HYDROGEN DEFECTS IN α-Al_2O_3 AND WATER WEAKENING OF SAPPHIRE AND ALUMINA CERAMICS BETWEEN 600 AND l000 C--II. MECHANICAL PROPERTIES

摘要

Hydrogen impurities in alumina have been introduced by hydrothermal annealing (see part I). In this paper, we report on reductions in the flow strength of α-Al_2O_3 single crystals and po1ycrystals as- sociated with hydrogen incorporation. Prior to deformation, α-Al_2O_3 single crystal and ceramic specimens were annealed in the presence of supercritical water at 850 deg. or 900 deg, C, under l500 MPa pressure. Sapphire and alumina ceramics were plastically deformed between 600 deg. and l000 deg. C under l500 MPa pressure, by the addition of a uniaxial stress. Flow stresses are reduced by a factor of two, due to the presence of water, for sapphire and large grain (30--50 μm) polycrystals, as a result of enhanced dislocation mobility. Flow stresses of fine-grained (3--5 μm) polycrystals are reduced by water by a factor of six. This large re- duction in strength is attributed to a change in mechanism from dislocation glide under dry conditions to grain boundary sliding under hydrothermal conditions.