Field of Invention

The invention relates to the field of cutting elements for tools, and methods of producing cutting elements for tools.

Background of Invention

Cutter inserts for machining and other tools typically comprise a layer of polycrystalline diamond (PCD) bonded to a cemented carbide substrate. PCD is an example of a super hard material, also called super abrasive material, which has a hardness value substantially greater than that of cemented tungsten carbide.

Components comprising PCD are used in a wide variety of tools for cutting, machining, drilling or degrading hard or abrasive materials such as rock, metal, ceramics, composites and wood-containing materials. PCD typically comprises a mass of substantially inter-grown cubic diamond grains forming a skeletal mass, which defines interstices between the cubic diamond grains. PCD material comprises at least about 80 volume % of diamond and can be made by subjecting an aggregated mass of diamond grains to an ultra-high pressure of greater than about 5 GPa, typically about 5.5 GPa, and temperature of at least about 1200°C, typically about 1440°C, in the presence of a sintering aid, also referred to as a catalyst material for diamond. Catalyst material for diamond is understood to be material that is capable of promoting direct inter-growth of diamond grains at a pressure and temperature condition at which diamond is thermodynamically more stable than graphite.

Examples of catalyst materials for diamond are cobalt, iron, nickel and certain alloys including alloys of any of these elements. PCD may be formed on a cobalt-cemented tungsten carbide substrate, which may provide a source of cobalt catalyst material for the PCD. During sintering of the body of PCD material, a constituent of the cemented-carbide substrate, such as cobalt from a cobalt-cemented tungsten carbide substrate, liquefies and sweeps from a region adjacent the volume of diamond particles into interstitial regions between the diamond particles. In this example, the cobalt acts as a catalyst to facilitate the formation of bonded diamond grains. Optionally, a metal-solvent catalyst may be mixed with diamond particles prior to subjecting the diamond particles and substrate to a high pressure high temperature (HPHT) process. The interstices within PCD material may at least partly be filled with the catalyst material. The inter-grown diamond structure therefore comprises original diamond grains as well as a newly precipitated or re-grown diamond phase, which bridges the original grains. In the final sintered structure, catalyst/solvent material generally remains present within at least some of the interstices that exist between the sintered diamond grains.

In drilling operations, a cutting tool insert is subjected to heavy loads and high temperatures at various stages of its useful life. In the early stages of drilling, when the sharp cutting edge of the insert contacts the subterranean formation, the cutting tool is subjected to large contact pressures. This results in the possibility of a number of fracture processes such as fatigue cracking being initiated. As the cutting edge of the insert wears, the contact pressure decreases and is generally too low to cause high energy failures. However, this pressure can still propagate cracks initiated under high contact pressures and can eventually result in spalling-type failures. In the drilling industry, PCD cutter performance is determined by a cutter's ability to achieve high penetration rates in increasingly demanding environments, and still retain a good condition post-drilling (hence enabling re-use). In any drilling application, cutters may wear through a combination of smooth, abrasive type wear and spalling/chipping type wear. Whilst a smooth, abrasive wear mode is desirable because it delivers maximum benefit from the highly wear-resistant PCD material, spalling or chipping type wear is unfavourable. Even fairly minimal fracture damage of this type can have a deleterious effect on both cutting life and performance.

With spalling-type wear, cutting efficiency can be rapidly reduced as the rate of penetration of the drill bit into the formation is slowed. Once chipping begins, the amount of damage to the diamond table continually increases, as a result of the increased normal force now required to achieve a given depth of cut. Therefore, as cutter damage occurs and the rate of penetration of the drill bit decreases, the response of increasing weight on bit can quickly lead to further degradation and ultimately catastrophic failure of the chipped cutting element.

