Effects of Porosity on the Threshold Strength of Laminar Ceramics

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 (σ_C), as measured by piezospectroscopy, is reduced and approaches zero at approximately 0.65 volume fraction of porosity. The elastic modulus mismatch ( E ₁/ E ₂) between the thin and thick laminate layers accounted for approximately one-half of the threshold strength for volume fractions of porosity ≤ 0.30 ( E ₁/ E ₂<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.