SURFACE-COATED BORON NITRIDE SINTERED BODY TOOL |
|||||||
| 申请号 | US14780141 | 申请日 | 2014-02-26 | 公开(公告)号 | US20160039010A1 | 公开(公告)日 | 2016-02-11 |
| 申请人 | SUMITOMO ELECTRIC HARDMETAL CORP.; | 发明人 | Makoto SETOYAMA; Katsumi OKAMURA; Nozomi TSUKIHARA; | ||||
| 摘要 | In a surface-coated boron nitride sintered body tool, at least a cutting edge portion contains a cubic boron nitride sintered body and a coating layer formed on a surface of the cubic boron nitride sintered body. A layer B of the coating layer is formed by alternately laminating one or more layers of each of two or more compound layers having different compositions. Each of the compound layers has a thickness not less than 0.5 nm and less than 30 nm. | ||||||
| 权利要求 | |||||||
| 说明书全文 | The present invention relates to a surface-coated boron nitride sintered body tool in which at least a cutting edge portion contains a cubic boron nitride sintered body and a coating layer formed on a surface of the cubic boron nitride sintered body. Tools in which the surface of a cubic boron nitride sintered body is coated with a coating layer of a ceramic material or the like exhibit excellent wear resistance, and thus, are used as cutting tools for cutting hardened steel. In recent years, high precision is demanded in such cutting, and improvement of the surface roughness of a workpiece surface is required. In order to meet this demand, WO 2010/150335 (PTD1) and WO 2012/005275 (PTD2), for example, propose tools in which the surface of a cubic boron nitride sintered body is coated with a coating film, wherein the coating film is composed of a lower layer made of multiple layers of a specific ceramic composition and an upper layer made of compound layers. Furthermore, as a base material of such a tool coated with multiple layers, cemented carbide, for example, is also used in place of the cubic boron nitride sintered body (Japanese Patent Laying-Open No 2008-188689 (PTD3) and Japanese National Patent Publication No. 2008-534297 (PTD4)). It is known that wear resistance is improved by using, as a cutting tool for machining steel, a tool in which the surface of a base material made of cemented carbide is coated with multiple layers of a ceramic composition. In the case of machining hardened steel, however, wear resistance has not been improved even though the surface of the cutting tool for use in this application that employs the cubic boron nitride sintered body as the base material is coated with multiple layers of a ceramic composition. Further improvement of the surface roughness of a workpiece surface is also demanded. The present invention was made in view of such circumstances, and an object of the invention is to improve the wear resistance of a tool in which at least a cutting edge portion includes a base material made of a cubic boron nitride sintered body, in the machining or the like of hardened steel. In order to solve the aforementioned problem, the present inventor investigated the condition of wear of a tool that occurs when machining hardened steel. Consequently, it was revealed that in addition to usual crater wear and flank-face wear, boundary wear occurs at the boundary of an end cutting edge, which is one end of wear portions, and this boundary wear most significantly affects tool life. As a result of extensive research based on this finding, the present inventor also found that in order to suppress this boundary wear, it is most effective to laminate layers of specific compositions in a specifically laminated manner, and conducted further research based on this finding to complete the present invention. In a surface-coated boron nitride sintered body tool according to the invention, at least a cutting edge portion includes a cubic boron nitride sintered body and a coating layer formed on a surface of the cubic boron nitride sintered body. The cubic boron nitride sintered body contains not less than 30% and not more than 80% by volume of the cubic boron nitride, and also includes a binder phase containing an aluminum compound, an inevitable impurity, and at least one compound selected from the group consisting of a nitride, a carbide, a boride, and an oxide of a group 4 element, a group 5 element, and a group 6 element of the periodic table of the elements, as well as a solid solution thereof. The coating layer includes a layer A and a layer B. Layer A is composed of MLaza1, where M represents one or more of Al, Si, as well as a group 4 element, a group 5 element, and a group 6 element of the periodic table of the elements; La represents one or more of B, C, N, and O; and za1 is not less than 0.85 and not more than 1.0. Layer B is formed by alternately laminating one or more layers of each of two or more compound layers having different compositions. Each of the compound layers has a thickness not less than 0.5 nm and less than 30 nm. A B1 compound layer as one of the compound layers is composed of (Ti1-xb1-yb1Sixb1M1yb1)(C1-zb1Nzb1), where M1 represents one or more of Al as well as a group 4 element, a group 5 element, and a group 6 element of the periodic table of the elements except for Ti, xb1 is not less than 0.01 and not more than 0.25; yb1 is not less than 0 and not more than 0.7; and zb1 is not less than 0.4 and not more than 1. A B2 compound layer as one of the compound layers different from the B1 compound layer is composed of (Al1-xb2M2xb2)(C1-zb2Nzb2), where M2 represents one or more of Si as well as a group 4 element, a group 5 element, and a group 6 element of the periodic table of the elements; xb2 is not less than 0.2 and not more than 0.77; and zb2 is not less than 0.4 and not more than 1. Layer A has a thickness not less than 0.2 μm and not more than 10 μm. Layer B has a thickness not less than 0.05 μm and not more than 5 μm. The coating layer has an overall thickness not less than 0.25 μm and not more than 15 μm. According to the invention, the wear resistance of a tool in which at least a cutting edge portion includes a base material made of a cubic boron nitride sintered body can be improved in the machining or the like of hardened steel. The present invention will be described in further detail below. In the following description of embodiments, the description is given in conjunction with the drawings, in which the same reference numerals represent the same or corresponding elements. <Structure of Surface-Coated Boron Nitride Sintered Body Tool> At least a cutting edge portion of a surface-coated boron nitride sintered body tool according to the invention includes a cubic boron nitride sintered body (hereinafter denoted as the “cBN sintered body”; the term “cBN” is an abbreviation of “cubic Boron Nitride”) and a coating layer formed on the surface of the cBN sintered body. The surface-coated boron nitride sintered body tool having such a basic structure can be effectively used particularly for mechanically machining (cutting, for example) sintered alloys and difficult-to-machine cast iron, or for machining hardened steel, and additionally can be suitably used for various types of machining of general metals other than the above. <cBN Sintered Body> Of the cutting edge portion of the surface-coated boron nitride sintered body tool, the cBN sintered body constitutes a base material of the tool, contains not less than 30% and not more than 80% by volume of cubic boron nitride (hereinafter denoted as “cBN”), and also includes a binder phase. As used herein, the binder phase contains