Moldable silicon nitride green-body composite and reduced density silicon nitride ceramic process

Abstract

A moldable green-body composite includes milling silicon nitride powder with a solvent and adding a surface modifier to the milled slurry to modify a surface of the silicon nitride particles. A polysiloxane in a solvent and a binder are also added to create a green body slurry. The solvents may be polar or non-polar solvents. A sintering aid, such as yttria-alumina, may be added to the slurry as well. A reduced density silicon nitride ceramic is made from the moldable green-body composite by molding the moldable green-body composite in a mold and curing at a curing temperature to convert the moldable green-body composite to a converted composite. The converted composite can then be sintered to form a reduced density silicon nitride ceramic that has a smooth surface finish and requires no post machining or polishing. The reduced density silicon nitride ceramic may also have very good dielectric properties.

Claims

1. A process for preparing a reduced density silicon nitride ceramic comprising the steps of: a) preparing a moldable green-body composite comprising: i) providing silicon nitride powder; ii) providing a surface modifier; iii) providing a binder comprising silicon containing polymer; wherein the silicon containing polymer comprises polysiloxane or polysilazanes iv) providing an anti-agglomerate; v) providing a sintering aid; vi) milling the silicon nitride powder and sintering aid with a first solvent to create a mill slurry of silicon nitride particles and milling said mill slurry to produce silicon nitride particles having a first average particle size of no more than 5 pm; vii) adding the surface modifier to the mill slurry and mixing at an elevated temperature to chemically modify a surface of the silicon nitride particles; viii) adding the anti-agglomerate to the mill slurry and mixing at a first temperature to create a dispersed slurry having dispersed silicon nitride particles; ix) combining the binder and a second solvent to produce a pre-dissolved binder having 25 to 50 volume percent binder; x) adding the pre-dissolved binder to the dispersed slurry and mixing at a second temperature that is effectively low to prevent curing of the binder, to create a green body slurry; wherein the second temperature is below 60° C. xi) removing the first and second solvents under a vacuum until the green body slurry comprises no more than about 2% by weight solvent; thereby producing a moldable green-body composite, having a binder concentration of between 30% and 50% by volume; b) molding the moldable green-body composite in a mold at a molding temperature and molding pressure to form a molded part; c) curing the molded part by heating said molded part at a curing temperature for an effective curing time to create a green body part; d) converting the green body part by heating the green body part in a reactive environment under a converting temperature profile to produce a converted composite; and e) sintering the converted composite at a sintering temperature in sintering atmosphere and a packed powder bed for a sintering time; wherein the packed powder bed is in contact with the converted composite during the sintering step to produce said reduced density silicon nitride ceramic.

2. The process for preparing a reduced density silicon nitride ceramic of claim 1, wherein the packed powder bed comprises: a) silicon nitride; and b) boron nitride.

3. The process for preparing a reduced density silicon nitride ceramic of claim 1, wherein the molding temperature is less than about 120° C.

4. The process for preparing a reduced density silicon nitride ceramic of claim 3, wherein the molding pressure is at least about 2000 psi.

5. The process for preparing a reduced density silicon nitride ceramic of claim 1, wherein the step of curing the green body is conducted at an effective temperature of at least 200° C. and time of at least 5 minutes to cross-linking the binder.

6. The process for preparing a reduced density silicon nitride ceramic of claim 1, wherein the converting temperature schedule ends in a maximum temperature of at least about 700° C. and a soak time is at least one hour.

7. The process for preparing a reduced density silicon nitride ceramic of claim 1, wherein the reactive environment of the converting step includes introduction of a reactive gas comprising nitrogen, hydrogen and ammonia.

8. The process for preparing a reduced density silicon nitride ceramic of claim 1, wherein the sintering temperature is no more than 1650° C.

9. The process for preparing a reduced density silicon nitride ceramic of claim 1, wherein the produced reduced density silicon nitride ceramic has a Ra surface roughness of no more than about 10 microns.

10. The process for preparing a reduced density silicon nitride ceramic of claim 1, wherein the produced reduced density silicon nitride ceramic has no more than 15% shrinkage from the molded green body.

Description

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

(1) The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention, and together with the description serve to explain the principles of the invention.

(2) FIG. 1 shows a preferred process diagram for making a moldable green-body composite.

(3) FIG. 2 shows a preferred process diagram for making a molded ceramic part from a moldable green-body composite.

(4) FIG. 3 shows a side view of a cured green body.

(5) FIG. 4 shows a front view of a cured green body.

(6) FIG. 5 shows a side view of a converted part.

(7) FIG. 6 shows a front view of a converted part.

(8) FIG. 7 shows a side view of a molded ceramic part.

(9) FIG. 8 shows a front view of a molded ceramic part.

(10) FIG. 9 shows a graph of the Elevated Temperature Response for a silicon nitride ceramic made according to the present invention.

(11) FIG. 10, shows a graph of silicon nitride particle size distribution for the milled ceramic slurry.

