Effect of α-Si3N4 initial powder size on the microstructural evolution and phase transformation during sintering of Si3N4 ceramics

By using the Si3N4 ceramic specimens prepared with fine and coarse α-Si3N4 powders, respectively, the phase transformation from α- to β-Si3N4 and concurrent microstructural evolution during sintering were monitored. For the compact prepared with fine powder, the α/β transformation was completed much earlier than the coarse powder. The higher fraction of pre-existing β-grains in fine powder and its higher reactivity compared to those of coarse one are likely to cause a rapid phase transformation. The growth rate of β-Si3N4 grains at the expense of α-Si3N4 during phase transformation stage was quite significant while that after they impinge each other was very limited. As a result, the specimens prepared with coarse and fine initial α-Si3N4 powders resulted in coarse and fine grained β-Si3N4 ceramics, respectively. The specimen prepared with mixture of fine and coarse α-Si3N4 powders exhibited the microstructure containing a few elongated large grains and showed an increased value of fracture toughness.