Hertzian indentation tests were performed to investigate the effect of porosity on the contact damage behavior of Si_3N_4 ceramics. Using a bonded-interface technique, the Hertzian contact damage patterns were examined. As a result of intragranular microfracture under Hertzian contact, a distributed subsurface damage region is developed in highly porous Si_3N_4 ceramics. In contrast, low-porosity Si_3N_4 shows a classical cone crack. Regardless of these two distinct damage modes, all the damaged specimens were observed to fail from the contact center in three-point bending tests. However, low-porosity Si_3N_4 exhibits an abrupt strength loss when the indentation load exceeds a critical value, while highly porous Si_3N_4 shows a gradual strength degradation.
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