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 well-defined 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 upward-propagating 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.