cemented carbides

cemented carbides

International Journal of Refractory Metals & Hard Materials 25 (2007) 237–243 www.elsevier.com/locate/ijrmhm Sintering behavior and properties of dia...

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International Journal of Refractory Metals & Hard Materials 25 (2007) 237–243 www.elsevier.com/locate/ijrmhm

Sintering behavior and properties of diamond/cemented carbides Hideki Moriguchi a,*, Katsunori Tsuduki a, Akihiko Ikegaya a, Yoshinari Miyamoto b, Yoshiaki Morisada b a

Electronics and Materials R&D Department, Sumitomo Electric Industries, Ltd., Itami, Hyogo 664-0016, Japan b Joining and Welding Research Institute, Osaka University, Ibaraki, Osaka 567-0047, Japan Received 18 January 2006; accepted 25 May 2006

Abstract Dense diamond/cemented carbides were fabricated at around 1300 °C, 41 MPa for 3 min under meta-stable conditions for diamonds by pulsed-electric current sintering. Diamond particles were coated with SiC to prevent the graphitization of diamond with molten binder during sintering. No graphitization of diamond was confirmed by microstructure observation and Raman scattering analysis. The combination of SiC coating of diamond and rapid sintering was effective to fabricate diamond/cemented carbides. These new materials, composed of diamond and cemented carbide, showed a 50% larger fracture toughness and 10 times better wear resistance compared with conventional cemented carbides, and five times better machinability compared with diamond compacts. The diamond/cemented carbides are applicable to wear-resistant sliding tools such as centerless blades, work rests, and stoppers. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Diamond particles; Cemented carbide; SiC coating; Pulsed-electric current sintering; Wear resistance

1. Introduction Cemented carbides are hard materials mainly composed of WC as a hard phase and Co as a binder phase. They are widely used for cutting, wear-resistant and mining tools because of the superior sinterability, hardness, Young’s modulus, flexural strength, toughness, and thermal conductivity [1]. W.D. Coolidge invented a manufacturing method of tungsten filaments in 1909. At that time, natural diamonds were used as dies for drawing of tungsten filaments, but they were expensive and their quality was not uniform. Schroter developed cemented carbides as an alternative material of natural diamond and applied a first patent on the manufacturing method in 1923. Cemented carbides were finally commercialized in 1926 under the name of ‘Widia’ (meaning ‘‘as hard as diamonds’’ in German) [1,2]. Later in 1953, using an ultra high-pressure vessel, artificial diamonds were developed at high-temperature and *

Corresponding author. Tel.: +81 72 772 4805; fax: +81 72 770 6727. E-mail address: [email protected] (H. Moriguchi).

0263-4368/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2006.05.006

high-pressure conditions of 1400–2400 °C and 5.5– 10 GPa [3,4]. As a result, diamond compacts with better wear resistance than cemented carbides were developed as the hardest materials on the earth. However, the production of these diamond materials requires expensive facilities such as ultra high-pressure vessels. In contrast, cemented carbides allow complex shapes to be produced easily at lower cost because a powder metallurgical process using liquid phase sintering around 1400 °C [1] is employed. Moreover, cemented carbides have superior flexural strength and toughness compared with diamond compacts, resulting in wide applications in various industries. We aimed to produce new materials having both the performance and characteristics of diamond and the low cost of cemented carbide. We succeeded in producing cemented carbides containing a dispersion of diamond particles without using an ultra-high-pressure vessel [5,6]. Recently, fast and low-temperature sintering processes such as microwave sintering [7], HFIHS (high frequency induction heated sintering) [8] and PECS (pulsed-electric current sintering) [9], which is also called SPS (spark

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plasma sintering), have been proposed. These fast sintering processes are effective to prevent graphitization of diamond under the meta-stable condition, such as high temperature and low pressure for diamond. Another effective way to prevent graphitization of diamond particles where a molten binder is present is to coat the diamond. In this study diamond particles were coated with SiC and mixed with WC and Co powders, then sintered rapidly within a few minutes by PECS [7]. This paper reports on the effect of SiC coating on diamond particles against the graphitization during sintering, as well as mechanical properties and performances for real applications of these diamond/cemented carbides composites.

