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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. Powders of a quasi-ternary crystalline compound (Gala compound) are employed as at least one sintering additive prepared, prior to mixing with the remaining components and to sintering, from gallium oxide, lanthanum oxide and aluminum oxide by heating to temperatures from 1500.degree. C. to 1850.degree. C. When using at least 70% by weight of silicon nitride powder, there can be used up to 30% by weight of a crystalline 'Gala' compound alone or mixed with other admixtures of oxides, carbides or nitrides, but altogether at least 1% by weight of a crystalline 'Gala' compound must be present. The sintered bodies are especially tough and very hard and can be used for the machining of iron materials at high cutting speeds.

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