EP1212511A1

EP1212511A1 - Method of mounting a tsp

Info

Publication number: EP1212511A1
Application number: EP01954158A
Authority: EP
Inventors: Stephen Martin Evans
Original assignee: Schlumberger Canada Ltd; Services Petroliers Schlumberger SA; Schlumberger Holdings Ltd
Current assignee: ReedHycalog UK Ltd
Priority date: 2000-08-23
Filing date: 2001-08-03
Publication date: 2002-06-12
Anticipated expiration: 2021-08-03
Also published as: EP1212511B1; WO2002016725A1; DE60101036T2; NO20021154L; NO20021154D0; DE60101036D1; AU2001276505A1

Abstract

A method of applying a wear-resistant material to a surface of a downhole component for use in subsurface drilling comprises forming a plurality of bearing elements, applying a layer of an electrically conductive, less hard material to each bearing element, and then bonding each bearing element to the surface of the component by welding or brazing to the surface of the component a part of the surface of the bearing element which comprises said less hard material, wherein the layer is of thickness greater than 0.05 mm.

Description

"Method of Mounting a TSP"

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to methods of applying a wear-resistant material to

a surface of a downhole component for use in subsurface drilling. The method

is suitable for use both with drill bits and with other downhole components.

The invention is applicable to downhole components of the kind which

include at least one surface which, in use, engages the surface of the earthen

formation surrounding the borehole. The invention relates particularly to rotary

drill bits, for example of the drag-type kind having a leading face on which

cutters are mounted and a peripheral gauge region for engagement with the

surrounding walls of the borehole in use or of the rolling cutter kind. The

invention will therefore be described with particular reference to polycrystalline

diamond compact (PDC) drag-type and rolling cutter type drill bits, although it

will be appreciated that it is also applicable to other downhole components

having bearing surfaces. For example, bearing surfaces may be provided on

downhole stabilisers, motor or turbine stabilisers, or modulated bias units for use

in steerable rotary drilling systems, for example as described in British Patent

No. 2289909. Such bias units include hinged paddles having bearing surfaces

which engage the walls of the borehole in order to provide a lateral bias to the bottom hole assembly.

In all such cases the part of the downhole component providing the

bearing surface is not normally formed from a material which is sufficiently

wear-resistant to withstand prolonged abrasive engagement with the wall of the

borehole and it is therefore necessary to render the bearing surface more wear-

resistant. For example, the bodies of rotary drag-type and rolling cutter type drill

bits are often machined from steel and it is therefore necessary to apply bearing

elements to the gauge portion of such drill bit to ensure that the gauge is not

subject to rapid wear through its engagement with the walls of the borehole.

This is a particular problem with steel bodied drill bits where the gauge of the bit

comprises a single surface extending substantially continuously around the whole

periphery of the bit, for example as described in British Patent No. 2326656.

2. Description of Related Art

One well known method of increasing the wear-resistance of the gauge of

a drag-type or rolling cutter type drill bit is to form the gauge region with sockets

in which harder bearing inserts are received. One common form of bearing insert

comprises a circular stud of cemented tungsten carbide, the outer surface of

which is substantially flush with the outer surface of the gauge. Smaller bodies

of natural or synthetic diamond may be embedded in the stud, adjacent its outer

surface. In this case the stud may comprise, instead of cemented tungsten

carbide, a body of solid infiltrated tungsten carbide matrix material in which the smaller bodies of natural or synthetic diamond are embedded. Bearing inserts

are also known using polycrystalline diamond compacts having their outer faces

substantially flush with the gauge surface.

Another known method of increasing the wear-resistance of the gauge

surface of a PDC drill bit is to cover the surface of the gauge, or a large

proportion thereof, with arrays of rectangular tiles of tungsten carbide. Such tiles

may be packed more closely over the surface of the gauge than is possible with

bearing inserts, of the kind mentioned above, which must be received in sockets,

and therefore allow a greater proportion of the area of the gauge surface to be

covered with wear-resistant material at lesser cost. However, it would be

desirable to use bearing elements which have greater wear-resistance than

tungsten carbide tiles.

