Abstract:
A method of making fully dense, crack-free silicon nitride articles using polysilanes as a binder. Polysilane is dissolved in a solvent and a silicon nitride composition including a densification aid is added to form a homogeneous mixture. The mixture is dried to form a powder, and molded at a temperature less than 100° C. to form a molded article. Alternatively, the slurry is poured into a mold and vacuum filtered to form a cake, then isostatically pressed at a temperature of approximately 90° C. The molded article or pressed cake is heated at a rate of approximately 5° C./min to about 900° C. in a nonoxidizing atmosphere and held at about 900° C. for a time sufficient to decompose the polysilane. The article is sintered in a nonoxidizing atmosphere at a temperature of about 1685°-1900° C. to form a silicon nitride article free of cracks and having a density greater than 3.5 g/cc.

Description:
This is a continuation of copending application Ser. No. 07/675,601 filed on Feb. 12, 1991, now abandoned, which is a continuation of application Ser. No. 07/494,891 filed on Mar. 12, 1990 and now abandoned, which is a continuation of application Ser. No. 07/092,270 filed on Aug. 31, 1987 and now abandoned. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a method for making silicon nitride articles. 
     More particularly, this invention relates to a method for making silicon nitride articles using polysilane, or polysilazane as a binder. 
     BACKGROUND OF THE INVENTION 
     In recent years, the search for cost-effective production of complex ceramic shapes used at elevated temperatures has stimulated the research and development of metal organic polymer precursors. Fine ceramics made from metal organic precursors have several advantages over the ceramics produced by the conventional processing. For example, low temperature forming processes can be used to produce complex shape by a variety of forming techniques. A wide range of purities can be achieved through careful balance of chemical stoichiometry. The opportunity to chemically purify starting materials and assure homogeneous mixing can improve the uniformity and reliability of the final product. 
     Strength-limiting factors in high-performance technical ceramics are not always directly related to composition. As the desired shapes get more complicated, it becomes more and more difficult to fabricate parts reliably and free of cracks. One of the problems encountered frequently in fabricating ceramic parts is the binder used in injection molding process. The binder&#39;s physical properties must satisfy stringent requirements to allow complete filling of complicated shaped molds without forming density gradients, and the binder must be completely removed prior to sintering without causing physical defects. Organic hydrocarbon-polymers are currently used for this purpose. However, there are problems such as low powder packing densities and the length of time necessary to remove the binders. In addition, the molded articles have a poor green strength after binder removal. As a result, there is excessive shrinkage when the molded article is sintered which makes it difficult to maintain the dimensional precision of the molded article after sintering. Therefore, the exploration of novel binder materials is needed to alleviate these problems. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, a new and improved method for making silicon nitride articles involves dissolving polysilane in a solvent and adding a silicon nitride composition to form a homogeneous mixture. The silicon nitride composition includes silicon nitride and one or more densification aids selected from aluminum oxide and yttrium oxide. The solvent is removed from the mixture by evaporation to form a powder. The powder is molded at a temperature less than 100° C. to form a molded article. The molded article is heated at a rate of approximately 5° C./min to a temperature of about 900° C. in a nonoxidizing atmosphere and the temperature is held at about 900° C. for a time sufficient to decompose the polysilane. The molded article is then sintered in a nonoxiding atmosphere at a temperature of about 1685° C. to about 1900° C. to form a densified silicon nitride article free of internal cracks and having a density greater than 3.05 g/cc. 
