Abstract:
A method for fabricating injection molded ceramic shapes suitable for densification by sintering or hot isostatic pressing has been developed. The method is applicable to ceramic articles of large, complex cross sections, greater than one centimeter, which are generally prone to external cracking during binder removal. The method requires use of a controlled starting powder, and a novel binder removal process to obtain an externally crack-free article.

Description:
The Government has rights in this invention pursuant to Contract No. DEN 3-168 awarded by NASA/DOE. 
    
    
     CROSS REFERENCE TO RELATED APPLICATION 
     A co-pending patent application, Ser. No. 716,274, filed concurrently herewith, entitled METHOD FOR FABRICATING OF LARGE CROSS SECTION INJECTION MOLDED CERAMIC SHAPES by Bandyopadhyay et al., and assigned to GTE Laboratories Incorporated, assignee of the present application, concerns related subject matter of this application. 
     FIELD OF THE INVENTION 
     This invention relates to a method for injection molding of ceramics. More particularly it relates to a method of injection molding high density silicon nitride articles having large cross sections. 
     BACKGROUND 
     Injection molding of ceramics has been described by several authors in the open literature (e.g. T. J. Whalen et al., Ceramics for High Performance Application-II, Ed. J. J. Burke, E. N. Lenoe, and R. N. Katz, Brook Hill Publishing Co. 1978, pp. 179-189, J. A. Mangels, Ceramics for High Performance Application-II, pp. 113-129, G. D. Schnittgrund, SAMPE Quarterly, p. 8-12, July 1981, etc.) and in patent literature (e.g. M. A. Strivens, U.S. Pat. No. 2,939,199, 1960, I. A. Crossley et al., U.S. Pat. No. 3,882,210, 1975, R. W. Ohnsorg, U.S. Pat. No. 4,144,207, 1979, etc.). Several papers have also been published by GTE authors (e.g. C. L. Quackenbush et al., Contractors Coordination Meeting Proceedings, 1981, G. Bandyopadhyay et al., Contractors Coordination Meeting Proceedings, 1983). The general process routing for injection molding is well known. It includes (a) compounding which involves mixing the high surface area ceramic with molten organic binder, (b) injection molding by which the powder/binder mix is formed into a given shape in a metallic mold, (c) binder removal which must be accomplished without disrupting the ceramic structure, and (d) consolidation of the part by sintering and/or by hot isostatic pressing. Significant effort has been made by various researchers to determine the effects of starting powder particle size and size distribution on moldability of powder, and to identify binder systems that allow easy compounding, molding and binder burnout (without disruption) from the part. Although volumes of patent literature now exist on powder requirements, different binder concepts and binder removal processes, it is generally recognized that injection molding and binder removal from a large, complex cross section part (e.g. rotors for turbine engines) poses a very difficult task because of internal and external cracking during burnout. An extensive evaluation of the patent literature reveals that in most cases only small cross section (less than one centimeter) parts were considered as examples, or that the cross sections and complexities were not revealed. None of these references described fabrication of large, complex cross section silicon nitride parts, specifically fabricated by injection molding and sintering. GTE Laboratories has developed a process routing which is highly successful for fabrication of good quality small cross section injection molded and sintered parts, such as turbine blades and vanes, in large quantities. Since this development, improvements in binder composition (K. French et al., U.S. Pat. No. 4,456,713) allowed further simplification of molding and binder removal procedures. This process routing however, failed to produce an externally crack-free injection molded ceramic article having a large cross section one centimeter or greater (e.g. turbine rotor and turbocharger sized test parts). It has been established that the use of a submicron starting powder, such as silicon nitride containing Y 2  O 3  and Al 2  O 3 , and a binder results in certain fundamental difficulties which causes external and internal cracking in large cross section parts. 
     OBJECTS OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a new and improved method of injection molding and binder removal from a ceramic article having a large cross section without forming external cracks. 
     SUMMARY OF THE INVENTION 
     This and other objects, advantages, and capabilities are provided in accordance with one aspect of the present invention wherein there is provided a method for fabricating a large cross-sectional ceramic article by injection molding comprising the following steps: 
     Step 1. A ceramic powder having a mean particle size from about 2 to about 12 microns is compounded with about 34 to about 42 v/o of a binder. 
