METHOD OF PRODUCING BRAZING METALS

Abstract: Active brazing metals that can join ceramic members mutu­ ally or a ceramic member and a metallic member to each other with high joint strength and improved availability can be provided in various shapes such as plate or wire, even when a metal of poor ductility is used as a raw material of the brazing metals. The jointing metals are a roughened substrate and a layer thereon of a metal of higher melting point which has a smaller standard free energy of reaction with the main non-metal element of the ceramics. The thickness ratio of the layer to substrate is in the range 0.005-0.1.

Description: The present invention relates to a method of brazing, particularly for jointing ceramic members mutually or a metallic member and a ceramic member to each other. Heretofore, as one method of jointing ceramic members mutually or a metallic member and a ceramic member to each other, a jointing method using an active brazing metal has been known. In this jointing method, superposed foils of alloys containing an active metal such as Ti or Zr, or a foil of such active metal superposed on a foil of metal such as Ni, are sandwiched between surfaces of ceramic members or surfaces of a metallic member and a ceramic member to be jointed, and heated to effect jointing of the members. For example, US-A-2,587,663 and JP-A-12,575/61 disclosed a brazing method wherein a foil of Ti group metal and a foil of Cu or Ni are sandwiched and heated between surfaces of a metallic member and a ceramic member or surfaces of ceramic members to be jointed. However, this method has the following drawbacks: (1) The active metals that can be used as brazing metals are limited to those metals that can be easily wrought to foils, so that metals of poor ductility can not be used. (2) The foils are liable to oxidize at their surfaces, so that an alloy can not be completely formed between the active metal foil consisting the brazing metal and other metal foil. (3) A plurality of foils are superposed, so that, jointing of members of complicated jointing surfaces can hardly be obtained. (4) Thicknesses of the foils are varied for varying the composition of the brazing metals, so that an alloy of low alloy content can not be produced. Therefore, the brazing metal become brittle, and the joint is weak in strength. US-A-4,471,026 disclosed ternary alloy solders consisting of Cu, Ti and a metal selected from Ag, Au, Sn or In for brazing ceramic members mutually, which discloses also methods of producing the solders by preliminarily melting three metals to form an alloy thereof, knitting three metals wires to form a strand, superposing three metals sheets, or mixing three metals powders. However, these methods also have the following drawbacks: (1) In the method of preliminarily melting three metals to form an alloy, Ti content in the alloy is high, so that only a fragile alloy can be obtained which is difficult to work to a sheet-like ductile brazing metal. (2) In the methods of using wires, sheets or powders of the metals as raw materials, costs of processing these raw materials are so expensive that practical use of the methods is hardly achieved. US-A-4,486,386 specification disclosed Pd-Au alloy solder containing at least one of Ti, V and Zr. However, it has disadvantages that special melting such as vacuum melting or the like is necessary for melting active metals such as Ti, V and Zr, and that the obtained alloys are difficult to work. JP-A-81,071/85 disclosed a metal sheet for jointing ceramic members, wherein a layer of brazing metals is coated with a layer of an active metal. However, this sheet has a shortcoming that thickness of the active metal layer is too large to thickness of brazing metals layers so that a joint of satisfactory strength can not be obtained. It is an object of the present invention to provide a method of producing active brazing metals for jointing ceramic members mutually or a ceramic member and a metallic member to each other, which is easy to produce and in handling and exhibits high joint strength. Another object of the present invention is to provide a method of producing active brazing metals for jointing ceramic members mutually or a ceramic member and a metallic member to each other, which can adjust the content of active metal easily. A further object of the present invention is to provide a method of producing active brazing metals in which metals of poor ductility are used as the active metals. The present invention is a brazing method using brazing metals composed of a metal substrate and at least one metal layer applied on at least one surface of the metal substrate for jointing ceramic members mutually or a ceramic member and a metallic member to each other, as set out in claim 1. Generally, the surface of the substrate which is in a planar or strand shape produced by at least one method selected from rolling, drawing and liquid quenching methods and which is scheduled for applying the metal layer thereon is adjusted to a preferred roughness of center line average height Ra of 0.1-2.0 µm, and then at least one metal element ("active metal") which, as compared with standard free energy of formation in reaction between a metal element and a non metal element which mainly constitute the ceramic member, has less standard free energy of formation of reaction with the non metal element and a higher m.p. than that of the metal substrate, is applied on the scheduled surface of the metal substrate such that a ratio of the thickness or diameter of the metal element to that of the metal substrate becomes 5×10⁻³ - 1×10⁻¹ by a method selected from the group consisting of physical vapor deposition (PVD) process, chemical vapor deposition (CVD) process and metal plating process. The metal elements constituting the metal layer is selected from metal elements satisfying the above mentioned condition, depending on the kinds of the ceramic members to be jointed by the brazing metals. For instance, if the ceramic members are nitride ceramics or sialon products, the metal element is preferably at least one metal element selected from the group consisting of Zr, Ti, B, Ta, Hf, La, Sc and V, and if the nitride ceramics is silicon nitride, the metal element is preferably at least one metal element selected from the above group, and most preferably at least one of Zr, Ti and Ta. If the ceramic members are carbide ceramics, the metal element is preferably at least one metal element selected from the group consisting of Zr, Ti, Ta, V, Cr and Mo, and if the carbide ceramics is silicon carbide, the metal element is preferably at least one metal element selected from the above group, and most preferably at least one of Zr, Ti, Ta and Cr. If the ceramic members are oxide ceramics, the metal element is preferably at least one metal element selected from the group consisting of Be, Ce, Zr and Ti, and if the oxide ceramics is at least one ceramics selected from the group consisting of zirconia, alumina, mullite, aluminum titanate and cordierite, the metal element is most preferably at least one metal element selected from the group consisting of Be, Ce, Zr and Ti. These metal elements are all selected from metal elements which, as compared with standard free energy of formation in reaction between the metal element and the non metal element which mainly constitute the ceramic member to be jointed by the brazing metals produced by the method of the present invention, have less standard free energy of formation in reaction with the non metal element. The metal elements in the metal layer have a property to decompose the ceramics into metal elements and non metal elements, and react with the non metal elements to form compounds thereof. The reaction between the metal element and the ceramic member proceeds easier when the metal element is in liquid state. Therefore, when jointing the metallic member and the ceramic member by means of the brazing metals produced by the method of the present invention, the metal element constituting the metal layer of the brazing metals is dissolved in a melt of the metal substrate so as to react with the ceramic member. Alloys containing the metal element in the metal substrate in situ are not preferable, because they are poor in ductility, and difficult to produce and control the amount of the metal element to be added or applied. If the metal elements in the metal layer have larger standard free energy of formation in reaction with a non metal element mainly constituting the ceramic member than standard free energy of formation in reaction between metal element and the non metal element both of which are main constituents of the ceramic member to be jointed by the brazing metals in this method, brazing can not be performed, because chemical reactions do not occur between the metal elements of the metal layer and the ceramic member. The metal layer comprised in the brazing metals used in the method of the present invention has a thickness of 5×10⁻⁴ - 0.1 mm, particularly 5×10⁻⁴ - 1×10⁻² mm. If the thickness of the metal layer is less than 5×10⁻⁴ mm, the amount of the metal element constituting the metal layer is so small that the reaction between the molten brazing metals and the ceramic member is weak, and jointed articles of weak joints strength can solely be attained. While, if the thickness of the metal layer is more than 0.1 mm, the amount of the metal element constituting the metal layer is so excessively large that the metal element is not perfectly dissolved and remains partially in a solid form in the melt of the metal substrate even when the metal substrate is melted, and the chemical reaction between the metal element and the ceramic member does not occur well. In the invention, the active metal layer constitutes 0.3 to 2.8 wt% of the brazing metal article. Thickness ratio or diameter ratio of the metal layer to the metal substrate constituting the brazing metals used in the method of the present invention is 5×10⁻³ - 0.1, particularly 0.01-0.1. If the ratio is less than 5×10⁻³, the amount of the metal element constituting the metal layer is so small that the reaction between the molten brazing metals and the ceramic member is weak, and jointed articles of low joint strength can only be attained. If the ratio is above 0.1, the amount of the metal element constituting the metal layer is so excessively large that the metal element is not perfectly dissolved and remains partially in a solid form in the melt of the metal substrate even when the metal substrate is melted, and the chemical reaction between the metal element and the ceramic member does not occur well. As the metal substrate, use is made of thin planar or strand shaped general metal, preferably commercial brazing metal, particularly a brazing metal selected from the group consisting of silver solders, copper solders, brass solders, gold solder, palladium solder and amorphous nickel solders. The metal substrate can be produced by rolling, drawing, liquid quenching or the like method used for producing planar or strand shaped usual metals. Selection of the metal substrate from the above metals depends on kinds of the metallic members to be jointed or temperatures of using the jointed articles. For example, as a metal substrate of brazing metals for jointing those articles of which the jointed parts are used at mild use condition of temperature of not