Protection of Archaeological Artefacts by Deposition of Parylene and SiOx Thin Films

Radka Balastikova, Michal Prochazka, Premysl Mencik, Jakub Horak,Radek Prikryl, Frantisek Krcma,

Brno University of Technology, Faculty of Chemistry, Czech Republice-mail: xcbalastikova@fch.vutbr.cz

To prevent oxidation process after the removal of corrosion, it is necessary to protect the surface with a barrier film preventing the penetration of oxygen (as well as the other corrosion agents) to the surface. This study focuses on possibilities of the archaeological artefacts (iron) protection by a thin film deposition of SiOx and Parylene thin films. Parylene coatings are prepared by the standard chemical vapor deposition (CVD) method. SiOx layers were deposited by PECVD in a low pressure reactor with capacitively coupled plasma discharge (13.56 MHz).

The coatings were characterized by various methods in order to obtain information about chemical structure (FTIR), elementar composition (XPS), surface morphology (LCSM, SEM) and barrier properties (OTR). Standard corrosion tests were performed to determine the effectiveness of corrosion protection.

PARALOID B44•Samples dried at 100°C for 4 hours under vacuum•2 layers of varnish (delay 6 hours), dried at ambient air•Solution of 4% for iron samples•3% of benztriazole in ethanol added for other materials

PARYLENE•Used modification Parylene C•Test – according to ISO 9227 in a cyclic salt spray chamber CC450 Ascot, time – 300 hours, temperature – 25°C

•pp-HMDSO•Used HMDSO as a precursor•Test – was performed as a immersion test (samples were dipped in pure water and in solution of sodium chloride), time – 336 hours, temperature – 20°C

Fig. 7: Infrared spectrum of Parylene C

SubstratePermeability

(cm3/m2atm·day))

PP-foil 665

PP-foil with

pp-HMDSO127

PP-foil with Parylene-C

32 Concentration (%)

O Si C

Surface 66 31 3

After etching

67 33 0

ConclusionWe have compared thin layers of Parylene C and pp-HMDSO which were used for the protection of archaeological artefacts. We determined that thin layers of pp-HMDSO did not contain carbon, which indicated that we had prepared the thin layer really similar to the SiO2 like layer. With respect to the standard corrosion test was found that the thin film of Parylene C appeared to be a better protection than the thin film of pp-HMDSO.

Fig. 6: Infrared spectrum of pp-HMDSO

Fig. 10 Comparison of iron samples before corrosion test (a) and aftercorrosion test (b) for pp-HMDSO, Paraloid B44 and Parylene

Fig. 1 Scheme of the apparatus for thedeposition of pp-HMDSO

Fig. 2 Apparatus for the depositionof pp-HMDSO (real view)

Fig. 3 Molecule of HMDSO

Fig. 4 Molecule ofParylene C

Tab. 1 Transmision rateof the PP-foil

Tab. 2 Concentration ofO, Si and C obtained fromthe XPS measurements

Fig. 8, 9 Micrographs of the surfaces of thin film of pp-HMDSO (a) and Parylene C (b), magnification of 10 000

a b

Fig. 5 Apparatus for the deposition ofParylene C (real view)

This work has been supported by the Ministry of Culture of the Czech Republic, project No. DF11P01OVV004.

a b

Tab. 3 The process of coating of the samples by Paraloid B44and conditions of corrosion tests for Parylene and pp-HMDSO