METHOD OF MANUFACTURING SINTERED COMPACT OF CUBIC BORON NITRIDE

Abstract: A method of manufacturing a sintered compact of cubic boron nitride mainly comprising the step of making 0.005 to 1.000 percent by weight of water adsorbed/diffused in a boron nitride compact containing alkaline earth metal boron nitride, thereby to enable manufacturing of a dense cubic boron nitride sintered compact of high purity under a relatively low temperature condition.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method of manufacturing a sintered compact of cubic boron nitride employed for a tool or a heat sink material for example, and more particularly, it relates to a method of manufacturing a sintered compact of cubic boron nitride (hereinafter referred to as cBN sintered compact) from hexagonal boron nitride (hereinafter referred to as hBN). Description of the Prior Art The cBN is almost the same as diamond in hardness. While diamond is oxidized and exhausts upon reaction with iron family elements when heated at a temperature within a range of 700 to 800°C or more, cBN is not oxidized, hardly converted into hBN and reacts with no iron family element under a high temperature of 1100 to 1300°C. Thus, cBN is chemically and thermally stable in the extreme, and suitable as a material for grinding a member of high-speed steel, alloy or cast iron having a substrate of nickel or cobalt etc., which cannot be ground by diamond. Further, cBN is next to diamond and by far higher than copper in heat conductivity, whereby the same is also suitable for a heat sink material. A cBN sintered compact for the aforementioned usage is preferably of high purity and high density with no admixture or an extremely small amount of admixture. However, it is extremely difficult to directly sinter cBN powder and hence metal, carbide, oxide or nitride must be generally admixed to the same as a binder, whereby the sintered compact is inuniformalized in structure and extremely reduced in mechanical strength and thermal stability, while heat conductivity is remarkably lowered. In order to solve the aforementioned problems, known are the following cBN sintering methods: U. S. Patent No. 3,212,852 and Japanese Patent Laying-Open Gazette No. 13731/1966 disclose a method of directly converting an hBN material into cBN without admixing a catalyst or binder, to obtain a dense cBN sintered compact. However, this method requires superhigh pressure and temperature conditions in excess of 100 Kbar and 2000°C, and the product as obtained is considerably restricted in configuration. Japanese Patent Publication Gazette No. 2625/1986 also discloses a method of directly converting hBN into cBN, which method requires operation under a high temperature in excess of 1600°C, leading to difficulty in production. On the other hand, U. S. Patent Nos. 4,188,194 and 2,947,617 and Japanese Patent Laying-Open Gazette No. 33510/1979 disclose a method of directly converting a material of pyrolytic boron nitride (pBN) into cBN, thereby to obtain a cBN sintered compact. However, pBN is still partially unknown in structure and the same is prepared by chemical vapor deposition through thermal decomposition of BCl₃ and NH gas, whereby the cost is increased. Further, this method requires high-temperature processing under a temperature in excess of 1800°C as described in Japanese Patent Publication Gazette No. 33510/1979, leading to a problem in manufacturing. Japanese Patent Laying-Open Gazette No. 128700/1976 discloses a method of directly converting a material of wurtzite boron nitride (hereinafter referred to as wBN), one of high-pressure phases of BN, into a cBN sintered compact. However, since wBN is generally in a thermodynamically instable non-equilibrium state, the state of the material must be controlled in high accuracy. Such strict control of the material state is extremely difficult in practice, and hence hBN or wBN is deposited in the obtained cBN sintered compact to irregularize the product quality. On the other hand, a method of manufacturing a cBN sintered compact containing no bonding phase with relatively relaxed conditions at a low cost is described in, for example, a report by Wakatsuki et al. in Material Research Bulletin, Vol. 7, 1972, p. 999. In this method, a cBN sintered compact is obtained by directly converting a parent material of low-crystalline hexagonal boron nitride (hBN). However, the low-crystalline hBN employed as the parent material is chemically instable and easily reacts with