Effect of internal oxidation on the interfacial morphology and residual stress in air plasma sprayed thermal barrier coatings

Significant internal oxidation was found in air plasma sprayed (APS) thermal barrier coating (TBC) samples with double-layered high velocity oxy-fuel (HVOF) NiCoCrAlY bond coats. The volumetric fraction of internal oxides within the bond coat increased from 5% to 38% during 799 h isothermal exposure at 1150 degrees C, indicating significant swelling of the internal oxides. This swelling behavior not only significantly increases the thickness of the bond coat layer from 175 mu m to 260 mu m, but also probably elevates the risk of interfacial failure in the TBCs, due to the roughness change at the interface and the huge residual stress generated. Thus, different finite element (FE) models were established to understand the role of the internal oxides in interfacial morphology and residual stress. The internal oxidation behavior was modeled on experimental results. The FE results showed that although the internal oxidation only slightly increased the interfacial roughness, significant changes were found in the patterns of the stress component S22 and the interfacial normal stress along the interface, increasing the risk of micro-cracks and changing the crack locations.