Sintering and microstructure of silicon nitride prepared by plasma CVD

Abstract

Ultrafine Si₃N₄ powder with average particle size of 30 nm prepared by a thermal plasma CVD was sintered at 1750 °C in nitrogen for 1 h. The sintering behaviour of the powder was characterized by the crystallization of the powder and the resultant sintered bodies were observed with microscopes. It was found that the sinterability depended strongly on the green density and the degree of crystallization. If the powder was homogeneously mixed with sintering additives, it sintered to 98% density at 1750 °C in a nitrogen atmosphere. The microstructure of the sintered bodies observed by SEM indicated that they consist of needle-like grains with an aspect ratio of about 4. The microstructure of a thin film of the sintered body observed by TEM indicated that the grains with crystal habits were wet with liquid phase. TEM also clarified that two kinds of grain boundaries were present; one was wet with liquid phase along a grain boundary and the other was a coincident one without liquid phase. The lattice fringes of liquid phase suggested the presence of Y-apatite which would be generated during cooling.