Indentation plasticity and fracture of Si3N4 ceramic alloys

The response of a series of one- and two-phase-Si₃N₄ ceramic alloy surfaces to sharp diamond microindentation has been examined by optical and electron microscopy. The microhardness (H), which obeys the load-independent relationH=P/a ² (whereP anda are load and indent size, respectively) is nearly constant within the alloy series, indicating a retention of high covalency at large (Al and O) substitution levels. Indentation results from severe localized plasticity which is characterized by the operation of the dominant dislocation Burgers vectora0 0 0 1] in the hexagonal lattice. The severe anisotropy in plasticity induces grain-boundary microcracking which is believed to nucleate median cracks which propagate away from the plastic zone on symmetry planes beneath the indenter. The relation between load, median crack size (c) and fracture toughness (K _c) is of the form,K _c=constant (P/c ^3/2) predicted theoretically. Values ofK _c rank correctly with those from notched-beam measurements, but there is uncertainty about the value of the constant.