Abstract:
A method of joining ceramics and metals to themselves and to one another is described using a brazing filler metal consisting essentially of 35 to 50 atomic percent copper, 15 to 50 atomic percent silver and 10 to 45 atomic percent titanium. This method produces strong joints that can withstand high service temperatures and oxidizing environments.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to the development of copper-silver-titanium brazing filler metals (or braze alloys) for direct brazing of ceramics to ceramics or ceramics to metals to form strong joints that can withstand high service temperatures. It was developed under a contract with the United States Department of Energy. 
     A key technology that will enhance or restrict the use of ceramic materials in high performance applications, such as advanced heat engines or high temperature heat exchangers, is the ability to reliably join simpleshape ceramic components to form complex assemblies or to join unit lengths of ceramic material to form large ceramic systems. Although ceramic joining technology has been highly developed over the past fifty years, very little has been done to develop brazing filler metals for joining ceramics for use at elevated temperatures, at high stress levels and in dirty environments. 
     There are basically two brazing processes that can be used for joining ceramics. One is indirect brazing in which the ceramic is coated with a metal such as molybdenum or titanium prior to brazing with a nonreactive commercial filler metal. In the widely used moly-manganese process a glassy phase forms a bond with the ceramic and the molybdenum particles that constitute the coating. In other indirect processes, the active metal titanium is applied to the ceramic by vapor deposition or from a slurry containing titanium hydride. The other major ceramic brazing process is direct brazing with filler metals specially formulated to wet and bond to the ceramics. Direct brazing is preferred since it avoids the development and application of what is, in many cases, the very sophisticated and expensive coating or metallizing treatment required for indirect brazing. Also, the inclusion of an active metal such as titanium within the filler metal more effectively protects the active metal from oxidation during storage or while brazing than when the pure active metal is first used to coat the ceramic. Finally, the strength of the bond between a coating and ceramic substrate, and the corrosion resistance of the coating do not have to be of concern in direct brazing. 
     In spite of the potential advantages of the direct brazing process, there are very few filler metals commercially available that will wet and adhere to structural ceramics. It is difficult to formulate such an alloy since ceramics, and particularly oxide-base ceramics, are notoriously difficult to wet. One filler metal that has been developed to braze ceramics is made by the WESGO Division of GTE Products Corporation and sold under the trade name &#34;Ticusil&#34;. This alloy contains 55 at. % silver, 36 at. % copper and 8 at. % titanium. However, the strengths of ceramic-ceramic brazements made with this filler metal tend to be low, and quite variable even within a given brazed sample. Therefore, in order to meet the requirements for joining various ceramics for high performance applications, there is a need to develop filler metals that can wet and strongly adhere to ceramics and also withstand high service temperatures and oxidizing conditions. 
     SUMMARY OF THE INVENTION 
     In view of the above-mentioned need, it is an object of this invention to provide brazing filler metals for direct brazing of ceramics and metals to themselves and to one another to form strong joints for use in high-performance applications at elevated temperatures, high stress levels and in oxidizing environments. Other objects and advantages of the invention will be apparent to those skilled in the art from the detailed description and the claims. 
     The invention is generally described as a method of joining ceramics and metals to themselves and to one another by brazing in a nonoxidizing atmosphere with a filler metal in the composition range of 35 to 50 at. % copper, 15 to 50 at. % silver and 10 to 45 at. % titanium. The invention is also a composite structure having at least two structural components of ceramics and metals joined to themselves or to one another with a brazing filler metal in the composition range of 35 to 50 at. % copper, 15 to 50 at. % silver and 10 to 45 at. % titanium. In addition, the invention is a brazing filler metal consisting essentially of 35 to 50 at. % copper, 15 to 50 at. % silver and 10 to 45 at. % titanium. These filler metals have the advantages of wetting and strongly adhering to ceramics and the capacity to withstand high service temperatures. They also have sufficiently low brazing temperatures (&lt;1,000° C.) to permit direct brazing of thermally sensitive structural materials such as nodular cast iron (NCI) and partially stabilized zirconia (PSZ) without damage to either the iron or the ceramic. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a ternary diagram indicating compositions in atomic percent of the alloys of this invention and compositions of two brazing filler metals tested for comparison to the alloys of this invention. 
     FIG. 2 is bar graph illustrating the superior wetting behavior of two filler metals of this invention compared with commercially available &#34;Ticusil&#34;. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Much work has been done in recent years to develop methods of joining ceramics to ceramics or ceramics to metals for use in high-temperature applications. A preferred technique is direct brazing wherein two components to be joined are brazed together using a filler metal that can wet and adhere to the respective surfaces of the components. One line of development for joining oxide-base ceramics is the use of filler metals containing oxide-forming active elements such as titanium. Oxide-base ceramics to which the brazing filler metals of this invention will adhere include partially stabilized zirconias, high purity high density aluminas, and alumina-matrix composites. 
     The filler metals studied were made by melt-spinning onto a rapidly rotating stainless steel wheel. Although brazing was done in vacuum ranging from 2×10 -4  mm Hg or lower, this would not preclude brazing in a high-purity, inert gas environment. Filler metals prepared and tested are described in Table 1. The alloys are lower in silver content and higher in titanium content than commercially available Ticusil manufactured by WESGO. 
     
