GB2058840A - Production of polycrystalline cubic boron nitride - Google Patents

Abstract

A charge comprising boron nitride (graphite-like, cubic and/or wurtzite-like) and 0.1 to 30% by weight, based on the weight of the charge, of at least one refractory compound (such as a transition metal nitride, e.g. titanium nitride, carbonitride or boronitride) is sintered at high temperature (at least 1200 DEG C) and high pressure (at least 40 kbars) so as to convert the charge to polycrystalline cubic boron nitride. The improvement is that the refractory compound has a smaller particle size than is conventional, namely, 50 to 1000 ANGSTROM . The resulting compacts can be used to make cutting tools suitable for machining very hard alloys, the tools being also suitable for machining softer alloys.

Description

SPECIFICATION Production of polycrystalline cubic boron nitride The present invention is concerned with the production of polycrystalline cubic boron nitride compacts (polycrystalline cubic boron nitride being known as "BZN" or "borazon" in U.S.A. or "composite" in U.S.S.R.).

A known processor the manufacture of cubic boron nitride (as disclosed in U.S. Patent 2947617) involves applying pressures of above 40 kbars and temperatures of above 100000 to a mixture of graphite-like boron nitride (GBN) and a catalyst (an alkali metal or an alkaline earth metal and/or a nitride thereof). However the resulting cubic boron nitride (CBN) has very small crystals (1 mm maximum) so that they are unsuitable for making blade-type cutting tools.

U.S. Patent 3192015 discloses a method of making large ( > 11 mm) single crystals of CBN, but this method is expensive and has a low yield, and the resulting crystals (just like diamonds) are anisotropic, which makes it difficult to make tools therefrom.

More efficient methods of making large aggregates (compacts) based on CBN involve sintering CBN powder with refractory compounds, such as B4C (U.S. Patent 3136615), Al203 or BeO (U.S. Patent 3233988) or sintering GBN with refractory compounds, such as nitrides, borides or carbides of high melting metals (U.S. Patents 3852078 and 3944398).

All these materials can be used for smooth cutting of hardened steels and other difficultly prncessible materials; however, such materials have a low wear-resistance upon machining under impact loads. This is due to the fact that they are all prepared using as binding additives, powders of refractory compounds with a particle size corresponding to that of CBN (1 to 10 millicentimetres and over), whereby the wearresistance of the resulting compacts is less than that of pure CBN, since all these compounds have lower abrasion-resistance than CBN. Furthermore, the crystal size in these materials is usually within the range of 1 x 10-3 to 1 x 10-2 cm, which affects their strength and, hence, wear-resistance.

The use of powders of compounds having low reactivity and a particle size above 1 millicentimetre results in the transition of GBN to CBN taking longer (cf. U.S. Patent 3852078).

This lowers the productivity of the process for the manufacture of compacts and reduces the service life of pressurised equipment thus considerably adding to the production costs.

Also known in the art is a process for the manufacture of a super-hard abrasive material according to French Patent 2174617, wherein a mixture of a highly-deficient GBN and wurtzite-like boron nitride (WBN) produced by impact compression of GBN is subjected to high temperatures and pressures within the range of stability of dense forms of boron nitride. Compacts produced from this material mainly have the structure of wurtzite-like boron nitride; these compacts can be used to process hardened steel under the conditions of discontinuous cutting.

However, the wear-resistance of this material is relatively low which is due to a reduced microhardness of the material (4,000 to 6,000 kg/mm2 (CBN has a microhardness of 7,000 to 8,000 kg/mm2) and a very high residual porosity (25%). Moreover, the presence of a large amount (up to 45%) of GBN in the reaction charge has a detrimental effect on the properties of the resulting compacts because of the risk of retaining GBN therein, since according to this process no initiators for the transition of GBN to CBN are included in the charge.

We have now developed an improved method of producing polycrystalline cubic boron nitride compacts, which comprises sintering a charge comprising boron nitride and 0.1 to 30% by weight, based on the weight of the charge, of at least one refractory compound of melting point above 20000C and of particle size 50 to 1000 A, the sintering being carried out at a temperature of at least 12000C and a pressure of at least 40 kbars, the temperature and pressure being within the range of stability of cubic boron nitride indicated in the Bundy-Wentorf diagram.

The Bundy-Wentorf diagram referred to above is given in J. Chem. Phys. vol. 63 (9), 1975, pp.

3812-3820, F. Corrigan and F. Bundy, the paper being entitled "Direct transitions among the allotropic forms of boron nitride at high temperatures and pressures".

