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The major objective of this study was to understand the mechanisms controlling the
durability of Thermal Barrier Coatings (TBCs) and use this understanding to develop new TBCs
with improved performance characteristics. The system studied consisted of the single crystal
superalloy René N5 as a substrate and yttria-stabilized zirconia as a ceramic topcoat prepared by
electron beam physical vapor deposition (EB-PVD). The parameters, which were varied, were
the type and fabrication condition of the bond coat. Two general types of bond coat were
studied; a platinum modified aluminide and a NiCoCrAlY overlay. A second objective of the
study was to ascertain the effects of exposure conditions on the failure of state-of-the-art TBCs.
The degradation behavior of the state-of-the-art systems during thermal cycling (one hour
cycles in a bottom-loading furnace) was evaluated in the first stage of the study. It was found
that the times to failure (TBC spallation) were shorter for the NiCoCrAlY bond coats than the
platinum aluminides. The failure mode for the NiCoCrAlY bond coats was found to involve
fracture initiation at defects at the TBC-thermally grown oxide (TGO) interface and at the TGO-
bond coat interface. The fractures then propagated along the interface between the bond coat and
the TGO. The failure of the Pt aluminide was found to involve a combination of fracture along
the TGO/ bond coat interface and a deformation mode of the bond coat known as “ratcheting”.
The effect of cycle frequency on the spallation failure of the state-of-the-art TBCs was also
investigated. The exposure conditions affect the lifetimes of the coatings and can even change
the relative performance of different bond coats. Exposure temperature was found to have a
strong effect, which is consistent with TGO growth being a first order variable in scale failure.
In the second stage of the study, processing methods to improve the TBC systems were
utilized. For the NiCoCrAlY bond coats the modifications were directed at minimizing the
effects of coating defects. These included overaluminizing and deposition of a thin platinum
overlayer on the bond coat prior to TBC deposition. Both techniques improved coating lives
with the Pt overlayer having the most dramatic effect. This is consistent with the proposed failure
mechanism. Layers of Pt deposited under the NiCoCrAlY resulted in increased TBC life,
although not to the extent of the Pt overlayers or overaluminizing.
SOURCE: N. M. Yanar, G. M. Kim, F. S. Pettit, and G. H. Meier. “Degradation of EBPVD YSZ Thermal Barrier Coatings on Platinum Aluminide and NiCoCrAlY Bond Coats During High Temperature Exposure. Proc. Of the Turbine Forum: Advanced Coatings for High Temperatures. Forum of Technology. Dorsten, Germany. April 2002. Section 14.
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