Field

This disclosure relates to super hard constructions and methods of making such constructions, particularly but not exclusively to constructions comprising polycrystalline diamond (PCD) structures and tools comprising the same, particularly but not exclusively for use in cutting applications.

Background

Polycrystalline super hard materials, such as polycrystalline diamond (PCD) or polycrystalline cubic boron nitride (PCBN) may be 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. The working life of super hard tool inserts may be limited by fracture of the super hard material, including by spalling and chipping, or by wear of the tool insert. Coatings may be applied to the super hard material and when applied to, for example, polycrystalline diamond (PCD), polycrystalline cubic boron nitride (PCBN) or tungsten carbide cutting tools, such coatings may improve wear resistance and increase the cutting life of the tools. This may be achieved by, for example, reducing chemical erosion of cBN during application, increase thermal stability of the cutting tool and reducing friction in use. Coatings may also be used to promote bonding between the tool insert and the braze material as these tools typically need to be brazed to a tool holder to render them suitable for use. The coatings are conventionally applied at the final stage of the tool manufacturing process, and the process may be both difficult and expensive.

There is therefore a need for a method of forming a coated polycrystalline super hard cutting tool which is economical, and non-complex to perform. SUMMARY

Viewed from a first aspect there is provided a method of forming a super hard polycrystalline construction, comprising: forming a pre-composite assembly comprising a skeleton formed of a first material, and a region of super hard particles or grains, the skeleton having one or more voids therein, the super hard particles being located in one or more of said voids in the skeleton; and treating the pre-composite assembly in the presence of a catalyst/solvent material for the super hard particles or grains at an ultra-high pressure of around 5 GPa or greater and a temperature to sinter together the super hard particles or grains to form a body of polycrystalline super hard material comprising a first region of super hard particles or grains, and an interpenetrating second region of a second material, the second material forming a coating on at least a portion of the first region, the second material comprising any one or more of the first material, or an oxide, a carbide, or a nitride of the first material.

Viewed from a second aspect there is provided a tool comprising a super hard polycrystalline construction formed according to the above defined method, the tool being for cutting, milling, grinding, drilling, earth boring, rock drilling or other abrasive applications.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example and with reference to the accompanying drawings in which:

Figure 1 is a schematic cross-section of a portion of a conventional PCD micro- structure with interstices between the inter-bonded diamond grains filled with a non- diamond phase material; Figure 2a is a schematic plan view of a plurality of coated super hard constructions according to a first example;

Figure 2b is a cross-section of a portion of hard material of a first example showing the micro-structure of the material; and

Figure 2c is a cross-section of a portion of hard material of a second example showing the micro-structure of the material.

The same references refer to the same general features in all the drawings. DESCRIPTION

As used herein, a "super hard material" is a material having a Vickers hardness of at least about 28 GPa. Diamond and cubic boron nitride (cBN) material are examples of super hard materials.

As used herein, a "super hard construction" means a construction comprising a body of polycrystalline super hard material. In such a construction, a substrate may be attached thereto or alternatively the body of polycrystalline material may be freestanding and unbacked.

As used herein, polycrystalline diamond (PCD) is a type of polycrystalline super hard (PCS) material comprising a mass 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 percent of the material. As used herein, "interstices" or "interstitial regions" are regions between the diamond grains of PCD material. In some examples of PCD material, interstices between the diamond grains may be at least partly filled with a binder material comprising a catalyst for diamond. In some examples of PCD material, interstices or interstitial regions may be substantially or partially filled with a material other than diamond, or they may be substantially empty. 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 "catalyst material" for a super hard material is capable of promoting the growth or sintering of the super hard material.

The term "substrate" as used herein means any substrate over which the super hard material layer is formed. For example, a "substrate" as used herein may be a transition layer formed over another substrate.

As used herein, the term "integrally formed" regions or parts are produced contiguous with each other and are not separated by a different kind of material.

