WO2017081649A1

WO2017081649A1 - Polycrystalline diamond cutting element

Info

Publication number: WO2017081649A1
Application number: PCT/IB2016/056799
Authority: WO
Inventors: Robert Grant Reid
Original assignee: University Of The Witwatersrand, Johannesburg
Priority date: 2015-11-13
Filing date: 2016-11-11
Publication date: 2017-05-18

Abstract

This invention concerns a cutting element (10) comprising a polycrystalline diamond (PCD) body (12) having a working surface (20), a flank (18) and a cutting edge (22). The cutting edge is defined as the interface between an operatively outer section of the flank and a radially outer section of the working surface. A formation, such as a depression (24), is formed in the working surface such that at least a portion of the polycrystalline diamond (PCD) body is provided between the cutting edge and the depression so that the depression has no effect on the geometry of the cutting edge. The depression effectively removes material from the PCD body so as to prevent propagation of spalling cracks, in use. The invention also concerns a method of obstructing propagation of spalling cracks in a PCD cutting element.

Description

POLYCRYSTALLINE DIAMOND CUTTING ELEMENT

BACKGROUND TO THE INVENTION

THIS invention relates to a polycrystalline diamond cutting element and more particularly, but not exclusively, to an improved polycrystalline diamond cutting element suitable for use in oil and gas well drilling.

Cutting elements are used to form, bore or degrade workpieces or bodies by removing material from them. Examples of cutting elements are turning, milling or drilling tools, rock boring tools such as bits for oil and gas drilling, and attack tools such as picks used for pavement degradation and soft rock mining. Such tools typically comprise one or more cutting inserts typically comprising at least one cutting edge. Hard or abrasive workpiece materials, such as metal alloys, ceramics, cermets, certain composite materials and stone may need to be worked using cutting tools having hard or superhard cutting tips or elements. Cemented tungsten carbide is the most widely used tool material for machining hard workpiece materials, and is both hard and tough. The use of superhard materials, such as polycrystalline diamond (PCD) and poly crystalline cubic boron nitride (PCBN) are, however, preferred when increased abrasion resistance is required.

Superhard materials are extremely hard and may have Vickers hardness of at least about 25 GPa. However, superhard materials typically have a lower fracture toughness than cemented carbide materials and consequently, they may be more prone to fracture, chipping and spelling than hard-metals, despite their superior resistance to wear. It is noted that PCD material can be made even more wear resistant by varying the diamond powder size and the matrix content. However, in general, when a PCD material is more wear resistant, it is more brittle or prone to fracture, and such PCD material will, therefore, have even less resistance to spalling in operation.

Superhard cutting elements typically comprise a superhard structure (PCD or PCBN) which is bonded to a support substrate, for example a structure formed of cemented tungsten carbide. Spalling-type wear of superhard cutting elements occurs when flakes or particles of material are broken off from a larger solid body, in this case the superhard structure or PCD layer of the cutting element. With such spalling type wear, the cutting efficiency of the cutting elements can rapidly be reduced, and hence the rate of penetration of the drill bit into the formation is adversely affected. Further, spalling is a random event, and can vary from cutter to cutter, in some it will be a small spall whereas, in other cutters, catastrophic spalling may occur. Such variable spalling can seriously affect the cutting efficiency of a cutting element included in, for example, an oil well drill bit. An embodiment of a PCD abrasive element, and in particular a cutting element, is described in WO2004/106004. The adverse effects of spalling- type wear are discussed in this application, and a cutting element is disclosed that is designed better to handle the damage brought about by spalling. The cutting element comprises a table of PCD material bonded to a substrate (for example a cemented carbide substrate) along a bonding interface. The PCD abrasive element is characterized by the interface being non-planar (for example having a cruciform configuration), and the PCD material having a region adjacent a working surface thereof that is lean in catalyzing material. It is suggested that the combination of these features will prevent the catastrophic failure of a cutting element exposed to excessive spailing. However, the configuration of the cutting element does not limit the occurrence of spalling perse.

