CRACK DEFLECTION AND PROPAGATION IN LAYERED SILICON-NITRIDE BORON-NITRIDE CERAMICS

Crack deflection and the subsequent growth of delamination cracks can be a potent source of energy dissipation during the fracture of layere d ceramics, In this study, multilayered ceramics that consist of silic on nitride (Si3N4) layers separated by boron nitride/silicon nitride ( BN/Si3N4) interphases have been manufactured and tested. Flexural test s reveal that the crack path is dependent on the composition of the in terphase between the Si3N4 layers. Experimental measurements of interf acial fracture resistance and frictional sliding resistance show that both quantities increase as the Si3N4 content in the interphase increa ses. However, contrary to existing theories, high energy-absorption ca pacity has not been realized in materials that exhibit crack deflectio n but also have moderately high interfacial fracture resistance. Signi ficant energy absorption has been measured only in materials with very low interfacial fracture resistance values. A method of predicting th e critical value of the interfacial fracture resistance necessary to e nsure a high energy-absorption capacity is presented.