The micro-structure of a conventional PCD material which may find application in conventional cutting tools, for example, is shown in Figure 1 . During formation of a conventional polycrystalline diamond construction, the diamond grains are directly interbonded to adjacent grains and the interstices 24 between the grains 22 of super hard material such as diamond grains in the case of PCD, may be at least partly filled with a non-super hard phase material. This non-super hard phase material, also known as a filler material, may comprise residual catalyst/binder material, for example cobalt, nickel or iron. The typical average grain size of the diamond grains 22 is larger than 1 micron and the grain boundaries between adjacent grains is therefore typically between micron-sized diamond grains, as shown in Figure 1 .

Polycrystalline diamond (PCD) is an example of a super hard material (also called a super abrasive material or ultra hard material) comprising a mass of substantially inter- grown diamond grains, forming a skeletal mass defining interstices between the diamond grains. PCD material typically comprises at least about 80 volume % of diamond and is conventionally made by subjecting an aggregated mass of diamond grains to an ultra-high pressure of greater than about 5 GPa, and temperature of at least about 1 ,200°C, for example. A material wholly or partly filling the interstices may be referred to as filler or binder material.

PCD is typically formed in the presence of a sintering aid such as cobalt, which promotes the inter-growth of diamond grains. Suitable sintering aids for PCD are also commonly referred to as a solvent-catalyst material for diamond, owing to their function of dissolving, to some extent, the diamond and catalysing its re-precipitation. A solvent-catalyst for diamond is understood be a material that is capable of promoting the growth of diamond or the direct diamond-to-diamond inter-growth between diamond grains at a pressure and temperature condition at which diamond is thermodynamically stable. Consequently the interstices within the sintered PCD product may be wholly or partially filled with residual solvent-catalyst material. Most typically, PCD is formed on a cobalt-cemented tungsten carbide substrate, which provides a source of cobalt solvent-catalyst for the PCD. Materials that do not promote substantial coherent intergrowth between the diamond grains may themselves form strong bonds with diamond grains, but are not suitable solvent - catalysts for PCD sintering.

A first example cutting tool is shown in Figures 2a and 2b. The cutting tool 100 comprises a template or skeleton 102 of a base material. Regions of superabrasive particles 104 are added to fill the empty volumes in the skeleton to form a pre- composite assembly. The pre-composite assembly is then placed in a suitable canister or container and is sintered under conventional superabrasive sintering conditions, for example at a pressure of around 6.8GPa, and temperature of around 1300 degrees C, to form a structure such as that shown in Figure 2b. It is possible to form multiple cutting elements in a single process, as shown in Figure 2a, which can be processed after sintering to separate the elements using EDM machining or laser cutting. Figure 2a shows four such elements denoted by the four superabrasive regions 104 which have been integrally formed, prior to separation into four individual elements.

As shown in the first example of Figure 2b, the sintered construction comprises a region of polycrystalline superhard grains 104, a first region 106 and a second region 108. During the sintering process, a reaction or bonding occurs between the base material of the template or skeleton 102, the superabrasive particles 104 and other constituents such as catalysts used during liquid phase sintering to assist in the sintering of the superhard material. The reaction may produce a stable compound such as an oxide, carbide or nitride at the interface of the base material and region(s) of superabrasive material. This stable compound is shown as a first region 108 in Figure 2b. The unreacted material in the base material is denoted by a second region 106. The first and second regions 108, 106 form a coating on the superabrasive material 104, the coating being formed in-situ on the material during the high temperature and high pressure sintering process.

Figure 2c shows the microstructure of a second example in which the first region comprises titanium carbide and the second region comprises titanium, the superabrasive material 104 being PCD.

In some examples, the base material is selected from any one or more of a group of oxides, nitrides, carbides, or metal alloys or metals that react during sintering to form an oxide, nitride or carbide. Examples of such materials include but are not limited to titanium, titanium carbide

The thickness of the coating comprising the first and second regions 108, 106 may be easily selected to suit the end application. In some examples, the thickness of the coating may be between around 10 microns to around 200 microns.

In some embodiments, the super hard material may be, for example, polycrystalline diamond (PCD) and the super hard diamond particles or grains may be of natural and/or synthetic origin.

