Hierarchical structuring of the surface of Fe-Al ferritic alloys through plasma nitration

Patterning is a way to optimize or promote unique surface properties. Depending on the applied technique the patterns can have different length scales. In this work, the feasibility of using selective low-temperature nitriding to promote hierarchical surface patterning (two structures at different length scales) in a single process was studied. For that, samples of pure iron and iron with aluminum in solid solution (in the contents of 1, 3, 5, 7 and 10 wt.% Al) were plasma nitrided at different temperatures (300 to 500 °C with 50 °C steps) in pulsed DC plasma and in the electron cyclotron resonance (ECR) plasma at temperature of 350 °C and 400 °C. For ECR plasma treatment, the variation of the gas mixture composition and nitriding time was also evaluated. A grid was placed on the surface of the samples with 3 and 10 wt% Al, in order to alternate plasma-exposed regions and covered regions by the grid mesh. Samples were characterized by X-ray diffraction, optical microscopy, scanning electron microscopy with electron microanalysis probe and electron back-scattered electron diffraction (EBSD), transmission electron microscopy with selected area electron diffraction and surface profilometry by white light interferometry and confocal laser scanning microscopy. The proposed method proved to be effective in promoting patterns at two different length scales, one self-formed due to the precipitation of a low-density nitrides with the planes family {100} coherent to the substrate grains. The strain promoted by this precipitation causes an additional surface displacement when compared the surface that was not exposed to plasma. The self-formed patterns are correlated to the alloy base composition and to the nitriding gas composition, with precipitation occurring in the very initial stages of nitriding and temperatures below 450 ° C are required for the occurrence of self-formed patterns that, for the studied conditions, can expand up to 500 nm in regions exposed to plasma, forming the hierarchical pattern. (author)