exceeding about 400°C, silver solders, copper solders or brass solders can be used. As a metal substrate for brazing metals for jointing those articles of which the jointed parts are used at use condition of temperature of exceeding about 400°C, gold solder, palladium solder, amorphous nickel solders or the like high m.p. metal substrate is preferably used. According to the method of producing the brazing metals in the present invention, on at least one surface of the metal substrate, a metal layer or layers consisting of the metal element or elements of m.p. higher than that of the metal substrate is applied. However, if the metal layer and the metal substrate is available for producing an alloy, they form an alloy and a melt of the alloy, even if the heating temperature at brazing or jointing is below the m.p. of the metal layer. In other word, if the brazing metal according to the present invention is heated, the metal substrate melts at first, and then the metal element constituting the metal layer is dissolved into the melt of the metal substrate to form a liquid brazing metals highly reactive with the ceramic member. In contrast, if the m.p. of the metal layer is below the m.p. of the metal substrate, the metal layer melts before the metal substrate melts. Therefore, metal element constituting the metal layer flows out from the jointed surfaces, and the reaction between the ceramic member and the metal element takes place before the alloy of the metal substrate and the metal element forms, so that jointed articles of low strength at jointed portions can only be produced. Thus, the metal layer consisting of a metal element of m.p. lower than that of the metal substrate is not preferable. When a plate shaped brazing metals are produced in a method of the present invention for jointing a metallic member and a ceramic member, the metal layer is provided on at least one surface of the plate shaped metal substrate. However, if the brazing metals according to the present invention is used for jointing ceramic members mutually, the metal layer is preferably be provided on both surfaces of the metal substrate. If the brazing metals according to the present invention is used for mutual jointing of different kinds of ceramic members, the kinds of the metal element constituting the metal layers which are to be provided on both surfaces of the metal substrate can be selectively varied, depending on the kind of the ceramic member to be jointed. In the method of producing the brazing metals of the present invention, on the surface of the metal substrate, the metal layer consisting of a metal element which has m.p. higher than m.p. of the metal substrate, is applied. In this case, if the metal layer is applied on the surface of the metal substrate after the substrate surface is roughened by a suitable means of such as sandblasting or grinding paper, adhesion strength of the metal layer to the metal substrate is enhanced, so that peeling-off of the metal layer from the metal substrate is eliminated. Thus, workability or work efficiency is improved. If the roughening is effected by means of a grinding paper,the grinding papers of No. 600 or less are preferable, and No. 600-240 are particularly preferable. Degree of the roughening or roughness is preferably a center line average height (Ra) of 0.1-2.0 µm, and particularly 0.3-0.6 µm. If the roughness Ra is below 0.1 µm, the adhesion strength of the metal layer to the metal substrate is weak, and peeling-off of the metal layer from the metal substrate occurs in handling the brazing metal. For applying the metal layer onto the metal substrate, any method of physical vapor deposition (PVD), chemical vapor deposition (CVD) and metal plating can be used. Among these methods, PVD is preferable in that thickness and area of the applying metal layer can be controlled and that metal elements of no ductility can be applied on the substrate. Among the vapor deposition, vacuum deposition, sputtering or ion plating process is particularly preferable. Jointing of ceramic members mutually or a ceramic member and a metallic member to each other by the brazing metals of the present invention is effected e.g. as follows. Surfaces of the members to be jointed are superposed with the brazing metals sandwiched therebetween, and heated in inert gas, reducing atmosphere or in vacuo at at least a temperature above m.p. of the brazing metals and below m.p. of the ceramic member and the metallic member. In this case, the brazing metals are preferably be disposed such that the surface of the metal layer covering the surface of the metal substrate contacts intimately with the surface of the ceramic member to be jointed. The heating to the brazing temperature can be effected by usual heating means. Cooling after the jointing is selected from cooling in a furnace or air cooling, depending on shapes of jointed articles and kinds of materials constituting the jointed articles. Other options and advantages of the present invention will be apparent from the ensuing descriptions and claims. For a better understanding of the present invention, reference is made to the accompanying drawings, in which: Fig. 1 is an explanational view showing a crosssection of a brazing metals produced by the present method; Fig. 2 is an explanational view showing a crosssection of another brazing metals produced by the present method; Fig. 3 is an explanational view illustrating a method of jointing a ceramic member and a metallic member, using the brazing metals obtained by the present method; and Fig. 4 is a crosssectional view showing a structure of a specimen for measuring a strength of a joint of a metallic member and a ceramic member obtained