Hybrid dual fiber matrix densified structure and method for making same
FIELD OF THE INVENTION 
     This invention relates to high strength structures, particularly ceramic and graphite fiber structures. 
     BACKGROUND AND DISCUSSION OF THE PRIOR ART 
     Heretofore it was known to make structures of various types of carbon fibers and composite materials. 
     U.S. Pat. No. 4,182,495, granted to Borgmeier et al. discloses a method of construction using graphite or carbon fiber preimpregnated tape having a low temperature cure epoxy resin system. U.S. Pat. No. 4,373,006 to Galasso et al. discloses composite materials containing silicon carbide coated fibers. 
     U.S. Pat. No. 3,853,506 and U.S. Pat. No. 3,900,675 to Olcott, and U.S. Pat. No. 3,980,105 to Myskowski disclose structures utilizing pyrolytic graphite. While U.S. Pat. No. 4,397,901 to Warren is directed to a method of making a composite article comprising a carbon fibrous substrate, including a pyrolytic carbon sheath formed about each fiber of the substrate, a metallic carbide, oxide or nitride coating over the coated fibers of the substrate, and an impermeable metallic carbide, oxide or nitride outer protective layer. 
     Woven composite articles are disclosed in O&#39;Brien et al., U.S. Pat. No. 4,063,684; Fukuta et al., U.S. Pat. No. 4,336,296; and Maistre, U.S. Pat. No. 4,366,658. 
     One major problem associated with the prior art structures was to provide a highly corrosion or oxidation resistant structure while providing a lightweight, high strength structure. 
     It is therefore a principal object of the present invention to provide a high strength structure which is lightweight, and yet has a corrosion or oxidation resistant interior, and to a method for forming same. 
     It is another object of the present invention to provide a structure as aforesaid which is useful in a broad number of heavy duty intensive environment applications. 
     The aforesaid as well as other objects and advantages will become apparent from a reading of the following description, the adjoined claims and the accompanying drawings, described below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front elevational partial sectional and schematic view of the braiding apparatus and operation of the present invention; 
     FIG. 2 is a partial sectional schematic view taken substantially along line 2--2 of FIG. 1; 
     FIGS. 3a-3e are illustrative views of the several steps in performing the method of the present invention; 
     FIG. 4a is a graph of the density through the wall from the outside toward the inside after step (1); and 
     FIG. 4b is a graph of the density across the composite wall of the inner and outer layers after step (3). 
     SUMMARY OF THE INVENTION 
     A high strength structure formed of an inner layer of one material fiber and an outer layer of another material fiber with a hybrid intermediate layer of both of the fibers, with a first composition densification of the outer layer and second composition densification of the inner layer to achieve an oxidative resistant, high strength structure. The different refractory ceramic and carbon material fibers, and densifying compositions complement this design, such as the chemical vapor deposition (CVD) of pyrolytic graphite matrix into the outer layer and a refractory carbide matrix into the inner structure. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 1 and 2, there is shown the novel zone controlled braiding of graphite and silicon carbide fibers in a simultaneous process. This process provides an effective hybrid zoned preform that can be produced on the circular braider 10. Circular braider 10 may be of the type as shown and described in U.S. Pat. No. 3,426,804, granted to Bluck, which disclosure is incorporated herein by reference thereto. The well known circular braider is modified to the extent as set forth in the following description. 
     Circular braider 10 is formed with braider bed 14, telescoping screw jack 37, braiding section 38 and tension ring 39. Circular braider 10 is specifically modified with respect to braider bed 14, in that bed 14 contains spools 11a and 12a for feeding graphite fiber 11 and silicon carbide fiber 12 respectively. In one aspect the graphite fiber 11/silicon carbide fiber 12 distribution may be 50/50 and that all 15 rows 13 of the braider bed 14 will be utilized. Thinning of the structure by using fewer rows of fibers on either side of the central boundary will not alter the hybrid zoned structure objective. 
     FIG. 2 presents the preliminary loading and braiding pattern. The numbered circular rows (1 thru 15) and radial slots for one quadrant (1 through 36) represent positions in which fiber carriers (spools) can be mounted. The circles in the outboard positions represent graphite fibers and the &#34;X&#39;s&#34; represent silicon carbide fibers. The loading is staggered in the radial direction of pairs of rows--slots 1 and 2 being loaded to the inboard boundary and slots 3 and 4 to the outboard boundary. The boundaries are formed by two fixed (immobile) circular rows consistent with the plan to perform braiding steps in a 2×2 pattern. These boundary rows will receive fiber carriers during radial shifts. The received carriers reside in the boundary rows during the subsequent reverse rotational shift of the delivery rows. Upon reentering the delivery rows they then proceed in the circular direction in which they were delivered. 
