Deformation Processes during Indentation of Mesoporous Silica Thin Films

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Deformation Processes during Indentation of Mesoporous Silica Thin Films M. Klotz, A. Perriot and E. Barthel Unité Mixte CNRS/Saint-Gobain “ Surface du Verre et Interface ”, UMR 125, Saint-Gobain Recherche, F-93303, Aubervilliers, Cedex, France. [email protected] ABSTRACT Nanoindentation was used to probe the mechanical response of mesoporous silica films as a function of processing treatment. A contrasted behaviour appears between low temperature processed films which retain the templating agent and are thus little compressible, and high temperature processed films, which are more condensed and also exhibit high porosity. We suggest that the behaviour of the former is dominated by shear flow, at least above a given threshold, while the latter are characterized by a yield strength in compression as is commonly found in porous materials. The response to indentation allows fast qualitative comparison of mechanical responses: in the present work, we show that a fast UV-O3 treatment results in mechanical properties which are undistinguishable from a much more cumbersome high temperature heat treatment. INTRODUCTION Ordered mesoporous thin films can be prepared by the combination of sol gel chemistry and lyotropic mesophases [1,2]. They are of interest for many applications ranging from optoelectronics to sensors and separation technologies. Even if extensive studies are undertaken to improve the understanding of the ordered structure and its formation mechanisms, work on the mechanical properties of the films is rather sparse [3,4]. However, the mechanical properties of the films are of paramount importance for the development of efficient and durable materials. In the present work, we study the mechanical response of ordered mesoporous silica layers exhibiting a P63/mmc structure as a function of the treatment used to remove the templating molecules of the ordered porosity.

EXPERIMENTAL DETAILS Mesoporous thin films are prepared using tetraethoxysilane as the precursor of the inorganic network and cetyltrimethylammonium bromide as the template. The detailed synthesis procedure can be found in [5,6]. The layers are obtained by spinning the solution at 4000rpm on glass slides. Three different treatments were used to remove the template and obtain the porous thin film: (a) Soxhlet extraction of the organic phase in absolute ethanol (6h), (b) a 3 hours UV-O3 treatment with a UV lamp at an oxygen flow of 3 l/minute and (c) heat treated at 450°C with steps at 50°C, 100°C, 150°C and 175°C to promote the condensation of the silica network. The typical film thickness is 300 nm. Transmission infra-red spectroscopy (Nicolet Nexus 670) was used to check that the various treatments were effective to remove the organic phase. The ordering of the mesoporous layers was studied using a Brucker D5000 X-ray diffractometer in Bragg-Brentano configuration. The

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samples were nanoindented with a Berkovich tip in a (pseudo) constant deformation rate mode to 1 micron depth. The force, displacement and contact s