     In accordance with another aspect of the invention, a new and improved method for making silicon nitride articles by slip isostatic pressing involves dissolving polysilane in a solvent and adding a silicon nitride composition to form a homogeneous mixture. The silicon nitride composition includes silicon nitride and one or more densification aids selected from aluminum oxide and yttrium oxide. The mixture is poured into a mold and the solvent is removed from the mixture by vacuum filtering to form a cake. The cake is isostatically pressed at a temperature of approximately 90° C. The pressed cake is heated at a rate of approximately 5° C./min to a temperature of about 900° C. in a nonoxidizing atmosphere and the temperature is held at about 900° C. for a time sufficient to decompose the polysilane. The pressed cake is then sintered in a nonoxidizing atmosphere at a temperature of about 1685° C. to about 1900° C. to form a densified silicon nitride article free of internal cracks and having a density greater than 3.05 g/cc. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     This invention utilizes polysilane, or polysilazane as a binder for the processing of Si 3  N 4  based composites, such as PY6 and AY6 materials. The PY6 composition comprises about 6 weight percent Y 2  O 3  as a densification aid and Si 3  N 4  which contains 1-4 weight percent SiO 2  preferably 3 weight percent SiO 2 . The AY6 composition comprises about 2 weight percent Al 2  O 3 , about 6 weight percent Y2O 3 , both densification aids, and Si 3  N 4  which contains about 1-4 weight percent SiO 2  preferably 3 weight percent SiO 2 . 
     The specific binders selected were MIT&#39;s thermal plastic polysilazane, and Union Carbide&#39;s thermoset vinylic polysilane, both having good flow properties and high ceramics yield upon pyrolysis. As illustrated in Tables I, II and III, silicon nitride based composite mixture was formulated at three different binder concentrations using polysilane or polysilazane. 
     The first mixture contained about 10 weight percent binder and about 90 weight percent AY6, the second contained about 25 weight percent binder and about 75 weight percent AY6, and the third contained 40 weight percent binder and 60 weight percent AY6. The mixture was blended either by dry blending or dispersing by sonicating a solution containing the binder, AY6, and toluene forming a slurry suspension. The slurry was dried to form a powder. The resulting powders were then sifted through a 200 mesh screen prior to pressing at 6,000 psi in a die press (1&#34; diameter disc or 1/2&#34; diameter pellets). The binder removal (burn out) was carried out in nitrogen, a non-oxidizing atmosphere. The composite pellets and discs were embedded in a setter powder made of the same material as the initial AY6 powder. The resulting pellets and discs of the composites using polysilane as a binder were heated at a rate of 3° C./min to 700° C., held for 11/2 hrs, and then cooled at a rate of 10° C./min to room temperature. A slightly different schedule was applied to the composites using polysilazane as the binder. They were heated at a rate of 5° C./min to 900° C., held for 3 hrs, and cooled at a rate of 5° C./min to room temperature. The volatile decomposition products from the pyrolysis step diffused out of the composite without causing internal cracking as noted by microfocus x-ray imaging analysis and scanning electron microscopy. 
    
    
     EXAMPLE 1 
     2.72 grams of polysilane were added to 32 ml of toluene and stirred until dissolved. 7.49 grams of AY6 powder (silicon nitride containing alumina and yttria sintering aids) was dispersed in the toluene mixture with a sonicator for 10 minutes. The toluene solvent was evaporated by heating in a nitrogen stream. The resulting powder was sifted through 150 mesh screen and pressed into 1 gram pellets utilizing a 6000 lb. per square inch pressure which was applied twice to form the pellet. The resulting pellet was then heated to 900° C. at a rate of 5° C. per minute and held for 1 hour forming an amorphous silicon nitride and silicon carbide. Then it was sintered at 1850° C. at 200 psi nitrogen pressure for 3 hours to form a densified silicon nitride pellet. 
     EXAMPLE 2 
     1.08 grams of polysilazane were dissolved in 30 ml of toluene and dispersed with 9 grams of AY6 (silicon nitride having alumina and yttria as sintering aids) and 0.5 grams (5 wt %) oleic acid. The mixture was sonicated for 10 minutes. The powder was dried with stirring and sieved through a 100 mesh screen. The sieved powder was then pressed into a 4 gram disc using 6000 lbs. per square inch pressure. The disc was then heated in a nitrogen atmosphere to a temperature of 900° C. at a rate of 5° C. per minute and held at temperature for 1 hour then cooled. This was then sintered at 1750° C. for 3 hours in a nitrogen atmosphere forming a densified silicon nitride disc having a density of 3.098 grams per cubic centimeter. 
