Effect of superalloy substrate on the lifetime and interfacial toughness of electron beam physical vapor deposited thermal barrier coatings

The effect of substrate composition on the lifetime of a thermal barrier coating (TBC) system was studied by comparing two TBCs applied to a CMSX-4 and a Rene N5 single crystal superalloy substrate, respectively. Both TBCs were applied by electron beam-physical vapor deposition (EB-PVD) on top of the Pt-diffused gamma/gamma' bond coats. Cyclic oxidation (1 h holding time at 1200 degrees C) test showed that TBCs deposited on the CMSX-4 substrates exhibited an average lifetime (30 cycles) 20% higher than that deposited on the Rene N5 substrate (24 cycles). Sub-critical cracks were found at the TGO/bond coat interface for the TBC with the Rene N5 substrate in the early stage of cyclic lifetime, while the interface for the TBC on the CMSX-4 substrate remained intact after the same cyclic treatment. Meanwhile, the TGO thickness and the roughness of the TGO/bond coat interface were found to be comparable for the two TBCs at each stage of cyclic oxidation. This indicated that the different cracking behaviour at the TGO/bond coat interface for the two TBCs may originate from the difference in the intrinsic interface toughness. To confirm this, a strain-to-fail test combined with 3D-DIC (digital image correlation) was employed to measure the bond coat/TGO interface toughness and its evolution for the two TBCs. The mode I interfacial toughness (Gamma(ic)) values were almost identical for the two TBCs (similar to 30 J/m(2)) in the as-deposited state. However, it decreased much faster for the TBC with a Rene N5 substrate after oxidation. The fast decrease of interface toughness was attributed to the sulfur segregation at the TGO/bond coat interface of this TBC as confirmed by the high-resolution STEM/EDX.