ZrO2/ZrO2 Layered Composites for Crack Bifurcation

Abstract

Laminates containing a thin layer (5 to 200 µm thick) sandwiched between two thick layers (2 mm thick) were fabricated by sequential slip casting in order to study crack bifurcation. The thin layer was formed with a mixture of a pure ZrO2 powder (MZ) and a Zr(Y)O2 powder containing 3 mol% Y2O3 (TZ). The thick layers were formed with TZ powder containing 0.05 volume fraction of Al2O3 powder to distinguish the interfaces between the different layers in the scanning electron microscope. Dilatometry data for monolithic specimens formed with the mixed MZ and TZ powders (0.30 to 1.00 volume fraction MZ) showed that the monoclinic‐to‐tetragonal transformation temperature and strain varied with the MZ content, suggesting the yttria in the TZ powder diffused into the MZ powder during processing at 1500°C/2 h. These data also showed that large compressive stresses developed in the thin layer due to the transformation. Conditions (thin layer composition and thickness) for observing edge cracks produced along the center line of the thin layer and for observing crack bifurcation during flexural failure were determined. Delamination occurred during cooling for layers fabricated with greaterthan equal to0.40 volume fraction of the MZ powder when the thin layers were 200 µm thick. Edge cracking, which occurred during cooling, and crack bifurcation, which occurred during flexural loading, occurred for thin layer compositions containing greaterthan equal to0.40 volume fraction MZ powder, when the thickness of the thin layer was between similar/congruent50 and 150 µm. Crack bifurcation was not observed in thinner layers. With decreasing layer thickness, thin layers fabricated with >0.60 volume fraction of the MZ powder contained multiple microcracks, parallel to the center line, on the surface, instead of a single edge crack. The flexural strength of all specimens depended on the strength of the thicker Zr(Y)O2 layers, regardless of whether bifurcation occurred.