METHOD FOR BONDING CUBIC BORON NITRIDE SINTERED COMPACT

Abstract: A method for bonding a cubic boron nitride sintered compact (1) to other cubic boron nitride sintered compact or to a body of shank material (2) is disclosed. The method comprises forming a Ti layer of 0.01-1 µm in thickness over a bonding interface between two cubic boron nitride sintered compacts or between a cubic boron nitride sintered compact and a body of shank material, forming a layer of Ni or Cu over the Ti layer to a thickness of 0.01-5 µm putting together the two cubic boron nitride sintered compacts or the cubic boron nitride sintered compact and the body of shank material with a 10-1,000 µm foil of Al, Al-Ni alloy or Ag-Cu-In alloy being placed over the boding interface, and heating the cubic boron nitride sintered compact structure to temperatures above the meeting point of the metal foil and not exceeding 750°C in an inert atmosphere or in a vacuum.

Description: Field of the invention This invention relates to a method for bonding a cubic boron nitride sintered compact, and in particular to a novel method for bonding a cubic boron nitride sintered compact to other cubic boron nitride sintered compact or to a metallic shank at temperatures in the neighborhood of 750°C. A cubic boron nitride sintered compact (hereinafter also referred to as cBN sintered compact) has a wide range of potential applications such as cutting tools, structural materials and heat sink materials because of its good thermal conductivity and superior hardness surpassed only by diamond. A cBN sintered compact with no binder material contained is particularly promising in that the excellent properties of the cBN compact are not impaired by the presence of a binding material. Various methods have been proposed to produce a binder-free cBN compact. For example, a reactive sintering process is disclosed in Japanese Laying-Open Gazette No. 28,782/1985. A number of proposals have also been made recently for bonding the cBN compact to metals or metal alloys. One of the most commonly employed method is to bond the cBN compact and other metallic material together using a suitable solder applied over the bonding interface thereof. Using a suitable solder material, the bonding of the cBN compact to a body of a metal or metal alloy is performed in an inert atmosphere or in a vacuum at elevated temperatures above 800°C. Therefore, for the successful solder bonding of the cBN compact, it is required that the cBN compact remains thermally stable at least up to a temperature of 800°C. On the other hand, in the binder-free cBN compact made by the reactive sintering process, a catalyst such as magnesium boron nitride which has been added to hexagonal boron nitride inevitably remains in small amounts in the cBN compact as a impurity component. This impurity residue has an adverse effect on the cBN compact of significantly impairing its thermal stability. While the cBN compact made using ceramics or a metal or metals as the binder is thermally stable at temperatures around 800-900°C, the cBN compact produced by the reactive sintering technique exhibits appreciable degradation of its properties such as thermal conductivity and hardness at temperatures over 750°C because fine cracks or fissures are caused in cBN crystal grain boundaries at the elevated temperatures. It is believed that the fine cracks are produced by the great thermal stress generated within the cBN compact during heating due to a substantial difference in thermal expansion between the cBN and the residual magnesium boron nitride. Accordingly, if it is desired to solder bond such a reactive sintered cBN compact, a soldering temperature of less than 750°C should be used. Also, when the cBN compact is utilized as the structural material, it is subject to a maximum temperature of between 500-600°C, making it necessary to employ a soldering material which does not melts out at the specified temperatures. Conventionally, Sn containing solders such as Cu-Sn-Ti have been proposed (see, for example, Japanese Laying-Open Gazette No. 136,605/1986). Because of their relatively lower soldering temperatures, the metal component e.g. Ti in the solder which imparts bonding capacity to the cBN compact does not undertake sufficient reaction with the cBN compact, resulting in poor bonding strength. Besides, such metallic components in the solder tends to be readily oxidized. In order to prevent the solder component from being oxidized during soldering, special care must be taken. For example, soldering must be conducted in a high vacuum of 0.133 Pa (10⁻³ torr) or in an ambient of high purity inert gas, or using a suitable oxide getter. FR-A- 2 395 237 discloses a method of bonding paticulate cBN to a support of cemented carbide by forming a Ti layer on the cBN; inserting a sheet of a brazing alloy and heating the assembly to a temperature of 675°C. SUMMARY OF THE INVENTION It is an object of the invention to provide a method for joining one cubic boron nitride sintered compact to other cubic boron nitride sintered compact with higher bonding strength at temperatures below 750°C where oxidation of bonding inducing Ti and deterioration of the cBN sintered compacts are avoided. It is still another object of the