Glass frit composition
TECHNICAL FIELD 
     The present invention relates generally to ceramic compositions and more particularly to a glass frit composition, i.e. in a powdered or comminuted form. 
     BACKGROUND ART 
     It is known that to obtain a glass frit having low firing temperatures, the alkali oxide content must be high. U.S. Pat. No. 2,925,351, issued Feb. 16, 1960 to D. M. Judd, discloses a glass frit having an alkali oxide content from 21% to 35% of the total composition; however, the composition therein disclosed was for application to aluminum and its alloys. The thermal expansion of typical aluminum alloys range from 12.8×10 -6  to 13.3×10 -6  /°F. (i.e. 23×10 -6  to 24×10 -6  /°C.). It is desirable to provide a glass frit composition having a coefficient of thermal expansion suitable for matching nickel-base alloys with varying amounts of molybdenum, chromium, manganese, copper, silicon and iron (e.g. Hastelloy X); for example in the range of 7.1×10 -6  to 7.9×10 -6  /°F. (i.e. 12.8×10 -6  to 14.2×10 -6  /°C.). 
     In the prior art, glass has been used for viscous damping at temperatures of 700° F. (about 370° C.) and higher. Somewhat below 700° F. (about 370° C.) organic materials have been used for damping, rubber being one example. It is desirable to provide a glass frit composition which would be useable for a damping material at temperatures below 700° F. (about 370° C.) or in a range intermediate existing damping materials. 
     The present invention is directed to meeting one or more of the objectives set forth above. 
     DISCLOSURE OF THE INVENTION 
     In accordance with one aspect of the present invention, there is provided a glass frit comprising about 60% by weight of glass formers, 30% by weight of monovalent fluxes and 10% by weight of divalent fluxes. The glass formers consist essentially of SiO 2  and B 2  O 3  ; and the monovalent fluxes consist essentially of Li 2  O, Na 2  O and K 2  O. The divalent fluxes are selected from the group consisting of CaO, SrO, BaO and PbO; but preferably CaO, SrO and BaO. Additional elements may be present not exceeding 10% by weight. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph plotting temperature vs. loss factor to show low temperature damping characteristics of a glass coating made from a glass frit composition made in accordance with the present invention; and 
     FIG. 2 is a three-dimensional graphical illustration of the damping characteristics of the glass coating in FIG. 1. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     A glass frit consists essentially of the following constituents calculated as weight percentages: 
     
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              Percent                                                     
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RO.sub.2 and R.sub.2 O.sub.3                                              
                60                                                        
R.sub.2 O.sub.1 30                                                        
R.sub.1 O.sub.1 10                                                        
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     where RO 2  represents the oxide of Si; R 2  O 3  represents the oxide of B; R 2  O 1  represents the oxides of Li, Na and K; and R 1  O 1  represents the oxides of alkaline earth elements and lead. 
     It has been found that a low melting glass frit having a coefficient of expansion suitable for application to nickel-base alloys and alloys with similar coefficients of thermal expansion, can be made if certain basic concepts are followed. One example of such an alloy is precipitation-hardening (PH) stainless steel 15-5PH, 17-4PH. First, the alkali oxide content must be high in order to make the composition relatively more fluid. As shown above, this is about 30% and may range from 27% to 33% of the total composition. Second, the B 2  O 3  should be in the range of 10% to 14% of the total composition. For low melting complexity of the formula is desirable; hence alkaline earth oxides are included. A CaO content in the range of 4% to 6% of the total composition is believed to be essential to minimize the overall movement of the alkali ions. The divalent Ca ion has been shown to be an effective element in blocking the movement of the monovalent and very mobile alkali ions, Na and K. 
     The formula of a preferred embodiment of a glass frit composition, in weight percentages, is as follows: 
     
         ______________________________________                                    
           Percent                                                        
______________________________________                                    
       SiO.sub.2                                                          
             49                                                           
       B.sub.2 O.sub.3                                                    
             12                                                           
       Li.sub.2 O                                                         
             3.6                                                          
       Na.sub.2 O                                                         
             14.4                                                         
       K.sub.2 O                                                          
             11.4                                                         
       CaO   5                                                            
       SrO   1.85                                                         
       BaO   2.75                                                         
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     This provides a coefficient of thermal expansion of 7.2×10 -6  /°F. (3.9×10 -6  /°C.) over the range from room temperature to 600° F. (316° C.). 
     Note that the molecular equivalent ratios in the preferred composition are as follows: 
     
