Effective Lifetimes of Atomic Layer Deposited Diffusion Barrier Films for Silver Artifacts*

We investigated using atomic layer deposition (ALD) to create dense, transparent oxide diffusion barrier coatings to reduce the tarnishing rate for silver art objects. An elevated H$_{\mathrm{2}}$S aging chamber was used for accelerated aging to directly compare the effectiveness of 5-100nm Al$_{\mathrm{2}}$O$_{\mathrm{3}}$ ALD thin films and nitrocellulose coatings, the current technique for silver preservation, at reducing the tarnishing rate of silver while minimally affecting the visual appearance of the silver. Reflectance spectroscopy and an integrated sphere spectrophotometer were used to measure the thickness of the tarnish layer and indicate the lifetimes of the ALD and nitrocellulose coatings. Electrochemical impedance spectroscopy (EIS) was used to determine the porosity and average pore size of ALD films. Failure mechanisms for the two types of films were observed, the ALD films failing in defects or pinholes in the films and the nitrocellulose failing due to non-uniform in coating thickness. Thin Al$_{\mathrm{2}}$O$_{\mathrm{3}}$ ALD films were found to be more porous than thick ALD films, sufficient in protecting silver five times longer and effected the overall color change of the object less than micron thick nitrocellulose films.