Technique for Monitoring the Etching Rate of Alumina
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Technique for Monitoring the Etching Rate of Alumina B. Deb, A. Altay, S. R. Gilliss, N.E. Munoz, C. B. Carter Department of Chemical Engineering and Material Science University of Minnesota, Minneapolis, MN 55455-0132 ABSTRACT The effect of chemical and thermal treatments on the grains and grain boundaries of polycrystalline α-Al2O3 has been examined using a combination of microscopy techniques. Commercially available alumina samples (Lucalox™) were chemically etched in phosphoric acid at 200°C in increments of 15 min. Thermal treatments were carried out at 1650˚C before chemical treatments. Using maps obtained by visible-light microscopy (VLM) as a guide, the same regions were re-examined using atomic force microscopy (AFM) after subsequent treatments. Variations in the dissolution rates of different grains and grain boundaries could then be studied using AFM. The geometry of the grain-boundary grooves was compared after thermal and chemical treatments. Electron backscattered diffraction (EBSD) patterns recorded in the scanning electron microscope (SEM) were used to obtain crystallographic orientations of the grains which enabled variations in dissolution rates between grains to be correlated to orientation. INTRODUCTION Among the various ceramics, aluminum oxide is one of the most thoroughly studied materials with a wide range of applications [1]. Many of the properties and applications of ceramic materials involve their surfaces. For example, their use in sensors, as catalyst supports or as materials for electronic packaging all depend on their surface behavior. Similarly, control of sintering, mechanical strength or the behavior of oxide films that may determine corrosion rates require knowledge of the surface and interface properties. Surface changes during postprocessing heat treatments or chemical treatments can alter the topography of a surface, which can have a significant impact on material performance. Two such thermally activated phenomena are surface faceting and grain-boundary grooving [2-5]. A surface may also be altered by chemical attack. Etching kinetics of different ceramics including alumina had been the subject of many studies [6-14]. The present study presents a technique which monitors changes of the surface of polycrystalline α–Al2O3 through a series of chemical and thermal treatments. By combining VLM, AFM, SEM and EBSD, the differences in the dissolution rates of the grains with various crystallographic orientations can be related to individual surface energies. The grain size of a polycrystalline ceramic typically varies between about 0.1 to 10 µm; for this range of grain size, X-ray diffraction cannot readily be used to determine local crystal orientations. Therefore, it is essential to have a characterization technique that can provide information such as grain size, orientation and the etching rate. EBSD combined with VLM, AFM and SEM can be used to obtain this range of data [15-18]. High-temperature treatments of polycrystalline alumina results in ridges forming at the periph
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