Recently, other PCD cutting elements have also been introduced into the market which are said to have improved spall resistance while retaining good wear resistance. These properties are achieved by providing a volume of PCD material adjacent to the cutting surface which is substantially free of the binder (catalyst) material. However, spailing in such cutters is still not consistent from cutter to cutter, and there remains the need to have more consistent spalling on the PCD cutters on an oil or gas well drill bit.

It is accordingly an object of the invention to provide a polycrystalline diamond cutting element that will, at least partially, alleviate the above disadvantages.

It is also an object of the invention to provide a polycrystalline diamond cutting element which will be a useful alternative to existing cutting elements. It is yet another object of the invention to provide a method of reducing or obstructing spelling crack propagation in PCD cutting elements.

It is a further object of the invention to provide a method of manufacturing a poiycrystalline diamond cutting element.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a cutting element including a poiycrystalline diamond (PCD) body having a working surface, a flank and a cutting edge which is the interface between an operatively outer section of the flank and a radially outer section of the working surface, wherein a formation, such as a depression, is formed in the working surface such that at least a portion of the poiycrystalline diamond (PCD) body is provided between the cutting edge and the depression so that the depression has no effect on the geometry of the cutting edge.

In one embodiment, the cutting element is cylindrical in shape.

The depression may also be cylindrical in shape. Preferably, the depression is concentric with the cylindrical poiycrystalline diamond (PCD) body.

The portion of the poiycrystalline diamond (PCD) body located between the cutting edge and the formation may be in the form of an annular rim or ring defining at least part of the working surface. The annular rim or ring may be shaped such that the working surface is perpendicular to the flank of the poiycrystalline diamond (PCD) body. The width of the annular rim or ring may be between about 0.5 mm and about 3 mm, preferably between about 1mm and 2.5 mm, most preferably about 1 mm.

The depth of the depression may be between about 0.25 mm and about 2 mm, preferably between about 0.5 mm and 1mm, particularly about 0.5 mm.

Alternatively, the portion of the polycrystalline diamond (PCD) body located between the cutting edge and the formation may be tapered such that the working surface may at least in part create an oblique angle with respect to the flank of the polycrystalline diamond (PCD) body. Alternatively, the portion of the polycrystalline diamond (PCD) body located between the cutting edge and the formation may carry a fillet radius, preferably extending the depth of the depression, between an internal surface and base of the depression.

In one embodiment the interface defining the cutting edge may be a ring, thereby defining a sharp cutting edge.

In an alternative embodiment the interface defining the cutting edge may be chamfered.

There is further provided for the working surface to be crescent shaped in one embodiment of the invention.

According to a second aspect of the invention there is provided a polycrystalline diamond (PCD) abrasive element, particularly a cutting element, including a polycrystalline diamond layer provided on a substrate, particularly a carbide tungsten substrate, wherein the polycrystalline diamond (PCD) layer has a working surface, a flank and a cutting edge that is defined by the interface between the flank and the working surface, and wherein the working surface carries a formation, such as a depression, which is displaced from the cutting edge such that a portion of the polycrystalline diamond (PCD) foody is provided between the cutting edge and the formation.

According to a third aspect of the invention there is provided a polycrystalline diamond abrasive element, particularly a cutting element, comprising a table of polycrystalline diamond that is bonded to a substrate, particularly a cemented carbide substrate, the table of polycrystalline diamond having a working surface having a depression, partially or completely surrounded by a ring of polycrystalline diamond at the edge of the cutter.

The depression in the polycrystalline layer can be of any shape, provided that there is a rim of polycrystalline diamond which is, in use, the active cutting edge in the drilling operation.

Typically, for a cylindrical cutter, the depression would be approximately cylindrical in shape, having a depth of about a sixth or more of the thickness of the polycrystalline diamond layer, and having a ring of polycrystalline diamond surrounding the depression with a radial dimension of about one to a few millimetres.

The depression in the polycrystalline diamond may be formed by using a suitable mould for the pressing operation in the manufacture of the PCD layer, or it can be formed by machining a standard cylindrical cutter using spark erosion or laser ablation, or similar, to form a depression in the upper working polycrystalline diamond (PCD) working surface.

