Silver artefacts: Plasma deposition of SiO x protective layers and tarnishing evolution assessment

Abstract

Silver artifacts suffer tarnishing when exposed to a sulphur containg atmosphere. Plasma deposited SiO_x thin films are proposed for the protection of silver artefacts, owing to their optical transparency and high barrier properties against vapours. The protective effectiveness of the SiO_x films was assessed by means of electrochemical impedance measurements performed in an Na₂S solution on a set of coated silver based alloy samples, and by submitting another set of samples to a tarnishing test in presence of H₂S vapours. The experimental findings reveal that the SiO_x deposition, performed in RF plasma fed with a tetraetoxysilane/oxygen/argon mixture, produces layers with excellent barrier effects against the aggressive agents. The protective effectiveness increases if the deposition is performed at increasing input powers and at decreasing tetraethoxysilane/oxygen ratios in the feeding gas. The tarnishing evolution onto the coated surface was assessed at the microscopical and macroscopical level by means of field emission scanning electron microscopy and by an easy to use diagnostic tool based on digital photography and image processing.