Similar problems exist in the machining industry. PCD may be used to machine non- ferrous materials in operations such as cutting and turning. Again, chipping greatly affects the lifetime of the PCD cutting element and also the quality and finish of the workpiece being machined. When optimising PCD cutter performance, increasing wear resistance in order to achieve better cutter life is typically achieved by manipulating variables such as average diamond grain size, overall catalyst/solvent content, diamond density and the like. Typically, however, as PCD material is made more wear resistant it becomes more brittle or prone to fracture. PCD elements designed for improved wear performance will therefore tend to have poor impact strength or reduced resistance to spalling. This trade-off between the properties of impact resistance and wear resistance makes designing optimised PCD structures, particularly for demanding applications, inherently self-limiting.

PCD cutting elements are typically provided with a usable lifetime (which may be measured in terms of time, metres cut, number of operations etc.). As chipping is a brittle process, the performance of an individual cutting element may greatly exceed that of another individual cutting element, and this effect is difficult to predict. In order to avoid damage to tooling or workpieces, this usable lifetime typically has a cautious value that is significantly lower than the actual lifetime a given tool may achieve.

There is currently a drive to apply sensors to tools to measure parameters such as temperature, chipping, vibration and so on. The data obtained by these sensors can be used to more accurately measure cutting element life, leading to less risk of damaging workpieces and a greater usable lifetime for each cutting element.

Summary of Invention

It is an object of the invention to provide a wear sensor for a PCD cutting element. According to a first aspect, there is provided a turning or milling tool cutting element. The cutting element comprises a high pressure-high temperature polycrystalline diamond body comprising a surface region. An electrically conductive wear sensor is formed at the surface region, the conductive wear sensor comprising graphite. During use, as the diamond body is worn away, portions of the wear sensor are worn away, thereby increasing the electrical resistance of the wear sensor. An indication of the degree in wear can be obtain from this increase in electrical resistance.

As an option, the electrically conductive wear sensor comprises an electrically conductive track. This path may define a tortuous path.

The electrically conductive wear sensor optionally comprises a plurality of electrically conductive tracks. As an option, the high pressure-high temperature polycrystalline diamond body comprises a substantially non-conducting surface region. This may be formed, for example, by leaching conductive material such as cobalt from the surface of the polycrystalline diamond body.

As an option, the cutting element further comprises a non-conducting protective layer disposed over the conductive wear sensor.

The conductive wear sensor is optionally disposed on any of a rake face and a clearance face of the cutting element.

As an option, the cutting element further comprises a data track for carrying an electrical signal between a data collection element and the electrically conductive wear sensor.

According to a second aspect, there is provided a turning or milling tool comprising the cutting element described above in the first aspect.

According to a second aspect, there is provided a method of making a turning or milling tool cutting. The method comprises providing a high pressure-high temperature polycrystalline diamond body comprising a surface region and forming an electrically conductive wear sensor at the surface region, the electrically conductive wear sensor comprising graphite, wherein the electrically conductive wear sensor has an electrical resistance substantially lower than that of the surface region.

As an option, the conductive track is formed by laser ablation of the polycrystalline diamond body at the surface region.

As a further option, the laser ablation is performed using a Nd:YAG laser having a power of between 8 and 14 x 10 ^® Wcnr ² .

As a further option, the laser ablation is pulsed at a frequency of between 20 and 100 kHz and a pulse duration of between 5 and 20 ps.

As a further option, laser ablation is performed at a speed of between 200 and 500 mms ¹ along the substantially non-conductive surface region. The method optionally comprises removing conductive binder material located at the surface region, the removal being performed by leaching. As a further option, conductive binder material is leached away from the entire polycrystalline diamond body.

As an option, the method comprises forming a plurality of conductive tracks to form the electrically conductive wear sensor at the surface region.

The method optionally comprises connecting the electrically conductive wear sensor to a data collection element.