oxygen in the air, and hence it is difficult to obtain a compact uniformly and sufficiently sintered as a whole. It is well known in the art that cBN can be obtained by superhigh pressure and temperature processing of an hBN material under presence of a catalyst. Typical examples of such a catalyst are alkaline metal, alkaline earth metal and nitride thereof. A method of synthesizing cBN from hBN in a relatively low temperature condition with a catalyst of water is described in Material Research Bulletin, Vol. 9, 1974, p. 1443. In this method employing water as a catalyst, however, a dense sintered compact having interparticle bonding cannot be obtained since borate is generated and cBN is extremely small in particle size. Japanese Patent Publication Gazettes Nos. 38164/1984 and 38165/1984 disclose alkaline earth metal boron nitride such as magnesium boron nitride, strontium boron nitride and barium boron nitride as catalysts for converting hBN into cBN. Further, Japanese Patent Publication Gazette No. 5547/1984 and Japanese Patent Laying-Open Gazette No. 52967/1984 describe a method of performing superhigh pressure and temperature processing on an hBN sintered compact carrying a small amount of alkaline earth metal boron nitride as disclosed in Japanese Patent Laying-Open Gazette No. 57966/1984 at a temperature in excess of 1350°C. However, this method requires high-temperature operation at a temperature in excess of 1350°C, and hence considerably high temperature processing is required in order to internally homogeneously sinter a large-sized sample. When temperature control is incorrectly performed in such high-temperature processing, unsintered parts are left in the sintered compact. Thus, it has been difficult to obtain a dense sintered compact. SUMMARY OF THE INVENTION An object of the present invention is to provide a method of manufacturing a dense cBN sintered compact of high purity under a relatively low temperature condition. In order to attain the object, the present invention is characterized in that a compact of boron nitride is made to carry alkaline earth metal boron nitride, 0.005 to 1.000 percent by weight of water is adsorbed and diffused in the boron nitride compact containing the alkaline earth metal boron nitride, and high temperature and pressure processing performs under thermodynamically stable conditions for cubic boron nitride. According to the present invention, the following effects are attained: (1) A dense and homogeneous cBN sintered compact can be readily manufactured in conditions advantageous for industrial production with excellent reproducibility. (2) A cBN sintered compact superior in mechanical characteristics to the conventional one can be obtained for application to a cutting tool or the like. (3) A sintered compact having high heat conductivity of 6 to 7 W/cm· °C and over at the room temperature can be obtained, to provide an optimum material for a heat sink material. These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic sectional view showing an example of a girdle type superhigh pressure generating apparatus employed for carrying out the present invention; Fig. 2 illustrates cBN generating regions; Fig. 3 is a sectional view for illustrating the structure of a sample employed in the apparatus of Fig. 1; and Fig. 4 is a graph showing the relation between the contents of water admixed to materials of hBN sintered compacts and heat conductivity of obtained cBN sintered compacts at the room temperature as the result of Example 4. DESCRIPTION OF THE PREFERRED EMBODIMENTS A compact of hexagonal boron nitride for carrying alkaline earth metal boron nitride, which can be prepared by an hBN compact or hBN powder, must be of high purity containing as small impurities, particularly B₂O₃ component, as possible. The alkaline earth metal boron nitride can be carried in the hBN compact by (a) a method of mixing hBN powder and alkaline earth metal boron nitride powder in an atmosphere of anhydrous inert gas, particularly of nitrogen gas, to pressurize and mold the same in an inert atmosphere or (b) a method of diffusing alkaline earth metal boron nitride in an hBN compact (embossed or sintered body of hBN powder) in a contact or noncontact manner in a nitrogen current. The hBN compact carrying the alkaline earth metal boron nitride can be obtained by either the method (a) or the method (b), while the method of thermally diffusing the alkaline earth metal boron nitride in the hBN compact in a contact state in a