     This hybrid cylinder loading is designed to form a medial boundary which blocks migration of graphite into silicon carbide and vice versa while assuring interlacing of these two fiber types at the interface. A computer program which models the spool motion was used to evaluate candidate braiding patterns. This program was utilized to assess several candidates for the hybrid preform until the figure plan was validated as providing an effective medial boundary with good interaction at the interface. As FIG. 2 illustrates, circular rows 7 and 9 have been immobilized, and one active row has been retained between these boundary rows. It should also be noted that the remaining mobile circular rows are shifted in pairs as indicated by the arrows. Inputting this layout and 2×2 shift plan into the computer program, the program verified that all positions performed in a repetitive manner (i.e., after some number of moves, each fiber returns to its starting position). 
     The next major consideration after the hybrid carbon-silicon carbide preform has been created, is the method for turning the preform skeleton 30 into a useable composite. Conventional chemical vapor deposition/infiltration (CVD/CVI) has been selected as the approach which can achieve the desired structure. The inside surface will be infiltrated with a refractory carbide to create the tough oxidation resistant flamefront wall 25. Both silicon carbide and hafnium carbide are preferred. The outer structure wall will be infiltrated with preferably either pyrolytic graphite (PG) or SiC 26. In either case, &#34;Coke bottling&#34;, or the density gradient effect, which has been the major drawback of infiltration densification by CVD/CVI, is now used advantageously in the present invention to achieve a desired densification. 
     Referring now to FIGS. 3a-3e, the hybrid zone braiding is accomplished on a male graphite mandrel 31. The reinforcement preform 30 and mandrel 31 are then positioned in a CVD furnace (not shown) and infiltrated with carbon 32. The process results in &#34;Coke bottling&#34;, whereby the interior pores tend to be closed off as the carbon closes off the pore entrances. This phenomenon results in a density gradient through the thickness of the structure which has plagued the rocket nozzle community. In this case, the phenomenon, while undesirable in the prior art, is a key to achieving the present invention. Since the graphite mandrel or mask 31 prevents CVD carbon densification from the inside, the carbon infiltrates from the outside diameter (O.D.) only. As the self-perpetuating &#34;Coke bottling&#34; takes place, we develop a gradient of 100% densification at the O.D. to 0% at the inside diameter (I.D.). See FIG. 4a which graphically illustrates the density gradient after the step of FIG. 3a. The CVD rates and times are determined experimentally for each proposed geometry. Silicon carbide can be substituted for the carbon in CVD with similar densification gradient results. 
     The partially densified structure is removed from the furnace, and the graphite mandrel 31 removed from the inside diameter (I.D.). The outside diameter is skim cut and overwrapped with graphite cloth or mask 35 to protect it from the next process step. The part is returned to the CVD furnace and infiltrated with one of the refractory carbides 22, such as silicon carbide 26 or hafnium carbide, depending on the service temperature requirements. The same gradient is developed, but from the inside diameter out in this case. FIG. 4b shows the idealized density of the finished structure. FIGS. 4a and 4b demonstrate how the prior art&#39;s undesirable &#34;Coke bottling&#34; effect is utilized in the present invention to achieve a positive result. It is to be borne in mind that the prior art directed one to CVD without maskings. The densified structure is removed from the furnace, and the protective overwrap 35 removed. This inside diameter should be net. Conventional finishing techniques may then be employed to finish the structure 50. (See FIG. 3e) 
     It should be noted that there are a multitude of equivalents in reinforcement compositions and reinforcement composition combinations which could be employed. In like manner, several matrices, all through CVD can be used. 
     A partial tabulation of useful combinations of fibers and CVD matrices is given below. 
     
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         Reinforcement                                                    
                      CVD Matrix                                          
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Inside Zone                                                               
           Silicon Carbide                                                
                          Silicon Carbide                                 
           Various Types  Hafnium Carbide                                 
           Carbon (Graphite)                                              
                          Other Carbides                                  
           Rayon          Nitrides                                        
           PITCH          Pyrolytic Carbon                                
           PAN                                                            
Outside Zone                                                              
           Carbon (Graphite)                                              
                          Silicon Carbide                                 
                          Pyrolytic Graphite                              
                          (Carbide)                                       
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     Important aspects of the present invention include (1) the hybrid character of the reinforcement and (2) the processing steps to achieve high levels of oxidation resistance on the inside of the structure with high strength on the outside of the structure, while minimizing the thickness and weight of the structure. The finished structure may be employed in a broad range of uses including combustion chambers, engine components and high pressure, corrosive chemical vessels. 
     It will be understood, therefore, that various changes and modifications may be made in the embodiments described without departing from the invention as defined in the appended claims, and it is intended, therefore, that all matter contained in the foregoing description shall be interpreted as illustrative only and not as limitative of the invention.