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FP5

EFESTUS Report Summary

Project ID: ICA3-CT-2002-10030
Funded under: FP5-INCO 2
Country: Italy

Plasma enhanced deposition of SiO2-like innovative coatings as protective barrier layers for bronze archaeological artefacts

New corrosion resistant coatings for the protection of bronze archaeological artefacts, by using clean and environmentally friendly processes, have been developed and optimised. The coatings deposition has been performed by PECVD technique (plasma enhanced chemical vapour deposition) in a home-made reactor with hexamethyldisiloxane-oxygen-argon mixtures of different compositions and at different input power values. The deposition has been carried out, in a first step, on Cu-based reference alloys produced by ISMN-CNR with microchemical and microstructural features similar to the ancient alloys, as-received and aged in different soils, and after the optimisation, in a second step, on archaeological bronze specimens, after restoration.

The optimisation of the PECVD treatment has been performed by changing the different parameters and has been accompanied by an accurate study of both the plasma phase and the characteristics of the deposits obtained; this information, in addition to assuring the exploration of all the possible operative conditions turns out to be indispensable for the reproduction of the results, for successive process scale-up and for the in situ process control. Starting from inorganic precursors (silane) or organic (organosilicons), it is possible to deposit thin film of SiO2-like containing variable quantities of carbon, hydrogen, and eventually nitrogen (if contained in the monomer); when these molecules are immersed in the plasma phase, they fragment due to electron impacts, generating ions and highly reactive radicals, which may react in a gas phase, producing films more or less thin (0.01 - 5 micron), with variable composition and property.

The reaction occurs at a relatively low temperature, 20 - 70°C and the fragments formed by the organosilicon precursors, on the whole, may react with possibly added oxidants (generally O2), both in the gas phase (homogeneous oxidation), and on the surfaces on which the organosilicon film grows (etherogeneous oxidation). Naturally, with the proper variations in the conditions of the process (substrate temperature, specimens position in the reactor, oxidant concentration) it is possible to vary with continuity the chemical composition and the properties of the deposit. The low temperatures and the low oxidant concentrations promote the production of deposits with high organic component, flexible, slightly hard, permeable to gases and to water, and at times, not very stable. The high temperatures, the ionic bombardment and the massive oxidant addition generate inorganic film (SiO2-like): stable, non porous, slightly permeable to water and to gases, hard, chemically resistant.

Even the temperature and the difference of potential between the bulk of plasma and surfaces are very important, since they induce the breakage of the bond Si-H, Si-CH and C-H; they increase cross-linking and reduce the tenor of carbon and hydrogen.
The elevated compactness, low porosity, hardness and low permeability to water and gases of SiOx deposits obtained in this project revealed them definitely suitable for the protection from corrosion of metals. Moreover the reversibility, due to the possibility of removal by means of plasma treatment and the good aesthetic appearance render them suitable also for cultural heritage artefacts of artistic, historical and archaeological interest.

The noticeable scientific and industrial interest of the PECVD technique is a highly versatile technique that can be employed for the production of a variety of coatings, on a avariety of substrates, from metals to textiles, paper and polymers, with a wide range of properties simply by means of a proper selection of the experimental conditions.

Related information

Reported by

Politecnico di Torino, Dipartimento di Scienza dei Materiali ed Ingegneria Chimica
Corso Duca degli Abruzzi 24
10129 Torino
Italy
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