nitrogen current is preferable since a homogeneous support having a large content of catalyst can be readily obtained. In this case, the content can be controlled by the density of the hBN compact material and the temperature and time of heating for thermal diffusion. The said content of the catalyst must be within a range of 0.01 to 5.0 mol % in case of expressing the alkaline earth metal boron nitride by Me₃B₂N₄ (Me: alkaline earth metal). When the content is less than 0.01 mol %, a dense cBN sintered compact cannot be obtained because of partially unconverted hBN. When the content is in excess of 5.0 mol % to the contrary, the catalyst is inuniformly dispersed and holes are defined by the catalyst remaining in the cBN sintered compact as obtained, whereby a dense cBN sintered compact cannot be obtained. In order to make the hBN compact carrying the alkaline earth metal boron nitride further carry water, the hBN compact may be stood in a closed vessel maintained in a constant temperature and humidity for a constant period so that water is stably and quantitatively adsorbed and diffused in the compact. The adsorption of water, which can be evaluated by increase in weight, must be within a range of 0.005 to 1.000 percent by weight of the hBN compact carrying the alkaline earth metal boron nitride. When the adsorption of water is less than 0.005 wt. %, water molecules are not sufficiently diffused in the alkaline earth metal boron nitride contained in the hBN compact to attain the object, whereby a dense sintered compact cannot be obtained since unconverted hBN remains in the sintered compact. When the adsorption of water is in excess of 1.000 wt. %, the alkaline earth metal boron nitride carried in the hBN compact is hydrolyzed to deposit large amounts of alkaline earth metal hydroxide and boric acid which are harmful for generation of cBN and to inhibit conversion from hBN to cBN, whereby unconverted parts are generated in the sintered compact as the result. Sintering must be performed at a temperature of 1200°C or more under cBN-thermodynamically stable conditions. In such cBN generating conditions, the pressure value is based on a press external pressure-pressure curve created with pressure fixed points, namely 2.55, 3.7 and 5.5 GPa. These points are phase transition points caused by pressure changes of bismuth, thallium and barium under a normal temperature. The sintering temperature was measured through a platinum-platinum rhodium thermocouple and evaluated by obtaining relation with electric power applied to a graphite heater. Fig. 1 shows an example of a superhigh pressure apparatus for carrying out the method according to the present invention. Referring to Fig. 1, the so-called girdle type superhigh pressure generating apparatus comprises a cylinder 1, a piston 2, an energizing tablet 3, a ring-shaped pressure medium 4, a cylindrical pressure medium 5 and a graphite heater 6. The piston 2 applies an AC or DC current to the graphite heater 6 through the energizing tablet 3 for performing heating. The apparatus for carrying out the inventive method is not restricted to that shown in Fig. 1, but a well-known superhigh pressure generating apparatus such as a belt type one can be employed so far as pressure and temperature conditions required for cBN generation can be obtained. In a process of manufacturing a cBN sintered compact, a constant amount of water is effectively admixed to an alkaline earth metal boron nitride (1) to remarkably improve the catalytic function of the alkaline earth metal boron nitride, (2) to suppress abnormal growth of cBN particles and (3) to prompt sintering of cBN particles. Namely, a constant amount of water is diffused in/admixed to an hBN compact carrying alkaline earth metal boron nitride, whereby a dense cBN sintered compact of high purity, in which particles are strongly bonded to each other, can be readily obtained in a low temperature condition. The obtained cBN sintered compact is remarkably improved in characteristics such as heat conductivity. Fig. 2 shows cBN synthesizing region according to the present invention. Referring to Fig. 2, a line A denotes an hBN-cBN equilibrium curve, and the part above the line A is a cBN-stable region while the part under the line A is an hBN-stable region. A line B denotes a cBN generating region according to the present invention, and a line C denotes a cBN generating region in case of admixing no water under conditions identical to those of the present invention. It