Oxidation Behavior of Nanostructured Ni-5Al Coating. A case Study on Monophase Coatings

Oxidation behavior of nanostructured Ni-5Al HVOF coatings was studied. For this purpose, isothermal oxidation of the free standing coatings was performed at 950 degrees C. Lattice parameter determination technique was used for evaluating aluminum depletion to characterize oxidation behavior. The results showed that Al-depletion rate of the nanostructured coating was less than that of the conventional one, suggesting superior oxidation resistance of the nanostructured one. One reason, besides the one usually ascribed to grain size refinement and distribution of Nano oxides, lies in the coating integrity which imply the absence of any notable discontinuities including inter-splat oxides and porosities. On the other hand, vacuum heat treatment revealed that the nanostructured coating exhibited a phenomenon called diffusional creep, which is thought to be the most effective one in all densification mechanisms responsible for metallurgical consolidation processes. It was argued that this mechanism must also be active during oxidation in air and therefore can help retain the coating integrity, providing a sound metallic base for durable surface supply of aluminum throughout oxidation process. Therefore, coating integrity also is central to the formation of the protective -Al2O3 subscale, as observed and argued in this paper for the nanostructured Ni-5Al coating.