Oxide Spallation during Post-Isothermal High Temperature Oxidation Cooling of Cr-rich Cast Alloys highly alloyed with Hf
Cobalt, nickel and iron-based alloys containing 25wt.%Cr and strengthened either by chromium carbides or by HfC were oxidized at 1000, 1100 and 1200°C to obtain external oxide scales. The spallation of these oxides during the post-oxidation cooling was studied by exploiting the cooling part of the thermogravimetry curves. The best resistant alloys against scale spallation were the iron-based alloys which did not lose oxide. The nickel-based alloys well behaved too. The worst alloys were the cobalt alloys, principally due to the complex composition of the oxide scales. Beside the base element, the thickness of the oxide was also identified as a major parameter. The presence of Hf with contents much higher than usual was also beneficial for the resistance against spallation.
Mechanical and Chemical Properties at High Temperature of {M-25Cr}-based Alloys Containing Hafnium Carbides (M=Co, Ni or Fe):Creep Behavior and Oxidation at 1200°C
This study concerns three alloys based on cobalt, nickel or iron, all containing chromium (25 wt.%), carbon 
and hafnium. The contents in the two last elements were chosen high enough to promote the formation of HfC carbides. 
All alloys were elaborated by casting and their microstructures preliminarily characterized. They were selected to be 
tested in three-points flexural creep at 1200°C under 20 MPa in inert atmosphere. All alloys contain hafnium carbides, 
together with chromium carbides in some cases. The HfC carbides are generally of two types: script-like eutectic and 
blocky pre-eutectic. The creep deformation is fast for the nickel-based and iron-based alloys, especially for the later one. 
In contrast the HfC-containing cobalt-based alloy behaves much better. It displays a creep resistance at 1200°C 
significantly higher than another cobalt-based alloy strengthened by chromium carbides added to the study for 
comparison. All alloys were also briefly tested in oxidation by air at 1200°C. The creep behaviors of the cobalt-based and 
iron-based alloys are not good and significantly worse than the nickel-based alloy’s one. The oxidation resistance of the 
HfC-strengthened cobalt-based alloy must be improved to take benefit of its superior creep strength.
High temperature oxidation behavior of hafnium modified NiAl bond coat in EB-PVD thermal barrier coating system
Abstract NiAl coatings doped with 0.5 at.% and 1.5 at.% Hf were produced by co-evaporation of NiAl and Hf ingots by electron beam physical vapor deposition (EB-PVD), respectively. The addition of 0.5 at.% Hf significantly improved the cyclic oxidation resistance of the NiAl coating. The TGO layer in the 1.5 at.% Hf doped NiAl coating is straight; while that in the 0.5 at.% Hf doped coating became undulated after thermal cycling. The doped NiAl thermal barrier coatings (TBCs) revealed improved thermal cycling lifetimes at 1423 K, compared to the undoped TBC. Failure of the 0.5 at.% Hf doped TBC occurred by cracking at the interface between YSZ topcoat and bond coat, while the 1.5 at.% Hf doped TBC cracked at the interface between bond coat and substrate.