Cutting elements for rotary drill bits

A preform cutting element for a rotary drill bit for use in drilling or coring holes in subsurface formations includes a cutting table 12, 13 of superhard material such as polycrystalline diamond. The rear face of the cutting table is bonded to a substrate 11 of less hard material, such as cemented tungsten carbide. A front portion 12 of the cutting table comprises a form of superhard material which is less wear-resistant than the superhard material forming the remainder 13 of the cutting table. The provision of the less wear-resistant superhard material at the front cutting faces reduces the tendency of the cutting table to spall.

BACKGROUND OF THE INVENTION 
The invention relates to cutting elements for rotary drill bits for use in 
drilling or coring holes in subsurface formations. 
In particular, the invention is applicable to cutting elements for use on 
rotary drill bits of the kind comprising a bit body having a shank for 
connection to a drill string and an inner passage for supplying drilling 
fluid to the face of the bit, the bit body carrying a plurality of cutting 
elements. Each cutting element comprises a preform element, often in the 
form of a circular tablet, including a cutting table of superhard material 
having a front cutting face and a rear face, the rear face of the cutting 
table being bonded to a substrate of material which is less hard than the 
superhard material. 
The cutting table, which is normally in the form of a single layer, usually 
comprises polycrystalline diamond, although other superhard materials are 
available, such as cubic boron nitride. The substrate of less hard 
material is often formed from cemented tungsten carbide, and the cutting 
table and substrate are bonded together during formation of the cutting 
element in a high pressure, high temperature forming press. 
Since the substrate is of less hard material than the cutting table, the 
two-part arrangement of the cutting element provides a degree of 
self-sharpening since, in use, the less hard substrate wears away more 
easily than the harder cutting table. 
The perform cutting element may be directly mounted on the bit body or may 
be bonded to a carrier, for example also of cemented tungsten carbide, the 
carrier being in turn received in a socket in the bit body. The bit body 
may be machined from metal, usually steel, or may be formed from an 
infiltrated tungsten carbide matrix by a powder metallurgy process. 
Such cutting elements are subjected to extremes of temperature and heavy 
loads, including impact loads, when the drill is in use down a borehole. 
It is found that under drilling conditions spalling of the diamond table 
can occur, that is to say the separation and loss of diamond material over 
the cutting surface of the table. Such spalling usually spreads from the 
cutting edge, probably as a result of impact forces. The spalling reduces 
the cutting efficiency of the element, and in severe cases can lead to 
delamination, that is to say separation of the diamond table from the 
substrate. 
It has been found that the incidence of spalling may be reduced by 
pre-bevelling the periphery of the diamond table. Reference in this regard 
may be made to U.S. Pat. No. Re. 32036. However, the pre-bevelling process 
is comparatively costly and time consuming, involving as it does the 
cutting or grinding of a significant amount of the superhard material from 
the periphery of the diamond table. Also, the pre-formed bevel becomes 
worn away after an initial period of use and the anti-spalling advantage 
it gives therefore also disappears after such period. 
The present invention sets out to provide an alternative and advantageous 
method of forming the cutting table of superhard material in a cutting 
element in a manner to inhibit spalling. 
SUMMARY OF THE INVENTION 
According to the invention there is provided a perform cutting element 
including a cutting table of superhard material having a front cutting 
face and a rear face, the rear face of the cutting table being bonded to a 
substrate of material which is less hard than the superhard material, 
wherein the cutting table includes a front portion which provides said 
cutting face and comprises a form of superhard material which is less 
wear-resistant than the superhard material forming at least one other 
portion of the remainder of the cutting table. 
As superhard material is made less wear-resistant it becomes more resistant 
to spalling and the formation of the front cutting portion of the cutting 
table from a less wear-resistant material thus reduces its tendency to 
spall. At the same time, however, the inclusion in the cutting table of a 
more wear-resistant form of superhard material, behind the cutting face, 
ensures that the cutting table as a whole still operates efficiently and 
does not wear away too quickly. 
The invention may therefore be regarded, in some of its aspects, as 
comprising the addition to a conventional two-part preform cutting element 
of a further front layer of superhard material which is less 
wear-resistant than the superhard material used for the remainder of the 
cutting table. 
The cutting table may comprise at least two distinct layers of superhard 
material bonded together and including a front layer which provides said 
cutting face and a second layer behind said front layer, the front layer 
comprising the form of superhard material which is less wear-resistant 
than the superhard material forming the second layer. 
In such an arrangement the distinct front and second layers may constitute 
the whole of the cutting table. Alternatively there may be provided a 
third layer of superhard material behind the second layer and bonded 
thereto. In this case the third layer may also comprise a form of 
superhard material which is less wear-resistant than the superhard 
material forming the second layer. For example it may be of the same 
composition as the front layer. 
