Patent Document

BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to forming silicon-carbide tooling and relates specifically to forming tooling used for layup and curing of composite structures. 
     2. Description of the Prior Art 
     Tooling for production of composite aircraft parts have close tolerances for dimensional control and are typically made from invar alloy. Invar can be characterized as an expensive material which is difficult to machine. However, an attractive feature of invar is a very low coefficient of thermal expansion (CTE) of approximately 1.5 μin./in./° F. at temperatures up to 400° F. For applications such as making flat laminates, other tooling materials are sometimes used, including aluminum and steel, and have CTE values of approximately 14 μin./in./° F. and 7 μin./in./° F., respectively. 
     A low CTE is necessary for producing high-temperature-cure polymer-matrix/carbon-fiber composites with precise dimensional accuracy. A mismatch in the CTE of the composite material and the tooling material will cause complications for maintaining dimensional accuracy. The CTE of a typical polymer-matrix/carbon-fiber composite is difficult to characterize precisely because it is a multi-component system. Carbon fiber has a small, negative CTE and a typical polymer matrix systems for structural aircraft composite components have a CTE in the range of 15-50 μin./in./° F. The CTE of a specific composite system will be dependent upon the lay-up construction and the composition. 
     Recently, technology has been developed at NASA Glenn Research Center to produce very economical, complex-shaped, silicon carbide (SiC) ceramic structures from wood precursors, called “ecoceramics.” To produce the ceramics, a wood preform is pyrolyzed in an inert atmosphere to convert the organic material into a carbonaceous form. The preform is infused with liquid silicon or silicon alloy, and the infiltrated preform is converted to a SiC in a high temperatures furnace. These materials have tailorable microstructures, low manufacturing costs, and can be easily machined in precursor stage before forming near-net shape ceramic structures. The targeted applications for this material have been seals, rings and filters for automotive applications; and armor, bricks, foundry crucibles and furnace components. These applications make use of the high service temperature of SiC (1350° C.). 
     One limitation of the NASA process, however, is that it is limited to small pieces of wood, especially those having a thickness of less than 1 in. With larger pieces of wood, cracking and warping are caused as the wood is dried and pyrolyzed, causing loss of tolerance and defects in the structure. 
     It is generally accepted that a material having a low CTE is desirable for use as composite tooling, and the CTE of SiC is approximately 2.5 μin./in./° F. in the temperature range of 70-2282° F. There exists a need for low-cost, easily-manufactured, SiC tooling for use in forming composite components of many sizes and thicknesses. 
     SUMMARY OF THE INVENTION 
     A method is provided for forming items from ecoceramic-based silicon-carbide. A wood preform is machined to a general shape having over- or undersized dimensions. The preform is pyrolyzed to transform the wood of the preform to a porous, carbonaceous material that retains the general shape of the preform. The preform is then machined to final, net-shape dimensions and immersed in liquid silicon or silicon alloy that penetrates and infuses the preform. The infused preform is held at a temperature sufficient to cause the transformation of the material in the preform to silicon carbide, completing formation of the item. Also provided is a method of forming ecoceramic-based tooling and composite components using the tooling. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed to be characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings. 
     FIG. 1 is a perspective view of a wooden preform for use in producing a tool in accordance with the present invention. 
     FIG. 2 is a perspective view of the wooden preform of FIG. 1 after the first machining step of the method of the present invention. 
     FIG. 3 is a perspective view of the preform of FIG. 1 after the pyrolyzing step of the method of the present invention. 
     FIG. 4 is a perspective view of the preform of FIG. 1 after the additional machining step of the method of the present invention. 
     FIG. 5 is a schematic end view of the preform of FIG. 1 immersed in liquid silicon according to the method of the present invention. 
     FIG. 6 is a perspective view of an ecoceramic tool formed from the preform of FIG.  1  and in accordance with the present invention. 
     FIG. 7 is a flowchart depicting the steps of a method of the present invention. 
     FIG. 8 is a perspective view of the tool of FIG. 6 after application of a mold release. 
     FIG. 9 is a perspective view of the tool of FIG. 6 with a composite component being formed on the tool. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a wood preform  11 , which maybe formed from one of several varieties of trees, e.g., black walnut or maple. As shown in the figures, preform  11  is a rectangular solid, though preform  11  may be of any shape capable of withstanding the process described below. Preform  11  has an upper surface  13  into which a recess  15  is machined, recess  15  having a rough, general shape of the desired negative mold, as shown in FIG.  2 . The negative mold is machined to have undersized dimensions, allowing for machining to the desired dimensions of the finished mold after subsequent steps. As illustrated in FIG. 2, the rough shape of recess  15  lacks the smooth contours of the desired shape (shown in FIGS.  4  and  6 ). Because wood is relatively soft when compared to normal tooling materials, such as invar alloy, machining preform  11  is quick and causes little wear on the tools used in the machining process. Though not shown, a positive mold would require an oversized rough shape to provide for additional material to be machined in later steps. 
