficient of thermal expansion (CTE) mismatch between the coating and substrate are the dominating factor leading to rumpling. Dilatometry measurements were made on bulk alloys representing depleted coatings and the superalloy substrate to determine CTE as a function of temperature. Finally, simulations were completed to help determine the role of CTE mismatch. It was found that these results compared closely to those collected during experimental cyclic exposure treatments; although, modifications to the current model were found to be needed in order to truly simulate rumpling.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
Joseph Peter Henderkott
Henderkott, Joseph Peter, "Mechanisms associated with rumpling of Pt-modified [beta]-NiAl coatings" (2007). Retrospective Theses and Dissertations. 14537.