an aluminum compound, an inevitable impurity, and at least one compound selected from the group consisting of a nitride, a carbide, a boride, and an oxide of a group 4 element, a group 5 element, and a group 6 element of the periodic table of the elements, as well as a solid solution thereof. The binder phase binds cBN together. When the cBN sintered body contains not less than 30% by volume of cBN, the wear resistance of the base material of the surface-coated boron nitride sintered body tool can be prevented from decreasing. Furthermore, when the cBN sintered body contains not more than 80% by volume of cBN, cBN can be dispersed in the cBN sintered body, which allows bonding strength of cBN attributed to the binder phase to be ensured. The volume content of cBN is herein determined in accordance with the following method. The cBN sintered body is mirror polished, and then a reflection electron image of the structure of a given region of the cBN sintered body is obtained with an electron microscope at 2000 times magnification. At this time, a particle made of cBN (hereinafter denoted as the “cBN particle”) is indicated in a black region, and the binder phase is indicated in a grey or white region. From the obtained image of the cBN sintered body structure, the cBN sintered body region and the binder phase region are binarized by image processing to determine the occupancy area of the cBN particles. The volume content of cBN can be determined by substituting the determined occupancy area of the cBN particles into the following equation: (volume content of cBN)=(occupancy area of cBN particles)/(photographed area of cBN sintered body structure)×100. Preferably, the cBN sintered body contains not less than 50% and not more than 65% by volume of cBN. When the cBN sintered body contains not less than 50% by volume of cBN, a base material of the surface-coated boron nitride sintered body tool having an excellent balance between wear resistance and defect resistance can be provided. Furthermore, when the cBN sintered body contains not more than 65% by volume of cBN, the bonding strength of cBN attributed to the binder phase can be increased. Preferably, at an interface between the cBN sintered body and the coating layer, the cBN particles protrude more toward the coating layer than the binder phase. This allows the adhesion between the cBN sintered body and the coating layer to be increased. More preferably, there is a difference in level of not less than 0.05 μm and not more than 1.0 μm between the cBN particles and the binder phase. When this difference in level is not less than 0.05 μm, an anchor effect can be achieved. Furthermore, when this difference in level is not more than 1.0 μm, the cBN particles can be prevented from falling off from the cBN sintered body. More preferably, there is a difference in level of not less than 0.1 μm and not more than 0.5 μm between the cBN particles and the binder phase. When this difference in level is not less than 0.1 μm, an anchor effect can be effectively achieved. Furthermore, when this difference in level is not more than 0.5 μm, the cBN particles can be further prevented from falling off from the cBN sintered body. The difference in level is herein measured in accordance with the same method as the method for measuring the overall thickness of the coating layer and the like described below. Preferably, the volume content of cBN in the cBN sintered body increases toward the inside of the cBN sintered body from the interface between the cBN sintered body and the coating layer. Thus, at the interface between the cBN sintered body and the coating layer, the volume content of the binder phase is higher than that of cBN, which allows the adhesion between the cBN sintered body and the coating layer to be increased. On the other hand, inside the cBN sintered body, the volume content of cBN is higher than that of the binder phase, which allows the defect resistance of the cBN sintered body to be improved. For example, the volume content of cBN is 40% near the interface to the coating layer (a region not less than 0 μm and not more than 20 μm distant from the interface between the cBN sintered body and the coating layer toward the inside of the cBN sintered body). Around the center of the cBN sintered body in a thickness direction (a region more than 20 μm and not more than 100 μm distant from the interface between the cBN sintered body and the coating layer toward the inside of the cBN sintered body), the volume content of cBN is 60%. Preferably, the particle size of the cBN particles contained in the cBN sintered body increases toward the inside of the cBN sintered body from the interface between the cBN sintered body and the coating layer. Thus, at the interface between the cBN sintered body and the coating layer, the particle size of the cBN particles is small, which allows the adhesion between the cBN sintered body and the coating layer to be increased. On the other hand, inside the cBN sintered body, the particle size of the cBN particles is large, which allows toughness to be increased. For example, the particle size of the cBN particles is not less than 0.1 μm and not more than 1 μm in the region not less than 0 μm and not more than 20 μm distant from the interface between the cBN sintered body and the coating layer toward the inside of the cBN sintered body. The particle size of the cBN particles is not less than 2 μm and not more than 10 μm in a region more than 20 μm and not more than 300 μm distant from the interface between the cBN sintered body and the coating layer toward the inside of the cBN sintered body. The particle sire of the cBN particles is herein determined in accordance with the following method. In the reflection electron image of the cBN sintered body structure obtained in determining the volume content of cBN, the diameter of a circle that circumscribes a cBN particle is measured, and the measured diameter is determined as the particle size of the cBN particle. It is noted that the cBN sintered body may be provided in the cutting edge portion of the surface-coated boron nitride sintered body tool. Therefore, the base material of the surface-coated boron nitride sintered body tool may include a cutting edge portion made of the cBN sintered body, and a base material body made of a material different from the cBN sintered body (cemented carbide, for example). In this case, the cutting edge portion made of the cBN sintered body is preferably adhered to the base material body with a braze material or the like interposed therebetween. The braze material can be selected in consideration of the bonding strength or the melting point. The cBN sintered body may also constitute the entire base material of the surface-coated boron nitride sintered body tool. <Coating Layer> The coating layer includes layer A and layer B. As long as the coating layer of the invention includes layer A and layer B, it may include other layers in addition to layers A and B. Examples of such other layers may include, but are not limited to, a layer C located between layers A and B, and a layer D that is the bottom layer, as described below. The thickness of the coating layer is not less than 0.25 μm and not more than 15 μm. When the thickness of the coating layer is not less than 0.25 μm, deterioration of the wear resistance of the surface-coated boron nitride sintered body tool due to the thickness of the coating layer being small