Hertzian Contact Response of Tailored Silicon Nitride Multilayers

The nature and degree of damage accumulation beneath Hertzian contacts in silicon nitride‐based laminates are studied. Specimens with alternating homogeneous and heterogeneous layers are fabricated by a tape‐casting route, with strong interlayer bonding. Homogeneous material consisting of relatively pure fine‐grain silicon nitride is used as the overlayers. Heterogeneous material containing 10 to 30 wt% boron nitride platelets in a silicon nitride matrix, with weak platelet/matrix interphase boundaries, forms the underlayers. Contact tests with spherical indenters are used to monitor the stress‐strain response of the laminates and to investigate the damage modes within the individual layers. The heterogeneous layer exhibits a distinctive “softening” in the stress‐strain curve, indicating a quasi‐plasticity in the silicon nitride associated with local microfailures at the platelet/matrix interfaces. In contrast to the welldefined cone cracks that develop within the tensile zone outside the contact area in bulk homogeneous silicon nitride, the damage in the laminates is widely distributed within the shear‐compression zone below the contact. Fractures form incompletely in the homogeneous layers, as downward‐propagating partial cone cracks and upwardpropagating stable cracks. Comparatively extensive, diffuse microscopic damage occurs in the heterogeneous layers, culminating in a macroscopic failure that traverses these layers at higher loads. A strong synergism between the interlayer damage modes is apparent. Implications concerning the design of composite laminates for improved damage tolerance, with retention of strength and wear resistance, are considered.