Drill bit insert and drill bit

Abstract

A drill bit insert attached to a tip portion of a drill bit to perform drilling, includes: an insert body (1) that includes: a rear end portion buried in a bit body of the drill bit; and a tip portion protruding from a surface of the drill bit and tapered toward a tip side of the insert body, in which a surface of at least the tip portion of the insert body (1) made of polycrystalline cubic boron nitride compact (4) sintered using a catalytic metal containing Al and at least one selected from the group consisting of Co, Ni, Mn, and Fe and containing 70 vol % to 95 vol % of cubic boron nitride.

Claims

1. A drill bit insert attached to a tip portion of a drill bit to perform drilling, the drill bit insert comprising: an insert body that has: a rear end portion buried in a bit body of the drill bit; and a tip portion protruding from a surface of the drill bit and tapered toward a tip side of the insert body, wherein a surface of at least the tip portion of the insert body is made of polycrystalline cubic boron nitride compact sintered using a catalytic metal containing Al and at least one selected from the group consisting of Co, Ni, Mn, and Fe and containing 70 vol % to 95 vol % of cubic boron nitride, and Hv hardness of the polycrystalline cubic boron nitride compact is 3.5 GPa to 4.4 GPa.

2. The drill bit insert according to claim 1, wherein a particle diameter of the cubic boron nitride is 0.5 m to 60 m in the polycrystalline cubic boron nitride compact.

3. The drill bit insert according to claim 2, wherein the polycrystalline cubic boron nitride compact contains a metallic additive containing at least one selected from the group consisting of W, Mo, Cr, V, Zr and Hf.

4. The drill bit insert according to claim 2, wherein a fracture toughness value K.sub.IC of the polycrystalline cubic boron nitride compact is 7 MPa.Math.m.sup.1/2 to 12 MPa.Math.m.sup.1/2.

5. The drill bit insert according to claim 1, wherein the polycrystalline cubic boron nitride compact contains a metallic additive containing at least one selected from the group consisting of W, Mo, Cr, V, Zr and Hf.

6. The drill bit insert according to claim 5 wherein a fracture toughness value K.sub.IC of the polycrystalline cubic boron nitride compact is 7 MPa.Math.m.sup.1/2 to 12 MPa.Math.m.sup.1/2.

7. The drill bit insert according to claim 1, wherein a fracture toughness value K.sub.IC of the polycrystalline cubic boron nitride compact is 7 MPa.Math.m.sup.1/2 to 12 MPa.Math.m.sup.1/2.

8. A drill bit comprising: a bit body; and the drill bit insert according to claim 1 attached to a tip portion of the bit body.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a cross-sectional view showing an embodiment of a drill bit insert of the present invention.

(2) FIG. 2 is a cross-sectional view showing an embodiment of a drill bit of the present invention with the drill bit insert of the embodiment shown in FIG. 1 attached to a tip portion thereof.

DETAILED DESCRIPTION OF THE INVENTION

(3) FIG. 1 is a cross-sectional view showing an embodiment of a drill bit insert of the present invention. FIG. 2 is a cross-sectional view showing an embodiment of a drill bit of the present invention having the drill bit insert of this embodiment attached thereto. The drill bit insert of this embodiment has an insert body 1. This insert body 1 includes: a body 2 made of a hard material such as a cemented carbide; and a hard layer 3 coated on a surface of at least a tip portion (upper portion in FIG. 1) of the body 2 and having higher hardness (Hv hardness) than that of the body 2.

(4) Hv hardness can be measured through a test method defined in Japanese Industrial Standards (JIS) Z2244.

(5) The insert body 1 includes: a rear end portion (lower portion in FIG. 1) formed in a cylindrical or disk shape centered on a center line C of the insert; and a tip portion formed in a hemispherical shape centered on the center line C of the insert with the same radius as that of the cylindrical or disk shape of the rear end portion in this embodiment and having a tapered shape with the outer diameter from the center line C of the insert gradually reduced toward an tip side. That is, the drill bit insert of this embodiment is a button insert.

