Investigation of the Formation Mechanism of Aligned Nano-Structured Siloxane Coatings Deposited Using an Atmospheric Plasma Jet

Abstract

In this study surfaces exhibiting a nano-structured morphology have been creating in a single step deposition process using an atmospheric pressure plasma jet. Plasma polymerized coatings were deposited from a HMDSO monomer which was introduced into a He/N2 plasma in the form of aerosolized droplets. By controlling the plasma discharge regime and monomer flow rate, the surface morphology was altered to create surfaces exhibiting an aligned fibrous morphology or a surface consisting of spherical agglomerates. Optical emission spectroscopy showed variation in photon emission energy depending on the He/N2 ratio. Both thermal imaging and numerical simulations indicate that vaporization of the monomer occurs, which facilitates monomer polymerization in the gas phase and subsequent particulate nucleation. XPS analysis identified a higher amount of carbon bound siloxy species at regions where greater particulate formation was observed on the substrate. Cross-sectional SEM and He-ion imaging indicate that the formation of fibrous structures is associated with localized surface charging, which gives rise to selective agglomeration of semi-amorphous particulates which become aligned on the substrate. Modeling of droplet behavior in the plasma indicates that under conditions of higher monomer flow rate, saturation within the plasma occurs which facilitates nucleation of particulates, while experimental and modeling results indicate surface driven growth mechanisms are more likely to occur at lower precursor flow rates.