Inhomogeneities In Sol-Gel Coatings
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INHOMOGENEITIES IN SOL-GEL COATINGS B. D. Fabes. D. L. Klein. and L. J. Raymond University of Arizona. Department of Materials Science and Engineering. Tucson. Arizona 85721
ABSTRACT The structure of sol-gel derived silica coatings was investigated using nanoindentation. chemical etching. and cross-sectional TEM. For coatings heated 0 at 100 C. the surfaces were harder, etched slower, and appeared less porous than the interior of the coatings. For samples fired at higher temperatures (400-800 °C). the surfaces remained harder and etched slower than the interior. even though the porosity appeared to be evenly distributed throughout the thickness of the coating. It is proposed that this difference is due to inhomogeneities on two scales in the coatings: bulk porosity and skeletal strain.
I.
BACKGROUND
It is often assumed that the structure of a sol-gel coating is the same near the surface as throughout the bulk of the coating. However. we recently found that the hardness of tetraethoxysilane (TEOS) derived silica coatings is significantly higher at the surface than within the bulk of the coating (1]. The goal of this study was to further characterize the structure of sol-gel coatings as a function of depth. and to gain insight into what causes the surface to be different from the interior. Three characterization techniques nanoindentation. chemical etching. and cross sectional TEM - were used to probe the structure of sol-gel coatings as a function of depth. These techniques are sensitive to different features of the coating structure, and therefore allowe the scale of the inhomogeneities to be probed. A description of the coating preparation process is given in the following Section. The details of the characterization techniques, a description of the features to which they are sensitive, and the results for the TEOS coatings are presented in Section III. These results are discussed in Section IV.
II.
COATING PREPARATION
TEOS derived silica coatings were prepared by hydrolyzing 1 mole of TEOS with 2 moles of water, diluted in 4 moles of ethanol. HCl was used to bring the solution pH to 2: the solutions were then allowed to react for at least two weeks before coating. Coatings were made by withdrawing silicon or sapphire substrates from the TEOS solution at a controlled rate. All substrates were cleaned in an ultrasonic bath using a warm detergent solution and rinsed in deionized water prior to coating. For the nanoindentation experiments, sapphire substrates were used so that there would be a large difference between the hardness of the coating and that of the substrate. Using a computer to continually vary the coating speed. a linear gradient was produced in the thickness of the coatings on sapphire so that a range of thicknesses could be tested on each sample. For etch rate and TEM studies, silicon wafers were withdrawn from the TEOS solution at a constant rate. resulting in a constant coating thickness.
Mat. Res. Soc. Symp. Proc. Vol. 180. @1990 Materials Research Society
606
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