Permanent magnetic bearing

Abstract

A permanent magnet bearing (e.g. a neodymium, magnet alloy, praseodymium, or samarium cobalt magnet) for a vacuum pump, wherein at least a surface of the bearing ring is coated in a coating of less th

Description

Field The present invention relates to a magnetic bearing for a vacuum pump, a vacuum pump, particularly a turbomolecular pump, comprising the same, and a method of manufacturing a vacuum pump. The invention further provides the use of a various materials in the ultrathin coating of a permanent magnetic bearing. Background A vacuum pump typically comprises an impeller in the form of a rotor mounted on a shaft for rotation relative to a surrounding stator. In a turbomolecular pump, for example, sets of moving rotor blades are located on a shaft and are separated and moveable between stationary blades, in use, to compress gas and typically deliver it to an outlet pump, e.g. a rotary pump. A shaft is supported by two bearing arrangements located at or intermediate respective ends of the shaft. Typically, the upper bearing (that nearest the pump inlet) may be in the form of a magnetic bearing, and the lower bearing is in the form of a rolling bearing. The magnetic bearing is typically a passive magnetic bearing comprising an inner stator bearing half and an outer rotor bearing half, the outer rotor bearing half forming a part of the rotor shaft of the vacuum pump. The outer rotor bearing half typically comprises one or more ring shaped permanent magnets coaxially aligned with the axis of rotation of the rotor shaft and the inner stator bearing half. Typically, the permanent magnet rings in the inner stator bearing half and outer rotor bearing half are magnetised and disposed relative to each other in order to create repulsive forces between the two halves. Thus they may suspend the rotor radially. Typically, the permanent magnet rings are made from a samarium-cobalt alloy, such as SmCos. Advantageously, these magnets are extremely resistant to demagnetisation, and show good temperature stability with maximum use temperatures typically between 250°C and 550 °C, and a Curie temperature from 700°C to 800°C. There is however a desire to move towards other magnet alloys, such as neodymium magnets and praseodymium magnets, as they are magnetically stronger, facilitating the use of small magnets and ultimately more compact vacuum pump designs. Known vacuum pump designs and methods of manufacture have however been found to be unsuitable for successfully accommodating these alternative magnets in their bearings. The present invention addresses at least to a degree some or all of these problems with the prior art. Summary Thus, in a first aspect, the present invention provides a permanent magnetic bearing ring for a vacuum pump. The bearing ring comprises a magnet alloy. A radially inward facing surface of the bearing ring and / or a radially outward facing surface of the bearing ring is coated by a substantially continuous ultrathin coating. A coating with a mean thickness of less than 10 microns will be considered “ultrathin” for the purpose of the invention. The ultrathin coating may comprise a single layer of material or a plurality of layers of material. When a plurality of layers are present, the material of each layer may be the same or different. Additionally, or alternatively, some surfaces of the bearing ring may be coated with a multilayer ultrathin coating and other surfaces may be coated with a single layer ultrathin coating. Multilayer coatings may provide flexibility in terms of coating properties, for instance improved resistance to corrosion. In embodiments, the ultrathin coating may comprise a phosphate priming layer and an organic layer. In some embodiments, one or more surfaces may be coated with a coating that has a mean thickness of greater than 10 microns, for instance to provide improved corrosion resistance. Preferably, however, a radially inward facing surface(s) and / or a radially outward facing surface(s) of the or each bearing ring has a coating with a total thickness which is ultrathin. The ultrathin coating may be employed to impart or to modify one or more surface properties of the magnet alloy. The ultrathin coating typically provides corrosion resistance or may otherwise protect the magnet alloy from the environment and / or process gases passing through the vacuum pump. The ultrathin coating may for instance increase the contact angle of water, i.e. decrease wettability. Preferably, the coating is more hydrophobic than the magnet alloy. The coating may be semi-impermeable, or preferably impermeable. In embodiments, the ultrathin coating may prevent, or reduce (preferably significantly), rusting of the magnet alloy. In embodiments, the magnet alloy may comprise a neodymium magnet or neodymium magnet alloy. In embodiments, the magnet may comprise a samarium cobalt magnet or magnet alloy. In embodiments, the magnet may comprise a praseodymium magnet or magnet alloy. Neodymium magnet or magnet alloy typically comprises neodymium, iron and boron. They may comprise further elements including those selected from the group consisting of