Claims
- 1. A method of producing polycrystalline superhard material from cubic boron nitride, wherein said superhard material comprises sintered particles of cubic boron nitride coated over their entire surface with a layer of crystalline compounds of the chemical formula B.sub.x N.sub.y C.sub.z, wherein x, y and z can assume any value from 0 to 1, with the proviso that the crystals of said coated layer adjacent to said boron nitride have said formula wherein x and y approach 1 and z approaches 0, while the compounds in said coating layer distal to said boron nitride particles have said formula wherein x and y each approach 0 and z approaches 1, said crystalline compounds binding together said particles of cubic boron nitride, the method comprising introducing particles of cubic boron nitride to a gas flow containing carbon, so that a carbon layer of 1 to 100 A thick is built up over the entire surface of the particles; and then sintering said particles of cubic boron nitride having the carbon layer built up thereon at a temperature and pressure corresponding to the region of the thermodynamic stability of cubic boron nitride.
- 2. A method of producing polycrystalline superhard material from cubic boron nitride as claimed in claim 1, wherein a carbon layer is built up in a flow of gaseous hydrocarbons under a pressure of from 0.1 to 760 mm Hg and at a temperature of from 700.degree. to 1,100.degree. C.
- 3. A method of producing polycrystalline superhard material from cubic boron nitride as claimed in claim 1, wherein particles of cubic boron nitride are sintered under a pressure above 50 kbar and at a temperature above 1,600.degree. C.
- 4. A method of producing polycrystalline superhard material from cubic boron nitride as claimed in claim 1, wherein methane is used as a gaseous hydrocarbon.
- 5. A method of producing polycrystalline superhard material from cubic boron nitride as claimed in claim 1 wherein said particles of cubic boron nitride are about 0.3 to 60 microns in size.
Priority Claims (1)
| Number |
Date |
Country |
Kind |
| 2340120 |
Jan 1976 |
SUX |
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SUMMARY OF THE INVENTION
This is a divisional of application Ser. No. 760,902, filed Jan. 21, 1977 now U.S. Pat. No. 4,148,964.
The present invention relates to superhard materials based on cubic boron nitride, said material being used for manufacturing wear-resistant inserts for cutting, dressing, and drilling tools, and for wire-drawing dies.
Known polycrystalline superhard materials based on cubic boron nitride can be classified into two groups. In the materials belonging to the first group the crystals of cubic boron nitride are strongly interconnected due to self-binding, i.e. by virtue of diffusion processes taking place in the zone of contact of the particles being sintered, without a binder (accepted Japanese Patent Application No. 49-39357). In the materials belonging to the second group the crystals of cubic boron nitride are interconnected by a binder.
U.K. Pat. No. 990,818 teaches a number of metals such as nickel, chromium, ziroconium, cobalt, manganese, copper, rhenium, titanium, and molybdenum as a binder in a polycrystalline superhard material based on cubic boron nitride.
Polycrystalline superhard materials are known which contain, in addition to crystals of cubic boron nitride. crystals of diamond; as a binder use is made of metals (accepted Japanese Patent Application No. 43-30409), refractory materials such as borides and oxides of magnesium and calcium (French Patent No. 2,201,268).
According to the known methods of producing polycrystalline superhard materials (see, for example, U.K. Pat. No. 990,818), it is possible to produce compact strong polycrystalline materials by sintering particles of boron nitride at high temperatures within the range from 1,200.degree. to 2,400.degree. C. and under pressures above 75 kbar.
Polycrystals produced by the known method exhibit relatively low wear resistance when tested on hardened steels in a cutting tool under dynamic loads (impact strength). This is caused by the fact that no strong bond is formed between the neighbouring crystals of cubic boron nitride in the process of sintering when pure powders of cubic boron nitrides are used.
A method is also known of producing polycrystalline superhard material (accepted Japanese Patent Application No. 43-30409) iron a mixture of graphite powder, metallic powder, and crystals of cubic boron nitride under pressures above 50 kbar and at a temperature of 1,200.degree. C. in the region of diamond formation; in this method a binder-metal catalyzes the formation of diamond, carbon-containing material transforms into diamond, particles of diamond and cubic boron nitride are fixed in the binder-metal.
A disadvantage inherent in the above-cited material is its low thermal stability caused by the presence of the remaining reaction products and the binder-metal.