invention to provide a method for joining a cubic boron nitride sintered compact to a metallic shank with high bonding strength at temperatures below 750°C where oxidation of bonding inducing Ti and deterioration of the cBN sintered compact are avoided. It is still another object of the invention to provide a method suitable for bonding a body of cubic boron nitride based material such as a polycrystalline sintered compact produced by sintering cubic boron nitride having an addition of binder material, and polycrystalline cubic boron nitride sintered compact having substantially no binding phase, to a body of different material. In a method for bonding the cBN sintered compact according to one aspect of the invention, a pair of cBN sintered compacts, each having a bonding surface, are prepared. A Ti layer having a thickness of 0.01-1 µm is developed over the bonding surface of each cBN compact, followed by the formation of a Ni layer having a thickness of 0.01-5 µm over each of the Ti layers. The pair of cBN sintered compacts are put together with a 10-1,000 µm metal foil of Al or Al-Ni alloy being interposed between the bonding surfaces of the cBN compacts. The cBN compacts thus assembled are then subject to a heat treatment at a temperature above the melting point of the metal foil and not exceeding 750°C in a vacuum or in a inert atmosphere. According to another aspect of the invention, a pair of cBN sintered compacts, each having a bonding surface are prepared. A Ti layer is formed over the bonding surface of each cBN compact to a thickness of 0.01-1 µm, followed by the formation of a Cu layer to a thickness of 0.01-5 µm over each of the Ti layers. The pair of cBN compacts are held together with a 10-1,000 µm metal foil of an Ag-Cu-In alloy being placed between the bonding surfaces of the cBN compacts. The cBN compacts thus assembled are then heated up to a temperature above the melting point of the alloy and not exceeding 750°C in a vacuum or in an inert atmosphere. According to still another object of the invention, a cubic boron nitride sintered compact having a bonding face and a metallic shank are prepared. A Ti layer is formed over the bonding surface of the cBN compact to a thickness of 0.01-1 µm, followed by the formation of a Ni layer to a thickness of 0.01-5 µm over the Ti layer. The cBN compact and the metallic shank are put together with a 10-1,000 µm metal foil of Al or Al-Ni alloy containing 90 % by weight or more of Al being interposed between the cBN compact and the metallic shank. The cBN compact structure is then heated up to a temperature above the melting point of the metal foil and not exceeding 750°C in a vacuum or in an inert atmosphere. DETAILED DESCRIPTION OF THE INVENTION Cubic boron nitride sintered compacts that can be suitably bonded according to a method of this invention are a polycrystalline sintered compact made by sintering cubic boron nitride mixed with an amount of a binding material, and a polycrystalline cubic boron nitride sintered compact having very few binding phase. When a cBN sintered compact is to be bonded to other cBN sintered compact or to a body of different material, thin layers of Ti and Ni or Cu are successively formed over the bonding surface or bonding interface of the cBN sintered compact by means of sputtering or ion plating techniques. The Ti layer is preferably deposited to a thickness of 0.01-1 µm. The Ti layer having a thickness of less than 0.01 µm cannot undergo a sufficient bonding reaction with the cBN sintered compact, resulting in poor bonding. The Ti layer having a thickness over 1 µm tends to retard the catalytic action of an intermediate material such as Al, Al-Ni and Ag-Cu-In to be applied over the bonding interface in the form a thin film. A sufficient catalytic action by the intermediate material also results in a poor bonding. On the other hand, the Ni layer should preferably have a thickness of 0.01-5 µm. The Ni layer less than 0.01 µm thick fails to effectively prevent the oxidation of the Ti layer, while the Ni layer more than 5 µm thick cannot be melted well with a catalytic intermediate layer of Al, an Al-Ni alloy or an Ag-Cu-In alloy during bonding at elevated temperatures, thereby effecting only an insufficient catalytic action with respect to the Ti layer. The result here is also poor bonding strength. This is true of the Cu layer. As briefly mentioned, a thin layer of Al, an Al-Ni alloy or an Ag-Cu-In alloy is used at the bonding interface as a catalytic intermediate layer for activating and promoting the bonding action that takes place at the interface between the Ti and cBN sintered compact. The catalytic intermediate layer having a thickness less than 10 µm is too thin to exhibit a satisfactory catalytic action. At thickness over 1,000 µm, while the intermediate layer produces the desired effect of promoting the bonding at the interface, it gets more fragile and causes the shear strength of the bonded cBN sintered compact structure to fall off. Accordingly, the thickness within the range of 10-1,000 µm should be utilized with respect to the catalytic intermediate layer. As is known to those skilled in the art, pure aluminum possesses a melting point of approximately 660°C. The melting point of an Al-Ni