A known method for increasing the wear-resistance of the rolling cone

cutter in rolling cutter bits is to include one or more rows of inserts on the gauge

reaming portion of the rolling cutter. Typically, the inserts are cylindrical bodies

which are interference-fitted into sockets formed on the gauge reaming surface

of the rolling cutter, as shown in US Patent No. 5,671,817. The inserts may be

formed of a very hard and wear and abrasion resistant grade of tungsten carbide,

or may be tungsten carbide cylinders tipped with a layer of polycrystalline

diamond. In addition, the gauge portion of each bit leg facing the borehole wall

may be provided with welded-on hard facing and/or the same type of tungsten carbide cylinders are as fitted into the rolling cutters.

A material which is significantly more wear-resistant than tungsten

carbide, and is also available in the form of rectangular blocks or tiles, is

thermally stable polycrystalline diamond (TSP). As is well known, thermally

stable polycrystalline diamond is a synthetic diamond material which lacks the

cobalt which is normally present in the polycrystalline diamond layer of the two-

layer compacts which are frequently used as cutting elements for rotary drag-

type drill bits. The absence of cobalt from the polycrystalline diamond allows

the material to be subjected to higher temperatures than the two-layer compacts

without sufficient significant thermal degradation, and hence the material is

commonly referred to as "thermally stable".

In one commercially available form of thermally stable polycrystalline

diamond the product is manufactured by leaching the cobalt out of conventional

non-thermally stable polycrystalline diamond. Alternatively the polycrystalline

diamond may be manufactured by using silicon in place of cobalt during the high

temperature, high pressure pressing stage of the manufacture of the product.

While TSP has the wear-resistance characteristics appropriate for a

bearing element on a downhole component, it has hitherto been difficult to

mount TSP on downhole components. Where blocks of TSP are to be used as

cutting elements on drag-type drill bits it is necessary either to mould the bit

body around the cutting elements, using a well-known powder metallurgy process, or to embed the blocks into bodies of less hard material which (are then

secured in sockets in the bit body. Where a bearing element is to be applied to

a surface of a downhole component for the purpose of wear-resistance, however,

it is preferable for the bearing element to be mounted on the surface of the

component, particularly if the component is formed by macluning, from steel or

other metal, so that the bearing element cannot readily be embedded in the

component. The present invention therefore sets out to provide novel methods

for mounting TSP bearing elements on to a bearing surface of a downhole

component.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a method of

applying a wear-resistant material to a surface of a downhole component for use

in subsurface drilling, the method comprising forming a plurality of bearing

elements, applying a layer of an electrically conductive, less hard material to

each bearing element, and then bonding each bearing element to the surface of

the component by welding or brazing to the surface of the component a part of

the surface of the bearing element which comprises said less hard material,

wherein the layer is of thickness greater than 0.05mm.

The layer of less hard material may comprise a thin coating pre-applied

to some or, preferably, all of the surface of the bearing element. Each bearing element preferably comprises a body of thermally stable polycrystalline

diamond. The layer is preferably formed from a material of high electrical

conductivity, such as nickel or nickel alloy. In this case the bearing element may

be held in position on the surface of the component by electrical resistance

welding. The body of thermally stable polycrystalline diamond may be pre-

coated with a layer of a carbide-fomiing metal before application ofthe coating

of less hard material, since the carbide-forming metal may form a stronger bond

with the TSP than does the nickel or nickel alloy alone.

Each bearing element may be inter engaged with a locating formation on

the surface ofthe component to which it is welded or brazed. For example, the

locating formation may comprise a socket or recess into which the bearing

element is at least partly received. The bearing element may be fully received

in the socket or recess so that an exposed surface of the bearing element is

substantially flush with the surface ofthe component surrounding the socket or

recess.

The use of a layer of less hard material of thickness greater than 0.05mm

is advantageous in that it is capable of carrying the electrical current applied

thereto during a resistance welding operation without breaking down. The

thickness of the layer preferably falls within the range of 0.1mm to 0.3mm.

More preferably, the layer thickness falls within the range 0.15mm to 0.25mm,

and conveniently within the range of 0.15 to 0.2mm. The use of a layer of thickness falling within the range 0.15mm to 0.2mm

is advantageous in that the resistance welding operation can be performed

relatively easily.

After securing the bearing elements in position, a layer of a hard facing

material may be applied over and around the bearing elements. The layer may

be of depth such that the bearing surfaces of the bearing elements are left

exposed, or the bearing surfaces may be covered , some of the hard facing

material subsequently being removed, either before or during use.

In any of the above arrangements the downhole component may, as

previously mentioned, comprise a drill bit, a stabiliser, a modulated bias unit for

use in steerable rotary drilling, or any other downhole component having one or

more bearing surfaces which engage the wall ofthe borehole in use.