(6) In this embodiment, as shown in FIG. 1, only the tip portion of the insert body 1 is coated with the hard layer 3, and the tip portion of the insert body 1 including the hard layer 3 is formed in the above-described hemispherical shape. In addition, in this embodiment, as shown in FIG. 1, the hard layer 3 has a two-layer structure composed of an outermost layer 4 and an intermediate layer 5 interposed between the outermost layer 4 and the body 2.

(7) Although not necessary, a maximum thickness of the outermost layer 4 is preferably 0.3 m to 1.5 m, and more preferably 0.4 m to 1.3 m.

(8) Similarly, although not necessary, a maximum thickness of the intermediate layer 5 is preferably 0.2 m to 1.0 m, and more preferably 0.3 m to 0.8 m.

(9) In the hard layer 3, the outermost layer 4 disposed on the surface of the tip portion of the insert body 1 is made of a polycrystalline cubic boron nitride compact sintered using a catalytic metal containing Al and at least one of Co, Ni, Mn, and Fe and containing 70 vol % to 95 vol % of cubic boron nitride. In this embodiment, the intermediate layer 5 is made of a polycrystalline cubic boron nitride compact sintered using the same catalytic metal, but the cubic boron nitride content thereof may be smaller than that of the outermost layer 4.

(10) Although not necessary, the cubic boron nitride content of the intermediate layer 5 is preferably 40 vol % to 70 vol %, and more preferably 45 vol % to 65 vol %.

(11) The particle diameter of the cubic boron nitride is 0.5 m to 60 m in the polycrystalline cubic boron nitride compact of the outermost layer 4. The particle diameter of the cubic boron nitride of the intermediate layer 5 is within the same range, but may be smaller than that of the cubic boron nitride of the outermost layer 4. Further, the polycrystalline cubic boron nitride compacts of the outermost layer 4 and the intermediate layer 5 may contain a metallic additive containing at least one of W, Mo, Cr, V, Zr and Hf in addition to the above-described catalytic metal.

(12) In this embodiment, the Hv hardness of the polycrystalline cubic boron nitride compact of the outermost layer 4 formed as described above is 3.5 GPa to 4.4 GPa, and the fracture toughness value K.sub.IC is 7 MPa.Math.m.sup.1/2 to 12 MPa.Math.m.sup.1/2. The three-point bending strength TRS of the outermost layer 4, measured using a specimen for TRS formed from a disk-like sample with the same composition as that of the outermost layer 4, is 1.2 GPa to 1.5 GPa.

(13) The fracture toughness value K.sub.IC can be measured through a test method defined in ASTM Standard (ASTM) E399.

(14) The outermost layer 4 can be formed by sintering hexagonal boron nitride under ultrahigh pressure and high temperature conditions, as described in Japanese Patent No. 5613970 of the inventors of the present invention. By integrally sintering the outermost layer 4, the intermediate layer 5, and the body 2 made of a cemented carbide, the insert body 1 of the drill bit insert according to this embodiment can be produced.

(15) The drill bit having such drill bit insert attached to the tip portion thereof has a bit body 11 made of steel or the like and having a substantially bottomed cylindrical shape centered on an axis O as show in FIG. 2. The bottomed portion thereof is the tip portion (upper portion in FIG. 2) to which the drill bit insert is attached. In addition, a female threaded portion 12 is formed on the inner periphery of the cylindrical rear end portion (lower portion in FIG. 2). A drill rod connected to a drilling apparatus is screwed into the female threaded portion 12, and by transmitting a striking force and an impelling force toward the tip side in the direction of the axis O and a rotating force around the axis O thereto, the drill bit insert crushes bedrock to form a borehole.

