Hard and tough sintered body

Abstract: A sintered body consists of high-pressure phase boron nitride, a residual binder phase and an unavoidable impurity, and contains high-pressure phase boron nitride powder which is previously coated with a binder before sintering. The coating binder consists of an element belonging to the group 4a, 5a or 6a of the periodic table or the like, and has an average thickness of 5 to 300 nm. Binder powder other than the coating binder consists of an element belonging to the group 4a, 5a or 6a of the periodic table or the like, and a contact ratio of high-pressure phase boron nitride is at least 20 % and less than 50 %. Thus provided is a high-pressure phase boron nitride-based hard and tough sintered body for a cutting tool which is improved in wear resistance and chipping resistance.

Description: The present invention generally relates to a sintered body, and more specifically, it relates to a high-pressure phase boron nitride-based hard and tough sintered body for a cutting tool such as a milling cutter or an end mill, which is improved in wear resistance and chipping resistance. Description of the Background Art High-pressure phase boron nitride represented by cBN (cubic boron nitride) is the next hardest substance to diamond and has lower reactivity with a ferrous material as compared with diamond. Thus, cBN sintered bodies are applied to various cutting tools. A hard and tough material is awaited in application to not only a cutting tool but a wear-resistant component such as a wear-resistant component provided on a sliding part or a wear-resistant component employed for a defensive wall, for example. Cubic boron nitride is hard to sinter similarly to diamond, and stable under a high pressure. Therefore, limited sintering conditions of at least 2000°C and at least 8 GPa are required for sintering only cBN particles. The cBN particles are not bonded to each other under industrial sintering conditions of about 1400°C and 4 GPa. In a sintered body consisting of cBN particles and a binder phase, on the other hand, reaction is caused in grain boundaries between the cBN particles and the binder phase. Thus, a cBN sintered body which is excellent in holding power for cBN particles can be obtained. However, a general cBN sintered body is obtained by mixing cBN powder with binder powder and sintering the mixture, and the cBN particles are not completely homogeneously mixed with the binder. In a partial region of the sintered body, therefore, cBN particles are directly in contact with each other or in contact with a binder phase hardly reacting with the cBN particles, to result in an unsintered part. Thus, the sintered body is inferior in wear resistance or chipping resistance. Japanese Patent Laying-Open Nos. 58-58247 (1983), 58-60678 (1983) and 5-186844 (1993) propose typical means for solving the aforementioned problem. Japanese Patent Laying-Open No. 58-58247 discloses a tough boron nitride-based superhigh pressure sintered material for a cutting or wear-resistant tool which is composed of cubic boron nitride and a binder phase consisting of a boride and a carbide of at least one of Ti, Hf, Zr and Mo so that a layer of the boride having an average thickness of 0.1 to 2 µm encloses the cubic boron nitride. Japanese Patent Laying-Open No. 58-60678 discloses a tough boron nitride-based superhigh pressure sintered material for a cutting or wear-resistant tool which is composed of cubic boron nitride and a binder phase consisting of a nitride and a carbide of at least one of Ti, Hf and Si so that a layer of a nitride having an average thickness of 0.1 to 2 µm encloses the cubic boron nitride. Japanese Patent Laying-Open No. 5-186844 discloses a tough boron nitride-based superhigh pressure sintered material for a cutting or wear-resistant tool which is composed of cubic boron nitride and/or wurtzite boron nitride boron nitride and a binder phase consisting of at least one of metals belonging to the groups 4a, 5a and 6a of the periodic table, carbides, nitrides, oxides and borides of Al, Si, Fe, Ni and Co, an oxide and a nitride of a rare earth metal and a solid solution thereof, and Fe, Ni and Co so that a layer of at least one of nitrides and borides of Ti, Hf, Zr, Mo, Al and Si and solid solutions thereof having an average thickness of 0.5 to 90 nm encloses the cubic boron nitride and/or the wurtzite boron nitride. The high-pressure phase boron nitride-based sintered bodies described in the aforementioned gazettes are obtained by coating high-pressure phase boron nitride