Effects of residual stresses on mechanical properties of three-layer alumina-based composites

Abstract

It is well known that the mechanical properties of ceramic components for structural applications can be enhanced by the introduction of surface compressive stresses during fabrication. For alumina ceramics, this goal can be attained by incorporating a second phase such as mullite into the surface region. Due to the mismatch of thermal expansion coefficient between mullite and alumina, a residual surface compressive stress will be created during cool-down after heat-treatment. Recently, an increase of 50--150% in fracture toughness has been obtained in the experiment by making a three-layer alumina composite in which the outer layers contain 40 vol% mullite. In other experiments where mullite was formed in the surface region by heating alumina in a bed of SiC powder or by annealing an Si-deposited alumina in air, the strength increases of > 25% have also been reported. These increases are attributed to the presence of surface residual compressive stresses. The existence of residual surface stresses can be confirmed by X-ray diffraction. The purpose of this work is to show that the X-ray diffraction technique can be used to determine the residual stresses in the layers of three-layered composites, and that the mechanical properties are closely related to the magnitudes of more » the residual stresses. « less