Sealing ring and method of forming a metal-to-metal seal

Abstract

A sealing gasket ring 301 between two opposing metal surfaces 303, 305 having a hollow expandable tube 311 and a metal jacket 315 around the expandable body. The sleeve 315 is a split tube having a di

Description

Technical Field The present invention relates to a sealing ring or gasket and a method of forming a metal-to-metal seal. In particular, but not exclusively, the present invention relates to a sealing ring for use in sealing together two sections of a vacuum chamber. Background Metal sealing rings are often used to provide a gas-tight or liquid-tight seal between two metal surfaces, such as between the flanged ports of two vacuum chambers that are being joined together. In some applications, the metal sealing ring is a flat, circular metal gasket made of a relatively soft metal such as a copper or aluminium, which is crushed between the metal surfaces or flanges such as “OF” flanges, which are generally made of a harder metal, such as stainless steel. OF flanges are each provided with a circular ridge or “knife-edge” which bites into respective faces of the gasket causing the metal of the gasket to be extruded around the knife-edges, thereby forming a seal between the two metal surfaces. Whilst flanges and metal gaskets can be used to form very effective seals, for example in ultrahigh vacuum systems, they suffer from a number of drawbacks, including the need for careful alignment (“mating”) of the flanges with the metal gasket and high clamping forces. The quality of the seal may also be severely reduced if the compression load on the gasket is uneven or too high. Metal surfaces typically contain microscopic asperities, i.e. irregularities in the height of the surface. When two hard metal surfaces are in contact, the asperities do not readily deform into each other, such that passages remain between the asperities through which fluids (e.g. gases or liquids) may be transmitted. Metal seals work on the principle of plastic deformation of a soft metal layer into the asperities of the hard metal surface, a process which may be termed “keying in”, such that passages between asperities are closed off or greatly reduced in size, thereby forming an effective seal. This process is disrupted if the soft metal surface is moved laterally over the hard metal surface. The sealing element (e.g. sealing ring) may also be damaged by such lateral movement such that it may not be possible to form an effective seal. Sealing surfaces are therefore generally brought into contact and compressed along a normal direction, i.e. a direction that is perpendicular to the surfaces, to allow correct engagement of the sealing surfaces and to prevent damage to the sealing element. In particular, ultrahigh vacuum (UHV) sealing technologies typically require the metal surfaces to engage one another in a direction that is perpendicular to their surfaces. Figure 1A shows a schematic cross section view of a metal sealing ring 101 that is deformed against an irregular metal surface 103 through a compressive force that is directed perpendicularly to the surface 103, which results in the sealing ring being “keyed in” to the irregularities of the metal surface 103. Figure 1B shows how the metal sealing ring fails to conform to the irregularities of the metal surface 103 when the compressive force comprises a component 107 directed transverse to the metal surface, thereby providing a plurality of potential leak paths 109 that may allow a fluid to be transmitted through the seal, i.e. between the sealing ring 101 and the metal surface 103. In some applications, it may not be possible for the surfaces to be brought together along a direction that is perpendicular to the surfaces because of geometrical constraints. For example, some vacuum chambers may be manufactured in angular segments (sectors) that need to be joined together, as may be the case for toroidal or spheroidal (e.g. spherical) vacuum chambers, e.g. where the segments are arranged like the segments of an orange. In such cases, it may not be possible to assemble the vacuum chamber without the segments having to move laterally with respect to one another, at least to some extent. Even when surfaces can be brought together along a perpendicular direction, lateral movement of the surfaces with respect to one another may still occur because of unbalanced forces being applied to the surfaces, e.g. unbalanced forces resulting from tightening bolts around the circumference of a flange one at a time. Figure 2 shows a segmented toroidal vacuum chamber 201, which is formed from 6 equiangular segments, of which only 3 segments 203, 205, 207 are shown in the figure. Sealing rings 209 are provided between the sealing faces 211 of the segments. During assembly of the vacuum chamber 201, the final segment 205 is installed along a radial installation direction, i.e. along a direction towards the central axis of the vacuum chamber 201. The relative motion of the final segment 205 with respect to each of the other two segments 203, 207 therefore comprises both a tangential component (i.e. a component that is perpendicular to the adjacent surfaces 209) and a radial component (i.e. a component that is parallel to the surfaces 209). The tangential component of the motion generates a compressive force