is obvious from Fig. 2 that the cBN generating region is extended to, for example a temperature region is extended to lower by 150°C, by making 0.005 to 1.000 wt. % of water adsorbed/diffused. It has been recognized that particles are strongly bonded to each other in the cBN sintered compact obtained under this condition, to substantially reach theoretical density of cBN as obvious from Examples as hereinafter described. Thus, a dense cBN sintered compact can be manufactured under lower pressure and temperature conditions as compared with the conventional case, by adding 0.005 to 1.000 wt. % of water. By such admixture of a small amount of water, further, the content of the catalyst required for obtaining a dense cBN sintered compact, i.e., the content of the alkaline earth metal boron nitride to be carried in the hBN compact, is widened as compared with the case of admixing no water. Namely, while at least 0.1 to 0.2 mol % of the catalyst must be admixed in the case of admixing no water, a dense and homogeneous cBN sintered compact can be obtained with 0.01 mol % of the catalyst at the minimum by admixing 0.005 to 1.000 wt. % of water. Thus, a cBN sintered compact of high purity having an extremely small residual amount of the catalyst can be manufactured by reducing the catalyst content to the utmost to, e.g., 0.01 to 0.1 mol %. However, no generation of cBN was observed in case where the catalyst content was less than 0.01 mol % or no catalyst was admixed, even if 0.005 to 1.000 wt. % of water was admixed (refer to Example 3). Several to several ten percent of water must be admixed in order to convert hBN into cBN with the catalyst being prepared by water alone. In this case, a large amount of borate is generated and the cBN particle size is extremely small as hereinabove described, whereby a strong sintered compact having interparticle bonding cannot be obtained. The small amount of water is also adapted to suppress abnormal growth of cBN particles while prompting sintering reaction between the cBN particles. If sintering is performed with no admixture of water when a large amount of catalyst is admixed or the sintering temperature is too high, the sintered compact is made heterogeneous in structure with local deposition of coarse cBN particles of several ten µm in particle size. In this case, mechanical characteristics such as hardness and toughness are lowered. On the other hand, a homogeneous cBN sintered compact having regular particles can be obtained with excellent reproducibility by admixing 0.005 to 1.000 wt. % of water. In this case, further, the cBN particles are strongly bonded to each other as compared with the case of admixing no water, with less disadvantages such as cracking after sintering or in working and droppage of the particles. Thus, the mechanical characteristics of the cBN sintered compact can be extremely improved by admixing 0.005 to 1.000 wt. % of water. Heat conductivity is one of the important characteristics of a cBN sintered compact employed as a heat sink material. According to the present invention, a highly heat-conductive cBN sintered compact having heat conductivity of 6 to 7 W/cm· °C or more at the room temperature can be stably manufactured by admixing a constant amount of water to the material. Namely, a BN compact is made to carry 0.1 to 2.0 mol % of catalyst (alkaline earth metal boron nitride) and 0.05 to 0.3 wt. % of water is diffused in/admixed to the same to be processed at a temperature of 1400°C or more in a cBN-stable region, thereby to obtain a highly heat-conductive cBN sintered compact having heat conductivity of 6 W/cm· °C or more at the room temperature. The content of the catalyst is 0.1 to 2.0 mol % for the following reason: When the catalyst content is less than 0.1 mol %, the obtained cBN sintered compact is small in unit particle size (about 1 µm or less), whereby mean free paths for phonons are decreased to lower heat conductivity. The cBN unit particle size is increased and heat conductivity is improved as the catalyst content is increased. While a large amount of catalyst remains in cBN grain boundaries when the catalyst content is in excess of about 2.0 mol %, which residual catalyst serves as a phonon scattering source to lower the heat conductivity. Thus, the catalyst content must be within a range of 0.1 to 2.0 mol %, most appropriately of about 1.0 mol %, in order to manufacture a cBN sintered compact of high heat conductivity. In order to manufacture such a cBN sintered