CUBIC BORON NITRIDE ABRASIVE BODY

Abstract: An abrasive body comprising a cubic boron nitride abrasive compact (14) presenting a cutting point or edge, the cutting point or edge being provided by a cubic boron nitride crystal (2) of about 1mm or larger in size.

Description: BACKGROUND OF THE INVENTION This invention relates to a cubic boron nitride abrasive body. Cubic boron nitride is an abrasive second only to diamond in hardness. It is produced by subjecting hexagonal boron nitride in the presence of a suitable catalyst to temperature and pressure conditions at which cubic boron nitride is crystallographically stable. With the technology currently available, small or fine cubic boron nitride particles of good quality can be produced without difficulty. However, difficulties are encountered in producing large cubic boron nitride crystals, i.e. crystals which are of good quality and sufficiently large, usually greater than 3mm, to enable them to be fixed to the end of a tool shank for turning and milling applications. Cubic boron nitride particles are extensively used in grinding wheels. In grinding wheels, a mass of small cubic boron nitride particles are held in a suitable matrix, generally a resin matrix, in the working portion of the wheel. Small cubic boron nitride particles are also used in the manufacture of cubic boron nitride abrasive compacts. Such compacts are manufactured under elevated temperature and pressure conditions and comprise at least 70 percent by volume cubic boron nitride particles bonded into a coherent, hard conglomerate. EP-A-0 024 757 discloses a compact comprising one or more single crystal diamonds, having a largest dimension of at least 1mm, embedded in a polycrystalline matrix selected from diamond; cubic boron nitride; and silicon and silicon carbide bonded diamond, cubic boron nitride, or mixtures of diamond and cubic boron nitride, wherein the single crystal diamond(s) comprise(s) 10 to 90 volume percent of the compact. SUMMARY OF THE INVENTION According to the invention, there is provided a method of producing a cubic boron nitride abrasive body including the steps of providing a mass of small cubic boron nitride crystals wherein the small particles have a size less than 60 µm (microns), dispersing a number of cubic boron nitride particles 1mm or larger in size through the mass of small particles, subjecting the mass to elevated temperature and pressure conditions suitable to form an abrasive compact, and sectioning the abrasive compact into a plurality of pieces, each of which has a cubic boron nitride crystal 1mm or larger in size which presents a cutting point or edge. DESCRIPTION OF THE DRAWINGS Figure 1 illustrates a disc-shaped cubic boron nitride abrasive body from which an abrasive body of the invention can be produced; and Figure 2 illustrates an example of an abrasive body of the invention. DETAILED DESCRIPTION OF THE INVENTION The cubic boron nitride abrasive body of the invention presents a cutting point or edge which is provided by a relatively large cubic boron nitride particle. The abrasive body is thus able to act as a single crystal tool enabling it to be used to produce fine finishes on work pieces. The cubic boron nitride compact, save for the presence of the large crystal, may be any known in the art. Preferably it will be a compact which has a second phase consisting essentially of aluminium nitride and/or diboride as described in British Patent No. 2048927. The abrasive bodies of the invention are preferably manufactured by mixing fine cubic boron nitride particles, i.e. particles having a size of less than 60 µm (microns), with a number of large cubic boron nitride crystals, i.e. having a size of about 1mm or larger. The large crystals will be uniformly dispersed through the mixture. A layer of aluminium is placed on the mixture which is then loaded into the reaction capsule of a conventional high temperature/high pressure apparatus. The capsule is placed in the apparatus and the contents of the capsule subjected to elevated temperature and pressure conditions suitable for compact manufacture. The elevated temperature is typically in the range 1400 to 1600°C, the elevated pressure is typically in the range 5 to 7 Gpa (50 to 70 kilobars) and these elevated conditions are typically maintained for a period of 10 to 30 minutes. Recovered from the reaction capsule is a cubic boron nitride compact of disc shape having uniformly dispersed therethrough a number of the large crystals. Any flaws in these crystals will have been annealed out during the compact manufacture. The abrasive bodies are then produced by sectioning the compact into a plurality of pieces each of which presents a cutting point or edge provided by a large crystal. Sectioning of the compact may take place by methods known in the art such as spark erosion or laser cutting. The pieces may have any of a variety of shapes such as triangular, cube, hexagonal, pentagonal, and the like. Each such piece is capable of acting as a single crystal cubic boron nitride tool insert and may be used in the fine finishing of a work piece, typically a steel work piece. Each piece has the further advantage that the large crystal, which may be suitably oriented to optimise its cutting life, is held in a matching, i.e. cubic boron nitride, matrix. Referring to the drawings, Figure 1 illustrates a disc-shaped cubic boron nitride compact 10 produced by the method described above and having uniformly dispersed therethrough a number of large crystals 12. The disc-shaped compact may be sectioned into a plurality of pieces, two of which are illustrated by the dotted lines. Figure 2 illustrates one such piece 14 which is of triangular shape and has a large crystal 16 at the apex of the triangle. This large crystal provides the cutting point for the body.

Claims: A method of producing a cubic boron nitride abrasive body including the steps of providing a mass of small cubic boron nitride crystals wherein the small particles have a size less than 60 µm (microns), dispersing a number of cubic boron nitride particles 1mm or larger in size through the mass of small particles, subjecting the mass to elevated temperature and pressure conditions suitable to form an abrasive compact, and sectioning the abrasive compact into a plurality of pieces, each of which has a cubic boron nitride crystal 1mm or larger in size which presents a cutting point or edge. A method according to claim 1 wherein the elevated temperature is in the range 1400°C to 1600°C, the elevated pressure is in the range 5 to 7 Gpa (50 to 70 kilobars) and the elevated conditions are maintained for a period of 10 to 30 minutes.

Inventors: PHAAL, CORNELIUS
Applicants: DE BEERS INDUSTRIAL DIAMOND DIVISION (PROPRIETARY) LIMITED
Application year: 1987
Main classification: C22C29
IPC codes: B23B27, B24D3, B23P15, C04B35