Plastic and nonmetallic article shaping or treating: processes – Including step of generating heat by friction
Patent
1983-07-19
1985-04-09
Parrish, John A.
Plastic and nonmetallic article shaping or treating: processes
Including step of generating heat by friction
C04B 3558
Patent
active
045101078
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The invention relates to the technology of making silicon nitride by reacting silicon powder under a nitrogenous atmosphere and then densifying the nitrided body by heat fusion.
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, pressures, and pressing times 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 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). Rarely have compounds other than oxides been introduced to Si.sub.3 N.sub.4 prior to hot pressing or sintering. One indirect instance is disclosed in copending U.S. application Ser. No. 444,251, PCT application No. 82/01372, commonly assigned to the assignee of this invention. In this latter case, oxides were added to a silicon metal mixture and the mixture was nitrided to react the oxides and form a composite of silicon nitride, oxynitrides, and a small amount of silicate. Under this disclosure, the nitriding step was not able to optimally fulfill the dual roles imposed upon it, that is: (a) to efficiently convert all of the silicon to silicon nitride, and (b) at the same time form only a selected silicon oxynitride in a controlled optimum amount.
What is needed is the ability to introduce chemical modifications to the mixture prior to nitriding or heat treatment, which modifications allow for: (a) a more efficient conversion of silicon to silicon nitride, (b) a reduction in the time and temperature required in subsequent steps to hot press or sinter the mixture to a fully densified object, (c) freedom to decrease the amount of silicate forming oxide to a much lower controlled amount to optimize physical characteristics in the final object, and (d) closer control and proportioning of the desired secondary phase chemistry in the final product without total reliance on the vagaries of chemical reaction during any reaction heating involved in the process such as during nitriding, hot pressing, or sintering.
Continuing research, as exemplified herein, underscores the need for selectivity of silicon oxynitrides and the need for greater silicon conversion efficiency. Certain of these oxynitrides are more desirable than others and more desirable than silicates or oxides in promoting lower temperatures and pressures needed for processing. This selectivity 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 at least one of the processing parameters (temperature, pressure, or hot pressing time) needed for full dissolution of the silicon nitride during the heat fusion step.
SUMMARY OF THE INVENTION
The invention is an improved method of making a more densified silicon nitride comprising object under less stringent heating conditions without sacrificing the physical properties now attainable by the most preferred prior art methods of making silicon nitride.
In a method where silicon powder is subjected to sequential heating, including first a nitriding heating to form silicon nitride and secondly a heating to fuse the resulting silicon nitride, the invention comprises having yttrium silicon oxynitride present with said silicon or silicon nitride prior to heating to fuse. A major proportion of the yttrium silicon oxynitride is of the Y.sub.10 Si.sub.6 O.sub.24 N.sub.2 phase. The yttrium silicon oxynitride serves to dissolve the silicon nitride during the second heating to fuse, the dissolved yttrium silicon oxynitride converting to a more stable phase as a result of the second heating to fuse.
Preferably, the Y.sub.1
REFERENCES:
patent: 4102698 (1978-07-01), Lange
patent: 4143107 (1979-03-01), Ishii
patent: 4341874 (1982-07-01), Nishida
patent: 4350771 (1982-09-01), Smith
patent: 4401617 (1983-08-01), Ezis
Galasso, "Sintering of Si.sub.3 N.sub.4.ISY.sub.2 O.sub.3 Under High N.sub.2 Pressure", Ceramic Bulletin, Aug. 1979, pp. 793-794.
Blair Howard D.
Ezis Andre
Ford Motor Company
Johnson Olin B.
Malleck Joseph W.
Parrish John A.
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