According to a fourth aspect of the invention there is provided a method of reducing spelling crack propagation in a cutting element, the method including providing a depression in a working surface of the cutting element such that at least a portion carrying at least a part of the working surface of the body is provided between a cutting edge of the cutting element and the depression, thereby, in use, allowing a spalling crack to propagate across the portion of the body provided between the cutting edge and the depression before propagation is obstructed by the absence of material in the depression.

The cutting element may be a polycrystalline diamond (PCD) cutting element. Preferably, the cutting element is a cutting element in accordance with the first, second or third aspects of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention is described by way of a non- limiting example, and with reference to the accompanying drawings in which:

Figure 1 shows a perspective view of a first embodiment of an abrasive element in accordance with the invention;

Figure 2 shows a cross-sectional view of the abrasive element of

Figure 1 ;

Figure 3 shows a perspective view of a second embodiment of an abrasive element in accordance with the invention;

Figure 4 shows a cross-sectional view of the abrasive element of

Figure 3; shows a perspective view of a third embodiment of an abrasive element in accordance with the invention; shows a perspective view of a fourth embodiment of an abrasive element in accordance with the invention; shows a plan view of an abrasive element of Figure 4 wherein the results of experimental testing is illustrated in Figure 7(a), which correspond to the photograph of Figure 7(b) in which the actual results can be seen; shows a plan view of an abrasive element of Figure 5 wherein the results of experimental testing is illustrated in Figure 8(a), which correspond to the photograph of Figure 8(b) in which the actual results can be seen; and

Figure 9 shows a graph plotting durability and abrasion resistance trends of an abrasive element according to the invention against control tests carried out using known abrasive elements.

DETAILED DESCRIPTION OF INVENTION

Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of "including," "comprising," or "having" and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Additionally, directional references such as lower", "upper", "upward", "down" and "downward" designate directions in the drawings to which reference is made. The terminology includes the words specifically mentioned above, derivatives thereof, and words or similar import. It is noted that, as used in this specification and the appended claims, the singular forms "a," "an," and "the," and any singular use of any word, include plural referents unless expressly and unequivocally limited to one referent.

Referring now to the drawings, in which like numerals indicate like features, a non-limiting example of an abrasive element in accordance with the invention is generally indicated by reference numeral 10.

The abrasive element 10 is typically a cutting element and more particularly a polycrystalline diamond (PCD) cutting element. It is envisaged that the cutting element 10 would be particularly suitable for use in oil and gas well drilling.

Turning now to Figure 1 , the cutting element 10 has a body 12 carried on a substrate 14. The body 12 is preferably made from a superhard material such as polycrystalline diamond (PCD) or polycrystalline cubic boron nitride (PCBN). In the preferred embodiment of the cutting element 10 the body 12 is made from PCD. The PCD body 12 is in the form of a table or layer of PCD that is bonded to the substrate 14 at an interface 16. In the preferred embodiment of the cutting element 10 the substrate 14 is in the form of a structure and is made from a hard material, such as a cemented carbide substrate, for example. It is envisaged that the substrate 14 could be made from a cemented tungsten carbide. In the illustrated embodiment of Figure 1, the cutting element 10 is substantially cylindrical. In the cutting element 10, the PCD body 12 is shaped complementary to the substrate structure 14 and together they determine the shape, and in particular the cross-sectional shape, of the cutting element 10. It therefore follows that in the cutting element 10 the PCD body 12 and substrate structure 14 are also substantially cylindrical in shape. It should be understood that the outer periphery of the cutting element and, accordingly, the PCD body 12 and substrate structure 14 have circular outer profiles.

Referring still to Figure 1, the PCD body 12 has a flank 18 and a working surface 20. As shown in Figures 1 and 2, the flank 18 is effectively the exterior surface around the periphery of the PCD body 12 while the working surface 20 is the end surface, i.e. the top surface as illustrated in Figures 1 and 2. A cutting edge 22 is defined by the interface between the flank 18 and working surface 20. More particularly, the cutting edge 22 is defined by the interface between an operatively outer section of the flank 18 and a radially outer section of the working surface 20.

In this first embodiment of the cutting element 10 the interface defining the cutting edge 22 is in the form of a single circular line. As best seen In Figure 2, the flank 18 and working surface 20 are substantially perpendicular to each other such that the interface where they meet or intersect is in the form of a circular line. It is therefore said that the interface defines a sharp cutting edge 22.