Polycrystals produced by this known method can be used only as abrasives but not as a cutting tool.
Likewise a polycrystalline superhard material is known in the art which is produced from a mixture of powders of cubic boron nitride and boron carbide, the latter component amounting to 25 wt.% in the resulting material (U.K. Pat. No. 975,316). The material is prepared under a pressure equal to at least 15 kbar and at a temperature of 1,050.degree. C.
This known method is disadvantageous in that the resulting material is fragile and therefore cannot be used in a cutting tool under conditions of dynamic loads when working heardened difficult-to-work steels and alloys.
Besides, the known method does not ensure highly uniform material, since simple mechanical stirring of the initial components, cubic boron nitride and boron carbide that have different densities makes it absolutely impossible to obtain strictly uniform distribution of particles of various materials, even in the case when one of the components is taken in a highly dispersed state. Heterogeneity of the material and non-uniform distribution of the components in the bulk of the material deteriorates its mechanical characteristics, in particular, resistance to impact thermal and mechanical loads.
It is an object of the invention to eliminate the above-cited disadvantages.
It is an object of the invention to provide such polycrystalline superhard material from cubic boron nitride which will possess imnproved mechanical properties, in particular, high wear-resistance.
Another object of the invention is to provide a method of producing a polycrystalline superhard material from cubic boron nitride, which will possess improved mechanical properties, in particular, high wear-resistance.
Said objects are accomplished by a polycrystalline superhard material derived from cubic boron nitride and produced by sintering particles at a temperature no less than 1,600.degree. C. and under a pressure no less than 50 kbar; according to the present invention, each particle of cubic boron nitride is coated over the whole surface with a layer of a crystalline compound of the formula B.sub.x N.sub.y C.sub.z, where x, y and z can assume any value from 0 to 1, said compound binding the particles of cubic boron nitride.
It is expedient, according to the present invention, that the carbon contained in the crystalline compound of the formula B.sub.x N.sub.y C.sub.z amount to 0.1-10 wt.% of the total weight of the polycrystalline material.
The polycrystalline superhard material proposed in the present invention is highly uniform and in each its point it displays mechanical properties superior to those of the known materials from cubic boron nitride. Thus, for example, under identical conditions of cutting, resistance of cutters made from the herein proposed material is 2-5 times higher than that of the cutters from the hitherto known materials based on cubic boron nitride.
According to the present invention, it is expedient that the polycrystalline superhard material from cubic boron nitride be produced by a method wherein the particles of cubic boron nitride are placed in a flow of a gas containing carbon in order to build up a layer 1-100 A thick over the whole surface of the particles, and then said particles are sintered at temperatures and pressures corresponding to the region of thermodynamic stability of cubic boron nitride.
To provide conditions enabling phase rearrangement of carbon in a boundary layer, it is expedient, according to the present invention, to build up a carbon layer in a flow of gaseous hydrocarbons under a pressure of from 0.1 to 760 mm Hg and at a temperature of iron 700.degree. to 1,100.degree. C.
It is expedient, according to the present invention, to carry out sintering of particles of cubic boron nitride under a pressure above 50 kbar and at a temperature above 1,600.degree. C.
For a more rapid and uniform building up of a carbon layer on the surface of particles of cubic boron nitride under similar conditions, methane is used as a gaseous hydrocarbon.
US Referenced Citations (2)
| Number |
Name |
Date |
Kind |
|
3351690 |
Stover |
Nov 1967 |
|
|
3852078 |
Wakatsuki et al. |
Dec 1974 |
|
Foreign Referenced Citations (2)
| Number |
Date |
Country |
| 784705 |
Oct 1957 |
GBX |
| 990818 |
May 1965 |
GBX |
Non-Patent Literature Citations (2)
| Entry |
| Samsonov et al., "Boron Carbonitride as a High-Temperature Electrically Ilating Refractory Material", in Chemical Abstracts, vol. 78, 127866c (1978), p. 228. |
| Andreeva et al., "Electrically Insulating Materials for Linings and Holders of High-Pressure Chambers", in Chemical Abstracts, vol. 84, 160911y, (1974), p. 514. |
Divisions (1)
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Number |
Date |
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| Parent |
760902 |
Jan 1977 |
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Patent Grant
4220677