Load

Pulsed-electric current Electrode Graphite punch

Powder

Optical r pyrometer Graphite die

2. Experimental procedure 2.1. SiC coating on diamond particles Fig. 1 shows a schematic diagram of the reaction container for producing a SiC coating on diamond particles [10,11]. Diamond powders with grain sizes of 8–16, 20– 30, 40–60 lm (Showa Denko KK) and SiO powders with an average grain size of 75 lm (Kojundo Chemical Lab. Co.) were used for the SiC coating. The molar ratio of SiO to diamond was fixed at 1:5. The diamond powders were placed on the SiO powder bed using a carbon felt. This assembly was covered with carbon sheets in an alumina crucible to maintain the SiO gas pressure. The coating was carried out at 1350 °C in vacuum (about 0.03 Pa) for 1 h. The SiC-coated diamond particles were characterized by XRD, SEM, and TEM (Hitachi H9000-UHR). A thin section of SiC coated diamond particles for TEM observation was prepared by using a focused ion beam system (Hitachi FB-2100). 2.2. Sintering of diamond/cemented carbide The powders of WC with an average grain size of 1.9 lm (ALMT Corp.) and Co with an average grain size of 1.4 lm (Union Muniere SA) were mixed with a ratio of WC and

Fig. 2. Schematic illustration of PECS device.

10 wt% Co, ball-milled in ethanol for 15 h, and dried. Then, SiC coated diamond particles with grain sizes of 8– 16 lm, 20–30 lm and 40–60 lm were dry-mixed with the WC–Co powder using a V-type mixer for 1 h. The content of diamond was 20 vol%. The mixed powder was charged into a graphite die with an inside diameter of 30 mm, and pre-pressed at 1.4 MPa. The powder content was adjusted to have a 4 mm thickness after sintering. Fig. 2 shows a schematic illustration of the PECS device. The mixed powder compact was heated quickly by passing a pulsed current through a graphite die including the sample under uniaxial pressure. The sample was vacuum sintered under the following conditions: heating rate of 6 °C/min, maximum temperature of 1150 °C, holding time of three minutes, and load of 41 MPa. The temperature was monitored on the surface of the graphite die by using an optical pyrometer. The real sintering temperature at the sample was 1300 °C, which was measured by inserting a WRe thermo couple into a hole through a graphite mold. In order to compare the sinterability and graphitization, the WC–10 wt%Co powder and 20 vol% diamond particles with a lager grain size of 200 lm (Showa Denko KK) were dry-mixed and pressed at 100 MPa. The pressed samples were sintered at 1350 °C in vacuum for 1 h by a conventional sintering process. 2.3. Characterization of mechanical properties

Fig. 1. Schematic diagram of reaction container for SiC coating on diamond particles.

Various mechanical properties of the SiC-coated diamond/cemented carbides were characterized. Vickers hardness and indentation fracture toughness were measured at 490 N load using a Vickers hardness tester [12]. Tribological properties were measured using a pin-on-disk testing equipment under the following conditions: a rotation speed of 3 m/min, a load of 10 N, air atmosphere, and Al2O3 balls with a diameter of 6 mm. The wear resistance was

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239

Work (SUJ2)

Load

Load

Sample Fig. 3. Schematic diagram of Tsuya tribometer.

Fig. 4. Schematic diagram of a work rest used for wear resistance test at an actual manufacturing line.

measured using a Tsuya tribometer under the following conditions: a rotation speed of 19 m/min, a load of 30 MPa, a time of 2 h, an air atmosphere, and work material of SUJ2 (Bearing steel) as shown in Fig. 3. The machinability was characterized by measuring the grinding force using a surface grinder with a # 230 diamond grind stone under the conditions: a rotation speed of grind stone of 1500 m/min, a cutting depth of 20 lm, and a feed rate of 6 m/min. The wear resistance at an actual manufacturing line was evaluated, where printed circuits board drills composed of cemented carbides were ground as shown in Fig. 4. The sectional area of the worn part was evaluated by using a stylus profiler.

Fig. 5. Optical micrograph of diamond/cemented carbide sintered by conventional sintering.

The protective coating of diamond is an effective way to prevent the graphitization. Various coatings of diamond have been reported [13,14]. Although diamond particles coated with Ni, Cr or Ti are commercially available [15], these coated layers are not stable against molten binder, and the thickness is not always uniform. A coating technique using Si powder is known [16], but the coated layer is rough and irregular. We prepared a dense and uniform SiC coating on diamond particles using a simple method, as shown in Fig. 1. SiO gas reacts with diamond and forms a SiC layer at 1350 °C. The coating mechanism was analyzed as follows [11]. Firstly a thin SiC layer of about 10 nm in thickness is formed on each diamond particle by the reaction (1). Next, the SiC is deposited through vapor phase reactions (2) and (3) with carbon felt: 2CðDiamondÞ þ SiOðgÞ ! SiCðsÞ þ COðgÞ