The cutting element 10 further has a formation, which is in the form of a depression 24 in this illustrated embodiment. The depression 24 is a sunken or hollowed out section formed in the working surface 20 of the PCD body 12. The depression 24 is intended to break the continuity of the working surface 20 along a diametrical line across the working surface. It can therefore also be said that the depression is a discontinuity formed in the working surface 20 of the PCD body 12.

In this first illustrated embodiment of the cutting element 10 of the invention the depression 24 is in the form of a cylindrical recess. The depression 24 Is concentric with the cylindrical PCD body 12 and is dimensioned so as to cause the working surface 20 to take the shape of an annular ring or rim. The depression 24 is dimensioned so that at least a portion 26 of the (PCD) body 12 is provided between the cutting edge 22 and the edge 28 of the depression 24. The result of this geometry of the cutting element 10 and in particular the PCD body 12 and its working surface 20, is that the depression 24 has no effect on the geometry of the cutting edge 22. In other words, the depression 24 is removed or displaced from the cutting edge 22 such that it has no effect on the geometry of the cutting edge.

Although the depression 24 is described above as a cylindrical recess it should be understood that the invention is not limited to this particular geometry. It is envisaged that depressions of other shape could be used. For example, the depression could be in the form of a groove. It is also envisaged the depression could be in the form of a groove and an interior mound.

As a result of the geometry of the PCD body 12 and the depression 24, the portion of the PCD body spanning between the cutting edge 22 and the edge 28 of the depression 24 comprises the annular rim. The width W of the rim, i.e. the portion of the PCD body spanning between the cutting edge 22 and the edge 28 of the depression 24, is between about 0.5 mm and about 3 mm, preferably between about 1 mm and 2.5 mm, most preferably about 1 mm. It has been found that a width W of greater than about 0.5 mm resulted in containing spelling to a localised region. The width W of the portion of the PCD body spanning between the cutting edge 22 and the edge 28 of the depression 24 is selected such that K is sufficient to prevent crumbling of the portion and, therefore, the cutting edge 22 while still removing enough PCD material to prevent propagation of the spalling cracks in accordance with the inventive concept of the invention.

In this first illustrated embodiment of the cutting element 10 the diameter of the PCD body 12 is between about 12.5 mm and about 40 mm, while the diameter of the depression 24 is between about 10 mm and about 39 mm. For example, in an embodiment wherein the PCD body 12 has a diameter of about 12.5 mm and a depression diameter of about 10 mm, the width W of the portion 26 is about 1.25 mm.

The diametrical ratio between the PCD body 12 and depression 24 is typically between about 0.8 and about 0.975. In this first embodiment of the cutting element 10 the ratio between the width W of the annular rim and the diameter of the depression 26 is typically between about 0.0125 and 0.1, preferably between about 0.05 and 0.09, and most preferably about 0.07.

Referring still to Figure 2, the depth D of the depression 26 is about a sixth or more of the thickness of the PCD body 12. In this first illustrated embodiment of the cutting element 10 the depth D of the depression is between about 0.25 and 2 mm, preferably between about 0.5 mm and 1 mm, and most preferably about 0.5 mm.

Probably best seen in Figure 2, the depression 26 creates a clear, particularly radial, discontinuity in the working surface 20 of the PCD body 12. In the cutting element 10 the internal surface 30 of the depression 26 is parallel to the flank 18 of the PCD body 12. Considering that the working surface 20 is perpendicular to the flank 18 in this embodiment, the internal surface 30 of the depression 24 is also perpendicular to the working surface 20. Similarly, the internal surface 30 of the depression is substantially perpendicular to the lower surface 32 of the depression 26. These right angles between the internal surface 30 and the working and lower surfaces 20 and 32 respectively create a sudden and drastic discontinuity in the working surface 20. This aggressive break in the working surface 20 serves to contain the propagation of spalling to a localised region of the PCD body 12. More about this is said below.

It is envisaged that the depression 24 in the PCD body 12, and in particular its working surface 20, may be formed by using a suitable mould in a pressing operation. Alternatively, the depression 24 could be formed by machining using spark erosion or laser ablation.

