Polycrystalline diamond cutting tool and method of manufacturing the same

Abstract: A polycrystalline diamond cutting tool comprises a tool material (3) of polycrystalline diamond formed by low-pressure vapor deposition, which is bonded to a shank (4) of cemented carbide through a brazing layer (5). The thickness of the polycrystalline diamond layer (3) is set at 0.1 to 1.0 mm, while that of the brazing layer (5) is set at 10 to 50 µm. The brazing layer (5) is made of a material having a melting point of 950 to 1300°C, which is in the form of an alloy layer containing at least one material selected from metals belonging to the groups IVa, Va, VIa and VIIa of the periodic table and carbides thereof and at least one material selected from Au, Ag, Cu, Pt, Pd and Ni. The polycrystalline diamond cutting tool is improved in heat resistance and tool strength. In order to improve deposition resistance of the cutting tool, the surface roughness of a tool rake face (10) is set to be not more than 0.2 µm in Rmax. A portion of the polycrystalline diamond layer (3) up to a depth of 10 µm from the rake face (10) contains 99 to 100 atomic percent of carbon elements, and 99 to 100 % of carbon atoms are diamond-bonded. A surface of the polycrystalline diamond layer which has been in contact with the substrate during formation of the polycrystalline diamond layer defines the rake face (10), whose surface is subjected to ion beam machining and thereafter treated in the atmosphere at a temperature of 300 to 500°C.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a bonding structure of a polycrystalline diamond cutting tool which has high strength and excellent heat resistance, and a method of manufacturing the same. It also relates to a polycrystalline diamond cutting tool which has excellent deposition resistance and a method of manufacturing the same. Description of the Background Art Diamond, which is excellent in hardness and thermal conductivity, exhibits high performance upon application to a cutting tool or a wear resisting tool, and is employed for various purposes. For example, Japanese Patent Publication No. 52-12126 (1977) describes a diamond cutting tool employing a diamond sintered body, which is prepared by sintering fine grains of diamond with a ferrous metal binder in order to suppress a defect caused by cleavage, being a disadvantage of monocrystalline diamond, in particular. However, such a tool employing a diamond sintered body has a problem of low heat resistance. In more concrete terms, the tool may be deteriorated in wear resistance and strength when the temperature of its cutting edge is increased, to lose practicability. For example, the diamond sintered body is reduced in wear resistance and strength when the same is heated to a temperature exceeding 750°C, and broken when the same is further heated to a temperature exceeding 900°C. This is conceivably because graphitization of diamond is caused at interfaces between the diamond grains and the ferrous metal binder, and high thermal stress is generated on the basis of difference in thermal expansion coefficient between the same under a high temperature. In order to improve heat resistance, there has been developed another tool employing a diamond sintered body, as described in Japanese Patent Laying-Open No. 53-114589 (1978), for example. According to this prior art, acid treatment is performed on the sintered body to remove most portions of a binder metal layer. In this technique, however, the tool is remarkably reduced in strength due to holes defined by the removed portions of the binder metal layer, although heat resistance is improved. Japanese Patent Laying-Open No. 59-161268 (1984) or 61-33865 (1986), for example, discloses a tool employing a heat-resistant diamond sintered body containing no holes, in order to solve the aforementioned problem. Such a heat-resistant diamond sintered body contains a binder of Si, SiC or an Ni-Si alloy. However, this sintered body is insufficient in wear resistance due to a small degree of cohesion between diamond grains and a large content of the binder. Thus, no satisfactory performance has generally been attained in relation to wear resistance and strength of a diamond tool, in spite of improvement in heat resistance. Under such circumstances, there has been developed a cutting tool employing polycrystalline diamond which is synthesized by low-pressure vapor deposition to contain no binder, as described in Japanese Patent Laying-Open No. 1-212767 (1989), for example, in order to solve problems in a diamond tool employing a diamond sintered body. In a mode of a cutting tool employing a tool material of polycrystalline diamond, a polycrystalline diamond layer is bonded to a tool holder so that a cutting edge is formed on this polycrystalline diamond layer. In relation to such a cutting tool which is prepared by brazing a tool material of polycrystalline diamond to a tool holder, however, the inventors have found that not only the performance of the tool material itself but that of a brazing layer for bonding the same to the tool holder serve as factors controlling the performance of the tool. They have also found that the heat resistance and the thickness of the brazing layer remarkably influence the tool performance in addition to the thickness of the polycrystalline diamond layer, particularly when the polycrystalline diamond layer is not more than 0.5 mm in thickness. In other words, strength of the tool is lowered as the thickness of the polycrystalline diamond layer is reduced, although the polycrystalline diamond layer is preferably minimized in thickness in view of the working cost. Even if the thickness of the polycrystalline diamond layer exerts no influence on the tool strength, further, the brazing layer may be remarkably deformed particularly when the cutting edge is heated to a high temperature if the brazing layer is inferior in heat resistance or excessive in thickness, to reduce the strength of the overall tool. On the other hand, polycrystalline diamond which is synthesized by low-pressure vapor deposition is excellent in strength, wear resistance and heat resistance since the same is substantially formed of only diamond, and is regarded as an ideal material for a diamond tool. In a cutting tool prepared from a polycrystalline diamond material which is synthesized by low-pressure vapor deposition, however, a workpiece may be deposited depending on cutting conditions, leading to a problem of insufficient work surface roughness. The document JP-A-1212767 discloses a highly wear-resistant polycrystalline diamond tool and its production. The European patent application EP-A-319 926 discloses a method for the preparation of a diamond-clad machining tool including the steps of forming a film of diamond having a thickness of 10 - 1000 µ m by chemical vapor phase deposition on the surface of a temporary substrate, the step of brazing the film of diamond on the substrate to the surface of a base body of a tool and the step of removing away a temporary substrate leaving the film of diamond based on the surface of the base body of the tool. The European patent application EP-A-449571 discloses a polycrystalline diamond tool and a method for producing the polycrystalline diamond tool, wherein the diamond near the rake face is of better quality than the diamond near the fixation surface. The European patent application EP-A-0480394 teaches a cutting tool of polycrystalline diamond and a method of manufacturing the same, whereby the quality of the diamond near the rake face is better than the quality of diamond near the fixation surface in the way that the density of the non-diamond ingredients increases along the direction of thickness from the rake surface to the fixation surface. However, the above mentioned four documents do not disclose a product or a method of producing the same which has the feature of that the grain sizes of the polycrystalline diamond layer being increased from a side of a rake face toward a brazing surface are at least 0.01 µ m and not more than 1 µ m on the side of the rake face and at least 5 % and not more than 15 % of the thickness of the