can be prevented. When the thickness of the coating layer is not more than 15 μm, the chipping resistance of the coating layer in the initial stage of cutting can be improved. Preferably, the thickness of the coating layer is not less than 1.5 μm and not more than 4.5 μm. The overall thickness of the coating layer and the thickness of each of the layers described below as well as the number of laminated layers are all determined herein by cutting the surface-coated boron nitride sintered body tool, and observing the cross section with an SEM (scanning electron microscope) or a TEM (transmission electron microscope). The composition of each of the layers as described below that constitute the coating layer is measured with an EDX analyzer (energy dispersive X-ray analyzer) equipped with an SEM or a TEM. While the coating layer may be provided only on the cutting edge portion of the surface-coated boron nitride sintered body tool, it may also be provided over the entire surface of the base material of the surface-coated boron nitride sintered body tool, or may not be provided on a portion of the section different from the cutting edge portion. Alternatively, in the section different from the cutting edge portion, the laminated structure of a portion of the coating layer may be partially different. <Layer A> Layer A is composed of MLaza1, where M represents one or more of Al, Si, as well as a group 4 element, a group 5 element, and a group 6 element of the periodic table of the elements; La represents one or more of B, C, N, and O; and za1 is not less than 0.85 and not more than 1.0. This allows layer A to experience smooth wear. In other words, layer A experiences wear without peeling, breaking, chipping, or the like. Therefore, the resistance to crater wear or the resistance to flank-face wear of the surface-coated boron nitride sintered body tool can be improved. Preferably, layer A is composed of (Ti1-xaMaxa)(C1-za2Nza2), where Ma represents one or more of Al, Si, as well as a group 4 element, a group 5 element, and a group 6 element of the periodic table of the elements except for Ti; xa is not less than 0 and not more than 0.7; and za2 is not less than 0 and not more than 1. When layer A contains Ti, peeling, breaking, chipping, or the like of layer A during wear can be further prevented. More preferably, the composition xa of Ma is not less than 0 and not more than 0.3. This allows peeling, breaking, chipping, or the like of layer A during wear to be further prevented. It is noted that where layer A is composed of (Ti1-xa(1)-xa(2)Ma(1)xa(1)Ma(2)xa(2))(C1-xa2Nza2), the sum of xa(1) and xa(2) is preferably not less than 0 and not more than 0.7, and more preferably not less than 0 and not more than 0.3. The same also applies to layers B, C, and D described below. Preferably, in layer A, the composition of N changes in a step-like or slope-like manner toward a surface-side of layer A from its cBN sintered body-side. For example, when the composition of N is high on the cBN sintered body-side of layer A, defect resistance and peeling resistance can be improved. When the composition of N is low on the surface-side of layer A, peeling, breaking, chipping, or the like of layer A during wear can be further prevented. As used herein, the expression “the composition of N changes in a step-like manner toward the surface-side of layer A from its cBN sintered body-side” means that the composition of N decreases or increases discontinuously toward the surface-side of layer A from its cBN sintered body-side, which means, for example, a structure obtained by laminating two or more layers having different compositions of N. As used herein, the expression “the composition of N changes in a slope-like manner toward the surface-side of layer A from its cBN sintered body-side” means that the composition of N decreases or increases continuously toward the surface-side of layer A from its cBN sintered body-side, which means, for example, a structure that is formed while continuously changing the flow rate ratio between the source gas for N and the source gas for C. Preferably, layer A has, on its surface-side, a region having a greater composition of C than on its cBN sintered body-side. This also allows the defect resistance and peeling resistance to be improved on the cBN sintered body-side of layer A, while allowing peeling, breaking, chipping, or the like of layer A during wear to be further prevented on the surface-side of layer A. As used herein, the “cBN sintered body-side of layer A” means a region not less than 0 μm and not more than 0.1 μm distant from a face of layer A situated nearest to the cBN sintered body toward the inside of layer A. The surface-side of layer A means a portion different from the cBN sintered body-side of layer A. Layer A has a thickness not less than 0.2 μm and not more than 10 μm. When layer A has a thickness not less than 0.2 μm, a surface-coated boron nitride sintered body tool having excellent resistance to crater wear and resistance to flank-face wear can be provided. On the other hand, when layer A has a thickness more than 10 μm, it may be difficult to further improve the resistance to crater wear or resistance to flank-face wear of the surface-coated boron nitride sintered body tool. Preferably, layer A has a thickness not less than 1 μm and not more than 3 μm. Preferably, layer A is located nearer to a surface-side of the surface-coated boron nitride sintered body tool than layer B. Thus, since layer A experiences smooth wear, the formation of cracks can be prevented. Even if a crack forms, layer B can prevent propagation of the crack toward the base material. <Layer B> Layer B is formed by alternately laminating one or more layers of each of two or more compound layers having different compositions. In the following, layer B having a structure in which one or more layers of each of a B1 compound layer and a B2 compound layer are alternately laminated will be described by way of example. Layer B of the invention, however, may include other layers in addition to the B1 compound layer and the B2 compound layer, as long as it includes the B1 compound layer and the B2 compound layer. Layer B has a thickness not less than 0.05 μm and not more than 5 μm. Preferably, the average value of the Si composition in layer B as a whole is not less than 0.005 and not more than 0.1. This allows the peeling resistance of layer B to be increased, which allows oxygen to be prevented from entering into the interface between layer B and layer A or the base material. The average value of the Si composition in layer B as a whole is more preferably not less than 0.01 and not more than 0.07, and even more preferably not less than 0.02 and not more than 0.05. The average value of the Si composition in layer B as a whole is herein determined using the following equation: (average value of Si composition in layer B as a whole)=[sum total of {(Si composition of each layer constituting layer B)×(thickness of each layer)}]/(overall thickness of layer B). The B1 compound layer is composed of (Ti1-xb1-yb1Sixb1M1yb1)(C1-zb1Nzb1), where M1 represents one or more of Al as well as a group 4 element, a group 5 element, and a group 6 element of the periodic table of the elements except for Ti, xb1 is not less than 0.01 and not more than 0.25; yb1 is not less than 0 and not more than 0.7; and zb1 is not less than 0.4 and not more than 1. The B1 compound layer has a thickness not