Plastic and nonmetallic article shaping or treating: processes – Pore forming in situ – By mechanically introducing gas into material
Patent
1983-07-19
1985-01-29
Parrish, John A.
Plastic and nonmetallic article shaping or treating: processes
Pore forming in situ
By mechanically introducing gas into material
264 65, 264 66, 264 86, 264176R, 264325, C04B 3532
Patent
active
044965036
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The invention relates to the technology of making silicon nitride by heat fusing a mixture of Si.sub.3 N.sub.4 powder and densification aids.
BACKGROUND OF THE INVENTION AND PRIOR ART STATEMENT
In the art of making silicon nitride it is conventional to add certain oxides to the raw material from which the fully dense silicon nitride body is constituted; such oxides act as pressing aids or sintering aids (see U.S. Pat. No. 4,143,107). The presence of these oxides has required higher temperatures and pressures to reach full densification during hot pressing or sintering than what is optimally desired for a more economical process.
Generally, compounds other than oxides have been introduced to silicon nitride only as a result of chemical reaction during hot pressing or sintering (see U.S. Pat. Nos. 4,102,698; 4,341,874; and 4,350,771).
Thus, the prior art has recognized that secondary phases form from the oxides and ingredients of the basic mixture during hot pressing or sintering to produce silicates, complex oxides, and oxynitrides. With continuing research herein, it has been discovered that certain of these oxynitrides are more desirable than other oxynitrides and more desirable than silicates or oxides in promoting lower temperatures and pressures needed for processing. This selectively is due in part to the fact that certain of these compounds have a higher degree of solubility for silicon nitride, thus tending to promote a reduction in temperature and pressure needed for full dissolution of the silicon nitride during the heat fusion step.
What is needed is the ability to introduce chemical modifications to the mixture prior to hot pressing or sintering, which modifications allow for: (a) a reduction in the time and temperature required to hot press or sinter the mixture to a fully densified object; (b) freedom to increase the amount of silicate forming oxide to a higher controlled amount to optimize physical characteristics in the final object; and (c) closer control or selection of the desired secondary phase chemistry in the final product without total reliance on the vagaries of the chemical reaction during hot pressing or sintering.
SUMMARY OF THE INVENTION
The invention is a method of making a densified silicon nitride comprising object by the steps which consist essentially of: (a) shaping a substantially homogeneous powder mixture of silicon nitride and yttrium silicon oxynitride, the latter being present in an amount sufficient to form a viscous solution with at least a portion of the silicon nitride during fusion heating, said shaping being carried out to form an object of less than required dimension and density; and (b) densifying the body by heat fusion, with or without the use of mechanical pressure, to a density and a dimension required for the final product.
The homogeneous powder mixture may be prepared by either mixing silicon nitride powder directly with independently prepared yttrium silicon oxynitrides, in the proportions required, or the mixture may be formed by mixing silicon nitride with oxides which are reactive during a preliminary nonfusing heating step to form in situ yttrium silicon oxynitride. The yttrium silicon oxynitride utilized or prepared by both modes should advantageously be at least 75% of the Y.sub.10 Si.sub.6 O.sub.24 N.sub.2 phase. The Y.sub.10 Si.sub.6 O.sub.24 N.sub.2 phase, sometimes referred to as the H phase, has been discovered to have a higher degree of solubility for silicon nitride than other readily formed secondary phases. The H phase can be more readily converted to the K phase (having the formula YSiO.sub.2 N) during the heat fusion step, the latter phase being particularly desirable because of a more stable crystalline characteristic.
The independently prepared yttrium silicon oxynitride should preferably have a purity of 98% or greater and should preferably be introduced in an amount of 6-18% by weight of the mixture. The mixture, for the cold formed mode utilizing a preliminary nonfusing heating step, preferably comprises b
REFERENCES:
patent: 4102698 (1978-07-01), Lange
patent: 4264548 (1981-04-01), Ezis
patent: 4384909 (1983-05-01), Layden
patent: 4401617 (1983-08-01), Ezis
Wills et al., "Phase Relationships in the System Si.sub.3 N.sub.4 -Y.sub.2 O.sub.3 -SiO.sub.2 ", J. Materials Science, 11, (1976), pp. 1305-1309.
Beckwith Elaine C.
Blair Howard D.
Ezis Andre
Ford Motor Company
Johnson Olin B.
Malleck Joseph W.
Parrish John A.
LandOfFree
Method of making a densified silicon nitride/oxynitride composit does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method of making a densified silicon nitride/oxynitride composit, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of making a densified silicon nitride/oxynitride composit will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-505827