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Patent Searching and Data


Title:
ABRASIVE COMPACT MATERIAL
Document Type and Number:
WIPO Patent Application WO/2007/113643
Kind Code:
A2
Abstract:
A method of producing an abrasive compact material includes providing a source of cBN particles, coating the cBN particles with one or more aluminium or titanium compounds, in particular aluminium nitride, aluminium boride, titanium nitride and/or titanium boride, and sintering the coated cBN particles with nanosized alumina at a temperature less than 1550°C. The sintering process is carried out in air at atmospheric pressure.

Inventors:
SIGALAS IAKOVOS (ZA)
Application Number:
PCT/IB2007/000852
Publication Date:
October 11, 2007
Filing Date:
April 03, 2007
Export Citation:
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Assignee:
ELEMENT SIX PRODUCTION PTY LTD (ZA)
SIGALAS IAKOVOS (ZA)
International Classes:
C04B35/117; C04B35/5831; C04B35/628; B24D3/14; C09K3/14; C04B35/63
Domestic Patent References:
WO2005056495A12005-06-23
Foreign References:
EP1006092A12000-06-07
EP1006093A12000-06-07
EP0571865A11993-12-01
GB2320725A1998-07-01
Attorney, Agent or Firm:
SPOOR & FISHER et al. (0001 Pretoria, ZA)
Download PDF:
Claims:

T/IB2007/000852

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CLAIMS

1. A method of producing an abrasive compact material including:

providing a source of cBN particles;

coating the cBN particles with one or more aluminium or titanium compounds; and

sintering the coated cBN particles with nanosized alumina at a temperature less than 155O 0 C.

2. A method according to claim 1 , wherein the one or more aluminium or titanium compounds are selected from aluminium nitride, aluminium boride, titanium nitride and titanium boride.

3. A method according to claim 1 or claim 2, wherein the alumina has an average particle size of 10 to 150 nm.

4. A method according to any one of claims 1 to 3, wherein the sintering is carried out in air at atmospheric pressure.

5. A method according to any one of claims 1 to 4, wherein the cBN particles have an average particle size of 0.25 to 5μm.

6. A method according to any one of claims 1 to 5, wherein the coating on the cBN particles is 5nm to 3μm.

7. A method according to any one of claims 1 to 6, wherein the volume fraction of cBN is 20% to 70%.

Description:

ABRASIVE COMPACT MATERIAL

BACKGROUND OF THE INVENTION

This invention relates to a method of producing an abrasive compact material, and to an abrasive compact material so produced.

Tool inserts comprising abrasive compacts are used extensively in cutting, milling, grinding, drilling and other abrasive operations. Wear parts are used as bearings, nozzles, pump impellers, milling media and the like. The abrasive compacts consist of polycrystalline diamond or cubic boron nitride particles bonded into a coherent hard conglomerate, with or without other binding phases. Abrasive compacts are typically made under elevated conditions of pressure and temperature, at which the abrasive particles, be they diamond or cubic boron nitride, are crystallographically stable.

Making such compacts at elevated pressures and temperatures has a number of disadvantages. The resulting pieces are confined to a few simple shapes, typically flat discs or cylinders of limited size. The resulting pieces are also not made to final dimensions, resulting in great costs being incurred in post-sintering processing operations.

One way of addressing such problems is to devise techniques that allow for the sintering of such compacts at reduced temperatures, so that the conversion of the high pressure diamond and boron nitride phases is avoided. European Patent EP 0 715 930 to Kramer et al discloses a composite containing diamond or cBN in a nanosized alumina matrix.

Nanosized alumina sinters at temperatures significantly lower than those of normal micron-sized alumina. Thus composites made with such alumina powders and cBN can sinter at atmospheric pressure and temperatures low enough to avoid hexagonalisation of the cBN phase.

However, such composites do not guarantee good wear resistance of the resulting part, as it is not possible for the ultrahard material particles to be well bonded onto the alumina matrix. This is caused by the presence of boron-oxides on the cBN surface, which inhibits good adhesion of the cBN particles to the alumina matrix. Such adhesion could, however, be improved if higher temperatures are used during sintering. This option is not attractive, as high temperatures may cause hexagonalisation of the cBN particles.

The combination of hardness and chemical inertness of alumina has not gone unnoticed by the cutting tool R&D community. A number of patents exist which disclose the use of alumina as a coating on PcBN, either as a PVD generated thin coating (e.g. US 2003027015 to Zywitzki et al, EP1 253 215 to Alfred et al, and US 6,382,951 to Littecke et al), or as a polycrystalline coating sintered on top of a PcBN layer at high pressures and temperatures (e.g. EP 1 120 387 to Junichi et al).

A number of researchers have also explored the possibility of improving the properties of alumina, either by doping, or by the addition of strengthening phases. In particular, it has been shown that addition of nanosized silicon carbide in alumina modifies the mode of fracture from intergranular to

transgranular, with a resulting 30% increase in fracture strength and an increase in slurry erosive wear resistance by a factor of 3.

