Super-hard structure, tool element and method of making same

Abstract

A method for treating a super-hard structure, the method including heating the super-hard structure to a treatment temperature of at least 500 degrees centigrade and cooling the super-hard structure from the treatment temperature to a temperature of less than 200 degrees centigrade at a mean cooling rate of at least 1 degree centigrade per second and at most 100 degrees centigrade per second to provide a treated super-hard structure. A PCBN structure produced by the method may have flexural strength of at least 650 MPa.

Claims

1. A method for treating a super-hard structure comprising PCBN material, the PCBN material comprising a plurality of cubic boron nitride (cBN) grains and a matrix, the matrix consisting of material selected from the group consisting of of titanium carbide, titanium nitride, titanium carbonitride, boride of aluminium, nitride of aluminium or combinations thereof, the matrix having a combined mean coefficient of thermal expansion of at least 4.5?10.sup.?6 per Kelvin (/K), the method including heating the super-hard structure to a treatment temperature of at least 500 degrees centigrade and cooling the super-hard structure from the treatment temperature to a temperature of less than 200 degrees centigrade at a mean cooling rate of at least 1 degree centigrade per second and at most 100 degrees centigrade per second to provide a treated super-hard structure, wherein the cooling is done by quenching in oil.

2. A method as claimed in claim 1, in which the super-hard structure is cooled by contacting it with said oil having a thermal conductivity of at most 0.4 Watts per meter Kelvin (W/mK) at 20 degrees centigrade.

3. A method as claimed in claim 1, in which the super-hard structure comprises at least 40 volume percent and at most 80 volume percent cBN grains.

4. A method as claimed in claim 1, in which the super-hard structure comprises cBN grains having a mean size of at most 25 microns.

5. A method as claimed in claim 1, wherein the matrix comprises material or a combination of materials having a combined mean Young's modulus of at most 350 Gigapascals (GPa).

6. A method as claimed in claim 1, including heating the treated super-hard structure to a temperature of at least 500 degrees centigrade for a period of at least 5 minutes.

7. A method as claimed in claim 1, including processing the treated super-hard structure to form an element for a tool.

Description

(1) Non-limiting examples are described below with reference to the accompanying drawings, of which:

(2) FIG. 1 shows a perspective view of an example PCBN construction;

(3) FIG. 2 shows a perspective view of an example PCBN structure;

(4) FIG. 3 shows a plan view of an example PCBN construction with and example PCBN element cut therefrom;

(5) FIG. 4 shows a perspective view of an example tool insert comprising a PCBN element;

(6) FIG. 5 shows a graph of measured flexural strength of a PCBN structure as function of quench temperature difference; and

(7) FIG. 6 shows a graph of measured flexural strength of a PCBN structure as function of quench temperature difference.

(8) The super-hard structure may consist essentially of a PCBN structure and may be joined to a substrate such as a cemented carbide substrate or the PCBN structure may be substantially free-standing and not joined to a substrate. With reference to FIG. 1, an example PCBN construction 10 comprises a PCBN structure 12 joined to a cemented carbide substrate 14. With reference to FIG. 2, an example PCBN structure 12 may have a generally disc-like shape and is free-standing (not joined to a substrate).

(9) Example methods of providing PCBN constructions can be found in U.S. Pat. No. 7,867,438.

(10) With reference to FIG. 3, a treated PCBN construction 10 may be cut and or otherwise processed to produce a PCBN element 30 for a tool insert. FIG. 4 shows a tool insert 40 comprising a PCBN element 30 joined to a carrier body 42.

(11) A non-limiting example is described below in more detail below.

(12) Two sets of 46 rectangular specimens of two respective grades of PCBN material were provided, each specimen having length 28.5 mm, width 6.25 mm and thickness 4.76 mm were prepared. The first set consisted of PCBN specimens of a first grade (or type) of PCBN material comprising about 50 volume percent cBN grains having mean size of about 0.7 micron to about 1 micron dispersed in a matrix consisting substantially of titanium carbonitride material and borides of aluminium. The second set consisted of PCBN specimens of a second grade of PCBN material comprising about 84 volume percent cBN grains having mean size of about 22 microns dispersed in a matrix comprising substantially nitrides and borides of aluminium. The PCBN specimens in both the first and second sets consisted only of a free-standing PCBN structure (i.e. they were not joined to a substrate). Properties of the matrix materials of the respective specimen PCBN materials are summarised in table 1 below.

(13) TABLE-US-00001 TABLE 1 Matrix material of Matrix material of the first grade the second grade of PCBN material of PCBN material Young's modulus, GPa 315 250 Poisson's ratio 0.2 0.2 Thermal expansion 4.8 9.4 coefficient, 10.sup.?6/K

(14) Fifteen specimens from each of the sets were subjected to a flexural strength test without having been treated. The flexural strength was measured using three point bending and calculated in accordance with the British Standard for determination of flexural strength of monolithic ceramics, standard number BS-EN 843:1. The mean flexural strength of the untreated specimens from the first set was found to be about 430 MPa and that of the untreated specimens from the second set was found to be about 630 MPa.

