Laser Densification of Sol-Gel Coatings
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LASER DENSIFICATION OF SOL-GEL COATINGS
D.J. TAYLOR, B.D. FABES and M.G. STEINTHAL University of Arizona, Department of Materials Tucson, Arizona 85721
Science and Engineering,
ABSTRACT
Alkoxide-derived silica coatings were deposited on fused silica by dip coating. The samples were covered with a metal film to absorb the infrared radiation from an Nd:YAG laser. Coupling the laser power to the coating By scanning the sample produced localized surface heating on the samples. across the beam's path, channels in the coating were formed. The channels varied from 150 pm to 600 pm wide and from 200 A to 1000 A deep depending on the laser power and the translation speed. The size and shape of the channels also depended on the composition of the sol-gel coating. Optical microscopy showed that there were no cracks in either the substrate or in the coating on the micron scale. SEM revealed cracks on the nanometer scale in the laser fired coatings and no cracks in the unfired coatings. Ellipsometry showed that the index of refraction increased with increasing depth of the channels.
INTRODUCTION
Sol-gel methods can be used to make glass and ceramic materials of high purity, novel compositions, and novel microstructures. Sol-gel derived coatings offer outstanding opportunities because the attractive features offered by such processing can be obtained without many of the problems associated with the formation of bulk pieces [I]. In nearly all applications of sol-gel coatings, the control or elimination of porosity in the coating is key to obtaining the desired properties. Dense coatings are usually desired because they have a higher index of refraction, higher hardness, increased abrasion resistance, and other desirable properties. Unfortunately, the high temperatures (typically > I000° C) required to densify the coatings often place severe restrictions on the choice of substrate material. In addition, unwanted diffusion between the substrate and the coating can occur during the firing process [2]. The goal of the present research is to densify highly refractory compositions on soft, less refractory substrates. We want to control densification not only normal to the substrate, but also in the plane of the coating. This type of process could open the door to many other applications, such as planar waveguides. The main issue to be addressed is how to heat the coating sufficiently to densify it without melting or warping the substrate. The approach taken here is to densify the sol-gel derived coating with a laser. Hench reported using a CO2 laser to form densified channels in bulk sol-gel samples [3]. The wavelength of the CO2 laser (10.6 pm) would be absorbed by an oxide substrate if proper care were not taken. Hence, using a CO 2 laser to densify coatings might work if the power and modulation of the laser were closely controlled. However, it would be ideal to have a coating that absorbs at the wavelength Mat. Res. Soc. Symp. Proc. Vol. 180. @1990 Materials Research Society
1048
This might be accomplished by of the laser
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