Abstract
The mechanisms governing the failure of multi-layer thermal barrier systems based on Pt-modified nickel aluminide bond coats and electron beam deposited thermal barrier coatings (TBCs) have been studied. The primary experimental variable is the morphology of the bond coat surface prior to application of the TBC, at constant multi-layer chemistry. The durability of these systems in a furnace cycle test has been measured and compared. The failure mechanisms, as well as the thickening of the thermally grown oxide (TGO), have been characterized for each of the surface morphologies. The major findings are that the durability is enhanced by removing imperfections on the surface of the bond coat, as well as by surface pre-treatments that diminish subsequent TGO thickening and by incorporating a reactive-element in the substrate that strengthens the bond coat upon inter-diffusion during manufacture. These effects are consistent with the expectations of a TGO instability mechanism, driven by a combination of growth and thermal expansion misfit strains in the TGO. The grain structure of the bond coat also affects failure through its influence on the TGO instability sites.
Published Version
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