dysprosium, praseodymium, cobalt, gallium, aluminium and copper. Praseodymium magnet or magnet alloy typically comprises praseodymium, iron and boron. For the purpose of the invention, a praseodymium magnet or magnet alloy is substantially free from neodymium. Magnets and magnet alloys comprising neodymium are herein referred to neodymium magnets or magnet alloys, as the case may be. Typically, the, or each, bearing ring prior to coating consists of, or consists essentially of, the magnet alloy. Once coated, the, or each, bearing ring may consist of, or consist essentially of, the magnet alloy and the coating(s). Typically, the, or each, magnetic bearing ring is of annular construction. The, or each, bearing ring may comprise a radially outward facing circumferentially extending longitudinal surface, which, in use, may be substantially vertically extending. This may be referred to herein as outward facing surface of the or each bearing ring or, similarly, the, or each, bearing half. The, or each, bearing ring may comprise a radially inward facing circumferentially extending longitudinal surface, which in use may be substantially vertically aligned. This may be referred to herein as an inward facing surface (radially inward facing surface) of the or each bearing ring or, similarly, the, or each, bearing half. Extending between the outward facing surface and inward facing surface of the or each bearing ring there may be a radially extending upper (distal) surface. Extending between the outward facing surface and inward facing surface of the or each bearing ring there may be a radially extending lower (proximal) surface. Typically, the permanent magnetic bearing ring is a part of a bearing half of a magnetic bearing. The magnetic bearing may comprise rotor bearing half and a stator bearing half. The, or each, bearing half may comprise one or more permanent magnetic bearing rings, for instance the, or each, bearing half may comprise three permanent magnetic bearing rings. The invention may therefore also provide a permanent magnetic bearing comprising a permanent magnetic bearing ring as described herein. Typically, the stator bearing half is an inner bearing half and the rotor bearing half is an outer bearing half. Although in embodiments the opposite arrangement may be preferred. Typically, the vacuum pump comprises a rotor shaft. The bearing halves are typically arranged substantially concentrically with the axis of rotation of the rotor shaft of the vacuum pump. The, or each, bearing half may be partially or, preferably, substantially entirely coated with the ultrathin coating. Preferably, the, or each, bearing half is entirely coated with the ultrathin coating. Where the, or each, bearing half-comprises a plurality of bearing rings, adjacent rings may be in direct contact with one another along one or more mutually opposing ring-to-ring engagement surfaces. The ring-to-ring engagement surface may be the radially extending upper surface and / or radially extending lower surface described hereinbefore. The, or each, ring-to-ring engagement surface may be coated with the ultrathin coating, or they may be uncoated. Preferably the radially extending upper surface and the radially extending lower surface of each bearing ring are both coated. Ring-to-ring engagement surface(s) may be coated because the ultra-thin coating does not affect tolerances and / or fit in a significant manner. Advantageously, it is easier to coat an entire bearing ring than to selectively coat specific surfaces. In embodiments, the, or each, individual permanent magnetic bearing ring may be substantially entirely coated with the ultrathin coating. Entirely coating the magnetic bearing ring is advantageous as it may be simpler to manufacture than partial coatings, whilst still providing the benefits described elsewhere herein. In embodiments at least a radially outward facing surface of the inner bearing half (e.g. the stator bearing half) is coated with the ultrathin coating and / or at least a radially inward facing surface of the outer bearing half (e.g. the rotor bearing half) is coated. Preferably, the radially outward facing surface(s) of the inner bearing half is(are) completely coated with the ultrathin coating and / or the radially inward facing surface(s) of the outer bearing half is(are) completely coated with the ultrathin coating. Additionally, or alternatively, the radially inward facing surface(s) of the inner bearing half is(are) completely coated with the ultrathin coating and / or the radially outward facing surface(s) of the outer bearing half is(are) completely coated with the ultrathin coating. When a given surface is coated, preferably substantially all of that surface is coated. The bearing halves are placed in close proximity to one another and are generally configured to be in mutual repulsion, in use. The rotor is thereby contactlessly supported, avoiding any need for lubrication or grease. Typically, the mutually opposing inward facing and outward facing surfaces of the outer and inner bearing halves are coated with the ultrathin coating. Exposed radially extending surfaces are typically coated with the ultrathin coating. Vacuum pump components are