Abstract:
A process for joining or coating carbon-carbon composite components. For joining such components, surfaces of two components are prepared so that they match. An interlayer comprised of a material having a melting point in excess of 2500° F. is sandwiched between the two surfaces and the assembly is held together under compression while it is heated to a temperature sufficient to melt the interlayer material or to a temperature sufficient to cause interdiffussion between the interlayer and the carbon-carbon composite material. In a preferred embodiment of this invention the interlayer material is a carbide former. For example, if zirconium is chosen as the interlayer material, a joint can be prepared at a temperature of 3400° F. which will retain its strength to temperatures in excess of 4350° F. If the second surface is a removable backing, the interlayer provides a coating of the surface of the carbon-carbon composite component.

Description:
This is a continuation of application Ser. No. 07/145,129, now abandoned, filed 1/19/88 Process for Joining or Coating Coating Carbon-Carbon Composite Components. 
    
    
     This invention relates to processes for joining or coating components and in particular high temperature carbon-carbon composite components. 
     BACKGROUND OF THE INVENTION 
     Carbon-carbon composite components are increasingly being proposed for use in very high temperature applications. The term carbon-carbon composite should be understood in this specification and the claims to refer to a composite comprised of fibers in a crystal graphitic form of carbon in a matrix which is also graphitic but in both crystal and amorphous forms of carbon. Carbon-carbon composites of this type are well known and are discussed is handbooks such as Engineer&#39;s Guide to Composite Materials and Engineered Materials Handbook. In many cases the need exists for joining carbon-carbon composite components for such use. Low temperature joints are made with epoxy materials and intermediate temperature joints are made with silver base braze alloys. Neither of these joining processes are suitable for structures designed to perform in the 2500 F. to 3000 F. temperature range. 
     Efforts to join high temperature components of other materials have been reported. Apparently, successful joints of silicon carbide were made by furnace brazing and diffusion welding using compounds such as molybdenum disilicide, titanium dicilicide and silicon hexaboride. Lower temperature graphite joints have been made using chromium carbide coatings and molybdenum coatings with copper interlayers. 
     SUMMARY OF THE INVENTION 
     The present invention provides a process for joining or coating carbon-carbon composite components. For joining such components, surfaces of two components are prepared so that they match. An interlayer comprised of a material having a melting point in excess of 2500 F. is sandwiched between the two surfaces and the assembly is held together under compression while it is heated to a temperature sufficient to melt the interlayer material or to a temperature sufficient to cause interdiffussion between the interlayer and the carbon-carbon composite material. In a preferred embodiment of this invention the interlayer material is a carbide former. For example, if zirconium is chosen as the interlayer material, a joint can be prepared at a temperature of 3400 F. which will retain its strength to temperatures in excess of 4350 F. If the second surface is a removable backing, the interlayer provides a coating of the surface of the carbon-carbon composite component. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is sketch showing an assembly prepared for joining two carbon-carbon composite components. 
     FIG. 1B is a section of an assembly with serrated surfaces. 
     FIG. 1C is a section of an assembly with smooth surfaces. 
     FIG. 2 is a sketch showing an enlargement of a section of the assembly shown in FIG. 1A. 
     FIGS. 3, and 4 are sketches each showing an assembly prepared for coating a carbon-carbon composite component using a backing. 
     FIG. 3A is a section of the assembly shown in FIG. 3. 
     FIG. 4A is a section of the assembly shown in FIG. 4. 
     FIGS. 5A, 5B and 5C are sketches showing steps of coating a component without the use of a backing. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention may be described by reference to the drawings. 
     Carbon-carbon composite components 1 and 2 are to be joined. The surfaces to be joined 3 and 4 are prepared so that they match. (The surfaces may be serrated to increase the joint area as shown in FIG. 1B.) The surfaces are cleaned and loose material is removed. A thin interlayer 5 is assembled between surfaces 3 and 4 as shown in FIG. 1C and the assembly is heated at a temperature at or above the melting point of the interlayer material or at a temperature sufficient to cause interdiffusion between the interlayer and the carbon-carbon composite. During the heating process the assembly is held together under compression by clamp 6. The heating is done in a vacuum, in an inert gas atmosphere or in an atmosphere which will produce or enhance the desired chemical reactions. The assembly is then cooled. 
     An enlargement of a typical completed joint is shown in FIG. 2. As shown at 7, the interlayer material has diffused into the material of components 1 and 2 and the composite material has diffused into the interlayer material. Interdiffusion is generally limited to about 0.010 inch, but in cases where the interlayer has been forced to melt, the inventor has observed evidence of the flow of liquid metal up to 0.060 inch into the carbon-carbon material. 
