IL47808A - Elongated cubic boron nitride particles and method of making them - Google Patents

Abstract

In order to prepare needle-shaped, cubic boron nitride which is suitable, in particular, for manufacturing grinding discs, a sleeve (12) which is made of hexagonal boron nitride and encircles a catalyst core (10), is arranged in a reaction zone in which the temperature and pressure conditions suitable for forming cubic boron nitride are generated. The mass ratio between the hexagonal boron nitride and the catalyst is in the range of from 10:1 to 3:1. The catalyst used can be a nitride of an alkali or an alkaline earth metal. The needle-shaped, cubic boron nitride particles are used to produce grinding discs. <IMAGE> [GB1505923A]

Description

Elongated cubic "boron nitride particlea and method of making them DE BEERS INDUSTRIAL DIAMOND DIVISION LIMITED C. 45391 •aip » β»3 na w i s»» Production of cubic boron nitride DE BEERS IliDUSTHIAl DIAMOHD DIVISION LIMITED THIS invention relates to cubic boron nitride.

Cubic boron nitride is a hard substance, second only to diamond in hardness. The substance forms the subject of U.S. patent specification No. 2,947,617. Cubic boron nitride (CBN) is produced by subjecting hexagonal boron nitride in the presence of a suitable solvent/catalyst to conditions of elevated temperature and pressure at which cubic boron nitride is crystal lographi cal ly stable. Cubic boron nitride is produced in the presence of a substance which acts as a solvent or catalyst or both. Such substances will hereinafter and in the claims be referred to simpTy as catalysts. Examples of suitable catalysts are given in the above mentioned U.S. specification and include alkali metals, alkaline earth metals, lead, antimony, tin and nitrides of these metals. Other catalysts have been developed and aluminium/iron alloys may be given by way of example.

The production of cubic boron nitride requires the use of very high temperatures and pressures. Such can be generated in an apparatus known as the "belt" apparatus which forms the subject of U.S. patent specification No. 2,941,248. This apparatus consists essentially of an annular belt or die member having a tapering aperture therethrough and a pair of concentric f rustoconi cal punches which are capable of moving into the aperture to define a reaction chamber or zone therein. A suitable gasket material such as pyrophyllite is employed between the punches and die members for sealing purposes and over the inner surface of the die facing the reaction zone to insulate thermally this portion of the die. The temperature of the reaction chamber may be raised by connecting the punch members to a source of electrical power thereby creating a resistance heating circuit through the punches and the contents of the reaction chamber.

Cubic boron nitride is a good abrasive for the grinding of steels such as high speed steels. For abrading operations, it generally forms part of a resin bond gri ndi ng wheel .

According to this invention, a method of making elongate cubic boron nitride particles includes the steps of providing a reaction zone, so placing essentially discrete layers of hexagonal boron nitride and catalyst in the reaction zone that when the contents of the reaction zone are subjected to conditions of temperature and pressure suitable for cubic boron nitride formation areas of weakness are created in the hexagonal boron nitride layer, and subjecting the contents of the reaction zone to conditions of temperature and pressure suitable for cubic boron nitride formation. It is believed that the elongate particles are produced as the solvent penetrates the areas of weakness. Of course, some particles which are not of elongate shape are produced simultaneously with the elongate particles.

The conditions of temperature and pressure and catalyst may be any known in the art for cubic boron nitride production. Examples of suitable temperatures and pressures and catalysts may be found in the abovementi oned U.S. patent specif cation No. 2,947,617. In general, however, the temperatures used will be in the range 1500°C to 2000°C and the pressures used will be in the range 50 kilobars to 100 kilobars. Preferred catalysts are the alkali and alkaline earth metal nitrides, particularly calcium nitride and lithium nitride.

The preferred arrangement for the hexagonal boron nitride V.

(HBN) and the catalyst is to. provide a sleeve of the HBN around a core of the catalyst. It has been found that radial areas of weakness are produced in the HBN sleeve 5 when it is subjected to the required temperature and pressure conditions and that the catalyst penetrates these areas of weakness to produce the elongate particles.

The HBN si eeve, havi ng the solvent core may be placed in a pyrophyllite sleeve and then in: the reaction zone of a 10 high temperature pressure apparatus in the conventional manner.

The sleeve of HBN may be a continuous sleeve or may consist of a plurality of segments which together defi ne the sleeve. The core and sleeve are preferably dimensioned 15 for a snug fit one inside the other. The core may be in the form of loose powder, but is preferably compacted into a coherent body. Any suitable compacting method known in the art may be used. A disc of HBN may be provided at each of opposite ends of the core. 20 The weight ratio of hexagonal boron nitride to catalyst, particularly for the sleeve/core arrangement, may be in the range 10:1 to 3:1 , preferably in the range 6:1 to 5:1 The elongate cubic boron nitride particles produced by the ) above described method have been found to be distinctive 25 and are characterised by having a long axis and a short transverse axis, the ratio of the long axis to the short ' axis being at least 3:1 and the long axis being in the l 11^> crystal 1 ographi c direction. The invention provides, according to another aspect, such elongate cubic boron 30 nitride particles produced by any method.

Examples of the elongate cubic boron nitride particles of the invention are shown in the photographs attached as Figures 1 and 2. The magnification of the photograph as Figure 1 is 269 and of the photograph as Figure 2 is 273. j It will be noted that the long axes of these particles are in the (ill) crystal 1 ograph i c direction and the particles have an irregular surface In determining the ratio of long axis to short axis one takes the long and short axes of greatest dimension for this determination.

