Graphite-glass composite laser mirror
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The field of art to which this invention pertains is composite optical elements of the reflecting type. 
     2. Description of the Prior Art 
     While there is a myriad of art covering laser mirrors (e.g., U.S. Pat. Nos. 3,836,236; 3,926,510; and 3,942,880) because of the many peculiar physical property requirements of such mirrors in this environment, both a variety of materials and designs have been employed in attempts to optimize the particular properties necessary for a composite used in this particular environment. For example, a laser mirror in this environment must not only have the requisite reflective properties but should be a relatively simple structure to permit rapid fabrication, both for time and cost purposes. Such mirrors should also desirably have low density for ease of use in the types of apparatus where they will be used. Furthermore, such mirrors ideally should have high elastic stiffness and high strength along with high fracture toughness. And stability is of the utmost importance both from the point of view of the fine resolution-type work environment the mirrors will be used in, and the inaccessibility of the apparatus which these mirrors would be used in, for example outer space applications. These stability properties include low thermal expansion, high thermal conductivity and environmental stability. Environmental stability includes such things as dimensional stability and mirror integrity regardless of moisture conditions, vacuum conditions, or ultraviolet light exposure, and mirror integrity and dimensional stability at both high and low temperatures. Currently, laser mirrors are basically either highly polished metal blocks (high energy laser application), or graphite reinforced resin matrix composites or low expansion glasses (low energy laser application). However, because such currently used composites and glasses fall off in one or more of the above-cited property areas, it is generally necessary to include a myriad of complicated designs to compensate for property defects in the particular areas under consideration. Note, for example, the complicated coolant designs in the above-cited references. Furthermore, the popular use of resins in conventional composites of the above type inherently suffer from dimensional changes due to absorption or desorption of moisture, evolution of organic constituents due to prolonged exposure to high vacuum, breakdown due to prolonged exposure to ultraviolet radiation, low thermal conductivity, high coefficients of thermal expansion, and rapid decrease in integrity when used above 300° C. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is directed to laser mirrors comprising graphite fibers in a glass matrix having a central plane of symmetry across their central plane. Both the graphite fibers and the glass matrix are selected and matched to provide, after hot pressing in a mold, a laser mirror of low density, high elastic stiffness, high strength, high fracture toughness, low thermal expansion, high thermal conductivity and environmental stability. The graphite fibers must have high strength and good modulus of elasticity. The glass matrix must have a low coefficient of thermal expansion matched closely to that of the graphite fiber. The mirror is made by laying alternate layers of fiber and glass and hot pressing at elevated temperatures to form a composite mirror having a central plane of symmetry across its central plane. The composite itself may provide the reflecting surface shape. However, in use the composite requires a laser reflecting surface layer. The thermal expansion characteristics, thermal conductivity and composite strength and fracture toughness of such composite resulting from the selection of such materials processed as described is far superior to any similar articles currently in use, which is quite surprising from the relatively brittle nature of the two starting materials. 
     The foregoing and other objects, features and advantages of the present invention will become more apparent from the following description of preferred embodiments and accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 demonstrates a typical 0°/90° cross-ply graphite fiber lay-up for the mirrors of the present invention. 
     FIG. 2 demonstrates an end view of a finished mirror of the present invention. 
     FIG. 3 demonstrates graphically thermal expansion characteristics of mirrors of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     While any graphite fiber with the requisite high strength and good modulus of elasticity can be used in the laser mirrors of this invention, such as Hercules HMS graphite fiber, Celanese GY-70 (formerly DG102) graphite fibers are particularly suitable. This fiber consists of 384 fibers/tow and has an oxidized finish. It is 8 microns in diameter, has a modulus of elasticity of 531 GPa (77(10) 6  psi). It has a tensile strength of 1724 MPa (250 ksi) and a density of 1.96 gm/cm 3 . The fiber is used at about 40 to 70% by volume based on the graphite-glass composite and preferably at about 60% by volume. 
     The glass used was particularly selected to have a very low coefficient of thermal expansion preferably matched closely, but not equal to that of the graphite fibers used since the graphite has a highly negative axial coefficient of thermal expansion and the glass has a positive but small coefficient of thermal expansion. Particularly suitable for the purposes of this invention is a borosilicate glass (CGW 7740) with an anneal point of 500° C., a softening point of 821° C., a liquidus temperature of 1017° C., a density of 2.23 grams per cubic centimeter, an index of refraction of 1.474, a dielectric constant of 4.6, a coefficient of linear expansion of 32.5 cm/cm°C.×10 -7  and a modulus of elasticity of 9.1×10 6  psi. The particle size of the glass should be such that at least 90% passes through a 36.0 mesh screen. 
