CUBIC BORON NITRIDE ABRASIVE BODIES

Abstract: A tool insert comprises an abrasive compact comprising a polycrystalline mass of cubic boron nitride particles present in an amount of at least 70 percent by volume and containing sub­ stantial direct particle-to-particle bonding and a second phase dispersed through the cubic boron nitride particle mass. The abrasive compact presents major surfaces on each of opposite sides thereof. The one major surface provides a cutting point for the insert and the other major surface has bonded thereto a layer of molybdenum of thickness less than 0,5 mm. The largest linear dimension of the compact is no more than 10 mm. The tool insert is designed for applications where a high removal rate of work piece is desired.

Description: BACKGROUND OF THE INVENTION This invention relates to cubic boron nitride abrasive bodies. Cubic boron nitride abrasive compacts are known in the art (e.g. EP-A1-0 101 259) and consist of a polycrystalline mass of cubic boron nitride particles present in an amount of at least 70 percent by volume bonded into a hard conglomerate. Such compacts may contain a second phase. When provided, the second phase is typically a ceramic phase. The second phase acts to some extent as a binder phase for the cubic boron nitride particles. One of the most successful cubic boron nitride abrasive compacts presently on the market is one wherein the second phase consists essentially of aluminium nitride and/or aluminium diboride. This compact is sold in the form of large discs which are clamped into a suitable tool holder and used in this manner for abrading various work pieces. A curved edge of the compact is used as the abrading edge. This compact is designed for applications where a high removal rate of workpiece is desired. Abrasive compacts comprising diamond or cBN abrasive particles or a mixture thereof, present in an amount of at least 50 volume percent, bonded into a hard conglomerate by means of a metal are known from US-A-4 063 909. According to this document the compacts are bonded to a support by a layer of high temperature braze with a thickness of less than 0.5 mm. Suitable high temperature braze metals include Ti, Ni, Co, Fe, Cr, Mn, V, Mo, Ta or Pt or an alloy containing one or more thereof, Ti or Ti-alloys are particularly preferred. SUMMARY OF THE INVENTION According to the present invention, there is provided a tool insert comprising an abrasive compact comprising a polycrystalline mass of cubic boron nitride particles present in an amount of at least 70 percent by volume and containing substantial direct particle-to-particle bonding and a second phase dispersed through the cubic boron nitride particle mass, the abrasive compact presenting major surfaces on each of opposite sides thereof, the one major surface providing a cutting point for the insert and the other major surface having bonded thereto, and being in direct contact with, a layer of molybdenum of thickness less than 0,5mm, the largest linear dimension of the compact being no more than 10mm. DETAILED DESCRIPTION OF THE INVENTION The tool insert of the invention is thus a small cubic boron nitride compact which has a molybdenum layer bonded to one major flat surface thereof. Because of its size, it is not practical to hold the insert mechanically in a suitable tool holder. However, the presence of the molybdenum layer renders the compact brazeable to a tool holder. It is preferable that the compact is first brazed to a cemented carbide support which is then brazed to the tool holder or mechanically held in a tool holder. Cemented carbide has a similar coefficient of thermal expansion to the compact and problems arising due to differences in thermal expansion of the compact and support are minimised. The abrasive compact of the insert preferably has a rectangular, triangular, pentagonal or hexagonal shape. Thus, the compact presents several points and it is these points which provide cutting points for the insert. The molybdenum layer is the thin layer, preferably having a thickness in the range 50 to 150µm (microns). The layer of molybdenum may have an outer nickel or like metal layer applied to it, that is to say, the layer of molybdenum is in direct contact with the abrasive compact and is between the abrasive compact and the nickel or like metal layer. The cubic boron nitride compact may be any known in the art but is preferably one wherein the second phase contains aluminium nitride and/or aluminium diboride. Second phases of this nature are produced when aluminium is present during manufacture of the compact. One particularly preferred cubic boron nitride of this nature has a second phase which consists essentially of aluminium nitride and/or aluminium diboride. The second phase may also contain other components such as silicon which is either in elemental or combined form. The tool inserts of the invention may be used for a wide variety of applications. They have particular application for light to medium turning, boring, grooving, threading and milling of various steels, irons and alloys such as: Ni-HARD white irons having a hardness of 55 to 58 HRC High chromium irons having a hardness of 55 to 62 HRC Cold work tool steels having a hardness of 60 HRC High speed steels having a hardness of 62 HRC Grey cast irons (pearlitic) having a hardness of 220 HB Sintered irons having a hardness of 100 HB Cobalt-based hard facing alloys Nickel-based hard facing alloys Meehanite white irons having a hardness of 55 HRC In all these applications the cubic boron nitride abrasive compact will be brazed to a cemented carbide support, preferably a cemented tungsten carbide support, by means of the readily brazeable molybdenum layer and using a braze such as a silver brazing alloy having a melting point between 660°C and 840°C. The thus bonded cubic boron nitride abrasive compact will then be held mechanically or brazed to the shank of a tool and used in this form for the various abrading applications mentioned above. As mentioned above, the tool insert is particularly useful for light to medium cut applications. This means that the infeed of the tool insert during turning, boring, grooving or milling will not exceed 0,3mm per revolution or pass of the insert across the workpiece. In an example of the invention, a mass of cubic boron nitride particles was placed in contact with an aluminium foil which itself was placed in contact with a foil of molybdenum. This unbonded assembly was placed in the reaction zone of a conventional high temperature/high pressure apparatus and subjected to a temperature of about 1500°C and a pressure of about 5.5Gpa (55 kilobars) for a period of 15 minutes. Recovered from the reaction zone using conventional techniques was an abrasive compact containing about 90 percent by volume of cubic boron nitride particles in which there was substantial direct particle-to-particle bonding and a second phase consisting essentially of aluminium nitride and/or aluminium diboride. The compact was produced in the form of a disc and had bonded to a major flat surface thereof a layer of molybdenum of thickness about 100µm (microns). This compact was cut into a plurality of equilateral triangles, some having a side dimension of 5mm and others having a side dimension of 7mm. Each triangle constituted a tool insert of the invention. The triangular-shaped inserts produced in this manner were found to be excellent for the boring of pearlitic grey cast iron, for the facing of Stellite 6 hard facing alloy and the threading of Ni-HARD white cast iron. The particular conditions used in each example are given below. EXAMPLE 1 - BORING Material Pearlitic grey cast iron Hardness 220 HB Speed 600m/min Feed 0,2mm/rev Cut 0,5mm Coolant None EXAMPLE 2 - FACING Material Stellite 6 hard facing alloy Hardness 30 HRC Speed 200m/min Feed 0,2mm/rev Cut 0,5mm Coolant None EXAMPLE 3 - THREADING Material Ni-HARD white cast iron Hardness 58 HRC Speed 40m/min Thread form ISO M16 Radial infeed 0,1mm/pass Coolant None