     EXAMPLE 3 
     Slip-Isostatically Pressed Billets were prepared by dispersing 85 grams of a AY6 powder in 100 ml of a toluene solution containing 15 grams of polysilane. The mixture was sonicated for 10 minutes to form a slurry. The slurry was then dried in air. The dried powder (20-25 grams) was added to 9 to 14 ml of isopropanol to make a slip. The slip was sonicated and poured into the cavity of a rubber mold (11/2&#34;×11/2&#34;×1/2&#34;) and vacuum filtered on a porous bronze filter to form a filter cake. The cake was then isostatically pressed at 23,000 psi for 2 minutes forming a pressed billet. The pressed billet was dried in a dissicator for 12 hours prior to burnout. The binder in the billet was burned out by heating in a nitrogen non-oxidizing atmosphere at a rate of 1.0 to 2.5° C./min to 900° C., and held at 900° C. for 5 hours then cool down to room temperature at a rate of 2.5° C./min. The billet was then sintered at 1850° C. in a 200 psi nitrogen atmosphere (overpressure) for 4 hours. 
     EXAMPLE 4 
     Billets were also prepared by dispersing 39 grams of a AY6 powder into a 30 ml of toluene solution containing 6.5 grams of polycarbosilane by sonicating the dispersion to form a slurry. The slurry was then poured into the cavity of a rubber mold (11/2&#34;×11/2&#34;×1/2) and vacuum filtered on a porous bronze filter to form a filter cake. The cake was either isostatically pressed or set for 2 hrs at 90° C. prior to the removal from the mold. The billet was then sintered at 1850° C. in a 200 psi (overpressure) nitrogen atmosphere (non-oxidizing atmosphere) for 4 hours. 
     Two slip-isostatically pressed billets containing 25 wt % polysilazane and 75 wt % AY6 powders were found to remain intact after the binder burnout cycle and were sintered to 3.1 to 3.2 g/cc nominal density at 1850° C. 200 psi N 2  overpressure. The similar results were obtained from billets containing 25 wt % polysilane and 75 wt % AY6 as illustrated in Table IV of the sintering results. Slip-cast billets prepared from the same compositions were also sintered to high density composites at 1850° C., 200 psi N 2  overpressure. The mechanical strength of these sintered, fully dense composites has been determined by 4 point MOR testing from 25° C. to 1400° C. The oxidation rate at 1000° C. was also determined after 600 hr. exposure. The results are comparable to those of the conventional AY6. 
     The mechanical strength and oxidation resistance properties, oxidation rate constant, obtained after 600 hrs at 1000° C. of samples 28-35, are illustrated in Table V. The sintering results are summarized in Table IV. 
     This invention provides a method for improving the uniformity and reliability of the final product. The green strength is improved and the amount of shrinkage upon sintering is reduced improving the dimensional precision of the molded article after sintering. In addition, the sintered article is free of internal cracking which is a serious problem with other methods. 
     While there has been shown and described what is at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims. 
