Abstract:
A new and improved method of brazing a ceramic-metal article is described. The ceramic-metal article comprises a ceramic article, such as a lanthana-strengthened yttria article, bonded to a metal article, such as niobium, by a layer of a brazing material, such as brazing alloy consisting essentially of 72% by weight silver, 28% by weight copper to form a joined ceramic-metal article. The method of brazing the ceramic article comprises coating a surface of the lanthana-strengthened yttria article with a group IVB metal, such as titanium, hafnium, and zirconium then placing a brazing alloy, such as a brazing alloy consisting essentially of 72% by weight silver and 28% by weight copper or a brazing alloy consisting essentially of 82% by weight gold and 18% by weight nickel, between the coated lanthana-strengthened yttria article and the niobium article and heating the layered structure in vacuum at about 50 degrees centrigrade above the liquidus temperature of the brazing alloy for a period of about 5 minutes.

Description:
CROSS-REFERENCE AND RELATED APPLICATIONS 
     Co-pending patent application, Ser. No. 324,844, entitled &#34;A BONDED CERAMIC-METAL ARTICLE,&#34; and co-pending patent application, Ser. No. 324,928 entitled &#34;A METHOD OF BONDING A CERAMIC ARTICLE TO A METAL ARTICLE,&#34; both filed concurrently herewith and both assigned to GTE Laboratories Incorporated, assignee of the present application, concern related subject matter of this application. 
     FIELD OF THE INVENTION 
     This invention relates to a method of brazing a ceramic article to a metal article. More particularly, this invention relates brazing an yttria based article to a metal article. 
     BACKGROUND OF THE INVENTION 
     In the joining of relatively brittle materials such as ceramics, brazing with ductile braze alloys is useful in accommodating residual stress from differential expansion. It has been previously demonstrated that niobium to yttria based material joints can be made successfully with a shrinkage fit and diffusion bonding as described in U.S. Pat. No. 4,545,799 to Rhodes et al. However. diffusion bonding has a number of disadvantages in that the joint configuration may make it difficult to apply the necessary pressure (shrinkage) and the tolerances needed for a reliable joint are very tight. 
     Frit seals based on calcium aluminate were also applied to the joining of a lanthana-strengthened yttria (LSY) article to another LSY article or to a niobium article (1 w/o Zr-doped) for high pressure sodium lamp applications However, this technique has been limited to relatively small parts (approximately 10 mm diameter by 0.8 mm thick tube). Although these LSY to Nb frit seals have remained vacuum tight after several thousand thermal cycles, it is believed that large joints associated with windows and domes may require a braze capable of undergoing plastic deformation. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention. a new and improved method of brazing a ceramic article to a metal article to form a ceramic-metal article comprises the following steps: 
     Step 1. A surface of a ceramic article is coated with a group IVB metal to form a coated surface of the ceramic article. 
     Step 2. A sheet of brazing material is placed between the coated surface of the ceramic article from Step 1 and a surface of a metal article forming a layered structure in which the sheet of brazing material is in contact with the coated surface of the ceramic article and in contact with the surface of the metal article. The ceramic article has a liquidus temperature, a solidus temperature, and a coefficient of thermal expansion. The ceramic article comprises an yttria based ceramic. The metal article has a liquidus temperature, a solidus temperature, and a coefficient of thermal expansion. The brazing material comprises a ductile metal and has a liquidus temperature less than the solidus temperature of the ceramic article and less than the solidus temperature of the metal article. The brazing material having a solidus temperature above a service temperature of the ceramic-metal article. 
     Step 3. The layered structure from Step 2 is heated in an atmosphere non-reactive to the ceramic article, the metal article, and the brazing material at a rate of temperature increase sufficiently slow to preclude thermal-shock fracturing of the ceramic article and to a temperature above the solidus temperature of the brazing material, less than the solidus temperature of the ceramic article, and less than the solidus temperature of the metal article for a period of time sufficient to chemically react the brazing material with the ceramic article and to react the brazing material with the metal article to form a heated layered structure. 
     Step 4. The heated layered structure of Step 3 is cooled to room temperature at a rate of temperature decrease sufficiently slow to preclude thermal-shock fracturing the ceramic article to form a bonded ceramic-metal article. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     In the drawing: 
     FIG. 1 is a perspective view of an infrared transparent lanthana-strengthened yttria dome bonded to a metal article in accordance with the present invention. 
     FIG. 2 is a cross-sectional view of the infrared transparent lanthana-strenghtened yttria dome bonded to a metal article depicted in FIG. 1 in accordance with the present invention. 
     FIG. 3 is a cross-sectional view of a brazing material in contact with a coated ceramic article and a metal article in accordance with the present invention. 
