Cutting elements for rotary drill bits

A cutting element for a rotary drag-type drill bit comprises a body of polycrystalline diamond incorporating a binder-catalyst selected from iron group elements, such as iron, cobalt and nickel, or alloys thereof. The body of polycrystalline diamond is unsupported by an integral substrate. The cutting element may be mounted directly on the body of the drill bit, or may be brazed to a substrate of a different, less hard material which is in turn mounted on the drill bit.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The invention relates to cutting elements for rotary drill bits and
 particularly to cutting elements for drag-type drill bits comprising a bit
 body having a leading surface to which the cutting elements are fixedly
 mounted.
 2. Description of the Prior Art
 As is well known, one common form of cutting element for a rotary drag-type
 drill bit is a two-layer or multi-layer cutting element where a facing
 table of polycrystalline diamond is integrally bonded to a substrate of
 less hard material, such as tungsten carbide. The cutting element is
 usually in the form of a tablet, usually circular or part-circular. The
 substrate of the cutting element may be brazed to a carrier, usually also
 of cemented tungsten carbide, which is received in a socket in the bit
 body, or the substrate itself may be of sufficient axial length to be
 mounted directly in a socket in the bit body.
 As is well known, polycrystalline diamond is formed by compressing diamond
 powder with a suitable binder-catalyst in a high pressure, high
 temperature press. In one common process for manufacturing two-layer
 cutting elements, diamond powder is applied to the surface of a preformed
 tungsten carbide substrate incorporating cobalt. The assembly is then
 subjected to very high temperature and pressure in a press. During this
 process cobalt migrates from the substrate into the diamond layer and acts
 as a binder-catalyst causing the diamond particles to bond to one another
 with diamond-to-diamond bonding, and also causing the diamond layer to
 bond to the substrate.
 Although cobalt is commonly used as the binder-catalyst, any iron group
 element, such as cobalt, nickel or iron, or alloys thereof, may be
 employed. Polycrystalline diamond using iron group elements, or alloys
 thereof, as a binder-catalyst will be referred to herein as "conventional"
 polycrystalline diamond. Other forms of polycrystalline diamond are
 sometimes used as cutters in rotary drag-type drill bits, for example
 silicon may be used as the binder-catalyst or a conventional binder
 catalyst such as cobalt may be leached out of the diamond after formation.
 Such forms of polycrystalline diamond are not usually formed on a
 substrate and are generally more thermally stable than conventional
 polycrystalline diamond. However, problems may arise in the use of such
 materials as cutting elements.
 When two-layer cutting elements using conventional polycrystalline diamond
 were first manufactured the polycrystalline diamond facing table was very
 thin in relation to the thickness of the substrate. More recently,
 however, the thickness of the diamond facing table has often been
 increased relative to the thickness of the substrate, particularly around
 the periphery of the cutting element. Such arrangements are shown, for
 example, in WO 97/30264. Also GB 2323110 suggests extending part of the
 diamond facing table through the thickness of the substrate, and up to the
 rear surface thereof, so that part of the diamond facing table engages the
 surface on which the cutting element is mounted so as to provide high
 modulus support (the modulus of elasticity of the diamond being greater
 than that of the substrate itself).
 According to the present invention, the advantages provided by such
 arrangements are enhanced by use of cutting elements which consist
 entirely of conventional polycrystalline diamond material and do not
 incorporate an integral substrate.
 SUMMARY OF THE INVENTION
 According to the invention, there is provided a cutting element for a
 rotary drag-type drill bit comprising a body of polycrystalline diamond
 incorporating a binder-catalyst selected from iron group elements or
 alloys thereof, said body of diamond being unsupported by an integral
 substrate.
 The term "iron group elements", as used herein, includes iron and those
 other elements, such as cobalt and nickel, which are in the same group as
 iron in the Periodic Table of the elements.
 The invention also provides a cutting element for a rotary drag-type drill
 bit comprising a body of polycrystalline diamond incorporating a
 binder-catalyst selected from iron group elements or alloys thereof, said
 body being brazed to a substrate by use of a brazing alloy.
