The prevention of the tarnishing of silver
The valuable properties of silver can be diminished or destroyed because the metal is susceptible to corrosion by certain atmospheric species. Particularly aggressive is H2S which 'tarnishes' the surface by reacting to form Ag2S. There have been many attempts in the past to produce a tarnish resistant silver either by surface coating or bulk alloying; none have so far proved to be entirely successful. This thesis describes the production of surface coatings on silver by inert and reactive sputtering for application in the Silversmithing Industry. They should, therefore, be indistinguishable from silver and contain no more than 7.5% (by weight) of alloying addition (to comply with the Sterling Standard). Initially, the tarnish behaviour of uncoated pure and Sterling silver was investigated. Results indicated that the degree of sulphidation is a strong function of alloy content and also of surface preparation. Oxides of tantalum, hafnium, niobium, tin, zirconium, yttrium, titanium and aluminium were produced by reactive sputtering and deposited onto sputtered silver substrates. Films were characterised by X-ray diffraction and tarnished in a controlled atmosphere. The degree of sulphidation was then assessed by Energy Dispersive X-ray Analysis. The protectiveness of the oxide films was related to film stress, thickness and sputter deposition conditions. Using a dual-target sputter system, alloys of silver-tantalum, silver-hafnium, silver-niobium, silver-titanium and silver-aluminium were produced. The tarnish behaviour of the alloys was investigated as a function of alloy composition. X-ray diffraction analysis demonstrated that the deposition technique was capable of producing non-equilibrium structures. For some of the alloys, selective oxidation resulted in a slight improvement in tarnish resistance. In order to establish whether selective oxidation might produce completely protective surfaces, further studies of the oxidation of such materials needs to be undertaken.