Development of New PCD Made Up of Boron Doped Diamond Particles and its Machinability by EDM

Kiyoshi Suzuki1,a, Yoichi Shiraishi1, Nobuhiro Nakajima2, Manabu Iwai3,b, Shinichi Ninomiya1,c, Yukinori Tanaka4, Tetsutaro Uematsu3

1 Nippon Institute of Technology, Miyashiro, Saitama 345-8501, Japan

2 Sodick Co., Ltd., Yokohama, Kanagawa 224-8522, Japan

3 Toyama Prefectural University, Imizu, Toyama 939-0398, Japan

4 San Seimitsu Kako Lab., Ltd., Kuki Saitama, 346-0034, Japan

a kiyoshi@nit.ac.jp, b iwai@pu-toyama.ac.jp, c ninomiya@nit.ac.jp

Keywords: PCD, Electrically conductive PCD, Boron doped diamond particle, wire EDM

Abstract. This paper deals with a new PCD named EC-PCD which is made up of boron doped

diamond particles and its properties related to EDM machinability. For the purpose of improving

various properties of standard PCD including resistance to heat, wear and reactivity, a new PCD

(EC-PCD) was manufactured on a trial basis using electrically conductive diamond particle as a

basic ingredient. Grain size, resistivity and thermal conductivity of the boron doped diamond used

are 10μm, 5~37×10Ω·m and 440~580W/m·K. In this report, machinability of newly developed

PCD (EC-PCD) by wire EDM was investigated in comparison with that of standard PCD. In wire

cutting of 2 types of PCD in water under the condition of open gap voltage: ue=80V, set peak

current: iP=0.8A and pulse condition: te/to=20/20μs, it was found that roughness of the first cut

surface of standard PCD was approximately 8μm Rz, while that of EC-PCD was far better such as

3μm. Also in finish cut (7th cut), the latter achieved the value of Rz=1.7μm while the former

achieved only the value of Rz=2.7μm. Expecting better performance, EC-PCD was tested also in oil.

As a result, the best achieved roughness was improved to Rz=0.4μm with no chipping on the edge.

To explore a reason for such a good roughness obtained, the cut samples were observed on the SEM,

which revealed that the diamond particles in EC-PCD were flattened by electro discharge.

Introduction

Polycrystalline Composite Diamond (PCD) is manufactured by sintering fine diamond particles at

high temperature together with a small amount of solvent (or catalyst) such as cobalt. With recent

development of manufacturing technique, available size of PCD has become over 70mm [1].

Though major applications of PCD are cutting tools and wear parts, applications to a grinding

wheel for hard to grind materials [2] and a zero-wear electrode for electric discharge machining [3]

are being developed recently. Furthermore, the process called “Hybrid Processing of EDM and

Grinding with the Same Tool [4]“ has been developed exploiting the advantages of PCD that can be

used as an EDM electrode and a grinding tool at the same time. Having a structure where diamond

particles are strongly bonded together (intergrowth), PCD has already become an essential tool

material for cutting hard materials such as high silicon aluminum alloy for its toughness. At the

same time, there are some disadvantages in PCD such as irrelevancy to be used for cutting ferrous

materials and difficulty to give precision machining to PCD for its hardness. In general,

manufactured PCD is first cut to a required shape and size by EDM or laser and then finished by

grinding with a wheel containing diamond abrasives. Since finishing PCD by grinding using

diamond abrasives is time consuming and costly, it is quite natural for one to think that EDM finish

using an inexpensive electrode is desirable. Being electrically conductive as a whole, PCD can be

processed by wire EDM or die sinking EDM. However, the diamond which makes up PCD is

electrically non-conductive. This gives bad effects on EDM performance (lower efficiency and high

Advanced Materials Research Vols. 76-78 (2009) pp 684-689online at http://www.scientific.net© 2009 Trans Tech Publications, Switzerland

Table 1 Characteristics of B doped diamond and expected properties of EC- PCD

1. Advantages of boron doped diamond in the new PCD

(1) Improvement of specific resistance up to 5~37×10Ω·m

(2) Improvement of oxidation temperature by around 200°C

2. Expected properties of new PCD

(1) Improvement in electrical conductivity of PCD

(2) Improvement in thermal stability of PCD

(3) Improvement in reactivity resistance of PCD

(4) Improvement in machinability of PCD by EDM

3. Expected applications of new PCD

(1) Cutting tool with superior performance

(2) Electrode for micro EDM with no wear

(3) Wear resistant part

wear rate of electrode) causing a problem in achieving high quality machining. In particular, this

gives a significant problem in making a fine hole in PCD by EDM. In this study, the authors made

an attempt to develop a PCD composed of electrically conductive diamond particles for the purpose

of solving the problems incidental to the existing PCD.