In the latter arrangement the presence of the third layer, being less 
wear-resistant than the second layer, enhances the self-sharpening 
property of the cutting element, since in use it wears away, rearwardly of 
the second layer, at a rate intermediate of the rate of wear of the second 
layer and the substrate. 
In addition, the third layer may act as a transition layer between the 
second layer and the substrate in a manner to facilitate manufacture of 
the cutting element. For example, the third layer, as well as being less 
wear-resistant, may have a coefficient of thermal expansion and modulus of 
elasticity intermediate those of the second layer and the substrate. This 
may enhance the bonding between the cutting table and the substrate and 
reduce the stresses incorporated in the cutting element in the region of 
the bond during its formation. 
Such transition layers have been employed in the construction of inserts 
for roller cone bits, as described in U.S. Pat. No. 4694918. In the 
arrangements described in that specification, however, the front, 
outermost layer is always the hardest layer, and there is no disclosure of 
the concept of the present invention where the front layer is less 
wear-resistant than one or more layers inwardly thereof. The transition 
layer is also not used for the purpose of enhancing self-sharpening since 
the inserts of a roller cone bit operate by impact crushing the formation 
and do not provide a cutting edge in the manner of cutting elements for a 
drag bit. It will be apparent that transition layers, for example as 
described in U.S. Pat. No. 4694918, may be employed in any of the 
arrangements according to the present invention. 
Instead of the provision of a single third layer, of less wear-resistance, 
behind the second layer, as described above, the cutting table may include 
a plurality of further layers stacked behind the second layer, the further 
layers being of decreasing wear-resistance as they extend away from the 
second layer towards the substrate. By providing a plurality of further 
layers of decreasing wear-resistance, the rate at which the layers are 
worn away, in use, increases towards the less hard substrate and this 
enhances the self-sharpening effect. 
In an alternative arrangement according to the present invention, the 
aforesaid third layer may be formed of a superhard material which is more 
wear-resistant still than the second layer. In a development of such 
arrangement there may be provided a plurality of layers stacked behind the 
second layer, the further layers being formed of increasingly 
wear-resistant superhard material as they extend away from the second 
layer. 
Instead of the superhard cutting table comprising two or more distinct 
layers bonded together, it may comprise a single layer, the composition of 
the single cutting layer varying throughout its thickness in a manner so 
as to provide said front portion which is less wear-resistant than at 
least one other portion of the remainder of the single layer. 
Various means may be employed to render the different portions or layers of 
the cutting table of different wear-resistance. For example, in the case 
where the cutting table is formed of polycrystalline diamond, the front 
portion or front layer of the cutting table may be rendered less 
wear-resistant by being formed of diamond particles which are, on average, 
of larger grain size than the diamond particles forming said other portion 
of the cutting table. 
Alternatively or additionally, the front portion of the front layer may be 
rendered less wear-resistant by being formed of diamond particles of lower 
packing density than the diamond particles forming said other portion of 
the cutting table. 
In a further alternative, the front portion or front layer of the superhard 
cutting table may comprise polycrystalline diamond material which is 
rendered less wear-resistant by the inclusion of an additive material 
prior to formation of the cutting element in the press. The additive 
material may, for example, be tungsten carbide particles or pre-cemented 
particles of tungsten carbide. 
The formation of a superhard composite by combining polycrystalline diamond 
particles with pre-cemented tungsten carbide particles is disclosed in 
U.S. Pat. No. 4,525,178.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, the cutting element, which is generally in the form of 
a circular tablet, comprises a front cutting table 10 of superhard 
material bonded to a thicker substrate 11 of less hard material, such as 
cemented tungsten carbide. As is well known, the components of the cutting 
element are bonded together during formation of the whole cutting element 
in a high pressure, high temperature forming press. 
Usually, in cutting elements of this type, the front cutting table 10 
comprises a single layer of polycrystalline diamond of substantially 
uniform composition. (The use in the specification of the description 
"substantially uniform" does not imply that the layer is formed of 
particles of substantially uniform size. In any part of the layer there 
will normally be particles of a range of sizes and other particles may be 
present in addition to the diamond particles. The description 
"substantially uniform" therefore means that the proportions of the 
particles of different sizes and/or compositions is approximately the same 
in all parts of the layer). The composition of the diamond making up the 
single layer is selected to provide a high degree of wear-resistance so as 
to provide the desired cutting efficiency and life of the cutting element 
in use on the drill bit. 
FIG. 1, however, shows a cutting element in accordance with the present 
invention where the front cutting table 10 comprises two layers 12 and 13 
of different compositions. The material forming the front layer 12 is less 
wear-resistant than the material forming the second layer 13, and as 
previously described it is found that this may inhibit spalling of the 
front cutting surface. 
