JOINING OF CVD DIAMOND BODIES TO METAL STRUCTURES

Abstract: The invention is concerned with a method of forming a joint between a CVD (chemical vapour deposition) diamond and a metal support structure, and with a joint formed by the method. In the method, a CVD diamond body (14) is bonded to a ceramic body (16) having thermal expansion characteristics compatible with those of CVD diamond. The ceramic body (16) is in turn bonded to a dimensionally compliant, intermediate metal element (18). The metal element is then secured to the metal supporting structure (12). Optimally, the intermediate metal element has thermal expansion characteristics between those of the ceramic body and the metal supporting structure. In one application, the joint may be used to mount the window in a gyrotron viewport flange.

Description: JOINING OF CVD DIAMOND BODIES TO METAL STRUCTURES BACKGROUND TO THE INVENTION THIS invention relates to the joining of a CVD diamond body, i.e. a diamond body formed by chemical vapour deposition, to a metal supporting structure. In one application the invention relates to the joining of a CVD diamond window to a metal gyrotron viewport flange structure. A requirement in high power vacuum RF sources such as gyrotrons which provide up to 1 MW power in the millimetre wave band is for windows or viewports which are transparent to the radiation and able to maintain high vacuum conditions in the tube. High power gyrotrons of this type are predominantly used in nuclear fusion plasma research. In such applications the windows must be able to withstand pressure differentials of up to five atmospheres and to contain highly radioactive gases such as tritium. In each case the attachment of the window to its supporting metal structure must be by way of a high strength, UHV compatible and impervious bond. A problem in this regard is that, as part of the window/tube conditioning, a high temperature bake-out treatment is made in an effort to drive off adsorbed gas from the internal surfaces. Out-gassing occurs faster at high bake-out temperatures exceeding 500°C. After bake-out the window must maintain a leak-up rate less than 10 "9 mbar.l.s "1 to remain effective, and it is essential that the high temperature treatment does not cause undue degradation to the seal. These requirements are made particularly problematical in the case of a CVD diamond/metal joint because of the thermal mismatch between them, CVD diamond having a coefficient of thermal expansion of approximately 1 χ10 "6 K "1 with that of metals generally exceeding 5χ10 "6 K "1 . In an effort to achieve a satisfactory joint an experimental attempt has been made to join a 50mm diameter CVD diamond window directly to an Inconel™ cuff using an active gold braze. The attempt was however unsuccessful because the large thermal mismatch between the CVD diamond and the Inconel™ placed the window, on cooling after the brazing operation, in a state of high compressive stress. The gold braze allowed only limited compliance resulting, after thermal cycling to 500°C, in an inability of the window to withstand applied vacuum. In another experiment, a 100mm diameter CVD diamond window was bonded to an Inconel™ cuff using a compliant, essentially pure aluminium bond. Although the residual stresses in the window were reduced compared to the experiment mentioned above because of the lower bond temperature and plastic deformability of the aluminium bond, this attempt also proved to be unsuccessful, once again because of the large thermal mismatch between the CVD diamond and the Inconel.™ In this case, after thermal cycling to 500°C, the window was observed to separate from the Inconel™. SUMMARY OF THE INVENTION According to the present invention there is provided a method of joining a CVD diamond body to a metal supporting structure comprising the steps of bonding the CVD diamond body to a ceramic body having thermal expansion characteristics compatible with those of CVD diamond, bonding the ceramic body to a dimensionally compliant, intermediate metal element, and securing the intermediate metal element to the metal supporting structure. In the preferred method, the intermediate metal element has thermal expansion characteristics between those of the ceramic body and the metal supporting structure. The method may, in one application of the invention, be used to join a CVD diamond window to the cylindrical metal structure, possibly an Inconel™ cuff, in the flange of a gyrotron viewport. In this application, the method includes the steps of bonding, to a CVD diamond window at or towards the periphery thereof, a ring of ceramic material having thermal expansion characteristics compatible with those of CVD diamond, and bonding a ferrule to the ceramic ring and to the metal structure, the ferrule being of metal and having thermal expansion characteristics between those of the ceramic ring and the metal structure. The bonds between the CVD diamond and the ceramic and between the ceramic and the metal ferrule may be aluminium-based diffusion bonds. If the thermal expansion match between the CVD diamond and the ceramic ring is very close it is possible to join the two components using a high temperature braze before joining the ceramic ring to the metal ferrule using a