alloy varies with the Ni content in the alloy. With the Ni less than 10 % by weight, Al-Ni alloy has a melting point of 640-720°C. By heating the bonding interface of the cBN sintered compact to elevated temperatures above the melting point of the Al or Al-Ni intermediate layer, both the intermediate layer and the Ni layer melt out to activate and catalize the bonding action between the Ti layer and the cBN compacts. The cBN compact sintered compacts are thus firmly joined together. Note that the heating of the bonding interface is performed in an inert atmosphere or in a vacuum to avoid the oxidation of Al or Ni. Since the Ti layer extremely susceptible to oxidation is protected by the Ni layer, it is not necessary to use a high purity inert gas or an ultra high vacuum for the heating process. Neither is it necessary to introduce an oxygen getter. The melting point of Ag-Cu-In alloy varies depending on its composition but it should be compatible with the maximum allowable temperature for the cBN compacts and the conditions under which the bonded cBN sintered compact structure is actually used. In this view, Ag-Cu-In alloy should possesses the melting point within the range of 600-740°C. As can be represented by the ternary diagram, it is desired that Ag-Cu-In alloy is composed of 15-25 % by weight of In, 25-35 % by weight of Cu and the balance of Ag. For example, a 52Ag-28Cu-20In alloy exhibits an melting point of approximately 650°C. If this ternary alloy is used in the form of a thin intermediate layer at the bonding interface between the cBN sintered compacts, heating of the compacts up to temperatures above the melting point of the alloy fuses the intermediate layer and the Ni layer or the Cu layer into a molten state, by which the bonding action between Ti and the cBN sintered compacts is effectively activated. Upon the completion of the heat treatment, both cBN sintered compacts are firmly joined together. Heating of the sintered compacts should be carried out in an inert atmosphere or in a vacuum to prevent Ag, Cu and In from being oxidized. However, Ti which is highly susceptible to oxidation being protected by the Cu layer, there is no need to used the high purity inert gas and ultra high vacuum or an oxygen getter. As described hereinabove, it is possible to bond one cBN sintered compact firmly to other cBN sintered compact or a metallic shank at temperatures in the neighborhood of 750°C by forming over the entire bonding interface a layer of Ti which imparts an extremely high bonding ability to the cBN sintered compact and an intermediate layer of Al, Al-Ni alloy or Ag-Cu-In alloy which functions to activate and promote the bonding action between Ti and a cBN compact. In addition, the use of an oxidation resistant Ni layer or Cu layer over the Ti layer makes it possible for a cBN sintered compact to be joined firmly to other cBN sintered compact or a body of different material without causing the oxidation of Ti even in an atmosphere having a certain amount of oxygen. Shank materials having a coefficient of thermal expansion closer to that of the cBN sintered compact are preferred for bonding with the cBN sintered compact. Examples of suitable shank materials are W alloy, Mo, W and cemented carbide. In accordance with the invention, the cBN sintered compact can be bonded tightly to other cBN sintered compact or a metallic shank by heating up to temperatures lower than those employed with the conventional brazing solders. Thus, it is possible to join the cBN sintered compact, which is heat resistant up to approximately 750°C, to other cBN sintered compact or a metallic shank without causing fine cracks under thermal stress in the cBN compact and without inviting any deterioration in physical and chemical properties of the cBN compact. In the past, the cBN sintered compact was used only a limited range of application in spite of its distinguished properties such as excellent hardness and good thermal conductivity because the lack of thermal stability made the solder bonding of the cBN sintered compact extremely difficult. However, the present invention, by effectively solving problem inherent in the prior art bonding process, makes possible the use of cBN sintered compact in a wide variety of applications. The invention itself as well as other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWING Fig. 1 is a perspective view of a bonding tool made in accordance with the method of this invention. EXAMPLES Example 1 Two cBN sintered compacts made by the reactive sintering process and having a size of 2.5x2.5x1.5 mm were used. A layer of Ti was first deposited over the bonding surface of 2.5x2.5 mm in each of the cBN sintered compact to a thickness of approximately 0.3 µm by means of sputtering. A layer of Ni was then grown in a similar way to a thickness of approximately 0.1 µm. The two cBN sintered compacts were put together with a 2.5x2.5x0.1 mm pure Al foil interposed between the bonding surfaces of the compacts. The cBN sintered compacts thus assembled were placed in a vacuum furnace. Within the furnace, a vacuum of 0.133 Pa (10⁻³ torr) was produced by a vacuum pump. The cBN sintered compacts were heated up to 700°C by raising the temperature within the furnace at a rate of 