- Email: [email protected]
Some properties and cutting performance of polycrystalline cubic boron nitride with no additives
International Journal of
REFRACTORYMETALS & HARDMATERIALS ELSEVIER
International Journal of Refractory Metals & Hard Materials 16 (1998) 403-407
Some properties and cutting performance of polycrystalline cubic boron nitride with no additives T. Ohashi,* K. Yamamoto, Y. Hamada, T. Tanase Central Research Instttute, Mttsubtshl Materials Corporatton, 1-297Kttabukuro-Cho. Omiya, Sa~tama 330-8508. Japan
Recewed 9 March 1998, accepted 8 September 1998
Abstract
Hexagonal BN disks made by chemical vapor deposition were treated under ultra high pressure of 6.8 GPa and at temperatures from 1800 to 2500°C for 1.8 ks. When treated at temperatures above 2100°C, only a c-BN phase was detected. The sample treated at 2100°C had a homogeneous microstructure having fine grain size less than 1/zm, whereas that treated at 2300°C had a heterogeneous one with some large grains The sample treated at 2500°C showed remarkable grain growth. Some physical properties and cutting performance of these compacts were investigated. Then hardness clearly decreased at 2500°C, and the thermal conductivity was higher when synthesized at 2500°C than at 2100°C. The fine grained compact of 2100°C exhibited better wear resistance than the coarse grained one of 2500°C in cutting of Co base super alloy and cemented carbide. © 1998 Pubhshed by Elsevier Science Ltd. All rights reserved Keywords. c-BN; Compact, Gram size; Physicalproperty; Cutting performance
1. Introduction
2. Experimental
Cubic boron nitride (hereafter c-BN) has high hardness and thermal conductivity second only to diamond and a low affinity to ferrous materials. Therefore, c-BN is widely used in grinding and cutting applications for ferrous materials. Convention c-BN based composites, commercially available as a cutting material, contain a ceramic or metallic binder in order to facilitate sintering, to optimize cutting performance, and so on. On the other hand, a polycrystalline c-BN compact with no additives (hereafter PCBN) synthesized by direct transformation from a low pressure phase of BN under ultra high pressure and at high temperature has been hitherto reported [1-6]. However, influences of synthesizing conditions on properties of PCBN such as microstructure, hardness and thermal conductivity have not yet been systematically studied. These interesting subjects were studied in detail in this report. In addition, its cutting performance with two sorts of work materials containing Co, Stellite No. 6 and cemented carbide, was investigated.
A hexagonal BN (hereafter h-BN) disk made by chemical vapor deposition was used as a starting material (purity > 99.9%). Its diameter and thickness were 9.5 and 1.5 ram, respectively. It was wrapped with Ta foil and treated under ultra high pressure of 6.8 GPa and at temperatures from 1800 to 2500°C for 1.8ks using a belt type high pressure apparatus (developed by National Institute for Research in Inorganic Materials, refer to Fig. 1). The treatment temperature was measured with a W - R e thermocouple, and the pressure effect was neglected. The samples obtained were ground with a diamond wheel and subsequently polished with diamond paste less than 2/~m. They were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Vickers hardness (HV) and thermal conductivity measurements which were carried out by a laser flash heated method. For microstructural examinations, they were dipped in molten N a O H for 60 s to etch the grain boundary. For cutting tests, ground samples were cut into 5 mm triangular pieces with a Y A G laser, brazed onto
*Corresponding author Tel +81-48-642-0514;fax +81-48-6497767, e-mad, ohashi@mmcco jp
0263-4368/98/$-- see front matter © 1998Pubhshed by Elsevier ScienceLtd All rights reserved PII: S0263-4368(98)00057-2
404
T. Ohasht et al /Internatmnal Journal of Refractory Metals & Hard Materials 16 (1998) 403-407
6
1
7
/,2
j
3 1
8
Fracture surfaces are shown in Fig. 3 for the samples treated at 2100°C and 2500°C. Though the fracture made of the sample of 2100°C is not clear, that of 2500°C is intragranular with river patterns which indicate cleavage fracture. Twins are observed in c-BN grains as above by SEM observation. This is also confirmed by TEM (Fig. 4). Twins are seen in the grown grains in the samples of 2300°C and 2500°C. They were occasionally observed in fine grains. Figure 5 illustrates hardness and thermal conductivity as a function of treating temperature. The
4
Fig. 1 The belt type high pressure apparatus used in the present experiment" (1) piston, (2) cylinder; (3) Ta capsule; (4) NaCI-(10 or 20)%ZRO2; (5) graphite heater; (6) NaCl-10%ZrOz, (7) steel nng; (8) gasket.
