Crack propagation in layered ceramic composites of alumina and mullite

Laminar composites, containing layers of Al2O3 and either mullite or a mixt ure of mullite and Al2O3, were fabricated using a sequential centrifuging t echnique of water solutions containing suspended particles. Controlled crac k growth experiments with notched beams of composites showed the significan t effect of barrier layer thickness and composition on the crack propagatio n path during fracture. Distinct crack deflection in mullite layers was obs erved. Also, an increase of the crack deflection angle with the mullite lay er thickness was found. In the case of barrier layers made of a mixture, cr ack deflection was not found independent of layer thickness. The observed c hanges were correlated with the distribution of residual stresses in layers created during cooling of sintered composites from their fabrication tempe rature. These residual stresses are a result of thermal expansion and subse quent sintering shrinkage mismatch of alumina and mullite.