Silicon nitride based ceramics and method

Claim 1

A method of preparing fully dense silicon nitride base ceramic articles comprising the steps of blending form about 1 to about 25 mole percent of a metal oxide selected from the group consisting of magnesium oxide, aluminum oxide, silicon dioxide, yttrium oxide, the lanthanide rare earth oxides, and mixtures thereof with the balance consisting essentially of silicon nitride including acicular crystals of said silicon nitride; said acicular crystals having an average aspect ratio greater than 3; said acicular crystals being present in sufficient quantity to yield a compact having a green density less than 60% of theoretical; comminuting the blend of metal oxide and silicon nitride to form a powder mixture; monitoring said comminuting to determine the degree of acicularity of said acicular crystals of said silicon nitride, said monitoring comprising removing a sample of said powder mixture being comminuted and pressing said sample into a pill, fracturing said pill forming a fractured surface, examining said fractured surface to determine said degree of acicularity of said acicular crystals; controlling said comminuting to obtain an average aspect ratio of said acicular crystals of said silicon nitride equal to or less than about 3; compacting the comminuted powder mixture to form a green compact having a density of at least 60% of theoretical; and pressureless sintering said compact to form an article having a density of at least 98% of theoretical.

Claim 2

A method of preparing fully dense silicon nitride base ceramic articles comprising the steps of blending form about 1 to about 25 mole percent of a metal oxide selected from the group consisting of magnesium oxide, aluminum oxide, silicon dioxide, yttrium oxide, the lanthanide rare earth oxides, and mixtures thereof with the balance consisting essentially of silicon nitride including acicular crystals of said silicon nitride; said acicular crystals having an average aspect ratio greater than 3; said acicular crystals being present in sufficient quantity to yield a compact having a green density less than 60% of theoretical; comminuting the blend of metal oxide and silicon nitride to form a powder mixture; monitoring said comminuting to determine the degree of acicularity of said acicular crystals of said silicon nitride, said monitoring comprising removing a sample of said powder mixture being comminuted and pressing said sample into a pill, fracturing said pill forming a fractured surface, examining said fractured surface to determine said degree of acicularity of said acicular crystals; controlling said comminuting to obtain an average aspect ratio of said acicular crystals of said silicon nitride equal to or less than about 3; compacting the comminuted powder mixture to form a green compact having a density of at least 60% of theoretical; and pressureless sintering said compact to form an article having a density of at least 98% of theoretical.

Claim 4

A method of preparing fully dense silicon nitride based ceramic atricles comprising the steps of comminuting silicon nitride, including acicular crystals of said silicon nitride having an average aspect ratio greater than 3 monitoring said comminuting to determine the degree of acicularity of said acicular crystals of said silicon nitride, said monitoring comprising removing a sample of said powder mixture being comminuted and pressing said sample into a pill, fracturing said pill forming a fractured surface, examining said fractured surface to determine said degree of acicularity of said acicular crystals; controlling said comminuting to obtain an average aspect ratio of said acicular crystals of said silicon nitride equal to or less than about 3; blending from about 1 to about 25 mole percent of a metal oxide selected from the group consisting of magnesium oxide, aluminum oxide, silicon dioxide, yttrium oxide, the lanthanide rare earth oxides, and mixtures thereof with the blance consisting of the comminuted silicon nitride wherein the average aspect ratio of said acicular crystals is equal to or less than about 3; compacting the comminuted powder mixture to form a green compact having a density of at least 60% of theoretical; and pressureless sintering said compact to form an article having a density of at least 98% of theoretical.

Claim 5

A method of preparing fully dense silicon nitride based ceramic atricles comprising the steps of comminuting silicon nitride, including acicular crystals of said silicon nitride having an average aspect ratio greater than 3 monitoring said comminuting to determine the degree of acicularity of said acicular crystals of said silicon nitride, said monitoring comprising removing a sample of said powder mixture being comminuted and pressing said sample into a pill, fracturing said pill forming a fractured surface, examining said fractured surface to determine said degree of acicularity of said acicular crystals; controlling said comminuting to obtain an average aspect ratio of said acicular crystals of said silicon nitride equal to or less than about 3; blending from about 1 to about 25 mole percent of a metal oxide selected from the group consisting of magnesium oxide, aluminum oxide, silicon dioxide, yttrium oxide, the lanthanide rare earth oxides, and mixtures thereof with the blance consisting of the comminuted silicon nitride wherein the average aspect ratio of said acicular crystals is equal to or less than about 3; compacting the comminuted powder mixture to form a green compact having a density of at least 60% of theoretical; and pressureless sintering said compact to form an article having a density of at least 98% of theoretical.