compact of high heat conductivity of 6 W/cm· °C or more, the water content must be within a range of 0.05 to 0.3 wt. % for the following reason: Fig. 4 shows the result of measurement of heat conductivity of various cBN sintered compacts prepared by admixing 1.0 mol % of magnesium boron nitride (Mg₃B₂N₄) to hBN compacts with various water contents (refer to Example 4). It is obvious from Fig. 4 that the cBN sintered compacts having water contents within a range of 0.05 to 0.3 wt. % show high heat conductivity values of 6 W/cm· °C or more. When the water content is small, the aforementioned effect of the water to the sintering reaction is so insufficient that the cBN particles are partially insufficiently bonded to each other, whereby the heat conductivity is lowered. When the water content is excessive to the contrary, a large amount of oxide, particularly MgO is deposited in cBN grain boundaries to serve as a phonon scattering source, leading to reduction in heat conductivity. Thus, the water content is preferably within a range of 0.05 to 0.3 wt. %, most appropriately of about 0.15 to 0.20 wt. % in order to manufacture a cBN sintered compact of high heat conductivity. Further, the sintering temperature must be of 1400°C or more for manufacturing a cBN sintered compact of high heat conductivity. When the sintering temperature is low, the cBN particle size is reduced to lower the heat conductivity. The cBN particle size is increased with increase in sintering temperature, to improve heat conductivity. Example 1 An hBN sintered compact (1.8 g/cm³ in bulk density) of high purity containing no binder was refined until the content of B₂O₃ was less than 0.03 wt. % by processing in high purity nitrogen gas at 2050°C for two hours. This hBN sintered compact was embedded in magnesium nitride powder to be heated in high purity nitrogen gas at 1165°C for 12 hours. After the heat treatment, the hBN sintered compact was subjected to chemical analysis and X-ray analysis, to find that Mg₃B₂N₄ was uniformly dispersed in the hBN sintered compact, and the amount thereof was 1.10 mol %. The hBN sintered compact containing Mg₃B₂N₄ thus obtained was placed on a water surface part of a closed vessel containing water and stood at a temperature of 25°C for 45 minutes, to adsorb a small amount of water. The amount of the adsorbed water was measured on the basis of the weight change of the sample, to find that 0.151 wt. % of water was adsorbed/diffused in the hBN sintered compact containing Mg₃B₂N₄. This sample was structured as shown in Fig. 3. Referring to Fig. 3, numeral 6 indicates a graphite heater, numeral 7 indicates an NaCl disk, numeral 8 indicates an NaCl cylinder, numeral 9 indicates a molybdenum vessel and numeral 10 indicates the sample. Then the sample was assembled in the girdle type superhigh pressure/temperature generating apparatus as shown in Fig. 1, to be subjected to superhigh pressure and temperature processing under a pressure of 5.0 GPa and a temperature of 1250°C for 30 minutes. Thus, obtained was a pale green translucent strong sintered compact, which was recognized as a cBN unit layer through X-ray analysis. The surface of this cBN sintered compact was observed by a scanning type electron microscope (SEM), to recognize that relatively regular cBN particles of 3 to 5 µm in particle size were densely bonded to each other. The cBN sintered compact had high heat conductivity and Vickers hardness of 6.1 W/cm· °C and 6000 to 6500 Kg/mm² at the room temperature. The density was 3.47 g/cm³, which was substantially equal to the theoretical value. For the purpose of comparison, Mg₃B₂N₄ was admixed in a similar content of 1.15 mol % to an hBN sintered compact similarly to the above, to be subjected to superhigh pressure and temperature processing under the same conditions as the above except for no admixture of water. This compact remained hBN with no generation of cBN. Example 2 Mg₃B₂N₄ and water were admixed to an hBN sintered compact under the same conditions as Example 1. The contents of Mg₃B₂N₄ and water were 1.14 mol % and 0.157 wt. % respectively. This sample as a parent material was subjected to superhigh pressure and temperature processing at a pressure of 5.5 GPa and a temperature of 1500°C for 30 minutes, similarly to Example 1. Thus obtained was an extremely dense cBN sintered compact consisting of only cBN particles of relatively regular particle sizes of 5 to 8 µm, of which density reached the theoretical value. Heat conductivity of this cBN