less than 0.5 nm and less than 30 nm. The B2 compound layer is composed of (Al1-xb2M2xb2)(C1-zb2Nzb2), where M2 represents one or more of Si as well as a group 4 element, a group 5 element, and a group 6 element of the periodic table of the elements; xb2 is not less than 0.2 and not more than 0.77; and zb2 is not less than 0.4 and not more than 1. M2 preferably represents at least one of Ti and Cr. The composition xb2 of M2 is preferably not less than 0.25 and not more than 0.5, and more preferably not less than 0.25 and not more than 0.4. The B2 compound layer has a thickness not less than 0.5 nm and less than 30 nm. The bottom layer of layer B may either be the B1 compound layer or the B2 compound layer. The top layer of layer B may either be the B1 compound layer or the B2 compound layer. Preferably, t2/t1, which is the ratio of an average thickness t2 of the B2 compound layers with respect to an average thickness t1 of the B1 compound layers, falls within 0.5<t2/t1≦10.0. This allows the resistance to boundary wear of the surface-coated boron nitride sintered body tool to be further improved. Average thickness t1 of the B1 compound layers is herein determined using the following equation. Average thickness t2 of the B2 compound layers is also similarly determined. (average thickness t1 of B1 compound layers)=(sum total of thicknesses of B1 compound layers)/(the number of B1 compound layers). Where it is difficult to measure the thickness of the B1 compound layers, average thickness t1 of the B1 compound layers can be determined using the following equation. Average thickness t2 of the B2 compound layers is also similarly determined. (average thickness t1 of B1 compound layers)=(thickness of layer B formed of lamination of B compound layers only)/(the number of laminated B1 compound layers). More preferably, t2/t1 falls within 1<t2/t1≦5.0. This allows the resistance to boundary wear of the surface-coated boron nitride sintered body tool to be further improved. Hence, a surface-coated boron nitride sintered body tool having excellent wear resistance against repeated shocks, vibrations, or the like can be provided. Furthermore, also for repeated machining or light interrupted cutting of small parts, the life of the surface-coated boron nitride sintered body tool can be extended. More preferably, t2/t1 falls within 1.1<t2/t1≦4.5. Even more preferably, where layer A is located nearer to the surface-side than layer B, t2/t1 falls within 1<t2/t1≦5 0 on a cBN sintered body-side, decreases toward layer A, and falls within 0.5<t2/t1<2 on a layer A-side. This allows the formation of cracks to be prevented on the layer A-side of layer B, while preventing the propagation of cracks toward the cBN sintered body on the cBN sintered body-side of layer B. As used herein, the “layer A-side of layer B” means the narrower one of a region not less than 0 μm and not more than ½ the thickness of layer B distant from a face of layer B situated nearest to layer A toward the inside of layer B, and a region not less than 0 μm and not more than 0.1 μm distant from the face of layer B situated nearest to layer A toward the inside of layer B. As used herein, the “cBN sintered body-side of layer B” means the narrower one of a region not less than 0 μm and not more than ½ the thickness of layer B distant from a face of layer B situated nearest to the cBN sintered body toward the inside of layer B, and a region not less than 0 μm and not more than 0 μm distant from the face of layer B situated nearest to the cBN sintered body toward the inside of layer B. <Layer C> Preferably, the coating layer further includes layer C located between layer A and layer B, and layer C is composed of McLczc, where Mc represents one or more of Al, Si, as well as a group 4 element, a group 5 element, and a group 6 element of the periodic table of the elements, Lc represents one or more of B, C, N, and O; and zc is not less than 0 and not more than 0.85. This allows the adhesion between layer A and layer B to be increased. Furthermore, where layer A is located nearer to the surface-side than layer B, the propagation of a crack formed in layer A toward the base material can be stopped in layer C. More preferably, layer C has a thickness not less than 0.005 μm and not more than 0.5 μm. When layer C has a thickness not less than 0.005 μm, a sufficient effect can be obtained by providing layer C. When layer C has a thickness not more than 0.5 μm, the thickness of the coating layer can be prevented from becoming excessively great due to layer C being provided. Even more preferably, layer C has a thickness not less than 0.01 μm and not more than 0.2 μm. More preferably, the composition zc of Lc is more than 0 and less than 0.7. When the composition zc of Lc is more than 0, the heat resistance and the chemical wear resistance of layer C can be improved, which allows the propagation of a crack formed in layer A to be effectively prevented in layer C. Even more preferably, the composition zc of Le is not less than 0.2 and not more than 0.5. More preferably, layer C includes at least one or more of the elements constituting layer A and layer B. When layer C includes at least one or more of the elements constituting layer A, the adhesion between layer A and layer C can be increased. When layer C includes at least one or more of the elements constituting layer B, the adhesion between layer B and layer C can be increased. More preferably, layer C includes at least one or more of the elements constituting a portion situated on a layer C-side of each of layer A and layer B. <Layer D> Preferably, the coating layer further includes layer D located between the base material and layer B, and layer D is composed of MdLdzd, where Md represents one or more of Al, Si, as well as a group 4 element, a group 5 element, and a group 6 element of the periodic table of the elements; Ld represents one or more of B, C, N, and O; and zd is not less than 0.85 and not more than 1.0. This layer D has excellent adhesion with the cBN sintered body. Thus, when the coating layer further includes layer D, the adhesion between the cBN sintered body and the coating layer can be increased. More preferably, Ld is N. More preferably, layer D is composed of (Al1-xdMd2xd)Ldzd, where Md2 represents one or more of Si as well as a group 4 element, a group 5 element, and a group 6 element of the periodic table of the elements; and xd is not less than 0.25 and not more than 0.45. When layer D includes Al, the adhesion between the cBN sintered body and the coating layer can be further increased. Even more preferably, Md2 is at least one or more of Ti, Cr, and V. More preferably, layer D has a thickness not less than 0.05 μm and not more than 1 μm. When layer D has a thickness not less than 0.05 μm, a sufficient effect can be obtained by providing layer D. When layer D has a thickness not more than 1 μm, the thickness of the coating layer can be prevented from becoming excessively great due to layer D being provided. Even more preferably, layer D has a thickness not less than 0.1 μm and not more than 0.5 μm. <Method for Manufacturing Surface-Coated Boron Nitride Sintered Body Tool> A method for manufacturing the surface-coated boron nitride sintered body tool according to the invention includes, for example, the steps of preparing a base material containing the cBN sintered body in at least a cutting edge portion, and forming a coating layer at least on a surface of