20°C per minute. After heating at 700°C for 10 minutes, the compacts were cooled down. Upon the completion of the heat treatment, the cBN sintered compacts were found to have been firmly joined together with a bonding strength of 127 MPa (13 kg/mm²). No degradation in the properties of the bonded compacts was found and their hardness and thermal conductivity was substantially the same as before the heat treatment. Example 2 Two cBN sintered compacts identical to those of example 1 were used. A 0.1 µm thick Ti layer and a 0.06 µm thick Ni layer were successively sputtered on the bonding surface of each cBN sintered compact. The pair of cBN sintered compacts were put together with a 2.5x2.5x0.1 mm foil of Al-Ni alloy being sandwiched between the bonding surfaces of the sintered compacts. The Al-Ni alloy comprised of 96 % by weight Al and 4 % by weight Ni. The sintered compact were then subject to a high-temperature treatment at 680°C in a vacuum of 0.133 Pa (10⁻³ torr) for a duration of 10 minutes in much the same way as in Example 1. Upon the completion of the heat treatment, the cBN sintered compacts were found to have been firmly joined to each other with the boding strength of approximately 127 MPa (13 kg/mm²), which was substantially the same value as in the example 1. No degradation of the bonded cBN sintered compacts was found. Example 3 A pair of cBN sintered compact made by the reactive sintering process and having a three dimensional size of 2.5x2.5x1.5 mm were used. A Ti layer of about 0.3 µm in thickness and a Cu layer of about 0.05 µm thick were successively deposited over the 2.5x2.5 mm bonding surface of each of cBN compact by sputtering technique. The two cBN compacts were held together with a 2.5x2.5x0.1 mm foil of 52Ag-28Cu-20In alloy placed between them. The compacts were then introduced into a vacuum furnace. While creating a vacuum of 0.133 Pa (10⁻³ torr) in the furnace by a vacuum pump, the temperature within the furnace was gradually raised up to 700°C at a rate of 20°C per minute. The cBN sintered compacts were heated at 700°C for 10 minutes and then cooled down. Upon the completion of the heat treatment, the cBN sintered compacts were firmly joined together with the bonding strength of 127 MPa (13 kg/mm²). No deterioration is witnessed in the bonded compacts and their hardness and thermal conductivity were much the same as before the heat treatment. Example 4 Two cBN sintered compacts similar to those used in example 3 were employed. A 0.1 µm thick Ti layer and a 0.1 µm Cu layer were successively sputtered on the bonding surfaces of the compacts, after which the compacts were held together with a 2.5x2.5x0.1 mm foil of 52Ag-25Cu-23In alloy being placed between them. The assembled compacts were then subject to a heat treatment at 670°C in a vacuum of 0.133 Pa (10⁻³ torr) for 10 minutes as in Example 3. Upon the completion of the heating cycle, the two cBN sintered compacts were firmly joined to each other showing the bonding strength of 127 MPa (13 kg/mm³), which was approximately the same as in Example 3. Also, there was found no degradation of the bonded cBN sintered compacts. Example 5 A cBN sintered compacts produced by the reactive sintering process and having a three dimensional size of 2.5x2.5x1.5 mm and a W-10 % Cu alloy to be used as a shank material were employed. A layer of Ti was sputtered to a thickness of approximately 0.3 µm over the bonding face of the cBN sintered compact, followed by the sputtering of a Ni layer to a thickness of approximately 0.1 µm over the same bonding face. The cBN sintered compact and the shank alloy were put together with a 2.5x2.5x0.1 mm foil of aluminum being interposed between them. The cBN compact and the shank alloy were next placed within the vacuum furnace where a vacuum of 0.133 Pa (10⁻³ torr) was created by a vacuum pump and the temperature was gradually elevated up to 700°C at the rate of 20°C per minute. The cBN compact and the shank alloy were exposed to an elevated temperature of 700°C for a 10 minutes and then cool down. The resulting structure is illustrated in Fig. 1 and showed that the cBN sintered compact 1 and the body 2 of the W-10 % Cu alloy were firmly joined together with the boding strength of 127 MPa (13 kg/mm²). The cBN sintered compact were found to have undergone no deterioration. Its hardness and thermal conductivity were substantially the same as before the heat treatment. Example 6 A cBN sintered compact and a shank alloy of WC-5 % Co similar to those used in Example 5 were employed. The bonding face of the cBN compact was sputter-coated with a 0.1 µm layer of Ti and then with a 0.06 µm layer of Ni. The cBN compact and the shank alloy were put together with a 2.5x2.5x0.1 mm foil of an Al-Ni alloy being interposed between them. The Al-Ni alloy contained 96 wt.% aluminum and 4 wt.% nickel. The cBN compact and the shank alloy were then heated at 680°C in a vacuum of 0.133 Pa (10⁻³ torr) for 10 minutes in much the same way as in the Example 5. After the heat treatment, the cBN sintered compact and the shank alloy were firmly joined together with the bonding strength of 127 MPa (13 kg/mm²) which was comparable to that obtained in the Example 5. No deterioration was witnessed in the cBN sintered compact.