Brief Description of the Drawings

For a better understanding of the present invention and to show how the same may be carried into effect, embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:

Figure 1 is a flow diagram showing exemplary steps;

Figure 2 is a perspective view of an exemplary cutting element;

Figure 3 is a side elevation schematic view of a further exemplary cutting element;

Figure 4 is a graph of measured resistance against number of cutting passes for an exemplary cutting element;

Figure 5 illustrates schematically a side elevation cross-section view of the distribution of graphite around a laser ablated region; and

Figure 6 illustrates schematically a plan view of a further exemplary cutting element.

Detailed Description

A wear sensor may be applied to PCD in the form of an electrically conductive track. If the track is disposed at a pre-determined location, and the resistance suddenly increases, it can be assumes that the track has been broken and wear has advanced to the location of the track. One way to apply a conductive track is to apply a layer of conductive metal, such as silver, to the surface of the PCD. A problem with this approach is that the adhesion between the silver and the PCD is poor. Silver can easily be scraped off or otherwise mechanically removed. In a challenging or aggressive environment, such as around a machine tool or during a downhole drilling operation, it is very difficult to maintain a continuous conductive pathway of silver on the surface of the PCD. This therefore does not make a very robust sensor.

It has surprisingly been found that laser ablation of a PCD surface causes sufficient graphitization at the surface to form a conductive track. It has also been surprisingly found that the graphite conductive region is sufficiently adhered to the underlying PCD body to form a continuous conductive pathway even under aggressive machining conditions.

Figure 1 is a flow diagram illustrating steps in forming a cutting element for a tool. The following numbering corresponds to that of Figure 1 .

51 . An HPHT PCD diamond cutting element is provided with a surface region.

52. Laser ablation is performed on the surface of the cutting element to form a wear sensor. The carbon in the PCD is in the form of diamond, and laser ablation converts some of the diamond carbon to the graphite form of carbon, thereby forming an electrically conductive track. The resistivity of graphite is around 1x10 ^-5 ohm cm, making graphite a good conductor of electricity. The graphite track can therefore be used to carry data in the form of an electric signal.

An exemplary cutting element 1 is shown in Figure 2. The cutting element 1 comprises a PCD layer 2 bonded to a support substrate 3. The support substrate 3 may be any suitable material, such as cemented tungsten carbide. The PCD layer 2 has a top (or rake) face 4, and a clearance face 5.

Initially, the PCD contains cobalt in the interstices between diamond grains. Prior to laser ablation, the cutting element 1 is leached in HCI to remove cobalt close to the surface of the top face 4 and the clearance face 5. A protective element such as acid resistant tape (e.g. polyamide tape with a silicone adhesive) is used to protect the cemented tungsten carbide substrate 3 from the HCI. In this example, it was found that leaching in equal volumes of deionized water and HCI at 107°C for several hours removed sufficient cobalt from the surface of the PCD to make it effectively nonconducting.

Electrically conductive tracks were applied by using a Nd:YAG Q-switched high frequency laser. A power of between 10 and 12 x 10 ⁶ Wcnr ² was used, at a frequency of 50 kHz, a pulse duration of 10 ps and a speed of 300 to 400 mms l

Two conductive tracks 6, 7 were applied using the pulsed laser. Track 6 was applied only on the top face 4 of the PCD layer 2. Track 7 was applied on both the top face 4 and the clearance face 5 of the PCD layer 2. The laser ablation energy was sufficient to form a trench in the surface of the PCD layer 2, and to convert diamond to graphite at the surface.

As PCD is a polycrystalline structure, and the removal of cobalt leaves pores, it is thought that the width of each ablated electrically conductive track 6, 7, 8 should be at least three times the average diamond grain size of the PCD to ensure that there are no breaks in the electrically conductive tracks that would otherwise affect the ability of the electrically conductive track to carry an electrical signal.

Note that the conductive track need not be disposed in a laser-cut trench, but can be on the surface of the PCD with no trench. An exemplary way to achieve this is to apply graphitic tracks as described above, and mask the surface of the track with an acid-resistant mask, such as an Au sputter-coating through a shadow mask. Subsequent leaching removes cobalt and undercuts the track to electrically isolate it.

