Abstract: A method of making a shaped part of sintered silicon nitride is disclosed which includes preparing a mixture of Si.sub.3 N.sub.4 having a BET specific surface area in the range of from 2 to 15 m.sup.2 /g, having an O.sub.2 content of <1.5% by weight, a .beta.-form content of <2% by volume with finely divided Y.sub.2 O.sub.3, Al.sub.2 O.sub.3 or HfO.sub.2 and/or ZrO.sub.2, the total additive content being in the range of from 6 to 13% by weight, based on the total weight of the mixture, then mixing and milling mixture in a liquid dispersion medium, drying and agglomerating the suspension so produced, subsequently pressing, injection molding or redispersing and casting the agglomerated material obtained to make shaped parts, and finally sintering the shaped parts at temperatures between 1725.degree. and 1850.degree. C. under nitrogen for a period of up to 2 hours. The shaped parts produced according to the method have high mechanical strength and include at least 87% by weight Si.sub.3 N.sub.

Abstract: In the process for producing a homogeneous and finely crystalline silicon nitride ceramic, ceramic powder is milled, mixed with shaping aids, the mixture is shaped into parts and the shaped parts are subsequently sintered in a nitrogen atmosphere. According to the invention, sintering is carried out in a multistage process at temperatures T.sub.max in the range from 1700.degree. to 1900.degree. C. using gas pressures in the range from 0.3 to 50 MPa. In a first temperature stage, a temperature from 0.9 to 0.96.multidot.T.sub.max is maintained over a time period from 10 to 50 minutes at an N.sub.2 pressure from 0.2 to 1.2 MPa, while in a second temperature stage a temperature from 0.97 to 0.985.multidot.T.sub.max is maintained over a time period from 20 to 80 minutes at an N.sub.2 pressure from 3 to 6 MPa. Then the temperature is increased to T.sub.max and the pressure is increased to from 7 to 50 MPa.

Abstract: A shaped part of sintered silicon nitride has high mechanical strength and comprises at least 87% by weight of silicon nitride and up to 13% by weight of an additive combination of Al.sub.2 O.sub.3 and Y.sub.2 O.sub.3. The Y.sub.2 O.sub.3 /Al.sub.2 O.sub.3 weight ratio is in the range of from 1.1 to 3.4. It can additionally contain HfO.sub.2 and/or ZrO.sub.2 in amounts of up to 1.0% by weight.The shaped part has a density of 98% of the theoretical maximum density, a mechanical strength of .gtoreq.850 MPa at room temperature and a mechanical strength of .gtoreq.800 MPa at a temperature of 800.degree. C. Its fracture toughness K.sub.IC is in the range .gtoreq.8 MPa.multidot..sqroot.m.