Development of low cost Si3N4 ceramics

Abstract

Development of low cost Si3N4 ceramics is studied by using low cost starting materials, b-Si3N4 powder with MgO and Al2O3 additives, and sintering in air atmosphere furnace. Fabrication processes were performed by mixing and grinding 90wt%Si3N4 + 5wt%MgO + 5wt%Al2O3 powders for 4 to 16 hours. Then, the mixture was uniaxially and isostatically pressed into specimens. The specimens were placed in a set of specially designed alumina crucibles with silicon nitride and alumina packing powders and sintered in air atmosphere furnace at temperature ranging from 1500 to 1700ฐC for 2 hours. Sintering of b-Si3N4 powder in N2 gas furnace at a temperature of 1750 to 1800 ฐC for 2 hours with N2 gas pressure of 0.1 MPa was also operated in order to compare with the specimens sintered in air furnace. The theoretical densities of all specimens sintered in air furnace were over 96 % at the sintering temperature of 1700 ฐC. Vickers hardness, fracture toughness, and flexural strength of specimens sintered from mixed powder for 4 to 8 hours were from 12.7 to 13.0 GPa, 5.5 to 6.1 MPa.m1/2, and 238 to 263 MPa, respectively. These values were less than the specimens sintered from the same powder in N2 gas furnace of which the Vickers hardness, fracture toughness, and flexural strength are 13.3 to 13.8 GPa, 6.1 to 6.8 MPa.m1/2, and 327 to 406 MPa, respectively. However, the values of flexural strength are relatively low, when compared with 500-600 MPa of commercial Si3N4 ceramics.