Role of different plasma gases on the surface chemistry and wettability of RF plasma treated stainless steel

Vacuum radio frequency (RF) plasma treatment was conducted on electropolished and passivated 316 LVM stainless steel (SS) surface by using Ar and O₂. Contact angle measurements revealed that both Ar and O₂ plasma treatments significantly increased the wettability of the SS surface by water. The physical and chemical effects induced by Ar and O₂ plasma on SS were investigated via Atomic Force Microscopy (AFM) and X-ray Photoelectron Spectroscopy (XPS). AFM analysis revealed that plasma process exerted similar effects on surface roughness and topography. Both Ar and O₂ plasma treatments resulted in smoother surface when compared to reference electropolished sample. XPS investigations of the surfaces with respect to Fe2p, Cr2p, Mo3d and O1s XPS spectra showed that the main role of Ar plasma was thinning of the oxide-hydroxide layer along with the removal of contamination without any definitive difference on the valence states of metal compounds. On the other hand, O₂ plasma treatment resulted in thickening of oxide layer along with oxidation of the species to their highest valence states (CrVI, FeIII and MoVI). After washing the plasma treated surfaces with water, contact angles for oxygen plasma treated samples increased dramatically indicating the substantial role of higher valent oxides on wetting process.