The boron nitride used in the method according to the invention may be, for example, GBN, CBN, WBN, or mixtures thereof.

When CBN is used, it may be a CBN powder such as that available in the USSR under the trademark "Elbor" or in the U.S.A. under the trademark "Borazon" having a particle size from fractions of a micron to 10 mcm. The ability to use such small particles constitutes one of the advantages of the present invention, since such powders cannot usually be employed for the manufacture of grinding wheels and are generally wastes resulting from the production of grinding powders from CBN.

When GBN is used, this may have a particle size from fractions of a micro to 100 micron. This is also an advantage of the process according to the present invention, since, for example, U.S. Patent 3852078 teaches the use of GBN powder with a particle size of below 3 mcm. Any commercial grade of adequately pure GBN can be employed.

WBN for use according to the invention can be produced by impact compression of GBN, as taught by U.S. Patent 4014979. It is preferable that the specific surface area of the WBN employed be at least 20 m2/g. It is also possible to use boron nitride similar to WBN produced by static compression of GBN, e.g. according to U.S.

Patent 3212852 which employs similar parameters, though in this case the time of application of high pressures and temperatures to the starting charge is somewhat increased.

When CBN or GBN are used, they are also preferably subjected to impact compression.

One of the factors lowering the cost of compacts produced in accordance with the present invention is the possibility of using a mixture of highly critical GBN and WBN which is obtained according to the process taught by U.S.

Patent 4014979 (in the manufacture of pure WBN by this method it is necessary to separate WBN and GBN after the impact compression, which results in considerable expense and great loss of WBN).

The refractory compound of particle size of from 50 to 1,000 A, which is preferably a singlecrystal powder with a particle size of from 100 to 1,000 A, may be, for example, a nitride, boride, siiicide, carbide, oxide, or a mixed compound thereof (such as a compound as described in the above-mentioned U.S. Patents 3852078 and 3944398), is preferably a transition metal nitride.

Preferred such nitrides include titanium nitride, titanium carbonitride of formula TiC N in which x = 0.1 to 0.9 and y = 0.9 to 0.1 or titanium boronitride of formula TiB N in which x = 0.05 to 0.30 and y = 0.95 to 0.70. Such materials can be produced conventionally for example, by the method of Hojo J. et al. Defect structure, thermal and electrical properties of Ti-nitride and V-nitride powders, "J. Less-Common Metals", vol. 53 (2), 1977, pp. 265-276.

The use of such powders gives the following advantages: 1. The small particle size of the refractory compounds substantially increases the number of particles in the compact (by several orders of magnitude) at the same percentage thereof thus substantially reducing the particle size of the recrystallized boron nitride located between the particles. Thus, with a TiN content in the charge of from 2 to 5% by weight, a particle size of about 500 A and a uniform distribution of TiN particles over the entire volume, the size of crystallites in the compact would not exceed 10-6 to 10-5 cm.

Decreasing the dimensions of boron nitride crystallites in compacts results in increased mechanical strength and wear-resistance thereof.

2. Reducing the particle size of the refractory compounds to the above-mentioned limits makes it possible to substantially enhance catalytic activity thereof for the transition of GBN to CBN which, in turn, makes it possible either to reduce the synthesis duration or lowers the percentage thereof, compared to conventional powders. On the one hand, this makes possible to lower production costs due to an increased service life of the pressurised equipment; on the other hand, this increases wear-resistance of the resulting compacts due to a reduced amount of compounds having lesser hardness as compared to CBN.

3. Reducing the particle size of the refractory compounds to 50-1,000 A makes it possible to considerably increase their mechanical strength (since all these particles are single crystals) as compared to particles of the same compounds but having dimensions above 1 millicentimetre and a mainly polycrystalline structure. This, in turn, minimizes the weakening effect of the refractory compounds on the mechanical strength of the resulting compacts, while retaining the positive effects resulting from incorporation of such compounds, namely increased impact strength, lowered chemical reactivity of the compact with respect to the surface to be processed (this is especially clearly pronounced when a titanium carbonitride is used), and the possibility of modification of the electro-physical properties of the compacts.This also makes it possible to enlarge the scope of materials to be processed with tools manufactured from polycrystalline boron nitride. Thus, known materials, for example, "BZN", have best cutting properties when machining hardened steels with the hardness exceeding 45 RC units, whereas in machining of steels of a lower hardness, their cutting properties decrease to become even inferior to those of hard alloys. Cutting tools made of polycrystalline boron nitride produced according to the present invention from a charge containing 25% by weight of titanium carbonitride have no such disadvantages.