polycrystalline diamond layer on the side of the brazing surface. Summary of the Invention An object of the present invention is to provide a polycrystalline diamond cutting tool which is superior in heat resistance to and higher in strength than a conventional tool, and a method of manufacturing the same. Another object of the present invention is to provide a polycrystalline diamond cutting tool which is superior in deposition resistance to a conventional tool, and a method of manufacturing the same. A polycrystalline diamond cutting tool according to the present invention as defined in claim 1 comprises a tool material, formed by a polycrystalline diamond layer being synthesized on a substrate by low-pressure vapor deposition, being bonded to a tool holder, through a brazing layer, said polycrystalline diamond layer being at least 0.1 mm and not more than 1.0 mm in thickness; wherein said brazing layer being made of a brazing filler metal having a melting point of at least 950°C and not more than 1300°C, wherein said brazing layer contains at least one element selected from metals belonging to the groups IVa, Va, VIa and VIIa of the IUPAC form of the periodic table and carbides of said metals, and at least one metal selected from Au, Ag, Cu, Pt, Pd and Ni; wherein grain sizes of said polycrystalline diamond layer, being increased from the side of a rake face toward a brazing surface, are at least 0.01 µm and not more than 1 µm on the side of said rake face and at least 5 % and not more than 15 % of the thickness of said polycrystalline diamond layer on the side of said brazing surface; and wherein a portion of said polycrystalline diamond layer up to a depth of 10 µm from said tool rake face being composed of at least 99 atomic percent of carbon and not more than 1 atomic percent of elements other than carbon, at least 99% of as-contained carbon atoms being diamond bonded. A method of manufacturing a polycrystalline diamond cutting tool according to the present invention as defined in claim 6 has the following steps: a step of forming a polycrystalline diamond layer on a substrate by low-pressure vapor deposition; a step of removing said substrate from said polycrystalline diamond layer; a step of cutting said polycrystalline diamond layer into a prescribed chip shape for forming a tool material; and a step of interposing a brazing filler metal of an alloy having a melting point of at least 950°C and not more than 1300°C and containing at least one metal selected from those belonging to the groups IVa, Va, VIa and VIIa of the IUPAC form of the periodic table and carbides thereof and at least one metal selected from Au, Ag, Cu, Pt, Pd and Ni and heating and melting the same, thereby bonding said tool material to said tool holder; wherein said step of forming a polycrystalline diamond layer is carried out on a surface of said substrate having surface roughness of not more than 0.2 µm in Rmax so that the mean grain size of said polycrystalline diamond is increased along the direction of crystal growth to be at least 0.01 µm and not more than 1 µm on the side of said substrate and at least 5 % and not more than 15 % of the thickness of said polycrystalline diamond layer on the side of an upper growth surface, said polycrystalline diamond layer being brazed to said tool holder so that a surface having been in contact with said substrate defines a tool rake face; and wherein said method further comprises a step of performing ion beam machining on a surface of said polycrystalline diamond layer having been in contact with said substrate during formation of said polycrystalline diamond layer, heating said polycrystalline diamond layer in the atmosphere at a temperature in a range of at least 300°C and not more than 500°C and thereafter forming a tool material of a prescribed shape from said polycrystalline diamond layer; a step of bonding said tool material to a tool holder so that a surface of said tool material being subjected to ion beam machining defines a tool rake face; and a step of forming a cutting edge on said tool material while maintaining the surface state of said tool rake face. From the results of various investigations, the inventors have found that the characteristics of the polycrystalline diamond layer itself and those of the brazing filler metal serve as factors influencing and controlling the strength and heat resistance of the polycrystalline diamond cutting tool. Consequently, it has been proved that the thickness of the polycrystalline diamond layer forming the tool material is preferably in a range of 0.1 to 1.0 mm. A large thickness is unpreferable in view of working efficiency of the tool material. If the thickness is smaller than 0.1 mm, the strength of the tool material is reduced. On the other hand, influential characteristics of the brazing filler metal are the material and the thickness thereof. According to the present invention, the brazing filler metal, having a melting point of 950 to 1300°C, still exhibits sufficient heat resistance under high temperature cutting conditions. When the thickness of the brazing filler metal is preferably in a range of 10 to 50 µm, further, it is possible to suppress reduction in tool strength caused by deformation of the brazing filler metal. In order to attain the object of improving deposition resistance, the inventors have analyzed the causes for deposition of workpieces which are cut with the polycrystalline diamond tool disclosed in Japanese Patent Laying-Open No. 1-212767 (1989). As the result, it has been clarified that workpieces of an aluminum alloy or the like are deposited when the tool contains impurities, such as the substrate material and carbides of elements forming the substrate material contained in a tool rake face, i.e., a surface of the polycrystalline diamond layer having been in contact with the substrate during synthesis, and non-diamond carbon which is co-grown in an initial stage of film formation by low-pressure vapor deposition in excess of constant amounts. It has also been proved that physical surface roughness of the tool rake face also influences the deposition resistance. In order to remove such factors, a diamond grindstone or the like may be employed to grind the tool rake face, thereby removing the impurities, other than diamond, from portions close to the tool rake face. In this method, however, a load is applied to the polycrystalline diamond layer by the grinding operation, to crack the polycrystalline diamond substance during the operation. In this method, further, it is difficult to control the thickness of a portion removed from the polycrystalline diamond layer. When the thickness of the removed portion is too large, therefore, the tool rake face is defined by a portion containing coarse polycrystalline diamond crystal grains, to reduce strength of the polycrystalline diamond tool. According to this method, in addition, the manufacturing cost is increased in consideration of wearout of the high-priced diamond grindstone which is employed for the grinding operation. In consideration of the aforementioned points, the present invention has been proposed on the basis of such discovery that it is possible to implement a polycrystalline diamond cutting tool which is excellent in deposition resistance when impurities other than diamond are accurately removed from a portion of a polycrystalline diamond layer by a desired thickness with no damage to the polycrystalline diamond layer by combining ion beam machining and heat treatment. In the polycrystalline diamond cutting tool according to the present invention, the surface roughness of the tool rake face is suppressed below a constant value, while the crystal grain sizes of the polycrystalline diamond layer are restricted in a constant range on the tool rake face side and on the side of a surface bonded to the tool holder. Further, a portion forming the polycrystalline diamond layer up to a depth of 10 µm from the tool rake face contains carbon elements of extremely high purity, while most of the contained carbon atoms are diamond-bonded. Thus, the tool rake face is formed by a polycrystalline diamond layer containing an extremely suppressed