         ______________________________________                                    
        Li.sub.2 O                                                        
              1                                                           
        Na.sub.2 O                                                        
              1.9                                                         
        K.sub.2 O                                                         
              1                                                           
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     Hence, on a molecular equivalent basis there is as much Li 2  O as K 2  O, and about as much Na 2  O as the sum of them. 
     The foregoing can also be expressed as weight percentages within the following ranges: 
     
         ______________________________________                                    
           Percent                                                        
______________________________________                                    
       SiO.sub.2                                                          
             44 to 54                                                     
       B.sub.2 O.sub.3                                                    
             10 to 14                                                     
       Li.sub.2 O                                                         
             3 to 4                                                       
       Na.sub.2 O                                                         
             13 to 16                                                     
       K.sub.2 O                                                          
             10 to 13                                                     
       CaO   4 to 6                                                       
       SrO   1 to 2                                                       
       BaO   2 to 3                                                       
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     If it is desired to adjust the amounts of the various ingredients of the glass frit for a particular result, these guidelines should be followed: (1) complexity of the formula is desirable; (2) the agressive monovalent fluxes (R 2  O) are generally interchangeable but should be present in ratios to each other of not less than 1 to 4 or more than 4 to 1; and (3) other ingredients may be present in reasonable amounts, depending upon their respective characteristics known to those skilled in the art, but the total generally should not exceed 10% by weight. 
     It will be understood that in addition to the ingredients specified the glass frit might in some cases contain trace impurities, such as those derived from impurities in the raw materials. The amount of such impurities or of any such &#34;other ingredients&#34; should, of course, never be allowed to be so large as to detract from the desired properties of the glass frit. 
     The raw materials used in preparation of the glass frit may take the form of oxides, carbonates, nitrates, silicates, etc. For example, technical/industrial grade SiO 2 , H 3  BO 3 , Na 2  CO 3 , K 2  CO 3 , CaCO 3 , SrCO 3  and BaCO 3  are acceptable. Higher purity grades may also be used. For melting, high purity fused silica crucibles are recommended. The raw materials are smelted at a temperature of between 1800° F. and 1850° F. (980° C. and 1010° C.), and fritted in the conventional manner. 
     Industrial Applicability 
     A glass frit made in accordance with this disclosure may be used to make a slip and applied to a suitable substrate in accordance with existing practices for a viscous damper or for a coating to form a matrix for holding other particulate materials. FIGS. 1 and 2 show the damping characteristics over temperature ranges from about 500° F. (255° C.) to about 1100° F. (595° C.). The drawings show a broad band of damping and suggest (at the left of FIG. 1) that there may be a second area of very effective damping. 
     FIG. 1 compares the loss factor of Hastelloy X, a nickel-base superalloy shown as line A, with that of a coating made for the above-disclosed glass frit shown as line B. The coating contains 17% alumina (Al 2  O 3 ). The broad range of use temperature for the coating versus the narrower range for a more conventional coating shown in dashed line C is noteworthy. 
     FIG. 2 shows isothermal scans over a frequency range of 25 to 525 Hz of the vibrational response of Hastelloy X beams coated with compositions based on the above-described glass frit. These scans give the data shown in FIG. 1. 
     Strain between a coating and a substrate is generated on cooling from the firing temperature, or slightly below at a termperature where flow of the coating ceases. Therefore, close approach of coefficients of expansion and low firing temperature combine to reduce the strain on cooling to room temperature. The combination of these factors achieved using the present glass frit permits thicker coatings to be applied and permits the coating to contain large amounts of filler added for various purposes. The damping coating shown in FIG. 1 contains 17% alumina in spite of the low expansion of alumina. Additions of fillers of matching expansion, such as metallic particles with an expansion matching that of the substrate, may be made to control other properties such as electrical conductivity, modification of flow characteristics, magnetic properties, reflectivity, etc. 
     While the invention has been described in preferred embodiments and specific compositions, it should be understood that modifications and variations are possible in the light of the above teaching and that the invention may be otherwise practiced within the scope of the appended claims.