Grazing incidence x-ray fluorescence modelling of complex surfaces as applied to diffusion barriers for cultural heritage objects

Diffusion barriers deposited by atomic layer deposition (ALD) have been shown to be effective in reducing the tarnishing rate of silver cultural heritage objects. Conformal, dense metal oxide thin films deposited with ALD meet conservation requirements of being more effective than current silver protection methods, reversible without significant substrate alteration, and optically transparent. ALD film lifetimes are limited by how quickly atmospheric H2S reaches the silver substrate, either by diffusion or fast transportation through film defects, to form a Ag2S tarnish layer. To better optimize barrier layer performance, diffusion of sulfur from accelerated aging under atmospheres with elevated H2S concentrations/ low temperature (50°C) was characterized by correlating secondary ion mass spectroscopy (SIMS) and grazing incidence x-ray fluorescence (GIXRF). Multiple GIXRF models were compared to determine the most adequate one for non-ideal samples with surface roughness and non-uniform sulfur distribution in the barrier layer. GIXRF analysis was used to quantify sulfur concentration throughout the ALD barrier layer deposited on to Ag-Cu alloy films with variable Ag/ Cu ratio after being aged for varying times. Sulfur dose was determined by a full-fit procedure of the angular scan using internally developed simulating software to simultaneously fit all the spectra composing the GIXRF angular scan. Given the complexity of these non-ideal samples, the accuracy of the following different models for treating sulfur inclusion were compared.