(16) The tip portion of the bit body 11 has a slightly larger outer diameter than the rear end portion, a plurality of discharge grooves 13 extending in parallel with the axis O are formed on the outer periphery of the tip portion with an interval in the circumferential direction. The drill cuttings generated from the bedrock crushed by the drill bit insert are discharged to a rear end side through the discharge groove 13. In addition, a blow hole 14 is formed along the axis O from the bottom surface of the female threaded portion 12 of the bit body 11 having a bottom. The blow hole 14 branches obliquely at the tip portion of the bit body 11, opens to a tip surface of the bit body 11, and ejects a fluid such as compressed air supplied via the drill rod to promote discharge of drill cuttings.

(17) Furthermore, the tip surface of the bit body 11 has a circular face surface 15 centered on the axis O perpendicular to the axis O on the inner periphery side, and a truncated conical gauge surface 16 located on the outer periphery of the face surface 15 and extending toward the rear end side to be closer to the outer periphery side. The blow hole 14 opens to the face surface 15 and the tip end of the discharge groove 13 opens to the outer periphery side of the gauge surface 16. Further, on the face surface 15 and the gauge surface 16, a plurality of fitting holes 17 having a circular cross-section are formed perpendicularly to the face surface 15 or the gauge surface 16 in a manner that the holes 17 avoid opening portions of the blow hole 14 and the discharge groove 13, respectively.

(18) In such fitting holes 17, in a state where the rear end portion of the insert body 1 is buried as shown in FIG. 2, the drill bit inserts are interference-fitted by press fitting, shrink fitting, or the like, or brazed thereby being fixed to the fitting holes 17, that is, the drill bit inserts are buried in the fitting holes 17 and attached thereto. The tip portion of the insert body 1 having the hard layer 3 formed thereon protrudes from the face surface 15 and the gauge surface 16 and crushes bedrock with the above-described striking force, impelling force, and rotating force.

(19) In the drill bit insert with the above-described configuration, the outermost layer 4 of the hard layer 3 coated on the surface of the tip portion of the insert body 1 involved with the drilling is made of a polycrystalline cubic boron nitride compact with a cubic boron nitride content as high as 70 vol % to 95 vol %. Such a polycrystalline cubic boron nitride compact has Hv hardness comparable to a polycrystalline diamond compact of a drill bit insert for a mining tool as described above, while having the fracture toughness value K.sub.IC higher than that (3 MPa.Math.m.sup.1/2 to 6 MPa.Math.m.sup.1/2) of the polycrystalline diamond compact and thus high toughness.

(20) Accordingly, even in a case of drilling a super-hard rock layer, there is little concern that the drill bit insert may be fractured or may chip unexpectedly, and thus the tool life is increased. Thus, it is possible to stably perform drilling over a long period of time. Therefore, in a drill bit having such a drill bit insert attached to a tip portion thereof, the frequency of exchange of the drill bit due to the damage of the drill bit insert is reduced, and thus the time and effort for an exchange operation can be reduced and drilling tasks can be efficiently performed.

(21) Here, in a case where the Hv hardness of the outermost layer 4 is less than 3.5 GPa or the fracture toughness value K.sub.IC is greater than 12 MPa.Math.m.sup.1/2, there is a concern that the wear resistance may be insufficient. In contrast, in a case where the Hv hardness is greater than 4.4 GPa or the fracture toughness value K.sub.IC is less than 7 MPa.Math.m.sup.1/2, there is a concern that the toughness may be impaired and thus sufficient fracture resistance may not be obtained. Therefore, as in this embodiment, it is desirable that the Hv hardness is 3.5 GPa to 4.4 GPa, and the fracture toughness value K.sub.IC is 7 MPa.Math.m.sup.1/2 to 12 MPa.Math.m.sup.1/2.

(22) In addition, the polycrystalline cubic boron nitride compact has low affinity to Fe or Ni, and therefore drilling can be stably performed over a long period of time even in Fe or Ni mines. Furthermore, since a heat resistant temperature is 1,100 C. higher than that of the polycrystalline diamond compact, the drill bit insert can be used even under drilling conditions where it is exposed to high temperatures. Moreover, the polycrystalline cubic boron nitride compact can be ground by a diamond grinding stone, and thus can be effectively reused by resharpening.