the cBN sintered body. The step of preparing the base material preferably includes the step of forming the cBN sintered body, and the step of forming the cBN sintered body preferably includes the step of sintering a mixture of cBN particles and a raw-material powder of the binder phase at a high temperature and a high pressure. More preferably, the step of preparing the base material further includes the step of binding the cBN sintered body to the base material body having a predetermined shape. The step of forming the coating layer preferably includes the step of forming a coating layer using an arc ion plating method (an ion plating method that causes a solid material to evaporate, using vacuum arc discharge), or the step of forming the coating layer using a sputtering method. With the arc ion plating method, the coating layer can be formed using a metal evaporation source containing a metal species for forming the coating layer, and a reactant gas such as CH4, N2, O2, or the like. Known conditions can be employed as the conditions for forming the coating layer. With the sputtering method, the coating layer can be formed using a metal evaporation source containing a metal species for forming the coating layer, a reactant gas such as CH4, N2, O2, or the like, and a sputtering gas such as Ar, Kr, Xe, or the like. Known conditions can be employed as the conditions for forming the coating layer. While the present invention will be described in more detail hereinafter with reference to Examples, the present invention is not limited thereto. <Manufacture of Surface-Coated Boron Nitride Sintered Body Tool> <Manufacture of Sample 1> <Formation of cBN Sintered Body A> First, a TiN powder having an average particle size of 1 μm and a Ti powder having an average particle size of 3 μm were mixed to give an atomic ratio of Ti:N=1:0.6. The resulting mixture was heat-treated for 30 minutes in vacuum at 1200° C., and then ground. An intermetallic compound powder made of TiN0.6 was thus obtained. Next, the intermetallic compound powder made of TiN0.6 and an Al powder having an average particle size of 4 μm were mixed to give a mass ratio of TiN0.6:Al=90:10. The resulting mixture was heat-treated for 30 minutes in vacuum at 1000° C. The compound obtained by the heat treatment was uniformly ground with a ball mill using ball media made of cemented carbide and having a diameter of 6 mm. A raw-material powder of the binder phase was thus obtained. Then, the raw-material powder of the binder phase and cBN particles having an average particle size of 1.5 μm were blended such that the cBN content in the cBN sintered body would be 30% by volume, and were uniformly mixed with a ball mill using ball media made of boron nitride and having a diameter of 3 mm. The resulting mixed powder was deposited on a support plate made of cemented carbide, and was loaded into an Mo capsule. The mixed powder was then sintered for 30 minutes using an ultrahigh pressure apparatus at a temperature of 1300° C. and a pressure of 5.5 GPa A cBN sintered body A was thus obtained. <Formation of Base Material> A base material body made of a cemented carbide material (equivalent to K10) and having the shape of ISO DNGA150408 was prepared. The above-described cBN sintered body A (shape: a 2-mm-thick triangular prism having isosceles triangles as bases whose vertex angle was 55° and sides sandwiching the vertex angle were each 2 mm) was bonded to an insert (corner portion) of the prepared base material body. A braze material made of Ti—Zr—Cu was used for bonding. The bonded body had its peripheral faces, upper face and lower face ground to form a negative land (having a width of 150 μm and an angle of 25°) on the insert. In this way, a base material 3 having a cutting edge portion made of cBN sintered body A was obtained. The obtained base material 3 was placed within a film deposition apparatus, the apparatus was evacuated and heated to 500° C., and then base material 3 was etched with Ar ions. The Ar gas was then exhausted from the film deposition apparatus. <Formation of Coating Layer> <Formation of Layer D> A layer D 20 was formed on base material 3 within the above-described film deposition apparatus. Specifically, layer D having a thickness of 0.5 μm was formed by vapor deposition under the following conditions: Target: contains 70 atomic % of Al and 30 atomic % of Cr Introduced gas: N2 Film deposition pressure: 4 Pa Arc discharge current: 120 A Substrate bias voltage: −50 V Table rotating speed: 5 rpm. <Formation of Layer B> A layer B 30 was formed on layer D 20 within the above-described film deposition apparatus. Specifically, layer B 30 having an overall thickness of 0.03 μm was formed by vapor deposition under the following conditions. At this time, the arc current for targets B1 and B2 and the rotation speed of the rotation table on which the base material was set were adjusted such that the thickness of a B1 compound layer 31 would be 7 nm, and the thickness of a B2 compound layer 32 would be 10 nm. Target B1: contains 75 atomic % of Ti, 15 atomic % of Si, and 10 atomic % of Cr. Target B2: contains 60 atomic % of Al, 10 atomic % of Cr, and 30 atomic % of Ti. Introduced gas: N2 Film deposition pressure: 1 Pa Substrate bias voltage: −50V <Formation of Layer C> A layer C 40 was formed on layer B 30 within the above-described film deposition apparatus. Specifically, layer C 40 having a thickness of 0.1 μm was formed by vapor deposition under the following conditions: Target: contains 50 atomic % of Ti and 50 atomic % of Al Introduced gas: N2 Film deposition pressure: 0.1 Pa Arc discharge current: 150 A Substrate bias voltage: −100 V Table rotating speed: 5 rpm. <Formation of Layer A> A layer A 50 was formed on layer C 40 within the above-described film deposition apparatus. Specifically, layer A having a thickness of 2 μm was formed by vapor deposition under the following conditions: Target: contains 50 atomic % of Ti and 50 atomic % of Al Introduced gas: N2 Film deposition pressure: 4 Pa Arc discharge current: 120 A Substrate bias voltage: −600 V Table rotating speed: 5 rpm. As described above, a coating layer 10 sequentially including layer D 20, layer B 30, layer C 40, and layer A 50 on base material 3 was formed, and thus, sample 1 was manufactured. <Manufacture of Samples 2 to 6> Samples 2 to 6 were manufactured following the method for manufacturing sample 1 described above, except that the number of B1 compound layers and the number of B2 compound layers were changed to the values shown in Table 2, and the overall thickness of layer B was changed. It is noted that the number of layers shown in Table 2 denotes a total of the number of B1 compound layers and the number of B2 compound layers. <Manufacture of Sample 7> A cBN sintered body D was obtained following the method for forming cBN sintered body A described above, except that the cBN particles and the raw-material powder of the binder phase were blended such that the cBN content in the cBN sintered body would become the value shown in Table 3. A base material of sample 7 was formed following the method for manufacturing the base material of sample 1 described above, using the obtained cBN sintered body D. Next, layer D and layer B were sequentially formed following the method for manufacturing sample 1 described above. Layer C was formed following the method for forming layer C of sample 1 described above, except that the film deposition pressure