Method of preparing a self-reinforced silicon nitride ceramic of high fracture toughness

Claim 1

A process for preparing a silicon nitride ceramic body having a fracture toughness greater than about 6 MPa (m) 1/2 and containing predominantly β-silicon nitride whiskers having a high average aspect ratio, the process comprising: preparing a powder mixture comprising: (a) silicon nitride in an amount sufficient to provide a ceramic body; (b) a densification aid, said densification aid being a non-oxide derivative of magnesium or a source of an element selected from the group consisting of beryllium, calcium, strontium, barium, radium, lithium, sodium, potassium, rubidium, cesium and francium, said source being present in an amount sufficient to promote densification of the powder; (c) a conversion aid, said conversion aid being a non-oxide derivative of yttrium or a source of an element selected from the group consisting of scandium, actinium, lanthanum, lithium, sodium, potassium, rubidium, cesium and francium, said source being present in an amount sufficient to promote the essentially complete conversion of the starting silicon nitride to β-silicon nitride; and (d) at least one whisker growth enhancing compound in an amount sufficient to promote the formation of β-silicon nitride whiskers, said compound being a non-oxide derivative of calcium, or a derivative of an element selected from the group consisting of sodium, potassium, scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, strontium, zirconium, niobium, barium, lanthanum, and mixtures thereof, or an oxide of an element selected from the group consisting of gallium, indium, hafnium, tantalum and boron; provided that (b), (c) and (d) are each derivatives or sources of different elements; and subjecting the powder mixture to conditions of temperature and pressure sufficient to provide for densification and in situ formation of β-silicon nitride whiskers having an average aspect ratio of at least about 2.5 and such that the silicon nitride ceramic body having a fracture toughness greater than about 6 MPa (m) 2 is formed.