Abstract
The depth profile of indented cracks in silicon nitride with MgO additive was determined by serial sectioning and observation of fracture areas. These cracks were of Palmqvist’s type, in contrary to the Si₃N₄ with Al₂O₃ + Y₂O₃ additives. A deformed/damaged zone was identified under the indent with a depth ranging from 420 to 490 µm. The volume of the indent represents on average 5 % of the volume of the deformed zone. The deformed zone of silicon nitride with MgO additive was approximately 3 times higher than in the silicon nitride sintered with additives of Al₂O₃ + Y₂O₃. The shape of the deformed zone was angular for both materials, although their character is different. In the case of specimens with Al₂O₃ + Y₂O₃ additives, the fracture area circumvents the deformed zone, whereas in the case of MgO additive the fracture passes through it. The number of minor cracks, initiated during indentation with Vickers indenter was lower in the case of silicon nitride with MgO additive, which renders the value of fracture toughness more reliable for this material.

Key words
silicon nitride, deformation under indent, depth profile of cracks, serial sectioning, fracture area

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