Atomic Layer Deposition Films as Diffusion Barriers for Silver Artifacts*

Atomic layer deposition (ALD) was investigated as a means to create transparent oxide diffusion barrier coatings to reduce the rate of tarnishing for silver objects in museum collections. Accelerated aging by heating various thicknesses (5 to 100nm) of ALD alumina (Al$_{2}$O$_{3})$ thin films on sterling and fine silver was used to determine the effectiveness of alumina as a barrier to silver oxidation. The effect of aging temperature on the thickness of the tarnish layer (Ag$_{2}$S) created at the interface of the ALD coating and the bulk silver substrate was determined by reflectance spectroscopy and X-Ray Photoelectric Spectroscopy (XPS). Reflectance spectroscopy was an effective rapid screening tool to determine tarnishing rates and the coating's visual impact. X-Ray Photoelectric Spectroscopy (XPS), and Time of Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) analysis showed a phase transformation in the Ag$_{2}$S tarnish layer at 177\r{ }C and saturation in the thickness of the silver sulfide layer, indicating possible self-passivation of the tarnish layer.