Although the cutting element 10 is shown to include a single depression 24 formed in a central region of the working surface 20, it is envisaged that in an alternative embodiment of the cutting element a number or series of depressions could be formed In its working surface. In one embodiment, a series of concentric depressions of increasing depth is formed in the working surface.

It is also envisaged that the depression or depressions could be filled by a filler material, which typically has a hardness of less than that of the PCD. Alternatively, when forming the depression by moulding, an insert used in the moulding process could be left inside the depression. It is therefore envisaged that the depression 24 could be at least partially filled, in some instances fully, so that the top surface of the cutting element 10 is essentially level even through the depression 24 is carried in the PCD body 12. It should therefore be understood that the reference to the depression 24 refers to the sunken or hollowed out section in the PCD material of the body 12. In use, the depression 24 acts to obstruct propagation of a spalling crack by effectively removing the PCD material through which the crack propagates.

Turning now to Figures 3 and 4 in particular, a second embodiment of the cutting element in accordance with the invention will be described. This second embodiment of the cutting element is indicated by the reference numeral 40. This second embodiment of the cutting element 40 is substantially similar the first embodiment of the cutting element 10, and like numerals indicate like features.

Similarly to the first embodiment of the cutting element 10, the depression of the second embodiment of the cutting element 40 is displaced from the cutting edge 46 such that a portion of the (PCD) body is provided between the cutting edge 46 and the depression. The most significant difference between the first and second embodiments of the cutting element 10 and 40 is the geometry of the discontinuity in the working surface 20 created by the depression. In Figures 3 and 4 the depression is indicated by the numeral 42 and its internal surface by the numeral 44. It can be seen that the internal surface of the depression 42 is not parallel to the flank 18 of the PCD body 12. instead, the internal surface 44 is formed by a fillet radius that is at least in part at an oblique angle relative to the flank 18, the working surface 20 and lower surface 32 of the depression 42. In this particular embodiment, the fillet radius extends the entire depth of the depression.

It is envisaged that in an alternative embodiment not shown in the drawings, the internal surface 44 could be chamfered. In this alternative embodiment, the fillet radius will therefore be replaced by a substantially straight, chamfered surface.

It is believed that the angles created between the internal surface 44 and the working and lower surfaces 20 and 32 respectively remain sufficient to create a sudden and drastic discontinuity in the working surface 20 in order to contain the propagation of spelling to a localised region of the PCD body 12. From Figures 3 and 4 it can further be seen that the cutting edge of the PCD body 12 is also chamfered. The cutting edge is indicated by the numeral 46 and spans between a radially inner cutting point 48.1 and a radially outer cutting point 48.2. It is envisaged that the cutting edge could carry a double chamfer. For example, the first chamfer at 30 degrees and the second one at 60 degrees.

Turning now to Figure 5, a third embodiment of the cutting element in accordance with the invention will be described. This third embodiment of the cutting element is indicated by the reference numeral 50. This third embodiment of the cutting element 50 is substantially similar to the second embodiment of the cutting element 50 and, again, like numerals indicate like features.

The only difference between the cutting elements 40 and 50 is the inclusion of a second formation, again in the form of a depression 52, in the PCD body 12 of the cutting element 50. The second depression 52 is formed in the lower surface 32 of the first depression 42, thereby creating a second discontinuity to contain the propagation of spelling to a localised region of the PCD body 12.

Turning now to Figure 6, a fourth embodiment of the cutting element in accordance with the invention will be described. This fourth embodiment of the cutting element is indicated by the reference numeral 60. This fourth embodiment of the cutting element 60 is substantially similar to the earlier embodiments, and in particular this first embodiment of the cutting element 10. Again, like numerals indicate like features.

Unlike the first cutting element 10, in the cutting element 60 its depression 62 is not concentric with the PCD body 12. From Figure 6 it can be seen that the depression 62 intersects the edge of the PCD body 12 at a region away from its cutting edge 64. In other words, the depression 64 is located off-centre on the top or working surface 66 of the PCD body 12. As a result of the geometry of the depression 62 the cutting edge 64 does not run around the complete periphery of the PCD body 12. Instead, the cutting edge 64 only runs around a portion of the periphery, or diameter, of the PCD body 12. in the cutting element 60 of Figure 6 the cutting edge 64 runs around a major portion of the PCD body 12.

