Method of making silicon nitride comprising objects

Details Method of making silicon nitride comprising objects Method of making silicon nitride comprising objects

1983-07-19

1984-10-16

Parrish, John A.

Plastic and nonmetallic article shaping or treating: processes

Including step of generating heat by friction

264325, 264332, B28B 716

Patent

active

044774028

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION AND PRIOR ART STATEMENT

In the art of making hot pressed or sintered ceramics, graphite has been frequently used to form the elements of the hot pressing or sintering fixture, the fixture being used to contain the ceramic materials for densification. Graphite is selected for such use because it is relatively strong at high temperature conditions (i.e., 1000.degree.-2000.degree. C.), it has a low modulus of elasticity at high temperatures, and, most importantly, it is excellent for coupling with an induction magnetic field to indirectly heat the ceramic material contained within the graphite elements (see U.S. Pat. Nos. 3,632,708 and 4,071,372). Unfortunately, the graphite frequently reacts with the ceramic material at the high temperature and/or pressure conditions of hot pressing or sintering, forming an unwanted material on the fixture which must be removed for subsequent processing. In sintering, this has been indirectly alleviated by packing of the ceramic material to be sintered in a loose, discardable powder (see U.S. Pat. No. 4,179,486) which may add to the cost of complexity of processing.
In the art of nitriding silicon particles to form silicon nitride by gas phase reaction in a heated furnace, graphite has been used only as a structural wall or felt material located remotely from the silicon to be nitrided, thus avoiding the unwanted reaction by intimate contact with the silicon powder (see U.S. Pat. No. 3,244,480).
It would be desirable in the processing of silicon (to a fully dense silicon nitride object) to combine the arts of cold compaction, nitriding, and hot pressing/or sintering in a continuous procedure. This would obviate frequent handling of the materials to be worked and frequent rearrangement of the processed materials, all of which is costly and can promote flaws if not carried out correctly.


SUMMARY OF THE INVENTION

The invention is a method of making a heat fused silicon nitride comprising object, which method improves the economy and dimensional accuracy of making the object. The method comprises: (a) containerizing at series of measured powder shots in a common assembly to be used for both nitriding and heat fusing steps, the containerization being carried out by (i) lining the internal wall of a graphite sleeve, treated for inertness, with a refractory metal foil essentially impervious to the migration of carbon ions an having a thickness of less than 0.020 inch, (ii) inserting a series of measured shots of a powder mixture of silicon and oxygen carrying powder agents into said lined sleeve, preselected groups of said powder shots being separated by an inert spacer, preferably a parting agent treated silicon nitride spacer; (b) heating the assembly with said shots of powder mixture therein in a nitriding atmosphere to a temperature below the melting point of silicon to convert each of said powder mixture shots to a nitrided silicon body consisting essentially of silicon nitride, 5-17% oxynitrides, and less than 0.5% unreacted oxygen carrying agents, said nitrided silicon body having a density less than that required and a dimension greater than required; and (c) heating the assembly with the nitrided silicon bodies therein with pressure normally associated with hot pressing to a temperature above the melting point of silicon and for a period of time to heat fuse the constituents of said silicon nitrided bodies together, forming a silicon nitride body of required dimension and density.
Preferably, the impervious foil is selected from the group consisting of Mo, Nb, Ta, and W, and preferably the oxygen carrying agents consist of by weight of the silicon metal mixture 1-3.5% SiO.sub.2, 0.4-4% Al.sub.2 O.sub.3, and 6.2-17.8% Y.sub.2 O.sub.3. Preferably, the shots of powder mixture are cold pressed into preformed discs prior to insertion into the sleeve. Preferably, the nitriding involves heating in an evacuated furnace to 650.degree. C., filling the furnace with a nitrogen mixture, and treating to a nitriding temperature above 1000.degree. C. at a s

REFERENCES:
patent: 3535132 (1970-10-01), Lunde
patent: 3589880 (1971-06-01), Clark
patent: 3632708 (1972-01-01), Mandorf
patent: 4377542 (1983-03-01), Mangels
Giachello et al., "Sintering of Silicon Nitride in a Powder Bed", J. Natural Science, 14 (1979), pp. 2825-2830.

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