WO2013124388A2

WO2013124388A2 - Bearing and bearing assembly

Info

Publication number: WO2013124388A2
Application number: PCT/EP2013/053503
Authority: WO
Inventors: Gerald Francis Flynn; Klaus Tank
Original assignee: Element Six Gmbh; Element Six Abrasives S.A.
Priority date: 2012-02-23
Filing date: 2013-02-21
Publication date: 2013-08-29
Also published as: GB2499902A; WO2013124388A3; GB201303097D0

Abstract

A composite bearing is provided comprising: (i) a continuous annular element (15) made from a silicon-carbide-cemented diamond (SCD) composite material or from silicon carbide; and (ii) an element (24) comprising another superhard material optionally diamond material which has been deposited or grown onto the silicon- carbide-cemented diamond (SCD) or silicon carbide element (15). The other superhard element (24) covers at least part of a surface of the SCD or SiC annular element (15) and provides at least part of the wear surface of the composite bearing.

Description

BEARING AND BEARING ASSEMBLY

FIELD

This disclosure relates to a bearing and to a bearing assembly, including a thrust bearing and thrust bearing assembly, and a radial bearing and a radial bearing assembly, especially such assemblies incorporating diamond composite materials, and including composite bearings comprising one element which is a diamond composite material and another element which is another superhard material. This disclosure also relates to a combined bearing incorporating not only a thrust bearing but also a radial bearing and to a combined bearing assembly.

BACKGROUND

Thrust bearings are a particular type of rotary bearing, and are designed not only to permit rotation between parts, but also to support and transmit a high axial load while doing this. Thrust bearing assemblies are used in a number of applications, for example in downhole drilling systems. In a typical downhole drilling system two mating thrust bearings, which are typically annular, rotate relative to one another. These are known respectively as a stator, which does not rotate, and a rotor which does rotate, the rotor being driven by a motor and being fixed relative to an output shaft which typically turns a drill bit head. Both rotor and stator thrust bearings are subject to an axial as well as a relative rotationary load during the drilling process. In a drilling application, thrust bearings have a primary function to transmit load in an axial direction in the direction of drilling

Radial bearings in contrast are bearings designed to support radial loads. They may typically be annular in configuration and arranged between relatively rotating cylindrical surfaces.

Thrust and radial bearings are generally both annular in shape, with opposed substantially flat surfaces, and substantially cylindrical curved inner and outer surfaces between those flat surfaces. In a thrust bearing it is one or both of the flat surfaces that generally provides the wear surface, and in a radial bearing it is one or both of the substantially cylindrical surfaces that generally provides the wear surface. Depending on the specific application, thrust bearings and radial bearings may be used independently, side by side, or combined in a single unit. In drilling applications it is often the case that two pairs of mating thrust bearings are used along the same drilling line, one pair being used to drill into the rock or other substrate, and the other pair being used to drill out of the rock or other substrate.

One example of a thrust bearing assembly is described in US7552782 which describes rotor and stator bearings each comprising a plurality of discrete spaced apart superhard compacts which provide the mating bearing surface of the rotor. Each superhard compact comprises a superhard table of a superhard material such as polycrystalline diamond (PCD) bonded to a substrate of a cobalt cemented tungsten carbide. Other materials mentioned for the superhard table include silicon carbide, a silicon carbide polycrystalline diamond composite, polycrystalline cubic boron nitride, and chemical vapour deposition diamond (CVDD). This type of bearing assembly using compacts of PCD diamond on a tungsten carbide substrate is typical of that commonly used in the field.

Diamond, for example the PCD used in US7552782, is an example of a superhard material. As other known examples of superhard materials, there may be mentioned (i) ceramic diamond composite materials comprising SiC and diamond made using low pressure methods (which are discussed in more detail later in the specification and referred to as SCD), (ii) ceramic diamond composite materials comprising SiC and diamond made using high pressure for example those known as Syndax™ , (iii) Chemical vapour deposition diamond (CVDD), and (iv) polycrystalline cubic boron nitride (PCBN) material.

US5368398 and US5364192 also describe diamond thrust bearing assemblies. In these references sintered tungsten carbide support elements support a series of composite PCD compacts which are located in end pockets of the support elements, the compacts typically being round cylindrical in shape and carried in a circumferentially spaced relationship to one another. US 5092687 describes similarly circumferentially spaced substantially cylindrical diamond bearing pads.

US7255480 describes thrust bearings comprising shaped PCD diamond bearing pads on support rings, the shape of the pads described being hexagonal, rhomboidal, triangular and circular, with a plurality of micro-channels between the bearing pads. US7896551 describes a thrust bearing assembly including a support ring that carries a plurality of circumferentially adjacent arcuately shaped bearing segments with a seam formed between each segment, so that the bearing segments collectively form a substantially continuous bearing element. This reference also envisages forming a substantially continuous superhard bearing surface by depositing a layer of diamond onto a generally planar surface of a steel support ring, for example by chemical vapour deposition. The use of CVD polycrystalline diamond coatings to enhance surface hardness on steel substrates has a number of technical issues that have prevented them from finding practical and wide spread applications. CVD diamond layers are deposited at temperatures in the range of 400 to 1 100°C, ideally above 700°C, in the presence of methane and hydrogen gases heated to above 2000°C. Under these conditions carbon is soluble in the steel and rather than a polycrystalline diamond film being deposited the steel substrate undergoes carburization and graphitic layers are deposited. The mitigation to this is usually to use some sort of interlayer coating that forms a stable carbide. However this does not solve the underlying issue with the temperature and hydrogen stability of the substrate steels at CVD diamond growth conditions, which leads to a degradation of the mechanical properties. Diamond has an extremely low expansion coefficient while steels are typically 3 to 4 times larger. At the operating temperatures of the coated component this mismatch gives rise to tensile and compressive residual stresses in the diamond layer and the substrate, which can result in delamination of the coatings when under loading in their chosen application. Bearing parts comprising polycrystalline diamond compacts are generally manufactured using a high pressure high temperature technique (HPHT) applied to diamond particles held in a binder including Group VIII metals such as cobalt, nickel iron or manganese. The resulting PCD compact therefore is typically rich in Group VIII metals, and it has been noted in US5560716 that where a diamond bearing assembly utilises a diamond material containing a Group VIII metal the friction behaviour of the contacting bearing surfaces tends to become quite unpredictable during high contact pressures. In some cases rapid seizure of the bearing assembly occurs without adequate warning, and this phenomenon is attributed in US5560716 to the presence of a Group VIII tribofilm which forms on the diamond surfaces during use of the bearing assembly. US5560716 attempts to overcome these problems by providing a diamond bearing assembly comprising opposed diamond bearing surfaces, at least one of which is free of group VIII metals. The one that is preferred is the superhard material mentioned above which is a ceramic diamond composite material comprising SiC and diamond made using high pressure, and which because of its method of manufacture contains substantial diamond to diamond bonding together with a second phase consisting essentially of silicon, the silicon being in the form of silicon and/or silicon carbide. The diamond content will generally be 80 to 90 percent by volume and the diamond to diamond bonding will generally be such as to form a coherent skeletal mass. As mentioned above an example of such a body is that sold under the trade name Syndax (trademark), Syndax composite being a diamond/silicon carbide composite formed under HPHT conditions and exhibiting diamond to diamond intergrowth.