 In this case, the substrate may comprise a body of diamond/tungsten
 carbide/binder-catalyst composite material, or a body of cemented tungsten
 carbide, or two bodies of said materials respectively, brazed together by
 use of a brazing alloy.
 The invention also provides a cutting element for a rotary drag-type drill
 bit comprising a body of polycrystalline diamond incorporating a
 binder-catalyst selected from iron group elements or alloys thereof which
 has been integrally bonded, in a high pressure, high temperature press, to
 a body of diamond/tungsten carbide/binder-catalyst composite material.
 Preferably a portion of the body of polycrystalline diamond which is
 nearer to the body of composite material includes a greater proportion of
 binder-catalyst than a portion thereof which is further from the composite
 material.
 In any of the cutting elements according to the invention, the cutting
 element may have an outer surface which is coated with a material to allow
 the cutting element to be brazed to another material. Alternatively or
 additionally, the outer surface of the cutting element may be formed with
 a plurality of projections and recesses, which in use, interlock with a
 material within which the cutting element is embedded.
 In any of the above arrangements the cutting element may be in the form of
 a tablet having generally parallel front and rear surfaces and a
 peripheral surface which may be circular, part circular, or of any other
 suitable shape.
 The invention also provides a method of manufacturing a cutting element for
 a rotary drill bit, comprising the steps of forming a preform element by
 bonding a body of diamond particles to a surface of a substrate
 incorporating tungsten carbide and a binder-catalyst selected from iron
 group elements or alloys thereof, in a high pressure, high temperature
 press, so that binder-catalyst from the substrate migrates into the
 diamond layer, then subsequently removing the preform element from the
 press and removing the substrate so as to leave only a body of
 polycrystalline diamond incorporating the binder-catalyst, unsupported by
 a substrate.
 The invention also provides a method of manufacturing a cutting element for
 a rotary drill bit comprising the steps of manufacturing a preform element
 by forming a mixture of diamond particles and particles of a
 binder-catalyst selected from iron group elements or alloys thereof and
 subjecting the mixture to high pressure and temperature in a press,
 sufficient to bond the particles together with diamond-to-diamond bonding.
 In this method a layer consisting of diamond particles alone may be applied
 to the mixture of diamond and binder-catalyst particles before it is
 subjected to high pressure and temperature in the press, so that, during
 pressing, some binder-catalyst from the mixture migrates into the diamond
 layer.
 The invention also provides a method of manufacturing a cutting element for
 a rotary drill bit comprising forming a mixture of diamond particles,
 tungsten carbide particles and particles of a binder-catalyst selected
 from iron group elements or alloys thereof, applying to the mixture of
 particles a layer of particles consisting of diamond alone, and subjecting
 the mixture and layer to high pressure and temperature in a press so that
 the particles bond to one another and some binder-catalyst from the
 diamond/tungsten carbide/binder-catalyst mixture migrates into the layer
 of diamond particles.
 In a modification of this method, there is disposed between the diamond
 layer and the diamond/tungsten carbide/binder-catalyst mixture an
 intermediate layer comprising a mixture of diamond and binder-catalyst
 particles so that it is binder-catalyst from the intermediate layer which
 migrates into the layer of diamond particles alone.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Referring to FIG. 1, the drill bit comprises a bit body on which are formed
 four primary blades 1 and four secondary blades 2. The blades extend
 generally radially with respect to the bit axis.
 The leading edges of the secondary blades are substantially equally spaced
 with respect to one another, but the leading edge of each secondary blade
 is closer to its associated preceding primary blade than it is to the
 following primary blade.
 Primary cutters 3 are spaced apart side-by-side along each primary blade 1
 and secondary cutters 4 are spaced apart side-by-side along each secondary
 blade 2. Each secondary cutter 4 is located at the same radial distance
 from the bit axis as an associated one of the primary cutters on the
 preceding primary blade.
