Effect of Nitrate Salts as Sintering Additives during the Ball‐Milling Process of Silicon Nitride Powders

A chemical adsorption method in a Si 3 N 4 slurry that contained a nitrate solution was studied during ball milling, with particular interest in increasing the oxide layer in the Si 3 N 4 powder and improving the distribution homogeneity of the sintering additives. The nitrate salts Al(NO 3 ) 3 .9H 2 O and Y(NO 3 ) 3 .6H 2 O were selected as sintering additives. The following characterization techniques were used: oxygen-nitrogen analysis, X-ray photoelectron spectroscopy, high-resolution electron microscopy (coupled with energy-dispersive X-ray spectroscopy), and X-ray imaging (using wavelength-dispersive X-ray spectroscopy). The thickness of the amorphous layer and the oxygen content of the Si 3 N 4 powder were greater for samples that were milled with nitrate additives, which were heat-treated at 600°C, than those of powders that were milled with oxide additives. The chemical composition of the oxygen-containing layer - that is, the amorphous layer that formed and/or changed on the Si 3 N 4 surface - was similar to Si 2 N 2 O in heat-treated Si 3 N 4 powder with nitrate additives, whereas the composition of heat-treated Si 3 N 4 powder with oxide additives was similar to SiO 2 . Furthermore, a homogeneous distribution of the additives was achieved via the incorporation of aluminum and yttrium into the amorphous layer on the Si 3 N 4 surface. The metal ratio (Y:AI) of the adsorbates was somewhat higher than that of the additives.