that deforms (keys in) the sealing ring 209 to form the seal, whilst the radial component of the motion may reduce the extent to which the sealing ring 209 is able to deform into asperities on the metal surface (e.g. as described above with reference to Figure 1B), thereby reducing the effectiveness of the seal. The thickness of the metal sealing ring 209 is also limited by the clearance between the segments 203, 205, 207 during assembly of the vacuum chamber. WO2020254543 describes an inflatable a metal sealing ring for forming a metal-to-metal seal between two opposing metal surfaces. The metal sealing ring comprises a tubular metal body and an inlet tube extending from the body for introducing an internal pressure into the body. The body is adapted to deform under the internal pressure against each of the opposing metal surfaces to form the seal. Summary of the Invention The present disclosure seeks to overcome or at least alleviate the issues mentioned above with existing approaches to forming metal-to-metal seals. According to a first aspect of the present invention there is provided a sealing ring or gasket for forming a metal-to-metal seal between two opposing metal surfaces, the gasket comprising: an inflatable body in the form of a closed loop and having an internal volume enclosed by walls of the inflatable body, the internal volume comprising (i) an inlet through which to introduce fluid into the internal volume to inflate the inflatable body, or (ii) a substance for introducing fluid into the internal volume by a physical or chemical change of the substance to inflate the inflatable body; and a metal jacket extending around the closed loop and at least partly around an outer surface of the inflatable body, the metal jacket having a discontinuity that extends around the closed loop, wherein, on expansion, the inflatable body is configured to force apart opposing sections of the metal jacket adjacent the discontinuity to form a seal in use. According to a second aspect of the present invention there is provided a gasket or sealing ring for forming a metal-to-metal seal between two opposing metal surfaces. The sealing ring comprises an inflatable ring-shaped body having an internal volume enclosed by walls of the inflatable body, the internal volume comprising (i) an inlet through which to introduce fluid into the internal volume to inflate the body, or (ii) a substance for introducing fluid into the internal volume by a physical or chemical change of the substance to inflate the body, and a substantially tubular metal jacket wrapped or fitted around the ring-shaped body. The metal jacket has a discontinuity (e.g. a break or opening). In other words, the metal jacket comprises a split tube around the ring of the ring-shaped body. The discontinuity extends around an inner or outer circumference of the ring-shaped body (i.e. an inner or outer perimeter of the ring-shaped body). The ringshaped body is adapted to expand, on inflation, towards or into the discontinuity to force opposing sections of the metal jacket adjacent the discontinuity against the metal surfaces to form the seal. Embodiments of the present invention provide a gasket having a body that can be expanded or inflated under an internal pressure to force a surrounding metal jacket against first and second metal surfaces to create a seal between the surfaces. Because the body is expanded in use, the body and the metal jacket can be contained entirely within a groove in the first surface such that the second surface can be brought into place against the first surface from any direction without contacting and damaging the gasket. Expansion of the body then forces open the metal jacket around the whole of the closed loop to create the seal. The discontinuity may allow a greater proportion of a force that is used to inflate the expandable body to go into deforming the metal jacket against the metal surfaces (as compared to a metal jacket that lacks such a discontinuity). Less metal may also be required to make the metal jacket in some cases. Respective contact areas between the metal jacket and the metal surfaces may be increased by expansion of the expandable body towards or into the discontinuity. For example, portions of the cross section of the metal jacket adjacent the discontinuity that initially curve away from the metal surfaces may be flattened or straightened against the metal surfaces as the expandable body expands to increase the contact area between the metal jacket and each of the metal surfaces. Increasing the contact areas may allow more effective (e.g. lower leak rate) and / or robust seals to be formed. After the metal-to-metal seal has been formed, the metal jacket provides an impermeable barrier preventing fluid (e.g. gas or liquid) from passing into or out of a central region enclosed by the inflatable body. The metal jacket extends continuously around the inflatable body. That is, the metal jacket does not have any discontinuities (e.g. breaks or openings) other than the discontinuity that extends around the loop of the inflatable body. Typically, the discontinuity in the metal jacket extends around an outer circumference (i.e. the radially outermost perimeter) of the inflatable body, such that the metal jacket extends between the inflatable body and a central region encircled or enclosed