cemented carbide substrates and finally ground to an insert of TNGA160408 form. Work materials tested were Stellite No. 6 (Co-28Cr-4W alloy in as-cast state) and cemented carbide with 11 mass% Co content (HV about 1.4 GPa). A commercially available c-BN-Co composite for multi-purpose application with about 85 vol% c-BN content and a Co alloy binder, was used for comparison. Cutting conditions are summarized in Table 1. 3. Results and discussion
In the sample treated at 1800°C, a so-called compressed h-BN phase [4] (which is reported as a monoclinic phase in a recent article [7]) is observed in addition to a c-BN phase by XRD. When treated over 2100°C, only a c-BN phase is detected. The microstructure by SEM is indicated in Fig. 2. When treated at 2100°C, the microstructure is homogeneous and as fine as, or less than, 1 gin, whereas it becomes heterogeneous with some coarse grains at 2300°C and shows remarkable grain growth at 2500°C. Twins are clearly observed in coarse grains. These results correspond qualitatively with those by Corrigan [4]. Table 1 Cutting conditions Work piece/condition
Stellite No. 6
Cemented carbide
Cutting speed (m/s) Depth of cut (mm) Feed rate (mm/rev) Coolant
0.83 0.15 0.1 Used
0 17 0.1 01 Used
F'lg. 2. SEM mlcrographs of etched surfaces Temperatures ln&cated are treatment temperatures
T Ohashi et al./lnternattonal Journal of Refractory Metals & Hard Materials 16 (1998) 403-407
405
hardness values at 2100°C and 2300°C are almost equal and clearly decrease at 2500°C. This result would be explained by the remarkable grain growth at 2500°C. The thermal conductivity is higher for samples treated at 2500°C than at 2100°C, especially at measuring temperature of 20°C. It decreases with increasing measuring temperature, irrespective of treatment temperature. These results seem reasonable because surface area of c-BN grains decreases with increasing treating temperature, i.e. with increasing grain size, and because phonons are scattered by lattice vibration at high measuring temperature. According to Corrigan [4], however, the influence of treatment temperature on thermal conductivity is not clear, but a thermal conductivity vs measuring temperature curve tends to have a peak at 100°C. This is somewhat different from the above results. Since the number of our data points regarding measuring temperature is restricted, further investigation is necessary to clarify the difference. For the cutting test results, Fig. 6 shows flank wear vs cutting time curves and the appearance of the cutting edges at the point indicated in the turning of Stellite No. 6, compared with a conventional c-BN-Co composite. It is seen that the sample treated at 2100°C, i.e. the sample composed of a c-BN single phase and having a uniform and fine microstructure, has excellent
wear resistance, while the sample treated at 2500°C with a single phase and coarse grains, has similar wear resistance to the conventional composites. Figure 7 shows the result of turning cemented carbide. It is also seen that the sample of 2100°C has the best cutting performance of the three. These results are considered as follows. Firstly, the hardness is listed. Hardness of PCBN is 52-60 GPa HV and higher than the c-BN-Co composite for which the hardness is about 45 GPa HV. Secondly, the difference in grain size should be taken into account when PCBN compacts treated at different temperatures are compared. It is considered that the coarser the grain size, the easier fracture can be
Fig. 3. SEM observation of fracture surface.
