Hard abrasive particle and method of producing same

Abstract: Hard abrasive particles are provided comprising diamond nuclei and single crystal or polycrystal cubic boron nitride layers coating the surfaces of said diamond nuclei, in which the diamond nuclei are bonded to the cubic boron nitride layers by the atoms thereof, to give materi­ als having the hardness of diamond and usable for grinding, abrasion and the like of the iron group metals. A method of producing such particles is also disclosed.

Description: The present invention relates to hard abrasive particles comprising diamond nuclei and cubic boron nitride layers coated on said diamond nuclei, and to a method of producing the same. As is widely known, diamond is the hardest substance known and cubic boron nitride (hereinafter referred to as cBN) is a substance having a hardness close to that of diamond. On account of their hardness, diamond and cBN have been widely used for grinding materials, abrasive compounds, cutting materials and the like. In particular, cBN has been widely used for the grinding and the like of the iron group metals on account of the superior chemical stability thereof. It has been known for a long time that diamond and cBN can be artificially synthesised under high temperature and pressure conditions and both are now synthesised industrially. As for a substance comprising diamond and cBN, there has been proposed in US-A-3,142,595 a P-P type or P-N type semiconductor junction crystal, in which P type diamond is grown on a cBN substrate of P type or N type semiconductor, and in US-A-4,734, 339 there has been proposed a substance in which a diamond layer is coated around boron nitride nuclei by the use of the gaseous phase synthetic method. However, a substance in which cBN is coated on the surface of diamond nuclei has not been obtained by the prior art methods. As above described, the problem is that there is no substance which has both the hardness of diamond and the required chemical stability against the iron group metals. It has been postulated that a substance which is composed of a diamond nucleus and the cBN layer grown thereon may have the features of both the highest hardness and chemical stability, but such a substance has not been obtained until now, beacuse the diamond nucleus is destroyed under high temperature and high pressure when cBN is grown around the nucleus. According to a first aspect of the present invention there is provided a hard abrasive particle comprising a diamond nucleus and a single crystal or polycrystalline cubic boron nitride layer coating the surface of said diamond nucleus, characterised in that said diamond nucleus and said cubic boron nitride layer are bonded directly by atoms. According to a further aspect of the invention there is provided a method of producing a hard abrasive particle, in which diamond abrasive particles are coated with cubic boron nitride at high temperatures and high pressures using hexagonal boron nitride as the source, characterised in that diamond abrasive particles having particle sizes of 1 µm to 1,000 µm are used as seed crystals for growing cubic boron nitride and in that one or more substances having an oxidation energy at high temperatures and high pressures lower than that of diamond are added to the mixture of solvent, hexagonal boron nitride source and diamond abrasive particles. There can thus be produced a substance which has the features of both the highest hardness and the chemical stability to the iron group metals. Preferred embodiments of the invention will now be described with reference to the accompanying drawings wherein: Fig 1 is a general drawing showing hard abrasive particles according to the present invention; Fig 2 is a diagram showing a crystal structure of diamond and single crystal cBN; Fig 3 is a diagram showing a range of manufacturing conditions of the hard abrasive particles according to the present invention; Fig 4 is a diagram showing an apparatus used for obtaining the hard abrasive particles according to the present invention; Fig 5 is a photograph (x200) showing the structure of the hard abrasive particles produced in the Example 1 taken by the scanning electron microscope ; and Fig 6 is an X-ray diffraction chart of the hard abrasive particles produced in the Example 1. Referring to Fig 1 a diamond nucleus 1 is surrounded by cBN layer 2, and each atom of the first cBN layer 2 is bonded to each atom of the surface of the Having a structure in which diamond, the hardest material on earth, is surrounded by a layer of cBN, which is inert to iron group metals, the hard abrasive particles of the present invention have the features of both the highest hardness and the chemical stability. Fig 2 illustrates the crystal structures of cBN and diamond, and in the Fig 2  indicates N atoms, ○ indicates B atoms, and O ○ indicates C atoms. As obvious from Fig 2 cBN has the zinc blend type structure similar to that of diamond. Furthermore, diamond and cBN has the