     
                                           TABLE I__________________________________________________________________________                 Density g/ccSample        Shape-Forming                 After BinderNo. Composition         Technique                 Burnout                        Sintering Conditions__________________________________________________________________________1   40 wt % polysilane         Die Pressed-                 1.61   1850° C. - 60 psi    60 wt % AY6         6,000 psi      N.sub.2 overpressure         1/2&#34; pellets2   Same      Same    1.65   1900° C. - 200 psi                        N.sub.2 overpressure3   25 wt % polysilane         Same    1.59   1750° C. - 60 psi    75 wt % AY6              N.sub.2 overpressure4   Same      Die Pressed-                 1.36   1850° C. - 60 psi         6,000 psi,     N.sub.2 overpressure         1&#34; Disc5   25 wt % polysilane         Same    1.55   1750° C. - 60 psi    75 wt % AY6 +            N.sub.2 overpressure5 wt % oleic acid    based on AY66   Same      Die Pressed-                 1.62   1850° C. 3 hrs.         6,000 psi      60 psi         1/2&#34; pellets   N.sub.2 overpressure7   10 wt % polysilane         Same    1.92   1850° C. - 75 psi    90 wt % AY6              N.sub.2 overpressure8   Same      Same    1.98   1700° C. - 12 psi                        N.sub.2 overpressure9   Same      Same    1.98   1900° C. - 200 psi                        N.sub.2 overpressure10  Same      Same    1.93   1700° C. - 12 psi                        N.sub.2 overpressure__________________________________________________________________________ 
    
     
                                           TABLE II__________________________________________________________________________                   Density g/ccSample          Shape-Forming                   After Binder                          SinteringNo. Composition Technique                   Burnout                          Conditions__________________________________________________________________________11  40 wt polysilazane           Die Pressed-                   1.87   1850° C. - 60 psi    60 wt % AY6 6,000 psi      N.sub.2 overpressure           1/2&#34; pellets12  Same        Same    1.94   1850° C. - 70 psi                          N.sub.2 overpressure13  Same        Same    1.90   1685° C. - 8 psi                          N.sub.2 overpressure14  25 wt % polysilazane           Die Pressed-                   2.23   1850° C., 3 hrs.    75 wt % AY6 6,000 psi,     60 psi           1&#34; Disc        N.sub.2 overpressure15  Same        Same    2.25   1850° C. - 60 psi                          N.sub.2 overpressure16  25 wt % polysilazane           Same    2.16   1850° C., 60 psi    75 wt % AY6 +              N.sub.2 overpressure5 wt % oleic acid    based on AY6 powders17  Same        Same    2.19   1850° C. - 60 psi                          N.sub. 2 overpressure18  10 wt % polysilazane           Same    2.18   1750° C. - 60 psi    90 wt % AY6                N.sub.2 overpressure19  Same        Same    2.17   1850° C. - 60 psi                          N.sub.2 overpressure20  10 wt % polysilazane           Same    2.12   1750° C. - 60 psi    90 wt % AY6 +              N.sub.2 overpressure5 wt % oleic acid    based on AY6 powder__________________________________________________________________________ 
    
     
                                           TABLE III__________________________________________________________________________                   Density g/ccSample         Shape-Forming                   After Binder                          SinteringNo. Composition          Technique                   Burnout                          Conditions__________________________________________________________________________21  20 wt % polysilazane          Isostatically                   --     1850° C./50 psi    80 wt % AY6          Pressed billet  N.sub.2 overpressure                          4 hrs.22  16 wt % polysilane          Same     2.06   1850° C./200 psi    84 wt % AY6                N.sub.2 overpressure                          4 hrs.23  20 wt % polysilane          Same     1.79   Same    80 wt % AY624  25 wt % polysilane          Same     1.79   Same    75 wt % AY625  Same       Same     1.79   Same26  10% wt polysilane          Same     1.91   Same    90 wt % AY627  15 wt % Polysilane          Same     1.73   Same    85 wt % AY628  15 wt % Polysilane          Slip-Isostatically                   1.54   Same    85% wt % AY6          Pressed Billet29  Same       Same     1.73   Same30  Same       Same     1.77   Same31  Same       Same     1.75   Same*32 Same       Same     1.64   Same*33 Same       Same     1.73   Same*34 Same       Same     1.75   Same*35 Same       Same     1.58   Same__________________________________________________________________________ *AY6 powders contain: 2.64 wt % Al.sub.2 O.sub.3 ; 7.68 to 8.47 wt % Y.sub.2 O.sub.3 
    
     
