The Influences Of Different Additives On Pressureless Sintering Of Si₃N₄ Ceramics

The construction, the property, the application and the preparative methods of silicon nitride ceramics were introduced hi this paper, and the pressureless sintering progress of Silicon Nitride ceramics was emphatically introduced.The pressureless sintering Si3N4 ceramics exhibits good use value and good performances in comparision with other sintered Si3N4 ceramics. The election of additives to increase density and to keep high-temperature strength is a key process of the pressureless sintering Si3N4 ceramics. The election of additives and pressureless sintering is the primary coverage of the paper. Si3N4 ceramics have been fabricated by pressureless sintering with rare-earth oxide composite additives at 1720 C , and the mechanical properties have been measured. The sintering process , composition of phase in Si3N4 ceramics and the microstructure have been studied by means of X-ray diffraction (XRD), Scanning electron microscopy (SEM) and transmission electron microscopy (TEM).The results show that the densification process was controlled by the mechanism of liquid phase sintering. The additives would reacted with the SiO2 which was on the surface of the Si3N4 particles to from silicate liquid phase at high temperature, and silicate liquid phase would from glassy phase at grain boundary which was harmful to the high temperature properties. The additives with MgO and rare-earth oxide was found to be much effective as a sintering aid for Si3N4 because it can effectively acceleration of sintering and reduce suiter point. The results indicated that the higher the relative density, the higher the mechanical properties, and 1% TiC can increases the bending strength and hardnessl. The pressureless sintered silicon nitride achieved a relative density of 97.8%, a bending strength of 903 MPa, a Vickers diamondhardness of 13.9 GPa and a fracture toughness of 7.5MPa.M1/2. Corresponding, the pressureless sintered silicon nitride achieved a relative density of 97.4%, a bending strength of 923 MPa, a Vickers diamond hardness of 15.0 GPa and a fracture toughness of 7.02MPa.M1/2.Micro-analysis indicated that the typical microstructure. of sintered SisN4 is made of equiaxed a-Si3N4 crystalline grains and cylindrical B- Si3N4 crystalline grains. The amount and shape of B- Si3N4 crystalline grains is relative of the in raw stuff power. Si3/be made ofa-Si3N4 raw stuff power contains abnormal growth cylindrical - Si3N4 crystalline grains which was helpful to the mechanical properties, intergranular cracks and transgranular cracks was found in sintered Si3N4 ceramics . The blow holes , abnormal grain growth and crack is the chief cause of ceramics damage.