WO2008132675A2

WO2008132675A2 - Boron suboxide composite material

Info

Publication number: WO2008132675A2
Application number: PCT/IB2008/051588
Authority: WO
Inventors: Anthony Andrews; Iakovos Sigalas; Mathias Herrmann
Original assignee: Element Six (Production) (Pty) Ltd
Priority date: 2007-04-26
Filing date: 2008-04-24
Publication date: 2008-11-06
Also published as: WO2008132675A3

Abstract

The invention provides a method of making a boron suboxide composite material including the steps of reducing the B2O3 content of a source of boron suboxide to produce a boron suboxide of reduced B2O3 content and sintering the boron suboxide in the presence of a material capable of producing a secondary phase to produce the boron suboxide composite. The B2O3 content is preferably reduced to as low a content as possible.

Description

BORON SUBOXIDE COMPOSITE MATERIAL

BACKGROUND OF THE INVENTION

The invention relates to a boron suboxide composite material.

The development of synthetic ultrahard materials which have hardness values approaching or even exceeding that of diamond has been of great interest to material scientists. With a Vickers hardness of between 70 to 100 GPa, diamond is the hardest materia! known, followed by cubic boron nitride (H_v ~ 60 GPa) and boron suboxide (H_v ~ 38 GPa)₁ herein referred to as B₆O. Hardness values of 53 GPa and 45 GPa were determined at 0.49 and 0.98N load respectively for B₆O single crystals, which are similar to those of cubic boron nitride⁸. It is known that B₆O may also be non-stoichiometric i.e. exist as B₆Oi_-X (where x is in the range 0 to 0.3). Such non-stoichoimetric forms are included in the term B₆O. The strong covalent bonds and short interatomic bond length of these materials contribute to the exceptional physical and chemical properties such as great hardness, low mass density, high thermal conductivity, high chemical inertness and excellent wear resistance [1 , 2], In U.S. Patent No. 5,330,937 to Ellison-Hayashi et a! the formation of boron suboxide powders of nominal composition B₃O, B₄O, B₆O, B₇O, B₈O, Bi₂O, B₁₅O and B_iaO was reported. Potential industrial applications have been discussed by Kurisuchiyan et al. (Japan Patent No. 7,034,063) and Ellison- Hayashi et al. (U.S. Patent No. 5,456,735) and include use in grinding wheels, abrasives and cutting tools.

Several techniques have been employed for producing novel boron suboxide and include such procedures as reacting elemental boron (B) with boron oxide (B₂O₃) under suitably high pressure and high temperature conditions [1]. In U.S. Patent No. 3,660,031 to Holcombe Jr. et al other methods of producing boron suboxides such as reducing boron oxide (B₂O₃) with magnesium, or by reducing zinc oxide with elemental boron are mentioned. With each of these known procedures however, there are drawbacks which retard the usefulness of the material in industry. For example, the reduction of B₂O₃ with magnesium produces a solid solution of magnesium and magnesium boride contaminants in the suboxide, while the reduction of magnesium oxide with boron produces only a relatively small yield of boron suboxide and is very inefficient, Holcombe Jr. et al. (U.S. Patent No. 3,660,031) produced B₇O by reducing zinc oxide with elemental boron at temperatures of between 1200⁰C to 1500^°C. A hardness vaiue of 38.2 GPa under 10Og load and density of 2.6 g.cm^"3 is reported for this material. The fracture toughness for this material is not discussed. No dense materials were produced. The hardness was determined only on particles (grit was produced)

Efforts have been made to enhance the mechanical properties of B₆O, especially its fracture toughness, by forming B₆O composites with other hard materials such as diamond [4], boron carbide [5], and cBN [6], The intention was to form pseudo-binary composite systems, stronger at the grain boundaries than those of pure B₆O. Even though high hardness values were recorded for the composites (H_v ~ 46 GPa), again, fracture toughness values did not exceed 1.8 MPa. m⁰⁵. The best value here was obtained with B₆O-CBN composites. Nevertheless, this was an improvement on the fracture toughness of pure sintered B₆O materials, indicating that the fracture toughness can be increased through the addition of other phases.

Shabalala et al. WO 2007/029102 [7]) describes the production of B₆O composites with aluminium compounds which resulted in an aluminium borate phase at the grain boundary. A fracture toughness of about 3.5 MPa. m^{0 5} with a corresponding hardness of 29.3 GPa was obtained. SUMMARY OF THE INVENTION

The invention provides a method of making a boron suboxide composite material including the steps of reducing the B₂O₃ content of a source of boron suboxide to produce a boron suboxide of reduced B₂O₃ content and sintering the boron suboxide in the presence of a material capable of producing a secondary phase to produce the boron suboxide composite. The B₂O₃ content is preferably reduced to as low a content as possible.

The B₂O₃ content may be reduced by contacting the boron suboxide with a solvent capable of dissolving B₂O₃ and separating the solvent from the boron suboxide.