lower temperature braze. The bond between the tantalum ferrule and the cuff may be provided by a high temperature braze. According to another aspect of the invention there is provided a CVD diamond to metal joint comprising a CVD diamond body, a ceramic body to which the CVD diamond body is bonded and which has thermal expansion characteristics compatible with those of CVD diamond, an intermediate, dimensionally compliant metal element to which the ceramic body is bonded, and a metal supporting structure to which the intermediate metal element is bonded. As indicated previously, the intermediate metal element preferably has thermal expansion characteristics between those of the intermediate body and the metal supporting structureThe joint may form part of the flange of a gyrotron viewport and the invention extends to such flange. The ceramic body may be of fused silica or any other suitable ceramic material having a sufficiently low coefficient of thermal expansion for thermal compatibility with CVD diamond, such as silicon nitride, boron nitride or the like. The ceramic body may itself be of diamond. The intermediate metal element may be of tantalum or other suitable metal having good dimensional compliance and thermal expansion characteristics between those of the ceramic material and the metal supporting structure, typical examples being copper, gold, platinum or the like. It is preferred that the ceramic body has a planar surface which is bonded to the CVD diamond body and a peripherally outer surface including a cylindrical region extending transversely to the planar surface and a conically tapered region, the metal element having a first portion which is complementally conically shaped and bonded to the conically tapered region and a second portion which extends alongside but is unconnected to the cylindrical region, the second portion being bonded to the metal supporting structure. There may be an annular gap between the second portion and the cylindrical region. The invention also extends to a method and joint in which opposite surfaces of a CVD diamond window are bonded to respective ceramic bodies, the ceramic bodies are bonded to respective, intermediate metal elements and each metal element is bonded to a metal support structure. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows a diagrammatic, cross-sectional view of a joint according to the invention; Figure 2 shows an enlarged detail of the zone 2 in Figure 1 ; and Figure 3 shows a double-sided joint according to the invention. DETAILED DESCRIPTION OF THE DRAWINGS Figure 1 shows a joint 10 according to this invention and diagrammatically illustrates relevant parts of the flange of a gyrotron viewport, also according to the invention. In this Figure the numeral 12 indicates a cylindrical Inconel™ cuff which is, in the assembled viewport flange, mounted within the bore of the flange. The remainder of the flange is not shown. Also shown in Figure 1 is a circular CVD diamond window 14 which may, for instance, have a diameter of up to 200mm and a thickness which typically does not exceed 2mm. As illustrated, the diameter of the window 14 is slightly greater than that of the cuff 12 and is located a short distance beneath the lower extremity of the cuff. A ceramic body 16, in this case an annular fused silica body of 10mm thickness, is bonded to a major surface of the window 14. The fused silica body 16 has an outer peripheral surface 18 composed of two regions 18.1 and 18.2. The region 18.2 is cylindrical in shape and extends normally to the planar surfaces of the window 14, while the region 18.1 is conically tapered as shown. The body 16 is bonded to the window 14 by means of an aluminium-based diffusion bond. This is achieved by diffusion bonding under pressure and a temperature which is typically less than 550°C. The joint 10 also includes an intermediate metal element, in this case in the form of a tantalum ferrule 20, which has a conically shaped portion 20.1 and a cylindrical portion 20.2. The portion 20.1 extends over and is bonded to the conically tapered surface region 18.1 of the fused silica body 16. This is once again typically achieved by means of an aluminium-based diffusion bond. The portion 20.2 extends alongside but is unconnected to the surface region 18.2 of the body 16 and is brazed by means of a high temperature braze to the lower extremity of the cuff 12. A small annular gap 22 is present between the portion 20.2 and the surface region 18.2. The ferrule 20 is prepared from tantalum by metal spinning to produced a thin-walled structure having a high degree of dimensional compliance. The coefficient of thermal expansion of tantalum is approximately 6*10 "6 K '1 , which is between that of the fused silica of the body 16 and the Inconel™ cuff. This, together with the physically compliant nature of the ferrule allows the fused silica body and the cuff to expand at different rates while limiting the build-up of stress between them. The complementary relationship between the profiles of the surface region 18.1 of the body 16 and of the ferrule portion 20.1 is advantageous in that with this geometry the body 16 and the ferrule can be bonded to one another under axial load which provides compression at the conical interface. This