Analyses of a Thermally Grown Oxide Scale in Thermal Barrier Coatings Containing a Platinum-Nickel-Aluminide BondcoatMichael R. Brickey - Purdue Graduate Student
Hot running turbine engine components made of such alloys as Rene N5 are treated with bondcoats of Ni, Pt, and Al alloy for oxidation protection. To prevent melting, the coated component is also coated with an insulative barrier coating of Yttria Partially Stabilized Zirconium (YPSZ). Between the two coatings, a very fine layer of "TGO" (Thermally Grown Oxide) is present. The initial development of this thin oxide layer is the topic of this presentation.
Most prior research has assumed this layer to be Al2O3. The bulk of this study was performed by CTEM and STEM studies of this layer, first at its initial state (immediately after deposition) and after up to 10 thermal cycles simulating use of the turbine.
In the initially deposited state, there was concern that metastable forms of oxide might be present. However, none were found. All of the Al2O3 detected was found to have the stable hexagonal structure.
Near the YPSZ layer there were increasing concentrations of ZrO2 found in the oxide layer. These were present as dispersoids with an average diameter of 0.03 micrometers.
In thermally cycled specimens, a "duplex" structure was noted. Near the YPSZ side, ZrO2 dispersoids were frequently present and the structure was equiaxed (rather than the columnar structure of the YPSZ iteself). Futher toward the bondcoat the structure was columnar and free of ZrO2. The equiaxed layer is probably the result of nucleation and growth of Al2O3 around ZrO2 at pinning sites within prior existing columnar grains. Recrystalization occurs to relieve stress.
It would be desirable to eliminate this region and the dispersoids which cause it. The author proposes two methods: 1) addition of a diffusion barrier coating (but this would further complicate the overall structure), or 2) using tailored surface preparation to discourage Zr migration to the TGO layer.