The Competitive Relation Between Grain Size, Phase Transformation and Oxide Selectivity in Nanostructured and Ultrafine Coatings

There is great interest in nanostructured coatings due to the non-conventional properties obtainable when nanostructured feedstocks are used. In particular, results showing improvement in wear and corrosion resistance are reported. This improvement in properties is generally attributed to the refinement of the grain size, which in addition to promoting an improvement in strength, allows for a modification of the surface native oxide scale. However, several questions remain to be addressed, such as the grain size threshold to observe the effect of oxide selectivity, influence of the grain size on the long term stability of the chemistry of the coatings and if the presence of a phase transformation has an effect on the novel properties. This presentation will focus on the results obtained on two systems: an Al-containing 316 stainless steel and NiCoCrAlY. The modification of the powders feedstock was carried out via cryomilling. Results obtained by FE-SEM, TEM and XRD will be used to highlight the relation between the modification of the starting powders and the evolution of the oxide composition as a function of grain size, temperature and exposure time.