INTRAGRANULAR CRACK DEFLECTION AND CRYSTALLOGRAPHIC SLIP IN SI3N4 SICNANO-COMPOSITES/

Crack deflection is a potential mechanism for toughening essentially b rittle materials. Deflection generally occurs along grain boundaries a nd is enhanced by particles with high aspect ratios. Si3N4/SiC ceramic /ceramic composites contain elongated betaSi3N4 grains as a major phas e, and provide a good illustration for this mechanism. As a result, su ch materials demonstrate a relatively good fracture toughness. It is s hown in this study, using transmission electron microscopy (TEM), that transgranular cracks, which develop at low temperatures and/or high d eformation rates (present case), may also be deflected by intragranula r SiC inclusions. Cracks use the 100 planes and propagate along the [1 00] directions of the silicon nitride hexagonal lattice. Further, afte r large plastic deformations at high temperature, slip defects are obs erved in silicon nitride grains, with many step-like deviations ('cros s' slip), and the slip system is found to be [001] {100}.