Interaction of Silicate Liquid with Sapphire Surfaces
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ABSTRACT The wetting behavior of a calcium aluminosilicate liquid in contact with sapphire of four crystallographically different orientations has been investigated using scanning electron microscopy and atomic force microscopy. Continuous layers of glass films heat-treated at 1600'C and 1700'C result in dewet pockets of glass on the surface. The heat treatment also causes the alumina surfaces to facet which presents interesting shapes to the glass pockets. The morphology of the pockets with respect to the orientation of the facets is useful in understanding qualitatively the liquid-solid interfacial energies relative to the orientation of the facets. This approach enables a sequential study of the interaction between the facets and the silicate liquid. Based on temperature, the equilibrium composition of the silicate changes by solution-precipitation and evaporation processes which affects the facet and step morphology. This experiment directly emphasizes the relevance of anisotropy in the oa-A12 0 3 crystal structure which may ultimately control some of the key issues of materials' processing. INTRODUCTION Polycrystalline alumina ceramics invariably contain siliceous phases along grain boundaries. Densification of alumina using liquid-phase sintering (LPS) involves solid-liquid interactions which are not well understood on a microscopic scale. Different approaches adopted recently to study the various aspects of a silicate interacting with cx-A1 20 3 include infiltration studies', alumina-glass-alumina bicrystal "sandwich" experiments 2,3 and in-situ crystallization studies 3 . The wetting and/or dewetting behavior of a silicate liquid at the grain boundaries of ot-A12 0 3 appears to have a dependence on the crystallography of the grain boundary 4 . In this regard, attempts have been made to assess the silicate-glass/alumina interfacial energy as a 2 function of the crystallographic orientation of cx-A120 3 . Anorthite-based composition (CaAl2 Si 2 O8 ) is commonly found as intergranular phase in LPS alumina and this composition is in chemical equilibrium with alumina at 1600'C. In this work, the interaction of liquid anorthite in contact with different orientations of monocrystalline (x-A120 3 has been studied. Study of liquid interaction with surfaces would be helpful in studying how the same liquid interacts with the grain boundaries. EXPERIMENTAL Thin films of calcium-aluminosilicate glass have been deposited onto optically polished surfaces of sapphire using pulsed-laser deposition (PLD). Four different orientations of sapphire obtained from Crystal Systems, Inc. Salem, MA were used as substrates for the deposition. The orientations were (0001), 11170), 1010} and 11102). The substrates were cleaned in acetone followed by methanol prior to the deposition.
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Mat. Res. Soc. Symp. Proc. Vol. 398 0 1996 Materials Research Society
Glass-film deposition Glass films were deposited by PLD using an excimer laser operating with KrF (248 nm). The laser was focused onto a rotating anorthite-based target. Deposition
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