particles with binders and sintering the same. Thus, these sintered bodies can reduce unsintered parts, and are improved in wear resistance and chipping resistance. In each of these sintered bodies, however, the coating binder is made of ceramics which is a brittle material, and this ceramics is so thermally and chemically stable that the same causes insufficient reaction with the high-pressure phase boron nitride to result in insufficient adhesion strength between the coating binder and the high-pressure phase boron nitride particles. Thus, the coating binder is readily chipped and separated from the high-pressure phase boron nitride particles when the same is mixed with another binder or pressurized by a superhigh pressure sintering device or a hot press. In the high-pressure phase boron nitride-based sintered body employing such powder materials, the high-pressure phase boron nitride particles are frequently in contact with each other to insufficiently homogenize the sintered body structure. The present invention has been proposed in order to solve the aforementioned problems, and an object thereof is to provide a high-pressure phase boron-nitride based hard and tough sintered body for a cutting tool such as a milling cutter or an end mill, which is improved in wear resistance and chipping resistance. The hard and tough sintered body for a cutting tool according to the present invention consists of: a high-pressure boron nitride phase comprising boron nitride particles, a residual binder phase comprising a coating binder, and an additional binder other than said coating binder, and an unavoidable impurity, wherein said coating binder consists of at least one material selected from Group 4a, 5a and 6a elements of the Periodic Table, Al, Si and an iron family element, and intermetallic compounds thereof, wherein said coating binder forms a coating on said boron nitride particles, with said coating having an average thickness of 5 to 300 nm, wherein said additional binder consists of at least one material selected from elements belonging to Groups 4a, 5a and 6a of the Periodic Table, a nitride, a carbide, a boride, an oxide of Si and solid solutions thereof, an Al compound, Al, and an iron family metal, and wherein from 20% to 50% of said high-pressure phase boron nitride particles in an arbitrary region of said sintered body containing at least 100 of said high-pressure phase boron nitride particles are in contact with each other, and wherein the sintered body includes both of AlN and AlB 2 . According to a preferred embodiment of the present invention, the aforementioned coating binder is obtained by infiltrating at least one material selected from a group of Al, Co, Ni, W and WC, which is the additional binder other than the coating binder, in high pressure phase boron nitride which is an intermetallic compound of at least one material selected from a group of Ti, Zr and Hf and Al, and sintering the same. It is also preferable that the aforementioned high-pressure phase boron nitride is cubic boron nitride, the aforementioned coating binder is an intermetallic compound of Al and at least one material selected from the group of Ti, Zr and Hf and Al, and the aforementioned additional binder consists of at least one material selected from a group of TiN, ZrN, HfN, TiC, ZrC, HfC and TiAlN and solid solutions thereof, Al and an Al compound. In this case, reactivity with the cubic boron nitride is further effectively improved due to interaction of Ti, Zr and/or Hf as compared with the case of coating the powder simply with Al. Preferably, the mean particle diameter of said high pressure phase boron nitride particles is not more than 20 µm, and the transverse rupture strength under a 4 mm span condition is not more than 20 kgf/mm 2 . The transverse rupture strength is measured by dissolving a rectangular test piece prepared from the sintered body of 6 mm in length, 3 mm in width and 0.4 to 0.45 mm in thickness in fluoronitric acid, said acid being prepared by mixing 40 ml of double-diluted nitric acid having a concentration of 60 to 65 % and 10 ml of hydrofluoric acid having a concentration of 45 to 50 % in a closed vessel at 120 to 150°C for 3 to 5 hours. 