(23) In a case where the cubic boron nitride content of the polycrystalline cubic boron nitride compact in the outermost layer 4 is less than 70 vol %, the ratio of direct bonding between cubic boron nitride particles decreases, and thus it is not possible to obtain Hv hardness necessary for drilling of a super-hard rock layer as described above. In a case where the cubic boron nitride content of the outermost layer 4 is greater than 95 vol %, the catalytic metal content is relatively reduced, the catalytic metal is not distributed over the whole sintered compact, unreacted cubic boron nitride particles are generated, and a nonuniform sintered compact is formed. Such unreacted cubic boron nitride particles fall off and the outermost layer 4 is worn early.

(24) Furthermore, as a catalytic metal, Al (essential) and at least one of Co, Ni, Mn, and Fe are contained. Since a polycrystalline cubic boron nitride compact sintered using such metal binders has higher wear resistance and toughness than a polycrystalline cubic boron nitride compact sintered using a ceramic binder such as TiC, TiN, AlN, and Al.sub.2O.sub.3, the above-described effects can be reliably achieved with, in particular, a drill bit insert used in percussion drilling. In addition, in a case where a metallic additive containing at least one of W, Mo, Cr, V, Zr and Hf is contained in addition to the catalytic metals, a sintering reaction of the polycrystalline cubic boron nitride compact can be promoted.

(25) In this embodiment, since the particle diameter of the cubic boron nitride particle is 0.5 m to 60 m in the polycrystalline cubic boron nitride compact of the outermost layer 4 of the hard layer 3, a sintered compact with a uniform fine structure can be formed, and toughness can be reliably retained. That is, in a case where the particle diameter of the cubic boron nitride particle of the outermost layer 4 is less than 0.5 m, there is a concern that the sintered compact has a nonuniform structure and a deviation may be partially caused in hardness and toughness. In a case where the particle diameter of the cubic boron nitride particle is greater than 60 m, the specific surface area of the particle is reduced, and thus there is a concern that the catalytic metal content is reduced and the toughness may be reduced.

(26) In this embodiment, the hard layer 3 has a two-layer structure composed of the outermost layer 4 and the intermediate layer 5. However, the hard layer 3 may have a single layer structure composed of the outermost layer 4 or a multi-layer structure composed of three or more layers. In a case where the hard layer 3 has a multi-layer structure composed of three or more layers, it is desirable that a layer with a cubic boron nitride content of less than 70 vol %, such as the intermediate layer 5 according to the embodiment, is interposed between the outermost layer 4 and the body 2, and it is desirable that the cubic boron nitride content of the intermediate layer 5 is gradually reduced, and thus the Hv hardness is reduced and the fracture toughness value K.sub.IC is increased toward the body 2 from the outermost layer 4. In a case where the hard layer 3 is formed on the tip portion of the body 2 made of a cemented carbide or the like as in this embodiment, it is desirable that the thickness of the hard layer 3 on the center line C of the insert is 0.8 mm or greater in order to retain a certain level of drilling distance, and also not greater than 2 mm in consideration of residual stress in the hard layer 3 caused by a difference in the shrinkage ratio from the cemented carbide during sintering.

(27) On the other hand, instead of coating the body 2 with the hard layer 3 to form the hard layer 3 on the tip portion of the insert body 1, the entire insert body 1 may be made of the same polycrystalline cubic boron nitride compact as the outermost layer 4. In this case, in order to prevent the insert body 1 from breaking or the like, it is desirable that the fracture toughness value K.sub.IC of the polycrystalline cubic boron nitride compact is set to 10 MPa.Math.m.sup.1/2 or greater. In a large drill bit insert with an outer diameter of the insert body 1 of 16 mm or greater and a length of the insert body in a direction of the center line C of the insert of 20 mm or greater, it is desirable that the three-point bending strength TRS is 1.3 GPa or greater.