Where the component is a drill bit, it may be a rotary drag-type drill bit

having a leading face on which the cutters are mounted and a peripheral gauge

region for engagement with the walls ofthe borehole, in which case the methods

according to the invention may be used to apply bearing elements to the outer

surface ofthe gauge region.

The methods ofthe invention may also be applied to increase the wear-

resistance of surfaces of roller-cone bits or other types of rock bit.

The invention also includes within its scope a downhole component, such

as a drill bit, having at least one surface to which bearing elements have been applied by any ofthe methods referred to above, and a coated bearing element

for use in the methods defined hereinbefore.

BRIEF DESCRIPTION OF THE DRAWINGS

The following is a detailed description of embodiments ofthe invention,

reference being made to the accompanying drawings in which:

Figure 1 is a perspective view of a PDC drill bit to the gauge sections of

which wear-resistant materials have been apphed in accordance with the method

ofthe present invention,

Figure 2 is a diagrammatic enlarged cross-section of a part of the gauge

section ofthe drill bit, showing the structure ofthe wear-resistant material,

Figure 3 is a similar view to Figure 2 showing an alternative method of

forming the wear-resistant material,

Figure 4 is an enlarged view illustrating the structure of a bearing element

mounted in position,

Figure 5 is a perspective view of a rolling cutter drill bit, to the gauge

sections of which wear-resistant materials have been applied,

Figure 6 is a view of a stabiliser unit to at least part of which a wear-

resistant material has been applied,

Figure 7 is a view of a bias unit to at least part of which a wear-resistant

material has been applied, Figure 8 is a view of a bottom hole assembly of a drill string having tools

or components with surfaces to at least some of which a wear-resistant material

has been applied, and

Figure 9 is a view of another bottom hole assembly having tools or

components with surfaces to at least some of which a wear-resistant material has

been applied.

DETAILED DESCRIPTION OF THE INVENTION AND THE PREFERRED

EMBODIMENT

Referring to Figure 1: the PDC drill bit comprises a bit body 10

machined from steel and having eight blades 12 formed on the leading face ofthe

bit and extending outwardly from the axis ofthe bit body towards the peripheral

gauge region 14. Channels 16a, 16b are defined between adjacent blades.

Extending side-by-side along each of the blades 12 is a plurality of

cutting structures, indicated at 18. The precise nature ofthe cutting structures

does not form a part ofthe present invention and they may be of any appropriate

type. For example, as shown, they may comprise circular preform PDC cutting

elements brazed to cylindrical carriers which are embedded or otherwise

mounted in the blades, the cutting elements each comprising a preform compact

having a polycrystalline diamond front cutting table bonded to a tungsten carbide

substrate, the compact being brazed to a cylindrical tungsten carbide carrier. In another form of cutting structure the substrate of the preform compact is of

sufficient axial length to be mounted directly in the blade, the additional carrier

then being omitted.

Back-up abrasion elements or cutters 20 may be spaced rearwardly of

some ofthe outer cutting structures, as shown.

Nozzles 22 are mounted in the surface of the bit body between the blades

12 to deliver drilling fluid outwardly along the channels, in use ofthe bit. One

or more of the nozzles may be so located that they can deliver diilling fluid to

two or more channels. All of the nozzles communicate with a central axial

passage (not shown) in the shank 24 ofthe bit, to which drilling fluid is supphed

under pressure downwardly through the drill string in known manner.

Alternate channels 16a lead to respective junk slots 26 which extend

upwardly through the gauge region 14, generally parallel to the central

longitudinal axis ofthe drill bit, so that drilling fluid flowing outwardly along

each channel 16a flows upwardly through the junk slot 26 between the bit body

and the surrounding formation, into the annulus between the drill string and the

wall ofthe borehole.

Each of the other four alternate channels 16b does not lead to a

conventional junk slot but continues right up to the gauge region 14 ofthe drill

bit. Formed in each such channel 16b adjacent gauge region is a circular opening

28 into an enclosed cylindrical passage which extends through the bit body to an outlet (not shown) on the upper side ofthe gauge region 14 which communicates

with the annulus between the drill string and the borehole.

Accordingly, the gauge region 14 of the drill bit comprises four

peripherally spaced bearing surfaces 30 each bearing surface extending between

two junk slots 26 and extending continuously across the outer end of an

intermediate channel 16b.