Claims: A method for bonding a cubic boron nitride sintered compact comprising the steps of: preparing a pair of cubic boron nitride sintered compacts, each having a bonding surface; forming a Ti layer of 0.01-1 µm in thickness over said bonding surface of each of said cubic boron nitride sintered compacts; forming a Ni layer of 0.01-5 µm in thickness over each of said Ti layers; holding said cubic boron nitride sintered compacts together with a metal foil of 10-1,000 µm in thickness being interposed between said bonding surfaces of said sintered compacts, said metal foil being made of a material selected from the group consisting of aluminum and Al-Ni alloy; and heating said sintered compacts to a temperature above the melting point of said metal foil and not exceeding 750°C in an inert atmosphere or in a vacuum. A method for bonding a cubic boron nitride sintered compact according to claim 1 wherein said Al-Ni alloy comprises 90 % by weight or more of Al and the balance of Ni. A method for bonding a cubic boron nitride sintered compact according to claim 2 wherein said cubic boron nitride sintered compact is heat-resistant up to a temperature of 750°C. A method for bonding a cubic boron nitride sintered compact according to claim 3 wherein said cubic boron nitride sintered compact is produced by a reactive sintering process. A cubic boron nitride sintered compact structure produced in accordance with the method of claim 1. A method for bonding a cubic boron nitride sintered compact comprising the steps of: preparing a pair of cubic boron nitride sintered compacts, each having a bonding surface; forming a Ti layer of 0.01-1 µm in thickness over said bonding surface of each of said cubic boron nitride sintered compacts; forming a Cu layer of 0.01-5 µm in thickness over each of said Ti layers; holding together said cubic boron nitride sintered compacts with a metal foil of Ag-Cu-In alloy having a thickness of 10-1,000 µm interposed between said bonding surfaces of said cubic boron nitride sintered compacts; and heating said cubic boron nitride sintered compacts to a temperature above the melting point of said Ag-Cu-In alloy and not exceeding 750°C in an inert atmosphere or in a vacuum. A method for bonding a cubic boron nitride sintered compact according to claim 6 wherein said Ag-Cu-In alloy is a ternary alloy comprising 15-25 % by weight of In, 25-35 % by weight of Cu and the balance of Ag. A method for bonding a cubic boron nitride sintered compact according to claim 7 wherein said cubic boron nitride sintered compact is heat resistant up to a temperature of 750°C. A method for bonding a cubic boron nitride sintered compact according to claim 8 wherein said cubic boron nitride sintered compacts are produced by a reactive sintering process. A cubic boron nitride sintered compact produced in accordance with the method of claim 6. A method for bonding a cubic boron nitride sintered compacts comprising the steps of: preparing a cubic boron nitride sintered compact having a bonding surface and a metallic shank; forming a Ti layer of 0.01-1 µm in thickness over said bonding surface of said cubic boron nitride sintered compact; forming a Ni layer of 0.01-5 µm in thickness over said Ti layer; holding together said cubic boron nitride sintered compact and said metallic shank with a metal foil of Al-Ni alloy having a thickness of 10-1,000 µm being interposed between said cubic boron nitride sintered compact and said metallic shank, said metal foil being made of Al or Al-Ni alloy comprising 90 % by weight or more of Al and the balance of Ni; and heating said cubic boron nitride sintered compact and said metallic shank to a temperature above the melting point of said Al or said Al-Ni alloy and not exceeding 750°C in an inert atmosphere or in a vacuum. A method for bonding a cubic boron nitride sintered compact according to claim 11 wherein said shank material is a metal selected from a group of W alloy, Mo, W and cemented carbide. A method for bonding a cubic boron nitride sintered compact according to claim 12 wherein said cubic boron nitride sintered compact is heat-resistant up to a temperature of 750°C. A method for bonding a cubic boron nitride sintered compact according to claim 13 wherein said cubic boron nitride sintered compact is produced by a reactive sintering process. A cubic boron nitride sintered compact produced in accordance with the method of claim 11.

Inventors: KUMAZAWA, YOSHIAKI ITAMI WORKS, SATOH, KEIICHI ITAMI WORKS, SUMIYA, HITOSHI ITAMI WORKS, TSUJI, KAZUWO ITAMI WORKS, URAKAWA, NOBUO ITAMI WORKS
Applicants: SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1988
Main classification: C04B37
IPC codes: C04B37, B23K35