lattice constant of 3.567Å and 3.615Å respectively, that is to say there is a difference of merely about 1.3% between them, and also the thermal expansion coefficient of diamond and cBN is 4.50 x 10⁻⁶K⁻¹ (700 °C) and 4.30 X 10⁻⁶K⁻¹ (750 °C) respectively, that is to say they are remarkably close to each other, so it has been thought that it may be possible to grow cBN on diamond epitaxially but it has never been realised. Until we invented the method no-one had succeeded in growing cBN under the condition which the diamond nuclei can exist stably. We have analyzed the causes of the failure to grow cBN on the diamond nuclei, and have found that the diamond nuclei are oxidized to disappear under the condition of cBN being synthesized. And we have confirmed this by the fact that if the oxygen is completely removed from inside the cell in which cBN is grown, cBN can be grown on diamond without the extinction of the diamond nuclei under the condition of both diamond and cBN are thermodynamically stable which is shown by the shaded range in Fig.3. And so the substances of which the oxidation energy at high temperatures is lower than that of diamond work for removing oxygen from inside the cell in which cBN is grown. Next, Fig.4 illustrates an apparatus for producing the hard abrasive particles according to the specific operative examples. The apparatus and its internal construction are same with those generally used for producing diamond and cBN. The substances of which the oxidation energy at high temperatures is lower than that of diamond are added to the mixture of the solvent and hexagonal boron nitride in the reaction cell 5 in Fig.4. The substances of which the oxidation energy at high temperatures is lower than that of diamond include Mg, Al, Ca, V, Zr and Ti. Thus the abrasive particles obtained through the present invention have the hardness equal to diamond by the direct bond of diamond and cBN atoms. However, since the diamond atoms have less effect on hardness with the increase of the thickness of the cBN layer, it is desirable that the thickness of said cBN layer is 1/5 times or less the particle sizes of the diamond nuclei. In addition, in the hard abrasive particles obtained through the present invention, cBN is grown around the diamond nuclei, so that this cBN layer serves as the protective layer against the iron group metals. Therefore, the hard abrasive particles mentioned and produced by the present invention have the features of both the highest hardness and the chemical stability against the iron group metals. Next, Fig.4, which illustrates the high temperature and high pressure generating apparatus used for producing the hard abrasive particles is explained in detail. Referring to Fig.4, inside the reaction cell 5 is heated up to the aimed temperature by supplying alternating or direct electric current through heater 3 made of graphite. The reaction cell 5 is surrounded by capsule 6 made of molybdenum. Capsule 6 is electrically insulated by pressure medium 4, usually made of hexagonal boron nitride (hBN). Pressure medium 4 is closed or sealed at each end by conducting end disks 7. The material of disks 7 is unimportant to the present invention, since the function of the disk is merely to serve as a means of conducting current to heater 3. Thus, any conducting metal may be employed for disks 7 which will withstand evaluated temperatures and pressures. Positioned adjacent each disk 7 is an insulating disk 8 made of pyrophyllite. An annular conducting ring 9 made of alloy steel surrounds each of disks 8. The electric current is supplied through punches 11 made of WC-Co alloy. Pressure medium 10 made of pyrophyllite insulates the electric current and the heat from die 12. The reaction cell 5 is staffed with hBN, the solvents, the substances having the oxidation energy lower than that of diamond, and the diamond nuclei. The substances having the oxidation energy lower than that of diamond are used in the form of powders, particles, or foils depending upon the forms in which they are generally used. That is to say, Ca, Mg, V are generally used in the form of powders or particles and they are added to the hBN source and/or the solvent. Furthermore, Al, Ti, Zr are generally used in the form of powders or foils and they are added to the hBN source and/or the solvent in the form of powders or wrapped around the pressed body of the hBN and the solvent mixure. Thus, if the temperature and pressure conditions are set within the shaded range in Fig.3 preferably 1300°C or more and 6.0 GPa or more, hBN can be dissolved into the solvents and then deposited on the diamond nuclei in the form of cBN because of the relative difference between hBN and cBN in the solubility to the solvents under the condition of cBN being stable. The following examples illustrate the present invention. EXAMPLE 1 hBN powders, Li₃BN₂ powders as the solvent and diamond abrasive particles of 0.1 carat having particle sizes of 200 to 300 µm were mixed and the mixture