PECVD coatings: analysis of the interface with the metallic substrate

Plasma enhanced chemical vapor deposition (PECVD) from silicon-containing organic compounds, such as SiOx coatings, appears to be an attractive treatment technique for the surface modification of metals in order to increase their corrosion resistance. This study deals with the characterization of SiNx thin films, in comparison with SiOx coatings, deposited on steels and aluminum alloys in a capacitively coupled (CC) parallel-plate reactor and in an inductively coupled (IC) reactor. The nitride-like coatings have been deposited in plasma fed with bis(dimethylamino)dimethylsilane (BIS) and Ar on as-received substrates and after a plasma treatment performed in oxygen or hydrogen plasmas. SiO xcoatings have been deposited starting from tetraethoxysilane (TEOS) in a mixture of Ar and O2. The substitution of oxygen by nitrogen can lead to higher tribological and mechanical properties, maintaining the insulating nature of the film. Surface and chemical characterization of the coatings have been carried out by means of X-ray photoelectron spectroscopy (XPS) and Fourier transformed infrared spectroscopy (FTIR). The protective effectiveness of PECVD coatings has been assessed by electrochemical impedance spectroscopy (EIS) and may be related to their barrier effect against the diffusion of water and gaseous aggressive agents from the environment toward the metal surface. Plasma pretreatments play an important role in determining the barrier properties of the film by reducing its defectiveness and enhancing its adhesion to the substrate.