Claims: A tool insert comprising an abrasive compact comprising a polycrystalline mass of cubic boron nitride particles present in an amount of at least 70 percent by volume and containing substantial direct particle-to-particle bonding and a second phase dispersed through the cubic boron nitride particle mass, the abrasive compact presenting major surfaces on each of opposite sides thereof, the one major surface providing a cutting point for the insert and the other major surface having bonded thereto, and being in direct contact with, a layer of molybdenum of thickness less than 0,5mm, the largest linear dimension of the compact being no more than 10mm. A tool insert according to claim 1 wherein the second phase contains aluminium nitride and/or aluminium diboride. A tool insert according to claim 1 or claim 2 wherein the second phase contains silicon in elemental or combined form. A tool insert according to any one of the preceding claims wherein the thickness of the molybdenum layer is in the range of 50 to 150µm (microns). A tool insert according to any one of the preceding claims wherein the compact has a triangular, rectangular, pentagonal or hexagonal shape . A tool insert according to any one of the preceding claims wherein the cubic boron nitride compact is brazed to a cemented carbide support by means of the molybdenum layer and a braze. A tool insert according to claim 6 wherein the braze is a silver brazing alloy having a melting point between 660°C and 840°C. A method of abrading a workpiece in a light to medium cut application, i.e. wherein the infeed of the tool insert does not exceed 0.3 mm per revolution or pass of the insert across the workpiece, including the step of causing the workpiece to be contacted in an abrading manner by the cutting point of a tool insert according to any one of claims 1 to 7. Use of a tool insert according to any one of claims 1 to 7 in a light to medium cut application, i.e. wherein the infeed of the tool insert does not exceed 0.3 mm per revolution or pass of the insert across the workpiece. A tool comprising the tool insert of any one of claims 1 to 7.

Inventors: DODSWORTH, JOHN, PHAAL, CORNELIUS
Applicants: DE BEERS INDUSTRIAL DIAMOND DIVISION (PROPRIETARY) LIMITED
Application year: 1986
Main classification: B24D3
IPC codes: B24D3, B22F7, C22C29, B23B27