Hydrogen defects in alpha-Al2O3 and water weakening of sapphire and alumina ceramics between 600 and 1000 degrees C - II. - Mechanical properties

Hydrogen impurities in alumina have been introduced by hydrothermal anneali ng (see part I). In this paper, we report on reductions in the Row strength of alpha-Al2O3 single crystals and polycrystals associated with hydrogen i ncorporation. Prior to deformation, alpha-Al2O3 single crystal and ceramic specimens were annealed in the presence of supercritical water at 850 degre es or 900 degrees C, under 1500 MPa pressure. Sapphire and alumina ceramics were plastically deformed between 600 degrees and 1000 degrees C under 150 0 MPa pressure, by the addition of a uniaxial stress. Flow stresses are red uced by a factor of two, due to the presence of water, for sapphire and lar ge grain (30-50 mu m) polycrystals, as a result of enhanced dislocation mob ility. Flow stresses of fine-grained (3-5 mu m) polycrystals are reduced by water by a factor of six. This large reduction in strength is attributed t o a change in mechanism from dislocation glide under dry conditions to grai n boundary sliding under hydrothermal conditions. (C) 2000 Acta Metallurgic a Inc. Published by Elsevier Science Ltd. All rights reserved.