often exposed to corrosive environments, which may be detrimental to the magnet alloys from which permanent magnetic bearings are made. For instances, protective coatings have been developed to resist hydrogen (H2) induced decrepitation. Similarly protective coatings have been employed to protect permanent magnetic bearings from corrosion caused by the environment, e.g. atmospheric air, which may be present when the vacuum pump is not in use. The inventors have however found that many coating methods, however, are themselves detrimental to the magnetic bearing. In particular, there is a need for precise tolerances between the permanent magnets and the corresponding cavity on the rotor shaft configured to receive them. Permanent magnet alloy rings are usually press fit inside a rotor shaft cavity. Precise tolerances are thus necessary to minimise the risk of damage to the magnet alloy, which are typically brittle and may fracture under tensile stresses, while ensuring an effective interference fit between the permanent magnetic bearing ring and the rotor shaft cavity is maintained during use. As used herein, the term “interference fit” refers to a fit between parts in which the external dimension of a first part is at least substantially the same as (or greater than) the internal dimension of a second part into which the first part is fitted. An effective interference fit may be one which minimises movement of the permanent magnetic bearing ring(s) relative to the rotor shaft cavity as vacuum pump components undergo centrifugal and, especially, thermal expansion and contraction during use. This is advantageous, particularly where at least one of the magnetic rings is a neodymium magnet or neodymium magnet alloy, because such a magnet or magnet alloy typically has a low coefficient of thermal expansion compared to the rotor making it desirable to apply a compressive force to the permanent magnet alloy ring. Thus, the magnetic ring may have a large minimum interference fit. However, it is relatively difficult to apply a strong interference fit because cooling the magnetic elements during a fitting process typically has a negligible thermal expansive effect (i.e. they do not shrink particularly) and the maximum temperature that the magnetic elements can be exposed to before demagnetisation takes place is relatively low, reducing the ability to heat the rotor before the magnet is inserted. Alternatively, or additionally, the rotor maximum heating temperature before its properties are adversely affected, or the maximum force that can be applied to the rotor, may also limit the interference fit achievable. Therefore, the inventors have found that reducing the tolerance of the fit may be very helpful The tolerance on the diameters of the magnets may be of the order of + / - from about 10 to about 20 microns. This may be critical in order provide the required compressive action when fitting the magnets on the rotor. Preferably the coating has a maximum thickness of less than 5 microns. Put simply, by using an ultrathin coating on a permanent magnet bearing ring, as claimed herein, the inventors have found that problems associated with tolerances of known coatings are avoided because the coating is sufficiently thin that any variation in the coating itself makes a negligible contribution to the tolerance stack. The present invention is therefore advantageous because seating of the permanent magnet alloy rings in the rotor shaft cavity is more easily achieved with a reduced risk of demagnetisation of or damage to the permanent magnetic alloy ring or the rotor and with a sufficient interference fit. Furthermore, typically the radial clearance between the inner and outer bearing halves of a magnetic bearing to prevent contact during operation is, with uncoated magnets, of the order of about 0.5 mm. Typically the thinner the airgap the better. A coating thickness of 50 microns means that to keep this clearance, the airgap will have to be increased to 0.6 mm. A coating thickness less than 10 microns may require a minimal increase of the airgap. A coating thickness in the order of 1 micron may require no adjustment of the magnetic airgap. In embodiments, the, or each, bearing ring may be substantially entirely coated with the ultrathin coating. Alternatively, only a radially inward facing surface and exposed radial surface of the bearing ring may be coated with the ultrathin coating or only a radially outward facing surface and exposed radial surface of the bearing ring may be coated ultrathin coating. In some embodiments, only those surfaces which are exposed during use are coated with the ultrathin coating. Additionally, or alternatively, an exposed surface(s) may include those surfaces of the bearing ring not in direct contact with another component of the vacuum pump. In embodiments the magnetic bearing may include one or more inner rings. Typically, the, or each, magnetic bearing ring is generally toroidal with a generally rectangular or square cross-section. Typically, where the bearing ring is an inner bearing half the inner diameter of the magnetic bearing ring is from about 1 cm to about 5 cm. The ultrathin coating may be substantially