amount of impurities other than carbon. Therefore, deposition of workpieces is effectively prevented during cutting. Since the physical surface roughness of the tool rake face is suppressed below a constant value, deposition resistance is not reduced. If the surface roughness of the tool rake face exceeds 0.2 µm in maximum height Rmax, workpieces are unpreferably remarkably deposited. If the polycrystalline diamond layer has crystal grain sizes of less than 0.01 µm on the tool rake face side, wear resistance of the tool is unpreferably reduced. If the crystal grain sizes exceed 1 µm, on the other hand, strength of the tool is unpreferably reduced. The lower limit of the crystal grain sizes of the polycrystalline diamond layer is defined as 5 % of the thickness of the polycrystalline diamond layer on the side of the surface bonded to the tool holder in consideration of wear resistance, while strength of the tool is unpreferably reduced if the crystal grain sizes exceed 15 % of the thickness of the polycrystalline diamond layer. If the amount of carbon elements contained in the portion of the polycrystalline diamond layer up to a depth of 10 µm from the tool rake face is less than 99 atomic percent, deposition of workpieces is unpreferably caused. If the rate of diamond bonding of the as-contained carbon atoms is less than 99 %, deposition of workpieces is unpreferably caused. In the inventive method of manufacturing a polycrystalline diamond cutting tool, ion beam machining is performed on the surface of the polycrystalline diamond layer forming the tool rake face, and thereafter the polycrystalline diamond layer is heat treated in the atmosphere at a temperature in a range of at least 300°C and not more than 500°C. The term "ion beam machining" indicates a method of ionizing a raw material gas which is introduced in a vacuum apparatus by discharge, and thereafter field-accelerating only positive charge ions contained in the ionized gas and applying the same to a target, thereby removing a part of the target by etching. When such ion beam machining is performed in a proper state, it is possible to remove impurities other than carbon, such as a substrate material and carbides of elements forming the substrate material contained in the portion of the polycrystalline diamond layer forming the tool rake face. However, this state unpreferably results in a residual of non-diamond carbon components which are co-grown during the ion beam machining. In order to remove such non-diamond carbon components, it is necessary to perform heat treatment in the atmosphere after the ion beam machining. Such heat treatment must be performed in a temperature range of at least 300°C and not more than 500°C. If the heating temperature is lower than 300°C, it is difficult to efficiently remove the non-diamond carbon components. If the heat treatment is performed at a higher temperature exceeding 500°C, on the other hand, the polycrystalline diamond layer itself is unpreferably damaged by the heat. The ion beam machining and the heat treatment in the atmosphere may be performed after the tool material is formed by the polycrystalline diamond layer and bonded to the tool holder in advance of working of a cutting edge, or in a final step after working of the cutting edge. In any case, the ion beam machining and the heat treatment in the atmosphere may be performed on the surface of the polycrystalline diamond layer, which has been in contact with the substrate during synthesis of the polycrystalline diamond layer, after the substrate is removed and the polycrystalline diamond layer is recovered. While the ion source for the ion beam machining may be prepared from any raw material, oxygen is preferably employed in the present invention. This is because activating reaction etching which is accompanied by oxidizing reaction is effective for efficiently removing impurities other than carbon in order to obtain a polycrystalline diamond layer which is substantially made of only diamond. While a raw material such as argon or nitrogen is used for simple etching, it is possible to implement high-speed etching accompanied by oxidizing reaction of diamond when the ion source is prepared from oxygen. The portion of the polycrystalline diamond layer removed by the ion beam machining is preferably at least 0.02 µm and not more than 2 µm in thickness from the surface which has been in contact with the substrate during the synthesis. If the thickness of the removed portion is less than 0.02 µm, impurities causing deposition are incompletely removed. While it is possible to remove a portion of the polycrystalline diamond layer in excess of 2 µm in thickness, on the other hand, this leads to increase in working cost since the impurities causing deposition are not contained in a portion exceeding 2 µm in thickness from the surface which has been in contact with the substrate during synthesis. If an excessive portion of the polycrystalline diamond layer is removed, coarse diamond crystal grains exceeding 1 µm in crystal grain size are exposed on the tool rake face, to unpreferably reduce strength of the cutting edge. In manufacturing of the inventive polycrystalline diamond cutting tool, the polycrystalline diamond layer can be synthesized by any well-known low-pressure vapor deposition. In other words, a method of decomposing and exciting a raw material gas through thermoionic emission or plasma discharge, or a film forming method employing a combustion flame can be applied. The raw material gas is generally prepared from a mixed gas which is mainly composed of an organic carbon compound of hydrocarbon such as methane, ethane or propane, alcohol such as methanol or ethanol, or ester, and hydrogen. In addition, the raw material may contain inert gas such as argon, oxygen, carbon monoxide, water and the like in ranges neither inhibiting the synthesis of diamond nor deteriorating its characteristics. The polycrystalline diamond layer preferably has a thickness of at least 0.1 mm and not more than 1.0 mm. If the thickness is smaller than 0.1 mm, strength of the polycrystalline diamond layer itself is reduced. A diamond tool which is employed for ordinary finishing may have a sufficient material thickness exceeding the width of flank wear land (not more than 0.1 mm) at the time of the tool life, i.e., 0.1 to 0.2 mm, for example. The aforementioned lower limit of the thickness of 0.1 mm is set in consideration of this. If the thickness of the polycrystalline diamond layer exceeds 1.0 mm, the working cost for manufacturing the tool is unpreferably increased. However, a polycrystalline diamond layer having a thickness exceeding 1.0 mm has no problem in performance. The substrate to be provided with the polycrystalline diamond layer preferably has a thermal expansion coefficient which is close to that of diamond, in order to reduce internal stress in the polycrystalline diamond layer. According to the present invention, the substrate is melted and removed by chemical treatment through hydrochloric acid, sulfuric acid, nitric acid, hydrofluoric acid and/or a mixed solution thereof so that only the polycrystalline diamond layer is recovered as a tool material. Thus, the substrate is preferably prepared from a material containing elements such as Mo, W, Si and the like, to be melted and removed. The surface of the substrate is finished as a mirror finished surface having surface roughness of not more than 0.2 µm in maximum height Rmax, since the surface roughness of the substrate influences that of the finally obtained tool rake face. Namely, the surface roughness of the substrate is transferred to the surface of the polycrystalline diamond layer which is in contact therewith during synthesis. According to the present invention, ion beam machining is performed on the surface of the polycrystalline diamond layer, in order to improve the surface roughness itself. If the surface roughness is in excess of 0.2 µm in Rmax before the ion beam machining, the finally obtained tool rake face cannot attain stable surface roughness of not more than 0.2 µm in Rmax. The tool material formed by the polycrystalline diamond layer can be bonded to