by the inflatable body. Alternatively, in some implementations, the discontinuity extends around an inner circumference of the inflatable body, such that the metal jacket extends between the inflatable body and a region radially outside the inflatable body. The inflatable body may be adapted to expand axially on inflation towards the opposing metal surfaces before or during expansion of the inflatable body towards or into the discontinuity of the metal jacket. For example, the inflatable body may be adapted to expand along an axial direction defined with respect to a central axis of the sealing ring (with the expansion of the sealing ring towards or into the discontinuity being along a radial direction with respect to the central axis). The inflatable body may be adapted such that the amount of axial expansion exceeds the amount of radial expansion at least prior to the metal jacket contacting the opposing metal surfaces. At least prior to inflation, the outer surface of the inflatable body may have a radial cross section (defined with respect to the central axis of the sealing ring) that comprises a concave portion facing the discontinuity of the metal jacket. In some implementations, the radial cross section of the outer surface of the inflatable body may be C-shaped. Preferably, the metal jacket contacts the inflatable body around an outer or inner circumference of the inflatable body opposite the discontinuity. For example, the discontinuity may extend around an outer circumference of the inflatable body, with the metal jacket contacting an inner circumference of the inflatable body. That is, the discontinuity may be located radially outwards with respect to where the metal jacket contacts the inflatable body prior to inflation. In some implementations, the metal jacket comprises one or more outwardly directed ridges (i.e. protrusions formed on an exterior surface of the metal jacket) for deforming against the opposing metal surfaces upon expansion of the inflatable body to form the seal. Each ridge is preferably provided on a respective one of the opposing sections of the metal jacket. Expanding the inflatable body may force one or more ridges provided on the exterior of the metal jacket into contact with the second metal surface and / or the sealing groove. The edge(s) may provide increased contact pressure between the metal jacket and the second metal surface and / or the sealing groove. In some implementations, the ridges(s) may be plastically deformable. The metal jacket and the inflatable body may be formed of different respective materials, the material of the metal jacket being softer (and / or more ductile) than the material of the inflatable body. Thus, the inflatable body may be able to exert a greater force on the metal jacket to cause the metal jacket to deform against the metal surfaces. The inflatable body may comprise an inlet into an internal volume enclosed by walls of the inflatable body through which to expand or inflate the inflatable body. For example, the inlet may comprise a tube extending from the inflatable body for connection to a pressure source. The inflatable body may be inflated by introducing a fluid into the internal volume through the inlet. Alternatively, the inflatable body may be inflated by a physical or chemical change of a substance contained within the internal volume. In such cases, the internal volume of the inflatable body may remain sealed (with no inlet) to prevent loss of pressure, at least during inflation of the inflatable body. According to a third aspect of the present invention there is provided a method of forming a metal-to-metal seal between opposing first and second metal surfaces. The method comprises: providing a gasket between the first and second metal surfaces, the gasket comprising: an inflatable body in the form of a closed loop; and a metal jacket extending around the closed loop and at least partly around an outer surface of the inflatable body, and having a discontinuity that extends around the closed loop; and inflating the inflatable body to cause expansion of the inflatable body towards or into the discontinuity in the metal jacket to force opposing sections of the metal jacket adjacent the discontinuity against the metal surfaces to form the seal. Preferably, at least prior to inflation the outer surface of the inflatable body has a radial cross section that comprises a concave portion facing the discontinuity in the metal jacket, and at least during an initial stage of inflation, the curvature of the concave portion of the radial cross section of the outer surface of the inflatable body reduces, thereby forcing the opposing sections of the metal jacket adjacent the discontinuity against the metal surfaces. In some implementations of the method, the first metal surface comprises a sealing grove and providing the sealing ring between the first and second metal surfaces comprises locating the sealing ring within the sealing groove. Inversion of the curvature of the concave portion of the radial cross section of the outer surface of the inflatable body may be limited by contact of the metal jacket with a sidewall of the sealing groove. Thus, the concave portion of the radial cross section of the inflatable body may be straightened against the sidewall of the sealing groove following inflation of the