Improvement in Oxidation Behavior of Nanostructured CoNiCrA1Y Bond Coat Dispersed with Nano-size Alumina Particles

The oxidation behavior of a nanostructured CoNiCrA1Y bond coat dispersed nano-sized alumina particles was compared to that of a conventional coarse-grained CoNiCrA1Y bond coat. It was found that after a 1000ºC/24 h oxidation a protective alumina layer was developed on the nanostructured coating, whereas a mixed and less protective oxide layer was observed on the conventional coating. Correspondingly, spallation of the oxide layer was not observed with the nanostructured coating but did occur with the conventional coating. The improvement in oxidation behavior observed in the nanostructured coating was suggested to be due to its fine-grained structure, which provides rapid pathways (grain boundaries) for A1 diffusion to form a protective scale of pure alumina. Furthermore, the dispersion of the nano-sized alumina particles was effective in suppressing grain growth and in promoting the rapid formation of a protective alumina scale and thus significantly improving the oxidation resistance of the bond coat

Synthesis and Oxidation Behavior of Nanocrystalline MCrA1Y Bond Coats

Thermal barrier coating (TBC) systems protect turbine blades against high-temperature corrosion and oxidation. They consist of a metal bond coat (MCrAIY, M = Ni, Co) and a ceramic top layer (ZrO2/Y2O3). In this work the oxidation behavior of conventional and nanostructured HVOF NiCrAIY coatings has been compared. Commercially available NiCrAIY powder was mechanically cryomilled and HVOF sprayed on a nickel alloy foil to form a nanocrystalline coating. Free-standing bodies of conventional and nanostructured HVOF NiCrAIY coatings were oxidized at a 1000ºC for different time periods in order to form the thermally grown oxide (TGO) layer. The experimants show an improvement in oxidation resistance in the nanostructured coating when compared to that of the conventional one. This behavior is a result of the formation of a continuous AI2O3 layer on the top surface of the nanostructured HVOF NiCrAIY coating. This layer protects the coating from further oxide protrusions present in the conventional coating

Epitaxial strain-engineered self-assembly of magnetic nanostructures in FeRh thin films

In this paper we introduce an innovative bottom-\up approach for engineering self-\assembled magnetic nanostructures using epitaxial strain-\induced twinning and phase separation. X-\ray diffraction, Fe-57 Mossbauer spectroscopy, scanning tunneling microscopy, and transmission electron microscopy show that epitaxial films of a near-\equiatomic FeRh alloy respond to the applied epitaxial strain by laterally splitting into two structural phases on the nanometer length scale. Most importantly, these two structural phases differ with respect to their magnetic properties, one being paramagnetic and the other ferromagnetic, thus leading to the formation of a patterned magnetic nanostructure. It is argued that the phase separation directly results from the different strain-\dependence of the total energy of the two competing phases. This straightforward relation directly enables further tailoring and optimization of the nanostructures' properties

Tailoring magnetic frustration in strained epitaxial FeRh films

We report on a strain-induced martensitic transformation, accompanied by a suppression of magnetic order in epitaxial films of chemically disordered FeRh. X-ray diffraction, transmission electronmicroscopy, and electronic structure calculations reveal that the lowering of symmetry (from cubic to tetragonal) imposed by the epitaxial relation leads to a further, unexpected, tetragonal-to-orthorhombic transition, triggered by a band-Jahn-Teller-type lattice instability. The collapse of magnetic order is a direct consequence of this structural change, which upsets the subtle balance between ferromagnetic nearest-neighbor interactions arising from Fe-Rh hybridization and frustrated antiferromagnetic coupling among localized Fe moments at larger distances