Mechanisms of lifetime improvement in thermal barrier coatings with hafnium and/or yttrium modification of CMSX-4 superalloy substrates

In modern turbine engines for propulsion and energy generation, thermal barrier coatings (TBCs) protect hot-section blades and vanes, and play a critical role in enhancing reliability, durability and operation efficiency. In this study, thermal cyclic lifetime and microstructural degradation of electron beam physical vapor deposited (EB-PVD) Yttria Stabilized Zirconia (YSZ) with (Ni,Pt)Al bond coat and Hf- and/or Y-modified CSMX-4 superalloy substrates were examined. Thermal cyclic lifetime of TBCs was measured using a furnace thermal cycle test that consisted of 10-minute heat-up, 50-minute dwell at 1135°C, and 10-minute forced-air-quench. TBC lifetime was observed to improve from 600 cycles to over 3200 cycles with appropriate Hf- and/or Y-alloying of CMSX-4 superalloys. This significant improvement in TBC lifetime is the highest reported lifetime in literature with similar testing parameters. Beneficial role of reactive element (RE) on the durability of TBCs were systematically investigated in this study. Photostimulated luminescence (PL) spectroscopy was employed to non-destructively measure the residual stress within the TGO scale as a function of thermal cycling. Extensive microstructural analysis with emphasis on the YSZ/TGO interface, TGO scale, TGO/bond coat interface and bond coats was carried out by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and scanning transmission electron microscopy (STEM) as a function of thermal cycling including after the spallation failure. Focused ion beam in-situ lift-out (FIB-INLO) technique was employed to prepare site-specific TEM specimens. X-ray diffraction (XRD) and secondary ion mass spectroscopy (SIMS) were also employed for phase identification and interfacial chemical analysis.; While undulation of TGO/bond coat interface (e.g., rumpling and ratcheting) was observed to be main mechanism of degradation for TBCs on baseline CMSX-4, the same interface remained relatively flat (e.g., suppressed rumpling and ratcheting) for durable TBCs on Hf- and/or Y-modified CSMX-4. The fracture paths changed from the YSZ/TGO interface to the TGO/bond coat interface when rumpling was suppressed. The geometrical incompatibility between the undulated TGO and EB-PVD YSZ lead to the failure at the YSZ/TGO interface for TBCs with baseline CMSX-4. The magnitude of compressive residual stress within the TGO scale measured by PL gradually decreased as a function of thermal cycling for TBCs with baseline CMSX-4 superalloy substrates. This gradual decrease corresponds well to the undulation of the TGO scale that may lead to relaxation of the compressive residual stress within the TGO scale. For TBCs with Hf- and/or Y-modified CMSX-4 superalloy substrates, the magnitude of compressive residual stress within the TGO scale remained relatively constant throughout the thermal cycling, although PL corresponding to the stress-relief caused by localized cracks at the TGO/bond coat interface and wihin the TGO scale was observed frequently starting 50% of lifetime.; A slightly smaller parabolic growth constant and grain size of the TGO scale was observed for TBCs with Hf- and/or Y-modified CSMX-4. Small monoclinic HfO2 precipitates were observed to decorate grain boundaries and the triple points within the alpha-Al2O3 scale for TBCs with Hf- and/or Y-modified CSMX-4 substrates. Segregation of Hf/Hf 4+ at the TGO/bond coat interfaces was also observed for TBCs with Hf and/or Y modified CMSX-4 superalloy substrates. Adherent and pore-free YSZ/TGO interface was observed for TBCs with Hf and/or Y modified CMSX-4, while a significant amount of decohesion at the YSZ/TGO interface was observed for TBCs with baseline CMSX-4. The beta-NiAl (B2) phase in the (Ni,Pt)Al bond coat was observe to partially transform into gamma'-Ni3 Al (L12) phase due to depletion of Al in the bond coat during oxidation. More importantly, the remaining beta-NiAl phase transformed into L10 martensitic phase upon c...