inflatable body. In some implementations, the concave portion becomes convex (at least to some extent) before contacting the sidewall. The internal pressure in the inflatable body may then be reduced to cause relaxation (i.e. partial contraction) of the inflatable body to provide an additional spring force to support the seal. In general, the sealing groove may have any cross section that can accommodate the metal sealing ring, e.g. a square or rectangular cross section or a curved cross section such as a semi-circle. The sealing ring may be accommodated within the sealing groove without protruding therefrom in a direction perpendicular to the first metal surface. As the sealing ring does not protrude from the sealing groove (at least along the direction perpendicular to the first metal surface), the first and second metal surfaces are not required to approach one another along the direction perpendicular to the first metal surface. For example, the first and second metal surfaces may approach one another closely or even contact one another as they are brought into position, e.g. the second metal surface may be able to slide over the first metal surface (or vice versa). The method therefore allows the coming together of the first and second metal surfaces to be decoupled from the deformation of the metal jacket. Scraping or scratching of the surface of the metal jacket is also avoided, which is particularly important for Ultra High Vacuum (UHV) seals, which are known for their sensitivity to such types of damage. Accordingly, more effective seals may be created, particularly in cases where there are geometric constraints on how the first and second metal surfaces can be brought together (i.e. mated) that mean that at least some lateral motion of the surfaces is needed as the sealing ring is engaged. The method may further comprise positioning the second metal surface over the first metal surface to cover the sealing groove, e.g. by moving the first and / or the second metal surface into position. The first and / or second metal surface(s) may be positioned by sliding, such the metal surfaces are (substantially) parallel to and in contact with one another during the positioning. The sealing ring may deform against a bottom surface of the sealing groove, such that the sealing ring is provided between the second metal surface and the bottom surface of the sealing groove. Metal jackets of varying respective thicknesses (i.e. jackets having walls of different thicknesses) may be used together with a common (e.g. the same) design of inflatable body to produce sealing rings of different heights. Thus, selection of a suitable metal jacket may allow the metal sealing ring to be adapted for sealing grooves of different depths. For example, a metal jacket may be selected to provide a pre-determined separation or clearance between the metal jacket and the second metal surface prior to the inflatable body being inflated. Thus, the thickness of the metal sealing ring can be accurately matched to the depth of the sealing groove whilst still allowing the first and second metal surfaces to slide over one another. Some separation or clearance between the metal jacket and the second metal face prior to expanding the inflatable body may be preferable to account for manufacturing tolerances (for example). Thus, in some implementations, the thickness of the metal sealing ring may be selected to provide a predetermined separation that is the same as or exceeds expected manufacturing tolerances for the metal sealing ring, the first and second metal surfaces, and / or the sealing groove. The method may additionally comprise coupling components (e.g. vacuum chamber parts) on which the first and second metal surfaces are provided to prevent relative movement between the first and second surfaces during the inflation of the inflatable body. The components may be bolted together, for example. The first and second metal surfaces may be provided on respective angular segments of a vacuum chamber. For example, the vacuum chamber may be a toroidal vacuum chamber comprising a plurality of angular segments that require sealing together end-to-end to provide a toroidal inner volume in which a vacuum can be maintained. The angular segments are aligned prior to forming a seal between them, which may comprise moving the second metal surface and / or the first metal surface to bring the surfaces into contact, with at least a component of that motion (or all of it) being parallel to the first and / or second metal surface. The first and second metal surfaces may be provided on respective angular segments of a vacuum chamber, e.g. a toroidal or spheroidal vacuum chamber. The vacuum chamber may be assembled, including the angular segments, before the sealing ring is expanded. According to a fourth aspect of the present invention, there is provided a metal-to-metal seal formed between two opposing metal surfaces using the method described above. According to a fifth aspect of the present invention, there is provided a vacuum chamber comprising one or more metal seals or sealing rings as described above. The first and second metal surfaces are provided on different respective segments or parts of the vacuum chamber. Each sealing ring may be located in a corresponding sealing groove provided in the metal surface of one of