Morphology, bond strength and thermal cycling behavior of (Ni, Pt)Al/YSZ EB-PVD thermal barrier coatings

• Bond coat surface contains a cellular network of ridges underlying grain boundary.
• Ridge is the main site of preferential oxidation and cracking with thermal cycling.
• Tensile adhesion of the (Ni, Pt)Al/YSZ EB-PVD TBCs is reduced with thermal cycling.
• The fracture mechanism tends to occur at the interface of ceramic coat/bond coat.

Thermal barrier coating systems (TBCs) including of chemical vapor deposited (Ni, Pt)Al bond coat with grit blasting process and electron beam physical vapor deposited Y2O3-stabilized-ZrO2 (YSZ) ceramic coating were investigated. The bond coat surface contains a cellular network of ridges corresponding to the underlying bond coat grain boundary structure. With thermal cycling, the ridges and associated grain boundaries are the main sites of preferential oxidation and cracking, which results in the formation of cavities that are partially filled with oxide and further produce internal oxidation. The fracture location of TBCs after tensile adhesion occurs either within the ceramic top coat or at the bond coat/ceramic coat interface. The spallation location of the TBCs probably occurs either at the interface of TGO layer and bond coat or inside of TGO layer. It is proposed that ridge top spallation leads to debonds of sufficient size to result in unstable fracture driven by the strain energy stored in the TGO layer. The lower is the aspect ratio of bond coat ridges formed at grain boundaries, the longer is the lifetime of (Ni, Pt)Al/YSZ TBCs.

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