was changed as described below. Specifically, at the beginning of the formation of layer C, the film deposition pressure was set to 3 Pa with the introduction of N2, and thereafter gradually reduced to 0.1 Pa, and then again gradually increased to 3 Pa. Layer A was then formed following the method for forming layer A of sample 1 described above, except that the film deposition pressure and the introduced gas were changed as follows. Specifically, the film deposition pressure was set to 3 Pa with the introduction of N2 only during the period of time from the beginning of the formation of layer A until the thickness of layer A reached 0.9 μm. N2 was then gradually reduced while gradually increasing CH4, further forming layer A in an additional thickness of 0.3 μm. At this time, N2 was gradually reduced while gradually increasing CH4 until the composition became TiC0.5N0.5. Then, layer A was further formed in an additional thickness of 0.3 μm without changing the amount of supply of each of CH4 and N2. Sample 7 was thus manufactured. It is noted that during the manufacture of sample 7, the targets were prepared, and the types of the introduced gases and the amounts of supply thereof were adjusted, such that the layers of the compositions shown in Tables 1 and 2 were obtained. As the introduced gases, Ar, N2, CH4, and the like were used as appropriate. The film deposition pressure was adjusted as appropriate within the range of 0.1 Pa to 7 Pa, the are discharge current was adjusted as appropriate within the range of 60 A to 200 A, and the substrate bias voltage was adjusted as appropriate within the range of −25 V to −700 V. The same also applies to samples 8 to 55 described below. <Manufacture of Samples 8 to 13> Samples 8 to 13 were manufactured following the method for manufacturing sample 7 described above, except that layers B were formed to have the values shown in Table 2 for the thickness of the B1 compound layers and the thickness of the B2 compound layers. <Manufacture of Sample 14> A cBN sintered body B was obtained following the method for forming cBN sintered body A described above, except that the cBN particles and the raw-material powder of the binder phase were blended such that the cBN content in the cBN sintered body would become the value shown in Table 3. A base material of sample 14 was formed following the method for manufacturing the base material of sample 1 described above, using the obtained cBN sintered body B. Next, layer D was formed following the method for manufacturing sample 1 described above. Layer B was then formed by changing the arc current for target B1 and the arc current for target B2 to give the ratio of the thickness of the B2 compound layers with respect to the thickness of the B1 compound layers shown in Table 2. Specifically, the arc current for target B1 was gradually increased at a certain rate, and the arc current for target B2 was gradually reduced at a certain rate. Layer A was then formed on layer B without forming layer C. Specifically, layer A was formed following the method for forming layer A of sample 1 described above, except that CH4, in addition to N2, was used as an introduced gas. At this time, layer A made of TiC0.3N0.7 was formed by adjusting the amount of supply of each of CH4 and N2. Sample 14 was thus manufactured. <Manufacture of Samples 15 to 19> Samples 15 to 19 were manufactured following the method for manufacturing sample 14 described above, except that the introduced gas during the formation of layer A was changed. Layer A of sample 15 was formed as follows, for example. Specifically, the film deposition pressure was set to 3 Pa with the introduction of N2 only during the period of time from the beginning of the formation of layer A until the thickness of layer A reached 1.6 μm. N2 was then gradually reduced while gradually increasing CH4, further forming layer A in an additional thickness of 0.3 μm. At this time, N2 was gradually reduced while gradually increasing CH4 until the composition became TiC0.3N0.7. Layer A was then further formed in an additional thickness of 0.1 μm without changing the amount of supply of each of CH4 and N2. The supply of CH4 was then stopped, and the amount of supply of N2 was increased, further forming layer A in an additional thickness of 0.5 μm. <Manufacture of Samples 20 to 25> A cBN sintered body C was obtained following the method for forming cBN sintered body A described above, except that the cBN particles and the raw-material powder of the binder phase were blended such that the cBN content in the cBN sintered body would become the value shown in Table 3. A base material of each of samples 20 to 25 was formed following the method for manufacturing the base material of sample 1 described above, using the obtained cBN sintered body C. Next, layers D, B, C, and A were sequentially formed following the method for manufacturing samples 1 and 14 described above. Samples 20 to 25 were thus manufactured. <Manufacture of Sample 26> A cBN sintered body E was obtained following the method for forming cBN sintered body A described above, except that the cBN particles and the raw-material powder of the binder phase were blended such that the cBN content in the cBN sintered body would become the value shown in Table 3. A base material of sample 26 was formed following the method for manufacturing the base material of sample 1 described above, using the obtained cBN sintered body E. Next, layer D and layer B were sequentially formed following the method for manufacturing sample 1 described above. Then, layer C was formed by vapor deposition under the following conditions, after which layer A was formed following the method for manufacturing sample 14 described above. Sample 26 was thus manufactured. Target: contains 80 atomic % of Ti, 10 atomic % of Cr, and 10 atomic % of W. Introduced gas: Ar Film deposition pressure: 4 Pa Arc discharge current: 150 A Substrate bias voltage: −30V. <Manufacture of Samples 27 to 32> Samples 27 to 32 were manufactured following the method for manufacturing sample 26 described above, except that layers C each having the composition shown in Table 1 were formed with the introduction of Ar and N2. <Manufacture of Samples 33 to 38> A cBN sintered body F was obtained following the method for forming cBN sintered body D described above, except that cBN particles having an average particle size of 0.5 μm and the raw-material powder of the binder phase were blended. A base material of each of samples 33 to 38 was formed following the method for manufacturing the base material of sample 1 described above, using the obtained cBN sintered body F. Next, layers D, B, C, and A were sequentially formed following the method for manufacturing samples 1 and 7 described above. Samples 33 to 38 were thus manufactured. <Manufacture of Samples 39 to 44> A cBN sintered body G was obtained following the method for forming cBN sintered body D described above, except that cBN particles having an average particle size of 3 μm and the raw-material powder of the binder phase were blended. A base material of each of samples 39 to 44 was formed following the method for manufacturing the base material of sample 1 described above, using the obtained cBN sintered body G. Next, layers D, B, and A were sequentially formed following the method for manufacturing samples 1 and 7 described above. Samples 39 to 44 were thus manufactured. <Manufacture of Samples 45 to 51> A