Abstract: The invention relates to polycrystalline sintered articles comprising at least 50% by weight of crystalline silicon nitride phase of which at least 90% by weight is in the .beta.-form, and at least one secondary crystalline intergranular silicate phase, and having a density of at least 99% of the theoretically possible density, a modulus of elasticity at room temperature of less than 300 kN/mm.sup.2, a short-term bending strength at room temperatue, measured according to the 4-point method, of at least 600 N/mm.sup.2 which, up to 1200.degree. C., decreases by less than 50%, and a long-term bending strength, measured according to the 4-point method, which is characterized by a numerical value of more than 40 at 1200.degree. C. for the crack growth resistance parameter n.

Abstract: Polycrystalline sintered bodies are provided consisting of at least 66% by weight of a crystalline Si.sub.3 N.sub.4 phase, of which at least 90% by weight is in the beta-modification, and of up to 34% by weight of secondary intergranular grain boundary phases of oxide, carbide and/or nitride. At least 25% by weight of the secondary grain boundary phases consists of a quasi-ternary crystalline compound in the E.sub.1 --E.sub.2 --E.sub.3 triangle of the Ga.sub.2 O.sub.3 --La.sub.2 O.sub.3 -Al.sub.2 O.sub.3 ternary system called "Gala", the "Gala" compound being prepared from gallium oxide, lanthanum oxide and aluminum oxide prior to producing the sintered bodies.The polycrystalline sintered bodies are produced by sintering with and without application of pressure from silicon nitride powders having a portion of impurities not exceeding 4.0% by weight, sintering additives of oxides and optionally other refractory admixtures of carbides and/or nitrides.