 Each cutter 3, 4 is generally cylindrical and of circular cross-section and
 comprises a front facing table of polycrystalline diamond bonded to a
 cylindrical substrate of cemented tungsten carbide. Each cutter is
 received within a part-cylindrical pocket in its respective blade.
 The primary cutters 3 are arranged in a generally spiral configuration over
 the drill bit so as to form a cutting profile which sweeps across the
 whole of the bottom of the borehole being drilled. The three outermost
 cutters 3 on each primary blade 1 are provided with back-up studs 5
 mounted on the same primary blade rearwardly of the primary cutters. The
 back-up studs may be in the form of cylindrical studs of tungsten carbide
 embedded with particles of synthetic or natural diamond.
 The bit body is formed with a central passage (not shown) which
 communicates through subsidiary passages with nozzles 6 mounted at the
 surface of the bit body. Drilling fluid under pressure is delivered to the
 nozzles 6 through the internal passages and flows outwardly through the
 spaces 7 between adjacent blades for cooling and cleaning the cutters. The
 spaces 7 lead to junk slots 8 through which the drilling fluid flows
 upwardly through the annulus between the drill string and the surrounding
 formation. The junk slots 8 are separated by gauge pads 9 which bear
 against the side wall of the borehole and are formed with bearing or
 abrasion inserts (not shown). This is just one example of a rotary
 drag-type drill bit, and many other designs are in use and will be know to
 those skilled in the art.
 The bit body and blades may be machined from metal, usually steel, which
 may be hardfaced. Alternatively the bit body, or a part thereof, may be
 moulded from matrix material using a powder metallurgy process. The
 methods of manufacturing drill bits of this general type are well known in
 the art and will not be described in detail.
 FIG. 2 shows a typical prior art cutting element in which conventional
 polycrystalline diamond is normally used. The polycrystalline diamond
 comprises the facing table 15 of a two-layer circular cylindrical cutting
 element 16 of generally tablet-like form. The diamond facing table 15 is
 integrally bonded to a significantly thicker substrate 17 of cemented
 tungsten carbide.
 As previously mentioned, such preform cutting elements are manufactured by
 applying to the surface of the substrate 17 a layer of diamond powder, the
 substrate and diamond layer then being subjected to extremely high
 pressure and temperature in a press. During the formation process, cobalt
 from the substrate 17 migrates into the diamond layer and acts as a
 catalyst, resulting in the diamond particles bonding together and to the
 substrate.
 Preform cutting elements may also be manufactured where the diamond layer
 is substantially thicker, as shown for example in FIG. 3.
 In order to achieve cutting elements which consist entirely of
 polycrystalline diamond in accordance with the invention, the substrate 17
 may be totally removed from the preform element, e.g. by grinding, EDM or
 other machining process, to leave just a tablet consisting solely of
 polycrystalline diamond, as indicated at 19 in FIG. 4.
 A preform element consisting of 100% polycrystalline diamond may also be
 formed by pressing a mixture of diamond and cobalt powder in the high
 pressure, high temperature press. In this case a substrate is not required
 since the cobalt powder incorporated in the mixture itself effects the
 bonding of the diamond particles together. The mixture might also include
 other powdered materials, such as powdered tungsten carbide, so that the
 preform element from which the abrasive particles are formed is a
 composite material.
 The present invention provides for the use of elements consisting entirely
 of conventional polycrystalline diamond material, e.g. as described in
 relation to FIG. 4, as preform cutting elements for drag-type rotary drill
 bits. Such elements may be formed by removing the substrate from two-layer
 polycrystalline diamond elements, or by moulding the elements in a high
 pressure, high temperature press from a mixture of powdered diamond and
 binder-catalyst, or a mixture or powdered diamond, tungsten carbide and
 binder-catalyst.
 FIGS. 5-10 show cutting elements of this kind.
 In the following arrangements and methods, the binder-catalyst is, for
 convenience, described as consisting of cobalt, since this is the material
 most commonly used for this purpose in the manufacture of conventional
 polycrystalline diamond on a substrate. However, in accordance with the
 present invention, the binder-catalyst in any of the following
 arrangements and methods may comprise any iron group element, such as
 iron, cobalt or nickel, or alloys thereof.