The PCD compacts described above and the Syndax composite parts, because of the manufacturing constraints imposed by the HPHT process used to make them, are limited in the size of composite parts that can be made. Both PCD and Syndax can be produced in a single disc format which may then be EDM processed to discrete units of the desired shape or alternatively can be produced as discrete parts. The max diameter of a single disc is determined by the HPHT reaction chamber. Typically single parts can be produced ranging in size from about 10-100mm. Another superhard material also mentioned above is silicon cemented diamond material, or SCD material, which is a ceramic diamond composite materials comprising SiC and diamond made using low pressure methods. SCD material has not hitherto been publicly disclosed for use as a bearing material. Manufacture of SCD is described for example in US6447852, US6709747, and US7008672. The process consists essentially of forming a workpiece by shaping diamond particles and a binder into what is known as a "green body", heating the workpiece so as to graphitise some of the diamond particles, and then infiltrating the body with silicon or a silicon alloy, thereby creating a final body in which most of the silicon has reacted with the graphite and/or some diamond to form silicon carbide. The final body consists of a composite in which diamond particles are embedded in a skeletal matrix of silicon carbide. In SCD material there is generally no diamond-diamond bonding and substantially no diamond-diamond contact given the skeletal matrix of silicon carbide and the typical relative mass and volume percentages of diamond material and silicon carbide material and free silicon. Advantageously SCD composite parts may be sintered at low pressures below 5000 kPa (50 bars). SUMMARY

In one embodiment of the invention there is provided a composite bearing comprising: (i) a continuous annular element made from a silicon cemented diamond (SCD) composite material; and (ii) an element comprising another superhard material which covers at least part of a surface of the SCD annular element and provides at least part of the wear surface of the composite bearing.

In other embodiments in place of the SCD composite element a continuous annular element of SiC could be used. The replacement of the SCD composite element with a SiC element of similar shape and size applies to all embodiments described in this specification. Therefore where reference is made to a SCD composite element it is understood that analogous embodiments in which the SCD composite element is replaced by SiC also are envisaged. Where a SiC annular element is used small amounts of additives may be present, typically less than 10% or less than 5% or 1 % by weight. The silicon carbide may be a or β phase SiC.

The SCD composite material when used in this and all other embodiments of the invention may be one that has been made using a low pressure method, typically less than 5000 kPa (50 bar).

When reference is made to silicon-carbide cemented diamond composite material or SCD material (sometimes also referred to as silicon cemented diamond material) we mean a diamond composite material in which diamond particles are embedded in a silicon carbide skeletal matrix with substantially no diamond- diamond contact, since diamond crystals in SCD material are substantially always separated from each other by a layer of silicon carbide, this being observable by SEM images of cross-sections of the material. Sometimes free silicon is also present, but it is silicon carbide that provides the embedding matrix.

SCD material typically contains a lower percentage by volume of diamond material compared with Syndax diamond material referred to in the Background section of this specification. The diamond content in SCD material in general, and in the SCD material used in the present invention is typically in the range 20-55 volume percent, while in Syndax material it is typically in the range 70-95 volume percent, the higher diamond content being possible due to the HPHT manufacture method. As used herein, a "superhard material" is a material having a Vickers hardness of at least about 28 GPa. Diamond, especially CVD diamond, cubic boron nitride (cBN), polycrystalline diamond (PCD) and polycrystalline cubic boron nitride (PCBN) material are examples of superhard materials.

Advantageously the use of SCD material (a hybrid super hard composite material) in bearings according to the invention removes the geometrical and size limitations for the design of a super hard bearing which are present when prior art high temperature high pressure (HPHT) diamond composites are used. It also provides significant benefits for performance and durability and at reduced costs compared with HPHT prepared diamond composite materials of the prior art. A SCD material (a

diamond/Silicon Carbide composite material) may be manufactured under conditions of low pressure (e.g. less than 1 ,000bar) and high temperature. The pre cursor starting material may be produced in granular form. The granules may be formed into the desired shape, size and geometry of the bearing part, thereby forming a green body, and the green body part then sintered at low pressure through a reaction bonded silicon infiltration process performed at low pressure (e.g. less than 1 kbar) and at temperatures in the region of, 1 ,400°C. The SCD material so formed contains silicon carbide formed by the infiltrating silicon reacting with graphitised diamond, the silicon carbide content typically being in the range 40-70 % by volume and diamond content in the range 30-60 percent by volume. The method of manufacture of SCD therefore involves the pressing of a green body to the desired size and shape required, followed by subsequent reaction bonded sintering in the presence of liquid Silicon.

The nature of the SCD material and the process of manufacture allows a super hard composite substrate material to be formed in a diverse range of sizes, shapes and formats e.g. convex, concave .continuous & arcuate profiles which would not otherwise be easily achievable using a super hard material produced using high pressure high temperature processes (HPHT), since using the latter processes only simple shapes, such as simple cylindrical shapes can easily be formed, and it other shapes are needed this involves a costly cutting or forming process in a separate subsequent step. In some embodiments the composite bearing provided is a thrust bearing and in other embodiments it is a radial bearing. In still further embodiments which are described in more detail later it is a combined thrust and radial bearing. In the embodiment described above, in addition to the annular SCD element the composite bearing is provided with another element made from a superhard material. In some embodiments this other superhard element comprises CVD (chemically vapour deposited) diamond material.

In some embodiments the said another superhard element provides the entire wear surface of the composite bearing; in other embodiments the said another superhard element in combination with the SCD element provides the entire wear surface of the composite bearing; in other embodiments another material also provides part of the wear surface. The wear surface may be a continuous element, or it may contain channels therein, for example to allow mud flow during operation where the bearing is used in oil and gas drilling applications. A continuous bearing surface has significant benefits in helping to achieving near hydrodynamic operating conditions which is generally desired in bearing assemblies as under these conditions interfacial bearing friction is minimised and bearing efficiency is maximised.

When we talk about the "wear surface" of the bearing we mean the surface of the bearing that first meets another bearing surface when placed in a bearing assembly comprising two mating thrust bearings. For example in the bearing assemblies described above with reference to the prior art which used PCD compacts on a support element the mating faces of the PCD compacts provide the wear surface. The skilled person understands that other parts of the bearing assembly away from the wear surface may also be subject to wear in operation, for example mud carrying particulate debris flowing through a bearing assembly in a drilling operation exerts an abrasive action on the support elements carrying the PCD compacts in the bearing assemblies discussed with reference to the prior art. In this specification we shall talk about wear away from the wear surface as subsidiary wear. Such subsidiary wear may be damaging in operation since for example wear of the support element holding the PCD compacts in the prior art may result in loosening and even loss of the PCD compacts from their support element.

CVD Diamond, another superhard material, has excellent adhesion to ScD through the combined mechanism of hetero and homo epitaxial growth of the CVD diamond onto the SCD. SCD material is particularly suited as a substrate for diamond overgrowth using CVD methods. SCD has a high resistance to thermal degradation, which is advantageous due to the excessive temperatures generally present during diamond CVD growth process and substantially prevents degradation of the SCD substrate. Also SCD material has a relatively low coefficient of thermal expansion (CTE) providing a close match to that of the relatively low CTE of a deposited CVD diamond layer. This matching of CTE of substrate and deposited layer minimises residual stresses between the SCD and diamond coating after the CVD coating process. Typically the CTE of the SCD composite material and the CVD diamond layer are about 1.0 - 2.5 x10^"6 K^"1.

The SiC present in SCD composite elements may be β phase. The structure of the β phase present in some SCD composite elements may be described as a diamond cubic structure. This stable structure accounts for the excellent thermal phonic conduction properties of the material.