4. The small particle size of the refractory compounds causes them to act as disperse reinforcing agents of the compact material, while retaining their dimensions in the compact formed during synthesis. This improves physicomechanical properties of the compact.

It should be noted that the catalytic activity of the transformation of GBN to CBN is higher in the case of titanium boron nitride, whereas the highest cutting properties of the material, especially in discontinuous cutting, are obtained when using titanium carbonitride.

To obtain a polycrystalline boron nitride in accordance with the present invention, use can be made of any pressurised apparatus such as those which are extensively employed in industry. The only requirement is that the apparatus must ensure the required temperatures and pressures within the range of stability of CBN as given in the Bundy-Wentorf diagram (as referred to above) over a period of, for example, from 1 to 10 minutes. As such apparatus use can be made of Belt chambers (cf. U.S. Patent 2941248), a tetrahedral apparatus (cf. U.S. Patent 2918699), as well as the chamber disclosed in U.S. Patent 3695797.

Calibration of the chambers for the required pressures and temperatures can be effected in a conventional manner (cf. paper by P. W.

Bridgman, Proceedings, American Academy of Arts and Science, vol.81, March1952, pp. 165-251).

Heating of the reaction volume may be effected by passing electric current through a tubular graphite heater, whereinto the reaction charge is fitted by compression. As the heater material - molybdenum, tungsten, tantalum and other highmelting metals can be used.

It should be noted that in the case of a graphite heater there is no need to use protective metal baffles as in U.S. Patent 3743489, since during sintering of the reaction charge under the conditions of high pressures and temperatures according to the present invention, diffusion of carbon from the heater to the reaction volume is inhibited by the reaction of the carbon with titanium nitride, carbonitride or boronitride resulting in the formation of new solid compounds which do not affect the physico-mechanical characteristics of the compacts. From this point of view, it is preferable to use titanium nitride in the charge, since this compound is the most reactive and upon reaction with carbon it forms titanium carbide possessing the highest mechanical strength.

It is known that the sintering of boron nitride or refractory compounds in the absence of a liquid phase occurs at the expense of recrystallization and in the case of a short sintering period a residual porosity is observed in the resulting compact as disclosed in French Patent 21 7461 7.

Furthermore, while having a high specific surface area, the powders of boron nitride and refractory compounds can absorb a substantial amount of water, oxygen, nitrogen and the like, which during the synthesis can result in the formation of boron oxide (B203) and other compounds which are detrimental to mechanical strength and, consequently, wear-resistance of a compact.

To avoid this, aluminium (either in the form of a fine powder or as a coating one one of the charge components) is preferably included in the charge.

This aluminium melts during sintering, fills pores, reacts with vapours of water and absorbed gases with the formation of very durable compounds (Al2O3 or AIN) which do not substantially reduce the mechanical strength of the compact.

Furthermore, the presence of aluminium which is one of initiators of phase transformations in boron nitride makes it possible to noticeably accelerate and stabilize the sintering process.

In selecting the amount of aluminium to be added to the charge, it should be noted that the higher the specific surface of the starting compounds, the greater amount of aluminium should be present in the charge and vice versa.

The amount of aluminium is preferably 0.1 to 5% by weight.

Germanium or silicon can be also employed instead of aluminium, though with a less pronounced effect.

Cutting tools produced from polycrystalline boron nitride produced in accordance with the present invention make it possible to efficiently perform machining of steels of both high (RC above 45 units) hardness and low (RC below 40) hardness, chilled cast iron, hard alloys (WC-Co) with a content of cobalt of above 8% by weight and a number of other materials which are difficult to machine. The surface of the machined article corresponds to 7-10 classes, while the wearresistance of these materials can be superior to that of a hard alloy by 10-100 times (depending on cutting parameters and hardness of the material being machined).

In order that the present invention may be more fully understood, the following Examples are given by way of illustration only.

EXAMPLE 1 A mixture of 45% by weight of wurtzite-like boron nitride with a particle size of from 0.1 to 1.5 mcm, 53% by weight of cubic boron nitride with a particle size of from 0.1 to 10 mcm and 2% by weight of titanium nitride (TiN) in the form of a single crystal powder with a particle size of from 100 to 1,000 A was simultaneously compressed at a pressure of 80 kbar and heated at 2,0000 C, whereafter the temperature was lowered to room temperature and the pressure was reduced to atmospheric pressure.

The resulting product comprises a polycrystalline compact consisting mainly of cubic boron nitride. Cutting tools (cutters) manufactured from such compacts were used for machining of a hardened steel with a hardness of 60-68 RC units under conditions of both smooth and discontinuous cutting at a cutting speed of 60-120 m/min, feed of 0.01 to 0.07 mm/rev.

and cutting depth of up to 1 mm. The service life of such cutters before resharpening was 80 to 120 minutes.