the tool holder by any method. According to the present invention, the tool material is preferably simply bonded to the tool holder with interposition of a brazing filler metal having a melting point in a range of at least 950°C and not more than 1300°C. If the melting point of the brazing filler metal is lower than 950°C, the brazing filler metal is fluidized due to influence by cutting heat depending on cutting conditions, to disadvantageously cause a defect. If the melting point of the brazing filler metal is higher than 1300°C, on the other hand, the polycrystalline diamond layer is thermally damaged when the same is bonded to the tool holder, to unpreferably reduce wear resistance and chipping resistance. The brazing filler metal optimumly contains at least one of metals belonging to the groups IVa, Va, VIa and VIIa of the periodic table and at least one of Au, Ag, Cu, Pt, Pd and Ni. While the cutting edge can be worked by grinding which is applied to a conventional diamond sintered body, it is possible to form a sharper cutting edge with smaller chipping by laser beam machining. A cutting tool provided with a laser-beam machined cutting edge having chipping of 0.5 to 5 µm is particularly superior in sharpness of the cutting edge to a conventional diamond sintered tool and a tool provided with a ground edge to which a tool material of vapor-deposited diamond is brazed. Formation of a cutting edge with laser beam machining is disclosed in Japanese Patent Application No. 2-271011 (1990) or 2-326277 (1990) (Japanese Patent Laying-Open No. 4-146007 (1992) or 4-193406 (1992)) by the inventors. In the polycrystalline diamond cutting tool according to the present invention, as hereinabove described, the brazing layer is prepared from a heat resistant material and the thickness of the polycrystalline diamond layer is set in a prescribed range, whereby it is possible to implement a polycrystalline diamond cutting tool which is superior in deposition resistance to a conventional tool. Thus, it is possible to implement a cutting tool which is particularly suitable for application requiring excellent work surface roughness. The foregoing 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 Figs. 1 to 5 are perspective views and partially fragmented sectional views successively showing steps in a method of manufacturing a polycrystalline diamond cutting tool according to the present invention; and Fig. 6 is a partially fragmented sectional view showing a principal step in another method of manufacturing a polycrystalline diamond cutting tool according to the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of a polycrystalline diamond cutting tool according to the present invention are now described. The polycrystalline diamond cutting tool according to the present invention is schematically described along manufacturing steps thereof. Referring to Fig. 1, a polycrystalline diamond layer 2 is formed on a surface of a substrate 1 which is made of a metal or an alloy, by low-pressure vapor deposition. The surface of the substrate 1 is finished in a mirror finished surface having surface roughness of not more than 0.2 µm in Rmax. The low-pressure vapor deposition is applied to deposit the polycrystalline diamond layer 2, which is substantially made of only diamond in such a sectional structure that the mean grain size of diamond is 0.01 to 1 µm on the substrate side in an initial stage of growth and increased to 5 to 15 % of the thickness of the diamond layer on the side of a growth surface, i.e., a growth completion surface, on the mirror finished surface of the substrate 1, to be 0.1 to 1.0 mm in thickness. The grain sizes of the polycrystalline diamond are defined in the aforementioned range, in order to prevent a defect. Then, cutting lines are-formed in the polycrystalline diamond layer 2, which is provided on the substrate 1, with a laser beam 12 along prescribed tool material shapes, as shown in Fig. 2. Then, chemical treatment is performed through hydrochloric acid, sulfuric acid, nitric acid, hydrofluoric acid or a mixed solution thereof to melt the substrate 1, thereby removing the same from the polycrystalline diamond layer 2, as shown in Fig. 3. The as-obtained polycrystalline diamond tool materials 3 are 0.1 to 1.0 mm in thickness along the direction of lamination. Then, each polycrystalline diamond tool material 3 is bonded to a tool holder 4 through a brazing filler metal 5, as shown in Fig. 4. In order to bond the polycrystalline diamond tool material 3 to the tool holder 4, it is important to arrange the same so that a surface which has been in contact with the substrate 1 during synthesis of the polycrystalline diamond layer 2 defines a rake face 10 of the tool. In advance of the synthesis of the polycrystalline diamond layer 2, the surface of the substrate 1 is finished in a mirror finished surface having surface roughness of not more than 0.2 µm in Rmax. Thus, the polycrystalline diamond layer 2 deposited on the mirror finished surface of the substrate 1 has a smooth surface which is similar to the substrate surface, as a transfer surface. When the tool material 3 is so arranged that this surface defines the rake face 10 of the tool and brazed to the tool holder 4, therefore, it is possible to obtain a smooth tool rake face without working the surface of the polycrystalline diamond tool material 3, which is regarded as being hard to work. As described above, further, the crystal grain sizes of the polycrystalline diamond layer 2 are smaller on the substrate side and increased toward an upper growth surface 31. Thus, it is possible to form a tool which is excellent in chipping resistance by defining the tool rake face 10 by the fine-grain diamond surface provided on the substrate side. The brazing filler metal 5 is prepared from an alloy containing 0.2 to 50 volume percent of at least one metal selected from those belonging to the groups IVa, Va, VIa and VIIa of the periodic table and the residue of at least one material selected from Au, Ag, Cu, Pt, Pd and Ni. The brazing filler metal 5 of such an alloy has a melting point of 800 to 1300°C, preferably 950 to 1300°C. This brazing filler metal 5 is interposed between the tool material 3 and a receiving surface 41 of the tool holder 4 and heated to a high temperature to be melted, and thereafter cooled to bond the tool material 3 to the tool holder 4. The composition of the brazing filler metal 5 is defined as 0.2 to 50 volume percent of at least one metal selected from those belonging to the groups IVa, Va, VIa and VIIa of the periodic table, since these metals or carbides thereof react with diamond to be bonded to the same. Among these metals, Ti, W, Ta and Zr or carbides thereof are particularly excellent in bond strength. The melting point of the brazing filler metal 5 is limited since the same may be softened and fluidized depending on cutting conditions if the melting point is below the aforementioned lower limit, while the polycrystalline diamond layer 2 is deteriorated if the melting point is higher than the upper limit. Thereafter a cutting edge 8 is worked by grinding or laser beam machining, as shown in Fig. 5. When the cutting edge 8 is formed by laser beam machining, it is possible to suppress chipping to 0.5 to 5 µm. A polycrystalline diamond cutting tool is manufactured through the aforementioned steps. Fig. 6 shows another embodiment of a method of bonding a tool material 3 to a tool holder 4. First, a covering layer 6 is formed on a polycrystalline diamond upper growth surface 31 of the tool material 3 in a thickness of about 0.1 to 5 µm. This covering layer 6 is formed by ion plating, sputtering or vapor deposition, for example The covering layer 6 is prepared from at least one element selected from metals belonging to the groups IVa, Va, VIa and VIIa of the periodic table or carbides thereof. The thickness of the covering layer 6 is defined in the range of 0.1 to 5 µm, since no effect of preventing oxidation is attained if the thickness is less than 0.1 µm. If the thickness exceeds 5 µm, on the other hand, the cost is unpreferably increased. After the covering layer 6 is formed on the tool material 