Various methods may be used to achieve the desired difference in 
wear-resistance of the two layers. For example, the wear-resistance of 
polycrystalline diamond material may be varied by varying the grain size 
of the diamond particles used in the formation of the layer. A smaller 
maximum or average particle size in a diamond layer will result in greater 
wear-resistance than a larger maximum or average particle size. Thus, to 
provide the effect required by the invention the front layer 12 may have a 
larger maximum or average particle size than the layer 13. 
In addition to, or instead of, varying the wear-resistance by varying the 
grain size, the wear-resistance may also be varied by varying the grain 
size distribution or packing density of the diamond particles. Thus, a mix 
having an appropriate range of different particle sizes will usually 
provide a higher packing density, and thus greater wear-resistance, than a 
mix of comparatively uniformly sized particles, since the small particles 
will fill the voids between the larger particles in the former (relatively 
non-uniform) mix. Accordingly, the variation in wear-resistance required 
by the invention may be achieved by forming the front layer 12 from 
diamond particles providing a lower packing density than the particles 
used for the layer 13. Due to the effect of pressure on packing density, 
during formation of a layer, it may be necessary to form the layers during 
separate pressings rather than simultaneously. 
In a further alternative method of varying the wear-resistance of the 
layers, this may be achieved by including with the diamond particles, 
before they are introduced into the forming press, an additive which 
alters the final wear-resistance. Such an additive may comprise, for 
example, particles of tungsten carbide or pre-cemented particles of 
tungsten carbide, mixed with the diamond particles. The front layer 12 
will thus contain a higher proportion of the additive than the second 
sub-layer 13, which may have no additive at all. 
The necessary properties of the additive are that it must be bondable to 
diamond and must be able to withstand the temperature and pressure to 
which it must be subjected in the press during the initial formation of 
the cutting element. It should also have a high Young's modulus and fairly 
high strength and a low coefficient of thermal expansion. Other suitable 
additives may be metallic tungsten or other refractory metal, or a ceramic 
such as boron carbide, silicon carbide, tantalum carbide, titanium 
carbide, titanium nitride, boron nitride, or titanium boride. 
FIG. 2 is an enlarged view, in the region of the cutting edge, of a cutting 
element according to FIG. 1 after an initial period of wear when it is 
mounted and in use on a drill bit, the bit body being indicated 14. It 
will be seen that an angled wear flat 15 becomes formed on the cutting 
edge. Where the wear flat 15 occurs on the harder layer 13, the wear flat 
is generally parallel to the surface of the formation 16 and is thus 
determined by the rake angle of the cutting element. However, due to the 
lower wear-resistance of the front layer 12 there is greater wear of this 
layer in the region of the cutting edge as indicated at 17. It will be 
seen that this provides a rounded edge to the cutting element where it 
engages the formation. As previously mentioned, it has been found that the 
incidence of spalling can be reduced by pre-bevelling the periphery of the 
diamond cutting table, and the rounding of the cutting edge achieved by 
the present invention has a similar anti-spalling effect. Accordingly, in 
addition to the two-layer form of cutting table per se reducing the 
tendency to spall, the present invention also results, after a period of 
use, in a structural shape of a kind which has been found further to 
reduce the tendency to spall. 
It should be mentioned that some slight rounding of the cutting edge of the 
diamond layer will normally occur, with wear, in a conventional preform 
cutting element. However, this slight rounding is normally insufficient to 
inhibit spalling, whereas the present invention increases the extent of 
rounding to a point where an anti-spalling effect is achieved. 
The arrangement according to the invention has advantage over the 
pre-bevelled arrangement referred to earlier and described in U.S. Pat. 
No. 32036. In the known arrangement, in order to provide the anti-spalling 
effect, the pre-bevelling must be at such an angle that the bevelled 
surface is inclined away from the surface of the formation when the 
cutting element is mounted at the required rake angle. However, this 
pre-bevelled edge wears away during use of the cutting element, so that 
eventually a point is reached when all the bevel has worn away. The wear 
flat, where it extends across the single layer of the cutting table, is 
then substantially parallel to the surface of the formation, due to the 
cutting table being of substantially uniform composition. When this point 
is reached, and the bevel has been worn away, the cutting element 
obviously has no more resistance to spalling than a similarly worn 
conventional non-pre-bevelled cutting element. 
In the arrangement according to the present invention, however, the rounded 
portion 17, extending away from the surface of the formation, is 
constantly renewed and persists during the whole life of the cutting 
element, and thus tends to inhibit spalling during the whole life of such 
element. Similar modifications can be made to other embodiments of the 
invention. 
