Influence of microstructure and temperature on the interfacial fracture energy of silicon nitride/boron nitride fibrous monolithic ceramics

The microstructure and interfacial fracture energy of silicon nitride/boron nitride fibrous monoliths, Gamma(BN), were determined as a function of sta rting silicon nitride composition and temperature using the method describe d by Charalambides. The glassy phase created by the sintering aids added to the silicon nitride cells was shown to migrate into the boron nitride cell boundaries during hot-pressing. The amount of glassy phase in the boron ni tride cell boundaries was shown to strongly influence Gamma(BN) at room tem perature, increasing the fracture energy with increasing amounts of glass. Similar trends in the interfacial fracture energy as a function of temperat ure were demonstrated by both compositions of fibrous monoliths, with a lar ge peak in Gamma(BN) observed over a narrow temperature range. For silicon nitride cells densified with 6 wt% yttria and 2 wt% alumina, the room-tempe rature interfacial fracture energy was 37 J/m(2), remaining constant throug h 950 degrees C. A sharp increase in Gamma(BN), to 60 J/m(2), was observed between 1000 degrees and 1050 degrees C. This increase was attributed to in teractions of the crack tip with the glassy phase in the boron nitride cell boundary. Measurements at 1075 degrees C indicated a marked decrease in Ga mma(BN) to 39 J/m(2). The interfacial fracture energy decreased with increa sing temperature in the 1200 degrees to 1300 degrees C regime, plateauing b etween 17 to 20 J/m(2). A crack propagation model based on linkup of existi ng microcracks and peeling/cleaving boron nitride has been proposed.