 FIG. 5 shows a circular cylindrical cutting element 20 which is formed
 entirely from polycrystalline diamond incorporating cobalt by any of the
 methods referred to above. In this case the axial length of the element is
 greater than its diameter and the element is secured in a bit body,
 indicated diagrammatically at 22.
 The cutting element 20 may be secured in the bit body 22 by shrink fitting
 or it may be brazed in the bit body 22. Since polycrystalline diamond
 cannot normally be wetted by brazing alloy, the element is preferably
 formed with a metallic coating prior to the brazing operation. For
 example, the surface of the cutting element may be treated by any known
 process which creates carbides on the surface of the element so as to
 permit brazing.
 In the arrangement of FIG. 6, the polycrystalline diamond cutting element
 23 is formed with peripheral ribs 24 and grooves 25 so that the cutter may
 be mechanically locked into the bit body. For example, the cutting element
 may be moulded into the bit body during its manufacture from solid
 infiltrated matrix by the above-described powder metallurgy process, a low
 temperature infiltrant alloy being used to prevent degradation of the
 diamond. Alternatively, the cutting element 23 could be brazed into a
 socket in a bit body, the provision of the ribs 24 and grooves 25 then
 increasing the braze area as well as providing some mechanical
 interlocking.
 In the arrangement of FIG. 7 the polycrystalline diamond cutting element 26
 is brazed to a co-extensive tablet 27 of a diamond composite material
 which is in turn brazed to a co-extensive tablet 28 of cemented tungsten
 carbide. The diamond composite tablet 27 is formed by pressing a mixture
 of diamond, tungsten carbide and cobalt particles in a high pressure, high
 temperature press.
 In the arrangement of FIG. 8, the polycrystalline diamond is incorporated
 in a cutting element comprising three integral layers: a front layer 29 of
 normal polycrystalline diamond, an intermediate layer 30 of
 polycrystalline diamond with a higher cobalt content and a rear layer 31
 comprising diamond, tungsten carbide and cobalt.
 The element of FIG. 8 is manufactured by pressing, in a high pressure, high
 temperature press, a composite of particulate materials in three layers.
 The first layer, corresponding to layer 29, comprising diamond particles
 alone, a second layer comprising an admixture of diamond particles and
 cobalt powder, and a third, deeper layer comprising a mixture of diamond
 particles, tungsten carbide particles, and cobalt powder. During the
 pressing operation cobalt from the second, intermediate layer migrates
 into the first diamond layer so as to create the layer 29 of bonded
 diamond particles. The layer 29, having received only cobalt which has
 migrated from the second layer, will contain less cobalt than the second
 layer 30. The lower proportion of cobalt in the first layer improves its
 abrasion resistance. This is desirable since the first layer provides the
 cutting face of the element.
 In the arrangement of FIG. 9 the cutting element 32 comprises a body 33 of
 diamond composite having along its front and outer surfaces a layer 34 of
 polycrystalline diamond. In this case, the element is manufactured by
 forming a body of diamond composite particles, comprising diamond,
 tungsten carbide and cobalt, and then applying thereto a layer of diamond
 particles alone to form the layer 34. In the press cobalt from the diamond
 composite body 33 migrates into the diamond layer 34 to form the layer of
 conventional polycrystalline diamond.
 FIG. 10 shows another form of cutting element manufactured by this method,
 but in this case the polycrystalline diamond provides the front layer 35
 of the cutting element and a column 36 of polycrystalline diamond which
 extends through the surrounding diamond composite 37 to the rear face 38
 of the cutting element. The column 36 of polycrystalline diamond thus
 provides a high modulus support for the front cutting table 35 of the
 element, transmitting loads applied to the front cutting table directly to
 the bit body.
 Whereas the present invention has been described in particular relation to
 the drawings attached hereto, it should be understood that other and
 further modifications, apart from those shown or suggested herein, may be
 made within the scope and spirit of the present invention.