the vacuum chamber parts (the metal surface of the other vacuum chamber part may also comprise a complementary sealing groove in some cases). In some implementations, the vacuum chamber may comprise a plurality of angular segments arranged to enclose an inner volume (e.g. a toroidal inner volume), the one or more metal sealing rings forming respective seals between pairs of the angular segments. Brief description of the drawings Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawing in which corresponding reference symbols indicate corresponding parts, and in which: Figure 1A is a schematic cross section view of a metal seal in which a compressive force is applied to a sealing ring along a direction that is normal to a surface; Figure 1B is a schematic cross section view of a metal seal in which a compressive force is applied to a sealing ring along a direction that has a component parallel to a surface; Figure 2 is a schematic top view of half of a segmented toroidal vacuum chamber; Figure 3A is a schematic vertical cross section view of a metal sealing ring according to an embodiment of the present invention; Figure 3B is an enlarged portion of Figure 3A; and Figure 4 is a schematic vertical cross section view of the metal sealing ring of Figure 3 at various stages of expansion. Detailed description Figure 3A shows a cross section through a metal gasket or sealing ring 301 for forming a metal-to-metal seal between two opposing metal surfaces 303, 305. The metal sealing ring 301 is provided in a sealing groove (or channel) 307 formed in a first of the metal surfaces 303. The cross section shown in Figure 3A is a radial cross section taken with respect to an axis, Z, perpendicular to a plane of the sealing ring 301. The metal surfaces 303, 305 are provided on different respective parts of a vacuum chamber, with a radially-inner region (on the left-hand side of Figure 3A, in which the Z-axis is provided) corresponding to an interior volume of the vacuum chamber. In use, the sealing ring 301 provides a seal that prevents gas (e.g. air) from being transmitted between the metal surfaces 303, 305 from a high pressure, radially-outer region (the right-hand side of Figure 3A) to the low pressure, radially-inner region (the left-hand side of Figure 3A). The metal sealing ring 301 should be understood as having a generally toroidal structure, e.g. a structure formed by rotating the cross section of the metal sealing ring 301 around the Z-axis. In general, the metal sealing ring 301 is not a circle around the Z-axis and may have different shapes depending on the application, e.g. the metal sealing ring 301 may be ellipsoidal or follow any shape of closed path that encloses the Z-axis. The metal sealing ring 301 is preferably substantially planar, i.e. has a uniform extent along the Z-axis, but can be non-planar in some cases. The sealing ring 301 can be used in any orientation such that the Z-axis may be vertical, as shown in Figure 3A, or aligned along any other direction, e.g. horizontal, depending on the orientations of the surfaces 303, 305 being sealed. The metal sealing ring 301 comprises an expandable or inflatable ring-shaped body 311 and a metal jacket 315. around the expandable body 311. The expandable body 311 comprises a ring-shaped conduit or tube describing a closed path. The expandable body 311 is hollow and has an internal volume 313. The internal volume 313 preferably has no walls such that the path of the conduit around the expandable body 311 is uninterrupted. The expandable body 311 is configured to expand or inflate when a pressure is generated inside the internal volume 313. For example, the expandable body 311 may have one or more inlets for introducing a fluid into the internal volume 313, and / or compressing a fluid contained in the internal volume 313, to generate an internal pressure. The fluid may be a liquid, such as a hydraulic fluid, or a gas. The (or each) inlet may be a tube connecting to the internal volume 313 and connectable to an external pressure source such as a pump or compressor. In some implementations, an internal pressure may instead be generated by heating a substance (solid, liquid or gas) trapped within the internal volume, or through a pressure-generating chemical reaction such as vaporising or decomposing a solid or liquid substance into a gas. The substance may be introduced into the internal volume 313 via an inlet before closing the inlet to seal the internal volume 313. The metal jacket 315 is wrapped or fitted at least partly around an outer surface of the expandable body 311. Expansion of the expandable body 311 deforms the metal jacket 315 and, in use, deforms the metal jacket 315 against the opposing metal surfaces 303, 305 to form a seal between them. The expandable body 311 may deform plastically such that the internal pressure can be relieved without the expandable body 311 returning to its initial form in order to maintain the seal. Alternatively, the internal pressure may be continuously maintained by an external pressure source, or the expandable body 311 may be sealed by, for example, closing the inlet to maintain the internal pressure. Alternatively, as described in more detail below with respect to Figures 4A-F, the expandable body 311 may be configured to provide a spring force that maintains the seal when the internal