AVS 50th International Symposium
    Surface Science Tuesday Sessions
       Session SS-TuP

Paper SS-TuP10
Secondary Phases on Oxide Surfaces

Tuesday, November 4, 2003, 5:30 pm, Room Hall A-C

Session: Poster Session
Presenter: C.B. Carter, University of Minnesota
Authors: C.B. Carter, University of Minnesota
S.R. Gilliss, University of Minnesota
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The wetting of a ceramic surface by a secondary phase is a technologically relevant problem that is encountered under many situations. Processing of ceramics using liquid-phase sintering relies on the wetting of the ceramic powder compact by a lower melting additive. The additive may be a non-metallic glassy phase, as in oxide ceramics, or can be a liquid metal as in the case of Co additives for WC ceramics. The wetting of ceramics by liquid metals is also encountered in joining applications like active metal brazing. Wetting and dewetting of liquid metals and glasses on ceramic substrates has been investigated using a combination of microscopy techniques. The influence of surface structure on dewetting behavior and the influence of the dewet droplets on the morphology of surface steps has been shown with the aid of experimental observations from many systems. The role of chemistry and kinetics on the wetting behavior will also be discussed. The manner in which the secondary phase wets the crystalline ceramic at processing temperatures dictates the microstructure and therefore the properties of the material. Model systems of anorthite and sapphire and silica and rutile will be emphasized. Thin films (100 nm) of the secondary-phase (anorthite or silica) are deposited onto a single-crystal substrate of sapphire or rutile by pulsed-laser deposition. The specimens are then heat-treated in air at high temperature (1400°C-1650°C). At high-temperature and during cooling steps and facets form on the oxide surface and the secondary-phase may dewet or continue to wet the surface. The effect of the secondary-phase on the kinetics of step/facet formation and an analysis of the wetting behavior will be presented. The degree of dissolution and reprecipitation of the substrate material within the secondary phase has been monitored by X-ray energy dispersive spectroscopy and electron energy-loss spectroscopy and will be discussed.