Hot pressing of silicon nitride using magnesium silicide

Claim 1

A method of producing a hot-pressed polycrystalline silicon nitride body having a density of at least 80% of the theoretical density of silicon nitride and which retains at least about 75% of its room temperature mechanical properties at elevated temperatures ranging up to at least 1350° C in air which consists essentially of providing at least a significantly homogeneous powder dispersion having an average particle size which is submicron of silicon nitride and magnesium silicide, and based on the total composition of the starting silicon nitride powder said silicon nitride powder containing up to about 0.1% by weight of metallic impurities which react with SiO 2 or Si and O 2 to form low melting intergranular silicate glassy phase, up to about 3% by weight of oxygen, up to about 4% by weight of elemental silicon, and up to about 1% by weight of halogen, said magnesium silicide being used in an amount ranging from about 0.5% by weight to about 3% by weight based on the amount of silicon nitride, and hot-pressing said dispersion in an atmosphere of nitrogen at a temperature ranging from about 1600° C to about 1850° C under a pressure of at least about 2000 psi, said body being at least substantially free of an intergranular silicate glassy phase at its grain boundaries.

Claim 2

A method of producing a hot-pressed polycrystalline silicon nitride body having a density of at least 80% of the theoretical density of silicon nitride and which retains at least about 75% of its room temperature mechanical properties at elevated temperatures ranging up to at least 1350° C in air which consists essentially of providing at least a significantly homogeneous powder dispersion having an average particle size which is submicron of silicon nitride and magnesium silicide, and based on the total composition of the starting silicon nitride powder said silicon nitride powder containing up to about 0.1% by weight of metallic impurities which react with SiO 2 or Si and O 2 to form low melting intergranular silicate glassy phase, up to about 3% by weight of oxygen, up to about 4% by weight of elemental silicon, and up to about 1% by weight of halogen, said magnesium silicide being used in an amount ranging from about 0.5% by weight to about 3% by weight based on the amount of silicon nitride, and hot-pressing said dispersion in an atmosphere of nitrogen at a temperature ranging from about 1600° C to about 1850° C under a pressure of at least about 2000 psi, said body being at least substantially free of an intergranular silicate glassy phase at its grain boundaries.