FIG. 3 shows an alternative arrangement where a third diamond layer 18 is 
provided between the second layer 13 and the substrate 11. The third layer 
18 is of less wear-resistance than layer 13, for example it may be of 
similar composition and hence wear-resistance to the front layer 12. Since 
the third layer 18 is less wear-resistance than the layer 13 it wears 
away, in use, more rapidly than the layer 13. It therefore contributes to 
the desirable self-sharpening effect referred to previously by allowing 
part of the superhard cutting table itself, as well as the substrate 14, 
to be worn away at a greater rate than the layer 13 of the cutting table. 
The less wear-resistant layer 18 may also act as a transition layer, as 
previously described, to improve the bonding between the substrate and the 
cutting table. 
FIG. 4 shows a modification of the arrangement of FIG. 3 in which there are 
provided a plurality of further layers behind the second layer 13, the 
further layers being indicated at 19. Any required number of further 
layers 19 may be provided and their wear-resistance is preferably graded, 
using any of the methods referred to earlier, so that the layers 19 
becomes less hard and wear-resistant as they extend from the layer 13 
towards the substrate 11. As in the previous arrangement the layers 19 may 
also act as multiple transition layers in the manner of U.S. Pat. No. 
4,694,918. 
In the arrangement of FIG. 4, the single less wear-resistant layer 12 in 
front of the layer 13 may be replaced by two layers of less 
wear-resistance than the layer 13, the outermost of the two layers being 
less wear-resistant than the next layer. 
FIG. 5 is an enlarged view of the cutting element shown in FIG. 4, in use 
and after a certain amount of wear of the cutting element has occurred. It 
will be seen that, as in the arrangement shown in FIG. 2, the front layer 
12 has worn away to a rounded shape which enhances the anti-spalling 
effect. However, it will also be seen that the further layers 19 behind 
the second layer 13 have also worn away as they extend rearwardly of the 
layer 13, thus providing clearance between the formation 16 and both the 
cutting table and the substrate 11 to the rear of the layer 13, thus 
enhancing the self-sharpening effect. 
FIG. 6 shows a cutting element of similar construction to the element shown 
in FIG. 3 in which the cutting table 20 comprises a front diamond layer 
21, a second diamond layer 22, a third diamond layer 23 and a substrate 24 
of cemented tungsten carbide. In accordance with the invention the front 
layer 21 is of less wear-resistance than the second layer 22, using any of 
the methods referred to, to provide the desirable anti-spalling effect. In 
the arrangement of FIG. 6, however, the third layer 23 is of greater 
wear-resistance than the second layer 22. In such an arrangement the 
resultant rounding of the cutting edge which occurs after some use of the 
cutting element may extend over both the front layer 21 and the second 
layer 22, since these are both less wear-resistant than the third layer 
23. 
A further development of the type of cutting element shown in FIG. 6 is 
shown in FIG. 7. In this case there are a plurality of diamond layers 25 
making up the cutter table, the layers being bonded to one another and the 
rearmost layer being bonded to the substrate 26. Each layer is less wear 
resistant than the one behind it. In use, rounding of the layers of the 
cutting table will be spread over all those layers which are in front of 
the layer which, for the time being, is bearing against the formation. 
In the arrangements shown in FIGS. 6 and 7 the layers are of essentially 
the same thickness but they may also be of different thicknesses and it 
will be appreciated that the profile shape of the resultant rounding will 
depend to a certain extent on the relative thickness of the layers, as 
well as their relative wear-resistance. 
In the arrangements of FIGS. 6 and 7, transition layers of diamond may also 
be provided between the rearmost of the layers shown and the substrate. 
In the arrangements so far described, the different portions of the 
superhard cutting table are in the form of discrete layers which are 
formed in the press, preferably simultaneously, when the cutting element 
is formed, the composition of each layer being substantially uniform (as 
hereinbefore defined). However, as shown in FIG. 8, it is also possible 
for the cutting table to comprise a single non-uniform layer 27 of 
polycrystalline diamond, bonded to the substrate 28, the composition of 
the single layer 27 varying from its front cutting face 29 to its rear 
surface 30 where it is bonded to the substrate 28. The varying composition 
of the layer 27 between these surfaces may be so arranged, using any of 
the methods referred to earlier, that the wear-resistance of the layer 
increases continuously as it extends from the cutting face 29 towards the 
substrate 28. This may be achieved by (a) decreasing the particle size 
and/or (b) increasing the packing density and/or (c) decreasing the 
proportion of additive in the layer as it extends from the surface 29 
towards the substrate. Such variation, and consequent increase in 
wear-resistance, may continue all the way up to the rear surface 30 where 
the diamond layer is bonded to the substrate. If desired a transition 
layer may be provided between the surface 30 and the substrate 28. 
Alternatively, the arrangement may be such that the wear-resistance of the 
layer 27 increases up to an intermediate position away from the front face 
29 whereafter the wear-resistance decreases again until the rear surface 
30 is reached.