Crack Bifurcation in Laminar Ceramic Composites

Abstract

Crack bifurcation was observed in laminar ceramic composites when cracks entered thin Al 2 O 3 layers sandwiched between thicker layers of Zr(12Ce)O 2 . The Al 2 O 3 layers contained a biaxial, residual, compressive stress of ∼2 GPa developed due to differential contraction upon cooling from the processing temperature. The Zr(12Ce)O 2 layers were nearly free of residual, tensile stresses because they were much thicker than the Al 2 O 3 layers. The ceramic composites were fabricated by a green tape and codensification method. Different specimens were fabricated to examine the effect of the thickness of the Al 2 O 3 layer on the bifurcation phenomena. Bar specimens were fractured in four‐point bending. When the propagating crack encountered the Al 2 O 3 layer, it bifurcated as it approached the Zr(12Ce)O 2 / Al 2 O 3 interface. After the crack bifurcated, it continued to propagate close to the center line of the Al 2 O 3 layer. Fracture of the laminate continued after the primary crack reinitiated to propagate through the next Zr(12Ce)O 2 layer, where it bifurcated again as it entered the next Al 2 O 3 layer. If the loading was stopped during bifurcation, the specimen could be unloaded prior to complete fracture. Although the residual stresses were nearly identical in all Al 2 O 3 layers, crack bifurcation was observed only when the layer thickness was greater than ∼70 μm.