Stainless steel patterning by combination of micro-patterning and driven strain produced by plasma assisted nitriding

Plasma-assisted nitriding treatments on austenitic stainless steel at low temperature produce the so called “expanded austenite”. The expansion of the nitrided layer that occurs from the initial surface of the substrate in a direction perpendicular to the surface is used here as a mean to produce patterned surfaces by selective nitrogen diffusion through masks. Using grids, a network of well defined square dots can be obtained. In this communication, the results of nitriding treatments on austenitic stainless steel substrates previously covered by a patterned silicon oxide layer are presented. The interactions mechanisms at the interface between fixed silicon oxide mask with several different shapes (circular and square dots) and the expanded austenite are also described. The role of nitrogen diffusion, consistent with the experimental conditions and the mask characteristics, is shown to be very important. Depending on the size of the dots, it leads to a simple uniform mask deformation or to a significant mask deformation with strong distortions at the edges. This phenomenon is noted as a toroidal-shell shape distortion. Optical cross-section seems to prove that it is only the result of the vertical force due to the austenite expansion induced by nitrogen diffusion just under the mask edges.