Fig. 4. TEM mlcrographs.
T Ohashi et al /InternattonaI Journal of Refractory Metals & Hard Matertals 16 (1998) 403-407
406
600 >
Measuringtemperature:20°C
-~ 400 ¢-,. 2• "~ ~
500°C
200
1ooo°c
0.3
c"
0 65 == "
¢0
E
60
"0
0.2
o3
:z:m 55 •-< 0
L
50
45 2100°C
I
2300°C
=0.1 2500°C
Treatment Temperature (°C)
Fxg. 5. Thermal conductwlty and Vickers hardness as a function of treating temperature.
produced. Thus, flank wear rapidly proceeds in a coarse grained PCBN compact.
4. Conclusion
Polycrystalline c-BN compacts with no additives were directly synthesized under ultra high pressure of 6.8 GPa at various temperatures from h-BN disks made by chemical vapor deposition. Their microstructures, physical properties and cutting performance for Stellite No. 6 and cemented carbide were examined. 1. It was confirmed that a compact composed of a c-BN single phase was obtained above 2100°C and
0
0
500
1000
1500
Cutting Time (sec) Fig. 7. Flank wear in cutting of cemented carbide: (e) PCBN treated at 2100°C, (a) PCBN treated at 2500°C, (A) c-BN-Co composite.
that c-BN grains began to grow at 2300°C, resulting in a coarse grained structure at 2500°C. 2. The coarse grains of c-BN fractured intragranurally; while the fracture mode of fine grains was not obvious. 3. The hardness clearly decreased at 2500°C and the thermal conductivity was higher when treated at 2500°C than at 2t00°C. 4. The polycrystalline c-BN compact synthesized at 2100°C showed the best cutting performance of the samples tested, Stellite and cemented carbide.
Acknowledgements
O.08
The authors would like to thank Dr F. Ueda for useful discussions and Mr S. Agawa for precise TEM observations.
0.06 h..
~ 0 . 04
References
e-
~.0.0 2 0
0
500 1000 1500 2000 Cutting Time (see)
Fxg 6. Flank wear m cutting of Stelhte No 6: (I) PCBN treated at 2100°C; (..) PCBN treated at 2500°C, (A) c-BN-Co composite.
[1] Wakatsukl M, Ichlnose K, Aokl T. Synthesis of polycrystalhne cubic BN Mater Res Bull 1972;7.999-1003. [2] Corrigan FR, Bundy FP. Direct transitions among the allotroplc forms of boron mtnde at high pressures and temperatures. J Chem Phys 1975;63'3812-20. [3] Ichmose K, Wakatuskl M, Aoki T, Maeda Y. Synthesis of polycrystalhne cubic BN (V). In: Proceedings of the 4th International Conference on High Pressure, Kyoto, Japan, Nov 1974:436-40
T Ohashl et al./Internattonal Journal of Refractory Metals & Hard Materials 16 (1998) 403-407
[4] Corrigan FR Thermal conductivity of polycrystalhne cubic boron nitride compacts In: Tlmmerhaus KD, Barber MS, editors High pressure and science technology New York: Plenum, 1979,1 994-99. [5] Onodera A, Inoue K, Yoshlhara H, Nakae H, Matsuda T, Hlral T Synthesis of cubic boron nitnde from rhombohedral form under high static pressure J Mater Sci 1990;25:4279-84.
407
[6] Akalshl M, Satoh T, Ishn M, Taniguchl T, Yamaoka S. Synthesis of translucent sintered cubic boron mtride. J Mater Sci Lett 1993;12'1883-85. [7] Horiuchi S, He LL, Onoda M, Akalsha M. Monoclinic phase of boron nitrlde appearing during the hexagonal cubic phase transmon at high pressure and high temperature. Appl Phys Lett 1996;68(2):182-4.
