Toughness of Si{sub 3}N{sub 4} ceramics obtained by precipitating sintering aids as hydroxides

Sintering aids were added to silicon nitride slurries through hydroxide precipitation from aluminum and yttrium nitrate solutions and subsequent calcination. Spray-dried powder and sintered products were characterized by light optical and electron microscopy. As a result of the homogeneous dispersion of the sintering aids, sintered density of the system approached the theoretical value even when the total amount of sintering aids was as low as 5%. Mechanical properties of the sintered product, however, were not significantly improved over samples obtained with traditional methods; this was probably due to the presence of hard agglomerates and/or to a microstructure which contained long, unattached grains of {beta}-silicon nitride.