(28) In this embodiment, the case where the present invention is applied to a button type drill bit insert in which the tip portion of the insert body 1 has a hemispherical shape as described above, is described. However, it is possible to apply the present invention to so-called ballistic type drill bit insert in which the tip portion of the insert body 1 forms a bullet-shape, and to a so-called spike type drill bit insert in which the rear end side of the tip portion has a conical surface shape and decreases in diameter toward the tip side, and of which a tip end has a spherical shape with a smaller radius than that of the cylindrical rear end portion of the insert body 1.

Examples

(29) Next, the effects of the present invention will be verified with examples of a drill bit insert and a drill bit of the present invention. In the examples, based on the embodiments, a hard layer in which a cubic boron nitride (cBN) content of a polycrystalline cubic boron nitride compact, a catalytic metal type, and a composition were changed was sintered integrally with a body made of a cemented carbide containing 94 wt % of WC and 6 wt % of Co under conditions where sintering pressure was 5.8 GPa, sintering temperature was 1,600 C., and sintering time 30 minutes, to produce 11 types of button tips with a radius of 5.5 mm and a length of 16 mm in a direction of a center line of the insert. The radius of the hemisphere formed by a tip portion of an insert body was 5.75 mm. The thickness of the hard layer in the direction of the center line of the insert is 1.5 mm. In all of Examples 1, 2, 5, 6, 9, 10, and 11, the hard layer is a single layer composed of an outermost layer. In Examples 3, 4, 7, and 8, the hard layer has an outermost layer and an intermediate layer as in the embodiment shown in FIG. 1. In Example 9, the particle diameter of cubic boron nitride in a polycrystalline cubic boron nitride compact is 60 m or greater, and in Example 10, the particle diameter is 0.5 m or less.

(30) As comparative examples with respect to Examples 1 to 11, two types of button inserts (Comparative Examples 1 and 2) having a hard layer composed of a single layer of a polycrystalline diamond compact with different diamond contents, a button insert (Comparative Example 3) of which an entire insert body was made of the same cemented carbide containing 94 wt % of WC and 6 wt % of Co as the body, a button insert (Comparative Example 4) having a hard layer composed of two layers of polycrystalline cubic boron nitride compacts where a cubic boron nitride (cBN) content of an outermost layer was less than 70 vol %, a button insert (Comparative Example 5) where a cubic boron nitride content of an outermost layer was greater than 95 vol %, a button insert (Comparative Example 6) sintered using a ceramic binder (TiC) in place of a catalytic metal, and a button insert (Comparative Example 7) having a hard layer composed of a single layer of a polycrystalline cubic boron nitride compact where a cubic boron nitride content of an outermost layer was greater than 95 vol % and a particle diameter of cubic boron nitride was greater than 60 m, were produced to have the same size as in Examples 1 to 11. Except for Comparative Example 3, the thickness of the hard layer in the direction of the center line of the insert was 1.5 mm the same as in Examples 1 to 11.

(31) For each type of the drill bit inserts of Examples 1 to 11 and Comparative Examples 1 to 7, two drill bit inserts were attached to a face surface of a bit body having a bit diameter of 45 mm as shown in FIG. 2, and five drill bit inserts were attached to a gauge surface thereof to produce 18 types of drill bits where a total of seven drill bit inserts are attached. Using these drill bits, drilling tasks were performed to form a plurality of boreholes with a drilling length of 4 m in a mine made of super-hard rock layers and having an average uniaxial compressive strength of 200 MPa. The total drilling length (m) until the drill bit insert reached the end of the tool life was measured, and a damaged state of the insert when the drill bit insert reached the end of the tool life was confirmed.