REFRACTORYMETALS & HARDMATERIALS ELSEVIER
International Journal of Refractory Metals & Hard Materials 16 (1998) 403-407
Some properties and cutting performance of polycrystalline cubic boron nitride with no additives T. Ohashi,* K. Yamamoto, Y. Hamada, T. Tanase Central Research Instttute, Mttsubtshl Materials Corporatton, 1-297Kttabukuro-Cho. Omiya, Sa~tama 330-8508. Japan
Recewed 9 March 1998, accepted 8 September 1998
Abstract
Hexagonal BN disks made by chemical vapor deposition were treated under ultra high pressure of 6.8 GPa and at temperatures from 1800 to 2500°C for 1.8 ks. When treated at temperatures above 2100°C, only a c-BN phase was detected. The sample treated at 2100°C had a homogeneous microstructure having fine grain size less than 1/zm, whereas that treated at 2300°C had a heterogeneous one with some large grains The sample treated at 2500°C showed remarkable grain growth. Some physical properties and cutting performance of these compacts were investigated. Then hardness clearly decreased at 2500°C, and the thermal conductivity was higher when synthesized at 2500°C than at 2100°C. The fine grained compact of 2100°C exhibited better wear resistance than the coarse grained one of 2500°C in cutting of Co base super alloy and cemented carbide. © 1998 Pubhshed by Elsevier Science Ltd. All rights reserved Keywords. c-BN; Compact, Gram size; Physicalproperty; Cutting performance
1. Introduction
2. Experimental
Cubic boron nitride (hereafter c-BN) has high hardness and thermal conductivity second only to diamond and a low affinity to ferrous materials. Therefore, c-BN is widely used in grinding and cutting applications for ferrous materials. Convention c-BN based composites, commercially available as a cutting material, contain a ceramic or metallic binder in order to facilitate sintering, to optimize cutting performance, and so on. On the other hand, a polycrystalline c-BN compact with no additives (hereafter PCBN) synthesized by direct transformation from a low pressure phase of BN under ultra high pressure and at high temperature has been hitherto reported [1-6]. However, influences of synthesizing conditions on properties of PCBN such as microstructure, hardness and thermal conductivity have not yet been systematically studied. These interesting subjects were studied in detail in this report. In addition, its cutting performance with two sorts of work materials containing Co, Stellite No. 6 and cemented carbide, was investigated.
A hexagonal BN (hereafter h-BN) disk made by chemical vapor deposition was used as a starting material (purity > 99.9%). Its diameter and thickness were 9.5 and 1.5 ram, respectively. It was wrapped with Ta foil and treated under ultra high pressure of 6.8 GPa and at temperatures from 1800 to 2500°C for 1.8ks using a belt type high pressure apparatus (developed by National Institute for Research in Inorganic Materials, refer to Fig. 1). The treatment temperature was measured with a W - R e thermocouple, and the pressure effect was neglected. The samples obtained were ground with a diamond wheel and subsequently polished with diamond paste less than 2/~m. They were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Vickers hardness (HV) and thermal conductivity measurements which were carried out by a laser flash heated method. For microstructural examinations, they were dipped in molten N a O H for 60 s to etch the grain boundary. For cutting tests, ground samples were cut into 5 mm triangular pieces with a Y A G laser, brazed onto
*Corresponding author Tel +81-48-642-0514;fax +81-48-6497767, e-mad, ohashi@mmcco jp
0263-4368/98/$-- see front matter © 1998Pubhshed by Elsevier ScienceLtd All rights reserved PII: S0263-4368(98)00057-2
404
T. Ohasht et al /Internatmnal Journal of Refractory Metals & Hard Materials 16 (1998) 403-407
6
1
7
/,2
j
3 1
8
Fracture surfaces are shown in Fig. 3 for the samples treated at 2100°C and 2500°C. Though the fracture made of the sample of 2100°C is not clear, that of 2500°C is intragranular with river patterns which indicate cleavage fracture. Twins are observed in c-BN grains as above by SEM observation. This is also confirmed by TEM (Fig. 4). Twins are seen in the grown grains in the samples of 2300°C and 2500°C. They were occasionally observed in fine grains. Figure 5 illustrates hardness and thermal conductivity as a function of treating temperature. The
4
Fig. 1 The belt type high pressure apparatus used in the present experiment" (1) piston, (2) cylinder; (3) Ta capsule; (4) NaCI-(10 or 20)%ZRO2; (5) graphite heater; (6) NaCl-10%ZrOz, (7) steel nng; (8) gasket.
