Plasma enhanced chemical vapor deposition (PECVD) of SiOx thin films on Portuguese limestone: An experimental study

Plasma Enhanced Chemical Vapor Deposition (PECVD) of SiO_x thin films has been applied to the stone surface under laboratory conditions in order to assess its feasibility as an alternative method for stone protection. SiO_x thin layers were deposited on oolitic limestone lithologies known to have been used for the construction and restoration of the Batalha Monastery in Portugal surface by PECVD from tetraethoxysilane (TEOS) as the reaction precursor, in capacitively coupled parallel-plate reactor. The thickness, morphologies and chemical properties of the deposited film were characterized by attenuated total reflectance -Fourier transform infrared (ATR-FTIR) spectroscopy and scanning electron microscopy (SEM+EDS). Laboratory simulated decay and natural aging tests were implemented to evaluate the protective effect of the deposited micron-size SiO_x thin films. Results suggested that, due to the good barrier effect, SiO_x films were able to isolate the stones from aggressive acidic solution, thus reducing possible dissolution-related damage. Natural aging tests proved that SiO_x thin films were also able to inhibit to a high extent bio-colonization while, at the same time reducing soiling of the stone appearance. Durability and wetting property of this SiO_x thin film were preliminarily assessed. This research tested, for the first time, the feasibility of applying micron-size SiO_x film quickly and directly via cold plasma deposition and its versatile protection performance on carbonate stone.