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 drawing. Fig. 1 illustrates the results of a cutting test. A sintered body according to the present invention can be obtained by sintering high-pressure phase boron nitride powder whose surface is coated with a binder by chemical vapor deposition (CVD), physical vapor deposition (PVD), electroless plating or mechanical mixing such as mechanofusion, for example, in a conventional plasma sintering device, a hot press or a superhigh sintering device. A binder other than the coating binder can be infiltrated in situ during sintering, or mixed into the binder-coated high-pressure phase boron nitride powder by a conventional powder metallurgical method represented by a ball mill method or the like, to be thereafter sintered in the aforementioned method. Examples of the present invention are now described with concrete numerical values. Example 1 Cubic boron powder materials having mean particle diameters of 15 µm were wet-mixed with various intermetallic compound powder materials in a planetary mill, in a mixed solution of PEG (polyethylene glycol), PAMA (polyalkyl methacrylate) and ethyl alcohol with drying. The dried mixed powder materials were vacuum-heated in a vacuum furnace under conditions of 1.3 × 10 -3 Pa (10 -5 Torr) and 1200°C for one hour. After the heat treatment, the powder surfaces were observed with an EPMA, to find that the cBN powder materials were homogeneously coated with the intermetallic compounds. Then, the coated cBN powder materials and binder powder materials were charged in Mo capsules provided with Al plates, and sintered in a belt-type superhigh pressure device under conditions of 5.2 GPa and 1480°C for 20 minutes with infiltration of an Al solution. For the purpose of comparison, a cBN sintered body coated with no binder was prepared from starting materials of the same composition as the above. After the sintering, the Mo capsules were recovered and removed by grinding, and the compositions and structures of the sintered bodies were analyzed by X-ray analysis, with an SEM and a TEM, and by EDXD and FE-AES with a beam diameter of 45 nm. Contact ratios of cBN particles were determined from surface images analyzed by FE-AES. Table 1 shows the details of the prepared sintered bodies. No. Starting Material Sintered Body Coated cBN Powder Component of Coating Binder Average Thickness of Coating Binder (nm) cBN Content (vol%) Identified Component Contact Ratio of cBN Particles (%) × 1 Al 3 92 cBN, AlN, AlB2 53 Inventive 2 Al 5 90 cBN, AlN, AlB2 49 Inventive 3 Al 295 88 cBN, AlN, AlB2 48 × 4 Al 310 87 cBN, AlN, AlB2 48 Inventive 5 Ti 25 88 cBN, AlN, AlB2, TiN, TiB2 48 Inventive 6 Zr 295 88 cBN, AlN, AlB2, ZrN, zrB2 48 Inventive 7 Hf 295 88 cBN, AlN, AlB2, HfN, HfB2 48 Inventive 8 W 295 88 cBN, AlN, AlB2, WB2, WN 48 Inventive 9 TiAl 295 88 cBN, AlN, AlB2, TiN, TiB2 48 Inventive 10 ZrAl 295 88 cBN, AlN, AlB2, ZrN, ZrB2 48 Inventive 11 HfAl 295 88 cBN, AlN, AlB2, HfN, HfB2 48 Inventive 12 TiAl3 295 88 cBN, AlN, AlB2, TiN, TiB2 48 Inventive 13 TiSi2 295 88 cBN, AlN, AlB2, TiN, TiB2, Si 3 N 4 48 Inventive 14 Co 295 88 cBN, AlN, AlB2, CoB 48 Inventive 15 CoAl 295 88 cBN, AlN, AlB2, CoB 48 Inventive 16 NiAl 295 88 cBN, AlN, AlB2, NiB 48 Comparative 17 - - 88 cBN, AlN, AlB2 93 x: cBN sintered body employing coated cBN particles out of the inventive scope Inventive: cBN sintered body employing coated cBN particles within the inventive scope Comparative: cBN sintered body coated with no binder prepared for the purpose of comparison Test pieces of 6 mm in length, 3 mm in width and 0.5 to 0.55 mm in thickness were cut out from the sintered bodies, for measuring transverse rupture strength (strength) under 4 mm span conditions. Further, sintered body tips for cutting (SNMG 120808 under ISO standards) were prepared for performing a cutting test under the following conditions: Workpiece: round bar of SKH 51 in hardness (HRC) of 60 Cutting Speed (V): 80 m/min. Depth of Cut: 0.3 mm Feed Rate: 0.15 mm/rev. Type: wet Cutting Time: 30 min. Table 2 shows the results of this cutting test. No. Transverse Rupture Strength (kgf/mm2) Flank Wear Width (mm) × 1 85 chipped in 25 min. Inventive 2 95 0.045 Inventive 3 95 0.044 × 4 90 chipped in 28 min. Inventive 5 95 0.043 Inventive 6 95 0.043 Inventive 7 95 0.043 Inventive 8 95 0.047 Inventive 9 100 0.042 Inventive 10 100 0.042 Inventive 11 100 0.042 Inventive 12 100 0.042 Inventive 13 100 0.042 Inventive 14 95 0.042 Inventive 15 100 0.042 Inventive 16 100 0.042 Comparative 17 65 chipped in 10 min. Example 2 High-pressure phase boron nitride powder materials having mean particle diameters of 0.5 µm were coated with various metals, intermetallic compounds, AlB 2 , TiB 2 and AlN as shown in Table 3 in an RF sputtering PVD device. At this time, the degree of vacuum in a furnace was set at 1.3 × 10 -2 (10 -4 Torr) thereafter Ar gas was introduced to maintain the