     Once recess  15  is cut into preform  11 , preform  11  is pyrolyzed in an inert atmosphere. To prevent combustion of preform  11 , an inert gas, preferably argon, is used within the furnace, the argon displacing oxygen-rich air. Because preform  11  has moisture within it, preform  11  is first slowly dried to prevent cracking of preform  11  that could occur during pyrolyzation. 
     The preferred method of drying preform  11  involves covering preform  11  with a vacuum bag, applying vacuum to the bag, then placing the bagged preform  11  in a pressurized autoclave and increasing the temperature within the autoclave. For example, the temperature in the autoclave is raised at up to 10° C. per minute to a temperature of 90° C. to 120° C., where it is held for several hours, allowing for moisture to be removed without damage to preform  11 . The pressurized atmosphere minimizes the temperature gradients in the autoclave and in preform  11 , which reduces the chance of preform  11  warping during drying. Also, the pressurized atmosphere within the autoclave, which may be up to 90 psi of nitrogen, applies pressure to the outer surfaces of preform  11 , reducing the amount of cracking occurring in larger preforms  11 . The vacuum bag allows for negative pressure to be applied to preform  11 , enhancing the process of moisture removal prior to the water turning to steam, which may cause cracking of preform  11 . 
     The drying step may also be divided into two steps to avoid cracking in thick preforms  11 . For example, the autoclave may be raised to approximately 90° C. and held for 2 hours to 24 hours for an initial drying. For best results, vacuum should be applied to the bag at the beginning of the cycle. Then the temperature can be raised at up to 1° C. per minute to between 100° C. and 120° C. and held for an additional 2 hours to 24 hours, ensuring a complete drying of preform  11 . 
     The next step is to remove preform  11  from the autoclave, remove the vacuum bag, then replace preform  11  in the autoclave with a pressurized nitrogen atmosphere, preferably 15 psi to 90 psi. The pressure in the autoclave minimizes thermal gradients in the autoclave and provides increased hydrodynamic pressure to maintain the dimensional stability of preform  11 . The temperature within the autoclave is slowly increased again at up to 5° C. per minute to preferably between 100° C. and 120° C. and is held for 1 hour to 10 hours, then is preferably ramped upward to 220° C. at the rate of approximately 0.28° C. per minute. 
     When preform  11  approaches 220° C., an oily residue, referred to as bio-oil, and vapors begin to emerge from preform  11 . Bio-oils are a mixture of chemicals resulting from the decomposition of organic matter within the wood of preform  11 . The vacuum bag is removed before this step to prevent bio-oil and vapors from entering vacuum lines and to obviate the need for providing bleed cloths within the bag to absorb the bio-oil as it is produced. 
     Once the temperature has reached 220° C., the rate of increase of the temperature is preferably reduced to approximately 0.17° C. per minute until the temperature reaches between 375° C. and 425° C., though the rate may be up to 1° C. per minute. Preform  11  is preferably held at approximately 400° C. for 1 hour to 10 hours, the ambient pressure assisting in extracting the bio-oil. Afterward, preform  11  is removed from the autoclave, cooled, then inserted into a furnace where preform  11  is heated to a higher temperature than in the autoclave. 
     The furnace preferably has a constantly-flowing argon or nitrogen atmosphere at 1 psig to 10 psig. The temperature in the furnace is raised to approximately 400° C. at 1° C. to 5° C. per minute, then held from 1 hour to 10 hours. The temperature is then raised to between 900° C. and 1100° C. at a rate of up to 1° C. per minute, and preform  11  is held at that temperature for approximately 1 hour to 10 hours. Preform  11  is then cooled to room temperature at a rate of approximately 1° C. to 5° C., preferably under constantly flowing nitrogen or argon. 
     At this point, all of the material within preform  11  is completely pyrolyzed. The entire pyrolyzation process may take approximately 90 hours, though the time may be longer or shorter for different woods, thicknesses, shapes, etc. A pyrolyzed preform  11  is shown in FIG. 3, a lower corner having been removed to reveal the carbonaceous, foam-like material remaining in preform  11 . 