was pressed to be turned into a pressed body having a diameter of 4 mm and a height of 9 mm. Zr foils having a diameter of 4 mm and a thickness of 0.2 mm were arranged top and bottom of the resulting pressed body as the deoxidizing agent and then staffed in the reaction cell 5 surrounded by the capsule 6 made of molybdenum in the apparatus shown in Fig 4. Then, the reaction cell 5 was held for 1 hour under the high-temperature and high-pressure conditions of 6.5 GPa and 1,700 °C to confirm the growth of polycrystal cBN around the diamond abrasive particles. The thus obtained hard abrasive particles were observed with scanning electron microscope (x 200) with the result shown in Fig.5. In addition, these hard abrasive particles were identified by the X-ray diffraction method with the results shown in Fig.6. The peak was detected in the vicinity of 2 ϑ = 43.2° and it was confirmed that the surface was coated with cBN. EXAMPLE 2 ∼ 10 The hard abrasive particles identical with those obtained in EXAMPLE 1 were obtained from the hBN powders, the Li₃BN₂ powders, the diamond abrasive particles having the particles sizes shown in Table 1 and deoxidizing agents under the same conditions as those in EXAMPLE 1. In addition, the surface of the abrasive particles having the particle sizes close to 1 mm obtained in EXAMPLE 8, 9 and 10 was made smooth by means of the diamond grindstone #8000 and then tested on the hardness at a load of 500 g and a measuring time of 15 seconds by means of the micro Vickers hardness tester with the results shown in Table 1. For comparison, 5 pieces of single crystal cBN were synthesized by the usually known synthetic method and tested on the hardness in the same manner as in the EXAMPLES. In addition, the Vickers hardness shown in Table 1 is a mean value of several values measured at several points of the same one abrasive particle. EXAMPLE No. Particle sizes of diamond abrasive particles(µm) Deoxidizing agent Vickers hardness Hv (kg/mm²) 2 1 to 6 Mg - 3 do. Al - 4 do. Zr - 5 3 to 10 V - 6 do. Ti - 7 do. Ca - 8 850 to 1,000 Mg 6,670 9 do. Al 6,130 10 do. Zr 6,520 COMPARATIVE EXAMPLE not using not using 4,150 It has been confirmed from EXAMPLES 2 to 7 in Table 1 that according to the present invention, cBN is grown around the diamond abrasive particles without extinction even the diamond abrasive particles having the particle sizes of several µm . In addition, it has been confirmed from EXAMPLES 8 to 10 and COMPARATIVE EXAMPLE that the abrasive particles according to the present invention were remarkably improved in hardness in comparison with the conventional cBN abrasive particles. As above described, the hard abrasive particle provided by the present invention which comprises the diamond nucleus and cBN grown around the said diamond nucleus have the features of both the hardness equal to diamond and the chemical stability against the iron group metals. And thus the hard abrasive particles can be effectively used as the grinding materials and abrasive compounds for the materials such as cast iron and hardened steels.

Claims: A hard abrasive particle comprising a diamond nucleus (1) and a single crystal or polycrystalline cubic boron nitride layer (2) coating the surface of said diamond nucleus, characterised in that said diamond nucleus and said cubic boron nitride layer are bonded directly by atoms. A hard abrasive particle as set forth in Claim 1 characterised in that said diamond nucleus has particle size of 1 µm to 1,000 µm. A hard abrasive particle as set forth in Claim 1 characterised in that the thickness of said cubic boron nitride layer is 1/5 or less times the particle size of said diamond nucleus. A method of producing a hard abrasive particle, in which diamond abrasive particles are coated with cubic boron nitride at high temperatures and high pressures using hexagonal boron nitride as the source, characterised in that diamond abrasive particles having particle sizes of 1 µm to 1,000 µm are used as seed crystals for growing cubic boron nitride and in that one or more substances having an oxidation energy at high temperatures and high pressures lower than that of diamond are added to the mixture of solvent, hexagonal boron nitride source and diamond abrasive particles. A method as set forth in Claim 4 characterised in that at least one metal selected from Mg, Al, Ca, V, Ti, and Zr is used as said substance having the low oxidation energy. A method as set forth in Claim 4 or claim 5 characterised in that said mixture of solvent, hexagonal boron nitride source, diamond abrasive particles as seed crystals, and said substances having the low oxidation energy is subjected to a temperature of 1300 °C or more and a pressure range of 6.0 GPa or more in which both diamond and cubic boron nitride are thermodynamically stable.

Inventors: TSUJI KAZUWO, YOSHIDA KATSUHITO, TSUJI, KAZUWO, YOSHIDA, KATSUHITO
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1990
Main classification: C09K3
IPC codes: C09C1, C09K3, B24D3