3, a material 7 of at least one element selected from Au, Ag, Cu, Pt, Pd and Ni is arranged on a bonding surface 41 of the tool holder 4. Then the tool material 3 and the tool holder 4 are arranged on bonding positions and heated, to melt a brazing filler metal. Thus, the tool material 3 is bonded to the tool holder 4 through the brazing filler metal. When the covering layer 6 is prepared from at least one material selected from Ti, W, Ta and Zr and carbides thereof, bond strength is particularly improved. Example 1 A diamond polycrystalline substance was synthesized on an Si substrate having a mirror finished surface of 0.08 µm in Rmax by microwave plasma CVD for 10 hours under the following conditions: Raw Material Gas (Flow Rate) H 2 300 sccm CH 4 20 sccm Gas Pressure (120 Torr) 15987,6 Pa. Microwave Oscillation Output 750 W After the synthesis, the substance was dipped in nitric-hydrofluoric acid to melt/remove only the Si substrate, whereby it was possible to recover the diamond polycrystalline substance of 0.2 mm in thickness. The diamond polycrystalline substance was in such a sectional structure that the mean grain size of diamond was 0.2 µm on a rake face side and increased to 20 µm on a brazing surface side. The surface which had been in contact with the substrate had surface roughness of 0.08 µm in Rmax. This diamond polycrystalline substance was brazed to a shank of cemented carbide with a heat resistant brazing filler metal containing 6 volume percent, 95 volume percent and 4 volume percent of Ag, Cu and Ti respectively. In this brazing step, a growth surface of the tool material was regarded as a bonding surface, while the brazing filler metal was heated to its melting point of 1050°C in a vacuum of 5 x 10 -5 Torr for 10 minutes. The as-recovered substance had a brazing layer of 30 µm in thickness. Only a flank was worked with a diamond grindstone of #1500 to be provided with a cutting edge, thereby preparing a sample A of a cutting tool. For the purpose of comparison, a diamond polycrystalline substance similar to the above was bonded to a shank of cemented carbide with a brazing filler metal containing 30 volume percent and 70 volume percent of Ag and Cu respectively and having a melting point of 700°C, to prepare a comparative sample B. This sample B had a brazing layer of 40 µm in thickness. On the other hand, sintered diamond of 5 µm in grain size containing 12 volume percent of Co as a binder was surface-finished into 0.09 µm in Rmax to prepare a tool material, which was bonded to a shank of cemented carbide with the same brazing filler metal as that for the sample B, thereby preparing another comparative sample C. The comparative samples B and C were also provided with cutting edges through grinding with the same diamond grindstone as that for the sample A. These samples A, B and C were applied to throw-away tipped tools, and subjected to performance evaluation under the following conditions: Cuttinq Conditions Workpieces round bar of A390-T6 (Aℓ-17%Si) Cutting Speed 800 m/min. Depth of Cut 1.5 mm Feed Rate 0.2 mm/rev. Cooling Solution water-soluble oil solution Evaluation Method Comparison of cutting edge states after cutting for 5 minutes and 60 minutes As the result, it was clarified that a sharp cutting edge was maintained in the inventive tool for a longer time as compared with the sintered diamond tool, as shown in Table 1. It was also clarified that the tool employing no heat resistant brazing filler metal caused a large defect after outflow of the brazing filler metal, leading to critical damage of the tool. Sample Width of Flank Wear Land (Im) After Cutting for 5 Minutes After Cutting for 60 Minutes Inventive Sample A 21 67 Comparative Sample B Brazing Filler Metal Flown Out Chipped - Comparative Sample C (Sintered Diamond) 28 123 Example 2 A linear tungsten filament of 0.5 mm in diameter and 100 mm in length was employed as a thermoionic emission material, to synthesize diamond on a surface of an Mo substrate having surface roughness of 0.03 Ìm in Rmax by thermal CVD for 20 hours, under the following conditions: Raw Material Gas (Flow Rate) H 2 500 sccm C 2 H 2 10 sccm Gas Pressure (70 Torr) 9326,1 Pa. Filament Temperature 2200°C Filament-to-Substrate Distance 6 mm Substrate Temperature 900°C After the synthesis, the substance was dipped in hot aqua regia so that only the Mo substrate was melted and removed, whereby it was possible to recover a diamond polycrystalline substance of 0.3 mm in thickness. This diamond polycrystalline substance was in such a sectional structure that the mean grain size of diamond was 0.1 µm on a rake face side and increased to 25 µm on a brazing surface side. The surface which had been in contact with the substrate had surface roughness of 0.03 µm in Rmax. This diamond polycrystalline substance was brazed to a shank of cemented carbide with a heat resistant brazing filler metal containing 35 volume percent of Au, 60 volume percent of Cu and 5 volume percent of Ta respectively. In this brazing step, a growth surface of the tool material was regarded as a bonding surface, while the brazing filler metal was heated to its melting point of 1100°C in a vacuum of 0,0053 Pa (4 x 10 -5 Torr) for 5 minutes. The as-recovered substance had a brazing layer of 40 µm in thickness. Thereafter a cutting edge was formed with a YAG laser, to prepare a sample D. For the purpose of comparison, a tool material and a brazing filler metal similar to those for the sample D were employed for bonding, to prepare a comparative sample E having a brazing layer of 100 µm in thickness. Further, sintered diamond of 10 µm in grain size containing 10 volume percent of Co as a binder was worked into a mirror finished surface of 0.06 µm in Rmax to form a tool material, which in turn was bonded to a shank of cemented carbide with a brazing filler metal containing 10 volume percent, 60 volume percent and 30 volume percent of Pd, Ag and Cu respectively and having a melting point of 800°C, to prepare another comparative sample F. The samples E and F were provided with cutting edges by grinding with a diamond grindstone of #1500. These samples were applied to throw-away tipped tools, and subjected to performance evaluation under the following conditions: Cutting Conditions Workpiece hard carbon Cutting Speed 1000 m/min. Depth of Cut 1.5 mm Feed Rate 0.2 mm/rev. Cutting Time 40 min. Cooling Solution water soluble oil solution Evaluation Method Comparison of cutting edge states after cutting for 5 minutes and 40 minutes As the result, it was clarified that the inventive tool maintained a sharp cutting edge for a long time as compared with the sintered diamond tool, as shown in Table 2. It was also clarified that the diamond polycrystalline substance was chipped in the sample E employing the brazing filler metal of the same composition as that of the inventive sample, due to extreme displacement caused by the thickness of the brazing layer. Sample Chipping Before Cutting (µm) Width of Flank Wear Land (µm) After Cutting for 5 Minutes After Cutting for 40 Minutes Inventive Sample D 1 23 91 Comparative Sample E 12 24 Chipped Comparative Sample F 20 27 125 Example 3 A linear tantalum filament of 0.3 mm in diameter and 120 mm in length was employed as a thermoionic emission material, to synthesize polycrystalline diamond on a surface of an Si substrate having surface roughness of 0.12 µm in Rmax by thermal CVD for 30 hours under the following conditions: Raw Material Gas (Flow Rate) H 2 400 sccm CH 4 10 sccm Gas Pressure (100 Torr) 13323 Pa. Filament Temperature 2100°C Filament-to-Substrate Temperature 5 mm Substrate Temperature 950°C After the synthesis, the substance was dipped in heated nitric-hydrofluoric acid so that only the Si substrate was melted and removed, whereby it was possible to recover a diamond polycrystalline substance of 0.4 mm in thickness. The diamond polycrystalline substance was in such a sectional structure that the mean grain size of diamond was 0.4 µm on a rake face side and increased to 30 µm on a brazing surface side. The surface which had been in contact with the substrate was 0.12 µm in Rmax. An upper growth surface of this diamond