                       TABLE I______________________________________Content in Atomic %Filler metal       Cu            Ag    Ti______________________________________Ticusil     37            55     8MWF-1       47            27    26MWF-2a      45            26    29MWF-2b      43            22    35MWF-2c      37            20    43MWF-92      44            43    13MWF-93      41            35    24______________________________________ 
    
     Filler metals MWF-2a through MWF-2c were prepared from the same alloy ingot but segregation effects in the ingot resulted in differing compositions. 
     An indication of the ability of a brazing filler metal to form strong joints is the wetting or contact angle that the molten metal makes with the ceramic substrate. Small wetting angles are characteristic of strong joints. This invention provides brazing filler metals with small wetting angles and tests further indicate the strength of the joints formed is high. Sessile drop wettability studies were conducted to study the wetting angle of the brazing filler metals on ceramic substrates as a function of time and temperature. The sessile drop apparatus is essentially a horizontal induction heating furnace, consisting of a fused silica tube 38 mm in diameter and 300 mm long, in which a vacuum of 5×10 -5  mm Hg and temperatures to 1750° C. can be obtained. After each run was made, the sample was removed from the tube, examined with a shadowgraph to measure the degree of wetting (contact angle) and then either sectioned for ceramographic examination or used for determination of the apparent shear strength of the bond between the drop and the ceramic substrate. 
     Tests to determine the apparent shear strength of the bond between the drop and the ceramic substrate were conducted by a generally accepted method, the Sutton push-off technique. In this test, a projection on a special test fixture in a mechanical test machine shears the solidified sessile drop off the ceramic substrate. The bond strength is calculated from the load at failure divided by the interfacial area. However, in the case of some of these filler metals, the wetting angles are so low that the test apparatus shears through the drop without causing failure at the interface. Therefore, flexural strength measurements were conducted on ceramic-ceramic brazement samples joined with these filler metals. Ceramic coupons were brazed in a butt configuration and then sliced into flexure bars using a diamond abrasive wheel. The specimens were tested in a side-bend configuration in a four-point bend fixture. Both room- and elevated-temperature tests (up to 600° C.) have been conducted. 
     Example I 
     Wetting angles on various substrates were measured for filler metals MWF-1 and MWF-2b. Substrates tested are identified in FIG. 2 as PSZ/MgO which is zirconia partially stabilized with magnesium oxide and sold under the trade name Nilsen&#39;s MS Grade; PSZ/Y 2  O 3  which is zirconia partially stabilized with diyttrium trioxide made by NGK Spark Plug Co., Japan and sold under the trade name NGK Z-191. AL-23 which is a 99.5% alumina ceramic manufactured by Degussa of the Federal Republic of Germany; and AD-99 which is a 99% alumina ceramic manufactured by Coor&#39;s Porcelain Company. FIG. 2 shows the relative wetting behavior of filler metals of this invention and WESGO&#39;s Ticusil on various substrates indicating that the filler metals MWF-1 and MWF-2b exhibit superior wetting behavior. 
     EXAMPLE II 
     MWF-2b was used in a series of experiments to join zirconia partially stabilized with MgO, (PSZ), having the trade name Nilsen&#39;s MS Grade, and various high density, high purity aluminas more specifically described as AD-998 a product of Coor&#39;s Porcelain Company having 99.8% Al 2  O 3 , and AL-23 a product of Degussa, Federal Republic of Germany, having greater than 99.5% Al 2  O 3 . The ceramics were brazed in vacuum in a butt joint configuration in the temperature range 980° C. to 1,090° C. Flexure bars cut from the brazed coupons were tested in four-point bending in air. A summary of brazing conditions, ceramic brazed and flexural strength data is given in Table II. 
     
