Surface analysis of SiO2-like high-barrier thin films for protection of silver artefacts

Surface engineering has been explored using plasma enhanced chemical vapur deposition (PECVD) of SiO2-like coatings on different metallic substrates in order to improve their resistance to corrosion. Specifically, the coatings have been deposited onto silver-based alloys which are prone to unaesthetic tarnishing, by means of PECVD in radio frequency (RF) plasma fed with tetraetoxysilane/oxygen/argon mixture. Microchemical and microstructural characterization of the coatings has been carried out by means of X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM) and field emission scanning electron microscopy (FESEM). The protective effectiveness against tarnishing of SiO 2-like layers has been evaluated by immersion tests in 0.1 M Na 2S aerated solution at room temperature. It has been found that SiO2-like coatings deposited by increasing the oxygen-to-monomer ratio in the feeding-gas mixture and the input power density possess excellent barrier effects, and the coating adhesion increases if the deposition process is performed after a hydrogen plasma treatment. The protective effectiveness of PECVD coatings may be related to their barrier effect against the diffusion of water and other gaseous aggressive agents present in the environment and coming into contact with the metal surface.