The discontinuous cutting was cutting of a shaft made of hardened steel with a diameter of 60 to 100 mm having a longitudinal groove with a width of 4-5 mm.

Cutters manufactured from a conventional hard alloy, BK-8 (92% WC, 8% Co), under these conditions had a service life of not more than 0.5 minute (for smooth cutting), whereas in discontinuous cutting these cutters break almost instantly.

EXAMPLE 2 A mixture of 95% by weight of graphite-like boron nitride and 5% by weight of titanium nitride (TiN) in the form of a single-crystal powder with a particle size of from 50 to 300 A was simultaneously compressed at 85 kbar and heated at 20000C for one minute.

X-ray analysis of the resulting product revealed the absence of residual graphite-like boron nitride therein.

X-ray analysis of a similar material produced by a prior art process (cf. U.S. Patent 3852078), in which for the synthesis a titanium powder with a particle size of from 3 to 5 mcm was employed, showed the presence of 5 to 7% by weight of a residual graphite-like boron nitride. Only after a synthesis time of 1 5 minutes was no residual graphite-like boron nitride detected in the final product.

A still greater difference between the properties of the materials produced according to Example 2 and the prior art material is shown in comparative tests of tools manufactured therefrom: passing cutters for discontinuous cutting of hardened steel with a hardness of 58 RC units.

At a cutting speed of 80 m/min, a feed rate of 0.04 mm/rev., and a cutting depth of 0.2 mm, the cutters made of prior art material (employing TiN with a particle size of 3-5 mcm) broke after 0.2 to 0.3 minutes of cutting, whereas the tools made of material produced according to Example 2 withstood cutting for 5 to 10 minutes.

EXAMPLE 3 A mixture of 90% by weight of wurtzite-like boron nitride produced by the impact compression method, 9% by weight of graphite-like boron nitride which had been subjected to shock-wave treatment and 1% by weight of titanium carbonitride powder TiCoAN0.6 with a particle size of from 50 to 500 A was compressed at a pressure of 70 kbar and heated at 1 ,7500C. The resulting product, which contained 5 to 10% by weight of residual wurtzite-like boron nitride, had a micro-hardness of 5.500 to 6,500 kg/mm2.

Cutters made of such compacts could be used for machining of a hard alloy (80% WC + 20% Co) at a cutting speed of 30 to 40 m/min, feed of 0.05 mm/rev. and cutting depth of 0.1 to 0.4 mm.

EXAMPLE 4 A mixture of 90% by weight of graphite-like boron nitride which had been subjected to shock waves (80--90 kbar), 85/o by weight of powder of titanium carbonitride TiCo 1No9 with a particle size of from 100 to 1,000 A and 2% by weight of aluminium powder was compressed at a pressure of 80 kbar and heated at 2,0000C.

No residual graphite-like boron nitride was present in the resulting compact, as proven by the X-ray analysis. The properties of the compact were 1.5 times better than the cutting properties of the material produced according to Example 2.

EXAMPLE 5 The procedure of Example 4 was repeated except that instead of titanium carbonitride of the formula TiCo 1No 9, titanium carbonitride of the formula TiCo gNo 1, with the same particle size, was used. The cutting properties of the resulting material were somewhat worse than those of the material produced according to Example 4 when used for discontinuous cutting of hardened steel, while in the case of smooth cutting, the service life of the material was 1.2 times higher than the material according to Example 4.

EXAMPLE 6 A mixture of 50% by weight of cubic boron nitride with a particle size of 3 to 5 mcm, 45% by weight of a mixture of wurtzite-like and graphitelike boron nitride (in the ratio of 20:1) formed by shock-wave treatment of graphite-like boron nitride and having a high defect content, and 5% by weight titanium boronitride powder TiBo,2No a with a size of single-crystal particles of from 50 to 500 , was sintered under a pressure of 70 kbar at 1 ,7000C in a cutter-shaped high-pressure chamber.After releasing the pressure and lowering the temperature, the cutter blank was fixed in a holder and sharpened with a diamond wheel and then tested in discontinuous cutting of hardened steel of hardness 58 RC units, at a cutting speed of from 60 to 80 m/min, a feed rate of 0.04 mm/rev. and a cutting depth of 0.2 mm.

Under these conditions the cutter's service life was 90 minutes and the wear of the rear face Ah was 0.25 mm.