Although such major improvements in the properties of alumina have been achieved, these advances have not been applied in the cutting tool and wear part industry, particularly where cBN or diamond composites are concerned, mainly as a result of the poor bonding of the abrasive particles to the alumina matrix.

Kukino et al, in EP 1 006 092 disclose coatings of cBN with a dual coating layer. The inner layer comprises nitrides and borides of Al and Ti, while the outer coating comprises nitrides, borides, carbonitrides, carbides and oxides of, among others, Al. The particles are sintered onto themselves. The material would only sinter at high temperatures and pressures.

Kukino et al, in EP 1 006 093, disclose the use of coatings of cBN. The coating is made from nitrides and borides of Ti, Zr, and Hf. The particles are surrounded by a matrix that should not contain more than 5% AI 2 O 3 . Such a material would not utilise the advantages derived by the high hardness and chemical stability of the alumina second phase.

US 5,104,422, to St. Pierre, discloses the use of alumina coatings in helping cBN particles adhere to the bond of a grinding wheel.

US 5,820,721 , to Beane et al, discloses the general principle of coating material A with material B in order to derive desired properties of a resulting composite material. No binder phase is involved in this patent. This arrangement, although quite generic, would not allow for the sintering of cBN-containing particles at atmospheric pressures, unless coating material B had a low enough melting point to allow for sintering of the resulting compact at temperatures low enough to avoid hexagonalisation of the cBN particles.

00852

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SUMMARY OF THE INVENTION

The invention provides a method of producing an abrasive compact material including the steps of providing a source of cBN particles, coating the cBN particles with one or more aluminium or titanium compounds, in particular aluminium nitride, aluminium boride, titanium nitride and/or titanium boride and sintering the coated cBN particles with nanosized alumina at a temperature less than 1550 0 C.

The sintering process is preferably carried out in air at atmospheric pressure.

The invention extends to an abrasive compact material produced by the method of the invention as defined above.

DESCRIPTION OF PREFERRED EMBODIMENTS

The invention addresses the problems of adhesion of cBN to an alumina matrix, while maintaining the advantages of low pressure sintering of cBN- containing compacts. Such a compact would have, over the advantages of low pressure sintering, the advantages conferred to it by the high hardness and chemical inertness of AI 2 O 3 .

Specifically, it is proposed to coat the cBN particles with aluminium compounds, in particular aluminium nitride and/or aluminium boride, or with titanium compounds, in particular titanium nitride and/or titanium boride, or mixtures of the two.

In accordance with the invention, the cBN particles are coated with aluminium and/or titanium compounds selected from aluminium nitride, aluminium boride, titanium nitride and titanium boride by, for example, heat treating them at an initial temperature of 700 to 1000 0 C, typically about 900°C, in the presence of NH 4 CI and Al or Ti. This is then followed by an

2

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excursion to significantly higher temperatures such as 1000 to 1550 0 C, typically 1550 0 C. The resulting layer is continuous, protects the cBN surface from chemical attack by the O 2 in the sintering atmosphere, and adheres very well to alumina. The resulting particles are then sintered in air with nanosized alumina, typically having an average particle size of 10 to 150 nm, preferably 25 to 90 nm, at temperatures not exceeding 155O 0 C, and at temperatures typically greater than 1200 0 C.

The cBN are typically provided with an average particle size of about 0.25 to about 5μm. The thickness of the aluminium compound coat is typically about 5nm to about 3μm, preferably about 50nm to 500 nm. In the abrasive compacts produced, the volume fraction of cBN is typically about 10% to about 80%, preferably about 30 to about 80%.

The invention will now be described in more detail, by way of example only, with reference to the following non-limiting example.

EXAMPLE

0.615 grams of aluminium was mixed with 1.38 grams of ammonium chloride and 8 grams of 4 micrometer average particle size cBN. The mix was placed in an aluminium boat with its surface coated with HBN. The boat was placed in a tube furnace, and a stream of argon was passed over it. The temperature of the furnace was raised to 350 0 C at a rate of 10°C per minute. The mix was allowed to dwell there for 60 minutes. The temperature was then raised to 1000°C at a rate of 3O 0 C per minute. The mix was allowed to dwell at that temperature for 300 minutes. Following this, the mix was allowed to cool down at room temperature. The powder was then recovered, and sieved through a 4.5 micrometer sieve in order to separate the coated cBN particles from the residue ammonium chloride and aluminium powders.

The resulting powder was mixed in an attritor mill for 1 hour at 400 rpm with nanosized alumina, of 75 nanometer average particle size. The resulting

slurry was dried in a rotovap drier, and passed through a 4.5 micrometer sieve. A 20mm diameter pill was pressed and placed inside an alumina crucible, whose surface had been coated with HBN. The pill was placed inside a tube furnace, and the temperature was raised to 1450 0 C at a rate of 15°C per minute. The pill was allowed to dwell at that temperature for 120 minutes. The resulting sintered ceramic has a 3% porosity and a hardness of 28GPa.

The cBN does not hexagonalise, and the resulting compact has superior hardness, strength and wear resistance.