In accordance with the present invention, there is apphed to each

peripheral bearing surface 30 in the gauge region a wear-resistant layer

comprising an array of rectangular bearing elements 32 in mutually spaced

relationship on the bearing surface 30, each bearing element being formed, at

least in part from thermally stable polycrystalline diamond.

In the example shown in Figure 1 the bearing elements 32 are rectangular

and closely packed in parallel rows extending generally axially of the drill bit.

However, this arrangement is by way of example only and many other shapes

and arrangements of bearing elements may be employed, but still using the

methods according to the present invention. For example the bearing elements

might be square, circular or hexagonal and may be arranged in any appropriate

pattern. Also, the bearing elements may be more widely spaced than is shown

in Figure 1 and may cover a smaller proportion ofthe surface area ofthe bearing

surface 30.

Referring now to Figure 5, a perspective view of a rolling cutter drill bit 100 is shown. The rolling cutter drill bit 100 has a body portion 112 and a

plurality of legs 114 which each support rolling cutters 116. A typical rolling

cutter 116 has a plurality of cutting inserts 118 arranged in circumferential rows

120. An orifice arrangement 122 delivers a stream of drilling fluid 124 to the

rolling cutter 116 to remove the drilled earth, in use. Weight is applied to the

rolling cutter drill bit 100, and the bit 100 is rotated. The earth then engages the

cutting inserts 118 and causes the roiling cutters 116 to rotate upon the legs 114,

effecting a drilling action.

The gauge portion 126 of each leg 114 may define a bearing surface

which engages the borehole wall during operation. This engagement often

causes excessive wear of the gauge portion 126 of the leg 114. In order to

minimise the wear, a plurality of rectangular bearing elements 32 are provided,

the elements 32 being spaced apart in either a vertical alignment 128 or

horizontal alignment 130 on the gauge portion 126 of the leg(s) 114. The

particular arrangement of bearing elements 32 used will depend upon several

factors, such as the curvature of the gauge portion 126, the amount of wear

resistance required, and the bit size. Although the vertical ahgnment 128 and the

horizontal alignment 130 are shown on separate legs in the figure, it is

anticipated that both may be used on a single gauge portion 126 of a leg 114.

Each rolling cutter 116 has a gauge reaming surface 132 which defines a

further bearing surface and also experiences excessive wear during drilling. The rectangular bearing elements 32 may be used on the gauge reaming surface 132

to rniiώnise this wear. The advantage of placing the rectangular bearing

elements 32 on the gauge reaming surface 132 ofthe rolling cutter 116 is that

they can be placed in a particularly dense arrangement compared to the

traditional interference fitted cylindrical cutting elements. The rectangular

bearing elements 32 may be placed in a circumferential manner on the gauge

reaming surface 132 of the rolling cutter 116 as indicated by numeral 134.

Alternately, the rectangular bearing elements 32 may be in a longitudinal

arrangement as indicated by numeral 136. It is anticipated that a combination of

longitudinal and ckcumferential aπangements of the rectangular bearing

elements 32 would also be suitable.

The method ofthe present invention also allows the rectangular bearing

elements 32 to be placed on the gauge reaming surface 132 ofthe rolling cutter

116 without particular regard to the placement ofthe cutting inserts 118. Prior

to the invention, great care was required to aπange the cylindrical cutting

elements ofthe gauge reaming surface 132 in a manner that prevented the bases

of their mating sockets from overlapping.

Figures 2 and 3 show diagrammatic cross-sections through the bearing

surface 30 and applied wear-resistant layer, and methods of applying the wear-

resistant layer will now be described with reference to these figures.

As will be seen from Figure 2, the bearing elements 32 lie on the outer bearing surface 30 ofthe gauge portion 14 ofthe drill bit and the spaces between

adjacent bearing elements 32 are filled with a settable hardfacing material 34.

In one method according to the invention, the bearing elements 32

comprise solid blocks or tiles of TSP and are first attached to the bearing surface

30 in the desired configuration. The settable hardfacing material 34 is then

applied to the spaces between the TSP blocks 32 so as to bond to the bearing

surface 30 of the drill bit and to the blocks themselves. Upon solidification, the

hardfacing material 34 serves to hold the TSP elements 32 firmly in position on

the surface 30.