The boron suboxide material may be sintered at conditions of high temperature and high pressure such as those used to synthesize diamond or cubic boron nitride or under milder temperature and pressure conditions such as by hot pressing, gas pressure sintering, hot isostatic pressure or spark plasma sintering conditions.

It has been found that a reduction in the B₂O₃ content of boron suboxide results in a composite material being produced which has a higher hardness than a similar composite material using boron suboxide containing B₂O₃. Further it has been found that the reduction in the B₂O₃ content does not adversely affect the fracture toughness of the composite material.

It has been found that the density of a boron suboxide composite material comprising granular or particulate boron suboxide in a second phase is influenced by the amount of oxygen incorporated in the boron suboxide lattice. Thus, the invention provides, according to another aspect, a method of controlling the density of the boron suboxide composite material by manipulating or controlling the amount of oxygen incorporated in the boron suboxide lattice.

The amount of oxygen incorporated in the boron suboxide lattice may be manipulated by any one of the methods described above.

DESCRIPTION OF EMBODIMENTS

The invention provides for a method of making a boron suboxide composite material including reducing the B₂O₃ content and sintering this boron suboxide in the presence of a material capable of producing a secondary phase to produce the boron suboxide composite.

The B₂O₃ content of boron suboxide may be reduced, according to one aspect of the invention, by contacting the boron suboxide with a solvent capable of dissolving B₂O₃. The B₂O₃ dissolves in the solvent which is then separated from the boron suboxide. The solvent will generally be an organic solvent, preferably one with a high polarity, or water.

The preferred solvent is an alcohol such as methanol, ethaπol, propanol, isopropanol or butanol.

When the solvent is an alcohol, the B₂O₃ reacts with the solvent forming an ester which can be removed by volatilization.

The solvent can be separated from the boron suboxide by filtration or centrifugation.

The solvent can be contacted with the boron suboxide by mixing or washing. Alternatively, if the boron suboxide material is subjected to a particle size reducing step, e.g. milling, the solvent can be included in the medium used to reduce the particle size. B₂O₃ will dissolve in the solvent which can then be separated from the boron suboxide.

The boron suboxide composite material comprises boron suboxide and a secondary phase in coherent, bonded form.

The secondary phase I may be that described in WO2007/029102. This publication describes a boron suboxide composite material comprising particulate or granular boron suboxide distributed in a binder or second phase comprising M_xB_yO_z wherein M is a metal, selected from the group comprising aluminium, zirconium, titanium, magnesium and gallium. The second phase preferably comprises less than about 30% by weight of the composite material, in particular from about 3% to about 15% by weight.

The secondary phase may contain an oxide or a mixture of oxides. Examples of suitable oxides are rare earth metal oxides such as yttrium oxide or scandium oxide, oxides of elements of the lanthanide series such as lanthanum oxide, oxides of a metal of Group IA, HA, INA, or IVA of the periodic table and mixtures of such oxides. The oxide will generally be present in the composite in an amount of up to 20 volume percent of the composite.

The secondary phase may contain a transition metal or platinum group metal boride. Examples of such borides are titanium, tungsten, molybdenum, hafnium tantalum, zirconium, chromium, iron, cobalt, platinum, palladium and rhenium.

The second phase may also contain a mixture or combination of an oxide or mixture of oxides and a boride. Boron suboxide composite materials having second phases of the type described above form the subject of co-pending International patent applications.

In general, in order to manufacture a boron suboxide composite material, boron suboxide particles or granules are brought into contact with a material for producing secondary phase, e.g. mixed with the material, to form an unbonded mass which is then sintered to form a composite material.

The invention will now be illustrated by the following examples. Table 1 summarises these materials and their measured hardness and toughness properties for comparative purposes.

Example 1

B₆O starting powder was milled using an attritor mill with steel balls for 50 hours. The iron contaminants were removed by washing in HCI. The powder was subsequently washed in methanol to remove any B₂O₃ present. The average particle size after milling was 500 nm.

The milled powder was admixed with 2 % by weight of AI₂O₃ and 2.65 % by weight of Y₂O₃ in methanol and milled for two hours using a planetary mill. The milled mixture was dried using a rotary evaporator and then placed in a boron nitride cell {inside a graphite die) and sintered using a hot press at a temperature of 1800⁰C and a pressure of 50 MPa, under an argon atmosphere for about 20 minutes. A fully densified composite material comprising boron suboxide particles uniformly dispersed in a second phase was produced. The second phase, also called grain boundary phase, was an amorphous phase containing Y₂O₃ and AI₂O₃ and some B₂O₃ A cross-section of the sample was polished and then tested for hardness and fracture toughness with Vickers indenter. The hardness was found to be about 33 GPa at a load of 5kg and a fracture toughness of about 6 MPa. m⁰⁵.

Table 1 summarises the measured properties of this boron suboxide composite material. The hot pressed B₆O composite material produced by the method of the invention had a higher hardness and fracture toughness compared to both pure B₆O and the composite material produced by Shabalala et al (WO 2007/029102).