ensures that any residual stress components are not purely radial shear stresses which would tend to separate the components and also achieving a measure of mechanical keying between the components. The complemental mating of the ferrule and body 16 also allows for the use of the aforementioned aluminium-based diffusion bond to secure the components to one another. The use of a bond of this nature, requiring a temperature generally less than 550°C, is advantageous compared to high temperature gold or silver based brazes which require temperatures generally exceeding 750°C. Furthermore the use of an aluminium based diffusion bond takes place under pressure thereby reducing the chance of uneven melting and also reducing stresses. The aluminium based bond which is formed anneals during the subsequent bake-out treatment which is used in gyrotron preparation, and is therefore not unduly prone to hardening through thermal ageing or repeated plastic deformation. The CVD diamond window 14 and the fused silica body 16 have similar thermal expansion characteristics, at least up to 600°C, and so are thermally compatible with one another. It is accordingly possible to use an aluminium based diffusion bond to secure these components to one another as well, with the attendant advantages of such a bond as discussed above. An added advantage of the illustrated structure is that it allows the peripheral edge of the CVD diamond window 14 to project beyond the extremity of the cuff 12. In the context of a high power gyrotron, this allows the window to project into a cooling water channel for direct cooling purposes. From the above, it will be understood that the provision of the thermally compatible body 16 and the ferrule 20 reduces the thermal mismatch between the CVD diamond window 14 and the metal cuff 12. It is anticipated that the illustrated joint will be able to withstand sustained temperature cycling to 550°C without undue degradation and while still maintaining the integrity of the vacuum seal which is obtained and which is critical for a high power gyrotron application. It is envisaged that vacuum leak-up rates below 10 "9 mBar.ls "1 can be maintained with the illustrated joint. Figure 3 illustrates a double-sided joint according to the invention. In this case, the arrangement of cuff 12, body 16 and ferrule 20 is duplicated on both major surfaces of the CVD diamond window 14. An arrangement of this type is suitable when both sides of the window or viewport have special atmosphere or vacuum containment requirements as, for instance, in nuclear research reactors where one side of the window or viewport has to contain radioactive tritium at pressures possibly as high as 5 atmospheres while the other side is connected to a waveguide containing, for example, dry nitrogen. Numerous variations are possible within the scope of the invention. For instance, as mentioned previously, other suitable ceramic materials could be used for the body 16 and other suitable metals could be used for the ferrule 20. Also, while a pure aluminium-based diffusion bond is preferred to secure the CVD diamond window to each body 16 and to secure each body to the ferrule 20, other bonds may be used. It is, for instance, envisaged that gold or silver-based alloys could also be used to form the bond. It was mentioned previously that if the thermal expansion match between the CVD diamond and the ceramic ring is very close it is possible to join the two components using a high temperature braze before joining the ceramic ring to the metal ferrule using a lower temperature braze. The advantages of this two-stage process are that the alignment of the components can be better controlled using simple mechanical fixtures and that the CVD diamond/ceramic interface is stronger and more creep resistant. It should also be noted that in appropriate cases a small step may be provided in the Inconel™ cuff to assist in the alignment of the metal ferrule during brazing. In each case it is preferred to maintain the joint under slight compressive load during temperature cycling, and to mount the window perfectly horizontally, to minimise the risk of separation of the bond. In the illustrated examples, the CVD diamond window 14 is planar. It will however be understood that the principles of the invention are equally applicable to CVD diamond windows of other shapes, such as hemispherical. It is also within the scope of the invention for the CVD diamond window to be mounted directly to the metal flange of the viewport rather than to a cuff in the flange. In either case, the gap 22 provides for free radial expansion of the ferrule during thermal cycling. The invention also envisages pre-treatment of the CVD diamond surfaces to which bonding is to take place, thereby to aid the bonding of the diffusion bond layer and provide greater adhesive strength and bond stability. Specific mention has been made of the use of the joint of the invention in a gyrotron viewport flange. Nevertheless it is within the scope of the invention for the joint to be used in other applications. For instance, with an optically flat CVD diamond window and an optically flat body 16 the invention could be used to provide a low distortion output coupler window for a high power laser.