ð1Þ

SiOðgÞ þ 3COðgÞ ! SiCðsÞ þ 2COðgÞ

ð2Þ

CðsÞ þ CO2 ðgÞ ! 2COðgÞ

ð3Þ

XRD patterns of the SiC coated diamond particles formed are shown in Fig. 6. The SiC coated layer was composed of b-SiC. It is composed of nanosized crystallites as seen in the SEM micrograph of Fig. 7. A TEM micrograph of SiC coated diamond particles is shown in Fig. 8. The SiC layer was dense and it covered uniformly the whole surface of each diamond particle. The thickness of SiC layer was about 100 nm.

3. Results and discussion 3.1. Microstructure and effect of SiC coating The microstructure of a diamond/cemented carbide sintered by conventional sintering is shown in Fig. 5. The outer part of each diamond particle transformed into graphite. The transformation is caused by attacks of molten Co–WC binder during sintering at over 1300 °C. The molten Co–WC binder dissolves diamond and precipitates as the graphite phase, which is stable under low pressure.

Fig. 6. XRD patterns of SiC coated diamond particles.

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Fig. 7. SEM micrograph of SiC coated 8–16 lm diamond particles.

Fig. 8. TEM micrograph of SiC coated 8–16 lm diamond particles.

Optical microphotographs of SiC-coated diamond/ cemented carbides with different particle size of diamond are shown in Fig. 9. Every sintered sample was dense and diamond particles were dispersed uniformly. Fig. 10 shows a Raman scattering spectrum for a sintered cemented carbide with SiC-coated diamond particles (20–30 lm). It shows a sharp scattering peak from diamond at around 1333 cm1 [17]. The graphite peak at around 1581 cm1 was not detected. These results suggest that the diamond was not converted to graphite by the molten Co–WC binder. When the sample was sintered by PECS, the real sintering temperature was at around 1300 °C. The liquid phase is produced at over 1320 °C by the eutectic reaction between Co and WC [1]. Although the liquid phase would appear at least partly in the SiC-coated diamond/cemented carbides, the graphitization of diamond particles could be suppressed by the combination of the low temperature, fast sintering and the SiC coating. 3.2. Mechanical properties and performances The hardness and indentation fracture toughness of SiCcoated diamond/cemented carbides with different diamond particles were measured. The results are shown in Fig. 11. The hardness of the diamond/cemented carbides was

Fig. 9. Optical micrographs of SiC coated diamond/cemented carbides. (a) 8–16 lm (b) 20–30 lm (c) 40–60 lm.

almost the same as conventional cemented carbides including no diamond particles, but the fracture toughness is higher. Diamond particles did not form a skeletal structure in the cemented carbide matrix because the diamond content is limited to 20 vol%. The diamond particles exist like isolated islands floating in a sea of cemented carbides matrix. This microstructure is thought to give the similar hardness to the conventional cemented carbide under such a large indentation load of 490 N. On the other hand, the diamond/cemented carbides exhibited about 50% higher fracture toughness than that of the conventional cemented carbides. To clarify why they show such high fracture toughness, the crack propagation behavior was observed.

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3000

Intensity a.u.

2500 2000 1500 1000 500 0 1100

1200

1300

1400

1500

1600

1700

Raman shift cm-1 Fig. 10. Raman scattering spectrum for a cemented carbides with SiC coated diamond particles (20–30 lm).

20 18 16

20

14 12

18

10 16

8

Hardness

monds [18]. Diamond/cemented carbides having large diamond particles demonstrated high fracture toughness as shown in Fig. 11 as expected. Thus, the superior fracture toughness of diamond/cemented carbides is most likely a result of both the crack deflection effect and the blocking effect of hard diamond particles, whereby the crack propagation energy is efficiently absorbed. The tribological performances of diamond/cemented carbides having 8–16 lm SiC-coated diamond particles and conventional cemented carbides were compared using a pin-on-disk test. The results are shown in Fig. 13. The coefficient of dynamic friction of the diamond/cemented carbides was 0.05. This value is the same as that of diamond compacts and about one-fifth of that of conventional cemented carbides [19,20]. The result suggests that the diamond particles control the coefficient of dynamic friction of the diamond/cemented carbides. Fig. 14 shows a schematic illustration of the cross sectional view of diamond/cemented carbides after grinding and lapping. The shallow grooves among diamond particles seemed to be formed

Toughness (MPa m1/2)

Hardness (GPa)

Toughness

241

6 0

10

20

30

40

50

Diamond particle size ( m) Fig. 11. Hardness and toughness of diamond/cemented carbides.

