Grain and grain boundary strength of silicon nitride ceramics with different thermal conductivities and microstructures

The grain and grain boundary strengths of polycrystalline Si 3 N 4 ceramics with different thermal conductivities, fabricated over different sintering times, were investigated using microcantilever beam specimens. The bending strength and variation of Si 3 N 4 grains were almost the same regardless of the thermal conductivity. However, specimens with higher thermal conductivities exhibited lower grain boundary strength than those with lower thermal conductivities, owing to the higher yttrium and oxygen contents in the intergranular glassy film (IGF) and lower oxygen content in the grains. Hence, during firing at high temperatures, a part of the liquid phase was evaporated and oxygen was diffused from the grains to grain boundaries, increasing the ratio of yttrium and oxygen in the latter. Moreover, yttrium attracted oxygen into the IGF from Si 3 N 4 grains, decreasing the amount of Si-O bonds and increasing the dangling bonds at the IGF/Si 3 N 4 interface. Thus, the high thermal conductivity achieved during sintering increased the yttrium and oxygen contents in the grain boundaries, reducing the grain boundary strength.