     FIG. 4 is a graph of contact angle as a function of time for a Nioro braze on lanthana-strengthened yttria coated with different metals in accordance with the present invention. 
     FIG. 5 is a graph of contact angle as a function of time for a Cusil braze on a lanthana-strengthened yttria article coated with different metals and Cusil ABA braze on an uncoated lanthana-strengthened yttria article in accordance with the present invention. 
     For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the above-described drawing. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Lanthana-strengthened yttria (LSY) is a material with great potential for 3-5 microns infrared-transmitting window or dome applications. An important attribute of La 2  O 3  -strengthened yttria is it high transmissivity at wavelengths from 300 nm to 8 microns. The absorption coefficients of the most recent LSY material measured by laser calorimetry are 0.005-0.007cm -1  at 2.9 microns, and 0.004-0.005 cm -1  at 3.9 microns, close to the intrinsic absorption coefficients (10 -3  to 10 -4  cm 31  1 ) indicated by the extrapolated multiphonon edge. High broadband transmittance and low scattering in the visible and infrared ranges (0.81±0.31% forward total integrated scattering, TIS. at 647 nm and 0.18±0.03% at 3.39 microns in 1.3 mm thick disks) have been achieved. The long wavelength cutoff and low absorption and scattering in LSY result in low emissivity, which leads to more tolerance for heating than other IR-transmitting oxides, without interference from phonon edge shift. 
     In addition. LSY has many attractive physical attributes. It has cubic symmetry and thus no birefringence, a high melting point (2464° C.), moderate thermal expansion coefficient (7.9×10 -6  /°C. between 20° C. and 1000° C.), and high strength. Toughening by means of controlled second-phase precipitation has been demonstrated. This has important implications for improving the resistance of LSY to thermal shock and rain and dust erosion. 
     The fabrication of LSY starts with high-purity submicron powders that can be pressed into windows or domes to near net shape and consolidated to full density using a transient second-phase sintering technique without pressure as described by W. H. Rhodes, &#34;Controlled Transient Solid Second-Phase Sintering of Yttria.&#34; J. Am. Ceram. Soc. 64[1] 12 (1981). This processing route results in efficient material usage and minimal grinding and polishing costs for the final. high-quality optical components. Progress in the processing technology of LSY in the past has led to successful fabrication of many, full-size, optical-quality domes and numerous property test specimens. The optical and thermomechanical properties have been characterized and a large data base established in the literature. The LSY material technology has reached the point where dome application feasibility has been demonstrated and full implementation requires (a) mechanical properties refinement, (b) improved process control, and (c) viable joint techniques for attaching domes or windows to other structural members of advanced systems. This invention is directed to the joining of LSY to metals using brazing alloys. 
     The joining of relatively brittle materials such as ceramics to metals is discussed in U.S. Pat. No. 4,545,799 to Rhodes et al. in which niobium to yttria joints can be made with a shrinkage fit and diffusion bonding. Diffusion bonding has a number of disadvantages in that the joint configuration may make it difficult to apply the necessary pressure (shrinkage) and the tolerances needed for a reliable joint are very tight. LSY has also been joined to niobium (1 w/o Zr-doped) using frit seals for high pressure sodium lamp applications. Although these LSY-to-Nb frit seals have remained vacuum tight after several thousand thermal cycles, it is believed that the larger joints associated with windows and domes may require a braze capable of undergoing plastic deformation. 
     Brazing is the preferred joining method because (1) it is hermetic, (2) braze ductility accommodates residual stress, and (3) joint tolerance requirements are not extremely stringent. The current invention teaches the brazing of yttria based ceramics to niobium yielding strong. tight joints with minimal preparation of fixturing. 
     Coatings were applied to LSY using electron beam evaporation to a thickness of about 3 microns at 300° C. Wetting behavior of the system was evaluated using conventional sessile drop tests. A drop of braze was melted on an LSY substrate in a vacuum furnace at a pressure of about 10 -5  torr. Contact angles were monitored as a function of time with a video system equipped with a telephoto lens. The metallic component of the brazed couple (i.e., the structural, parent material) was screened on the basis of properties. In the case of brazing to LSY. matching of expansion coefficients was judged to be the most significant property. Brazing was accomplished in a vacuum furnace with a typical thermal cycle of 5 minutes holding time at a temperature of about time at 50° C. above the braze liquidus temperature. Candidate systems of materials were brazed and shear tested to determine properties of coupled systems of materials. 
     The braze alloys considered were three GTE alloys, Cusil, Cusil ABA, and Nioro. Cusil, Cusil ABA, and Nioro are trademarks of Wesgo Division of GTE Products Corporation, Belmont, Calif. 
     