     Step 2. The product from step 1 is injection molded to form a molded ceramic article having a cross section greater than one centimeter. 
     Step 3. The product from step 2 is embedded in a setter bed containing a setter powder. 
     Step 4. A binder retarding layer of the setter powder is formed on the molded ceramic article by heating the product of step 3 in a non-oxidizing environment at a heating rate equal to or greater than 1° C. per hour to 450° C. The setter powder retards the removal of the binder from the molded ceramic article sufficiently to maintain equal to or greater than 80 w/o of the binder within the molded ceramic article until 400° C. is obtained. 
     Step 5. Between 400° and 450° C. the binder is removed primarily by vaporization from the part surface by increasing the temperature from step 4 to 450° C. at a rate equal to or greater than 1° C. per hour in a non-oxidizing environment. 
     Step 6. The heating of the product of step 5 is increased to 600° C. and the product is maintained at that temperature in air for a period sufficient to completely remove the binder from the molded ceramic article, and 
     Step 7. The product from step 6 is cooled to room temperature to obtain an externally crack-free injection molded and binder free ceramic article having a cross section greater than one centimeter. 
     This method can be used for fabrication of small parts having cross sections less than one centimeter. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The cause of cracks in an injection molded ceramic article appears to be due to capillarity-driven migration of liquid binder from the interior of the molded part to its surface. The liquid binder appears to carry fine submicron silicon nitride powder from the bulk of the molded part to the surface causing a density gradient, a shrinkage gradient and cracking. In addition to fine particle migration, the capillary forces exert an outward drag on all particles causing them to start compaction from the molded part surface. As a result, the surface becomes rigid and prevents shrinkage of the part. Thus as the binder loss continues, the interior shrinks away from the rigid surface region causing the formation of internal cracks. If a region of the surface becomes hard before another region (e.g. thick vs. thin section, or top vs. bottom section) external cracks form. This understanding of binder loss and cracking mechanism is the basis for the present invention. The elimination of the external cracks can be affectuated by: 
     Reducing or eliminating the fine particles (one micron or less) from the starting powder to reduce or eliminate their preferential migration to the surface. 
     Increasing the particle size of the starting powder to reduce the capillary forces which, in turn, reduce the outward particle drag. 
     Developing a burnout setter powder composition and a thermal cycle, which allows liquid migration at the highest possible temperatures so that the liquid viscosity, when it is removed, is at its lowest. 
     To obtain a ceramic powder such as Si 3  N 4  +6 w/o Y 2  O 3  +2 w/o Al 2  O 3 , designated here as AY6, having a desired mean particle size of about 2 to about 12 microns, preferably from about 5 to about 10 microns from a conventionally processed ceramic powder having a mean particle size less than one micron, the conventionally processed ceramic powder is calcined followed by comminuting to obtain the desired particle size. In the case of AY6 powder, the calcining temperature was from about 1400° C. to about 1800° C. followed by milling for about 6 to about 36 hours as illustrated in Table I as powder numbers 2, 3 and 4. 
     Table I summarizes the surface area, mean and median particle sizes (as measured by x-ray sedigraph) and the weight fraction of less than one micron particles in the powder of the calcined and milled AY6 powders, powder numbers 2, 3 and 4, compared to a control AY6 powder which was not calcined. As Table I indicates, the calcining followed by milling of AY6 powder produces the desired mean particle sizes. 
     The calcined and milled AY6 ceramic powder is compounded with about 34 v/o to about 42 v/o, preferably from about 37 v/o to about 40 v/o of a wax based binder such as 90 w/o paraffin wax (Astor Chemical 1865Q), 5 w/o of surfactant (Fisher oleic acid A-215), and 5 w/o of epoxy thermosetting material (Acme 5144). The compounding is done in a Bramley two bladed dispersion mixer. The mixing chamber is heated to 80° C. Mixing is continued until the material has a creamy, homogenous appearance. 
     About 2 hours of mixing time subsequent to the initial blending of the particulate and binder materials is sufficient. At this point a vacuum is applied and the mixing continued approximately 45 minutes to remove any entrapped air. The resulting mixture has rheological properties comparable to a thermoplastic material with a softening range of 40° to 75° C. It can be pelletized or granulated according to well known techniques to a uniform particle size suitable as a feed material for injection molding apparatus. 