As a result of the geometry of the PCD body 12, and in particular the depression 62 formed therein, the portion of the PCD body 12 located between the cutting edge 64 and the edge of the depression 62 is not of uniform width. In other words, the width of the portion located between the cutting edge 64 and the edge of the depression 62 varies circumferentially around the working surface 66. The shape of the PCD body 12 creates a crescent shaped working surface.

Having described the four embodiments of the cutting element 10, 40, 50 and 60 above, some experimental results obtained during testing will now be described with reference to Figures 7 to 9. During testing, the spall resistance of the cutting element in accordance with the invention was assessed using the laboratory testing apparatus called a vertical borer (VBX). VBX testing is known to be the most trusted testing method that mimics actual conditions associated with drilling and, accordingly, the results obtained from VBX testing are believed to be representative of the performance of the cutting element in the field.

Turning to Figure 7, which shows a used cutting element 40 according to the second embodiment of the invention, the effects of spalling on the PCD body 12 are clearly visible. From this figure it can be seen that the cutting element, and in particular the depression 42, was effective in localising spelling, i.e. obstructing or substantially preventing spelling from propagating across the PCD body 12. As a result of the discontinuity in the working surface 20 created by the depression 42 spalling was substantially contained by the edge 28 of the depression, and more particularly by the internal surface 44 of the depression. From the test results shown in Figure 7 H can be seen that spelling cracks did not propagate beyond the internal surface 44 or across the lower surface 32 of the depression 42.

Spelling was further contained to an angular section of the working surface 20 in the localised region of the radial line running through the centre of the contact point between the cutting edge 46 and the surface that was being worked. Due to this localised containment of spelling the cutting element according to the invention can simply be rotated, in use, so that different radial sections of the cutting edge can be used. It should be understood that, in use, when spelling has rendered one redial section of the cutting edge unusable, the cutting element can simply be rotated so that a different radial section of the cutting edge is used. This process can be repeated until substantially the entire cutting edge is rendered unusable due to spalling. The advantage of being able to use different radial sections of the cutting edge is that the lifespan of the cutting element is improved significantly.

In a second experimental test the results of which are shown in Figure 8, the effect on spalling of the second depression 52 in the cutting element 50 was tested. However, as shown in Figure 8, spelling was contained by the first depression 42. Spading did not propagate across the lower surface 32 of the first depression 42 and, accordingly, the second depression 52 proved redundant in this particular test. It is however envisaged that in some applications the second depression 52 could prove useful in restricting spalling in the event that spalling propagates across the inner surface 32 of the first depression 42, i.e. past the discontinuity in the working surface 20 created by the first depression 42. The second depression 52 does however provide an additional cost saving advantage as it reduces the volume of PCD used in the manufacturing of the cutting element 50. The VBX testing was expanded to benchmark the durability and abrasion resistance of the cutting element in accordance with the invention against known cutting elements. In these tests the cutting elements 40 and 50 were compared to a control cutting element that was in all respects identical to it apart from the absence of the depression 42 from the control cutting element. In total three tests were carried out. In the first two tests the cutting element 40 was used and in the third and final test the cutting element 50 was used.

The results of the durability and abrasion resistance tests are shown in Figure 9. The results using the cutting elements 40 in accordance with the invention are indicated by the reference numerals 70 and 72, and the results using the cutting element 50 in accordance with the invention are indicated by the numeral 74. The results using the control cutting elements are indicated by the reference numerals 100, 102 and 104 respectively. From Figure 9 it can be seen that the results are represented graphically as the total wear scar area (mm²) against cutting length (km).

From the first curve 70 in Figure 9 it can be seen that the cutting element 40 started spelling after about 6.8km to a spall length of about 1mm. The test was stopped at 20.5 km with a spall length of about 1mm. From the second curve 72 in Figure 9 it can be seen that the cutting element 40 started spalling after about 6.8km to a spall length of about 1mm. The test was stopped at about 16.7 km with a spall length of about 1mm. From the third curve 74 in Figure 9 it can be seen that the cutting element 50 started spalling after about 4.6 km to a spall length of about 1mm. The test was stopped at about 18.2 km with a spall length of about 1mm.