base material of each of samples 45 to 51 was formed following the method for manufacturing the base material of sample 1 described above, using the cBN sintered body shown in Table 1. Next, layers D, B, C, and A were sequentially formed following the method for manufacturing samples 1 and 7 described above. Samples 45 to 51 were thus manufactured. <Manufacture of Sample 52> First, a TiCN powder having an average particle size of 1 μm and a Ti powder having an average particle size of 3 μm were mixed to give an atomic ratio of Ti:C:N=1.0.3:0.3. The resulting mixture was heat-treated for 30 minutes in vacuum at 1200° C., and then ground. An intermetallic compound powder made of TiC0.3N0.3 was thus obtained. Next, the intermetallic compound powder made of TiC0.3N0.3 and an Al powder having an average particle size of 4 m were mixed to give a mass ratio of TiC0.3N0.3:Al=90:10. The resulting mixture was heat-treated for 30 minutes in vacuum at 1000° C. The compound obtained by the heat treatment was uniformly ground with a ball mill using ball media made of cemented carbide and having a diameter of 6 mm. A raw-material powder of the binder phase was thus obtained. Thereafter, following the method for forming cBN sintered body D described above, a cBN sintered body H was obtained. A base material of sample 52 was formed following the method for manufacturing the base material of sample 1 described above, using the obtained cBN sintered body H. Next, layers D, B, C, and A were sequentially formed following the method for manufacturing samples 45 to 51 described above. Sample 52 was thus manufactured. <Manufacture of Sample 53> First, a TiC powder having an average particle size of 1 μm and a Ti powder having an average particle size of 3 μm were mixed to give an atomic ratio of Ti:C=1.0.6. The resulting mixture was heat-treated for 30 minutes in vacuum at 1200° C., and then ground. An intermetallic compound powder made of TiC0.6 was thus obtained. Next, the intermetallic compound powder made of TiC0.6 and an Al powder having an average particle size of 4 μm were mixed to give a mass ratio of TiC0.6:Al=90:10. The resulting mixture was heat-treated for 30 minutes in vacuum at 1000° C. The compound obtained by the heat treatment was uniformly ground with a ball mill using ball media made of cemented carbide and having a diameter of 6 mm. A raw-material powder of the binder phase was thus obtained. Thereafter, following the method for forming a cBN sintered body D described above, a cBN sintered body I was obtained. A base material of sample 53 was formed following the method for manufacturing the base material of sample 1 described above, using the obtained cBN sintered body I. Next, layers D, B, C, and A were sequentially formed following the method for manufacturing samples 45 to 51 described above. Sample 53 was thus manufactured. <Manufacture of Sample 54> Sample 54 was manufactured following the method for manufacturing sample 1 described above, except that layers B. C, and D were not formed. <Manufacture of Sample 55> Sample 55 was manufactured following the method for manufacturing sample 1 described above, except that layers A and C were not formed. Tables 1 and 2 show the compositions and thicknesses of layers A, B, C, and D contained in samples 1 to 55. TiCN*01 to TiCN*08 in Table 1 are as shown in Table 4. TiN*11 in Table 1 is as shown in Table 5. <Measurement of Flank-Face Wear Amount VB and Surface Roughness Rz> Using the manufactured samples 1 to 55, cutting was performed (cutting distance: 4 km) under the following cutting conditions. Then, a flank-face wear amount VB was measured with an optical microscope, and surface roughness Rz of the surface of a workpiece was measured in accordance with the JIS standard. Measured results of flank-face wear amounts VB are shown in the “VB (mm)” column of Table 6, and measured results of surface roughness Rz of surfaces of workpieces are shown in the “Rz (μm)” column of Table 6. A smaller VB indicates higher resistance to flank-face wear of the surface-coated boron nitride sintered body tool. A smaller Rz indicates higher resistance to boundary wear of the surface-coated boron nitride sintered body tool. In the present Examples, an Rz of 3 μm or smaller is satisfactory. (Cutting Conditions) Workpiece: high hardness steel (SCM415H/HRC60) Cutting speed: 200 m/min Feed rate: f=0.1 mm/rev Cutting depth: ap=0.1 mm Cutting oil: an emulsion (manufactured by Japan Fluid System under the trade name “System Cut 96”) 20-fold diluted (wet state). <Measurement of Tool Life> Using the manufactured samples 1 to 55, cutting was performed under the above-described cutting conditions. Specifically, the following procedure was repeated: the workpiece was cut to a certain cutting distance, and then surface roughness Rz of the workpiece was measured with a surface roughness meter. When surface roughness Rz of the workpiece exceeded 3.2 μm, cutting was stopped, and a cutting distance at that moment {(certain cutting distance)×(number of times n of cutting when surface roughness Rz of workpiece exceeded 3.2 μm)} was determined. A cutting distance immediately before surface roughness Rz of the workpiece exceeded 3.2 μm {(certain cutting distance)×(n−1)} was also determined. Then, using the specific value of surface roughness Rz of the workpiece when surface roughness Rz of the workpiece exceeded 3.2 μm and the cutting distance at that moment, as well as the specific value of surface roughness Rz of the workpiece immediately before surface roughness Rz of the workpiece exceeded 3.2 μm and the cutting distance at that moment, the relationship between the cutting distance and surface roughness Rz of the workpiece was linearly approximated to determine a cutting distance at the point of time when surface roughness Rz of the workpiece reached 3.2 μm. The results are shown in the “Cutting Distance (km)” column of Table 6. A longer cutting distance indicates higher resistance to flank-face wear, higher resistance to crater wear, and higher resistance to boundary wear of the surface-coated boron nitride sintered body tool. In the present Examples, a cutting distance of 8 km or longer is satisfactory. <Results and Consideration> The results are shown in Table 6. Samples 2 to 5, 8 to 12, 14 to 19, 21 to 24, 26 to 38, 40 to 43, and 45 to 53 each have a small VB, an Rz of 3 μm or smaller, and a cutting distance of 8 km or longer. This has revealed that these samples have excellent resistance to flank-face wear, resistance to crater wear, and resistance to boundary wear. On the other hand, samples 1, 6, 7, 13, 20, 25, 39, 44, 54, and 55 each have an Rz of 3 μm or greater, and a cutting distance of about 3 to 4 km. This has revealed that these samples have poor resistance to flank-face wear, resistance to crater wear, and resistance to boundary wear. <Samples 3, 10, 54, and 55> First, samples 10 and 55 are considered. Samples 10 and 55 are very similar in the composition of layer B, the composition of layer D, the thickness of layer D, and the thickness of the coating layer. However, sample 10 includes layer A, while sample 55 does not include layer A. Sample 10 has an Rz of 2.5 μm or smaller and a cutting distance of 12 km or longer, while sample 55 has a VB about twice as high as that of sample 10, an Rz of 3 μm or greater, and a cutting distance of about 4 km. Next, samples 3 and 54 are considered. Samples 3 and 54 are very similar in the composition and