pressure is relieved. The expandable body 311 is preferably made of a high strength and relatively high ductility material, e.g. a metal, such as an austenitic nickel-chromium based alloy, such as Iconel 718. Other (non-metal) materials may be used additionally or alternatively depending on the application. The metal jacket 315 is preferably made of a soft or ductile material so that it deforms easily (e.g. more easily than the expandable body 311) to form an effective seal. For example, a non-ferrous metal (preferably a substantially pure non-ferrous material), such as aluminium or copper, may be used. The metal jacket 315 comprises a split tube and extends around the entire length of the expandable body 311 in a closed loop. The metal jacket 315 has a discontinuity, preferably only a single discontinuity, that extends around its length such that, in radial cross section, the jacket 315 does not form a continuous, closed loop around the expandable body 311. In the example shown in Figure 3A, the discontinuity is an opening 317 formed between edges 318A,318B of the metal jacket 315 that are spaced apart from one another, e.g. such that the metal jacket 315 has a C-shaped or U-shaped cross section. Alternatively, the edges 318A.318B may touch each other or overlap and extend past one another such that, in radial cross section, the jacket 315 fully surrounds or wraps around the expandable body 311.The edges 318A.318B are nevertheless able to move relative to one another when the metal jacket 315 is deformed by inflation of the expandable body 311. The opening 317 or other discontinuity preferably extends around the metal jacket 315 at the substantially the same relative position relative to the expandable body 311 to ensure even deformation of the metal jacket 315. More preferably, the discontinuity is located at either of substantially the inner or outer circumference of the expandable body 311 such that the metal jacket 315 deforms substantially evenly in both directions along the Z-axis. More preferably, the discontinuity extends around substantially the outer circumference of the metal jacket 315, i.e., is directed away from the Z-axis and away from a space enclosed by the metal sealing ring 301, as shown in Figure 3A. The opening 317 is therefore directed away from a high vacuum (i.e. low pressure) side of the seal such that the metal jacket 315 forms a barrier between the high vacuum side and the expandable body 311. The metal jacket 315 comprises ridges 319 on its outer surface, i.e. externally directed protrusions for deforming, preferably plastically, against the second metal surface 303 and the sealing groove 307. Each ridge may extend all the way around the metal jacket (i.e. enclose the Z-axis). In the present example, opposing ridges 319 are formed on either side of the opening 317. However, ridges may be formed in other locations, e.g. closer to the Z-axis, or on an opposite side of the metal jacket 315 to the opening 317. Ridges 319 are generally optional however, and the metal jacket 315 may, for example, comprise none, 1, or 2 or more ridges. In some implementations, the ridges may be “knife edges” that are able to cut into the metal surfaces 303, 305, e.g. the ridges 319 may be coated with aluminium (or another relatively hard metal) to create the knife-edge. The expandable body 311 has a radial cross section that comprises a concave portion 321 facing the opening 317 or other discontinuity in the metal jacket 315. Thus, the expandable body 311 comprises an indentation or channel that extends circumferentially around the expandable body (i.e. around the Z-axis), and which is directed towards the opening 317 of the metal jacket 315. In the present example, the radial cross section of the expandable body 311 is otherwise convex, i.e. there is a single concave region 321. As described, below in connection with Figures 4A-F, the concave region 321 causes the expandable body 311 to expand preferentially towards the opposing metal surfaces 303, 305 as the concave region straightens, which increases performance of the seal by increasing the pressure exerted on the metal jacket 315 as the seal is being formed. Prior to inflation, the metal sealing ring 301 is configured to fit into the sealing groove 307 formed in the first metal surface 303. In the present example, the sealing groove 307 has a square cross section, but other shapes of cross section can be used, e.g. rectangular, ellipsoidal, semi-circular, etc. Before the seal is formed, the metal sealing ring 301 does not protrude from the sealing groove 307, i.e. does not extend past the first metal surface 303 in a direction perpendicular to the first metal surface 303 (which, in the figures, corresponds to the direction of the Z-axis). Thus, the second metal surface 305 may be moved over the first metal surface 303 to cover the sealing groove 307 without contacting the metal sealing ring 301, i.e. the second metal surface 305 may approach the first metal surface 303 from any direction (limited only by the surfaces 303, 305 coming into contact), or slid into position over the first metal surface 301, with the two surfaces in contact. As can be appreciated from the enlarged view provided by Figure 3B, a small amount of clearance 309 (measured in a direction perpendicular to the first metal surface 303) may be