CRACK BIFURCATION IN LAMINAR CERAMIC COMPOSITES

Crack bifurcation was observed in laminar ceramic composites when crac ks entered thin Al2O3 layers sandwiched between thicker layers of Zr(1 2Ce)O-2. The Al2O3 layers contained a biaxial, residual, compressive s tress of approximate to 2 GPa developed due to differential contractio n 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 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-poi nt bending. When the propagating crack encountered the Al2O3 layer, it bifurcated as it approached the Zr(12Ce)O-2/ Al2O3 interface. After t he crack bifurcated, it continued to propagate close to the center lin e of the Al2O3 layer. Fracture of the laminate continued after the pri mary crack reinitiated to propagate through the next Zr(12Ce)O-2 layer . where it bifurcated again as it entered the next Al2O3 layer. If the loading was stopped during bifurcation, the specimen could be unloade d prior to complete fracture. Although the residual stresses mere near ly identical in all Al2O3 layers, crack bifurcation was observed only when the layer thickness was greater than approximate to 70 mu m.