                       TABLE 1______________________________________Characteristics of Selected GTE Wesgo Brazes                 LIQUIDUS    SOLIDUSALLOY    COMPOSITION  TEMP. (°C.)                             TEMP. (°C.)______________________________________Cusil    72 wt. % Ag  780         780    28 wt. % CuCusil ABA    63 wt. % Ag  810         780    35.25 wt. % Cu    1.75 wt. % TiNioro    82 wt. % Au  950         950    18 wt. % Ni______________________________________ 
    
     A series of sessile drop tests were performed on coated lanthana-strengthened yttria (LSY) substrates. The coatings used were Hf, Ta, Ti, and Zr. Contact angle versus time data were generated for the above coatings using Nioro braze, 82 w/o Au and 18 w/o Ni. Nioro is a trademark of Wesgo Division of GTE Products Corporation Belmont, Calif. These results are shown in FIG. 4. All four coatings, Hf, Ta, Ti, and Zr, exhibited good wetting behavior at the end of the three minute test. Ti coating yielded a contact angle of about nine degrees in about 120 sec. Both the Ti and Hf coatings adhered well to the substrates prior to brazing with a uniform, shiny appearance. In contrast, the Ta and Zr coatings were patchy and non-uniform. However, the Ta and Zr coatings resulted in reasonable wetting angles of about 20 and 45 degrees, respectively. The wetting behavior of Cusil braze, 72 w/o Ag and 28 w/o Cu, on coated LSY substrates was also evaluated. Hf and Ti coatings were evaluated using the procedures described above. Ti coating yielded excellent wetting with a contact angle of zero degree in 120 seconds. Hf coating resulted in wetting behavior with a contact angle of 55 degrees after 180 seconds. A test was made using Cusil ABA, 63 w/o Ag, 35.25 w/o Cu. and 1.75 w/o Ti. This brazing material offers the advantage of avoiding the necessity of coating the LSY which may be useful in the case of complex geometrics of the articles. This material gave a wetting angle of 60 degrees after 180 seconds, indicating acceptable wetting behavior. These data are summarized in FIG. 5. 
     Brazed coupon pairs of LSY to metal were made to provide a means of measuring braze strength. Previous studies have demonstrated that residual stress reduces the level of shear strength obtained from brazed pairs. Consequently, candidate metals were selected on the basis of the closeness of their thermal expansion coefficients to that of LSY. Incoloy 909 (Huntington Alloys Fe-Ni-Co-Nb alloy) was considered to be a reasonable prospect while niobium was judged to be excellent. Relevant expansion coefficients are given in Table 2. 
     
                       TABLE 2______________________________________Coefficient of Linear Thermal Expansion(Room Temp. - 1000° C.) Reported in the LiteratureAlpha for LSY = 7.9 × 10.sup.-6 /°C.Alpha for Incoloy 909 = 7.8 to 9.8 × 10.sup.-6 /°C.Alpha Nb = 8.2 × 10.sup.-6 /°C.______________________________________ 
    