provided between the metal sealing ring 301 and the second metal surface 305. The clearance 309 may be selected to account for manufacturing tolerances, e.g. to ensure that there is a high likelihood that the metal sealing ring 301 does not protrude from the sealing groove 307. In some cases, the clearance 309 may be from 0.1 mm to 0.5 mm. The sealing ring may have a thickness of around 5 mm in some cases (the thickness being the extent of the sealing ring in a direction parallel to the Z-axis). Figure 4 shows the metal sealing ring 301 (without ridges 319) at different stages A-F during formation of the seal. In this example, the seal is formed by progressively increasing the pressure within the internal volume of the expandable body 311. In step A, the metal sealing ring 301 is accommodated within the sealing groove 307 in the first metal surface 303, with clearances 309 between the metal sealing ring 301 and the second metal surface 305 and the bottom of the sealing groove 307. In step B, the vacuum chamber parts on which the first and second metal surfaces 303, 305 are provided are coupled to one another using fixings 401 (e.g. bolts) to prevent relative movement between the metal surfaces 303, 305 as the expandable body 311 is expanded. In step C, the metal jacket 315 expands until there is zero clearance 309 between the metal jacket 315 and the second metal surface 305 and the bottom of the sealing groove 307. The metal jacket 315 therefore begins to deform against the second metal surface 305 and the bottom of the sealing groove 307. The configuration of the expandable body 311 (i.e. the inclusion of the concave region 321) means that the initial expansion of the expandable body 311 acts to straighten the concave region 321 and thereby force the metal jacket 315 towards the second metal surface 305 and the bottom of the sealing groove 307. Following contact of the metal jacket 315 with the opposing metal surface 303, 305, the expandable body 311 begins or continues to expand radially (i.e. parallel to the first metal surface 303, and away from the Z-axis in the embodiment illustrated in the Figures). In step D, contact areas 403, 405 between the metal jacket 315 and the second metal surface 305 and the bottom of the sealing groove 307 increases as the expandable body 311 expands radially. The radial expansion of the expandable body 311 reduces the curvature of the edges 318A,B of the metal jacket 315 adjacent the opening 317 in the metal jacket 315, causing these sections to unroll against the second metal surface 305 and the bottom of the sealing groove 307. The curvature of the concave region 321 also reduces, i.e. the concave region 321 straightens to generate an increased force to deform the metal jacket 315 against the second metal surface 305 and the bottom of the sealing groove 307. In step E, the radial expansion of the expandable body 311 continues until the body contacts a sidewall 405 of the sealing groove 307, and the concave region 321 has become substantially straightened. The sidewall 405 acts as a stop to prevent the expandable body 311 from over-expanding, which may lead to reduction of contact pressure between the seal and the sealing faces, thereby weakening the seal, which may ultimately cause the metal jacket 315 to lose contact with the second metal surface 305 and the bottom of the sealing groove 307. In step F, once the pressure in the expandable body 311 reaches a maximum, it may be reduced, e.g. down to atmospheric pressure, which may in some cases cause the expandable body 311 to stop contacting the sidewall 405 as a result of elastic strain in the expandable body 311 being relieved. Such contraction may increase the contact pressure between the metal jacket 315 and the second metal surface 305 and the bottom of the sealing groove 307. Elastic strain stored in the expandable body 311 may therefore act as an energising force to help prevent loss of sealing capability, e.g. during small relative motion of the first and second metal surfaces 303, 305 (fretting, shock loading, etc.). In some examples, the expandable body 311 is configured and positioned with the sealing groove 307 such that the concave region 321 becomes convex during inflation of the expandable body 311 and then becomes substantially straight when the internal pressure in the expandable body 311 is relieved. Alternatively, in some implementations, the pressure may be maintained e.g. so that there may be no contraction of expandable body 311. Once the seal has been formed, the parts may be heated (e.g. to 300°C), which may reduce the contact pressure to some extent, but it has been found that the seal remains effective nonetheless. Thus seal integrity may be maintained even at “bake out” temperatures used in high vacuum application. One or more seals formed using metal sealing rings 301 as described above in connection with Figures 3A, 3B and 4 may be used in the toroidal vacuum chamber 201 described above in connection with Figure 2, for example. The metal sealing ring 301 may be formed entirely of metal, which makes it particularly suited to harsh chemical and physical environments that may cause sealing rings made of other materials to degrade. 5 While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described 10 without departing from the scope of the claims set out below.