by using a brazing metals produced by the present method. Throughout different views of the drawings, the reference numerals indicate the followings; 1 ... metallic substrate, 2, 3 ... metal layers, 4 ... ceramic member, 5 ... metal plating, 6 ... metallic member, 7 ... ceramic disc, 8 ... jointed metals. Hereinafter, the present invention will be explained in more detail with reference to the attached drawings. Example 1 Referring to Fig. 1 which shows a schematic view of a crosssection of the brazing metals 10 produced by the present method, in which a surface of a plate shaped metal substrate 1 is coated with a metal layer 2. An illustrative example of producing the brazing metals 10 is as follows. A plate or substrate 1 of a silver solder of a thickness of 0.1 mm produced from an ingot by rolling is roughened at a surface by a grinding paper of No. 400, and then purified. Thereafter, the silver solder plate 1 is positioned in a sputtering device in such a fashion that the roughened surface thereof opposes to a target made of titanium metal. Sputtering is effected to apply a titanium metal layer of a thickness of 0.5-10 µm on the brazing silver plate to obtain brazing metals composed of the metal substrate 1 of the silver solder and the metal layer 2 of titanium. In this method, the metal constituting the target is applied or adhered on the metal substrate, so that the metal to be applied can be changed simply by changing the kind of the target metal. The target can be composed of at least one metal or alloy thereof. Amount of the metal to be applied on the substrate is adjusted by controlling the sputtering time. Example 2 Referring to Fig. 2 which shows a schematic view of a cross-section of the brazing metals 20 produced by the present method, in which both surfaces of a plate shaped metal substrate 1 are coated with metal layers 2 and 3 which may be a same or different kinds of metal. The metal to be applied is selected depending on the kinds of ceramic member being used. An illustrative example of producing the brazing metals 20 is as follows. A metal substrate or plate of a brazing metal produced by rolling is roughened at both surfaces thereof by a sandblasting, and purified at the surfaces. Thereafter, one surface of the metal substrate is coated with the metal layer 2 in a sputtering device. Then, the metal substrate is taken out from the sputtering device. The other surface of the metal substrate is coated with the metal layer 3 by the sputtering process, other physical vapor deposition process, chemical vapor deposition process or metal plating process. If the metal layers 2 and 3 are the same kinds of metal, the metal layers 2 and 3 may be applied on the substrate surfaces simultaneously. Example 3 Referring to Fig. 3 which shows a schematic view in cross-section illustrating a method of jointing a ceramic member 4 and a metallic member 6 by using the brasing metals 10 produced by the present method. The jointing is effected for example as follows. The ceramic member 4 and the metallic member 6 are prepared, and their surfaces to be jointed are cleaned. If necessary, the surface of the metallic member 6 scheduled for jointing is coated with a metal plating 5. The brazing metals 10 produced by the present method are disposed and sandwiched between the surfaces of the both members 4, 6 scheduled for jointing. The two members are aligned in their jointing positions, fixed firmly by a suitable fixing means, and heated in a furnace in vacuo at a temperature above the m.p. of the metal substrate constituting the brazing metals 10 and below the melting points of the metallic member 6 and the ceramic member 4, to melt the brazing metals 10. After holding at the temperature for an appropriate time, the furnace temperature is lowered below the m.p. of the brazing metals to solidify the same thereby to complete the jointing. The holding time at the heating temperature is a requisite time for alloying the metal substrate 1 and the coated metal layer 2 and for reacting the brazing metals 10 with the ceramic member 4 and the metallic member 6, and usually 1-10 min, optionally beyond this range. Example 4 Plate shaped metal substrates as described in the following Table 1 are roughened at their one side surfaces by a No. 400 grinding paper, and cleaned with acetone and alcohol at the roughened surfaces. Metal layers of metal elements and thicknesses described in the following Table 1 are applied on the roughened substrates surfaces by sputtering to produce brazing metals 10 composed of the metal substrate and the metal layers in combinations as described in Table 1, and of the structure as shown in the attached Fig. 1. The brazing metals are used to joint metal rods described in Table 1 on the both surfaces of discs of a diameter of 11 mm and a height of 10 mm made of the ceramics described in Table 1 at jointing temperatures described in Table 1 to produce metal-ceramics jointed articles specimen Nos. 1-13 of a shape as shown in the attached Fig. 4 and a total length of 100 mm wherein metal rods 8 corresponding to the metallic members are jointed integrally on both surfaces of the ceramic discs 7. The jointed articles are finished or ground by machining at their outer peripheries to a diameter of 10 mm, and then tested for joint strength by tensile test. The results are shown in Table 1. For comparison, referential specimens Nos. 101-102 were attempted using a Kovar metallic member and a ceramic member of silicon nitride which were beyond the numerical limitations of the present invention, and a referential specimen No. 103 was tried using a metallic member of Kovar and a ceramic member of alumina which combination was outside