EXAMPLE 7 The procedure of Example 6 was repeated, but instead of titanium boron nitride of the formula TiBo.2No 8t titanium boron nitride of the composition TiBo 05No 95 with the same particle size was used. The properties of the thusproduced material were slightly inferior to those of the material of Example 6.

EXAMPLE 8 A mixture of 50% by weight of cubic boron nitride with particle size of from 1 to 30 mcm, 40% by weight of graphite-like boron nitride which had been subjected to shock-wave treatment (pressure 60-70 kbar), 8% by weight of titanium boron nitride TiBo 3No 7 with a particle size of from 200 to 800 A and 2% by weight of aluminium powder was simultaneously compressed at a pressure of 85 kbar and heated at a temperature of 2,0500C so that the whole of graphite-like boron nitride was transformed to cubic boron nitride.

The wear-resistance of the resulting material under smooth cutting conditions was 20% better than that of the material produced in Example 6.

EXAMPLE 9 A mixture of 68% by weight of wurtzite-like boron nitride, 30% by weight of titanium carbonitride powderTiC05N05 with a particle size of from 100 to 1,000 A and 2% by weight of aluminium was compressed at a pressure of 60 kbar and heated at 1,6000C.

In machining of chilled cast iron at a speed of 600 m/min, a feed rate of 0.07 mm and a cutting depth of 0.3 mm, a cutting tool produced from this material was operated for 1 50 minutes with insignificant wear.

EXAMPLE 10 A mixture of 99.8% by weight of wurtzite-like boron nitride and 0.2% by weight of titanium carbonitride powder TiCo 6No with a particle size of from 100 to 500 A was treated under the conditions described in Example 9.

The properties of the thus-produced compact were similar to those of the material produced as described in Example 1.

Claims (8)

1. A method of producing polycrystalline cubic boron nitride compacts, which comprises sintering a charge comprising boron nitride and 0.1 to 30% by weight, based on the weight of the charge, of at least one refractory compound of melting point above 20000C and of particle size 50 to 1000 A, the sintering being carried out at a temperature of at least 1 2000C and a pressure of at least 40 kbars, the temperature and pressure being within the range of stability of cubic boron nitride indicated in the Bundy-Wentorf diagram (as defined herein).

2. A method according to claim 1, in which the refractory compound is a single-crystal powder comprising a transition metal nitride with a particle size of at least 100 A.

3. A method according to claim 2, in which the nitride is titanium nitride.

4. A method according to claim 2, in which the nitride is titanium carbonitride having the formula: TiCXNy, whereinx=0.1 to 0.9; y = 0.9 to 0.1.

5. A method according to claim 2, in which the nitride is titanium boron nitride having the formula: TiBXNy, wherein x = 0.05 to 0.30; y =0.95 to 0.70.

6. A method according to any of claims 1 to 5, in which the charge contains 0.1 to 5% by weight of aluminium.

7. A method according to any of claims 1 to 6, in which the boron nitride in the charge has been impact compressed.

8. Polycrystalline cubic boron nitride compacts, whenever prepared by a method according to any of claims 1 to 7.

8. A method of producing polycrystalline cubic boron nitride compacts, substantially as described herein in any of Examples 1 to 10.

9. Polycrystalline cubic boron nitride compacts, whenever prepared by a method according to any of claims 1 to 8.

New claims or amendments to claims filed on 4/9/80 New or amended claims:

1. A method of producing polycrystalline cubic boron nitride compacts, which comprise sintering a charge comprising boron nitride and 0.1 to 30% by weight, based on the weight of the charge, of at least one refractory single crystal powder comprising a transition metal nitride ot melTing point about 20000C and of particle size 100 to 1000 A, the sintering being carried out at a temperature of at least 1 2000C and a pressure of at least 40 kbars, the temperature and pressure being within the range of stability of cubic boron nitride indicated in the Bundy-Wentorf diagram (as defined herein).

2. A method according to claim 1, in which the transition metal nitride is titanium nitride.

3. A method according to claim 1 in which the transition metal nitride is titanium carbonitride having the formula: TiC N whereinx=0.1 to 0.9; y=0.9 to 0.1.

4. A method according to claim 1, in which the transition metal nitride is titanium boron nitride having the formula: TiBXNy, wherein x = 0.95 to 0.30; y = 0.95 to 0.70.

5. A method according to any of claims 1 to 4, in which the charge contains 0.1 to 5% by weight of aluminium.

6. A method according to any of claims 1 to 5, in which the boron nitride in the charge has been impact compressed.

7. A method of producing polycrystalline cubic boron nitride compacts, substantially as described herein in any of Examples 1 to 10.