Multi-pass deposition of organosilicon-based superhydrophobic coatings in atmospheric pressure plasma jets

Atmospheric-pressure plasma polymerization of organosilicon precursors such as hexamethyldisiloxane is a well-known process used in the development of SixOyCz thin films on various substrates in different plasma configurations. Typically, long treatments or multi-pass treatments are used to achieve a sufficient thickness. However, the effects of multiple deposition passes on surface properties are rarely considered. In this paper, the development of a superhydrophobic organosilicon-based coating on pre-treated micro-roughened Al-6061 substrates through multiple deposition passes with an atmospheric-pressure plasma jet is reported. It is shown that besides the expected effect on coating's thickness, multiple passes of plasma deposition can also alter surface morphology and surface chemistry through a mechanism similar to the activation of organosilicon substrates with oxygen-containing plasmas. While the increase in coating thickness enhances coating stability in aggressive conditions, the rise of oxygen content reduces the hydrophobic behaviour of the coating.