polycrystalline substance was covered with a Ti layer of 2 µm in thickness, which in turn was further covered with an oxidation inhibiting film of Ni of 3 µm in thickness. The metal-coated diamond polycrystalline substance was brazed to a shank of cemented carbide with a brazing filler metal of Pt, Pd and Ni. In this brazing step, the growth surface of the tool material was regarded as a bonding surface, while the brazing filler metal was heated to its melting point of 980°C in the atmosphere for 2 minutes. The as-recovered substance had a brazing layer of 30 µm in thickness. Thereafter a cutting edge was formed with a YAG laser, to prepare a sample G. For the purpose of comparison, a metal-coated tool material similar to that in the sample G was bonded to a shank of cemented carbide with a brazing filler metal of Ag and Cu having a melting point of 700°C, to prepare a comparative sample H. On the other hand, sintered diamond of 3 µm in grain size containing 12 volume percent of Co as a binder was worked into a mirror finished surface of 0.12 µm in Rmax to form a tool material, which in turn was bonded to a shank of cemented carbide with a brazing filler metal containing 10 volume percent, 60 volume percent and 30 volume percent of Pd, Ag and Cu respectively and having a melting point of 800°C, to prepare another comparative sample I. The samples H and I were provided with cutting edges by grinding with a diamond grindstone of #1500. These samples were applied to throw-away tipped tools and subjected to performance-evaluation under the following conditions: Cutting Conditions Workpiece round bar of Aℓ-25%Si Cutting Speed 1000 m/min. Depth of Cut 1.2 mm Feed Rate 0.15 mm/rev. Cutting Time 30 min. Cooling Solution none (dry type) Evaluation Method Comparison of cutting edge states after cutting for 10 minutes and 30 minutes As the result, it was clarified that the inventive tool maintained a sharp cutting edge for a longer time as compared with the sintered diamond tool, as shown in Table 3. It was also clarified that the tool prepared with no heat resistant brazing filler metal caused a large defect after outflow of the brazing filler metal, leading to critical damage of the tool. Sample Chipping Before Cutting (µm) Width of Flank Wear Land (µm) After Cutting for 10 Minutes After Cutting for 30 Minutes Inventive Sample G 2 22 59 Comparative Sample H 10 24 Chipped Comparative Sample I 15 30 125 Example 4 Under conditions shown in Table 4, a gas obtained by mixing H 2 , C 2 H 6 and Ar in the ratios of 7:2:1 was supplied into reaction tubes in which W substrates having mirror finished surface of 0.05 µm in Rmax were placed, at a flow rate of 500 sccm, to adjust pressures to 15987,6 Pa (120 Torr). Then, a high frequency of 13.56 MHz was supplied from a high frequency oscillator at an output of 700 W to excite the mixed gas and generate plasma, thereby synthesizing diamond polycrystalline substances. After the synthesis, only the substrates were melted and removed by acid treatment, whereby it was possible to recover only diamond polycrystalline substances with mirror finished surfaces, which had been in contact with the substrates, of 0.05 µm in Rmax. Table 4 also shows characteristics of these substances. Tool materials were formed by these diamond polycrystalline substances and growth surfaces thereof were regarded as bonding surfaces, to be brazed to shanks of cemented carbide with brazing filler metals shown in Table 4. Thereafter cutting edges were formed with diamond grindstones and by laser beam machining. The as-formed samples were applied to throw-away tipped tools and subjected to performance evaluation under the following conditions: Cutting Conditions Workpiece round bar of Aℓ-22%Si provided with eight axial grooves having V-shaped sections Cutting Speed 900 m/min. Depth of Cut 0.8 mm Feed Rate 0.12 mm/rev. Cooling Solution water-soluble oil solution Evaluation Method Comparison of cutting edge states up to cutting for 60 minutes Table 4 shows the results. The samples K, L, N and P were chipped in initial states. On the other hand, the samples J, M and O exhibited excellent wear resistance, chipping resistance and heat resistance, while it was clarified that these samples maintained sharp cutting edges for a long time. Example 5 A polycrystalline diamond layer was synthesized on a mirror finished surface of an Si substrate having surface roughness of 0.08 µm in Rmax for 25 hours by microwave plasma CVD under the following conditions: Raw Material Gas (Flow Rate) H 2 250 sccm CH 4 8 sccm Gas Pressure (80 Torr) 10658,4 Pa. Microwave Oscillation Output 700 W After the synthesis, the substance was dipped in nitric-hydrofluoric acid so that only the Si substrate was melted and removed, whereby it was possible to recover a polycrystalline diamond layer of 0.3 mm in thickness as a simple substance. The surface of the polycrystalline diamond layer, which had been in contact with the substrate during the synthesis, was 0.08 µm in Rmax. Further, the diamond layer was in such a sectional structure that the mean grain size was 0.2 µm on a tool rake face side and increased to 20 µm on a brazing surface side. This polycrystalline diamond layer was charged in an ion beam machining apparatus, and oxygen gas was introduced into this apparatus. This gas was ionized to generate oxygen cations, which in turn were field-accelerated at 500 V and applied to the surface of the polycrystalline diamond layer, which had been in contact with the substrate during the synthesis. This treatment was performed for 1 hour to remove a portion of the polycrystalline diamond layer up to a depth of 1 µm from its surface, while the polycrystalline diamond was converted to generate amorphous carbon. Then, the polycrystalline diamond layer was heated in an atmospheric furnace at a temperature of 450°C for 2 hours, to remove the amorphous carbon. It was proved by analysis that a portion of the polycrystalline diamond layer up to a depth of 10 µm from the treated surface was composed of 99.9 atomic percent of carbon elements and 0.1 atomic percent of Si elements, while 99.8 % of carbon atoms were diamond-bonded. Further, the surface roughness was improved to 0.07 µm in Rmax. This polycrystalline diamond layer was cut with a laser beam into an equilateral triangular tool material having edges of 5 mm. The tool material was brazed to a shank of cemented carbide with a heat resistant brazing filler metal containing 6 volume percent, 95 volume percent and 4 volume percent of Ag, Cu and Ti respectively. This brazing step was carried out by heating for 10 minutes the brazing filler metal to its melting point of 1050°C in a vacuum of 0,0067 Pa 5 x 10 -5 Torr so that the ion beam machined surface of the tool material defined a tool rake face. Thereafter only a flank was worked with a diamond grindstone of #1500 to be provided with a cutting edge, thereby preparing a sample Q. For the purpose of comparison, a tool material was prepared from a polycrystalline diamond layer which was similar to the above with neither ion beam machining nor heat treatment in the atmosphere, and brazed to a shank of cemented carbide to prepare a comparative sample R. In this sample R, it was confirmed by analysis that a portion of the polycrystalline diamond layer up to a depth of 10 µm from a tool rake face was composed of 98 atomic percent of carbon elements and 2 atomic percent of Si elements, while 98 % of the as-contained carbon atoms were diamond-bonded. On the other hand, sintered diamond of 5 µm in grain size containing 12 volume percent of Co as a binder was worked into a mirror finished surface of 0.09 µm in Rmax to form a tool material, which in turn was bonded to a shank of cemented carbide with interposition of a brazing filler metal having a melting point of 650°C, thereby preparing another comparative sample S. The comparative samples R and S were also provided with cutting edges by grinding with the same diamond grindstone as that for the sample Q. These samples were applied to throw-away tipped tools and subjected to performance evaluation under the following conditions: Cutting Conditions Workpiece round bar of ADC12 (Aℓ-12%Si) Cutting