(32) Drilling conditions were as follows: a drilling apparatus was model No. H205D manufactured by TAMROCK Co., Ltd., striking pressure was 160 bar, feed pressure was 80 bar, rotational pressure was 55 bar, and a water with pressure of 18 bar was supplied from the blow hole. The results of Examples 1 to 4 are shown in Table 1, the results of Examples 5 to 11 are shown in Table 2, and the results of Comparative Example 1 to 7 are shown in Table 3, together with compositions of hard layers of the respective drill bit inserts, and Hv hardness and fracture toughness values K.sub.IC of the outermost layers thereof.

(33) TABLE-US-00001 TABLE 1 Fracture toughness Drilling length Hv hardness of value K.sub.IC of until end of tool Composition of outermost Composition of outermost layer outermost layer life was reached Insert damaged No: layer intermediate layer (GPa) (MPa .Math. m.sup.1/2) (m) state Example 1 90 vol % cBN None 4 9 288 Normal wear (80 vol % 20/40 + 20 vol % 2/4) + 10 vol % (70Co20W10Al wt %) Example 2 82 vol % cBN None 4.2 10 320 Normal wear (0.5/1.3) + 18 vol % (60Co20Ni13V7Al wt %) Example 3 85 vol % cBN 55 vol % cBN 3.7 10.5 256 Normal wear (4/8) + (2/4) + 15 vol % 35 vol % WC + 10 vol % (50Co20Cr16Mo14Al wt %) (70Co20W10Al wt %) Example 4 92 vol % cBN 55 vol % cBN 3.9 8.5 308 Normal wear (10/20) + (2/4) + 8 vol % 35 vol % WC + 10 vol % (47Fe21Mn19Zr13Al wt %) (70Co20W10Al wt %)

(34) TABLE-US-00002 TABLE 2 Fracture toughness Drilling length Hv hardness of value K.sub.IC of until end of tool Insert Composition of outermost Composition of outermost layer outermost layer life was reached damaged No: layer intermediate layer (GPa) (MPa .Math. m.sup.1/2) (m) state Example 5 86 vol % cBN None 3.7 8.7 216 Normal wear (80 vol % 20/40 + 20 vol % 2/4) + 14 vol % (50Co50Al wt %) Example 6 89 vol % cBN None 3.9 11.2 336 Normal wear (0.5 to 1.3) + 11 vol % (80Co10V10Al wt %) Example 7 75 vol % cBN 55 vol % cBN 3.5 10.8 236 Normal wear (4 to 8) + 25 vol % (2/4) + 35 vol % (70Co22Cr8Al wt %) WC + 10 vol % (70Co20W10Al wt %) Example 8 79 vol % cBN 55 vol % cBN 3.6 8.1 204 Normal wear (10 to 20) + 21 vol % (2/4) + 35 vol % (60Ni40Al wt %) WC + 10 vol % (70Co20W10Al wt %) Example 9 90 vol % cBN None 4.2 4.6 184 Chipping (60/80) + 10 vol % (47Fe21Mn19Zr13Al wt %) Example 10 85 vol % cBN None 3.8 9.1 192 Normal wear (0/0.5) + 15 vol % and partial (47Fe21Mn19Zr13Al wt %) chipping Example 11 82 vol % cBN None 3.6 9.8 232 Normal wear (6/12) + 18 vol % (82Co6Hf12Al wt %)