Example 2

A boron suboxide composite material was produced using the same components and conditions set out in Example 1 , save that the amount Of Y₂O₃ and AI₂O₃ components was reduced by half; with the ratio between the two being kept the same. The composite material produced was fully densified and was also found to contain Y₂O₃ AI₂O₃ and some B₂O₃ in the second phase. It had a hardness of 30.4 GPa and a fracture toughness of 6.0 MPa.m⁰⁵.

Example 3

A boron suboxide composite material was produced using the same components and conditions set out in Example 1 , save that the components for the second phase included an additional 1.0 weight % SiO₂. The composite materia! produced was fully densified and was found to contain Y₂O₃ AI₂O₃ and some B₂O₃ in the second phase. It had a hardness of 33.5 GPa and a fracture toughness of 5.0 MPa.m^{0 5}. Example 4

A boron suboxide composite material was produced using similar components and conditions to those set out in Exampfe 3, save that the components for the second phase did not include the rare earth oxide, Y₂O₃. The binder components therefore included 2 weight% AI₂O₃ and 1 weight % SiO₂. The composite material produced was fully densified and found to contain the complex Al-B-O phase AI₁₈B₄O₃₃; and have a hardness of 29.5 GPa and a fracture toughness of 4.8 MPa. m⁰⁵.

Example 5

A boron suboxide composite material was produced using B₆O and 1.2 weight% residual Fe and Cr and using a similar preparation method to Example 1. The sintering conditions were increased to 1900⁰C. The composite material produced was fully densified and was found to contain no discernible crystalline second phase by XRD. It had a hardness of 34.7 GPa and a fracture toughness of 3.7 MPa. m^{0 5}.

Table 1

Example 6

A boron suboxide composite material was produced using the same components and conditions set out in Example 1 , save that the washing of the B₂O₃ was done separately from the milling step. The boron suboxide powder was washed with methanol after mixing with the components for the second phase and sintered at the same conditions as example 1. A second composite material was produced in the same manner, except that the boron suboxide powder was not washed. Table 2 shows a comparison of hardness, fracture toughness and density results of the two composite materials. It can be seen from these results that the washing of the boron suboxide powder resulted in a composite material with superior hardness and fracture toughness properties.

Table 2

REFERENCES

1. H. Hubert, L. Garvie, B. Devouard, P. Buseck, W. Petuskey, P. McMillan, Chem. Mater; 10; (1998); pp.1530-1537

2. H. Itoh, I. Maekawa, H. Iwahara, J. Soc. Mat. Sci., Japan; 47(10); (1998);pp.1000-1005

3. R.R Petrak, R. Ruh, G.R. Atkins, Cer. Bull.; 53(8); (1974); pp.569-573

4. R. Sasai, H. Fukatsu, T. Kojima, and H. Itoh, J. Mater. Sci.; 36; (2001 ); pp.5339-5343

5. H. Itoh, I. Maekawa, and H. Iwahara; J. Mater. Sci.; 35; (2000); pp.693- 698

6. H. Itoh, R. Yamamoto, and H. Iwahara; J. Am. Ceram. Soc; 83(3); (2000); pp.501-506

7. T.C. Shabalala, D.S. Mclachlan, I.J. Sigalas, M. Herrmann; Advances in Sci and Tech.; 45; (2006); pp.1745-1750

8. He, D., Zhao, Y., Daemen, L, Qian, J., Shen, T.D. & Zerda, T.W. Boron suboxide: As hard as cubic boron nitride, Appl. Phys. Lett. Vol.81, 4, 643-645 (2002)

Claims

1. A method of making a boron suboxide composite material including the steps of reducing the B₂O₃ content of a source of boron suboxide to produce a boron suboxide of reduced B₂O₃ content and sintering this boron suboxide in the presence of a material capable of producing a secondary phase to produce the boron suboxide composite.

2. The method of claim 1 wherein the B₂O₃ content of a source of boron suboxide is reduced by contacting the source of boron suboxide with a solvent capable of dissolving B₂O₃; and separating the solvent from the boron suboxide.

3. The method of claim 2 wherein the solvent is an organic solvent with a high polarity.

4. The method of claim 2 or claim 3 wherein the solvent is an alcohol.

5. The method of claim 4 wherein the alcohol is selected from methanol, ethanol, propanol, isopropanol and butanol.

6. The method of claim 2 wherein the solvent is water.

7. The method of any one of claims 2 to 6 wherein the solvent is contacted with the boron suboxide material by mixing.

8. The method of claim 4 or claim 5 wherein the alcohol reacts with the B₂O₃ to form an ester which is removed by volatilization.

9. The method of any one of claims 2 to 7 wherein the solvent is separated from the boron suboxide by filtration or centrifugation.

10. The method of any one of the claims 2 to 8 wherein the source of boron suboxide is subjected to a particle reducing step in the presence of a particle-reducing medium and the solvent is included in the medium.

11. The method of any one of the preceding claims wherein the source of boron suboxide is in particulate or granular form,

12. A method of claim 1 substantially as herein described with reference to any one of the examples.