The hardfacing material 34 may be of any of the kinds commonly used in

providing a hardfacing to surface areas of drill bits, and particularly to steel

bodied drill bits. For example, the hardfacing material may comprise a powdered

nickel, chromium silicon, boron alloy which is flame sprayed on to the surface

30 using a well known hardfacing technique. The hardfacing may also be

provided by other known techniques such as electrical plating, PDC, and metal

spraying.

In the arrangement shown in Figure 2 the hardfacing material 34 is in the

form of a broken layer of generally the same depth as the TSP bearing elements

32 so that the outer surfaces ofthe bearing elements are substantially flush with

the outer surface ofthe hardfacing layer. In the alternative arrangement shown

in Figure 3 the hardfacing layer 34 is applied to a depth which is greater than the depth ofthe elements 32 so as to overlie the outer faces ofthe bearing elements,

as indicated at 36. The overlying layer 36 can be left in position so that, during

use of the bit the layer 36 will wear away exposing the surfaces of the TSP

bearing elements 32 which will then bear directly on the surface ofthe wall of

the borehole. However, if required, the layer 36 may be ground away to expose

the outer surfaces ofthe bearing elements before the bit is used.

The bearing elements 32 are attached to the bearing surface 30 by

electrical resistance welding. Since it is extremely difficult to weld or braze

TSP directly to steel using conventional techniques, such as electrical-resistance

welding, the TSP blocks are coated with a less hard material, of higher electrical

conductivity, before welding or brazing them to the surface 30. For example, the

blocks may be coated with a thin layer of nickel or a nickel alloy, for example

by using the techniques of electroless plating, CVD, or immersion in a molten

alloy. Before coating the TSP with the nickel or nickel alloy, the TSP blocks

may first be coated with a suitable carbide-forming metal, since such metal will

bond to the TSP forming a firmly attached base surface to which the nickel or

nickel alloy coating may subsequently be applied. Once the TSP blocks have

had a suitable coating layer applied thereto, the blocks may more readily be

welded or brazed to the surface 30, for example by using electrical-resistance

spot welding.

As, during the electrical resistance welding process, high currents are applied and must be conducted by the nickel or nickel alloy coating, in order to

ensure that the coating is able to withstand the applied cuπent, the coating is of

thickness greater than 0.05mm. In order to withstand the cuπent applied in a

typical electrical resistance welding process, the coating thickness is preferably

within the range 0.1mm to 0.3mm and is preferably within the range 0.15mm to

0.25mm. More preferably, the layer thickness falls within the range 0.15mm to

approximately 0.2mm, and the layer thickness is conveniently approximately

0.2mm.

Figure 4 illustrates a bearing element comprising a block 38 of thermally

stable diamond coated with a layer 40 of nickel of thickness approximately

0.2mm. Prior to applying the nickel layer 40, a carbide forming material 42 is

applied to the block 38. The coated block is then secured in position on a bit

body 44 by electrical resistance welding, and a hard facing material 46 apphed.

In any of the aπangements described the bearing surface 30 may be

preformed with appropriate formations to assist in locating or holding the TSP

elements 32 on the surface 30. For example, each element 32 may be partly

received in a suitably shaped groove in the bearing surface 30 or in an individual

recess which matches the shape of the element. In another arrangement the

undersides of the elements 32 are preformed with shaped formations which

mechanically inter-engage with coπesponding shaped formations on the surface

30. In any ofthe described aπangements the sides ofthe elements 32 may be

so shaped that they mechanically interlock with the suπounding hardfacing

material. For example, the elements may increase in width towards the surface

30.

In the above-described aπangements, the hardfacing layer 34 serves to

hold the TSP elements 32 on the bearing surface 30, the welding or brazing of

the elements 32 to the surface 30 merely serving to locate the elements

temporarily in the desired configuration on the bearing smface while the

hardfacing layer is applied. However, since the above-described coating ofthe

TSP elements enables them to be welded or brazed to the bearing smface 30,

arrangements are also possible where the TSP elements are welded or brazed to

the bearing surface with sufficient strength that the hardfacing layer 34 may be

dispensed with, each element 32 being held on the bearing surface 30 by the

welded or brazed joint alone. In this case it may be desirable for the elements

32 to be wholly or partly received in recesses or grooves in the bearing surface

30 in order to improve the strength of the attachment of the elements to the

surface.

Similar techniques to these described hereinbefore are suitable for use in

securing the bearing elements 32 to the bearing surfaces ofthe drill bit illustrated

in Figure 5.

