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 polycry stalline ceramic composites was calculated with a simple thin boundary laye r model and a criterion for the self-cracking of the boundary phase was der ived under a certain assumption. From the proposed model, the toughness of the materials can be increased by both tensile and compressive stress at bo undaries when the crack propagates transgranularly; and will be increased w hen the stress at boundary is compressive for intergranular fracture mode. The maximum increase is predicted to be achieved at the boundary phase cont ent not higher than 33%. The experimental results with Y-TZP doped with dif ferent 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-infini te matrix model, the average stress in matrix and in particle for possible practical system was estimated and compared with the thin boundary layer mo del. The criterion for the self-cracking in matrix and in particle or at th e particle-matrix interface was derived with stress intensity factor approa ch. From the existing periodic stress field model for particulate composite , the toughness increase is found not to increase monotonously with the con tent 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. (C) 1998 Elsevier Science Limited All rights reserved.