REACTION BONDING OF METALS TO NON-OXIDE CERAMICS

Abstract: A method for bonding a body of a carbide or nitride ceramic selected from the carbides and nitrides of Groups III and IV of the second and third periods of the Periodic Classification of the Elements, to a body of a metal selected from the Transition Series and having an atomic number of from 21 to 29 inclusive, 31 to 47 inclusive or 57 to 79 inclusive, characterised in that selected surfaces of the bodies are disposed in intimate abutting relationship and at least the abutting surfaces are heated to a temperature below the melting point of the lowest melting component of the system but sufficient to bring about the formation of a bond between the surfaces. The examples include bonding of carbide or nitride ceramic (2) to metal (1) in CO/CO2 or NH3, N, H, Ar atmosphere using an induction heated alumina tube furnace under pressure exerted by alumina rod (9).

Description: REACTION BONDING OF METALS TO NON-OXIDE CERAMICS" This invention relates to the reaction bonding of metals to refractory non-oxide ceramics, specifically the refractory carbides and nitrides. The term "reaction bonding" has been used to describe the direct bonding of metals to oxide ceramics by the method described in Australian Patent No. 452,651. As shown in that patent, metal and ceramic oxide bodies can be bonded together by placing the bodies together in an abutting relationship and heating at least the abutting surfaces to a temperature which is below the melting point of the lowest melting component (usually the metal) but sufficient to bring about a chemical reaction between the metal and ceramic oxide leading to the formation of the bond. Reaction bonding involving oxide ceramics can often be carried out in the ordinary atmosphere, but in some cases better results are achieved using air under reduced pressure or an inert atmosphere. Bonding is also assisted by the application of light clamping pressure to the bodies during heating to improve contact at the metal/oxide interface and/or by ensuring the surfaces to be bonded are flat within optical tolerances. The nature of the reaction between metals and oxide ceramics, which gives rise to the bond has been discussed in the literature. (See for example H.J. de Bruin, et al., Silicates Industriels (1981) 201 and 219. H.J. de Bruin, S.P.S. Badwal, and P. . Slattery Proceedings of the 4th International CIMTEC Conference (1979) St. Vincent, Italy). Patent No. 452,561 and the subsequent literature on reaction bonding involving oxide ceramics disclose that there are two types of bonds which may form. The so-called Type 1 bonds are formed typically between the noble metals and oxide ceramics and are characterised by a sharp discontinuity at the metal/oxide interface with no discernable intervening third phase. Type 2 bonds, which are formed between reactive metals and oxide ceramics are typified by a prominent third phase at the interface and there may be some discernable diffusion of the metal into the oxide. In the subsequent literature it was noted that type 1 bonds are commonly stronger than type 2 bonds. It has now been found that reaction bonding may also be achieved between metals and refractory carbide-based and nitride-based ceramics by methods described herein. The differences between type 1 and type 2 in the reaction bonding involving carbide and nitride ceramics are not obvious in terms of the presence or absence of a third phase. The need for this classification is therefore not of great significance other than distinguishing between bonds with noble metals (type 1, i.e., elements 45, 46, 47, and 77, 78, and 79 in the periodic classification of the elements) , and those with reactive metals (type 2) . In accordance with its broadest aspect, the present invention provides a method for bonding a body of a carbide or nitride ceramic (as defined below) to a body of a metal selected from the Transition Series (as defined below) , which method comprises disposing selected surfaces of the bodies in intimate abutting relationship and heating at least the abutting surfaces to a temperature below the melting point of the lowest melting component of the system but sufficient to bring about the formation of a bond between the surfaces. As used herein, the terms "carbide ceramic" and "nitride ceramic" refer respectively to refractory materials having as a major component a carbide or nitride of elements of groups III and IV of the second and third periods of the periodic classification of the elements, in particular the carbides and nitrides of boron, aluminium, and silicon. These materials may consist of the chemically pure carbide or nitride or they may contain other carbides or nitrides in