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, loss in their performances, and often they are limited in achieving a uniform and conformal coverage across a stone’s topographical features. In this work, atomic layer deposition (ALD) has been explored to address these issues by growing protective amorphous alumina coatings on limestone. 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. The protective effects of the ALD coatings were investigated by aqueous acid immersion. The solution pH, along with [Ca+], was tracked over time for a constant volume of acid solution with an initial pH of 4 with the stone samples immersed. We find the protective effect of ALD alumina coatings are extremely promising, slowing the average rate of pH evolution significantly. The eventual failure of the ALD coatings during immersion were also investigated, with interesting morphological findings which point to the role of defects in the coatings, and suggesting new directions for improving the use of ALD coatings in future research and applications.