Nanosize silicon nitride: characteristic of doped powders and of the related sintered materials

The paper reports on both the characteristics of ultrafine silicon nitride powder produced by plasma synthesis and the microstructure and properties of the relative sintered material. The powder, already containing yttria and alumina as sintering aids, has a bimodal particle size distribution and it is partly amorphous. The chemical composition and morphology of the particles are shown. Yttria and alumina were not found in separate particles but the elements constituting them (i.e., Y, Al, O) are either in solid solutions in the crystalline particles or dispersed within the amorphous portion of the powder. Dense materials were obtained by pressureless sintering at 1750 degrees C. Microstructure and composition of silicon nitride grains and of grain boundary phases are analyzed and discussed. When compared to a micro-sized Si3N4, nanoindentation tests clearly revealed the inverse Hall Petch relation. The nanosize Si3N4 shows a Young's modulus which is almost independent on the peak load.