Also SCD material has a relatively high elastic modulus (about 400-750 GPa) which advantageously provides a stiff substrate for the deposited CVD diamond layer and substantially avoids cracking and delamination of the CVD diamond layer and provides mechanical integrity for assembly and handling and durability during application. Furthermore optimum deposition of a CVD layer onto a substrate occurs when homo- epitaxial growth takes place, and this is possible with an SCD substrate. Homo epitaxial growth on a SCD substrate occurs as a result of the exposed individual diamond crystals on the surface of the SCD composite providing potential nucleation and growth sites for the subsequent deposition of the CVD diamond layer. Between the exposed diamond particles on the surface of SCD is SiC binder phase, typically β phase SiC binder phase. Those surface regions consisting of the β phase SiC binder phase compliment the homo-epitaxial growth process through the promotion of hetero-epitaxial growth on the SiC phase. This mixed mode growth of the CVD diamond layer over the preformed SCD substrate material on both the exposed diamond crystals and interstitial Silicon carbide phase takes place to varying degrees and ensures excellent adhesion to the underlying SCD substrate material. The unique combination of SCD characteristics e.g. low coefficient of thermal expansion value, excellent thermal conductivity, thermal stability and high value of elastic modulus combine to provide a robust and durable interface between the two super hard materials. These embodiments of the present invention which provide a hybrid material composite bearing component consisting of a pre-formed superhard SCD substrate portion overgrown with a CVD diamond over layer have several benefits over the prior art. The CVD diamond over layer significantly increases the resistance to abrasive wear beyond that of the SCD substrate material alone whilst the SCD substrate (which has good thermal conductivity of about 400-600 Wm^"1K^"1) allows excellent conduction of heat away from the working bearing surfaces and thereby significantly reduces the opportunity for thermal damage to occur of the bearing parts; this can further be enhanced by introducing profiles e.g. channels or mosaic features on the surface of the SCD bearing element prior to CVD Diamond coating.

Profiling of the bearing segments may also be needed for the operation of some bearing components. For example radial bearings, which as mentioned above are typically annular with the curved inner and outer surfaces providing the wear surfaces, frequently have convex and concave mated profiles. SCD materials, because of their manufacturing process which involves a low pressure formation of a green part as described in the background to this specification, allows convex and concave mated profiles and other complex profiles to be easily achieved during the green body stage of manufacturing of the SCD element.

Another advantage of the composite bearing provided by embodiments of the invention is that the component elements of the bearing provide different features optimising the performance of the bearings in applications where they need the operate in a mixed regime for example in regimes where the bearing is exposed to several wear mechanisms simultaneously, which can include, for example, both sliding abrasive wear and slurry erosion abrasive action e.g. mud flow often experienced in downhole Oil and Gas applications. In such applications, the CVD diamond coating provides a highly inert protective coating which, in addition to the benefits described earlier, also serves as a physical and chemical barrier against corrosive agents which may be encountered in certain application types e.g. oil and gas subterranean drilling, thus allowing the desirable wear properties to be maintained by substantially preventing the corrosive attack on the SiC binder phase and premature erosion of the "softer" SiC phase. In some embodiments the thickness of the said another superhard element on the SCD element is in the range 10-50 microns. By over growing the SCD material with for example a CVD diamond layer, typically of a thickness in the range 10-50 microns the less hard SiC phase in the SCD material may be protected from both chemical and physical attack during a range of application conditions under a variety of abrasive, erosive wear mechanisms and chemical environments. Composite bearings according to certain embodiments of the invention which comprise another superhard element over a SCD element have advantages over a SCD element used alone for some applications. For example SCD bearing elements used alone may be vulnerable to corrosive attack of the Silicon Carbide phase in the SCD composite material. This class of diamond composite materials can be vulnerable to corrosive attack in the presence of certain chemical species e.g. H₂S0₄ and hydroxyl containing groups which may result in a reduction of material strength.

Another embodiment of the invention provides a thrust bearing comprising an annular element made from a silicon cemented diamond composite material (SCD material). This may be used alone, without an additional covering superhard element, or with an additional covering superhard element.

In certain embodiments of bearings according to the invention the SCD or SiC annular element of the bearing is supported on an annular carrier element. This may comprise steel or a cemented carbide or another material.

In embodiments where the SCD part is covered at least in part by another superhard element, the SCD annular ring provides part of the bearing, but is acting at least partly as a carrier, being at least partially covered by another superhard material.

In certain embodiments a composite bearing is provided which is a thrust bearing, wherein the annular SCD element presents a substantially flat surface at least part of which is covered by the said another superhard material to provide at least part of the wear surface of the composite thrust bearing. In other embodiments a composite bearing is provided in which annular SCD element presents a non-planar shaped surface which is covered at least in part by the said another superhard material which provides at least part of the wear surface of the composite bearing. Where the bearing is a radial bearing, this non planar surface of the annular SCD element may be, for example the curved, substantially cylindrical inner or outer surfaces of the SCD element. Where the bearing is a thrust bearing, the substantially non planar surface may be for example a channelled, or a castellated shaped surface. For example, for a thrust bearing, the SCD ring may be substantially flat with circumferential ridges of another superhard material extending around it projecting up from the substantially flat surface. In another embodiment the SCD annular ring may have a thrust bearing surface which has been pre-shaped (either by moulding in that configuration or by machining in its green state), so that it has for example a shape that is castellated in cross section with the castellated turrets extending as circumferential ridges around the annular SCD element. In this case the other superhard material may be laid down as a layer on the tops of the castellated turrets, or over the entire, or any part of the surface of the castellated annular SCD element. For example the other superhard material may be CVD (chemically vapour deposited) diamond material (polycrystalline). For example to provide the ridged arrangement as discussed or any other partial covering of the SCD annular element a process involving masking part of the annular SCD element and then carrying out CVD of diamond material may be used. Where the superhard material is provided as circumferential ridges on the annular SCD element the arrangement will appear to be castellated looking through a cross-section of the partially covered SCD annular ring. CVD diamond being grown by hetero-epitaxial growth bonds well to SCD material. The SCD ring also provides subsidiary wear resistance in these embodiments.

In one embodiment the SCD annular element has opposed flat ring-shaped surfaces and the said another superhard material that provides at least part of the wear surface of the composite bearing is provided as circumferentially-extending or radially-extending ridges on the ring-shaped flat surfaces of the SCD element.

Where the said another superhard material is CVD diamond material this may, for example, be laid down on the annular SCD element by a process involving masking part of the annular SCD element and then carrying out continuous vapour deposition of diamond material.

As mentioned above, in some embodiments the SCD or SiC annular element is secured to a carrier element. The SCD or SiC annular element may be profiled and an additional intermediate layer mechanically secured thereto between it and the carrier element to assist or effect the securement. In one embodiment, which is a thrust bearing, where the annular SCD or SiC element of the composite bearing has inner and outer curved surfaces which do not provide the wear surface, these curved surfaces are provided with projections or depressions arranged circumferentially therearound; a metal layer has been pressed into mechanical engagement with the said projections or depressions on the said inner and outer curved surfaces of the SCD or SiC annular element; and the metal layer has been secured to the carrier element, thereby securing the composite bearing to the carrier element. In another embodiment, which is a radial bearing secured to a carrier element via an intermediate metal layer, the annular SCD or SiC element of the composite bearing has opposed substantially planar surfaces which do not provide the wear surface, and which are provided with projections or depressions thereon; a metal layer has been pressed into mechanical engagement with the said projections or depressions on the SCD or SiC annular element; and the metal layer has been secured to the carrier element, thereby securing the composite bearing to the carrier element.

Other embodiments of the invention provide a composite bearing which is a combined bearing providing not only a thrust bearing but also a radial bearing, the continuous annular element made from silicon cemented diamond (SCD) composite material or SiC being substantially L shaped if viewed in cross section through any part of the annulus, at least part of the base of the "L" providing a thrust bearing wear surface and at least part of the stem of the "L" providing a radial bearing wear surface. The base of the "L" that provides the thrust bearing wear surface may be substantially planar or not. For example it may be shaped, for example to have a castellated cross-section, for example when the SCD body is in its green state prior to sintering. The stem of the "L" that provides the radial bearing wear surface may be concave or not, to match a mating radial bearing surface.