     After pyrolyzing preform  11 , recess  15  in upper surface  13  is machined to net-shape dimensions. By machining again after the pyrolyzation, dimensional changes in recess  15  caused by the pyrolyzation can be accounted for while also removing the additional material in recess  15  due to the undersize dimensions. FIG. 4 shows recess  15  as having the desired smooth contours of the finished mold. The machining of the pyrolyzed preform  11  requires very little effort and causes little to no wear on machine tools. 
     To provide silicon and convert preform  11  into a SiC material, pyrolyzed preform  11  is immersed in a tank  17  containing liquid silicon or silicon alloy  19 , shown in FIG.  5 . Preform  11  is held in tank  17  and at a temperature from approximately 900° C. to 1450° C. for 20 to 90 minutes. Liquid silicon  19  is drawn into preform  11  by capillary action, filling the micropores of preform  11 . The infusion may also be assisted by vacuum. Liquid silicon  19  readily infiltrates the pores of preform  11 , where the silicon reacts with the carbon of preform  11  to form SiC. If a silicon alloy, such as silicon-refractory metal alloys, is used, refractory disilicide is precipitated as the silicon reacts with the carbon. In either case, the final result is a dense matrix comprising silicon carbide and some free silicon or, in the case of alloy infiltration, some additional precipitated disilicide. 
     FIG. 6 shows the finished tool  21  formed from preform  11 . A corner of tool  21  has been removed to illustrate the ceramic structure throughout tool  21 . While it is desirable for recess  15  to have net-shape dimensions after the immersion and heating steps, some machining may be required to dimension recess  15  to within desired tolerances. After typical tooling preparation, tool  21  maybe used to form components from composite materials. 
     FIG. 7 shows a flowchart containing the steps for creating a composite layup tool using the method described above. In addition, the method includes layup of a composite component as an optional last step of the method. The step of block  23  is the rough shaping of the preform, which is then vacuum bagged and heated in an autoclave, as described in block  25 . In the step of block  27 , the bag is removed, and the preform is heated to a higher temperature, preform releasing vapors and bio-oil. The preform is completely pyrolyzed in the step of block  29 , then preform is machined to net-shape dimensions in the step of block  31 . The step of block  33  is the immersion of the preform in liquid silicon at approximately 900° C. to 1450° C. to cause the formation of SiC. These steps may be used to form any type of ecoceramic part, component, or tooling, and the step of block  35  provides for layup of composite parts on the tooling, as shown in FIGS. 7 and 8. 
     To prevent composite components formed on tool  21  from adhering to upper surface  13  and mold details such as recess  15 , a mold release, or mold sealant, is applied to upper surface  13 , as shown in FIG.  8 . Mold release may be a wax or other form of release that coats surface  13  to limit the difficulty of removal of a composite component after the resin in the component is cured. 
     FIG. 9 shows a composite component  37  being formed on tool  21 . Component  37  is formed from composite materials, typically multiple layers of woven fabric, though other types of fiber layers maybe used, for example, fiber mats having short fibers in random orientations. The layers are preferably impregnated with an uncured resin prior to layup, but resin may be brushed on or otherwise applied to dry layers after each layer is placed on tool  21 . Layers of component  37  are laid on surface  13 , conforming to the contours of recess  15 . A debulking process may be performed during layup to remove excess resin and to compact the layers. After the desired number of layers is applied, component  37  is cured while remaining on tool  21 , curing typically occurring within an autoclave or other type of oven. Component  37  is then removed from tool  21 . 
     The advantages of using the present invention to form large ecoceramic components, such as large tooling structures, is that limitations to the size of wood preforms are determined only by the size of the furnaces used, not by the cracking or warping problems of prior methods. Furnaces exist which are large enough to accommodate any current composite tooling structure used in aerospace manufacturing. Also, techniques have been developed for joining multiple SiC components using the same heating process that converts the infused carbonaceous material to SiC. Therefore, very complex tooling structures can be formed from several pieces. 
     There are several advantages to using ecoceramics for composite tooling. Since all the machining is done in the wood or the carbonized state of the material, ecoceramics provide a faster and more economical alternative to machining tooling from metal, especially when considering the difficulty in machining invar alloy. Silicon and silicon alloys are inexpensive materials, and heating costs are relatively insignificant. The ecoceramic material has other advantages over the traditional tooling materials in that it is more dent resistant, can be repaired, and has a capability of withstanding higher temperatures. 
     While the invention has been shown in only one of its forms, it should be apparent to those skilled in the art that it is not so limited but is susceptible to various changes without departing from the scope of the invention. For example, wood particles, such as sawdust, can be mixed with binders and used to form the preform. The binders are carbonized along with the wood during the pyrolyzation step.

Technology Category: c