atmosphere at 0.13 Pa (10 -3 Torr), and the furnace was heated to 300°C, for coating the materials under conditions of power of 5 KW and frequency of 15 MHz for 10 hours. Then, mixed powder materials which were prepared by blending the coated high-pressure phase boron nitride powder materials and binder powder materials to attain the structures shown in Table 3 were ball-milled and thereafter sintered in a belt-type superhigh pressure device similarly to Example under conditions of 4.5 GPa and 1300°C for 10 minutes, with no infiltration. For the purpose of comparison, some cBN sintered bodies were prepared from starting materials coated with no binders. Table 3 shows the compositions and structures of the prepared sintered bodies analyzed similarly to Example 1. Similarly to Example 1, transverse rupture strength (strength) was measured and a cutting test was made under the following conditions: Tip: SNGN 120808 Workpiece: case-hardened SCM 420H in hardness (HRC) of 60 having V-shaped grooves in six portions on its outer periphery Cutting Speed (V): 150 m/min. Depth of Cut: 0.2 mm Feed Rate: 0.1 mm/rev. Type: dry Table 4 shows the results of the cutting test. No. Transverse Rupture Strength (kgf/mm2) Number of Impacts up to Chipping Flank Wear Width with 50.000 Impacts Inventive 1 140 77.000 0.122 Inventive 2 130 70.000 0.101 Inventive 3 135 79.000 0.107 Inventive 4 140 81.000 0.103 × 5 105 49.000 chipped Inventive 6 133 79.000 0.097 Inventive 7 130 78.000 0.111 Inventive 8 125 69.000 0.096 Inventive 9 140 83.000 0.129 Inventive 10 118 62.000 0.085 Inventive 11 125 72.000 0.121 Inventive 12 125 79.000 0.102 Inventive 13 150 95.000 0.149 Inventive 14 125 72.000 0.098 Comparative 15 115 54.000 0.119 Comparative 16 113 52.000 0.128 Comparative 17 110 47.000 chipped Comparative 18 100 29.000 chipped Example 3 Cubic boron nitride powder materials having mean particle diameters of 10 µm were coated with Al through an RF sputtering PVD device. At this time, the degree of vacuum in a furnace was set at 1.3 × 10 -2 Pa (10 -4 Torr), Ar gas was introduced to maintain the atmosphere at 0.13 Pa (10 -3 Torr), and the furnace was heated to 200°C, for coating the materials under conditions of power of 5 KW and a frequency of 12 MHz for 300 minutes. After the mixed powder materials were recovered, the powder surfaces were observed with a TEM, to find that the cBN powder materials were homogeneously coated with Al in average layer thicknesses of 150 nm. Then, the cBN powder materials coated with binders were charged in Mo capsules provided with Al plates, and sintered in a belt-type superhigh pressure device under conditions of 4.8 GPa and 1450°C for 15 minutes. For the purpose of comparison, cBN sintered bodies coated with no binders were prepared from starting materials of the same composition as the above. After the sintering, the Mo capsules were recovered and removed by grinding, and the compositions and structures of the sintered bodies were analyzed by X-ray analysis, with an ICP and a TEM, and by AES. Table 5 shows the details of the prepared sintered bodies. No. Mean Particle Diameter of cBN (µm) cBN content (vol%) cBN Contact Ratio (%) Identfied Component 1 Inventive 15 87 40 cBN, AlN, AlB 2 2 Comparative 15 87 82 cBN, AlN, AlB 2 3 Comparative 15 88 82 cBN, AlN, AlB 2 4 Comparative 3 85 85 cBN, AlN, W 2 Co 21 B 6 W 3 Co 3 B, WCo Inventive: cBN sintered body employing coated cBN particles within the inventive scope Comparative No. 2: cBN sintered body coated with no binder prepared for the purpose of comparison Comparative No. 3: commercially available cBN sintered body Comparative No. 4: commercially available cBN sintered body For the purpose of comparison, commercially available cBN sintered bodies were also investigated. Then, rectangular test pieces of 6 mm in length, 3 mm in width and 0.4 to 0.45 mm in thickness were prepared from these sintered bodies and dissolved in fluoronitric acid prepared by mixing 40 ml of double-diluted nitric acid having a concentration of 60 to 65 % and 10 ml of hydrofluoric acid having a concentration of 45 to 50 % with each other in a closed vessel at 140°C for 3 hours. Thereafter the transverse rupture strength was measured under a 4 mm span condition. At this time, the transverse rupture strength was also measured before the dissolution. Table 6 shows the results. No. Transverse Rupture Strength before Dissolution (kgf/mm 2 ) Transverse Rupture Strength after Dissolution (kgf/mm 2 ) 1 Inventive 130 fell to powdery pieces to allow no measurement of transverse purture strength 2 