Another embodiment in which the SCD or SiC annulus provides part of the bearing but is acting as a carrier partially covered by another superhard material is a second embodiment of combined bearing providing not only a thrust bearing but also a radial bearing. In this second embodiment of combined bearing the continuous annular element made from silicon cemented diamond (SCD) composite material or SiC is substantially L shaped if viewed in cross section through any part of the annulus, the base of the "L" being covered partly only by PCD composite segments or compacts to provide a thrust bearing wear surface and the stem of the "L" being covered partly only by PCD composite segments or compacts to provide a radial bearing wear surface.

Other embodiments of the invention provide a composite bearing which is a combined bearing providing not only a thrust bearing but also a radial bearing, the continuous annular element made from silicon cemented diamond (SCD) composite material or SiC being substantially L shaped if viewed in cross section through any part of the annulus, the base of the "L" being covered partly only by PCD composite segments or compacts to provide a thrust bearing wear surface and the stem of the "L" being covered partly only by PCD composite segments or compacts to provide a radial bearing wear surface.

Embodiments of the invention also provide a bearing assembly comprising first and second bearings with mating bearing surfaces, one or both of the first and second bearings being a composite bearing of the types described hereinbefore, and in particular comprising an annular element made from a SCD composite material or SiC.

In other embodiments there are provided bearing assemblies comprising a first bearing which is a composite bearing of the type described herein before comprising a continuous annulus of SCD material or SiC, and a second bearing which has a different construction (typically of the type described in the prior art) providing a plurality of segments made from a superhard material at least the wear surface of these segments being separated circumferentially from each other.

In some embodiments both the first and second bearings comprise an annular element made from a silicon cemented diamond composite material or SiC. In other embodiments only the first bearing comprises an annular element made from a silicon cemented diamond composite material or silicon carbide and the other bearing has a different construction providing a plurality of segments made from a superhard material at least the wear surface of these segments being separated circumferentially from each other. The plurality of segments may also be supported on a carrier annular element, which may comprise, for example steel or a cemented carbide. Each segment may comprise a carrier segment of a cemented carbide which is arranged circumferentially adjacent its neighbouring segments leaving a small gap (less than 0.51 mm) to allow for thermal expansion at elevated temperatures, and also comprise an intermediate cemented carbide element that is shorter in length circumferentially than the carrier segment, so as to leave a substantial gap(in the range 3mm to 60mm) between itself and a corresponding intermediate cemented carbide element on its neighbouring segments. The intermediate cemented carbide element may be provided with a PCD wear surface layer; that PCD wear surface layer may or may not be coterminous with the intermediate cemented carbide layer. This substantial gap permits mud flow and prevent overheating where the bearing is used in drilling or similar operations. Any mud (which may be carrying abrasive particles) contacts only PCD or WC material when it flow through the gap and there is substantially no mud/steel contact which could be damaging and result in failure at the junction with the steel. Embodiments of the invention also provide the use of a thrust bearing, a radial bearing, or a thrust or radial bearing assembly or a combined bearing or a combined bearing assembly of the type disclosed herein in a drilling system. When so used the drilling system may comprise first and second mating bearings of the same or different construction. One of the bearing may be arranged as a plurality of segments made from a superhard material at least the wear surface of these segments being separated circumferentially from each other, the separation of the segments being sufficient to allow mud from the drilling operation to flow through the bearing system. Other embodiments of the invention provide a method of making a composite bearing of the type described above, comprising a continuous annular element, the method comprising forming the SCD annular element by forming a work piece having a predetermined annular shape and size by pressing diamond particles and a binder together in a mould thereby creating an intermediate annular body, optionally machining the intermediate body, then infiltrating silicon into the intermediate body to form a diamond/silicon carbide/silicon composite finished part; then optionally applying the said another superhard element to the formed SCD annular element . The intermediate body and the finished part may be any suitable shape, not only simple shapes, for example castellated in cross section.

The SCD composite material or SiC may provide a continuous annular wear surface for the bearing. There may or may not be an additional superhard element covering all or part of the wear-facing surface of the SCD or SiC element. Several embodiments of the invention recognise that it is possible to use a SCD composite material or SiC within a thrust or radial or combined bearing, and moreover that it is possible to modify the conventional circumferentially spaced diamond compact design of bearing used in the prior art and by using an SCD composite material, with or without a superhard covering element, or SiC with another superhard covering element, provide a continuous hard wearing diamond containing surface for the bearing. While the wear properties of SCD material are lower than that of PCD diamond (SCD has a Knoop hardness of about 30-40 GPa whereas PCD has a hardness of about 50 GPa), they are still sufficiently high to provide a long lifetime bearing surface. The continuous surface of the SCD thrust bearing avoids one of the problems that is sometimes encountered with the more traditional circumferentially spaced PCD compact bearing surfaces, and alluded to above, which is that as a result of high pressure flow of drilling fluid carrying abrasive particulate material the region between the super hard PCD compact is frequently eroded resulting in reduced service life of the bearing assembly and premature loss or failure of the bearing elements. With the continuous surface provided by the SCD bearing (with or without an additional superhard element covering), or by SiC with an additional superhard covering element, such inter-compact erosion areas are avoided. The use of SCD material is particularly advantageous compared with prior art HPHT processes as it allows bearings of the size frequently used in drilling and other applications to be made in a continuous part due to its low pressure manufacturing process. In contrast PCD material has not been made and could not easily be made in sufficiently large sizes to form a continuous bearing of the type used in drilling application. Typically SCD annular bearings exemplified in this disclosure may be made having an outer diameter in the range 25 mm to 250mm but smaller or indeed larger bearings are also envisaged. Particularly when considering large sizes, it is noted that unlike in the manufacture of PCD or any other diamond by a HPHT process, the usual tooling restrictions imposed by the HPHT process to the maximum size of the part do not apply for SCD bearings, since the SCD process is a low pressure process. The ability to make large bearings is an advantage of the presently disclosed SCD bearings, making them suitable in applications which it is not possible to solve using conventional HPHT-manufactured parts. Therefore particular embodiments of bearings according to the present disclosure have an outer diameter of at least 100 mm, even at least 150 mm, or at least 200 mm. Typical SCD annular bearings exemplified in this disclosure may be made having an inner diameter in the range 20mm to 240mm, and a thickness in the range 5mm to 50mm, but again larger, smaller, and thicker and thinner bearings are also envisaged. In contrast solid cylindrical PCD compacts are generally made having an outer diameter of about 13mm-16mm.

As mentioned above the manufacture of SCD parts typically comprises forming a workpiece by shaping diamond particles and a binder into what is known as a "green body", heating the workpiece so as to graphitise some of the diamond particles, and then infiltrating the body with silicon or a silicon alloy, thereby creating a final body in which most of the silicon has reacted with the graphite and/or some diamond to form silicon carbide. At the green body stage, the part is readily machinable, and therefore there is the option to machine the green body prior to sintering which presents advantages for the design of the superhard SCD bearings, and provides for example the opportunity to introduce non planar, and non-curved surface features for example irregular surface features on the bearings or regular or irregular upstanding or depressed projections or channels or undercut portions in the surface; for example the bearing may be shaped to be castellated in cross or side section. By "castellated" we mean a shape similar to battlements with spaced upper surfaces separated by intervening lower surfaces with upper and lower surfaces being joined by substantially vertical surfaces, upper, lower and vertical all being used as relative terms rather than to denote any actual specific orientation.

For bearings as described herein, the SCD or SiC annulus may be substantially rectilinear e.g. rectangular in cross-section, or L-shaped in cross section, or have any other regular or irregular cross-section. Where the SCD or SiC annulus is substantially rectilinear in cross section corners may or may not be smoothed, curved or tapered. That cross-section may be uniform or non-uniform measured around the annulus. The SCD or SiC annulus may comprise inner and outer closed loop curved surfaces and opposed flat surfaces which flat surfaces either provide the wear surface for thrust bearing, or act as a carrier being covered partially only by another superhard material which itself provides the wear surface. However, the wear surface is not necessarily flat, and may have another shape for example convex or concave. Where the disclosure relates to a combined bearing with a radial bearing element, then the wear surface of the radial bearing element will typically be concave for the outer bearing which has an inner wear surface), and convex for its mating inner bearing since has an outer wear. The wear surface may be concave or convex in profile, and this is the case for radial bearings.