     While a variety of methods may be used to produce the laser mirror of the present invention, the preferred method comprises continuously unwinding a tow of graphite fibers from a spool at a moderate rate of speed and passing such fibers through a slip of powdered glass, solvent and plasticizer to impregnate the tow. The impregnated fibers are then rewound onto a larger rotating spool. An exemplary slip composition may be composed of 250 grams of powdered glass in 780 ml of propanol. An alternative composition may comprise 85 grams of the glass and 200 grams of propanol, 10 grams of polyvinyl alcohol and 5 drops (approx. 1 cc) of a wetting agent, Tergitol®. The receiving drum is preferably run at 1 revolution per minute or linear speed of 5 feet per minute. Excess glass and solvent can be removed by pressing a squeegy against the drum as it winds. Preferably the ground glass is sized so that 90% of it passes through a 325 mesh sieve. The thus impregnated tape is then dried either at ambient temperature or with a radiant heating source to remove solvent. 
     Following the impregnation the fiber is removed from the drum and cut into strips up to the diameter of the mirror to be fabricated. While the typical test samples made were about 10 cm in diameter, mirrors up to 20 cm in diameter have also been made by the processes of the present invention. However, mirrors of even larger diameters can be made according to the present invention. The fibers are then preferably laid in alternating ply stack-up sequence of 0 and 90° as illustrated by FIGS. 1 and 2. The assembled composite is then hot pressed, either under vacuum or inert gas such as argon, in metal dies coated with colloidal boron nitride, or graphite dies sprayed with boron nitride powder, at pressures of 6.9 to 13.8 MPa (1000-2000 psi) and temperatures of 1050°-1450° C. Additional glass in the form of powder may also be inserted between each layer as it is laid in an attempt to achieve a preferred 40-70% by volume loading of graphite fiber in the composite. Also, the mold can be vibrated to insure uniform distribution of the glass over the laid fiber surfaces. 
     It is important for obtaining the above-cited mirror properties that a central plane of symmetry be maintained across the central plane of the graphite glass composite. Thus, while alternating 0 and 90° fiber laying was the most common test sample arrangement used, 0 and 45°; 0, 45 and 90°; 0, 30 and 90°; 0 and 60°, etc., fiber laying can also be used as long as a central plane of symmetry is maintained. In fact, the 0, 45 and 90° and 0 and 60°  fiber laying give additional advantage of isotropy of elastic stiffness. By central plane of symmetry is meant that if the composite is sliced in half parallel to what will be the reflecting surface and perpendicular to the mirror&#39;s focal axis, each half will be a mirror image of the other half. Such symmetry especially affects the warpage characteristics of the composite. In addition, it is preferred that the ply lay-up sequence be balanced. By balanced it is meant that there are equal numbers of composite plies, and hence fibers, directed in each of the principal fiber directions chosen. Such balance affects the strength and modulus properties of the composite. This is specifically demonstrated by FIG. 2 which is the end view of a 0°/90° specimen where 1 indicates the glass matrix, 2 indicates the graphite fibers and 3 indicates the central plane of symmetry. 
     As can be seen from FIG. 3, the thermal expansion characteristics of a 0° and 90° graphite-glass lay up are on a par with other conventional materials known for their dimensional stability. Curve A represents unreinforced fused silica (N.B.S. reference number 739 measurement). Reference B represents unreinforced ULE glass (Corning Code 7971 measurement). And Reference C represents the 0° and 90° graphite glass of the present invention. Because of its superiority in other properties of strength, stiffness, thermal conductivity, and toughness, the graphite reinforced glass is the overall superior material. 
     The finished mirror composite can be so molded as to provide the reflecting surface shape itself, e.g., by using a highly polished releasable mold surface. However, in use it is necessary to provide the mirror composite with a separate laser reflecting surface such as are conventionally used in the laser mirror art (e.g., a chromium-gold alloy). Such surface layers can be applied by conventional methods commonly used to deposit thin uniform layers (e.g., less than 1 mil) of such material such as spraying, vapor deposition and cathode sputtering. 
     As stated above, in the Summary of the Invention, process parameters and materials are specifically selected in the laser mirror application to provide ease of rapid fabrication, low density, high elastic stiffness, high strength, high fracture toughness, low thermal expansion, high thermal conductivity and environmental stability. Most important of these characteristics are the thermal expansion, thermal conductivity, and composite strength and fracture toughness. Note also Table I for a comparison of coefficients of thermal expansion for composites of various matrices. 
     
                       TABLE I                                                     
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                                     Tem-                                 
                   0° CTE                                          
                             90° CTE                               
                                     perature                             
Fiber    Matrix    (10.sup.-6 /°C.)                                
                             (10.sup.-6 /°C.)                      
                                     (°C.)                         
