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, GBN, were determined as a function of starting silicon nitride composition and temperature using the method described 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 nitride cell boundaries was shown to strongly influence GBN at room temperature, increasing the fracture energy with increasing amounts of glass. Similar trends in the interfacial fracture energy as a function of temperature were demonstrated by both compositions of fibrous monoliths, with a large peak in GBN observed over a narrow temperature range. For silicon nitride cells densified with 6 wt% yttria and 2 wt% alumina, the room-temperature interfacial fracture energy was 37 J/m2, remaining constant through 950°C. A sharp increase in GBN, to 60 J/m2, was observed between 1000° and 1050°C. This increase was attributed to interactions of the crack tip with the glassy phase in the boron nitride cell boundary. Measurements at 1075°C indicated a marked decrease in GBN to 39 J/m2. The interfacial fracture energy decreased with increasing temperature in the 1200° to 1300°C regime, plateauing between 17 to 20 J/m2. A crack propagation model based on linkup of existing microcracks and peeling/cleaving boron nitride has been proposed.