Claims: CLAIMS 1. A method of joining a CVD diamond body to a metal supporting structure comprising the steps of bonding the CVD diamond body to a ceramic body having thermal expansion characteristics compatible with those of CVD diamond, bonding the ceramic body to a dimensionally compliant, intermediate metal element, and securing the intermediate metal element to the metal supporting structure. 2. A method according to claim 1 wherein the intermediate metal element has thermal expansion characteristics between those of the ceramic body and the metal supporting structure. 3. A method according to either one of the preceding claims wherein the bond between the CVD diamond body and the ceramic body is an aluminium- based diffusion bond. 4. A method according to any one of the preceding claims wherein the bond between the ceramic body and the intermediate metal element is an aluminium-based diffusion bond. 5. A method according to either one of claims 1 or 2 wherein the CVD diamond body and the ceramic body have similar coefficients of thermal expansion and the bond between them is formed by a high temperature braze. 6. A method according to claim 5 wherein the bond between the ceramic body and the intermediate metal element is formed by a low temperature braze. 7. A method according to any one of the preceding claims wherein the intermediate metal element is brazed to the metal supporting structure, ramie body is 8. A method according to any one of the preceding claims wherein the ceramic body is of fused silica, silicon nitride, boron nitride or diamond. 9. A method according to any one of the preceding claims wherein the intermediate metal element is of tantalum, copper, gold or platinum. 10. A method according to any one of the preceding claims wherein the CVD diamond body is a flat or curved diamond window. 11. A method according to any one of claims 1 to 9 wherein the metal supporting structure is cylindrical in shape. A method according to any one of claims 1 to 9 wherein the CVD diamond body is in the form of a round CVD diamond window, the metal supporting structure is in a form of a round cylinder, the ceramic body is annular in shape and the diamond window is bonded to it at or near the periphery of the window, and the ceramic body is bonded to an intermediate metal element in the form of an annular metal ferrule. 13. A method according to claim 12 wherein the ceramic body has a planar surface to which the CVD diamond body is bonded and a peripherally outer surface including a cylindrical region extending transversely to the planar surface and a conically shaped region, the metal ferrule having a first, complementally conically shaped portion to which the conically shaped region of the ceramic body is bonded and a second portion which extends alongside but is unconnected to the cylindrical region of the ceramic body, the second portion of the ferrule being bonded to the metal supporting structure. 14. A method according to claim 13 wherein the ceramic body is bonded to the ferrule with clearance between the cylindrical region of the ceramic body and the second portion of the ferrule. 15. A method according to any one of claims 12 to 14 wherein the metal supporting structure is a cylindrical cuff in a flange of a gyrotron viewport. 16. A method according to any one of claims 12 to 14 wherein the metal supporting structure is a flange of a gyrotron viewport. 17. A method according to any one of the preceding claims wherein opposite surfaces of a CVD diamond window are bonded to respective ceramic bodies, the ceramic bodies are bonded to respective, intermediate metal elements and each metal element is bonded to a metal support structure. 18. A CVD diamond to metal joint comprising a CVD diamond body, a ceramic body to which the CVD diamond body is bonded and which has thermal expansion characteristics compatible with those of CVD diamond, an intermediate, dimensionally compliant metal element to which the ceramic body is bonded, and a metal supporting structure to which the intermediate metal element is bonded. 