                       TABLE II______________________________________Summary of flexural strength data for ceramic-ceramic brazementsmade with experimental brazing filler metal MWF-2a,Cu--26Ag--29Ti, at. % - all tests in airSpeci-                            Stan-men   Temperature            Number    Mean   dard  LocationNum-  (°C.)            of        Strength                             devia-                                   ofber   brazing test   Specimens                        (MPa)  tion  failure______________________________________AD-998 Alumina414    980     25    4       222.sup.c                               33    Ceramic394   1000    400    4       215.sup.d                               37    Mixed.sup.e395   1000    600    4       213.sup.d                               24    JointAL-23 Alumina356   1090     25    4        91.sup.c                               49    Ceramic392   1000    200    2       145.sup.c                                1    Mixed392   1000    400    2       110.sup.c                               28    Mixed415    980    400    4       165.sup.d                               41    MixedPSZ (83-064 MS, MgO Stab.)348   1040     25    4       350.sup.c                               41    Joint362   1000     25    4       339.sup.c                               53    Joint391   1000    200    2       159.sup.c                               26    Joint391   1000    400    2       132.sup.c                               70    JointSintered α-SiC433   1000     25    3        81.sup.f                               21    Joint______________________________________ .sup.a All brazing in vacuum of &lt;5 × 10.sup.-5 mm Hg at start of cycle, 2 × 10.sup.-4 mm Hg at temperature. .sup.b 30 min. at temperature prior to testing .sup.c Crosshead speed of 0.085 mm/s, 4point bend, 6.35  and 25.4 mm spans. .sup.d Load rate of 22.7 kg/s, 4point bend, 6.35  and 19.0 mm spans. .sup.e Some samples failed in braze joint, others in ceramic. .sup.f Crosshead speed of 0.085 mm/s, 4point bend, 6.35  and 19.0 mm spans. 
    
     Outstanding strength values were achieved at a brazing temperature of 1,000° C. or below for test temperatures up to 600° C. This temperature is 250° C. above the expected service temperature at the interface between a ceramic piston cap and nodular cast iron piston in the uncooled diesel engine. In some cases failure occurred in the ceramic, and in some cases there was &#34;mixed&#34; failure. For comparison purposes, the room temperature flexural strength of AD-998 alumina is about 210 MPa (30.5 ksi), of Degussit AL-23 alumina is 155 MPa (22.5 ksi) and of PSZ is 600 MPa (87 ksi). 
     The SiC data is included to show that this filler metal also has significant promise for brazing SiC. Although the flexural strength is relatively low (81 MPa), it is still twice that of a braze material used in industry to join silicon carbide. 
     Air-oxidation tests were conducted on ceramic specimens brazed with the MWF-2b filler metal. Air exposure was used as air is more oxidizing than the atmosphere in an engine combustion chamber. The specimens were held for over 5,000 hours at 500° C. A tightly adhering oxide film formed on the surface, and there was no catastrophic oxidation. 
     EXAMPLE III 
     This experiment was conducted to determine if either the 1,000° C. brazing temperature, or exposure to an active-element-containing filler metal, were harmful to the mechanical properties of either the NCI piston material or the PSZ ceramic. The iron undergoes a typical allotropic phase transformation on heating above about 750° C. The PSZ is susceptible to an overaging phenomena at elevated temperatures. 
     Bar/pad shear test specimens were brazed with MWF-2b. The bars were nodular cast iron (NCI) or type 446 stainless steel (more specifically described as a ferritic stainless steel containing about 25 wt. % chromium). Type 446 stainless steel is a potential transition material for accommodating mismatches in coefficients of thermal expansion between NCI and PSZ. The pads were MgO-stabilized PSZ. The metal bars had dimensions of 3.18×9.52 mm and overlapped the ceramic pad by 6.35 mm. The PSZ pad was 9.52×9.52×7.62 mm. Brazing was conducted in the temperature range 920° C. to 1,000° C. under a vacuum of 5×10 -5  mm Hg. 
     The PSZ ceramic was emphasized in these experiments since PSZ has low thermal conductivity and is therefore most desirable to insulate the combustion chamber in the uncooled diesel engine. Room temperature shear strengths for these brazements averaged an outstanding 200 MPa (29 ksi). Also significant was the finding that the microstructure of the NCI material after brazing was very similar to that of the as-received iron, and the hardness of the NCI was not reduced by this thermal cycle, an important factor in engine design. 
     Other experiments demonstrated that the PSZ is not harmed by brazing with this filler metal. The flexural strength and fracture toughness of the PSZ material was 652±44 MPa and 11.5±0.8 MPa·m1/2, respectively, after brazing as compared to 704±2 MPa and 14±2 MPa·m1/2 for the unbrazed materials. These differences are not considered to be significant as far as use of this ceramic in an uncooled diesel engine is concerned. 
     EXAMPLE IV 
     Fracture toughness of ceramic joints was examined since the fracture toughness of a braze joint (and particularly of ceramic braze joints) is increasingly considered to be a critical parameter for the design of components. The results were very encouraging considering the critical fracture toughness of the samples brazed with MWF-1 was at least as great as that of the bulk ceramic indicating that the brazement would not be a &#34;weak link&#34; in the system, see Table III. 
     
