Amorphous Alumina ALD Coatings for the Protection of Limestone Cultural Heritage Objects

From natural erosion to pollution-accelerated decay, stone cultural heritage deteriorates constantly through interactions with the environment. Common protective treatments such as acrylic polymers are generally prone to degradation and loss of performance, and they are often limited in their ability to achieve uniform and conformal coverage across a stone’s topographical features. In this work, atomic layer deposition (ALD) was explored to address these issues by growing protective amorphous alumina coatings on compact carbonate (Istria) stone. ALD protective coatings, unlike coatings produced by traditional methods, do not significantly alter morphology by filling open pores or accumulating on the surface in more compact areas. Our morphological and spectroscopic investigations revealed that the ALD alumina films deposited uniformly over the surfaces of Istria stone, without significantly altering the topography or appearance. The protective effects of the ALD coatings were investigated by aqueous acid immersion. The solution pH, along with the Ca2+ concentration, was tracked over time for a constant volume of acetic acid solution with an initial pH of 4 with the stone samples immersed. We found that the protective effects of ALD alumina coatings were extremely promising, slowing the average rate of pH evolution significantly. The eventual failure of the ALD coatings during immersion was also investigated, with interesting morphologicalfindings that point to the role of defects in the coatings, suggesting new directions for improving the use of ALD coatings in future research and applications.