Large deflections and blockings of crack propagation are seen in Fig. 12. Diamond particles with the lower thermal expansion coefficient than that of the cemented carbides matrix seemed to induce a tensile stress around diamonds resulting in deflection of crack propagation toward dia-

Fig. 12. SEM micrograph of crack propagation behavior of a diamond/ cemented carbide with 8–16 lm SiC coated diamond particles.

Fig. 13. Coefficients of dynamic friction of a conventional, a diamond/ cemented carbide with 8–16 lm SiC coated diamond particles and a diamond compact.

Fig. 14. Schematic illustration of cross sectional view of diamond/ cemented carbides during a pin-on disk test and Tsuya tribometer test.

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low

5000

Conventional cemented carbide WC-10wt%Co

400

4000 Wear (um2)

Wear ( μ m2)

Wear resistance

300

200

Diamond/cemented carbide

100

Diamond compact

3000 2000 1000 0

high

0

50 Grinding force (N)

high

WC-3wt%Co

100 low

Machinability Fig. 15. Wear resistance and machinability of a conventional cemented carbide, a diamond/cemented carbide with 8–16 lm SiC coated diamond particles and a diamond compact.

by grinding and lapping soft cemented carbides more deeply than hard diamond particles. Therefore, an Al2O3 ball contacted mainly with diamond particles during the measurement of tribological performance as illustrated in Fig. 14, and the coefficient of dynamic friction of diamond/cemented carbides became very low value similar to that of diamond compacts. The wear resistance of diamond/cemented carbides having SiC coated 20–30 lm diamond particles, conventional cemented carbides with 10 wt% Co, and diamond compacts were evaluated using a Tsuya tribometer. The results are shown in Fig. 15. The diamond/cemented carbides have 10 times higher wear resistance than the conventional cemented carbides. The diamond particles contact mainly with SUJ2 work in the wear resistance test as shown in Figs. 3 and 14. The machinability of materials was estimated by measuring their grinding force. High grinding force leads to low machinability. Cemented carbides with SiC coated 20–30 lm diamond particles have five times higher machinability than diamond compacts as shown in Fig. 15. Diamond compacts have superior wear resistance and inferior machinability because of the direct bonding of diamond particles. On the contrary, diamond/cemented carbides have superior machinability because of no direct bonding of diamond particles. 4. Applications Diamond/cemented carbides have superior characteristics compared with conventional cemented carbides and diamond compacts, which are expected to be useful as new materials for achieving intermediate performance and cost between cemented carbides and diamond compacts. However, diamond particles in the diamond/cemen-

Diamond/cemented carbide

Fig. 16. Wear of a low-binder cemented carbide and a diamond/cemented carbide with 8–16 lm SiC coated diamond particles.

ted carbides are not bonded directly. Therefore, diamond/ cemented carbides are not suitable for applications requiring cutting edges, such as cutting tools, because diamond particles are easily removed from the cutting edges. On the other hand, these carbides are expected to offer good wear resistance for uses such as centerless blades, work rests and stoppers which make contact with work materials at their faces. The wear resistance of cemented carbides with SiC coated 20–30 lm diamond particles as a work rest was evaluated in an actual manufacturing line. The compared material was a low-binder cemented carbide with 3 wt% Co. The hardness of the low-binder cemented carbide was 20 GPa, and the wear resistance was superior compared with other grades of cemented carbides. Fig. 16 shows the test results. The diamond/cemented carbide with 16 GPa in hardness showed 25 times higher wear resistance than the low-binder cemented carbide with 20 GPa. This result shows the interesting phenomenon that the local hardness of diamond particles rather than the average hardness of these materials control the wear resistance. 5. Conclusions Dense diamond/cemented carbides could be produced under meta-stable conditions for diamonds by using pulsed-electric current sintering and a new technique of coating SiC on diamond particles. In other words, even under thermodynamically meta-stable conditions for diamonds, such diamond/cemented carbides can be produced by preventing diamond particles from transforming into graphite by suppressing reactions between diamond and molten binder. Though there are large differences in performance and production cost between cemented carbides and diamond compacts, the diamond/cemented carbides are expected to bridge the gap. Further improvement of the bonding strength between diamond particles and the cemented carbide matrix will extend applications of these diamond/cemented carbides.

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