continuous in the sense that the coating is free from perforations or apertures or any other such uncoated regions within its perimeter. For the avoidance of doubt, any such uncoated areas or regions are not included in the measurement of the mean thickness of the coating because it is continuous. Discontinuities which are less than below the detection threshold of a coating gauge (e.g. less than 1 nm) and / or are otherwise so small that they do not affect the efficacy of the ultrathin coating are not considered apertures within the context of the invention. Preferably the coating has a mean thickness of from about 0.01 microns to about 10 microns, preferably from about 0.1 microns to about 5 microns. Preferably the maximum thickness of the coating is less than 20 microns, preferably less than 15 microns, preferably less than 10 microns. The thickness of the coating may be measured by measuring the thickness of the article before and after coating. A Thrubeam micrometer such as the optocontrol 2520 may be employed. Typically, in use, the magnetic bearing and / or the or each magnetic bearing ring sit(s) within a cavity of the rotor shaft of the vacuum pump. Typically, the rotor shaft cavity is located at or towards a distal, or lower pressure, end of the rotor shaft. Although, in embodiments, the rotor shaft cavity may be present at or towards the proximal, relatively higher pressure, end of the rotor shaft, or at or towards both ends of the rotor shaft. The rotor shaft cavity may be configured to slidably receive the bearing and / or the or each bearing ring to achieve an intermate engagement about the circumference of the bearing and / or bearing ring. Typically, the rotor shaft cavity has a generally circular cross-section. The fit between the rotor shaft cavity and the bearing may be permanent or semi-permanent. Typically, the, or each, permanent magnetic bearing half may comprise from about 1 to about 10 permanent magnet alloy rings, more preferably from about 2 to about 6 permanent magnet alloy rings, 2 to 4 permanent magnet alloy rings are particularly preferred. Preferably the coating is selected from the group consisting of conversion coating(s), such as phosphate conversion coating(s); fluoropolymer coating(s); graphene coatings; electroplating; and plasma coating(s). Conversion coating(s), and in particular phosphate conversion coating(s), are particularly preferred. As described herein, a phosphate conversion coating is a chemical treatment applied to the magnet alloy that creates a thin adhering layer of iron, zinc, or manganese phosphates, preferably zinc phosphate(s). The invention thereby improves corrosion resistance. A phosphate coating is usually obtained by applying to the substrate a dilute solution of phosphoric acid, possibly with iron, zinc, and / or manganese salts dissolved therein. The solution may be applied by sponging, spraying, or immersion. Immersion (or dip) coating is in particular preferred. In embodiments the bearing ring comprises a magnet alloy wherein an inward facing surface of the bearing ring and / or an outward facing surface of the bearing ring is a phosphate conversion coated magnet alloy wherein the phosphate conversion coating has a mean thickness of less than 10 microns. Where uncoated areas are desirable masking techniques may be employed. Where the coating is not a reaction coating, the coating may be applied using a method selected from the group consisting of spin coating, dip coating, slot die coating, and spray coating. In a further aspect, the invention provides a method of manufacturing a permanent magnetic bearing for a vacuum pump. The method comprises the steps of a) providing a bearing ring comprising a magnet alloy, b) coating a surface of the magnet alloy with a coating having a mean thickness of less than about 10 microns, and c) magnetising the magnet alloy. Typically step c) is performed after step b). Thus, the magnet alloy may be unmagnetized before step c) and / or in step b). In embodiments, step b) is preferably performed using a technique selected from the list consisting of reaction coating, spin coating, dip coating, slot die coating, and spray coating, or combinations thereof. As will be appreciated, in spin coating, a coating liquid (e.g. solution) is place onto a substrate. The substrate is then rotated at high speed and the centrifugal force from this constant acceleration, along with viscous drag and surface tension, causes the solution to spread evenly across the substrate. The thickness of the film is determined by the rate of rotation. Advantageously, spin coating reliably produces uniform films and works well with a wide range of liquid coatings to produce many types of film. This technique is particularly useful for producing micron and sub-micron thickness films. In dip coating, the substrate is immersed in the coating liquid. As the substrate is withdrawn, a liquid layer forms on the substrate, which in turn forms the coating. The final dry film thickness is dependent on for instance withdrawal speed, air flow, viscosity, and evaporation rate. This technique is also useful for producing micron and sub-micron thickness films. In slot