Although the invention has been described with particular reference to applying a wear-resistant surface to the gauge section of a drag-type or rolling

cutter type steel-bodied drill bit, as previously mentioned the invention is not

limited to this particular application and may be used for applying TSP-

incorporating bearing elements to a bearing surface of any other downhole

component, such as a stabiliser, or a modulated bias unit, as described below.

The description below is intended to be illustrative of the parts of the

components to which a wear-resistant layer should preferably be applied rather

than to take the form of a detailed description of these components.

Figure 6 illustrates a stabiliser unit for use in a bottom hole assembly.

The stabiliser unit 200 illustrated in Figure 6 includes a plurality of radially

outwardly extending blades 202, the outer surfaces 204 of which engage, in use,

the wall ofthe borehole in which the bottom hole assembly is located. These

surfaces 204 must be able to withstand the severe abrasion and loads apphed

thereto, in use. In order to improve the wear resistance ofthe blades 202, these

surfaces 204 are provided with wear-resistant materials using the methods

described hereinbefore to secure bearing elements 206 to the surfaces 204 and,

if desired, to apply a layer of a hard facing material over or around the bearing

elements 206.

The component illustrated in Figure 7 is a rotary steerable unit 208 having

a bias pad 210. The bias pad 210 repeatedly engages the wall ofthe bore, in use

to push an associated drill bit to one side as directed by a control unit. It will be appreciated that the bias pad 210 is subject to severe loads and so is subject to

wear. In order to improve the wear-resistance ofthe bias pad 210, a plurality of

bearing elements 212 are secured thereto using the method described

hereinbefore. If desired, a hard facing material may also be applied to the bias

pad using the technique described hereinbefore.

Referring now to Figures 8 and 9, are shown other applications utilising

downhole tools 214, 216 having a wear-resistant material applied using the

method described hereinbefore. In Figure 8 a number of different tools 214, 216

are shown in the drill string 218. These tools 214, 216 may include, but are not

limited to, downhole motors, measuring while drilling tools, logging tools,

vibration dampers, shock absorbers, and centralisers. These tools 214, 216

benefit from wear-resistant materials applied by the process of the present

invention. In particular, the bottom hole assemblies 220, as shown in Figure 9,

are often operated while gravity is pushing them against the borehole wall. Once

again the extreme abrasion and loads applied to the sides of these tools make

them benefit from the application of wear-resistant materials using the process

ofthe present invention.

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 ofthe present invention.

Claims

CLAIMSWe claim:

1. A method of applying a wear-resistant material to a surf ace of a downhole

component, the method comprising forming a plurality of bearing elements,

applying a layer of an electrically conductive, less hard material to each bearing

element, and then bonding each bearing element to the surface ofthe component

using one of a welding process and a brazing process to bond to the smface of

the component a part ofthe surface ofthe bearing element which comprises said

less hard material, wherein the layer is of thickness greater than about 0.05mm.

2. A method as claimed in Claim 1, wherein each bearing element comprises

a body of thermally stable polycrystalline diamond.

3. A method as claimed in Claim 1, wherein the layer of less hard material

comprises a coating applied to at least part of each bearing element.

4. A method as claimed in Claim 1, wherein the layer of less hard material

is of high electrically conductivity.

5. A method as claimed in Claim 4, wherein the layer of less hard material

is formed from a material selected from a group consisting of nickel and alloys

containing nickel.

6. A method as claimed in Claim 1, wherein each bearing element is bonded

to the surface using an electrical resistance welding technique.

7. A method as claimed in Claim 1, further comprising applying a layer of a carbide forming metal to each bearing element prior to the application ofthe

layer of less hard material thereto.

8. A method as claimed in Claim 1, wherein the layer of less hard material

is of thickness of between about 0.1mm and about 0.3mm.

9. A method as claimed in Claim 8, wherein the layer of less hard material

is of thickness of between about 0.15mm and about 0.25mm.

10. A method as claimed in Claim 9, wherein the layer of less hard material

is of thickness of between about 0.15mm and about 0.2mm.

11. A method as claimed in Claim 1, further comprising a step of applying a

layer of a hardfacing material over and around the bearing elements.

12. A downhole component having a surface to which a plurality of bearing

elements are bonded, each bearing element having previously had a layer of a

less hard, electrically conductive material applied thereto, the layer of less hard

material having a thickness greater than about 0.05mm.

13. A downhole component as claimed in Claim 12, and shaped to act as one

of a roller cone bit, a fixed cutter bit, a stabiliser unit and a bias unit.