Speed 800 m/min. Depth of Cut 0.8 mm Feed Rate 0.05 mm/rev. Cooling Solution none Evaluation Method Comparison of cutting edge states and work surface roughness after cutting for 5 minutes and 60 minutes As the result, it was clarified that the inventive cutting tool remarkably improved deposition resistance of the conventional tool employing vapor-deposited diamond, and maintained a sharp cutting edge for a long time as compared with the sintered diamond tool, as shown in Table 5. Sample Works Surface Roughness : Rmax (µm) Deposition After Cutting for 5 Min. After Cutting for 60 Min. Inventive Sample Q None 1.2 1.5 Comparative Sample R Slightly Deposited 2.5 2.7 Comparative Sample S (Sintered Diamond) None 2.3 3.8 Example 6 A linear tantalum filament of 0.4 mm in diameter and 200 mm in length was employed as a thermoionic emission material to synthesize a polycrystalline diamond layer on a surface of an Mo substrate having surface roughness of 0.12 µm in Rmax by thermal CVD for 40 hours, under the following conditions: Raw Material Gas (Flow Rate) H 2 350 sccm C 2 H 2 18 sccm Gas Pressure (100 Torr) 13332,2 Pa Filament Temperature 2200°C Filament-to-Substrate Distance 8 mm Substrate Temperature 980°C After the synthesis, the substance was dipped in hot aqua regia so that only the Mo substrate was melted and removed, whereby it was possible to recover a polycrystalline diamond layer of 0.4 mm in thickness. The surface of the polycrystalline diamond layer, which had been in contact with the substrate during the synthesis, was 0.12 µm in Rmax. Further, the diamond layer was in such a sectional structure that the mean crystal grain size was 0.1 µm on a tool rake face side and increased to 30 µm on a brazing surface side. This polycrystalline diamond layer was cut with a laser beam to form a tool material, which in turn was brazed to a shank of cemented carbide with a heat resistant brazing filler metal containing 5 volume percent, 93 volume percent and 2 volume percent of Ag, Cu and Ti respectively. This brazing step was carried out so that the surface which had been in contact with the substrate during the synthesis defined a tool rake face. After the brazing step, a cutting edge was formed with a YAG laser, to prepare a sample T. In order to improve the surface of the rake face of this tool, surface treatment was performed under the following conditions: First, this tool was charged in an ion beam machining apparatus, and argon gas was introduced into this apparatus. This gas was ionized to generate argon cations, which in turn were field-accelerated at 450 V and applied to the surface of the polycrystalline diamond layer which had been in contact with the substrate during the synthesis. This treatment was performed for 3 hours, to remove a portion of the layer up to a depth of 1.7 µm from its surface, while the polycrystalline diamond was converted to generate amorphous carbon. Then the tool was heated in an atmospheric furnace at 420°C for 3 hours, to remove the amorphous carbon. It was proved by analysis that a portion of the polycrystalline diamond layer up to a depth of 10 µm from the tool rake face was composed of 99.2 atomic percent of carbon elements and 0.8 atomic percent of Mo elements in the as-treated sample U, while 99.5 % of carbon atoms were diamond-bonded. Further, the surface roughness was improved to 0.1 µm in Rmax. On the other hand, sintered diamond of 10 µm in grain size containing 8 volume percent of Co as a binder was worked into a mirror finished surface of 0.1 µm in Rmax, to form a tool material. Thereafter the tool material was bonded to a shank of cemented carbide with a brazing filler metal having a melting point of 680°C, to prepare a comparative sample V. This sample V was provided with a cutting edge by grinding with a diamond grindstone of #1500. These samples were applied to throw-away tipped tools, and subjected to performance evaluation under the following conditions: Cutting Conditions Workpiece AC2A (Aℓ-5%Si) Cutting Speed 1000 m/min. Depth of Cut 0.5 mm Feed Rate 0.1 mm/rev. Cutting Time 90 min. Cooling Solution water-soluble oil solution Evaluation Method Comparison of cutting edge states after cutting for 5 minutes and 60 minutes As the result, it was confirmed that the inventive sample U was improved in deposition resistance as compared with the conventional vapor-deposited diamond tool which was subjected to no surface treatment, to sufficiently effectuate its characteristic of a sharp cutting edge, as shown in Table 6. It was also clarified that the inventive tool has excellent performance as compared with the sintered diamond tool. Sample Work Surface Roughness: Rmax (µm) Deposition After Cutting for 5 Min. After Cutting for 60 Min. Comparative Sample T Slightly Deposited 2.8 3.2 Inventive Sample U None 1.8 1.9 Comparative Sample V (Sintered Diamond) None 2.7 3.8 Example 7 Under conditions shown in Table 7, a gas obtained by mixing H 2 , C 2 H 6 and Ar in the ratios of 8:1:1 was supplied into reaction tubes, in which W substrates having mirror finished surfaces of 0.15 µm in Rmax were placed, at a flow rate of 550 sccm, to adjust pressures to 26664,5 Pa (200 Torr). Then, a high frequency of 13.56 MHz was supplied from a high frequency oscillator at an output of 900 W to excite the mixed gas and generate plasma, thereby synthesizing polycrystalline diamond substances. After the synthesis, only the substrates were melted and removed by acid treatment, whereby it was possible to recover only polycrystalline diamond substances having mirror finished surfaces, which had been in contact with the substrates, of 0.12 µm in Rmax. Table 7 also shows the characteristics of these substances. The polycrystalline diamond substances were employed as tool materials, which were bonded by brazing to shanks of cemented carbide with brazing filler metals shown in Table 7, so that the surfaces which had been in contact with the substrates during the synthesis defined tool rake faces. Thereafter cutting edges were formed with a diamond grindstone or by laser beam machining. Further, surface treatment was performed under conditions shown in Table 7. The as-formed tools were applied to throw-away tipped tools and subjected to analysis of the rake faces and performance evaluation of the tools under the following conditions: Cutting Conditions Workpiece A2218 (Aℓ-4%Cu-2%Ni-1.5%Mg) provided with four longitudinal grooves having V-shaped sections Cutting Speed 1200 m/min. Depth of Cut 0.5 mm Feed Rate 0.08 mm/rev. Cooling Solution none Evaluation Method Comparison of work surface roughness values up to cutting for 60 minutes Table 7 shows the results. Samples Y and b , which were surface-treated at temperatures out of the temperature range for the heat treatment in the atmosphere according to the inventive method of manufacturing a polycrystalline diamond cutting tool, caused deposition with inferior work surface roughness. It is conceivable that the polycrystalline diamond layer was thermally deteriorated in the sample Y, due to the high temperature employed for the heat treatment. In the sample b , on the other hand, it may have conceivably been impossible to remove amorphous carbon due to the low temperature employed for the heat treatment. The samples other than Y and b exhibited excellent deposition resistance, with excellent values of work surface roughness in initial cutting. However, the sample X was chipped after cutting for a long time, conceivably because the thickness of the polycrystalline diamond layer was small. The sample a was also chipped after cutting for a long time, conceivably because the brazing filler metal employed for bonding the tool material to the tool holder had a low melting point and hence the same was softened and fluidized during cutting. On the other hand, the samples W, Z and c exhibited excellent deposition resistance and had excellent wear resistance, chipping resistance and heat resistance, while it was clarified that these samples maintained sharp cutting edges for a long time. 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 scope of the present invention being limited only by the terms of the appended claims.