In the embodiment of Figure 2, the conductive tracks 6, 7 form a sensor to monitor wear or chipping of the PCD layer 2 of the cutting element 1 . An electric current is passed through each conductive track and the resistance of each conductive track is measured. If the resistance suddenly increases, an operator can be made aware that the electrically conductive track has been broken. The position of each electrically conductive track relative to the cutting edge of the cutting element 1 is known, and so wear or chipping of the tool can be monitored.

PCD is a brittle material and chipping can occur at almost any time. For this reason, the working life of a cutting element is based on recommended times, at which the cutting element is changed. However, a cutting element may, after this time, still have many potential hours of use. By using electrically conductive tracks to monitor wear of the cutting element 1 , the tool life can be more accurately monitored and the tool replaced shortly before an unacceptable amount of wear has occurred. This greatly increases the working life of cutting elements.

Referring to Figure 3, this embodiment shows two electrically conductive tracks 9, 10 used as a wear sensor and disposed on the clearance face 5 of the PCD layer 2. The conductive tracks 9, 10 terminate at a data collection element 10, which is effectively a terminal block that is connected to a device such as a computer (not shown). In this way a current can be applied to each conductive track 9, 10, and the resistance of each conductive track can be measured.

It should be noted that while the above embodiments describe a conductive track being either unbroken or broken, progressive wear can be seen to have an effect on the resistance of an electrically conductive track on a tool cutting element 1 during a cutting operation. Referring to Figure 4, there is shown a graph of cut number against measured resistance of a conductive track. The test was performed using a workpiece of Titanium Ti-6al-4v, using a NT 5400 DCG Machine. The tool geometry was CNGN 12 07 08. The electrically conductive track was located on the clearance face at a distance of 150 from the edge. The track thickness was around 50 pm, and the track depth was around 20 pm. Cutting was performed under continuous outer diameter turning (OD 200 mm) using a cut depth of 0.25 mm, a cutting speed of 250 mm/min, a feed rate of 0.15 mm/rev and a pass length of 10 mm.

The graph of Figure 4 shows that the electrical resistance of the electrically conductive track steadily increases with cut number, which is indicative of gradual wear at the cutting edge. In cut number 14, the resistance suddenly increase by a factor of almost 5, indicating that the wear has proceeded to cut the electrically conductive track. Using a graph like this, an operator can judge when a cutting element 1 should be changed. It may be possible to relate the electrical resistance of the electrically conductive track to the wear depth.

While not wishing to be bound by any specific theory, the inventors believe that gradually increasing resistance with gradual tool wear may be explained by graphitization of diamond in a region beyond the ablated region of the cutting element surface. Figure 5 illustrates schematically a cross-section view of an electrically conductive track 9 in a clearance face 5 of a PCD layer 2. The laser ablation has created a trench to a desired thickness, and the graphitization of carbon has occurred in a region around the trench. A graphitized region 12 is shown as the shaded area around the electrically conductive track 9. It is expected that the ratio of graphite to diamond gets smaller with increasing distance from the electrically conductive track.

During a machining operation, the cutting edge 13 is in contact a workpiece. As the edge wears away, in the direction shown by arrow 14, some of the graphitized region 12 is also worn away. It is known that the resistance of the conductive track 12 is inversely proportional to the cross-sectional area of the conductive track 9, and so wearing away the graphitized region 12 close to the cutting edge 13 will cause an increase in the resistance of the electrically conductive track 9. In this way, the graph of Figure 4 shows a gradual increase in resistance as the cutting edge gradually wears away. Once the graphitized region 12 has been entirely removed (either through gradual wear or sudden chipping) the resistance shows a very rapid increase.