the combinations of the present invention, as described in Table 1, and their strengths in terms of tensile strengths were measured. The results are also shown in Table 1. As seen from Table 1, the specimen Nos. 1-13 jointed by using the brazing metals of the present invention exhibit high joint strengths, while the referential specimen Nos. 101-102 using metal and ceramic members outside the scope of the present invention could not produce joints between the ceramic member and the metallic member, namely, between the ceramic disc made of silicon nitride and the metal rod made of Kovar, and the referential specimen No. 103 using a brazing metals of a combination of metal and ceramic members which metal is beyond the scope of the present invention, also could not produce joints between the ceramic member and the metallic member, because a reaction could not occur between the ceramic member and the brazing metals. As explained in detail in the foregoing, the method Of producing the brazing metals of the present invention applies a thin metal layer of specific active metal element on a surface of a metal substrate having ductility, so that ductility of the brazing metals is not damaged, even if the metal constituting the metal layer is a metal element of poor ductility. Thus, the active metal element constituting the metal layer can be selected by paying attention solely to the reactivity thereof with the ceramic member. This is the reason why the brazing metals produced by the present method can be of easy handling and high joint strength. In the method of producing the brazing metals according to the present invention, an active metal element highly reactive with the ceramic member is applied on the surface of the metal substrate by PVD process, CVD process or metal plating process, etc., so that the amount of the active metal to be applied can be adjusted easily. Accordingly, the present method can easily produce the brazing metals of a low content of active metal element. Therefore, if the brazing metals produced by the present method is used, formation of a brittle alloy layer in the joint can be prevented, so that metal-ceramics jointed articles and ceramics-ceramics jointed articles of high joint strengths can be obtained. Metal-ceramics jointed articles and ceramics-ceramics jointed articles jointed by using the brazing metals produced by the present method, have characteristic properties of ceramics of heat resistive, wear resistant, high strength, anticorrosive and low density properties. Therefore, they can be used as parts of turbines or parts of internal combustion engines, such as pistons, tappets, intake valves, exhaust valves, turbochargers, rocker arms, cams and other parts which repeatedly receive high temperatures, loads or impact loads.

Claims: A method of forming a joint between two ceramic members or between a ceramic member and a metallic member, comprising: (i) providing a multi-layered brazing metal article made by applying at least one active metal layer (2,3) on at least one surface of a metal substrate (1), the ratio of the thickness of the or each active metal layer (2,3) to that of the metal substrate (1) being 0.005-0.1 and the or each active metal layer (2,3) being applied on at least one surface of the substrate (1) to a thickness of 0.5-100 µm by physical vapour deposition, chemical vapour deposition or by a metal plating process, so that the resulting metal layer (2,3) constitutes 0.3-2.8 wt% of the brazing metal article; (ii) positioning the brazing metal article between the members to be joined, and (iii) heating the assembly to melt the entire brazing metal article to a molten alloy and braze the members together. A method according to claim 1, wherein the metal substrate (1) has a thickness of 0.1 mm. A method according to claim 1 or claim 2, wherein at least one surface having a surface roughness of centre line average height Ra in the range of 0.1-2.0 µm is provided on the metal substrate (1) and the or each active metal layer (2,3) is applied on that surface. A method according to claim 3, wherein said at least one surface of the substrate (1) is provided with a surface roughness of centre line average height Ra in the range 0.3-0.6 µm. A method according to claim 3 or claim 4, wherein said at least one surface is roughened using sandblasting or grinding paper. A method according to any one of the preceding claims, wherein the metal substrate (1) is a plate or wire produced by at least one method selected from rolling, drawing and liquid quenching process. A method according to any one of the preceding claims, wherein a ceramic member to be joined is made of at least one ceramics selected from silicon nitride, silicon carbide and sialon, and the corresponding active metal layer (2,3) is made of at least one metal element selected from Zr, Ti, B, Ta, Hf, V, Cr, La, Sc, Y and Mo. A method according to any one of claims 1 to 6, wherein a ceramic member to be joined is made of at least one ceramics selected from zirconia, alumina, mullite, aluminium, titanate and cordierite, and the active metal layer (2,3) is made of at least one metal element selected from Be, Ce, Zr and Ti. A method according to any one of the preceding claims, wherein an active metal layer (2) is applied on only one surface of the metal substrate (1). A method according to any one of the preceding claims, wherein the or each active metal layer (2,3) is applied by a physical vapour deposition process selected from vacuum deposition, sputtering and ion plating.

Inventors: TSUNO, NOBUO
Applicants: NGK INSULATORS, LTD.
Application year: 1986
Main classification: C04B37
IPC codes: C23C14, C04B37, C23C16, B23K35, B23K1, F02F7