EXUDATION OF GLASS FROM GRAIN BOUNDARIES IN ALUMINA

Alumina is one of the most studied ceramic systems, mostly due to its application in corrosive environments, its resistance to abrasion, and reasonable fracture strength. Industrial polycrystalline alumina often contains secondary phases due to dopants and impurities, and the presence of Si and Ca as impurities can often lead to the formation of anorthite glass (Al₂O₃·CaO·2SiO2). At small amounts this can result in the formation of pockets of glass at triple junctions and adsorption of Ca and Si to the alumina grain boundaries. At larger amounts this can lead to wetting films of anorthite at the grain boundaries in addition to triple junctions.

Preparation of alumina surfaces for microscopy usually includes polishing followed by thermal etching (grooving) to provide contrast from the grain boundaries. Annealing at elevated temperatures, or applications at elevated temperatures, can lead to exudation of the glass to the free surface, significantly altering the surface properties. This study focused on the exudation phenomena in different atmospheres (air, carbon rich, or Ar). Fully sintered commercial polycrystalline samples were polished and then thermally etched (annealed) in an air furnace, a graphite furnace (under flowing He), or in an alumina tube furnace (Ar ambient) at 1250°C for 2 hours. The dihedral angle at glass-containing grain boundary grooves (triple junctions) or grooved grain boundaries without glass were characterized in cross-section using focused ion beam microscopy. In this presentation the influence of adsorbed carbon on wetting of alumina by the glass will be presented and discussed.