                       TABLE III______________________________________Fracture toughness of brazements in PSZ ceramic.sup.ajoined with experimental brazing filler metalMWF-1 Cu--27Ag--26Ti, at. %    Brazing      Time at    FractureSpecimen temperature  temperature                            toughnessnumber   (°C.) (min)      (MPa · m1/2)______________________________________103      1010         5          7.2104      1010         5          6.6105      1040         4          5.9107      1010         20         6.1______________________________________ .sup.a Nilsen 8294159N having fracture toughness of 6.0 MPa · m1/2. 
    
     EXAMPLE V 
     MWF-92 and MWF-93 were used in a series of sessile-drop experiments test wetting angle and shear strength on Degussit AL-23 alumina and a ceramic-matrix composite of alumina reinforced with silica whiskers. The results are given in Table IV. 
     
                       TABLE IV______________________________________Wetting and bonding behavior of twoexperimental brazing filler metals on structural ceramics.    Brazing                ShearSpecimen Temperature   Wetting  StrengthNumber   (°C.)  Angle    (MPa)______________________________________Brazing filler metal: MWF-92 Cu--43Ag--13TiCeramic Substrate: Degussit AL-23 Alumina490.4    800           60       110491.4    850           30       81493.5    850           30       73493.6    850           30       71Brazing filler metal: MWF-93 Cu--35Ag--24TiCeramic Substrate: Degussit AL-23 Alumina490.5    800           --       28488.4    850           22       --Brazing filler metal: MWF-93 Cu--35Ag--24TiCeramic Substrate: Al.sub.2 O.sub.3 + 10 vol. % SiC whiskers509.6    850           40       95______________________________________ 
    
     The results of the tests described in Examples I through V indicate that these experimental filler metals are very viable candidates for direct brazing of insulating ceramics to NCI components of the uncooled diesel engine. The brazing temperatures are suitable for joining NCI, stainless steel and various ceramics, and the joints thus formed exhibit high strength as well as effective resistance to oxidation at temperatures of at least 500° C. MWF-1, MWF-2a, MWF-2b and MWF-2c exhibited better wetting angles an shear strength than the MWF-92 and MWF-93 alloys having higher silver content, although the high silver alloys still have wetting angles considerably less than 90°. 
     The compositional range of the Cu-Ag-Ti alloys claimed is schematically indicated with a heavy line in FIG. 1 and embraces the compositions MWF-1 (Point E), MWF-2a (Point F), MWF-2b (Point G), MWF-2c (Point H), MWF-92 (Point C) and MWF-93 (Point D). In comparison, brazing with an alloy designated Point A on FIG. 1 and having a composition of 28Cu-60Ag-12Ti resulted in braze joints between nodular cast iron and partially stabilized zirconia having shear strengths of only 72 MPa and 3 MPa in two tests, considerably lower than the alloys having low silver content. The composition of Ticusil is indicated at Point B. Flexural strength data for AD-998 alumina samples brazed at two temperatures with Ticusil are given in Table V. Although these brazements were excellent in appearance, they were inferior to samples brazed with filler metals MFW-2a (Table 11) from the standpoint of both overall low strength as well as wide variation from sample to sample. 
     
                       TABLE V______________________________________Flexural strength of ceramic-ceramic brazements joined withcommercial brazing filler metalSample    Temperature (°C.)                     Flexural strength.sup.cNo.       Brazing.sup.a               Test.sup.b                         (MPa)  (ksi)______________________________________AD-998 brazed ceramic brazed with Ticusil522.1     900        25       114    16.5522.2     900        25       62     9.0522.3     900       400       62     9.0522.4     900       400       82     11.9526.1     950        25       42     6.1526.2     950        25       153    22.2526.3     950       400       237    34.4526.4     950       400       40     5.8______________________________________ .sup.a Brazed in vacuum of &lt;1 mPa (&lt;9 × 10.sup.-6 mm Hg). .sup.b Samples held for 30 minutes at temperature in air prior to test loading. .sup.c Loading rate 22 kg/s.