Thin two-phase, AI,O, /t-Zr(3Y)O2 layers bounded by much thicker Zr(3Y)O, layers were fabricated by co-sintering powders. After cooling, cracks were observed along the center of the two-phase, A1,0, /t-Zr(3Y)02 layers. Although the AI,O, /t-Zr(3Y)O2 layers are under residual, biaxial compression far from the surface, tensile stresses, normal to the center line, exist at and near the surface. These highly localized tensile stresses can cause cracks to extend parallel to the layer, to a depth proportional to the layer thickness. A tunneling /edge cracking energy release rate function is developed for these cracks. It shows that for a given residual stress, crack extension will take place only when the layer thickness is greater than a critical value. A value of the critical thickness is computed and compared with an available experimental datum point. In addition, the behavior of the energy release rate function due to elastic mismatch is calculated via the finite element method (FEM). It is also shown how this solution for crack extension can be applied to explain cracking associated with other phenomena, e.g., joining, reaction couples, etc.

During the processing of laminar ceramic, biaxial residual stresses can arise due to differential thermal contraction between unlike layers. A tensile stress can cause preexisting flaws to extend across the layer and into the adjacent layers and then tunnel until they meet either another crack or a free surface. A previous analysis has shown that for a given residual stress there is a critical layer thickness, below which no tunnel cracks will exist, regardless of initial flaw size. Here, the previous analysis was modified to take into account the crack extension into adjacent layers. To determine the validity of the analysis, laminates composed of alternating layers of zirconia and alumina/zirconia were fabricated by a sequential centrifugation technique. The composition of the alumina/zirconia layer was varied to change the biaxial, tensile stresses in the zirconia layer. Observations were then made to determine the critical layer thickness for tunnel cracks and their extension into the adjacent layers. These observations were compared to the theoretical predictions.

Crack bifurcation was observed in laminar ceramic composites when cracks entered thin Al2O3 layers sandwiched between thicker layers of Zr(12Ce)O2. The Al2O3 layers contained a biaxial, residual, compressive stress of ∼2 GPa developed due to differential contraction upon cooling from the processing temperature. The Zr(12Ce)O2 layers were nearly free of residual, tensile stresses because they were much thicker than the Al2O3 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 Al2O3 layer on the bifurcation phenomena. Bar specimens were fractured in four‐point bending. When the propagating crack encountered the Al2O3 layer, it bifurcated as it approached the Zr(12Ce)O2/ Al2O3 interface. After the crack bifurcated, it continued to propagate close to the center line of the Al2O3 layer. Fracture of the laminate continued after the primary crack reinitiated to propagate through the next Zr(12Ce)O2 layer, where it bifurcated again as it entered the next Al2O3 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 Al2O3 layers, crack bifurcation was observed only when the layer thickness was greater than ∼70 μm.