Effects of Porosity on the Threshold Strength of Laminar Ceramics

Abstract

Ceramic laminates exhibiting a threshold strength have been fabricated by dip‐coating thick tape‐cast Al2O3 layers into slurries containing mixtures of Al2O3 and either unstabilized zirconia (MZ‐ZrO2) or mullite to produce thin compressive layers via both a molar volume change and a differential thermal contraction. Porosity was introduced into the thin compressive layers by adding rice starch to the dip‐coating slurries, which decomposed during densification of the laminate. As the volume fraction of porosity is increased, the residual compressive stress (σC), as measured by piezospectroscopy, is reduced and approaches zero at approximately 0.65 volume fraction of porosity. The elastic modulus mismatch (E1/E2) between the thin and thick laminate layers accounted for approximately one‐half of the threshold strength for volume fractions of porosity ≤0.30 (E1/E2<0.4). Above 0.40 volume fraction of porosity, the strength significantly increased as did the scatter in strength values, and it was observed that the highly porous layers completely arrested crack extension; these materials no longer exhibited a threshold strength. For these laminates, failure occurred by the independent, sequential failure of one layer after another, followed by catastrophic failure due to delamination.