| The effect of the tailoring of the grain boundary phase chemistry on the room and elevated temperature mechanical properties of silicon nitride was investigated.;A ultrafine, high purity, nano-sized amorphous silicon nitride powder was synthesized and exposed to a H;The monolithic ;To improve the mechanical properties, ;A hypothesis was developed to explain the transgranular crack propagation in these high purity materials. It was hypothesized that the presence of residual stresses, as a result of the difference in thermal expansion between the ;In the high purity materials, the residual compressive stress in the intergranular phase promoted transgranular crack propagation. In conventional silicon nitride materials, the intergranular phase is under a residual tensile stress, which led to an intergranular crack path and activation of the toughening mechanisms.;Addition of 100 or 500 ppm of Y or Mg did not change the microstructure, but the crack path was changed to a more circumventing, intergranular crack path. These low levels of doping increased the thermal expansion of the intergranular phase and lowered the glass transition temperature. Thus the residual compressive stress in the intergranular phase was reduced, and this led to more intergranular crack propagation. The increase in the fracture toughness was insignificant due to the small grain size and tensile residual stress state in the ;These small percentages of dopants also resulted in a faster decrease in hardness with temperature than for the undoped material. |