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50% Hercules                                                              
         7740 Pyrex                                                       
                   -0.51      4.5    27                                   
HMS      Glass                                                            
50% Hercules                                                              
         7740 Pyrex                                                       
                   -0.62      4.5    -125                                 
HMS      Glass                                                            
50% Hercules                                                              
         Epoxy     -0.18     37.4    24 to +180                           
HMS                                                                       
50% Hercules                                                              
         Epoxy     -0.56     24.3    24 to -200                           
HMS                                                                       
Hercules Polyimide -0.306    --      22                                   
HMS                                                                       
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     The thermal conductivity of a laser mirror composite formed according to the present invention is also high. For example, the thermal conductivity through a plane of a two-dimensional composite is clearly better than conventional epoxies and nearly twice that for the same composite using a conventional polyimide matrix rather than a glass matrix. Attention is directed to Table II for comparison of the superior thermal conductivity of the glass composite of the present invention as compared to that of resin matrix composites. 
     
                       TABLE II                                                    
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                           Thermal                                        
                           Conductivity                                   
Material      Orientation  (cal/sec-cm-°C.)                        
______________________________________                                    
HMS Fiber (0/90)-                                                         
 7740 Glass   ⊥       0.0053                                         
              //           0.0700                                         
HMS Fiber (0/90)-                                                         
              |                                                  
 Polyimide Resin                                                          
              ⊥       0.0032                                         
              //           0.0207                                         
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     In Table II, the measurements were taken in planes normal and parallel to the fibers and the conductivity was measured through the composite. ⊥ refers to the measurements made in a direction normal to the plane of the fibers and ∥ refers to the measurements made in a direction parallel to the plane of the fibers. 
     Because of the low temperature environment which high performance laser mirrors may be used in (e.g. outer space applications), testing was performed at 150 K and 300 K to demonstrate the composite strength and fracture toughness of the laser mirror under such adverse temperature conditions. In all cases, the mirrors were able to withstand high loads with increasing strain after initial fracture, and in all cases the mirrors remained substantially intact at the conclusion of testing. Attention is directed to Table III for demonstration of the superior strength and toughness of the mirror composites of the present invention. 
     And as far as high temperature applications of the mirrors of the present invention, the only limitation would be the upper limits of the reflective layers which are applied. Currently, most protective coatings have an upper limit of about 300° C. before they begin to release from the surface. However, because of the high temperature stability of the composites of the present invention, reflective layers with temperature limits greater than this could also be used, resulting in a coated mirror with higher use temperature than currently used. 
     Presently, the two most popular materials for laser mirror use are molybdenum substrates with intricate cooling passages and gold or silver coated ULE glass. However, the composites of the present invention are far superior to either of these commercially known materials. The mirrors of the present invention have a thermal conductivity sufficient for high flux loads, a low thermal expansion, are low density and lightweight. 
     
                       TABLE III                                                   
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Three Point Flexural Strength of Unnotched                                
0/90 Cross Ply Reinforced Glass                                           
                      Flatwise                                            
          Test Temperature                                                
                      Flexural Strength                                   
Specimen Number                                                           
            K             MPa      10.sup.3 psi                           
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1           300           294      42.6                                   
2           300           316      45.9                                   
3           300           367      53.3                                   
4           300           301      43.6                                   
 5*         300           259      37.5                                   
 6*         300           331      48.0                                   
            Average       311      45.5                                   
7           150           192      27.8                                   
8           150           354      51.3                                   
9           150           214      31.1                                   
10          150           485      70.3                                   
            Average       311      45.1                                   
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 *Specimens 5 and 6 were tested with machined surface in tension while all
 others had the asfabricated surface in tension.                          
 
    
     Such properties provide low distortion with increased flux, minimum coolant system requirements, and superior optical component durability. Such properties can result in mirrors fabricated with a thermal expansion only 3% that of molybdenum, with a lightweight density less than aluminum, and a cost only 10%, for example, of conventional molybdenum mirrors. Furthermore, the mirrors of the present invention can be made with a 90% reduction in distortion, 60% reduction in coolant supply, and 90% reduction in weight over conventionally used molybdenum mirrors. 
     It should also be noted that while this disclosure is specifically directed to laser mirrors, it would be within the purview of one skilled in this art to use the mirrors of the present invention to reflect other wavelengths of radiation such as optical light. 
     It is quite surprising that two brittle materials such as graphite and glass can produce in combination such a fracture tough, dimensionally stable mirror. For example, all glass mirrors have to be cooled slowly to avoid cracking. However, with the present invention the mirrors have such a high degrees of fracture toughness that such slow cooling is not necessary. Furthermore, because of the high thermal conductivity of such mirrors, cooling takes place not only much more uniformly, but much more quickly as well. And the dimensional stability of such mirrors is such that they are particularly suitable for close tolerance work. For example, a two-inch wide, two-inch thick mirror made according to the present invention would change only 0.05 mil in width and only one mil in thickness over a 100° C. change in temperature. And the high fracture toughness allows machining to almost any design. The presence of the fibers inhibits microcrack formation during machining and during use. 
     Although this invention has been shown and described with respect to a preferred embodiment, it will be understood by those skilled in this art that various changes in form and detail thereof may be made without departing from the spirit and scope of the claimed invention.