Comparative 110 15 3 Comparative 100 10 4 Comparative 120 20 Further, sintered body tips for cutting (SNMG 120812 under ISO standards) having rectangular major surfaces were formed and subjected to a cutting test under the following conditions: Workpiece: round bar of FCD 450 in hardness (HRB) of 180 having two V-shaped grooves in two portions on its outer periphery Cutting Speed (V): 200 m/min. Depth of Cut: 0.5 mm Feed Rate: 0.2 mm/rev. Type: wet Fig. 1 shows the results of the cutting test. Only the inventive cBN sintered body fell to pieces after the dissolution, while the remaining cBN sintered bodies maintained sufficient strength (transverse rupture strength) for handling. It is inferred that fluoronitric acid capable of dissolving components other than cBN was not able to completely infiltrate into the comparative sintered bodies containing cBN particles which were directly in contact with each other. In other words, it is inferred that the inventive sintered body containing cBN particles whose outer peripheries were covered with binder layer components dissolved in fluoronitric acid fell into powdery pieces upon dissolution. 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 hard and tough sintered body for a cutting tool consisting of: a high-pressure boron nitride phase comprising boron nitride particles, a residual binder phase comprising a coating binder, and an additional binder other than said coating binder, and an unavoidable impurity, wherein said coating binder consists of at least one material selected from Group 4a, 5a and 6a elements of the Periodic Table, Al, Si and an iron family element, and intermetallic compounds thereof, wherein said coating binder forms a coating on said boron nitride particles, with said coating having an average thickness of 5 to 300 nm, wherein said additional binder consists of at least one material selected from elements belonging to Groups 4a, 5a and 6a of the Periodic Table, a nitride, a carbide, a boride, an oxide of Si and solid solutions thereof, an Al compound, Al, and an iron family metal, and wherein from 20% to 50% of said high-pressure phase boron nitride particles in an arbitrary region of said sintered body containing at least 100 of said high-pressure phase boron nitride particles are in contact with each other, and wherein the sintered body includes both of AlN and AlB 2 . A hard and tough sintered body in accordance with claim 1, wherein the content of said high pressure phase boron nitride particles in said sintered body is at least 80 percent and not more than 99.9 percent by volume. A hard and tough sintered body in accordance with claim 1 or claim 2, wherein the mean particle diameter of said high pressure phase boron nitride particles is not more than 20 µm, and the transverse rupture strength under a 4 mm span condition is not more than 20 kgf/mm 2 , wherein the transverse rupture strength is measured by dissolving a rectangular test piece prepared from said sintered body of 6 mm in length, 3 mm in width and 0.4 to 0.45 mm in thickness in fluoronitric acid, said acid being prepared by mixing 40 ml of double-diluted nitric acid having a concentration of 60 to 65 % and 10 ml of hydrofluoric acid having a concentration of 45 to 50 % in a closed vessel at 120 to 150°C for 3 to 5 hours. A hard and tough sintered body in accordance with any one of claims 1 to 3, wherein said coating binder is obtained by infiltrating at least one material selected from Al, Co, Ni, W and WC, said additional binder being other than said coating binder, in high pressure phase boron nitride being an intermetallic compound of at least one material selected from Ti, Zr and Hf and Al and sintering the same. A hard and tough sintered body in accordance with any one of claims 1 to 4, wherein said high pressure phase boron nitride is cubic boron nitride. A hard and tough sintered body in accordance with any one of claims 1 to 5, wherein said coating binder is an intermetallic compound of at least one material selected from Ti, Zr and Hf and Al. A hard and tough sintered body in accordance with any one of claims 1 to 6, wherein said additional binder consists of at least one material selected from TiN, ZrN, HfN, TiC, ZrC, HfC and TiAlN and solid solutions thereof, Al and an Al compound.

Inventors: FUKAYA TOMOHIRO, KUKINO SATORU, NAKAI TETSUO, SHIRAISHI JUNICHI, UESAKA SHINYA, FUKAYA, TOMOHIRO, KUKINO, SATORU, NAKAI, TETSUO, SHIRAISHI, JUNICHI, UESAKA, SHINYA
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1997
Main classification: C04B35
IPC codes: C04B35