Abstract: The specification describes a method for producing high density sintered silicon nitride (Si.sub.3 N.sub.4) having a relative density of at least 98%. In a first step, silicon nitride powder is compacted into a desired shape. It is then pre-sintered in a second step, generally, under normal pressure to obtain a presintered body having a relative density of at least 92%. In a third step, the presintered body is subjected to a hot isostatic pressing (HIP) in an inert gas atmosphere of 1500.degree.-2100.degree. C. and of nitrogen gas partial pressure of at least 500 atm. Since the presintering does not require any capsule, it is possible to produce high density sintered Si.sub.3 N.sub.4 of complex configurations. As a sintering aid, Y.sub.2 O.sub.3 --Al.sub.2 O.sub.3 --MgO system sintering aid is particularly effective. To improve the strength of sintered Si.sub.3 N.sub.4, it is effective to add a heat treatment step after the HIP and maintain the temperature of the sintered Si.sub.3 N.sub.4 above 500.degree. C.

Abstract: The specification describes a method for producing high density sintered silicon nitride(Si.sub.3 N.sub.4) having a relative density of at least 98%. In a first step, silicon nitride powder is compacted into a desired shape. It is then presintered in a second step, generally, under normal pressure to obtain a presintered body having a relative density of at least 92%. In a third step, the presintered body is subjected to a hot isostatic pressing(HIP) in an inert gas atmosphere of 1500-2100.degree. C. and of nitrogen gas partial pressure of at least 500 atm. Since the presintering does not require any capsule, it is possible to produce high density sintered Si.sub.3 N.sub.4 of complex configurations. As a sintering aid, Y.sub.2 O.sub.3 -Al.sub.2 O.sub.3 -MgO system sintering aid is particularly effective. To improve the strength of sintered Si.sub.3 N.sub.4, it is effective to add a heat treatment step after the HIP and maintain the temperature of the sintered Si.sub.3 N.sub.4 above 500.degree. C. for a while.

Abstract: A mixed powder containing: 5-30 wt % of SiC whiskers; 3-30 wt % of a sintering auxiliary agent composed of one kind or more of oxides of Mg, Si, Ca, Ti, Zr, Cr, Ni, Y and rare earth elements; and an Al.sub.2 O.sub.3 powder as a base material; or further containing 0.5-40 wt % of one kind or more of compounds composed of one kind or more of transient elements in the groups IVa, Va, and VIa of the periodic table and one kind or more of C, N and B, are compacted and subjected to pressureless-sintering in a nitrogen containing inert gas atmosphere at a temperature between 1500.degree. and 1900.degree. C. With this method, the SiC whiskers containing Al.sub.2 O.sub.3 based ceramic material, which is densified and has a sufficient strength and toughness, can be formed in a complex shape and a large size.

Abstract: The specification describes a method for producing high density sintered silicon nitride(Si.sub.3 N.sub.4) having a relative density of at least 98%. In a first step, silicon nitride powder is compacted into a desired shape. It is then presintered in a second step, generally, under normal pressure to obtain a presintered body having a relative density of at least 92%. In a third step, the presintered body is subjected to a hot isostatic pressing(HIP) in an inert gas atmosphere of 1500-2100.degree. C. and of nitrogen gas partial pressure of at least 500 atm. Since the presintering does not require any capsule, it is possible to produce high density sintered Si.sub.3 N.sub.4 of complex configurations. As a sintering aid, Y.sub.2 O.sub.3 --Al.sub.2 O.sub.3 --MgO system sintering aid is particularly effective. To improve the strength of sintered Si.sub.3 N.sub.4, it is effective to add a heat treatment step after the HIP and maintain the temperature of the sintered Si.sub.3 N.sub.4, above 500.degree. C.

Abstract: In Al.sub.2 O.sub.3 composites used for machining tools for difficult-cutting steels, etc., strength and toughness are increased by dispersing a Ti compound with an average grain size of not greater than 0.1 .mu.m into Al.sub.2 O.sub.3 particles with the average grain size of not greater than 1 .mu.m, and SiC whiskers are uniformly dispersed between each of Al.sub.2 O.sub.3 particles to form a nanocomposite structure.For the above-mentioned purpose, SiC whiskers containing 0.3 to 1.5% by weight of oxygen, dissolved uniformly in a solvent, are mixed with a starting Al.sub.2 O.sub.3 series powder containing Ti compounds to form a uniform mixture, which is then molded and sintered.

Abstract: A method for producing a high strength sintered silicon carbide, the method comprising: molding a mixture of silicon carbide powder and a sintering aid into a green compact of a predetermined shape: preliminarily sintering the green compact to obtain a preliminarily sintered compact having a relative density higher than 80%; charging the preliminarily sintered compact in a HIP equipment without using a capsule; and subject the preliminarily sintered compact to a HIP treatment in a high-temperature and high-pressure N.sub.2 gas atmosphere maintaining a pressure higher than 500 atms and a temperature in the range of 1400.degree.-2300.degree. C.

Abstract: A nitrogen containing powder metallurgical tool steel comprising at least 0.40% N, 1.6 - 15% V, C in an amount satisfying the relationship of0.2 + 0.2 V(%).ltoreq.(C + N)<0.5 + 0.2 V(%),at least one element selected from the group consisting of up to 15% Cr, up to 10% Mo, up to 20% W and up to 15% Co, with balance iron.

Abstract: The present invention relates to a nitrogen containing high speed steel obtained by the powder metallurgical process, which comprises at least 0.40% N, 1.6 to 15% V, C in an amount satisfying the relationship of 0.5 + 0.2V (%) .ltoreq. (C + N) .ltoreq. 0.8 + 0.2V (%), at least one element selected from the group consisting of up to 15% of Cr, up to 10% of Mo, up to 20% of W and up to 15% of Co with the balance iron.