chemical combination, solid solution, and/or physical admixture, for example mixed carbides or nitrides or mixed carbide/nitride alloys, or mixed carbide/nitride/oxide alloys. Such alloys may include minor additions of metal compounds other than those in Groups III and IV of the second and third periods of the Periodic Classification of the Elements. Many such combinations as well as other individual carbides and nitrides are described in the literature, as summarized for example in the relevant section of irk-Othmer "Encyclopedia of Chemical Technology". Commercial carbide and nitride ceramics contain additives such as yttria (Y_,0_) and/or alumina (Al-,0-) to improve densification and their mechanical properties. Furthermore proprietary modifications to commercial carbide- and nitride-based ceramics are made from time to time to improve fracture toughness, lightweight and resistance to erosion. For example carbide and nitride ceramics used in diesel engines and other internal combustion engines, and in turbochargers. Such modified carbide and nitride ceramics are included in the above definition for the purpose of these- specifications. The metals concerned are broadly those of the so-called "Transition Series" of the Periodic Classification of the Elements. For the purposes of these specifications the elements concerned are those having atomic numbers between 21 and 29 inclusive in the first transition series: between 39 and 47 inclusive in the second transition series; and between 57 and 79 inclusive in the third transition series. Preferred metals in the method of the invention are those of Groups lb, Illb and VIII of the Periodic Classification, that is the metals copper (Cu) , -silver (Ag) and gold (Au) , aluminium (Al) , iron (Fe) , cobalt (Co) , nickel (Ni) , ruthenium (Ru) , rhodium (Rh) , palladium (Pd) , iridium (Ir) , platinum (Pt) . Metal alloys are of particular significance in the present teachings, and are included in the definition of metals as used herein. Examples are ferrous alloys such as carbon steels, series 300, 400, and 500 stainless steels, cobalt, nickel, and chromium alloys and special alloys such as kovar and invar. In reaction bonding between carbide and nitride ceramics and metals, the composition of the bonding atmosphere may vary from air at normal or reduced pressures, inert atmospheres such as nitrogen and argon, to mildly reducing conditions such as added hydrogen or cracked ammonia. It is usually necessary to provide an atmosphere which is compatible with the carbide or nitride ceramic used, that is to say an atmosphere which does not react adversely with the carbide or nitride ceramic (and incidentally the metal) . Particularly suitable atmospheres are carbon monoxide and/or dioxide for reaction bonding of carbide ceramics, and cracked ammonia, nitrogen or argon for nitride ceramics. Some reactive metals may require inert gases with small additions of hydrogen or cracked ammonia. Reaction bonding of carbide and nitride ceramics involves complex reactions at the ceramic-metal interface in which reaction products related to all original components present can be identified. This is illustrated by reference to the bonded couple, cited in Example 6 below, namely between kovar (composition: 53.5% Fe, 29% Ni, 17% Co, 0.3% Mn, and .2% Si) and a commercial silicon nitride with an yttria binder (2%) . Electron micrographic investigation shows high structural integrity of the bonded interface. Dispersive analysis across the interface shows that compositional changes of the various elements vary widely in this region as shown in the following table: TABLE 1 Variation of elemental composition within 3μm from the bonded interface between kovar and commercial silicon nitride. Element Concentration Originally present in: range Yttrium 2 - 46% silicon nitride Iron 5 - 54% kovar Cobalt 0 .3 - 17% kovar Nickel 0 .6 - 29% kovar Silicon 0 .5 - 60% silicon nitride Manganese 0.3 - 5.6! kovar Carbide and nitride ceramics are generally covalent compounds. The chemical reactions are therefore fundamentally different from those involved in bonding oxide ceramics. This is a further reason why reference to type 1 and type 2 bonds is only nominally significant in the present teachings. The thickness of a third phase in both types of bonds is commonly less than 3μm. In contrast to reaction bonding with oxide ceramics, the bond strength for couples between reactive metals and carbide or nitride ceramics is usually much greater than those involving noble metals. It is important to note that the reaction between, and the concomitant bonding of the metal and the refractory material, occurs at temperatures below the melting point of any component of the system. Melting