die coating, a coating liquid is coated directly onto the substrate. The solution flows through a ‘head’ at a determined rate as the substrate moves relative to the head or vice versa. The wet film thickness is determined by the amount of solution placed onto the substrate. All other parameters, such as the solution flow rate, the coating width, speed, and viscosity, can be optimized to improve the uniformity and stability of the thin film deposition. This technique is also useful for producing micron and sub-micron thickness films. Spray coating is an alternative deposition technique used in research and industry. In this method, the coating solution is broken up by a stream of pressurized gas, then dispensed in a continuous flow of fine droplets. The final film thickness may depend on the surface tension and viscosity of the solution, the properties of the gas flow and nozzle, the wetting of the solution, and the coating distance and speed. The skilled person will be able to optimise the technique for a particular use. This technique is also useful for producing micron and sub-micron thickness films. As discussed in the adjoining aspect, conversion coating, and in particular phosphate conversion coating, is particularly preferred as a coating technique for use in step b). As described herein, phosphate conversion coating creates a thin adhering layer of iron, zinc, or manganese phosphates, preferably zinc phosphate(s), on the surface of the magnet alloy. The invention may thereby provide corrosion resistance. A phosphate coating is usually obtained by applying to the substrate a dilute solution of phosphoric acid, possibly with iron, zinc, and / or manganese salts dissolved therein. The solution may be applied by sponging, spraying, or immersion. Immersion (or dip) coating is in particular preferred. Preferably, step b) is performed by reaction coating the magnet alloy in a phosphoric acid solution. In a further aspect, the present invention provides a method of manufacturing a vacuum pump, preferably a turbomolecular pump, the vacuum pump comprising a rotor shaft configured to have one or more rotor blades coupled thereto and defining a cavity for receiving a magnetic bearing ring for a rotor half of a permanent magnetic bearing and / or a stator defining a seat for seating a magnetic bearing ring for a stator half of the permanent magnetic bearing. The method comprises the steps of: i) carrying out the method of any of any preceding aspect or providing a magnetic bearing ring according to any preceding aspect; and ii) inserting a said permanent magnetic bearing ring for the rotor half into the rotor shaft cavity of the vacuum pump in an interference-fit configuration and / or onto the stator seat of the vacuum pump. Alternatively, the coated magnets can be fitted to a holder, which is then fitted to the rotor or stator. An additional coating can also be applied to the exposed surfaces of the magnets once they are fitted in the holders, if required, for example to provide additional resistance to corrosion. The total coating thickness may be greater than 10 microns, for instance on said exposed surfaces. In embodiments a phosphate coating may be provided on the ring-to-ring and / or unexposed surfaces as a sole coating, which is ultrathin, and / or a phosphate coating may be used on exposed surfaces which are then coated with a further coating, which is not ultrathin, such as those described herein. The total coating thickness of the exposed surfaces with multiple coatings may be up to 200 microns in thickness. By coating the permanent magnetic bearing ring prior to inserting it into the rotor shaft cavity or attaching it to the stator of the vacuum pump, a greater range of coating processes may be utilised, and improved coating of the permanent magnetic bearing may be achieved, as described above. Moreover, the magnet alloy may be protected during manufacture and storage. Typically, the, or each, permanent magnetic alloy ring is magnetised prior to step ii), usually during step i). Although, in embodiments, the method may comprise the step of magnetising the or each permanent magnet alloy ring after step ii). In a further aspect, the present invention provides a vacuum pump manufactured in accordance with any preceding aspect. In a further aspect the invention provides the use of phosphoric acid to reaction coat a magnet alloy, preferably an unmagnetized magnet alloy, to provide permanent magnet bearing alloy ring of a vacuum pump. The invention further provides the use of a neodymium magnet or neodymium magnet alloy or a praseodymium magnet or magnet alloy in a permanent magnetic bearing of a vacuum pump wherein at least a portion of the neodymium magnet or neodymium magnet alloy or a praseodymium magnet or magnet alloy is coated with a coating and said coating has a mean thickness of less than about 10 microns, preferably wherein the coating is a phosphate coating, such as described herein. Brief Description of Figures Preferred features of the present invention will now be described, by way of example, with reference to the accompanying drawing, in which: Figure 1 illustrates a cross-sectional view of a permanent