When comparing the results 70, 72 and 74 of the cutting elements of the invention to the results 100, 102 and 104 of the control cutting elements, it is found that the repeatability of the cutting elements in accordance with the invention is improved. The overall durability of the cutting elements in accordance with the invention is also improved by about 50%. As expected, the cutting elements in accordance with the invention had comparable abrasion resistance with the control cutting elements. This is in line with the finding that the depression in the working surface 20 acts to restrict spalling to a localised region in the working surface as opposed to preventing spelling from taking place altogether.

From the above description it should be understood that the cutting elements in accordance with the invention provides improved performance due to the localisation of spalling as a result of the inclusion of the depression in its working surface. Reduced and/or more consistent spelling is/are obtained by removing PCD material from the PCD body 12 of the cutting elements 10, 40, 50, 60. From the above description it should be understood that localised and controlled spelling is achieved by forming a depression in a central region of the working surface 20 of the cutting element such that the depression starts some distance away from the cutting element's cutting edge and particularly its cutting point. Accordingly, spalling is limited to a reduced volume of the PCD body 12, and is therefore more consistent and predictable. It is believed that this will be of great benefit to the application of such PCD cutting elements in drilling, specifically oil well and gas well drilling.

It will be appreciated that the above is only one embodiment of the invention and that there may be many variations without departing from the spirit and/or the scope of the invention. It is further envisaged that a wide variety of cutting element geometries could be used to achieve the desired effect of localising spalling of the working surface in use. It should be understood that the invention is not limited to the specific embodiments described above and illustrated in the accompanying drawings.

Claims

1. A cutting element including a polycrystalline diamond (PCD) body having a working surface, a flank and a cutting edge which is the interface between an operativery outer section of the flank and a radially outer section of the working surface, wherein a depression is formed in the working surface such that at least a portion of the polycrystalline diamond (PCD) body is provided between the cutting edge and the depression so that the depression has no effect on the geometry of the cutting edge.

2. A cutting element according to 2, wherein the cutting element is cylindrical in shape.

3. A cutting element according to either claim 1 or 2, wherein the depression is cylindrical in shape.

4. A cutting element according to claim 3, wherein the depression is concentric with the cylindrical polycrystalline diamond (PCD) body.

5. A cutting element according to any one of claims 1 to 4, wherein the portion of the polycrystalline diamond (PCD) body located between the cutting edge and the depression is in the form of an annular rim or ring defining at least part of the working surface.

6. A cutting element according to claim 5, wherein the annular rim or ring is shaped such that the working surface is perpendicular to the flank of the polycrystalline diamond (PCD) body.

7. A cutting element according to claim 5, wherein the portion of the polycrystalline diamond (PCD) body located between the cutting edge and the depression is tapered such that the working surface at least in part creates an oblique angle with respect to the flank of the poly crystalline diamond (PCD) body.

8. A cutting element according to claim 5, wherein the portion of the polycrystalline diamond (PCD) body located between the cutting edge and the depression carries a fillet radius between an internal surface and base of the depression.

9. A cutting element according to any one of claims 5 to 8, wherein the width of the annular rim or ring is between about 0.5 mm and about 3 mm.

10. A cutting element according to any one of claims 1 to 9, wherein the depth of the depression is about 0.5 mm.

11. A cutting element according to any one of claims 1 to 10, wherein the interface defining the cutting edge is a ring, thereby defining a sharp cutting edge.

12. A cutting element according to any one of claims 1 to 10, wherein the interface defining the cutting edge is chamfered.

13. A method of reducing spalling crack propagation in a cutting element, the method including providing a depression in a working surface of the cutting element such that at least a portion carrying at least a part of the working surface of the body is provided between a cutting edge of the cutting element and the depression, thereby, in use, allowing a spalling crack to propagate across the portion of the body provided between the cutting edge and the depression before propagation is obstructed by the absence of material in the depression. A method according to claim 13, wherein the cutting element is a polycrystalline diamond (PCD) cutting element according to any one of claims 1 to 12.