of the metal is inimical to the method as the liquid phases of the metals concerned generally do not wet the surfaces of refractory materials. The bonds formed by the method of the invention when utilizing the so-called "noble" metals, such as platinum, palladium, silver and gold, are less readily distinguished from other types of bonding by microscopic or other examination of sections through the bond. The metal-refractory interface is quite sharply defined for all metals including the reactive metals. In the absence of ionic bonding in the carbide and nitride ceramics, some diffusion of noble metal into the ceramic component is now feasible, thereby blurring the interface somewhat. This is in contrast with the type 1 bond in the case of the oxide ceramics. There are no indications of glassy intermediate phases. In the case of the reactive metals, e.g., Fe, Co, Ni, or their alloys, there is still a relatively sharp discontinuity at the metal-ceramic interface. However the intermediate phase that is formed is only 3 - 4 μm thick (up to 50 μm in the case of some oxide ceramics) . It is believed to be substantially covalent with only a limited amount (less than 5%) of ionic material due to the presence of oxide binders. Utilizing the method of the invention, massive bodies of the metals concerned may be thus bonded to carbide or nitride ceramic bodies. Metal foils or layers of the metal in powder form may be applied and sintered in situ. Metals which themselves do not reaction bond satisfactorily to a carbide or nitride ceramic body may be bonded by first applying a layer of one of the selected metals which do reaction bond to the unreactive metal by standard metallurgical techniques such as soldering, brazing, welding, sintering, flame spraying or the like, or by electrolytic deposition, and then bonding the layer to the carbide or nitride ceramic body by the method of the invention. Under some circumstances this type of refractory-metal-metal composite structure may be produced in one operation by simply heating an assembly of the three components to bring about the essential bonding processes. Similarly, a metal-ceramic composite, produced by the method of the invention, may be bonded to a further refractory body (which may be the same as or different to the first) by abutting a surface of the second ceramic body to the metal part of the composite and heating the structure according to the method of the invention. The method of the invention also extends to the bonding of bodies of identical or dissimilar carbide and/or nitride ceramic by simultaneous reaction of a metal layer with both bodies. The minimum temperature for the bonding reaction cannot be specified precisely, however to achieve acceptable reaction rates it is common to approach, but not' exceed the melting point of the metals employed. For practical purposes, the usual working maximum is set at about 100°C below the melting point unless a destructive phase transformation dictates a lower bonding temperature. No advantage appears to be gained by using temperatures above this maximum. In most cases the bonding reaction proceeds at a satisfactory rate and good bonds are formed well below this maximum working temperature. In carrying out the method of the invention the selected surfaces of the refractory (and metal) bodies to be joined should normally be smooth and finished to match each other closely. Although slight surface imperfections or mismatching as well as differences in thermal expansion for dissimilar bodies may be accommodated by using ductile metals such as platinum or gold, best results are obtained with surfaces finished to optical flatness, i.e., within 2 or 3 interference fringes of sodium light. A light clamping pressure is usually applied to the components during bonding to ensure close contact between the surfaces. For optically flat surfaces pressures of about 100 kPa are suitable. Stress induced creep may need to be invoked to encourage bonding between poorly prepared surfaces. Pressure up to 1 MPa have been used to bond 304 stainless steel to silicon nitride. Heating may be carried out with any suitable means, for example, a furnace such as a tubular resistance furnace, a high frequency induction heating device, or a plasma torch or furnace, or laser, or direct flame. The time required for the bonding process depends on the choice of the refractory material involved, as well as the bonding metal and the temperature. The time may vary from a few minutes up to several hours. Bonding is usually complete within one to five hours at temperatures above 1100°C. The method of the invention can be used in any applications where bonds are required, between metals and carbide or nitride