cemented carbide substrates and finally ground to an insert of TNGA160408 form. Work materials tested were Stellite No. 6 (Co-28Cr-4W alloy in as-cast state) and cemented carbide with 11 mass% Co content (HV about 1.4 GPa). A commercially available c-BN-Co composite for multi-purpose application with about 85 vol% c-BN content and a Co alloy binder, was used for comparison. Cutting conditions are summarized in Table 1. 3. Results and discussion
In the sample treated at 1800°C, a so-called compressed h-BN phase [4] (which is reported as a monoclinic phase in a recent article [7]) is observed in addition to a c-BN phase by XRD. When treated over 2100°C, only a c-BN phase is detected. The microstructure by SEM is indicated in Fig. 2. When treated at 2100°C, the microstructure is homogeneous and as fine as, or less than, 1 gin, whereas it becomes heterogeneous with some coarse grains at 2300°C and shows remarkable grain growth at 2500°C. Twins are clearly observed in coarse grains. These results correspond qualitatively with those by Corrigan [4]. Table 1 Cutting conditions Work piece/condition
Stellite No. 6
Cemented carbide
Cutting speed (m/s) Depth of cut (mm) Feed rate (mm/rev) Coolant
0.83 0.15 0.1 Used
0 17 0.1 01 Used
F'lg. 2. SEM mlcrographs of etched surfaces Temperatures ln&cated are treatment temperatures
T Ohashi et al./lnternattonal Journal of Refractory Metals & Hard Materials 16 (1998) 403-407
405
hardness values at 2100°C and 2300°C are almost equal and clearly decrease at 2500°C. This result would be explained by the remarkable grain growth at 2500°C. The thermal conductivity is higher for samples treated at 2500°C than at 2100°C, especially at measuring temperature of 20°C. It decreases with increasing measuring temperature, irrespective of treatment temperature. These results seem reasonable because surface area of c-BN grains decreases with increasing treating temperature, i.e. with increasing grain size, and because phonons are scattered by lattice vibration at high measuring temperature. According to Corrigan [4], however, the influence of treatment temperature on thermal conductivity is not clear, but a thermal conductivity vs measuring temperature curve tends to have a peak at 100°C. This is somewhat different from the above results. Since the number of our data points regarding measuring temperature is restricted, further investigation is necessary to clarify the difference. For the cutting test results, Fig. 6 shows flank wear vs cutting time curves and the appearance of the cutting edges at the point indicated in the turning of Stellite No. 6, compared with a conventional c-BN-Co composite. It is seen that the sample treated at 2100°C, i.e. the sample composed of a c-BN single phase and having a uniform and fine microstructure, has excellent
wear resistance, while the sample treated at 2500°C with a single phase and coarse grains, has similar wear resistance to the conventional composites. Figure 7 shows the result of turning cemented carbide. It is also seen that the sample of 2100°C has the best cutting performance of the three. These results are considered as follows. Firstly, the hardness is listed. Hardness of PCBN is 52-60 GPa HV and higher than the c-BN-Co composite for which the hardness is about 45 GPa HV. Secondly, the difference in grain size should be taken into account when PCBN compacts treated at different temperatures are compared. It is considered that the coarser the grain size, the easier fracture can be
Fig. 3. SEM observation of fracture surface.
