Sintered self-reinforced silicon nitride

A process for preparing a dense, self-reinforced silicon nitride ceramic body by pressureless or low pressure gas sintering, the process comprising subjecting a powder mixture including:

• (a) silicon nitride in an amount sufficient to provide a ceramic body;
• (b) a combination of a source of magnesium oxide and a source of silicon dioxide, the silicon dioxide being present in an amount sufficient to provide a total silicon dioxide content of at least 3 weight percent, and the combination being present in an amount sufficient to aid densification of the powder;
• (c) a source of yttrium oxide in an amount sufficient to promote essentially complete conversion of the silicon nitride to .beta.-silicon nitride;
• (d) a catalytic amount of a source of zirconium oxide;
• and
• (e) at least one whisker growth enhancing compound in an amount sufficient to promote the formation of .beta.-silicon nitride whiskers, said compound(s) being selected from the group consisting of a source of titanium dioxide and a source of aluminum oxide;
• to a pressure within a range of from 1 atmosphere (0.1 MPa) to 100 atmospheres (10 MPa) and to a temperature sufficient to provide for densification to a value which is at least 98 percent of theoretical and sufficient to provide in situ formation of .beta.-silicon nitride whiskers having a high average aspect ratio, such that the ceramic has a fracture toughness greater than about 6.0 MPa.(m).sup.1/2 as measured by the Chevron notch technique at 23.degree. C.