ceramics, especially where high strength of the bond is required. Carbide and nitride ceramics have been developed for entirely different applications than oxide ceramics. They are particularly suitable as lightweight, tough and hard structural materials with considerable erosion resistance. Applications of reaction bonding carbide and nitride ceramics therefore mostly involve the welding of massive bodies of metals and particularly alloys to ceramic bodies. However, as shown in Examples 1 and 2, the insulating type applications in electronic and scientific equipment typical for oxide ceramics using (noble) metal foils are feasible, but uncommon. Obvious applications include those in the automotive industries such as turbochargers consisting of metal shafts and ceramic turbo blades, ceramic cylinder sleeves and composite metal/ceramic pistons, the production of abrasive and cutting tools using carbides and nitrides bonded to metallic substrates. The ease of formation of the bonds and their stability also suggest their use in the fabrication of fuel elements and other components in nuclear technology. The thermal insulating properties and high melting points of the refractory materials in the present invention further suggest its application in the bonding of protective ceramic tiles to the outside of reentry space vehicles. The invention also includes bonded composites of metals and carbide and nitride ceramics when produced by the method of the invention. The formation and characterization of metal-refractory bonds in accordance with the invention is illustrated but not limited by the following examples. Reference will also be made to the accompanying drawings, in which: Figure 1 shows the apparatus used in the experiments described herein; Figures 2 - 7 show sections of bonded specimens obtained by the method of the invention. In Figure 1 a cylinder of the metal (1) and a cylinder of the carbide or nitride ceramic (2) to be bonded are placed on an alumina stool (3) inside a tube furnace. The furnace comprises an outer alumina tube (4) which is sealed at the top and bottom with brass assemblies (5 and 6 respectively) to allow compositional control of the atmosphere within the furnace on the basis of compatibility with the metal and ceramic components under the bonding conditions. Assembly 6 is provided with gas inlet means (7) . A clamping pressure is applied to the couple (1 and 2) via an alumina 5 pressure transmission rod (9) , guided to transmit pressure to the center of the upper surface of the couple to be bonded via an alumina guide (10) , which is held in position by the inner alumina tube (11) . The brass pressurehead assembly (12) is provided with gas ■i_0 outlet means (8) and seals the pressure transmission rod (9) without interfering with the applied clamping pressure. The outer alumina tube (4) is either wound with platinum or nickel alloy wire (13) (e.g. Kanthal Al, Nichrome or similar) as shown in the drawing, or -, c placed in the center of a silicon carbide rod assembly (not shown) . The furnace is insulated inside a furnace casing (14) . The potential generated by the thermocouple (15) is used to sense and control the temperature of the bonding process. 20 Example 1 Using the apparatus of Figure 1, two discs (21) of silicon nitride (Si-.N.) 25 mm diameter and 5mm thick and 25 lapped flat to within two interference fringes of sodium light, were reaction bonded with nickel foil (22), 0.1mm thick, sandwiched between them, as shown in Figure 2. The components had been ultrasonically cleaned prior to assembly. The couple was heated at 1150°C for 3 hours 30 under a clamping pressure of lOOkPa in an atmosphere of nitrogen. Exartiple 2 Two cylinders (31) of silicon carbide (SiC) , 15mm diameter and 15mm thick were reaction bonded with gold foil (32), 0.1mm thick, sandwiched between them as shown in Figure 3. The couple was heated in the apparatus of Figure 1 at 975°C for 5 hours in an atmosphere of air. The bond was formed under a clamping pressure of 100 kPa. As in the previous example the components had been lapped flat and ultrasonically cleaned prior to bonding. Example 3 A tube (41) of 310 stainless steel 5mm inside diameter was reaction bonded to a rod (42) of silicon carbide 5.02mm diameter and 50mm long at 1250°C in an atmosphere of carbon dioxide for a period of 3 hours. The assembly is shown in Figure 4. Example 4 A tube (51) of copper 8mm outside diameter was reaction bonded to a tube (52) of silicon carbide 8mm outside diameter and 50mm long at 1000°C in an atmosphere of carbon dioxide (4 hours) . The assembly is shown in Figure 5. The conical joint (53) is preferred in vacuum tube applications, where it increases the surface area of contact between the