     Interlayer materials are selected from reactive and refractory metals and compounds or other materials that will react during the joining process to produce refractory, high temperature compounds. The interlayers may be applied in the form of foils, compound powders, mixtures of elemental powders, or mixtures of compound and/or elemental powders. The interlayer materials may have inherently good strength at the intended use temperature or may develop this strength during reaction to form high strength compounds. An example of the latter material is Zr which reacts with C in the carbon-carbon composite to form ZrC which has good high temperature strength. An example of the former is MoSI (2) which has good initial high temperature strength and keeps it through the process. 
     In order to improve bonding, carbon-carbon surfaces may be modified by ion implantation or ion mixing. Ion implantation involves propelling ions toward the surfaces. The ions are embedded to a depth of up to several hundred angstroms into the carbon-carbon microstructure. Ion mixing involves first coating the surfaces by a process such as sputtering or chemical vapor deposition then bombarding the surface with high energy ions. Coating and component atoms are thus mixed together. 
     EXAMPLES 
     The composite material used was obtained from General Dynamics/Convair and is known as Kaiser K-Carb and consisted of WCA fabric (a rayon based fabric) and a Code 88A pitch/resin combination, fully graphitized. The material was 1/4 inch thick and was cut into specimens of 3/8 by 3/8 inch. Surfacing was done with a milling cutter to a surface finish of about 8 rms. After machining, the specimens were washed and brushed in anhydrous ethyl alcohol. After washing and drying, all samples were vacuum furnace outgassed at 3600 F. for one hour at a vacuum of 1×10 -4  torr or better. 
     Interlayer material used along with the results of Examples 1 through 10 are listed in Table 1. In some cases as indicated in the table, surfaces were bombarded with ions. In some cases a sputter coating was applied prior to bombardment. The ion source was the MEVVA ion source at the Lawrence Berkeley Laboratory. 
     In addition to the interlayer materials listed in Table 1, the following materials and their carbides show potentially good high temperature strength: Si, B, Nb, W, Hf, Cr, V, Ta. 
     In addition, the following compounds show potentially good high temperature strength: 
     TiC, W(2)C, WC, ZrC, VC, V(2)C, TaC, Ta(2)C, SiC, NbC, Nb(2)C, MoC, Mo(2)C, HfC, Cr(3)C(6), Cr(3)C(2), Cr(7)C(3), B(4)C, HfB)2), HfSi(2). Mixtures of HfB(2)+TaB(2). Mixtures of ZrB(2)+SiC MoB+MoC MoC+MoSi(2) WSi(2). 
     
                                           TABLE 1__________________________________________________________________________MATRIX OF EXPERIMENTS AND SUMMARY OF RESULTS__________________________________________________________________________Ex-perimentInterlayer       Surface Modification                  Interlayer          ProcessNumberMaterial       Treatment  Thickness                         Joining Process                                      Parameters                                               Other__________________________________________________________________________                                               Variables1    Titanium       None       .002&#34;-2 foils                         Vacuum Furnace Brazing                                      3200° F./5                                               Acetone clean foils2    Titanium       None       .002&#34;-2 foils                         Vacuum Furnace Brazing                                      3200° F./60                                               Acetone clean foils3    Titanium       None       .004&#34;-4 foils                         Vacuum Furnace Brazing                                      3200° F./5                                               Acetone clean foils4    Titanium       None       .004&#34;-4 foils                         Vacuum Furnace Brazing                                      3200° F./60                                               Acetone clean foils5    Titanium       None       .001&#34;-1 foil                         Vacuum Furnace Brazing                                      3200° F./60                                               Abraded foil, slow                                               cooling after                                               brazing6    Titanium       None       .002&#34;-2 foils                         Vacuum Furnace Brazing                                      3200° F./60                                               Cleaned foils in HF/                                               HNO.sub.3 solution7    Titanium       Ion Implantation w Ti                  .002&#34;-2 foils                         Vacuum Furnace Brazing                                      3200° F./60                                               Cleaned foils in HF/                                               HNO.sub.3 solution8    Titanium       Ion Mixing w Ti                  .002&#34;-2 foils                         Vacuum Furnace Brazing                                      3200° F./60                                               Cleaned foils in HF/                                               HNO.sub.3 solution9    Titanium       Ion Mixing w Ti                  .002&#34;-2 foils                         Vacuum Furnace Brazing                                      3500° F./60                                               Cleaned foils in HF/                                               HNO.sub.3 solution10   Zirconium       None       .0016&#34;-2 foils                         Vacuum Furnace Brazing                                      3450° F./60                                               Cleaned foils in HF/                                               HNO.sub.3 solution11   Zirconium       Ion Implantation w Zr                  .0016&#34;-2 foils                         Vacuum Furnace Brazing                                      3450° F./60                                               Cleaned foils in HF/                                               HNO.sub.3 solution12   Zirconium       Ion mixing w Zr                  .0016&#34;-2 foils                         Vacuum Furnace Brazing                                      3450° F./60                                               Cleaned foils in HF/                                               HNO.sub.3 solution13   Molybdenum       None       .001&#34;-1 foil                         Diffusion Welding                                      3500° F./60                                               Cleaned foils in HF/                                               HNO.sub.3 solution14   Molybdenum       Mo ion implantation                  .001&#34;-1 foil                         Diffusion Welding                                      3500° F./60                                               Cleaned foils in