Purpose of Developing EC-PCD Made up of EC Diamond Particles

Purpose of the research is to develop a new PCD that possesses proper electrical conductivity,

higher oxidation temperature and superior reactivity resistance with a work-piece compared with an

existing PCD (See Table 1). These natures of a new PCD are thought to be derived from boron

doped diamond used as a basic ingredient [5], for boron doped diamond possesses electrical

conductivity of 5~37×10Ω·m and improved oxidation temperature of about 200°C. Here, the

authors named a new PCD made up of electrically conductive diamond by doping boron “EC-PCD”.

PCD made up of electrically conductive diamond particles is expected to be machined more easily

by EDM process.

Main intended applications of EC-PCD will be a machining tool with high thermal stability and

anti-reactivity e.g. a drill bit for mining an oil hole, and an efficient electrode for EDM.

Experiment

EC-PCD. Boron doped diamond particles with a grain size

of φ10μm were sintered together with cobalt as a solvent

(catalyst). Properties of boron doped diamond particles

shown in Figure 1 are low resistivity such as 5~37×10Ω·m,

and high thermal conductivity of 440~580W/m·K.

Thickness of the PCD and carbide layer is 0.6mm and

2.0mm, respectively. Surface of the PCD was finished by

polishing. For comparison, commercially available standard

PCD (S-PCD hereinafter) of φ10μm was also tested.

Since diameter of diamond particle, sintering condition,

constituent of the binder material (solvent or catalyst), and

thickness of the PCD are different between the two samples

because manufacturer of PCD is different, accurate

comparison cannot be expected, but a certain level of the

tendency is thought to be seen. Properties of both PCD are

summarized in Table 2.

Figure 1 Boron doped diamond

particles used for EC-PCD

Advanced Materials Research Vols. 76-78 685

Experimental Conditions. Electro Discharge machinability of PCD was investigated using wire

EDM machine. In this experiment, a rectangular-shaped PCD workpiece was set with the PCD

layer facing downward as shown in Figure 2, and was cut with a φ0.2mm brass wire electrode.

Cutting condition used was according to the recommendation by the machine maker. Typically,

deionized water of 6.6×104Ω·cm was used as a working fluid. Other conditions of wire EDM are

shown in Table 3.

Results of wire cutting experiments were summarized after first cut, second cut and Nth cut.

Roughness of the cut surface was measured by Talysurf-120, and surface integrity was observed on

the SEM. Evaluation items were number of times of wire breakage, machining speed, surface

roughness and surface integrity. As for electrically conductive diamond PCD (EC-PCD), cutting

experiments were carried out also in oil.

Experimental Result

Wire Breakage and Cutting Speed. In cutting experiment in water on standard PCD (S-PCD) and

EC-PCD of 2.2mm width, wire breakage occurred 5 times with standard PCD (S-PCD), while no

breakage occurred with EC-PCD. This is thought to be because the diamond used in EC-PCD is

electrically conductive. Line removal rate was compared in the same operation on the same sample.

As a result, line removal rate for standard PCD was 2.9mm/min, while the same for EC-PCD was

4.4mm/min being higher by approximately 30%. This also can be attributed to electrical

conductivity of the diamond particles in the EC-PCD.

Table 2 Comparison between two kinds of PCD

S-PCD (Standard PCD) EC-PCD

Basic ingredient (diamond) Conventional HPHT diamond B doped HPHT diamond

Specific resistance of diamond (0.4~1×10‐3) Ω·m 5~37×10 Ω·m

Thermal conductivity of diamond (500~600) W/m·K 440~580 W/m·K

Particle size of the diamond 10(6-12) μm 10 μm

Binder material Cobalt Cobalt

Specific resistance of PCD 1.4×10-4 Ω·m -

Thermal conductivity of PCD 459 W/m·K -

Thickness of PCD layer 0.5 mm 0.6 mm

Thickness of backup metal (WC) 1.1 mm 1.4 mm

Feed

Wire

feed

WC

PCD layer

Wire

2nd cut

1st cut

Nth cut

Figure 2 Cutting status of PCD in water

wire

PCD

vice

686 Advances in Abrasive Technology XII

Surface Roughness When W-EDMed in Water.