As mentioned above, in certain embodiments the SCD or SiC annular element is supported on a carrier annular element, which may comprise, for example steel or a cemented carbide. The SCD or SiC part may be directly secured e.g. brazed to the carrier ring, but SCD is not readily brazeable to materials such as steel or cemented carbide, so in some embodiments a different method of securement is used; the annular SCD element is provided with projections or depressions arranged circumferentially around both its inner and outer curved surfaces and then a thin layer of a metal part, e.g. mild steel, is pressed into mechanical engagement with the projections or depressions arranged circumferentially around both the inner and outer curved surfaces of the SCD annular part. A convenient means of applying that pressure is cold isostatic pressing (CIPing), though other means may be used. Typically the thickness of the metal layer is in the range 0.10mm to 0.25 mm, for example it may have a thickness of about 0.2 mm. The metal layer may then be readily secured e.g. brazed to the carrier element which may be for example steel or a cemented carbide carrier element. The braze secures the metal layer to the carrier element in two main ways: firstly the braze itself forms a chemical bond between the parts; and secondly, the braze once cooled and hardened typically forms a mechanical engagement with the parts. This mechanical engagement typically arises because the metal layer has been pressed to follow the profile of the SCD part (with its surface depressions or projections), and it follows that there is therefore generally a recess between the CIPed metal part and the typically smooth carrier element to which it is brazed, and the braze typically occupies this recess, providing an element of mechanical engagement. In some embodiments both the inner and outer surface of the metal part follows the profile of the underlying part onto which it has been pressed.

As mentioned above, in certain embodiments of the invention a bearing is provided comprising a continuous annular element made from a silicon cemented diamond (SCD) composite material or SiC, but the wear surface of the bearing is provided not by the SCD or SiC material but by another superhard material that covers part only of the annular SCD or SiC element. In this case in operation in a bearing assembly the first point of contact of the bearing with another bearing surface in operation is the other superhard material that covers part only of the annular SCD or SiC element. The SCD or SiC element in this case is providing subsidiary wear resistance, and also acting as a carrier for the other superhard material. In these cases the SCD or SiC annular element may be secured to the other superhard material in any suitable manner, for example by brazing, soldering adhesives or mechanical fixing. In these embodiments the presence of an annular SCD or SiC part is advantageous because of its easy of formability and machinability, its ability to provide subsidiary wear resistance, for example against any abrasive particles brought into contact with it, for example by mud flow, during operation of the bearing, and its ability for easy bonding to other superhard materials. For example, CVD Diamond, another superhard material, has excellent adhesion to ScD through the mechanism of hetero-epitaxial growth of the CVD diamond onto the SCD. One example where the SCD or SiC annular ring provides part of the bearing but is acting as a carrier partially covered by another superhard material is in a bearing in which the other superhard material is provided as a coating layer on the SCD or SiC annular ring.

The SCD annular bearing elements disclosed herein may have a lapped surface lapped to a surface roughness R_A value in the range 0.3 to 1 .5 μ.

It will be appreciated by the skilled person that embodiments in which SCD rather than PCD is used as part of, e.g. as part or all of the wear surface, for a thrust bearing, a radial bearing, or a combined thrust radial bearing are particularly advantageous in high temperature applications since the thermal degradation resistance of SCD is better than that of PCD, with SCD capable of withstanding temperatures up to the order of 1 ,400°C without thermal degradation, whereas PCD composite material is typically limited to temperatures up to about 750°C.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments will now be described in more detail, by way of example only, with reference to the accompanying drawings in in which:

Figure 1 is a perspective, partly cut away view of a downhole drilling arrangement including a thrust bearing assembly; Figures 2a is a plan view of a thrust bearing that may be used as the rotor in the embodiment shown in Figure 1 , or in other applications;

Figure 2b (Figures 2b.1 and 2b.2) shows alternative cross sectional views through part of the bearing shown in Figure 2a;

Figure 2c is a cross sectional view showing part of a bearing assembly incorporating the thrust bearing of Figures 2a and 2b and another thrust bearing;

Figure 3a is a plan view of another thrust bearing that can be used in combination with the thrust bearing of Figures 2a and 2b; Figure 3b and 3c show in plan view and side elevation view part of the wear components used in the bearing of Figure 3a;

Figure 3d shows a bearing assembly including the bearing of Figures 3a to 3c in combination with the bearing of Figures 2a and 2b;

Figures 4a and 4b are cross sectioned partial perspective views through two other thrust bearing embodiment; Figure 4c is a cross sectional view of the bearing of Figure 4b mating with a corresponding bearing in a particular way to form a bearing assembly with thrust and radial bearing capability;

Figure 5a is a cross-sectional view through a combined thrust and radial bearing assembly;

Figure 5b is a perspective view of one bearing from the assembly shown in Figure 5a; Figure 6 is a cross-sectional view through another embodiments of combined thrust and radial bearing assembly;

Figure 7 is a longitudinal sectional view through two pairs of bearings assemblies (thrust and radial) in a drilling arrangement; and

Figure 8 is a longitudinal sectional view through a pair of radial bearings in a radial bearing assembly.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring now to the drawings, Figure 1 is a perspective view showing an application in a downhole drilling arrangement for a thrust bearing assembly embodiment 1 of the invention. The thrust bearing assembly 1 comprises a rotor 3 and a stator 5, the rotor 3 being driven by a motor (not referenced, and thereby rotationally driving output shaft 7 and in turn the drill-bit head 9. The rotor 3, output shaft 7 and drill-bit head 9 all turn together relative to stationary stator 5 and housing 1 1 . Rotor 3 and stator 5 may have different embodiments according to different embodiments of the invention. In the embodiment shown in Figure 1 and also in Figure 2 (which is discussed below) one bearing (rotor 3) comprises a support continuous ring of cemented tungsten carbide 13, and a continuous annulus or ring of SCD material 15 secured thereto. Similarly the mating bearing (stator 5) comprises a continuous ring of cemented tungsten carbide 17, and a continuous annulus or ring of SCD material 19 secured thereto. Thus the mating bearing wear surfaces of rotor bearing 3 and stator bearing 5 are two continuous rings of SCD material 15 and 19 respectively. The continuous nature of the SCD rings 15 and 19 means that any flow of drilling fluid carrying abrasive particles acts only on the wear surface of the SCD rings 15 and 19, and does not affect their point of attachment to the supports rings 13 and 17.

Figures 2a is a plan view of a bearing that is used as the rotor in the embodiment shown in Figure 1 , or in other applications. It shows a cemented tungsten carbide support ring 13 which has an annular channel 14 in which the SCD annular wear part 15 is embedded. This embedment of the SCD annulus 15 in the tungsten carbide support ring13 and the manner of securement of the SCD ring 15 to the carbide support ring 13 is most readily understood by reference to Figure 2b (Figure 2b.1 or Figure 2b.2) which is are alternative possible cross sectional views of Figure 2a taken along AA. Here it can be seen that the SCD annulus 15 includes circumferential grooves 21 and 22 extending around the inner and outer curved surfaces of the annular wear part 15. Such grooves may be incorporated into the SCD part either during formation of the green body, by pressing the green body in an appropriately shaped mould, or by machining the green body once it has been formed. A thin channel shaped steel part 23 having a thickness of about 0.2 mm is then cold isostatically pressed (CIPed) into place around the SCD part so it is mechanically secured thereto. The metal channel 23 can then be secured within the annular channel 14 in the cemented tungsten carbide carrier ring by brazing, soldering, welding or the like. Thus the SCD annulus 15 is secured to the tungsten carbide support 13. Direct brazing between cemented carbide 13 and the SCD annulus 15 may be used but the brazing of these materials is known to be difficult and involve complex brazing considerations, so the intermediate metal channel layer 23 described here provides a convenient method of securement for the SCD part 15 to the support ring 13. Figure 2b.1 shows one possible construction of the bearing which comprises the groove SCD annular part 15, whereas Figure 2.b.2 shows an additional annular element 24 made from CVD diamond material that has been deposited onto the wear surface of the annular SCD annular element, the CVD element 24 providing the wear surface of the bearing. In the embodiment of Figure 2b.2 the SCD element 15 may be replaced by a similarly sized and shaped SiC element.