metal and ceramic components and is used in electron optical equipment, mass spectrometers, and other scientific equipment to insulate vacuum lines running into operating chambers. Example 5 A rod of carbon steel (61) was bonded to a silicon nitride rod (62) as shown in Figure 6. The steel had been gold plated at the conical joint (thickness of gold layer = 0.05mm). The assembly was placed in a furnace similar to Figure 1 and heated for 4 hours at 975°C in an atmosphere of N,-/10%H- while pressure of 500 kPa was applied via the alumina pressure transmission rod (9 in Figure 1) . This configuration was developed specifically for the construction of turbochargers consisting of silicon carbide (or nitride) turbo fans bonded to a carbon steel shaft. A reaction bond was formed between the gold layer and the silicon nitride. A thermal diffusion bond was made in the process between the gold and the steel. Example 6 Eighteen identical specimens, as shown in Figure 7 were bonded as follows. Cylinders of silicon nitride (71) and kovar (72) each 20mm long and 15mm diameter were bonded in the furnace of Figure 1, under a clamping pressure of 245 kPa, in an atmosphere of nitrogen containing 10% hydrogen for 3 hours. The surfaces had been lapped flat to within 4 interference fringes of sodium light with a diamond lapping wheel of 1000 mesh. The bonded specimens were subjected to 4-point bend tests to determine their modulus of rupture. Each bonded couple was supported by bearings (73) and (74) and compressed at bearings (75) and (76) . The modulus of rupture was calculated from the applied force, and varied between 26.4 and 186 MPa. A low magnification (14X) electron micrograph of a non-tested couple was obtained by cutting and polishing it across plane ABCD in Figure 7 showed a high structural integrity of the bonded materials.

Claims: CLAIMS 1. A method for bonding a body of a carbide or nitride ceramic selected from the carbides and nitrides of Groups III and IV of the second and third periods of the Periodic Classification of the Elements, to a body of a metal selected from the Transition Series and having an atomic number of from 21 to 29 inclusive, 31 to 47 inclusive or 57 to 79 inclusive, characterised in that selected surfaces of the bodies are disposed in intimate abutting relationship and at least the abutting surfaces are heated to a temperature below the melting point of the lowest melting component of the system but sufficient to bring about the formation of a bond between the surfaces. 2. A method as claimed in Claim 1, characterised in that the ceramic is a carbide or nitride of boron, aluminium or silicon. 3. A method as claimed in Claim 1 or Claim 2, characterised in that the carbide or nitride ceramic contains other carbides or nitrides, in chemical combination and/or solid solution and/or physical admixture. 4. A method as claimed in any one of Claims 1 to 3, characterised in ' that the carbide or nitride ceramic contains non-carbide or non-nitride additives. 5. A method as claimed in any one of Claims 1 to 4, characterised in that the metal is a member of Groups lb, Illb or VIII of the Periodic Classification or an alloy of any such metal. 6. A method as claimed in Claim 5, characterised in that the metal is copper (Cu) , silver (Ag) and gold (Au) , aluminium (Al) , iron (Fe) , cobalt (Co) , nickel (Ni) , ruthenium (Ru) , rhodium (Rh) , palladium (Pd) , iridium (Ir) or platinum (Pt) , or an alloy of any of said metals. 7. A method as claimed in any one of Claims 1 to 6, wherein the ceramic is a carbide, characterised in that the bonding is carried out in an atmosphere of carbon monoxide and/or carbon dioxide. 8. A method as claimed in any one of Claims 1 to 6, wherein the ceramic is a nitride and the bonding is carried out in an atmosphere of cracked ammonia, nitrogen, hydrogen, argon, or mixtures containing any of these gases. 9. A method as claimed in any one of Claims 1 to 8, wherein the bonding temperature is at least 100°C below the melting point of the lowest melting component of the system, or below the temperature at which any destructive phase transformation of any component occurs. 10. A bonded composite comprising a body of a carbide or nitride ceramic selected from the carbides and nitrides of Groups III and IV of the second and third periods of the Periodic Classification of the Elements, bonded to a body of a metal selected from the Transition Series and having an atomic number of from 21 to 29 inclusive, 31 to 47 inclusive or 57 to 79 inclusive.

Inventors: DE BRUIN HENDERIKUS JOHANNES, DE BRUIN, HENDERIKUS, JOHANNES
Applicants: REACTION BONDING PTY LTD, REACTION BONDING PTY. LTD.
Application year: 1985
Main classification: C04B37
IPC codes: C04B37