Boundary stress and its effect on toughness in thin boundary layered and particulate composites: Model analysis and experimental test on Y-TZP-based ceramic composites

The average thermal residual stress in continuous boundary phase of polycrystalline ceramic composites was calculated with a simple thin boundary layer model and a criterion for the self-cracking of the boundary phase was derived under a certain assumption. From the proposed model, the toughness of the materials can be increased by both tensile and compressive stress at boundaries when the crack propagates transgranularly; and will be increased when the stress at boundary is compressive for intergranular fracture mode. The maximum increase is predicted to be achieved at the boundary phase content not higher than 33%. The experimental results with Y-TZP doped with different kinds of grain boundary phase show a qualitative agreement with the prediction by the model but the toughness increase is largely dependent on the distribution feature of glass phases. From the ideal particle-in-infinite matrix model, the average stress in matrix and in particle for possible practical system was estimated and compared with the thin boundary layer model. The criterion for the self-cracking in matrix and in particle or at the particle-matrix interface was derived with stress intensity factor approach. From the existing periodic stress field model for particulate composite, the toughness increase is found not to increase monotonously with the content of second phase. Alternatively a maximum toughness increase is found, which is predicted to be achieved at the particulate phase content of 14·3 vol%. The experimental results on Y-TZP/Al₂O₃ composites were compared with the prediction of the model.