HF/                                               HNO.sub.3 solution15   Molybdenum       Mo ion mixing                  .001&#34;-1 foil                         Diffusion Welding                                      3500° F./60                                               Cleaned foils in HF/                                               HNO.sub.3 solution16   TiSi.sub.2       None       .003&#34;  Vacuum Furnace Brazing                                      3300° F./60                                               Slurry brushed on17   SiB.sub.6       None       .002&#34;  Vacuum Furnace Brazing                                      3550° F./60                                               Slurry brushed on18   SiB.sub.6       None       .002&#34;  Vacuum Furnace Brazing                                      3550° F./60                                               Partial pressure                                               argon19   MoSi.sub.2       None       .003&#34;  Vacuum Furnace Brazing                                      3550° F./60                                               Slurry/vacuum__________________________________________________________________________ExperimentNumber Metallographic results         Microhardness                                         Other Observations__________________________________________________________________________1      Good bonded, some void formation, cracking in interlayer                                 --2      Good bonded, some void formation, cracking in interlayer                                 --3      Good bonded, some void formation, cracking in interlayer                                 --      Excess interlayer-surface                                         coating4      Good bonded, some void formation, cracking in interlayer                                 --      Excess interlayer-surface                                         coating5      Good bonded, some void formation, cracking in interlayer                                 &gt;1284 DPH                                         Needs more inerlayer                                         material6      Good bonding, less void formation, some cracking in                                 &gt;1284 DPHr                                         Good quantity of                                         interlayer7      Good bonding, less void formation, some cracking in                                 &gt;1284 DPHr                                         Good quantity of                                         interlayer8      Good bonding, less void formation, some cracking in                                 &gt;1284 DPHr                                         Good quantity of                                         interlayer9      Good bonding, less void formation, some cracking in                                 &gt;1284 DPHr                                         Good quantity of                                         interlayer10     Good bonding, significant void formation some cracking                                 &gt;1284 DPH                                         More interlayer needed11     Good bonding, less void formation, some cracking                                 &gt;1284 DPH                                         Less interlayer needed12     Good bonding, less void formation, some cracking                                 &gt;1284 DPH                                         Least additional interlayer                                         needed13     Good bonding, diffusion some formation                                 &gt;1284 DPH                                         Dark phase formation14     Some bonding, significant laying surface damage                                 &gt;1284 DPH                                         Dark phase formation15     Some bonding, good diffusion zone formation                                 &gt;1284 DPH                                         Dark phase formation16     Good bonding, lost considerable void formation in                                 &gt;1284 DPHr                                         Good crack filling/surface                                         coating17     Most of interlayer absent, some bonding to surfaces                                 Not taken                                         Poor crack filling18     More interlayer present, good bonding to surface                                 Not taken                                         Some crack filling19     Good bonding to surface and interlayer integrity                                 &gt;1284 DPH                                         Good crack__________________________________________________________________________                                         filling 
    
     The elemental powders may also be selected from powders that react exothermically during heating to form a high temperature compound. An example would be the use of a stochiometric mixture of molybdenum and silicon powders, heated and reacted to form MoSi(2). The advantage is that a lower joining temperature can be used, i.e. a temperature below the melting temperature of the final compound. 
     SURFACE PROTECTION 
     The interlayer material and process described above turns out to be an excellent process for coating carbon-carbon composites for oxidation protection in high temperature applications. However, as shown in FIG. 3, the second carbon-carbon composite component 2 is replaced with a nonporous high temperature backing 13 such as zirconium oxide. As shown in FIG. 3A, the interlayer material 11 is prevented from adhering to the backing by a sacrificial zirconium oxide coating 12. The component is removed after the heating process leaving the carbon-carbon composite component coated with a high temperature protective coating. 
     As shown in FIG. 4 and FIG. 4A a refractory coating 14 is joined to the component. In this case the refractory coating might be a zirconium oxide sheet and the interlayer material zirconium. 
     The interlayer material can also be applied to a free surface without the use of a backing as shown in FIGS. 5A, B and C. The interlayer material 16 is applied to component 18 by any of a number of methods common to coating applications such as brushing or spraying 20 (as shown in FIG. 6A) on a slurry of powder and volatile carrier such as alcohol. Heating is done as previously described for joining. The coating is consolidated or melted and reacted with the carbon-carbon substrate to cause good adherence. The results can be total conversion to carbide as shown in FIG. 5B or partial conversion as shown in FIG. 5C. 
     The process can also be used to attach carbon-carbon composite components to structural members such as metallic structures. Furthermore, it could be applied to join other high temperature carbon based materials such as pyrolytic graphite. 
     The products of this process are particularly useful in aerospace applications where light very high temperature components are needed.