thickness of layer A; however, sample 3 includes layer B, while sample 54 does not include layer B. Sample 3 has an Rz of about 2.5 μm and a cutting distance of about 10 km, while sample 55 has an Rz of 3 μm or greater and a cutting distance of about 4 km. The foregoing has revealed that a surface-coated boron nitride sintered body tool not including any one of layers A and B has poor resistance to flank-face wear, resistance to crater wear, and resistance to boundary wear. Surprisingly, however, a surface-coated boron nitride sintered body tool including both layers A and B is excellent in terms of all of resistance to flank-face wear, resistance to crater wear, and resistance to boundary wear. This finding was made for the first time by the present inventors. <Samples 1 to 6> Since samples 1 to 6 differ in the number of B1 compound layers and the number of B2 compound layers, they differ in the thickness of layer B. Samples 1 and 6 each have an Rz greater than 3 μm and a cutting distance of about 4 km. On the other hand, samples 2 to 5 each have an Rz of about 2.5 μm and a cutting distance of 8 km or longer. It has been revealed that when the overall thickness of layer B is not less than 0.05 μm and not more than 5 μm, the tool performance is enhanced in high-precision machining using the surface roughness of the workpiece as a criterion of determining the life of the surface-coated boron nitride sintered body tool. Furthermore, samples 3 and 4 each have an Rz of about 2.5 μm and a cutting distance of 9.5 km or longer. Therefore, it has also been revealed that the overall thickness of layer B is preferably not less than 0.1 μm and not more than 5 μm, and more preferably not less than 0.5 μm and not more than 2 μm. <Samples 7 to 13> Samples 7 to 13 differ in the thickness of B1 compound layers and the thickness of B2 compound layers. Sample 7 has an Rz greater than 3 μm and a cutting distance of about 3.5 km. With respect to samples 8 to 12, as each of the thickness of B1 compound layers and the thickness of B2 compound layers increases, Rz decreases and the cutting distance increases. However, when each of the thickness of B1 compound layers and the thickness of B2 compound layers further increases, Rz gradually increases and the cutting distance gradually decreases. Specifically, sample 10 has the smallest Rz and the longest cutting distance. From these facts, it has been revealed that there is a preferred upper limit value for the thickness of B1 compound layers and the thickness of B2 compound layers. Furthermore, sample 13 has an Rz greater than 3 μm and a cutting distance of about 3.5 km. The foregoing has revealed that when each of the thickness of B compound layers and the thickness of B2 compound layers is not less than 0.5 nm and less than 30 nm, the tool performance is enhanced in high-precision machining using the surface roughness of the workpiece as a criterion of determining the life of the surface-coated boron nitride sintered body tool. Furthermore, samples 9 to 12 each have an Rz of 2.5 μm or smaller and a cutting distance of 10 km or longer. Furthermore, samples 9 to 11 each have an even smaller Rz and a cutting distance of 11 km or longer. From these facts, it has also been revealed that each of the thickness of B1 compound layers and the thickness of B2 compound layers is preferably not less than 1 nm and not more than 28 nm, more preferably not less than 1 nm and not more than 15 nm, and even more preferably not less than 3 nm and not more than 10 nm. <Samples 14 to 19> While samples 14 to 19 have different compositions of layer A, they each have an Rz of 3 μm or smaller and a cutting distance of 9 km or longer. This has revealed that samples 14 to 19 have excellent resistance to flank-face wear, resistance to crater wear, and resistance to boundary wear. <Samples 20 to 25> Samples 20 to 25 differ in the composition of B1 compound layers. Samples 20 and 25 each have an Rz of 3.5 μm or greater and a cutting distance of about 3 km. On the other hand, samples 21 to 24 each have an Rz of 3 μm or smaller and a cutting distance of 8 km or longer. From these facts, it has been revealed that when the Si composition of B1 compound layers is not less than 0.01 and not more than 0.25, the tool performance is enhanced in high-precision machining using the surface roughness of the workpiece as a criterion of determining the life of the surface-coated boron nitride sintered body tool. Furthermore, samples 21 to 23 each have an Rz of 2.5 μm or smaller and a cutting distance of 10 km or longer. Furthermore, samples 22 and 23 each have an even smaller Rz and a cutting distance of 12 km or longer. It has therefore been revealed that the Si composition is preferably not less than 0.01 and not more than 0.2, and more preferably not less than 0.05 and not more than 0.15. <Samples 26 to 32> While samples 26 to 32 have different compositions of N of layer C, they each have an Rz of 3 μm or smaller and a cutting distance of 8 km or longer. Furthermore, samples 28 to 31 each have an Rz of 2.5 μm or smaller and a cutting distance of 10 km or longer. From these facts, it has been revealed that the composition of N of layer C is preferably not less than 0 and not more than 0.85, more preferably more than 0 and less than 0.7, and even more preferably not less than 0.2 and not more than 0.5. <Samples 33 to 38> While samples 33 to 38 differ in the thickness of layer C, they each have an Rz of 3 μm or smaller and a cutting distance of 8 km or longer. Furthermore, samples 34 to 36 each have an Rz of 2.5 μm or smaller and a cutting distance of 11 km or longer. From these facts, it has been revealed that the thickness of layer C is preferably not less than 0 μm and not more than 1 μm, more preferably not less than 0.005 μm and not more than 0.5 μm, and even more preferably not less than 0.01 μm and not more than 0.2 μm. <Samples 39 to 44> Samples 39 to 44 differ in the composition of Al of the B2 compound layers. Samples 39 and 44 each have an Rz greater than 3 μm and a cutting distance of about 3 to 4 km. On the other hand, samples 40 to 43 each have an Rz of 2.6 μm or smaller and a cutting distance of 9 km or longer. From these facts, it has been revealed that when the Al composition of the B2 compound layers is not less than 0.23 and not more than 0.8, the resistance to flank-face wear, the resistance to crater wear, and the resistance to boundary wear of the surface-coated boron nitride sintered body tool are improved. Furthermore, samples 41 to 43 each have an Rz of 2.4 μm or smaller and a cutting distance of 11 km or longer. It has therefore been revealed that the Al composition of B2 compound layers is preferably not less than 0.5 and not more than 0.75, and more preferably not less than 0.6 and not more than 0.75. <Samples 45 to 53> While samples 45 to 53 differ in the composition of their cBN sintered body, they each have a VB not significantly high, an Rz of 3 μm or smaller, and a cutting distance of 8 km or longer. Furthermore, samples 47 to 53 each have an Rz of 2.5 μm or smaller and a cutting distance of 10 km or longer. From these facts, it has been revealed that the volume content of cBN in the cBN sintered body is preferably not less than 30% and not more than 85% by volume, and more preferably not less than 50%/o and not more than 65% by volume. It should be understood that the embodiments disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.
|
||||||