As used herein, a PCD grade is a PCD material characterised in terms of the volume content and size of diamond grains, the volume content of interstitial regions between the diamond grains and composition of material that may be present within the interstitial regions. A grade of PCD material may be made by a process including providing an aggregate mass of diamond grains having a size distribution suitable for the grade, optionally introducing catalyst material or additive material into the aggregate mass, and subjecting the aggregated mass in the presence of a source of catalyst material for diamond to a pressure and temperature at which diamond is more thermodynamically stable than graphite and at which the catalyst material is molten. Under these conditions, molten catalyst material may infiltrate from the source into the aggregated mass and is likely to promote direct intergrowth between the diamond grains in a process of sintering, to form a PCD structure. The aggregate mass may comprise loose diamond grains or diamond grains held together by a binder material and said diamond grains may be natural or synthesised diamond grains.

Different PCD grades may have different microstructures and different mechanical properties, such as elastic (or Young's) modulus E, modulus of elasticity, transverse rupture strength (TRS), toughness (such as so-called KiC toughness), hardness, density and coefficient of thermal expansion (CTE). Different PCD grades may also perform differently in use. For example, the wear rate and fracture resistance of different PCD grades may be different.

All of the PCD grades may comprise interstitial regions filled with material comprising cobalt metal, which is an example of catalyst material for diamond.

The structure of the examples, as shown in Figures 2a to 2c, may comprise two or more PCD grades.

The grains of super hard material may be, for example, diamond grains or particles. In the starting mixture prior to sintering they may be, for example, bimodal, that is, the feed comprises a mixture of a coarse fraction of diamond grains and a fine fraction of diamond grains having a smaller average grain size than the coarser fraction. By "average particle or grain size" it is meant that the individual particles/grains have a range of sizes with the mean particle/grain size representing the "average". The average particle/grain size of the fine fraction is less than the size of the coarse fraction.

In some examples, the binder catalyst/solvent may comprise cobalt or some other iron group elements, such as iron or nickel, or an alloy thereof. Carbides, nitrides, borides, and oxides of the metals of Groups IV-VI in the periodic table are other examples of non-diamond material that might be added to the sinter mix. In some examples, the binder/catalyst/sintering aid may be Co. The construction of Figures 2a to 2c are discussed in more detail below with reference to the following example, which is not intended to be limiting.

Example

A titanium metal skeleton of the desired shape is formed using conventional 3D laser sintering of titanium metal powder. The skeleton is introduced into a niobium cup and diamond powder is added to fill the voids in the titanium skeleton structure. A bimodal diamond powder mixture was used which comprised 15wt% of diamond particles having an average grain size of around 2 microns and 85wt% of diamond particles having an average grain size of around 22 microns. A pre-formed tungsten carbide substrate comprising around 13wt% cobalt was then added into the niobium cup on top of the diamond particles to form a pre-form assembly. The assembly was subjected to mechanical vibration to ensure the loose powders filled the empty spaces in the titanium skeleton. The pre-composite was then sintered at a pressure of greater than 5GPa and a temperature of about 1400 degrees C in the presence of cobalt infiltrated from the WC-Co substrate. The sintered PCD construction was then removed from the canister. The sintered construction comprises an intergrown PCD skeleton with a three dimensionally continuously interpenetrating structure of titanium as shown in Figure 2c.

While various versions have been described with reference to a number of examples, those skilled in the art will understand that various changes may be made and that the examples are not intended to limit the particular examples or versions disclosed.

For example, in some embodiments of the method, the PCD material may be sintered for a period in the range from about 1 minute to about 30 minutes, about 2 minutes to about 15 minutes, or from about 2 minutes to about 10 minutes.

In some examples of the method, the sintering temperature may be in the range from about 1 ,200 degrees centigrade to about 2,300 degrees centigrade, about 1 ,400 degrees centigrade to about 2,000 degrees centigrade, about 1 ,450 degrees centigrade to about 1 ,700 degrees centigrade, or about 1 ,450 degrees centigrade to about 1 ,650 degrees centigrade.