Microstructure and fracture toughness of Si3N4 ceramics: Combined roles ofgrain morphology and secondary phase chemistry

Silicon nitride materials that contained different mixtures of sintering ai ds were investigated with respect to microstructure development and resulti ng fracture toughness. Postsintering annealing at 1850 degrees C for variou s times was adopted in order to coarsen the respective microstructures, Alt hough constant processing conditions were used, a marked variation in fract ure toughness of the Si3N4 materials was evaluated. With a larger grain dia meter of the Si3N4 grains, an increase in fracture resistance was generally observed. However, a correlation between fracture toughness and apparent a spect ratio could not be established. The observed changes in microstructur e were in fact caused by the difference in secondary-phase chemistry. Si3N4 , grain growth was dominated by diffusion-controlled Ostwald ripening and w as hence affected by the viscosity of the liquid at processing temperature. In addition, crystallization at triple pockets also depends on the sinteri ng additives employed and was found to influence fracture toughness by alte ring the crack-propagation mode as a consequence of local residual microstr esses at grain boundaries. The stress character (compressive vs tensile) is governed by the type of crystalline secondary phase formed. Moreover, a va riation in interface chemistry changes the glass network structure on the a tomic level, which can promote transgranular fracture, i,e,, can result in a low fracture resistance even in the presence of favorable large Si3N4 mat rix grains. Therefore, secondary-phase chemistry plays a dominant role with respect to the mechanical behavior of liquid-phase-sintered Si3N4, Fractur e toughness is, in particular, influenced by (i) altering the residual glas s network structure, (ii) affecting the secondary-phase crystallization at triple pockets, and (iii) changing the Si3N4 grain size/morphology by affec ting the diffusion rate in the liquid. The first two effects of secondary-p hase chemistry are superimposed on the merely structural parameters such as grain diameter and apparent aspect ratio.