The particles may vary in size but will generally be in the range 60/200 U.S. mesh. . The particles are preferably in the range 80/170 U.S. mesh. - The novel particles of the invention are generally friable rendering them particularly suitable for grinding operations where their friability allows for fresh cutting surfaces to be continually produced during the grinding operation. The particles may be incorporated into the abrading portion of abrasive tools such as resin bond and metal bond abrasive tools. It is preferred that the particles are so orientated in the abrading portion of the tool that their long axes are substantially normal to the working face.

Because of their friability, the particles are preferably used in resin bond grinding wheels. The irregular surface of the particles assists in keying the particles to the resin matrix. To improve this keying further, it is preferred that the particles are metal , preferably nickel , coated. For maximum effectiveness the particles, as mentioned above, are preferably so orientated in the grinding portion of the wheel that their long axes are substantially normal to the working face thereof.

Resin bond grinding wheels are well known in the art, as are their methods of manufacture. Briefly, resin bond^ grinding wheels are made by providing a suitable mo §* around a hub portion, generally of a material such bakelite, introducing a mixture of powdered resin starter ingredients, cubic boron nitride and filler into the mould, and applying pressure and heat to the mixture in the mould to cause the resin to cure and set. The resin may be a phenol formal dehyde or polyimide resin or any other resin known in the art of grinding wheels.

The amount of cubic boron nitride in the operative grinding ■'< portion of the wheel will vary according to the type of wheel . The cubic boron nitride content of the grinding portion will generally constitute about 10 to 25 volume percent.

The particles may be orientated by means of an impressed field of force using known techniques. The impressed field of force may be electrostatic. Alternatively, the particles may be coated with a magnetic material such as a ferromagnetic metal and the particles orientated by means of an externally impressed magnetic field.

In an example of the invention a core of compacted lithium nitride was placed inside a sleeve of hexagonal boron nitride. The dimensions of the core and sleeve were such that there was a snug fit. Hexagonal boron nitride discs were then placed on the top and bottom of the core.

Figure 3 of the attached drawings illustrates schematically the arrangement. Referring to this figure, the core is shown at 10 and the sleeve and discs at 12 and 14, respectively. The weight ratio of the hexagonal boron nitride to lithium nitride was about 6:1.

The l thium nitride/HBN composite was then placed in a sleeve of pyrophyllite and the whole placed in the reaction zone of a high temperature/pressure apparatus of the type described in U.S. Spec fication No. 2,941 ,248.

The temperature and pressure of the reaction zone were raised to 1500°C and 55 kilobars by first raising the pressure, and then the temperature, to the desired values. These conditions were maintained for about ten minutes and then released by first allowing the temperature, and then the pressure, to drop to ambient conditions . The cubic boron nitride content of the reaction capsule was recovered using conventional techniques. The cubic boron nitride produced contained a large percentage of elongate or needle-shaped particles of the type illustrated by Figures 1 and 2.

The elongate particles produced were mostly in the range 60/170 U.S. mesh.

Claims (7)

1. A method of making elongate cubic boron nitride particles including the steps of providing a reaction zone, so placing essentially discrete layers of hexagonal boron nitride and catalyst in contact with each other in the reaction zone that when the contents of the reaction zone are subjected to conditions of temperature and pressure suitable for cubic boron nitride formation areas of weakness are created in. the hexagonal boron nitride layer, and subjecting the contents of the reaction zone to conditions of temperature and pressure suitable for cubic boron nitride formation.

2. A method according to claim 1 wherein the hexagonal boron nitride is provided in the form of a sleeve around a core o the catalyst .

3. A method according to claim 2 wherein the weight ratio of hexagonal boron nitride to catalyst is in the range 10:1 to 3: 1.

4. A method according to claim 2 wherein the weight ratio of the hexagonal boron nitride to catalyst is in the range 6 : 1 to 5 : 1. A method according to any one of the preceding claims wherein the catalyst is selected from alkali and alkaline earth metal nitrides . 6. A method according to any one of claims 1 to 4 wherein the 5 catalyst is selected from calcium nitride and lithium nitride. 7. f A method according to any one of the preceding claims > — ' o o wherein the temperature is in the range 1500 C to 2000 C and the pressure is in the range 50 kilobars to 100 kilo- 10 bars. 8· A method of making elongate cubic boron nitride particles substantially as herein described with reference to Figure 3 of the accompanying drawings. ongate cubic boron nitride particles made by a method cording to any one of the preceding claims. 10. An elongate cubic boron nitride particle having a long axis and a short transverse axis, the ratio of the long axis to the short axis being at least 3:1 and the long axis being in the λ 11 ^ crys tal 1 ograph i c direction An elongate cubic boron nitride particle, according to claim 10 having an irregular surface. 12. An elongate cubic boron nitride particle according to claim 10 or claim 11 which is friable. 13. An elongate cubic boron nitride particle according to any one of claims 10 to 12 which is in the range 60/200 U.S. mesh. 14. An elongate cubic boron nitride particle according to any one of claims 10 to 12 which is in the range 80/170 U.S. mesh. 1

5. An elongate cubic boron nitride particle according to any one of claims 10 to 14 which is metal coated. 1

6. An elongate cubic boron nitride particle according to claim 15 wherein the metal is nickel . 1

7. An elongate cubic boron nitride particle substantiall as herein described with reference to Figures 1 and 2 of the accompanying drawings. \