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Novel protective coatings for silver and copper alloy cultural heritage objects using atomic layer deposited metal oxide barrier films
Marquardt, Amy Elizabeth
Phaneuf, Raymond J.
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Preserving silver and bronze alloy cultural heritage objects is a monumental challenge for museums as they house thousands of objects that are sensitive to atmospheric corrosion. Metal alloy objects are currently preserved by applying cellulose nitrate lacquers, however, these coatings are limited by short lifetimes, non-uniform application, and appearance changes. This thesis describes investigations of a new method for preserving silver and bronze alloy art and cultural heritage objects using atomic layer deposition (ALD) diffusion barriers. My research focused on determining the potential of ALD films to protect cultural heritage objects while adhering to the strict requirements of art conservation, which require the coatings to minimally change object appearance, have a long effective lifetime, and be completely reversible without significant substrate alteration. ALD coatings were structurally engineered to minimize color changes on silver, silver alloys and bronze patinas by modeling the simulated reflectance spectrum of single and multilayer ALD film stacks. Optimal ALD film structures for minimal appearance change depend on the substrate, requiring different films for silver alloys and bronze. ALD films protected silver at least 15 times longer than cellulose nitrate films, for an expected lifetime of more than 100 years. ALD film porosity decreases with increasing film thickness with no correlation to “dirty” surface treatment, indicating a thick ALD film could effectively protect a non-ideal surface or real cultural heritage object. Al2O3 ALD films were removed with weak solutions of NaOH, however, more Cu than Ag was removed. XPS depth profiling on combinatorial Ag-Cu alloy libraries showed Cu removal was limited to the top 2-3 nm of the alloy surface. ALD films can be deposited on bronze alloys and patinas with and without wax protective layers to prevent changes due to environmental exposure. Bronze patinas have a more complicate surface roughness and chemistry that causes difficulty in achieving a uniform, high quality ALD film. Plasma cleaning is an effective method to ensure high-quality ALD coatings on complex surfaces with tortuous paths, chemical heterogeneity, and topography. ALD films are promising as cultural heritage diffusion barriers as they are reversible with negligible effect on substrate composition.