Claims: A polycrystalline diamond cutting tool comprising a tool material (3), formed by a polycrystalline diamond layer (2) being synthesized on a substrate (1) by low-pressure vapor deposition, being bonded to a tool holder (4), through a brazing layer (5), said polycrystalline diamond layer (2) being at least 0.1 mm and not more than 1.0 mm in thickness; wherein said brazing layer (5) being made of a brazing filler metal having a melting point of at least 950°C and not more than 1300°C, wherein said brazing layer (5) contains at least one element selected from metals belonging to the groups IVa, Va, VIa and VIIa of the IUPAC form of the periodic table and carbides of said metals, and at least one metal selected from Au, Ag, Cu, Pt, Pd and Ni; wherein grain sizes of said polycrystalline diamond layer, being increased from the side of a rake face (10) toward a brazing surface (31), are at least 0.01 µm and not more than 1 µm on the side of said rake face (10) and at least 5 % and not more than 15 % of the thickness of said polycrystalline diamond layer on the side of said brazing surface (31); and wherein a portion of said polycrystalline diamond layer up to a depth of 10 µm from said tool rake face (10) being composed of at least 99 atomic percent of carbon and not more than 1 atomic percent of elements other than carbon, at least 99% of as-contained carbon atoms being diamond bonded. A polycrystalline diamond cutting tool in accordance with claim 1, wherein the thickness of said brazing layer (5) is at least 10 µm and not more than 50 µm. A polycrystalline diamond cutting tool in accordance with claim 1, wherein an upper growth surface (31) of said polycrystalline diamond layer (2) is bonded to said brazing layer (5) and said tool rake face (10) is defined by a surface of said polycrystalline diamond layer having been in contact with said substrate (1) during synthesis, the surface roughness of said tool rake face (10) being not more than 0.2 µm in maximum height Rmax. A polycrystalline diamond cutting tool in accordance with any of the preceding claims, wherein said polycrystalline diamond layer has chipping of at least 0.5 µm and not more than 5 µm on a cutting edge (8) along an intersection line of the rake face (10) and a tool flank. A polycrystalline diamond cutting tool in accordance with any of the preceding claims, wherein said tool holder (4) is made of cemented carbide. A method of manufacturing a polycrystalline diamond cutting tool, comprising: a step of forming a polycrystalline diamond layer (2) on a substrate (1) by low-pressure vapor deposition; a step of removing said substrate from said polycrystalline diamond layer; a step of cutting said polycrystalline diamond layer into a prescribed chip shape for forming a tool material (3); and a step of interposing a brazing filler metal (5) of an alloy having a melting point of at least 950°C and not more than 1300°C and containing at least one metal selected from those belonging to the groups IVa, Va, VIa and VIIa of the IUPAC form of the periodic table and carbides thereof and at least one metal selected from Au, Ag, Cu, Pt, Pd and Ni and heating and melting the same, thereby bonding said tool material (3) to said tool holder (4); wherein said step of forming a polycrystalline diamond layer (2) is carried out on a surface of said substrate (1) having surface roughness of not more than 0.2 µm in Rmax so that the mean grain size of said polycrystalline diamond is increased along the direction of crystal growth to be at least 0.01 µm and not more than 1 µm on the side of said substrate and at least 5 % and not more than 15 % of the thickness of said polycrystalline diamond layer on the side of an upper growth surface (31), said polycrystalline diamond layer (3) being brazed to said tool holder (4) so that a surface having been in contact with said substrate defines a tool rake face (10); and wherein said method further comprises a step of performing ion beam machining on a surface of said polycrystalline diamond layer having been in contact with said substrate during formation of said polycrystalline diamond layer, heating said polycrystalline diamond layer in the atmosphere at a temperature in a range of at least 300°C and not more than 500°C and thereafter forming a tool material (3) of a prescribed shape from said polycrystalline diamond layer; a step of bonding said tool material (3) to a tool holder (4) so that a surface of said tool material being subjected to ion beam machining defines a tool rake face (10); and a step of forming a cutting edge (9) on said tool material (3) while maintaining the surface state of said tool rake face (10). A method of manufacturing a polycrystalline diamond cutting tool in accordance with claim 6, wherein said brazing filler metal (5) contains at least 0.2 volume percent and not more than 50 volume percent of at least one metal selected from those belonging to the groups IVa, Va, VIa and VIIa of the IUPAC form of the periodic table. A method of manufacturing a polycrystalline diamond cutting tool in accordcance with claim 6, wherein said brazing filler metal (5) is formed to have a thickness of at least 10 µm and not more than 50 µm. A method of manufacturing a polycrystalline diamond cutting tool in accordance with any of the claims 6 to 8, wherein said tool material (3) is formed to have a thickness of at least 0.1 mm and not more than 1.0 mm. A method of manufacturing a polycrystalline diamond cutting tool in accordance with any of the claims 6 to 9, wherein said substrate (1) is melted in at least one solution of hydrochloric acid, sulfuric acid, nitric acid and hydrofluoric acid to be removed from said polycrystalline diamond layer (2). A method of manufacturing a polycrystalline diamond cutting tool in accordance with one of claims 6 to 10, wherein said bonding of said tool material (3) to said tool holder (4) is performed by a step of covering an upper growth surface (31) of said polycrystalline diamond layer forming said tool material with at least one material selected from metals belonging to the groups IVa, Va, VIa and VIIa of the IUPAC form of the periodic table and metal carbides thereof for forming a covering layer (6), and a step of interposing a brazing filler metal (7) containing at least one material selected from Au, Ag, Cu, Pt, Pd and Ni between a bonding surface of covered said tool material and a tool material receiving surface (41) of a tool holder (4) and heating and melting the same thereby bonding said tool material (3) to said tool holder (4). A method of manufacturing a polycrystalline diamond cutting tool in accordance with claim 11, wherein said covering layer (6) is formed to be at least 0.1 µm and not more than 5 µm in thickness. A method of manufacturing a polycrystalline diamond cutting tool in accordance with claim 6, wherein said step of forming a polycrystalline diamond layer (2) includes a step of forming a polycrystalline diamond layer being at least 0.1 mm and not more than 1.0 mm in thickness. A method of manufacturing a polycrystalline diamond cutting tool in accordance with claim 6, wherein said substrate (1) contains at least one element selected from Si, Mo and W, a surface of said substrate (1), being provided with said polycrystalline diamond layer (2), having surface roughness of not more than 0.2 µm in maximum height Rmax. A method of manufacturing a polycrystalline diamond cutting tool in accordance with claim 6, wherein an ion source employed for said ion beam machining is oxygen. A method of manufacturing a polycrystalline diamond cutting tool in accordance with claim 6, wherein the thickness of a portion of said polycrystalline diamond layer removed by said ion beam machining is in a range of at least 0.02 µm and not more than 2 µm from its surface.

Inventors: KANADA YASUYUKI, NAKAI TETSUO, NAKAMURA TSUTOMU, KANADA, YASUYUKI, NAKAI, TETSUO, NAKAMURA, TSUTOMU, KANADA, YASUYUKI, C/O ITAMI WORKS OF SUMITOMO, NAKAI, TETSUO, C/O ITAMI WORKS OF SUMITOMO, NAKAMURA, TSUTOMU, C/O ITAMI WORKS OF SUMITOMO
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LIMITED
Application year: 1992
Main classification: C23C16
IPC codes: B23B27, C23C16, C04B37