Figure 2c shows a bearing assembly including the bearing 3 shown from Figure 2b in combination with a mating symmetrical bearing 5, comprising support ring 17 of cemented carbide, and SCD annular ring 19 embedded and secured thereto by a channel shaped metal part 25 CIPed in place in the manner described with reference to part 3 in Figure 2a. The bearing assembly shown in Figure 2c therefore provides a bearing assembly with mating SCD annular bearings where for each bearing the SCD ring provides the bearing wear surface indicated by reference numeral 20. The SCD annular ring 19 may optionally be coated with a CVD diamond layer (not illustrated) over part or all of its surface to provide the wear surface 20, and in this case may be replaced by a SiC element.

Mud flow through a bearing system sometimes takes place, and this is in some cases advantageous for a number of reasons. Firstly mud flow has a cooling effect on the bearing system; secondly the mud flow has a lubricating effect on the bearing system; and thirdly the mud flow has a flushing effect on the bearing system removing any debris that might otherwise accumulate, for example debris from a substrate that is being drilled in the case of bearings used in a drilling process, or debris from worn parts of the bearing system, or other parts of associated tooling. Therefore for certain applications it may be useful to include the provision for mud flow through a bearing system.

We have found that while the bearing assembly embodiment shown in Figures 1 and 2 may be suitable for some applications it may be less desirable where there is substantial mud flow generated in the drilling process since the double continuous SCD wear surfaces mate together to leave minimal space for mud flow through the bearing assembly and may result in overheating during the drilling operation. For these applications another embodiment of bearing assembly is proposed with one bearing in the assembly providing a continuous SCD bearing surface and the other bearing in the assembly providing a bearing surface with some gaps therein to allow mud flow therethrough. This other bearing is illustrated with reference to Figures 3a to 3d. For this embodiment of the invention one bearing (the bottom thrust bearing or rotor 3 as illustrated in Figure 3d) is the same as the bottom thrust bearing 3 illustrated with reference to Figures 1 and 2. This bottom thrust bearing may or may not have the CVD diamond coating element 46 on the SCD annulus 15. Where a coating is used the SCD annulus 15 may be replaced by a similarly sized and shaped SiC element. The other bearing, which is the upper thrust bearing 31 , has a different construction. As shown in Figures 3a and 3d, in common with the bottom thrust bearing 3, the upper thrust bearing 31 comprises an annular steel carrier ring 33 into which a plurality of wear segments 35 are arranged. The wear segments 35 are arranged circumferentially relative to each other with a gap 40 at the wear surface of the bearing between the PCD elements 43/44 on adjacent segments 35, the gap 40 being in the range 3mm to 60mm. The gap 40 is sufficient to allow mud flow during operation of the bearing assembly and prevent or substantially alleviate overheating. Figures 3 b and 3c show the construction of each of the wear segments 35. Each consists of a lower tungsten carbide carrier element 39, an intermediate tungsten carbide element 41 of shorter circumferential length than that of the tungsten carbide carrier element 39, and a polycrystalline diamond table 43 on the upper wear surface of the intermediate tungsten carbide carrier element 41 , and coterminous therewith. The intermediate tungsten carbide elements 41 are coterminous at one end with the carrier element 39 but fall short of the tungsten carrier element at the other end. The PCD table 43 is arranged to align with the end of the intermediate WC element 41 at the end where it is short of the carrier element 39, and at this end the PCD table 43 has a tapered surface 44 extending at an angle of about 5 degrees to the horizontal for a distance of about 5mm (exaggerated in the Figure 3c). The tungsten carbide carrier segments 39 are arranged around the steel carrier ring 33 so that there is a minimal gap 37 between adjacent carrier segments 39, sufficient to allow for thermal expansion during operation of the bearing system at elevated temperatures. This gap 37 is illustrated by a thick line in Figure 3a. The bigger gap between the intermediate tungsten carbide elements 41 and the PCD diamond tables they support, is indicated as 40 in Figure 3a and 3b. The tungsten carbide carrier elements 39 may be secured to the steel ring 33 by a low temperature braze (typically lower than 600-700 °C) while the intermediate tungsten carbide carrier elements 41 may be secured to carrier tungsten carbide segments 39 by a high temperature braze, (typically in the range 800 to 1 100 °C). This arrangement can advantageously function where mud flow carrying abrasive particles is occurring, with minimal damage to the bearing system since the mud is flowing between a continuous SCD annular ring on one surface (the lower surface as illustrated with reference to Figure 3d), and between tungsten carbide channels between the PCD elements on the other bearing surface (upper surface as illustrated in Figure 3d). There is substantially no mud flow in contact with a steel surface where damage would be most detrimental to the lifetime of the bearing system. The tungsten carbide carrier segments 39 can be brazed by braze layer 45 to the steel carrier ring 33.

Figure 4a shows a cross section through another embodiment of bearing 150. As before there is a continuous SCD annular element, but in this embodiment the SCD annulus has been shaped or formed while the body is in its green state and prior to sintering to form a cross-sectional shape which is castellated in cross section, with "turret" annular ridges 152 of SCD material being upstanding (in the orientation shown) from the lower surface (in the orientation shown) 154 of SCD. Onto the top (in the orientation show) of each of the turrets 152, a layer 156 of CVD (chemically vapour deposited) diamond has been deposited. This can conveniently be achieved by masking the intervening areas of the SCD annular ring (the sides and tops of turrets 152 before the CVD process is carried out. In this case the bearing 150 comprises an annular SCD ring, but the wear surface is provided by the CVD annular ridges 53. While the wear resistant properties of SCD are good (SCD Knoop hardness is about 40 GPa, those of CVD diamond are even better (SCD Knoop hardens of about 85-100 GPa, so this embodiment provides a bearing that is hard wearing, thermally stable, easy to assemble, and very wear resistance since the coefficient of friction between the wear surfaces (CVD diamond to CVD diamond) are very low with a coefficient of friction for diamond to diamond of about 0.05-0.1 . As before the annular SCD ring may conveniently be provided with a annular grooves (not illustrated) on its inner and outer curved surfaces and a thin mild steel metal annular channel shaped element (157) CIPed into mechanical engagement therewith, the metal element 157 then being brazed within a carrier steel element 59. As an alternative the CVD layer 156 may be laid down not only on the top of the turrets of the SCD annular ring, but also on the sides of those turrets, or on the lower SCD flat surface 154. These alternative embodiments can be carried out by appropriate masking techniques.

