Correlation between structure and properties of Si3N4 materials with Y2O3/Al2O3 sintering additives

The chemical composition of the grain boundary phase and the concentration of Al3+ ions dissolved in the Si3N4 grains were determined by analytical tr ansmission electron microscopy for gas-pressure-sintered Si3N4 materials wi th different ratios of the sintering additives Y2O3 and Al2O3. The concentr ation of Al dissolved in the Si3N4 grains, i.e. the substitution parameter z, increases with increasing amount of Al2O3 in the starting powder. The li quid phase is enriched by Y, Si, and N as gas pressure sintering progresses . For z in the range 0,32 < z < 0,35, a maximum of bending strength, which was more pronounced at a testing temperature of 1000 degrees C, was achieve d. The materials with a high strength level at room and at higher temperatu res (up to 1000 degrees C) are characterised by a higher substitution param eter z, finer grains, lower thermal expansion coefficients and smaller diff erences in thermal strain between the amorphous grain boundary phase and th e Si3N4 matrix.