sintered compact was 6.4 W/cm· °C and Vickers hardness was 6500 to 7500 Kg/mm². For the purpose of comparison, a cBN sintered compact was prepared under the same conditions as the above except for that no water was admixed. As the result, while a dense sintered compact of a cBN unit phase was obtained, a large amount of cracking was recognized. The structure of this compact was observed through the SEM, to recognize that the particles, being 1 to 10 µm in particle size, were rather irregular, while the cBN particles were bonded to each other. An uncracked part of the sample was cut to measure heat conductivity and hardness, which were in lower values of 4.5 W/cm· °C and 5000 to 6500 Kg/mm² as compared with Example 2. Example 3 Mg₃B₂N₄ and water were admixed to an hBN sintered compact similarly to Example 1. The processing time for admixing Mg₃B₂N₄ was reduced to 30 minutes. As the result, the content of Mg₃B₂N₄ was reduced to 0.06 mol %. It has been confirmed through EPMA that a small amount of the admixed Mg₃B₂N₄ was uniformly dispersed in the hBN sintered compact. 0.05 wt. % of water was admixed to the hBN sintered compact containing the small amount of Mg₃B₂N₄. The processing time for admixing water was 12 minutes. This sample as a parent material was subjected to superhigh pressure and temperature processing at a pressure of 5.5 GPa and a temperature of 1450°C for 30 minutes similarly to Example 1. As the result, a dense sintered compact substantially consisting of only cBN particles was obtained with extremely fine average unit particle size of 1 to 2 µm. No Mg₃B₂N₄ or MgO was detected through chemical analysis, and the cBN sintered compact was of extremely high purity. Although heat conductivity of 5.2 W/cm· °C was slightly low, this sintered compact, having high Vickers hardness of 7000 to 7500 Kg/mm² and high bending resistance of 100 to 120 Kg/mm², was excellent in mechanical characteristics. For the purpose of comparison, 0.06 mol % of Mg₃B₂N₄ was admixed to an hBN sintered compact to be subjected to superhigh pressure and temperature processing under the same conditions as the above except for no admixture of water, whereby the compact remained hBN with no generation of cBN. Further, only water was admixed to hBN sintered compacts with no admixture of Mg₃B₂N₄ to be subjected to superhigh pressure and temperature processing under the same conditions of 5.5 GPa and 1450°C for 30 minutes, whereby no cBN generation was recognized in any water contents of 0.05, 0.153 and 0.5 wt. %. Example 4 About 1.0 mol % of Mg₃B₂N₄ was admixed to nine hBN sintered compacts similarly to Example 1. Then, samples having different water contents were prepared by changing the processing times for admixing water. These nine samples as parent materials were subjected to superhigh pressure and temperature processing under a pressure of 5.0 GPa and a temperature of 1450°C for 30 minutes similarly to Example 1. Table 1 and Fig. 4 show the result of measurement of heat conductivity (W/cm· °C) and Vickers hardness (Kg/mm²) of the cBN sintered compacts thus obtained at the room temperature. No. Parent Material Heat Conductivity (w/cm·°C) Vickers hardness (kg/mm²) Mg₃B₂N₄ Content (mol %) Water Content (wt. %) 1 1.02 0.102 6.3 6000∼6500 2 1.05 0.142 6.2 6000∼7000 3 1.08 0.153 6.9 5500∼6000 4 0.98 0.182 7.3 5500∼6000 5 1.01 0.184 7.0 5500∼6000 6 1.02 0.192 7.8 5000∼6000 7 0.95 0.213 7.5 5500∼6000 8 1.05 0.293 6.1 4500∼5500 9 0.96 0.302 4.6 4000∼4500 10 1.02 0.400 4.3 4000∼4500 It is understood that the inventive cBN sintered compact shows high heat conductivity of 6 W/cm·°C or more with the water content about 0.3 wt. % or less, and particularly that containing 0.15 to 0.25 wt. % of water shows an extremely high heat conductivity value of 7 W/cm·°C. Hardness is decreased with increase in the water content, to less than 5000 Kg/mm² with the water content in excess of about 0.3 wt. %. It has been recognized through X-ray analysis that a slight amount of MgO was generated in the cBN sintered compact when the water content exceeded 0.3wt. %. Example 5 An hBN powder material of 5 to 10µm in particle size containing 0.2 wt. % of B₂O₃ was compacted by CIP (cold hydrostatic pressing method), to prepare an hBN compact. This hBN compact was refined in high purity nitrogen gas at 2050°C for two hours, to obtain a high purity hBN compact of 1.7 g/cm³ in bulk density containing less than 0.03 wt. % of B₂O₃. Mg₃B₂N₄ was admixed for 210 minutes and water was admixed for 30 minutes