Figure 4b shows a cross section through a similar embodiment of bearing to that of Figure 4a. In this case the bearing is indicated generally by reference numeral 50 and it comprises a continuous SCD annular element 51 which has inner and outer curved cylindrical surfaces and flat end surfaces. In this case annular ridges of CVD (chemically vapour deposited) diamond 53 have been directly deposited onto the flat surface of the SCD annular element 51. This can conveniently be achieved by masking the intervening areas of the SCD annular ring before the CVD process is carried out. In this case the bearing 50 comprises an annular SCD ring, but the wear surface is provided by the CVD annular ridges 53. As before the annular SCD ring may conveniently be provided with annular grooves 55 on its inner and outer curved surfaces and a thin mild steel metal annular channel shaped element 57 CIPed into mechanical engagement therewith, the metal element 57 then being brazed within a carrier steel element 59. The bearing of Figures 4b (and also that of Figure 4a) can be used in combination with a corresponding bearing. That corresponding bearing may be arranged relative to the first bearing so that CVD diamond sections bear against CVD diamond sections, in other words so the turrets in Figure 4a embodiments line up with corresponding turrets in a mating version positioned against it, or so the upstanding ridges in the Figure 4b embodiment line up with the upstanding ridges in the corresponding bearing positioned against it. As an alternative, for both the Figure 4a and the Figure 4b embodiment two corresponding bearings may be arranged so the upstanding parts are offset relative to each other. This is illustrated in Figure 4c which shows the bearing 50 of Figure 4b mating with a corresponding upper bearing 50'. For the upper bearing similar parts are given the same reference numeral for the upper bearing as they have for the lower bearing but with an additional prime' notation. As seen in Figure 4c the upstanding ridges 53' of the upper bearing fit between the upstanding ridges 53 of the lower bearing. In this case thrust bearing wear is between the CVD diamond ridge 53 of the lower bearing and the SCD surface 51 'of the upper bearing, and similarly between the CVD diamond ridge 53' of the upper bearing and the SCD surface 51 of the lower bearing. In the embodiment shown the width of the upstanding ridges 53 and 53' are chosen to be substantially the same width as the separation between the ridges, or slightly, smaller than the separation between the ridges. This means that the CVD ridges 53 and 53' abut against each other sideways (in the orientation shown) and can provide a radial bearing wear surfaces if that bearing is subject not only to thrust but also to radial forces. It will be appreciated that the same configuration could also be used for mating offset pairs of bearings corresponding to the embodiment of Figure 4a. For simplicity in Figure 4c the metal layer CIPing the steel carrier ring to the SCD bearing is not illustrated. ln the bearings of Figures 4a and 4b, the SCD element may be replaced by a similarly sized and shaped SiC element

Figure 5a shows a combined bearing 61 made from SCD material. The bearing is annular and L shaped in cross section. The base of the L 63 provides a thrust bearing wear surface, and the stem of the L 65 provides a radial bearing surface. The other bearing 69 that mates with combined bearing 61 in operation may comprise PCD composite segments of the type described with reference to the earlier figures. Alternatively it may comprise a unitary castellated SCD annular bearing 70 as illustrated in Figure 5b. Such a unitary bearing 70 may have substantially flat thrust wear sections as castellated turret tops 72 which co-operate in use with the flat surface of the base 63 of the L shaped bearing 61 of Figure 5a, and ridged sections 74 extending across the thickness of the annular bearing in an axial direction, each ridge 74 having a circumferentially directed convex surfaces so as to match the corresponding concave surface of the stem 65 of the L of the bearing 61 of Figure 5a. A CVD diamond coating layer may optionally be provided on either the thrust bearing surface or the radial bearing surface of the combined bearing. This is not illustrated in Figure 5a or 5b. The SCD element in Figures 5a and 5b may be replaced by a similarly sized and shaped SiC element

Figure 6 shows an alternative design of combined bearing 71 comprising an annular SCD element 73 having a base 75 and a stem 77. In this case, the SCD element acts as a carrier and subsidiary wear element for PCD composite segments 79 and 81 carried on base 75 and stem 77 of the SCD element respectively. As with Figure 5, the other bearing 83 that mates with combined bearing 71 in operation may comprise PCD composite segments of the type described with reference to the earlier figures. As before, in this embodiment the SCD element may be replaced by a similarly sized and shaped SiC element. The SCD bearings 61 and 71 in Figures 5a and 6 are held in a steel carrier ring 67, 78 respectively. A similar steel carrier ring may also be provided for the SCD bearing 70 of Figure 5b. Attachment to the steel carrier rings 67 and 78 may be done by providing protrusions or recesses in the SCD parts, e.g. channels, and using a CIPed metal layer to mechanically lock the SCD part to the metal layer, and then subsequently brazing the metal layer to the steel carrier ring 67/78, in the manner described with reference to earlier embodiments. Figure 7 shows a drilling arrangement with an upper thrust bearing pair 91 and a lower thrust bearing pair 93, together with an upper radial bearing pair 101 , and a lower radial bearing pair 103. Thrust bearing pair 91 and 93 each comprises an SCD annular bearing ring 90 which mates with a PCD thrust bearing 92 mounted on a WC substrate 94. Each radial bearing pair 101 and 103 comprises a SCD radial bearing 100 (with a convex wear surface which mates with a radial PCD bearing 102 which has a convex wear surface. Also shown in the figure are a centralising ring 104 and a spacer ring tub 106. Figure 8 shows a radial bearing assembly comprising inner and outer radial bearings 61 and 63 respectively. Radial bearing 61 comprises a cylindrical SCD or SiC part 65 which in section has a straight edge 67 and a convex shaped surface 69. The SCD or SiC annular part 65 is covered with a CVD diamond annular element 71 which has been deposited thereon by a CVD process, and which itself has curved inner and outer surfaces. Inner bearing 61 is secured to shaft 71 and in operation the two rotate together and relative to the outer bearing 63 which is fixed. Outer bearing 63 also comprises an annular SCD or SiC element 73. This element 73 has a concave curved inner surface and is covered with an annular CVD diamond element 75 which has a concave inner and outer surfaces in section , the surface 77 of the CVD diamond layer 75 on the outer bearing 63 being co-operatively shaped to mate with the convex curved surface of the CVD element 71 on the inner radial bearing 61

It will be appreciated by the person skilled in the art that thrust and radial bearings and thrust and radial bearing assembly embodiments and combined thrust and radial bearings and bearing assemblies as described above with reference to a drilling arrangement, for example a downhole drilling arrangement may have many other applications. For example renewable energy technologies such as Wind Turbine and Marine Hydro Kinetic sectors present opportunities to employ the types of bearing described above These applications present challenges for conventional bearings due to the environments in which they are required to operate e.g. highly variable bearing loading , abrasive ambient conditions . In the case of submerged marine turbines, the bearing systems are required to operate in a corrosive environment with water lubrication which inevitable contains highly abrasive particulate material. The flexibility of the bearing systems in terms of flexibility of shaping, and contrasting profiles allowing for controlled mud flow and controlled presentation of superhard bearing surfaces against any debris in that mud flow make the bearings of the present disclosure applicable in these new market sectors. It is noted that the embodiment shown herein relating to their application in downhole drilling is merely by way of example.

Claims

A composite bearing comprising: (i) a continuous annular element made from a silicon-carbide-cemented diamond (SCD) composite material or from silicon carbide; and (ii) an element comprising another superhard material which covers at least part of a surface of the SCD or SiC annular element and provides at least part of the wear surface of the composite bearing.

A composite bearing according to claim 1 which is a thrust bearing.

A composite bearing according to claim 1 which is a radial bearing.

A composite bearing according to any preceding claim, wherein the said another superhard element comprises CVD (chemically vapour deposited) diamond material.

A composite bearing according to any preceding claim, wherein the said another superhard element provides the entire wear surface of the composite bearing, or the said another superhard element in combination with the SCD or SiC element provides the entire wear surface of the composite bearing.

A composite bearing according to any preceding claim, wherein the SCD or SiC annular element is supported on an annular carrier element.

A composite bearing according to claim 6, wherein the annular carrier element comprises steel or a cemented carbide.

A composite bearing according to any preceding claim which is a thrust bearing, wherein the annular SCD or SiC element presents a substantially flat surface at least part of which is covered by the said another superhard material to provide at least part of the wear surface of the composite thrust bearing.

