Plastic and nonmetallic article shaping or treating: processes – Including step of generating heat by friction
Patent
1982-10-12
1990-08-07
Derrington, James
Plastic and nonmetallic article shaping or treating: processes
Including step of generating heat by friction
51307, 51309, 264325, 264332, 423344, 501 97, C04B 3558
Patent
active
049466305
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION AND PRIOR ART STATEMENT
This invention is directed to the art of making silicon nitride from silicon powder as a starting material. The powder is subjected to a reactive nitriding gas to form a mixed phase (alpha and beta) silicon nitride body, the body being then hot pressed. Within this art there are known two ways to agglomerate the powder materials: (a) by hot pressing as the last stage in the process, and (b) by cold compacting as an early stage in the process.
The prior art predominantly employs the first way. Agglomerating by hot pressing involves nitriding a loose supply of uncompacted silicon powder to form a cake of unconsolidated silicon nitride. The cake is broken up by primary and secondary reduction techniques to form a free powder supply of silicon nitride. The powder is then poured into a hot pressing assembly where it is agglomerated by heat and pressure. Unfortunately, the reduction techniques applied to the cake introduce contaminants to the silicon nitride powder. Such contaminants can form flaws within the final product. In addition, this powder technology often transfers an unreacted portion of powder ingredients directly into the hot pressing steps, requiring high hot pressing temperatures to bring about sintering under pressure.
The second way uses cold compacting as a step prior to the nitriding treatment. This results in a minimum green density for the material prior to nitriding of at least 1.1 gm/cm.sup.3, and particularly 1.1-1.7 gm/cm.sup.3 (which is approximately 53-85% of full silicon nitride theoretical). This is particularly illustrated in U.S. Pat. No. 3,839,540. The consolidation of the powder, by compacting, adversely affects nitriding by reducing the available pore network and associated exposed surface area. This must be undesirably countered by a greatly increased nitriding time to reduce incomplete nitriding. This in turn adversely affects the temperature of reaction within the material and the type of crystallographic structure formed as a result of nitriding. Without the right crystallographic structure, hot pressing requirements are aggravated if full density is to be achieved.
What is needed is a process which increases the gas phase formation of silicon nitride, dramatically cuts the amount of time required to carry out effective nitriding of a silicon mixture with its accompanying additives, reduces substantially the temperature requirements for hot pressing, and substantially reduces the possibility for flaws in the resulting hot pressed product.
SUMMARY OF THE INVENTION
The invention is a method of making a silicon nitride comprising object by the steps of first heating an uncompacted mixture of silicon powder and a fluxing agent for aiding the pressure sintering of the mixture, said mixture having an increased pore network to increase the inward migration of the atmosphere and having an increased surface area exposed to the atmosphere to decrease the nitriding diffusion distance. The heating is carried out in a nitrogen atmosphere, without the use of pressure normally associated with hot pressing, to fully and substantially react said mixture with said atmosphere to form a body consisting substantially of silicon nitride and having a dimension greater than the required finished product and a density less than the required finished product. Second, the nitrided mixture is hot pressed to produce a silicon nitride comprising object of desired dimension and density which material is useful as a cutting tool material for machining metals.
Preferably, the increased pore network is sufficient to increase the flow of gases through the mixture, carrying heat away, and thereby controlling the exothermic reaction of said atmosphere and mixture to favor the formation of alpha silicon nitride in an amount of at least 60% of said mixture. It is also preferable if the increased pore network is sufficient to compensate for internal volume expansion during nitriding without adversely affecting the increase in inward gas migration and nitridin
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Ceradyne, Inc.
Derrington James
Tachner Leonard
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