to the hBN compact similarly to Example 1, to obtain a parent material containing 1.15 mol % of Mg₃B₂N₄ and 0.18 wt. % of water. This parent material was subjected to superhigh pressure and temperature processing under the same conditions as Example 1, to obtain a cBN sintered compact similar to that of Example 1. The cBN sintered compact thus obtained was 6.2 W/cm· °C in heat conductivity and 6000 to 6500 Kg/mm² in Vickers hardness. Example 6 An hBN sintered compact identical to that in Example 1 was embedded in strontium nitride powder and then retained in nitrogen gas at 1150°C for 12 hours. The hBN sintered compact was subjected to chemical analysis and X-ray analysis, to recognize that 1.05 mol % of Sr₃B₂N₄ was uniformly dispersed in the hBN sintered compact. 0.11 wt. % of water was admixed to the hBN sintered compact in a method and conditions similar to those of Example 1. The sample thus obtained was structured as shown in Fig. 3, to be subjected to superhigh pressure and temperature processing in the superhigh pressure generating apparatus as shown in Fig. 1 at a pressure of 4.8 GPa and a temperature of 1200°C for 30 minutes. This sample was recognized to be a high purity cBN sintered compact through X-ray analysis and chemical analysis. The section of this sample was observed through an SEM, to recognize that particles were strongly bonded to each other to provide a dense sintered compact, which was 3.47 in density equally to the theoretical value of cBN. This sample was 6.2 W/cm· °C in heat conductivity and 5500 to 6500 Kg/mm² in Vickers hardness. Example 7 An hBN sintered compact carrying Ca₃B₂N₄ was prepared under the same conditions as Example 6. The content of Ca₃B₂N₄ was 1.10 mol %. Then water was adsorbed/diffused under the same conditions as Example 6. This hBN sintered compact carrying Ca₃B₂N₄ contained 0.12 wt. % of water, through measurement by weight change. This sample was subjected to superhigh pressure and temperature processing under conditions of 50 Kbar and 1250°C for 15 minutes. Thus obtained was a dense cBN sintered compact of high purity. This cBN sintered compact was 3.48 in density equally to the theoretical value, 6.0 W/cm· °C in heat conductivity and 6000 to 6500 Kg/mm² in Vickers hardness. Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.

Claims: A method of manufacturing a sintered compact of cubic boron nitride comprising the steps of making a compact of boron nitride carry alkaline earth metal boron nitride, making 0.005 to 1.000 percent by weight of water adsorbed/diffused in said boron nitride compact containing said alkaline earth metal boron nitride and performing high temperature and pressure processing under thermodynamically stable conditions for cubic boron nitride. A method of manufacturing a sintered compact of cubic boron nitride in accordance with claim 1, wherein said boron nitride compact is an embossed or sintered body of hexagonal boron nitride powder. A method of manufacturing a sintered compact of cubic boron nitride in accordance with claim 1, wherein the content of said alkaline earth metal boron nitride carried in said boron nitride compact is within a range of 0.01 to 5.00 mol % in case of expressing said alkaline earth metal boron nitride by Me₃B₂N₄ (Me: alkaline earth metal). A method of manufacturing a sintered compact of cubic boron nitride in accordance with claim l, wherein said high temperature and pressure processing is performed at a temperature of 1200°C or more. A method of manufacturing a sintered compact of cubic boron nitride in accordance with claim 1, wherein the amount of said alkaline earth metal boron nitride carried in said boron nitride compact is within a range of 0.1 to 2.0 mol % in case of expressing said alkaline earth metal boron nitride by Me₃B₂N₄ (Me: alkaline earth metal) and the amount of water to be adsorbed/diffused is within a range of 0.05 to 0.3 wt. %, while said high temperature and pressure processing is performed at a temperature of 1400°C or more. A method of manufacturing a sintered compact of cubic boron nitride in accordance with claim 1, wherein said water is adsorbed/diffused by a method of standing a boron nitride compact containing alkaline earth metal boron nitride in a closed vessel maintained in a constant temperature and humidity for a constant period.

Inventors: OTA, NOBUHIRO, SUMIYA, HITOSHI, YAZU, SHUJI
Applicants: SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1987
Main classification: C04B35
IPC codes: C04B35, B01J3