(12) Corresponding reference characters indicate corresponding parts throughout the several views of the figures. The figures represent an illustration of some of the embodiments of the present invention and are not to be construed as limiting the scope of the invention in any manner. Further, the figures are not necessarily to scale, some features may be exaggerated to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

(13) As used herein, the terms “comprises,” “comprising,” “includes,” “including,” “has,” “having” or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a process, method, article, or apparatus that comprises a list of elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Also, use of “a” or “an” are employed to describe elements and components described herein. This is done merely for convenience and to give a general sense of the scope of the invention. This description should be read to include one or at least one and the singular also includes the plural unless it is obvious that it is meant otherwise.

(14) Certain exemplary embodiments of the present invention are described herein and are illustrated in the accompanying figures. The embodiments described are only for purposes of illustrating the present invention and should not be interpreted as limiting the scope of the invention. Other embodiments of the invention, and certain modifications, combinations and improvements of the described embodiments, will occur to those skilled in the art and all such alternate embodiments, combinations, modifications, improvements are within the scope of the present invention.

(15) As shown in FIG. 1, a moldable green-body resin is produced by making a mill slurry that combines a ceramic powder with a solvent. Other components may be added and the ceramic powder is milled to produce a ceramic powder having a reduced particle size. Other components that may be added to the mill slurry include sintering aids, surface modifier and anti-agglomerates. A binder is mixed with a solvent to pre-dissolve the binder, which is added to the mill slurry to produce a combined slurry. The combined slurry is mixed and dried to produce a green body slurry that is further dried and ground to produce a moldable green-body composite.

(16) As shown in FIG. 2, a green-body composite or resin is molded into a molded green-body composite and then cured by the application of heat and pressure to produce a green body part. The green body part is then converted by the application of heat to produce a converted part. The converted part is then sintered by the application of heat to make a ceramic part from a moldable green-body resin. An exemplary moldable green-body resin is molded in a mold at a molding temperature and molding pressure to form a molded part, the molding pressure may be at least about 2000 psi or about 2500 psi or more or about 3000 psi or more. An exemplary molding temperature is less than about 120° C. An exemplary step of curing the green body is conducted at an effective temperature of at least 200° C. and time of at least 5 minutes or at least 10 minutes to cross-linking the binder. The time will depend on the size of the part. An exemplary converting temperature schedule ends in a maximum temperature of at least about 700° C. and a soak time is at least one hour but may be at least 800° C. or about 900° C. for a longer soak time, such as at least 4 hours, or at least 6 hours, or at least 8 hours. An exemplary reactive environment of the converting step includes introduction of a reactive gas comprising nitrogen, hydrogen and ammonia. An exemplary sintering temperature is no more than 1650° C.

(17) As shown in FIGS. 3 and 4, a cured green body 10 comprises ceramic particles 12 that are retained by a binder 14. The space between fastener holes is shown as 18.

(18) As shown in FIGS. 5 and 6, a converted part 20 comprises ceramic particles 12. The space between fastener holes is shown as 28.

(19) As shown in FIGS. 7 and 8, a molded ceramic part 30 comprises ceramic particles 12. The space between fastener holes is shown as 38. The space between fastener holes may have less than 10% reduction from the converted green body and the molded ceramic part.

(20) As shown in FIG. 9, an exemplary silicon nitride ceramic made according to this invention from a moldable green-body composite has very good dielectric properties. The parts tested here have a density of 75% of theoretical.

(21) FIG. 10, shows a graph of silicon nitride particle size distribution for the milled ceramic slurry. The silicon nitride may have an average particle size of about 0.5 to 2 microns with a preferred particle size of about one micron with a quantity of larger particles to enhance packing. As shown, a quantity of larger particles trails off from the smaller particle sizes.

EXAMPLE

(22) An exemplary moldable green-body composite was made according to the present invention with the composition as described in Table 1. The concentration of the silicon nitride was about 82% by weight. The concentration of the binder was about 15% by weight and the concentration of surface modifier, methyltriethoxysilane, (MTES) of about 1% by weight. Anti-agglomeration agent, stearic acid is included in a concentration of about 0.18% by weight. The sintering aids are include in a combined concentration of about 2% by weight.

(23) TABLE-US-00001 TABLE 1 Mass % of Component (g) Total Si.sub.3N.sub.4 300 82 Al.sub.2O.sub.3 3.2 0.88 Y.sub.2O.sub.3 4.3 1.18 Binder 54 14.77 MTES 3 0.82 Stearic acid 1.2 0.33

(24) It will be apparent to those skilled in the art that various modifications, combinations and variations can be made in the present invention without departing from the spirit or scope of the invention. Specific embodiments, features and elements described herein may be modified, and/or combined in any suitable manner. Thus, it is intended that the present invention cover the modifications, combinations and variations of this invention provided they come within the scope of the appended claims and their equivalents.