Figure 6 shows a schematic plan view of a further exemplary embodiment in which two electrically conductive wear sensors 15, 16 are applied to the top face 4 of the PCD layer 2. In this example, the wear sensors 15, 16 each comprise an electrically conductive track that follows a tortuous path such that the conductive tracks are much longer than the largest linear dimension of the wear sensors 15, 16.

While the above embodiments all describe the use of electrically conductive tracks to monitor the wear of a cutting element, it will be appreciated that they may be used to carry data over the PCD layer 2 surface from a tool sensor to a data collection element 1 1 . For example, a thermocouple or a vibration sensor may be applied to the tool, and the electrically conductive track may be used to carry information.

Certain terms and concepts as used herein will be briefly explained.

As used herein, super-hard or ultra-hard material has Vickers hardness of at least 25 GPa. Synthetic and natural diamond, polycrystalline diamond (PCD), cubic boron nitride (CBN) and polycrystalline CBN (PCBN) material are examples of super-hard materials. Synthetic diamond, which may also be called man-made diamond, is diamond material that has been manufactured. A PCD structure comprises or consists of PCD material. Other examples of super-hard materials include certain composite materials comprising diamond or CBN grains held together by a matrix comprising ceramic material, such as silicon carbide (SiC), or by cemented carbide material such as Co-bonded WC material. For example, certain SiC-bonded diamond materials may comprise at least about 30 volume per cent diamond grains dispersed in a SiC matrix (which may contain a minor amount of Si in a form other than SiC).

In general and as used herein, catalyst material for super-hard material is capable of promoting the sintering of polycrystalline material comprising grains of the super-hard material, at least at a pressure and temperature at which the super-hard material is thermodynamically stable. The catalyst material may be capable of promoting the direct inter-growth of grains of the super-hard material and or more generally the sintering of the grains of the super-hard material to form the polycrystalline material. In some examples, the catalyst material may function as a binder material capable of forming a sintered matrix, on its own or in combination with other suitable material, within which the super-hard grains may be dispersed and not necessarily directly inter-bonded with each other. For example, catalyst material for synthetic diamond is capable of promoting the growth of synthetic diamond grains and or the direct intergrowth of synthetic or natural diamond grains at a temperature and pressure at which synthetic or natural diamond is thermodynamically more stable than graphite. Examples of catalyst materials for diamond are Fe, Ni, Co and Mn, and certain alloys including these. Catalyst or binder material for PCBN material may comprising a Ti- containing compound, such as titanium carbide, titanium nitride, titanium carbonitride and or an Al-containing compound, such as aluminium nitride, and or compounds containing metal such as Co and or W, for example.

As used herein, polycrystalline diamond (PCD) material comprises a mass (an aggregation of a plurality) of diamond grains, a substantial portion of which are directly inter-bonded with each other and in which the content of diamond is at least about 80 volume per cent of the material. Interstices between the diamond grains may be at least partly filled with a binder material comprising a catalyst material for synthetic diamond, or they may be substantially empty. Bodies comprising PCD material may comprise at least a region from which catalyst material has been removed from the interstices, leaving interstitial voids between the diamond grains.

A machine tool is a powered mechanical device, which may be used to manufacture components comprising materials such as metal, composite materials, wood or polymers by machining, which is the selective removal of material from a body, called a work-piece. A machine tool may comprise a cutter insert (or simply “insert”) comprising a cutter structure, and the insert may be indexable and or replaceable.

When a machine tool is in use machining a work-piece, pieces of the work-piece will likely be removed and these pieces are referred to as“chips”. Chips are the pieces of a body removed from the work surface of the body by a machine tool in use. Controlling chip formation and directing chip flow are important aspects of tools for high productivity machining and or high surface finish machining of advanced alloys of aluminium, titanium and Nickel. The geometry of chip-breaker features may be selected according to various machining factors, such as the work piece material, cutting speed, cutting operation and surface finish required.

While this invention has been particularly shown and described with reference to embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appended claims.