Pressureless Sintering of Si3N4 Ceramic Using AlN and Rare-Earth Oxides

Using intermediate, liquid-forming compositions in the approach focuses on the Si3N4–R2O3 binary, where R2O3 is any (Y,La)2O3–AlN system as additives, fully dense Si3N4 ceramrare-earth oxide (or Y2O3) used singly or severally. After sinics with high strength at high temperature have been tering, this composition forms -Si3N4 and some refractory rareobtained by pressureless sintering. The ceramics contain earth silicon oxynitrides (e.g., melilite (M-phase) R2Si3O3N4, rod-shaped -Si3N4 with M or K solid solutions as grainwollastonite (K-phase) RSiO2N, apatite (H-phase R5(SiO4)3N, boundary phases. The strength of these ceramics is 1150 or rare-earth silicates, e.g., R2Si2O7 and R2SiO5), which tend to MPa at 1200 C, and the room-temperature toughness is exhibit higher strength at high temperatures. The sinterability maintained at }7 MPa m. Phase relations that are pertiof this formulation, however, is poor, and hot pressing or hot nent to the new additive compositions are delineated to isostatic pressing is usually required to effect densification. rationalize their beneficial effects on sinterability and Also, because the -Si3N4 network of elongated grains is less mechanical properties. well-developed because of the more refractory liquid, the roomtemperature toughness of these ceramics also is lower. The second approach focuses on the Si3N4–R2O3:9AlN binary. This