     Brazed pairs of Incoloy 909 to LSY were made using Nioro on LSY (coated with either Ti or Hf). Additional pairs were made with Cusil ABA on uncoated LSY. The braze alloy foil was 0.0051 cm thick. 
     The brazed coupon pair samples were sheared in a specially designed fixture in order to minimize torque on the coupons and tensile component of stress on the braze. Nioro braze was ineffective with Incoloy 909 with the samples fracturing by thermal shock on cooling to room temperature. The Cusil ABA samples survived the brazing cycle and were sheared with a mean strength of about 1600 psi. This is considered reasonable compared with the shear strength values between 1600 psi and 3500 psi reported in the literature for molybdenum to silicon carbide brazed joints. All of these samples fractured in the ceramic, not at either the braze-ceramic or braze-Incoloy interface. This observation confirms the soundness of the braze and suggests that the joint strength is dependent on ceramic properties together with those factors in the braze joint which contribute to residual stress. The low yield stress of Cusil ABA made it relatively forgiving compared with Nioro. This proved to be critical given the differential expansion between Incoloy 909 and LSY. 
     An additional series of brazed pairs was made using niobium coupons brazed to LSY. Shown in FIG. 3 is layered structure 50 before heating. Layered structure 50 comprises LSY article 60 coated with Ti layer 70. Nioro braze 80, and niobium article 90 in which Nioro braze 80 is in contact with Ti layer 70 and niobium article 90. Use of Nioro braze with 3 microns Ti coated LSY resulted a shear strength level of about 1300 psi. Cusil braze was used with Ti coated LSY and Hf coated LSY yielding shear strength levels of 1600 and 1500 psi, respectively. Cusil ABA braze on uncoated LSY yielded a strength level of 1300 psi based on four samples. Table 3 gives a summary of the shear test results. An average of two specimens for each value was reported, except where noted. It can concluded that it is possible to obtain shear strengths of about 1500 psi in niobium to LSY joints brazed under a variety of conditions. 
     
                       TABLE 3______________________________________Summary of Brazed Coupon Shear Test Results                      SHEAR    BRAZE   COATING   STRENGTH (psi)______________________________________LSY toIncoloy 909Joints      Nioro     Hf        0      Nioro     Ti        0      Cusil ABA None      1600LSY to NiobiumJoints      Nioro     Ti        1300      Cusil     Hf        1500      Cusil     Ti        1600      Cusil ABA None      1300______________________________________ 
    
     A new and improved ceramic-metal article 10, shown in FIG. 1, is one preferred embodiment of the infrared transparent lanthana-strengthened yttria dome 20, approximately 7.1 centimeters outside diameter, bonded to niobium metal sheet 30 with brazing material 40, such as Cusil ABA, a copper-silver alloy containing titanium. Cusil ABA is a brazing alloy trademark of the Wesgo Division of GTE Products Corporation, Belmont Calif. Niobium was the metal selected because of the excellent thermal expansion match to lanthana-strengthened yttria. Brazing alloy, Cusil ABA, was used as the brazing material because of its good adhesion, wetting capability, and titanium content which eliminates the need to coat the edge of the LSY dome with titanium. The service temperature of ceramic-metal article 10 is greater than 250 degrees centigrade and less than the solidus temperature of brazing material 40. 
     A 4.57 cm diameter ring of Incoloy 909 was fabricated for brazing to a small LSY dome (4.06 cm outside diameter). This attempt was unsuccessful. Although adhesion between the dome and the metal ring was good, the dome shattered on cooling. The ring thickness was 0.17 cm which, together with the differential expansion of the materials, resulted in an excessive level of residual stress. 
     A demonstration braze was undertaken in which a fullsize LSY dome (7.1 cm outside diameter) was joined to a 0.25 mm thick niobium disk. The metal selection was made on the basis of the excellent thermal expansion match to LSY and consideration of shear test results. Cusil ABA was used because of the demonstrated adhesion and wetting capability of Ti in this system and the need to avoid the difficulties of coating the edge of a dome. The sample was put through a thermal cycle in which heating and cooling were less than 10° C./min to minimize thermal shock to the dome. 
     The full-size LSY dome brazed to niobium is shown in FIG. 1. Good wetting and adhesion was obtained around the periphery of the dome as evidenced by the uniformity of the fillet. No evidence of physical damage or loss of transparency of the ceramic as a result of the brazing operation was observed. 
     The feasibility of brazing LSY to metals was demonstrated. Wetting and adhesion between both Nioro and Cusil braze alloys and LSY were obtained by coating the ceramic with either Ti or Hf. Incorporation of Ti in the braze as in Cusil ABA was effective. Niobium was selected the most appropriate metal to bond to LSY because of its expansion match. The shear strengths of coupon samples of niobium brazed to LSY were about 1500 psi and are considered reasonable. Finally, a full-size LSY dome was successfully brazed to niobium sheet. 
     While there has been shown and described what is at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.