Fig. 4. TEM mlcrographs.
T Ohashi et al /InternattonaI Journal of Refractory Metals & Hard Matertals 16 (1998) 403-407
406
600 >
Measuringtemperature:20°C
-~ 400 ¢-,. 2• "~ ~
500°C
200
1ooo°c
0.3
c"
0 65 == "
¢0
E
60
"0
0.2
o3
:z:m 55 •-< 0
L
50
45 2100°C
I
2300°C
=0.1 2500°C
Treatment Temperature (°C)
Fxg. 5. Thermal conductwlty and Vickers hardness as a function of treating temperature.
produced. Thus, flank wear rapidly proceeds in a coarse grained PCBN compact.
4. Conclusion
Polycrystalline c-BN compacts with no additives were directly synthesized under ultra high pressure of 6.8 GPa at various temperatures from h-BN disks made by chemical vapor deposition. Their microstructures, physical properties and cutting performance for Stellite No. 6 and cemented carbide were examined. 1. It was confirmed that a compact composed of a c-BN single phase was obtained above 2100°C and
0
0
500
1000
1500
Cutting Time (sec) Fig. 7. Flank wear in cutting of cemented carbide: (e) PCBN treated at 2100°C, (a) PCBN treated at 2500°C, (A) c-BN-Co composite.
that c-BN grains began to grow at 2300°C, resulting in a coarse grained structure at 2500°C. 2. The coarse grains of c-BN fractured intragranurally; while the fracture mode of fine grains was not obvious. 3. The hardness clearly decreased at 2500°C and the thermal conductivity was higher when treated at 2500°C than at 2t00°C. 4. The polycrystalline c-BN compact synthesized at 2100°C showed the best cutting performance of the samples tested, Stellite and cemented carbide.
Acknowledgements
O.08
The authors would like to thank Dr F. Ueda for useful discussions and Mr S. Agawa for precise TEM observations.
0.06 h..
~ 0 . 04
References
e-
~.0.0 2 0
0
500 1000 1500 2000 Cutting Time (see)
Fxg 6. Flank wear m cutting of Stelhte No 6: (I) PCBN treated at 2100°C; (..) PCBN treated at 2500°C, (A) c-BN-Co composite.
[1] Wakatsukl M, Ichlnose K, Aokl T. Synthesis of polycrystalhne cubic BN Mater Res Bull 1972;7.999-1003. [2] Corrigan FR, Bundy FP. Direct transitions among the allotroplc forms of boron mtnde at high pressures and temperatures. J Chem Phys 1975;63'3812-20. [3] Ichmose K, Wakatuskl M, Aoki T, Maeda Y. Synthesis of polycrystalhne cubic BN (V). In: Proceedings of the 4th International Conference on High Pressure, Kyoto, Japan, Nov 1974:436-40
T Ohashl et al./Internattonal Journal of Refractory Metals & Hard Materials 16 (1998) 403-407
[4] Corrigan FR Thermal conductivity of polycrystalhne cubic boron nitride compacts In: Tlmmerhaus KD, Barber MS, editors High pressure and science technology New York: Plenum, 1979,1 994-99. [5] Onodera A, Inoue K, Yoshlhara H, Nakae H, Matsuda T, Hlral T Synthesis of cubic boron nitnde from rhombohedral form under high static pressure J Mater Sci 1990;25:4279-84.
407
[6] Akalshl M, Satoh T, Ishn M, Taniguchl T, Yamaoka S. Synthesis of translucent sintered cubic boron mtride. J Mater Sci Lett 1993;12'1883-85. [7] Horiuchi S, He LL, Onoda M, Akalsha M. Monoclinic phase of boron nitrlde appearing during the hexagonal cubic phase transmon at high pressure and high temperature. Appl Phys Lett 1996;68(2):182-4.