(35) TABLE-US-00003 TABLE 3 Fracture Drilling length Hv hardness toughness until end of of outermost value K.sub.IC of tool life Insert Composition of outermost Composition of layer outermost layer was reached damaged No: layer intermediate layer (GPa) (MPa .Math. m.sup.1/2) (m) state Comparative 85 vol % Diamond None 4.4 4.5 176 Chipping Example 1 (6/12) + 15 vol % Co Comparative 55 vol % Diamond None 3.5 9.1 168 Partial Example 2 (6/12) + 35 vol % WC + chipping 10 vol % Co Comparative WC 94 wt % + Co 6 wt % None 1.3 14.7 16 Normal wear Example 3 Comparative 60 vol % cBN 55 vol % cBN 2.5 12 64 Normal wear Example 4 (0.5/1.3) + 40 vol % (2/4) + 35 vol % WC + (60Co20Ni13V7Al wt %) 10 vol % (70Co20W10Al wt %) Comparative 98 vol % cBN 55 vol % cBN 1.9 6.5 24 Fall-off of Example 5 (0/1) + 2 vol % (2/4) + 35 vol % WC + particles (47Fe21Mn19Zr13Al wt %) 10 vol % (early wear) (70Co20W10Al wt %) Comparative 50 vol % cBN None 3.6 7.2 20 Chipping Example 6 (1/2) + 40 vol % TiC + 6 vol % WC + 4 vol % Al Comparative 98 vol % cBN None 4.4 4.2 8 Early wear and Example 7 (80/100) + 2 vol % chipping (47Fe21Mn19Zr13Al wt %)

(36) From the results, in the drill bits having the drill bit inserts of Comparative Examples 1 to 7 attached thereto, respectively, the drilling length was 176 m even in Comparative Example 1 in which the hard layer was a polycrystalline diamond compact and the drilling distance was long, the end of the tool life was reached due to chipping in Comparative Examples 1 and 2, and the drilling length did not reach 100 m in Comparative Examples 3 to 7. Among these, even in Comparative Examples 4 to 6 where the hard layer was a polycrystalline cubic boron nitride compact, in Comparative Example 4, wear was significantly occurred since the polycrystalline cubic boron nitride compact had a small cubic boron nitride content. In contrast, in Comparative Examples 5 and 7 where the cubic boron nitride content was tool large, the catalytic metal content was insufficient, and a nonuniform structure was thus formed. Therefore, cubic boron nitride particles fell off and early wear occurred. Moreover, chipping also occurred in Comparative Example 7 where the cubic boron nitride particles had a large particle diameter. Furthermore, also in Comparative Example 6 where the polycrystalline cubic boron nitride compact was sintered using a ceramic binder in place of a catalytic metal, the end of the tool life was reached due to chipping. In Comparative Example 6, the drilling length until the end of the tool life was reached was 20 m, and two reasons for this are considered. A first reason is that the polycrystalline cubic boron nitride compact is sintered using a ceramic binder in place of a catalytic metal in Comparative Example 6. A second reason is that a intermediate layer is not provided in Comparative Example 6. In this case, an outermost layer having a thermal expansion rate extremely different from the drill bit insert body is directly provided on a tip portion of the insert body. Therefore, large stress is generated at an interface between the tip portion and the outermost layer due to heat generated during drilling, and causes chipping.

(37) In the drill bits having the drill bit inserts of Examples 1 to 8 and 11 attached thereto, respectively, the wear state was normal wear in all of the cases. Even in Example 8 where the drilling length was the shortest, 200 m or more of drilling was possible, and in Examples 2, 4, and 6, 300 m or more of drilling was possible. In Example 9 where the particle diameter of the cubic boron nitride particle in the polycrystalline cubic boron nitride compact was 60 m or greater and in Example 10 where the particle diameter was 0.5 m or less, chipping was recognized and the drilling length did not reached 200 m. However, the drilling length is longer than in Comparative Examples 1 to 7.

INDUSTRIAL APPLICABILITY

(38) As described above, according to the present invention, it is possible to satisfy both of wear resistance and fracture resistance and thereby prevent a drill bit insert from being fractured or chipping unexpectedly even in a super-hard rock layer. Additionally, it is possible to use the drill bit insert under a wide range of drilling conditions, and effectively reuse the drill bit insert by resharpening.

REFERENCE SIGNS LIST

(39) 1: INSERT BODY 2: BODY 3: HARD LAYER 4: OUTERMOST LAYER 5: INTERMEDIATE LAYER 11: BIT BODY C: CENTER LINE OF INSERT O: AXIS OF BIT BODY 11