Microhardness Characterization of 80/20 MOL % TiO 2 /SiO 2 Sol-Gel Films
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MICROHARDNESS CHARACTERIZATION OF 80/20 MOL % TiO 2/SiO 2 SOL-GEL FILMS S. M. MELPOLDER, A. W. WEST AND C. L. BAUER Eastman Kodak Company, Manufacturing Research and Engineering Organization, Rochester, New York, 14650-2019. ABSTRACT Knoop hardness and nanoindentation studies were performed on single and multilayered 80/20 mol % TiO 2/SiO 2 films spin-coated onto [100] silicon wafers. The Knoop microhardness results indicated that a total sol-gel thickness (h) to indentation depth (h*) ratio of >2 was sufficient to obtain reliable thin film properties independent of substrate influence. Electron microscopy techniques were used to determine the sample's absolute film thickness, to examine the morphology of the indentations, and to determine the phase of the sol-gel thin films. INTRODUCTION It is desirable to extrapolate bulk material properties from the testing of thin films of similar composition [1-5]. The subject of this paper is the investigation of an acceptable ratio of total film thickness and indent depth for 80/20 mol % TiO2/SiO 2 sol-gel films to obtain reliable hardness values that are independent of substrate influence. The use of Knoop or Vickers microhardness techniques, limit testing to individual or composite film thickness >1 micron due to the instrument's 0.5 g minimum load. For sol-gel films, the absolute thickness of a single defect-free layer that can be coated is usually less than 200 nm. The preparation of 10-100 micron freestanding films by molding or casting techniques for a completely inorganic sol-gel system is limited by the inherent brittleness of the materials at these thicknesses. It is possible to prepare densified sol-gel monoliths, but often with great difficulty. In the case of TiO 2/SiO 2 binary compositions, it has not been possible to prepare consolidated amorphous glasses by traditional melt techniques, or from densified sol-gel aerogel or xerogel monoliths when the titania component exceeds approximately 15 mol % [6,7]. Therefore, in order to investigate the hardness of this amorphous glass, a film format must be used. Single and multilayer films from 100-600 nm were coated onto [100] silicon substrates. TEM cross-sections defined a morphology change from a homogeneous amorphous single layer sample to a nanocomposite network containing crystalline anatase within an amorphous silica matrix for multilayer samples of >3 layers. A nanoindentation study of the same single and multilayer sol-gel films was initiated to confirm the Knoop microhardness of the 5 and 7-layered TiO 2/Si0 2 films and to observe sensitivity of both techniques to microstructural changes. EXPERIMENTAL Thin Film Preparation and Processing The synthesis of 80/20 mol % TiO 2/SiO 2 sol-gel is described in reference [8]. The sol-gel solutions were spin-coated onto [ 1001 silicon wafers using a Headway spin-coater. A detailed description of the coating conditions are cited in [9]. The coated films were immediately processed in a Fisher 495 Isotemp Ashing Furnace and heated at 50 /min to 150 0 C, held for 3 min and
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