The Failure of Thermal Barrier Coatings at Elevated Temperatures
In this study, the current state of the art thermal barrier coating (TBC) systems (heavy grit blasted Pt aluminide and NiCoCrAlY Bond Coats with EBPVD TBCs) were investigated first followed by TBC systems which were modified based upon the results obtained on the failure of the state of the art TBC systems. The specimens were subjected to cyclic oxidation testing, mostly at 1100aC in a bottom loading furnace in laboratory air. Optical and scanning electron microscopy ( SEM) were used for characterization of the as-processed and failed specimens.
The state of the art TBC systems with NiCoCrAlY bond coats failed in the presence of defects which were identified as TBC defects, transient oxides, surface defects and reactive element (RE) rich oxide protrusions. On the other hand, the failure of the state of the art TBC systems with Pt aluminide bond coats were due to deformation of the bond coat by a mechanism known as ratcheting. The stored strain energy in the TGO was also a factor that contributed to the failure of both systems. Most of the modifications performed on the state of the art TBC systems improved their lives to some extent. In the case of NiCoCrAlY systems, elimination or at least minimization of the identified defects was responsible for the improvements whereas the prevention of the ratcheting type of failure was the main reason for the improvement in lives in the case of Pt aluminide systems. On the other hand, other issues such as slower growth of the TGO as well as improved interfacial toughnesses with some of the modifications were observed to be contributing factors in the improved lives.
Based on the observations on the failure of both the state of the art as well as the modified TBC systems, the surface condition of the bond coats as well as the morphology of the TBCs close to the TGO were found to have a first order effect on the failure of TBC systems. The characteristics of the TGO, such as composition, growth rate and adherence both to the bond coat and the TBC, as well as the characteristics of the bond coats were also observed to have an effect on the failures. Recommendations for future work that should be pursued to better define the conditions necessary for optimized TBC performances are given .
Advisor:Jack Beuth; Judith Yang; Scott Mao; Frederick S. Pettit; Gerald H. Meier
School:University of Pittsburgh
School Location:USA - Pennsylvania
Source Type:Master's Thesis
Keywords:materials science and engineering
Date of Publication:06/09/2004