Claims

1. A gasket for forming a metal-to-metal seal between two opposing metal surfaces, the gasket comprising:an inflatable body in the form of a closed loop and having an internal volume enclosed by walls of the inflatable body, the internal volume comprising (i) an inlet through which to introduce fluid into the internal volume to inflate the inflatable body, or (ii) a substance for introducing fluid into the internal volume by a physical or chemical change of the substance to inflate the inflatable body; anda metal jacket extending around the closed loop and at least partly around an outer surface of the inflatable body, and having a discontinuity that extends around the closed loop,wherein, on expansion, the inflatable body is configured to force apart opposing sections of the metal jacket adjacent the discontinuity to form a seal in use.

2. A gasket according to claim 1, wherein the inflatable body comprises a tube configured to expand under an internal pressure.

3. A gasket according to claim 1 or 2, wherein the discontinuity extends around one of an inner circumference or an outer circumference of the closed loop.

4. A gasket according to any one of the preceding claims, wherein the inflatable body is configured to expand towards or into the discontinuity.

5. A gasket according to claim 4, wherein the inflatable body is configured to expand along an axis substantially perpendicular to a plane of the inflatable body before or during expansion of the inflatable body towards or into the discontinuity in the metal jacket.

6. A gasket according to any one of the preceding claims, wherein, on expansion, the inflatable body is configured to force the metal jacket against adjacent metal surfaces to form a seal in use.

7. A gasket according to any one of the preceding claims, wherein the inflatable body is configured to force apart opposing sections of the metal jacket adjacent thediscontinuity in opposite directions along an axis substantially perpendicular to a plane of the closed loop.

8. A gasket according to any one of the preceding claims, wherein, at least prior to inflation the outer surface of the inflatable body has a radial cross section that comprises a concave portion facing the discontinuity in the metal jacket.

9. A gasket according to claim 8, wherein the radial cross section of the outer surface of the inflatable body is a hollow C-shape.

10. A gasket according to claim 3, wherein the metal jacket contacts the inflatable body around an outer or inner circumference of the inflatable body opposite the discontinuity.

11. A gasket according to any one of the preceding claims, wherein the metal jacket comprises one or more outwardly directed ridges.

12. A gasket according to any one of the preceding claims, wherein the metal jacket and the inflatable body are formed of different respective materials, the material of the metal jacket being softer than the material of the inflatable body.

13. A gasket ring according to claim 1, wherein the inlet comprises a tube extending from the inflatable body for connection to a pressure source.

14. A gasket according to any one of the preceding claims wherein the inflatable body is configured to expand under an internal pressure and, following expansion of the inflatable body and relief of the internal pressure, the inflatable body is configured to maintain a spring force against the opposing sections of the metal jacket.

15. A gasket for forming a metal-to-metal seal between two opposing metal surfaces, the gasket comprising:an inflatable ring-shaped body having an internal volume enclosed by walls of the inflatable body, the internal volume comprising (i) an inlet through which to introduce fluid into the internal volume to inflate the body, or (ii) a substance for introducing fluid into the internal volume by a physical or chemical change of the substance to inflate the body; anda substantially tubular metal jacket fitted around the ring-shaped body and having a discontinuity that extends around an inner or outer circumference of the ring-shaped body, and wherein the ring-shaped body is adapted to expand, on inflation, towards or into the discontinuity to force opposing sections of the metal jacket adjacent the discontinuity against the metal surfaces to form the seal.

16. A method of forming a metal-to-metal seal between opposing first and second metal surfaces, the method comprising:providing a gasket between the first and second metal surfaces, the gasket comprising:an inflatable body in the form of a closed loop;a metal jacket extending around the closed loop and at least partly around an outer surface of the inflatable body, and having a discontinuity that extends around the closed loop; andinflating the inflatable body to cause expansion of the inflatable body towards or into the discontinuity in the metal jacket to force opposing sections of the metal jacket adjacent the discontinuity against the metal surfaces to form the seal.

17. A method according to claim 16,wherein, at least prior to inflation the outer surface of the inflatable body has a radial cross section that comprises a concave portion facing the discontinuity in the metal jacket, and at least during an initial stage of expansion, the curvature of the concave portion of the radial cross section of the outer surface of the inflatable body reduces, thereby forcing the opposing sections of the metal jacket adjacent the discontinuity against the metal surfaces.

18. A method according to claim 16, wherein the first metal surface comprises a sealing grove and providing the gasket between the first and second metal surfaces comprises locating the gasket within the sealing groove.

19. A method according to claim 18, wherein inversion of the curvature of the concave portion of the radial cross section of the outer surface of the inflatable body is limited by contact of the inflatable body with a sidewall of the sealing groove.

20. A method according to claim 19, wherein the in the concave portion of the radial cross section of the inflatable body is straightened against the sidewall of the sealing groove during expansion of the inflatable body.

21. A method according to any one of claims 17 to 20, wherein the gasket is accommodated within the sealing groove without protruding therefrom in a direction perpendicular to the first metal surface, the method further comprising positioning the second metal surface over the first metal surface to cover the sealing groove.

22. A method according to any one of claims 17 to 21, further comprising coupling components on which the first and second metal surfaces are provided to prevent relative movement between the first and second metal surfaces during the expansion of the inflatable body.

23. A method according to any one of claims 17 to 22, wherein the first and second metal surfaces are provided on respective angular segments of a vacuum chamber.

24. A metal-to-metal seal formed between two opposing metal surfaces using the method of any one of claims 17 to 23.

25. A vacuum chamber comprising one or more metal-to-metal seals according to claim 24.

26. A vacuum chamber according to claim 25 and comprising a plurality of angular segments arranged to enclose an inner volume, the one or more metal-to-metal seals being formed between pairs of the angular segments.

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