magnetic bearing and magnetic bearing ring(s) according to the present invention, located in a turbomolecular vacuum pump. Detailed Description Figure 1 shows a bearing 1 of a turbomolecular vacuum pump 100. In particular, a permanent magnetic bearing 3 within a turbomolecular vacuum pump 100. The vacuum pump 100 comprises a housing having an inlet and an outlet (no shown). A stator 4 forms part of or is connected to the housing of the turbomolecular pump (100) and a rotor 5 is disposed and rotatably supported by the bearing 3. The rotor 4 comprises a plurality of rotor vanes (not shown) which are configured to interact with a plurality of stator vanes (not show), in use, in order to convey a gaseous medium from the inlet to the outlet. The illustrated permanent magnet bearing comprises a rotor half 2 and a stator half 6. The rotor and stator bearing halves 2, 6 each comprise one or more permanent magnets in the form of permanent magnet alloy rings 7. In Figure 1, the rotor and stator bearing halves 2, 6 each include three vertically adjacently arranged permanent magnet alloy rings 7. In use, the bearing halves 2, 6 are placed in close proximity to one another and are configured to be in mutual repulsion in order that the rotor is substantially contactlessly supported by the permanent magnetic bearing 3. The rotor shaft defines a cavity 8 configured to receive the permanent magnetic bearing rings 7 in an interference-fit configuration. The rotor half of the permanent magnetic bearing comprises three permanent magnet alloy rings 7. Generally, the, or each, of the permanent magnet alloy rings 7 have a substantially continuously annular configuration. The permanent magnetic bearing 3 is coated with an ultrathin coating (i.e. a coating with a mean thickness of less than 10 microns). The ultrathin coating is configured to prevent or at least reduce corrosion of the permanent magnet alloy rings 7. The selection of the ultrathin coating may be dependent on the material of the permanent magnet alloy rings 7 and / or one or more process gases of the vacuum pump 100. The coating may be selected in order to protect the magnet alloy rings 7 from corrosion when pump 100 is not operating, or / and from process gases and / or chemicals that could damage the integrity or performance of the permanent magnetic bearing. For example, a coating may be selected to protect the permanent magnet alloy rings 7 against hydrogen (or other) decrepitation, as discussed above. Preferably however the coating is a phosphate conversion coating. In this instance, the magnet alloy rings 7 may each have been entirely coated using phosphate coating to provide an ultrathin coating thereon. They may thus show increased resistance to rusting. In the embodiment of Figure 1, each bearing ring 7 is entirely coated with an ultrathin coating (i.e. substantially none of the magnet alloy is exposed). Thus, those surfaces which would otherwise be exposed to process gases within the vacuum pump 100 when the permanent magnetic bearing is received therein are coated with the ultrathin coating. In the embodiment of Figure 1, because each bearing ring 7 is substantially entirely coated, the mutually opposing circumferential surface of each permanent magnet alloy ring (i.e. those which face the other bearing half) and also the radial faces are coated with the ultrathin coating. Preferably, the, or each, permanent magnetic bearing half or rings thereof are coated prior to being installed. By doing so, the coating may protect the magnet alloy during manufacture. The, or each, permanent magnetic bearing half or rings thereof are typically coated outside of a vacuum pump. Any surfaces to be provided without a coating may be masked by a temporary masking component, which may be removed with any coating thereon. Individual permanent magnet alloy rings may be masked and / or selectively coated separately (in this case, the single magnet may be substantially unmagnetised during coating), or a plurality of permanent magnet alloy rings may be masked and / or coated concurrently. The permanent magnetic bearing may be coated by: a) providing each of the permanent magnet alloy rings 7; and d) applying the ultrathin coating to the permanent magnet alloy rings so that their surfaces are substantially entirely coated. Coating the surfaces of the permanent magnet alloy rings 7 with an ultrathin coating improves the interference-fit configuration of the permanent bearing ring 7 in the rotor shaft cavity 8. In particular, because those surfaces which are configured to interface another surface of the permanent bearing 3 or the rotor shaft cavity 8 are coated with an ultrathin coating, the tolerance on the radial outer diameter of the permanent magnet alloy rings is not unnecessarily increased. Thus, tolerances between the permanent magnetic bearing 3 and the rotor shaft cavity are more precise, and the risk of damage to the permanent magnet alloy rings 7 or unseating of the permanent magnet alloy rings 7 may be minimised. Reference 1. Bearing 2. Rotor bearing half 5 3. Permanent magnet bearing 4. Stator 5. Rotor 6. Stator bearing half 7. Permanent magnet alloy hng(s) 10 8. Magnet bearing cavity