A composite bearing according to any of claims 1 to 7, wherein the annular SCD or SiC element presents a non-planar shaped surface which is covered at least in part by the said another superhard material which provides at least part of the wear surface of the composite bearing.

10. A composite bearing according to claim 9, wherein the shaped surface is castellated in shape.

1 1 . A composite bearing according to claim 10, which is a radial bearing wherein the shaped surface is a convex or concave curved shape. 12. A composite bearing any preceding claim, wherein the SCD or SiC annular element has opposed flat ring-shaped surfaces and the said another superhard material that provides at least part of the wear surface of the composite bearing is provided as circumferentially-extending or radially-extending ridges on the ring-shaped flat surfaces of the SCD or SiC element.

13. A composite bearing according to any preceding claim, wherein the said another superhard material is CVD diamond material which is laid down on the annular SCD or SiC element by a process involving masking part of the annular SCD or SiC element and then carrying out continuous vapour deposition of diamond material.

14. A composite bearing according to any preceding claim which is a thrust bearing secured to an annular carrier element via an intermediate metal layer, wherein: (i) the annular SCD or SiC element of the composite bearing has inner and outer curved surfaces, which do not provide the wear surface and which are provided with projections or depressions arranged circumferentially therearound; (ii) the metal layer has been pressed into mechanical engagement with the said projections or depressions on the said inner and outer curved surfaces of the SCD or SiC annular element; and (iii) the metal layer has been secured to the carrier element, thereby securing the composite bearing to the carrier element.

15 A composite bearing according to any of claims 1 to 13 which is a radial bearing secured to a carrier element via an intermediate metal layer, wherein: (i) the annular SCD or SiC element of the composite bearing has opposed substantially planar surfaces, which do not provide the wear surface, and which are provided with projections or depressions thereon; (ii) the metal layer has been pressed into mechanical engagement with the said projections or depressions on the SCD or SiC annular element; and (iii) the metal layer has been secured to the carrier element, thereby securing the composite bearing to the carrier element.

16. A composite bearing which is a combined bearing providing not only a thrust bearing but also a radial bearing, the composite bearing comprising a continuous annular element made from silicon cemented diamond (SCD) composite material or SiC which is substantially L shaped if viewed in cross section through any part of the annulus, at least part of the base of the "L" providing a thrust bearing wear surface and at least part of the stem of the "L" providing a radial bearing wear surface.

17. A composite bearing which is a combined bearing providing not only a thrust bearing but also a radial bearing, the composite bearing comprising a continuous annular element made from silicon cemented diamond (SCD) composite material or SiC, being substantially L shaped if viewed in cross section through any part of the annulus, the base of the "L" being covered partly only by PCD composite segments or compacts to provide a thrust bearing wear surface and the stem of the "L" being covered partly only by PCD composite segments or compacts to provide a radial bearing wear surface.

18. A composite bearing according to claim 16 or 17, comprising an element comprising another superhard material which covers at least part of a surface of the SCD or SiC annular element and provides at least part of one or both of the thrust or radial wear surfaces.

19. A composite bearing according to claim 18, wherein the said another superhard element comprises CVD (chemically vapour deposited) diamond material.

20. A bearing assembly comprising first and second bearings with mating bearing surfaces, one or both of the first and second bearings being a composite bearing according to any preceding claim.

21 . A bearing assembly according to claim 20, wherein the first bearing comprises a composite bearing according to any of claims 1 to 19, but the second bearing has a different construction providing a plurality of segments made from a superhard material at least the wear surface of these segments being separated circumferentially from each other.

22. Use of a thrust bearing assembly according to claim 20 or 21 , in a drilling system.

Use of a bearing assembly according to claim 20 or 21 , in a drilling system wherein the first bearing comprises a composite bearing according to any of claims 1 to 17 but the second bearing has a different construction providing a plurality of segments made from a superhard material at least the wear surface of these segments being separated circumferentially from each other, the separation of the segments being sufficient to allow mud from the drilling operation to flow through the bearing system.

A method of making a composite bearing according to any of claims 1 to 19including a SCD annular element, the method comprising forming the SCD annular element by forming a work piece having a predetermined annular shape and size by pressing diamond particles and a binder together in a mould thereby creating an intermediate annular body, optionally machining the intermediate body, then infiltrating silicon into the intermediate body to form a diamond/silicon carbide/silicon composite finished part, then applying the said another superhard element to the formed SCD annular element .

A method according to claim 24, wherein the intermediate body and the finished part is castellated in cross section.

A bearing which is a thrust bearing comprising a continuous annular element made from a silicon cemented diamond (SCD) composite material.

27. A bearing according to claim 26, wherein the SCD composite material

provides a continuous annular wear surface for the thrust bearing.

28. A bearing according to claim 26, wherein the wear surface of the bearing is provided by another superhard material that covers at least part of, or part only of, the annular SCD element.

29. A bearing according to any of claims 26 to 28, wherein the SCD annular element is supported on an annular carrier element.

30. A bearing according to claim 29, wherein the annular carrier element comprises steel or a cemented carbide.

31 . A bearing according to any of claims 26 to 30, wherein the annular SCD element is provided with projections or depressions arranged circumferentially around both its inner and outer curved surfaces. 32. A bearing according to claim 31 additionally comprising a layer of a metal part that has been pressed into mechanical engagement with the projections or depressions arranged circumferentially around both the inner and outer curved surfaces of the SCD or SiC annular part. 33. A bearing according to claim 32 when dependent on claim 29 or any claim dependent thereon, wherein the metal part has been secured to the carrier element.

A bearing according to any of claims 26 to 33, wherein the annular SCD element present a substantially flat surface which is (a) a continuous annular wear surface of the thrust bearing, or (b) a surface at least part of which is covered by another superhard material , or part only of which is covered by another superhard material.

A bearing according to any of claims 26 to 34, wherein the annular SCD element presents a shaped surface which (i) provides at least part of the wear surface of the thrust bearing or (ii) is covered at least in part by another superhard material which provides at least part of the wear surface of the thrust bearing.

A bearing according to claim 35, wherein the shaped surface is castellated in shape.

37. A bearing according to claim 29 or any claim dependent thereon, wherein the SCD or SiC annular ring comprises opposed flat ring shaped surfaces and another superhard material that provides the wear surface is provided as circumferential ridges on the ring shaped flat surfaces of the SCD element.

A bearing according to any of claims 34 to 37, wherein the superhard material is CVD diamond which is laid down on the annular SCD element by a process involving masking part of the annular SCD element and then carrying out CVD of diamond material

A bearing assembly comprising first and second bearings with mating bearing surfaces, at least one of the first and second bearings being a bearing according to any of claims 24 to 38.

A bearing assembly according to claim 39, wherein both the first and second bearings comprise a bearing according to any of claims 26 to 38.

A bearing assembly according to claim 39, wherein the first bearing comprises a bearing according to any of claims 26 to 38, but the second bearing has a different construction providing a plurality of segments made from a superhard material at least the wear surface of these segments being separated circumferentially from each other. 42. Use of a thrust bearing assembly according to claim 39 to 41 in a drilling system.

Use of a thrust bearing assembly according to claim 39 to 41 in a drilling system wherein the first bearing comprises a bearing according to any of claims 26 to 38 but the second bearing has a different construction providing a plurality of segments made from a superhard material at least the wear surface of these segments being separated circumferentially from each other, the separation of the segments being sufficient to allow mud from the drilling operation to flow through the bearing system.

A method of making a bearing according to any of claims 26 to 38, comprising forming a work piece having a predetermined annular shape and size by pressing diamond particles and a binder together in a mould thereby creating an intermediate annular body, optionally machining the intermediate body, then infiltrating silicon into the intermediate body to form a diamond/silicon carbide/silicon composite finished part.

45. A method according to claim 44, wherein the intermediate body and the finished part is castellated in cross section.