                       TABLE IV______________________________________   SinteredSample  Densityno.     g/cc        XRD Phase Identification______________________________________ 1      3.12        beta-Si.sub.3 N.sub.4 2      3.21        Major: beta-Si.sub.3 N.sub.4,               Trace: SiC 3      3.04 4      3.11        Major: beta-Si.sub.3 N.sub.4               Minor: Si.sub.3 N.sub.4 --Y.sub.2 O.sub.3 --SiO.sub.2               7 5      2.95        beta-Si.sub.3 N.sub.4 6      3.05        Major: beta-Si.sub.3 N.sub.4               Minor: Si.sub.3 N.sub.4 --4Y.sub.2 O.sub.3 --SiO.sub.2 7      3.18        Major: beta-Si.sub.3 N.sub.4               Minor: Si.sub.3 N.sub.4 --SiO.sub.2 --4Y.sub.2               O.sub.3               Trace: YNSiO.sub.2 8      3.11        Major: beta-Si.sub.3 N.sub.4               Minor: YNSiO.sub.2, Y.sub.2 Si.sub.2 O.sub.5 9      3.15        Major: beta-Si.sub.3 N.sub.4               Minor: Y.sub.2 Si.sub.2 O.sub.5, YNSiO.sub.210      3.03        Major: beta-Si.sub.3 N.sub.4               Minor: Y.sub.2 Si.sub.2 O.sub.5, YNSiO.sub.211      3.02        Major: beta-Si.sub.3 N.sub.4               Minor: Y.sub.2 O.sub.3 --Si.sub.3 N.sub.412      2.96        Major: beta-Si.sub.3 N.sub.4               Minor: Y.sub.2 O.sub.3 --Si.sub.3 N.sub.413      2.94        Major: beta-Si.sub.3 N.sub.4               Minor: Y.sub.2 O.sub.3 --Si.sub.3 N.sub.4               Trace: alpha-Si.sub.3 N.sub.414      3.14        Major: beta-Si.sub.3 N.sub.4               Minor: Si.sub.3 N.sub.4 --Y.sub.2 O.sub.315      3.16         --16      3.12         --17      3.16        Major: beta-Si.sub.3 N.sub.4               Minor: Si.sub.3 N.sub.4 --Y.sub.2 O.sub.318      3.21        Major: beta-Si.sub.3 N.sub.4               Minor: Si.sub.3 N.sub.4 --Y.sub.2 O.sub.319      3.20        Major: beta-Si.sub.3 N.sub.4               Minor: Si.sub.3 N.sub.4 --Y.sub.2 O.sub.320      3.10        beta-Si.sub.3 N.sub.421      3.05        Major: beta Si.sub.3 N.sub.4               Minor: YSi.sub.2 ON22      3.16        Major: beta Si.sub.3 N.sub.4               Minor: 5Y.sub.2 O.sub.3 --Si.sub.3 N.sub.4 --Al.sub.2               O.sub.323      3.24        Major: beta-Si.sub.3 N.sub.4               Minor: 5Y.sub.2 O.sub.3 --Al.sub.2 O.sub.324      3.07        Major: beta Si.sub.3 N.sub.4               Minor: Y.sub.2 O.sub.3 --Si.sub.3 N.sub.4               Weak: SiC25      3.03        Major: beta Si.sub.3 N.sub.4               Weak: 5Y.sub.2 O.sub.3 --Si.sub.3 N.sub.4 --Al.sub.2               O.sub.326      3.13        beta-Si.sub.3 N.sub.427      3.10        Major: beta-Si.sub.3 N.sub.4               Minor: 5Y.sub.2 O.sub.3 --Si.sub.3 N.sub.4 --Al.sub.2               O.sub.3,               SiC28      3.16        Major: beta-Si.sub.3 N.sub.4               Minor: YSi.sub.2 ON29      3.25         --30      3.25         --31      3.26         --32      3.36         --33      3.30         --34      3.29         --35      3.28        beta-Si.sub.3 N.sub.4______________________________________ 
    
     
                                           TABLE V__________________________________________________________________________MECHANICAL STRENGTH AND OXIDATION RESULTSAVERAGE MODULUS OF RUPTURE  OXIDATION(KSI) AT                    RATE CONSTANTSample No. Room Temp.        1000° C.             1200° C.                  1400° C.                       (Kg.sup.2 M.sup.-4 sec.sup.-1)__________________________________________________________________________28-31 109    99.5 78.4 38   2.14 × 10.sup.-3                       to                       5.08 × 10.sup.-332-35 110    111  71   28   1.20 × 10.sup.-3                       to                       .sup. 3.50 × 10.sup.-13__________________________________________________________________________