Claims

1. A permanent magnetic bearing ring of a vacuum pump, the bearing ring comprising a magnet alloy and wherein an inward facing surface of the bearing ring and / or an outward facing surface of the bearing ring is coated by a substantially continuous coating with a mean thickness of less than 10 microns.

2. The permanent magnetic bearing ring according to claim 1 wherein the coating has a mean thickness of from about 0.01 microns to about 10 microns, preferably from about 0.1 microns to about 5 microns3. The permanent magnetic bearing ring according to claim 1 or 2 wherein the magnet alloy is selected from the group consisting of a neodymium magnet or magnet alloy, praseodymium magnet or magnet alloy, and a samarium cobalt magnet or magnet alloy.

4. The permanent magnetic bearing ring according to claim 3 wherein the bearing ring is magnetised.

5. The permanent magnetic bearing ring according to any preceding claim wherein the coating is selected from the group consisting of conversion coating(s), such as phosphate conversion coating(s); fluoropolymer coating(s); graphene coatings; electroplating; and plasma coating(s).

6. The permanent magnetic bearing ring according to any preceding claim wherein the coating is a phosphate coating, preferably a phosphate coating applied to the magnetic bearing ring by reaction coating the magnet alloy of the bearing ring in a solution comprising phosphoric acid.

7. The permanent magnetic bearing ring according to any preceding claim wherein substantially all of the outer surface of bearing ring is coated with the coating.

8. A permanent magnetic bearing of a vacuum pump comprising one or more bearing rings according to any preceding claim, wherein the bearing comprises an inner bearing half and an outer bearing half and wherein substantially all of the outersurface of the or each inner and / or outer bearing half is or are coated with the coating.

9. A vacuum pump comprising a permanent magnetic bearing ring according to any one of claims 1 to 7 or a permanent magnetic bearing according to claim 8, preferably wherein the vacuum pump is a turbomolecular pump.

10. A method of manufacturing a permanent magnetic bearing for a vacuum pump, the method comprising the steps ofa) providing a bearing ring comprising a magnet alloy,b) coating a surface of the magnet alloy with a coating having a mean thickness of less than about 10 microns, andc) magnetising the magnet alloy.

11. The method according to claim 10 wherein the bearing ring is provided in a bearing insert holder for locating the bearing ring in bearing cavity of a vacuum pump, preferably wherein an additional coating is applied to an or each exposed surface of the magnet alloy once the bearing ring is fitted in the insert holder.

12. The method according to claim 10 or 11 wherein step b) is performed by a technique selected from the list consisting of reaction coating, including phosphate coating, dip coating, spray coating, slot die coating, spray coating and combinations thereof, preferably wherein step b) is performed by reaction coating the magnetic bearing ring in a phosphoric acid solution.

13. The use of phosphoric acid to reaction coat a magnet alloy to provide a bearing ring for a vacuum pump, preferably wherein the magnet alloy is a neodymium magnet or neodymium magnet alloy or a praseodymium magnet or magnet alloy.

14. The use of a neodymium magnet or neodymium magnet alloy and / or a praseodymium magnet or magnet alloy, in a permanent magnetic bearing of a vacuum pump wherein at least a portion of the neodymium magnet or neodymiummagnet alloy or praseodymium magnet or magnet alloy is coated with a coating and said coating has a mean thickness of less than about 10 microns.

15. The use of a neodymium magnet or neodymium magnet alloy or5 praseodymium magnet or magnet alloy according to claim 14 wherein the coating is a phosphate coating.s

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