(15) The remaining specimens in each set were subjected to a variety of heat and subsequent quench (cooling) treatments, including quench by means of water and quench by means of light oil. Quenching was carried out on different specimens from various temperatures in the range from 240 degrees centigrade to 1,100 degrees centigrade, to which temperature the respective specimens had been heated in a furnace for a period of about ten minutes. Quenching was carried out by removing the specimens from the furnace and plunging them into a large reservoir containing water or light grade oil at a temperature of 20 degrees centigrade. The range of temperature differences used was therefore from 220 degrees centigrade (i.e. reduction in temperature from 240 to 20 degrees centigrade) to 1,080 degrees centigrade (i.e. from 1,100 to 20 degrees centigrade).

(16) The quench rate where water was used is estimated to be about 1,000 degrees centigrade per second and the quench rate where light oil was used is estimated to be about 10 degrees centigrade per second. The treated specimens were then subjected to flexural strength tests to measure their flexural strengths.

(17) In the case of the specimens from the first set, all treatments including heating the them to 1,080 degrees centigrade for two hours followed by quenching with water resulted in reduction of the flexural strength by about 20% as compared to the untreated specimens of the first set, while quenching by means of oil resulted in no change in the flexural strength.

(18) Some of the specimens quenched using water were subsequently annealed by heating them back up to the temperature from which they were quenched and maintained at that temperature for about one hour, after which their flexural strength was again measured. The flexural strength of the specimens from the first set following quenching by water and subsequent annealing from the quench temperature as a function of quench temperature difference is shown in FIG. 5.

(19) In the case of the specimens from the second set, treatments including heating the specimens to 1,080 degrees centigrade for two hours followed by quenching with water also resulted in reduction of flexural strength, but quenching with oil resulted in a substantial increase in the mean flexural strength, from about 630 MPa to about 830 MPa. The flexural strength of the specimens of the second set following quenching in water and (different) specimens of the second set quenched in oil is shown in FIG. 6 as functions of the quench temperature difference.

(20) Rapid quenching of the specimens of the second set with water, in which the quench temperature difference was between 240 degrees centigrade and 375 degrees centigrade, resulted in a large and discontinuous decrease in flexural strength. No substantial change in flexural strength was evident when the quench temperature difference was less than 240 degrees centigrade. Where the quench temperature difference using water was 780 degrees centigrade, the resultant flexural strength was about 25 GPa, which is very low. It was observed that the failure of these last mentioned specimens corresponded to very low energy and the fracture initiated in a central region of the specimen. The failed material also exhibited long unopened cracks extending from the initiation site to the edge of the sample. A significant change in surface colour from dark grey to light blue was also observed, and microscopic investigation revealed significant microscopic voids close to the surface of the specimens.

(21) Certain terms as used herein will briefly be explained.

(22) As used herein, super-hard or ultra-hard material has Vickers hardness of at least about 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 is also called man-made diamond, is diamond material that has been manufactured. 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, as described in U.S. Pat. No. 5,453,105 or 6,919,040). For example, certain SiC-bonded diamond materials may comprise at least about 30 volume percent diamond grains dispersed in a SiC matrix (which may contain a minor amount of Si in a form other than SiC). Examples of SiC-bonded diamond materials are described in U.S. Pat. Nos. 7,008,672; 6,709,747; 6,179,886; 6,447,852; and International Application publication number WO2009/013713).

(23) PCBN material comprises grains of cubic boron nitride (cBN) dispersed within a matrix comprising metal or ceramic material. For example, PCBN material may comprise at least about 35 volume percent or at least about 50 volume percent cBN grains dispersed in a matrix material 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. Some versions (or grades) of PCBN material may comprise at least about 80 volume percent or even at least about 90 volume percent cBN grains.

(24) Polycrystalline diamond (PCD) material comprises a mass (i.e. 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 percent of the material. Interstices between the diamond grains may be at least partly filled with a binder material comprising catalyst material for synthetic diamond, or they may be substantially empty. A 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 stable. Examples of catalyst materials for diamond are Fe, Ni, Co, Mn and certain alloys including these. Super-hard structures 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. 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. Different PCD grades may have different microstructure and different mechanical properties, such as elastic (or Young's) modulus E, modulus of elasticity, transverse rupture strength (TRS), toughness (such as so-called K1C 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.

(25) Thermally stable PCD material comprises at least a part or volume of which exhibits no substantial structural degradation or deterioration of hardness or abrasion resistance after exposure to a temperature above about 400 degrees centigrade, or even above about 700 degrees centigrade. For example, PCD material containing less than about 2 weight percent of catalyst metal for diamond such as Co, Fe, Ni, Mn in catalytically active form (e.g. in elemental form) may be thermally stable. PCD material that is substantially free of catalyst material in catalytically active form is an example of thermally stable PCD. PCD material in which the interstices are substantially voids or at least partly filled with ceramic material such as SiC or salt material such as carbonate compounds may be thermally stable, for example. PCD structures having at least a significant region from which catalyst material for diamond has been depleted, or in which catalyst material is in a form that is relatively less active as a catalyst, may be described as thermally stable PCD.

(26) As explained above, PCD material and PCBN material may be provided by sintering a plurality of diamond or cBN grains respectively in the presence of a suitable binder or catalyst material onto a substrate, such as a cemented carbide substrate. The PCD or PCBN structure thus produced is likely to be formed joined to the substrate, being an integral part of a construction comprising the PCD or PCBN structure bonded to the substrate during the process in which the respective structure formed into a sintered body.