19. A joint according to claim 18 wherein the intermediate metal element has thermal expansion characteristics between those of the intermediate body and the metal supporting structure. 20. A joint according to either one of claims 18 or 19 wherein the bond between the CVD diamond body and the ceramic body is an aluminium-based diffusion bond. 21. A joint according to any one of claims 18 to 20 wherein the bond between the ceramic body and the intermediate metal element is an aluminium- based diffusion bond. 22. A joint according to either one of claims 18 or 19 wherein the CVD diamond body and the ceramic body have similar coefficients of thermal expansion and the bond between them is formed by a high temperature braze. 23. A joint according to claim 22 wherein the bond between the ceramic body and the intermediate metal element is formed by a low temperature braze. 24. A joint according to any one of claims 18 to 23 wherein the intermediate metal element is brazed to the metal supporting structure. 25. A joint according to any one of claims 18 to 24 wherein the ceramic body is of fused silica, silicon nitride, boron nitride or diamond. 26. A joint according to any one of claims 18 to 25 wherein the intermediate metal element is of tantalum, copper, gold or platinum. 27. A joint according to any one of claims 18 to 26 wherein the CVD diamond body is a flat or curved diamond window. 28. A joint according to any one of claims 18 to 27 wherein the metal supporting structure is cylindrical in shape. 29. A joint according to any one of claims 18 to 28 wherein the CVD diamond body is in the form of a round CVD diamond window, the metal supporting structure is in a form of a round cylinder, the ceramic body is annular in shape and the diamond window is bonded to it at or near the periphery of the window, and the ceramic body is bonded to an intermediate metal element in the form of an annular metal ferrule. 30. A joint according to claim 29 wherein the ceramic body has a planar surface to which the CVD diamond body is bonded and a peripherally outer surface including a cylindrical region extending transversely to the planar surface and a conically shaped region, the metal ferrule having a first, complementaliy conically shaped portion to which the conically shaped region of the ceramic body is bonded and a second portion which extends alongside but is unconnected to the cylindrical region of the ceramic body, the second portion of the ferrule being bonded to the metal supporting structure. 31. A joint according to claim 30 wherein the ceramic body is bonded to the ferrule with clearance between the cylindrical region of the ceramic body and the second portion of the ferrule. 32. A joint according to any one of claims 29 to 31 wherein the metal supporting structure is a cylindrical cuff in a flange of a gyrotron viewport. 33. A joint according to any one of claims 29 to 31 wherein the metal supporting structure is a flange of a gyrotron viewport. 34. A joint according to any one of claims 18 to 33 wherein opposite surfaces of a CVD diamond window are bonded to respective ceramic bodies, the ceramic bodies are bonded to respective, intermediate metal elements and each metal element is bonded to a metal support structure. 35. A gyrotron viewport which includes a joint according to any one of claims 18 to 31 or 34. 36. A method of joining a CVD diamond body to a metal supporting structure substantially as herein described with reference to figures 1 and 2 or Figure 3 of the accompanying darwings. 37. A CVD diamond to metal joint substantially as herein described with reference to Figures 1 and 2 or Figure 3 of the accompanying drawings.

Inventors: BRANDON JOHN ROBERT, HANKS SIMON, SUSSMANN RICARDO SIMON, BRANDON, JOHN, ROBERT, HANKS, SIMON, SUSSMANN, RICARDO, SIMON
Applicants: BRANDON JOHN ROBERT, DE BEERS IND DIAMOND, HANKS SIMON, SUSSMANN RICARDO SIMON, BRANDON, JOHN, ROBERT, DE BEERS INDUSTRIAL DIAMONDS (PROPRIETARY) LIMITED, HANKS, SIMON, SUSSMANN, RICARDO, SIMON
Application year: 2000
Main classification: C04B37
IPC codes: B23K1, B23K20, C04B37, H01J25, H01P1