     The molding is accomplished by known injection molding techniques. Injection molding is usually carried out utilizing the transfer method or the direct injection method. In the transfer method a hydraulic press forces the material from a heated storage chamber, by means of a plunger, through sprues or runners, into a mold. In the direct injection method, the heated mixture is forced directly into the mold, through runners and gates, by either a hydraulic plunger or by reciprocating screw equipment. Either method may be utilized. The compounded material was injection molded into turbocharger sized shapes having cross sections up to 1.9 cm utilizing a 30 ton Trubor injection molding machine. Granulated material was loaded into the storage chamber and preheated to the molding temperature. Optimum molding temperature is usually just above the melting point of the binder composition. Where paraffin wax is the major binder component and epoxy is a minor component, the chamber temperature was 70° -72° C. The die was maintained at room temperature (24° C.). Molding pressure must be sufficient to force the preheated mixture into all areas of the die. A pressure of 3,000 to 10,000 psi is adequate for these materials, die and molding conditions. The shot was injected into the die cavity and the pressure held for 1/2 minute. The pressure was released, the die opened, and the parts removed from the die. 
     The injection molded green turbocharger sized parts were placed in a tray and embedded in a setter powder of calcined AY6 powder having a surface area (BET) of 0.2m 2  /g. 
     The binder was removed from the molded parts by heating the embedded parts in a non-oxidizing environment such as nitrogen up to a temperature of 450° C. to completely remove the binder. During initial heating at 10° C./hr or greater in which 15 w/o to 20 w/0 of the binder was removed the setter powder formed a thick cake around the part. The cake prevented further binder loss until the temperature was sufficiently high, approximately 400° C. up to 450° C., to break down the barrier by the thermal decomposition and vaporization of the binder. Thus, the majority of the binder loss occurred after a temperature of 400° C. was obtained and up to 450° C. The temperature of 450° C. was then raised to 600° C. and the heating was continued at 600° C. for up to 20 hours in air to remove the residual binder or carbon from the part. For turbocharger sized test parts about 3 days of thermal treatment was sufficient to completely removed the binder. For larger than turbocharger sized cross section parts a substantially lower heating rate, as low as 1° C./hr may be required or a total thermal treatment of approximately 17 days. The part was then cooled to room temperature. The resulting turbocharger sized part was free of external cracks. 
     Other low surface area powder could behave similar to silicon nitride setter powder and thus may be equally effective for burnout, binder removal, purposes. The use of a calcined and milled powder (as specified in Table I) and a silicon nitride setter powder successfully eliminated external cracks from more than 30 turbocharger sized test parts having cross sections up to 1.9 cm. 
     This method was used to successfully eliminate external cracks from more than 30 parts having cross sections up to 1.9 cm. These turbocharger sized test parts are suitable for subsequent processing and densification. 
     In summary, fabrication of an externally crack-free, injection molded and binder free silicon nitride parts of large, complex cross section requires controlled starting powder and a novel binder removal procedure. 
     
                       TABLE I______________________________________INJECTION MOLDING SILICON NITRIDEPOWDER CHARACTERISTICS              X-ray SedigraphPow-               Particleder                        Mean   Median % lessNum-   Processing  BET     Size   Size   than 1.0ber    Condition   (m.sup.2 /g)                      (micron)                             (micron)                                    micron______________________________________1.     Standard Pro-              11.06   1.78    .88   65.0  cessed AY6*  (Si.sub.3 N.sub.4 + 6 w/o  Y.sub.2 O.sub.3 + 2 w/o  Al.sub.2 O.sub.3)2.     AY6 Calcined.              5.00    5.61   2.06   31.0  1400° C. for 4 h.  milling for 6 h3.     AY6 Calcined.              4.62    5.51   3.94   19.5  1650° C. for 4 h.  milling, for 36 h.  Classified4.     AY6 Calcined.              3.36    11.54  6.34   25.0  1800° C. for 15  mins. milling  for 36 h______________________________________ *Data represents average of 5 different powder lots 
    
     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.