First Cut. In wire EDM of standard PCD (S-PCD) and EC-PCD in water, condition of the cut

surface was compared. Comparison of surface condition resulted are shown in Figure 3 and Figure

4, respectively. Surface roughness values with the standard PCD were Rz=7.7μm、Rzjis=3.6μm and

Ra=0.97μm, while those with the EC-PCD were Rz=2.9μm, Rzjis=1.3μm and Ra=0.43μm. It is

guessed that the difference in roughness between two types of PCD depends on whether or not the

diamond particles in the PCD are cut by electro discharge.

Table 3 Experimental device and conditions

Wire ED Machine AQ325L + LQ33W for water (Sodick Co. Ltd.)

AP200L + LQ1W for oil (Sodick Co. Ltd.)

Wire electrode Brass wire (φ0.2mm, Sodick Co. Ltd.)

Workpiece Standard PCD (diamond size=10µm), EC-PCD (diamond size=10µm)

Working fluid (1) Deionized water (Electrical resistivity: 6.6×104Ω·cm), (2) Oil

ED conditions for

1st cut

(1) In water: Open gap voltage: ue=80V, Set peak current: iP=250A,

Pulse condition: te/to=300ns/8µs

(2) In oil: Open gap voltage: ue=80V, Set peak current: iP=290A,

Pulse condition: te/to=400ns/88µs

Finish cut in water Amount of wire offset is from 0.135mm (2nd cut) to 0.104mm (7th cut)

1st

cut

7th

cut

(a) S-PCD (b) EC-PCD

Figure 3 Surface roughness of the 2 types of PCD (in water)

Advanced Materials Research Vols. 76-78 687

Finish Cut. Finish cut was conducted setting the value for the amount of wire offset as

previously shown in Table 3. Here, EDM conditions were set according to SODICK’s

recommendation. By giving the 7th cut, surface roughness of the standard PCD (S-PCD) was

improved to Rz=2.7μm, but far more improvement was shown on the EC-PCD achieving Rz=1.7μm.

This difference here again can be guessed to depend on whether top part of the diamond particles is

removed by electro discharge or not.

Surface Quality of EC-PCD by W-EDM in Oil. To obtain far better surface quality, EC-PCD

was wire EDMed in oil. Figure 5 compares surface roughness of EC-PCD machined in different

working fluid, water and oil. Roughness after first cut in water was Rz=2.8μm, but it was improved

to Rz=1.5μm in final cut (7th cut). In the case of finish cut in oil, roughness was drastically

improved to Rz=1.3μm in first cut and Rz=0.40μm in final cut (12th cut).

SEM photos of cross section and edge part are shown in Figure 6. Smooth surface of the

diamond and no chipping on the edge were found.

0

1

2

3

4

56

7

8

9

10

S-PCD EC-PCD S-PCD EC-PCD S-PCD EC-PCD

1st cut 2nd cut 7th cut

surf

ace

rou

gh

nes

s µ

m Ra

Rz

Rzjis

Figure 4 Comparison of surface roughness of 1st and finish cut in water between different PCDs

0

0.5

1

1.5

2

2.5

3

3.5

1st cut 2nd cut 7th cut 1st cut 12th cut

In water In oil

surf

ace

rou

gh

nes

s µ

m Ra

Rz

Rzjis

Figure 5 Comparison of surface roughness of EDMed EC-PCD between in water and in oil

688 Advances in Abrasive Technology XII

Conclusion

In order to improve various properties of existing PCD, a new PCD (EC-PCD) using boron doped

diamond particles having electrical conductivity was developed, and machinability in wire EDM

was investigated. As a result, it has been made clear that EC-PCD is superior to existing PCD in

cutting speed and surface roughness. In addition, drastically good surface roughness such as

Rz=0.4μm was achieved on the EC-PCD by finish cutting following the first cut in oil.

Though measurement was not made in this study, it is guessed that electric resistivity of

EC-PCD in whole is lower because diamond particles used as a basic ingredient have electrical

conductivity. Consequently, it is expected that EC-PCD shows excellent properties in EDM also as

a zero-wear electrode material. In addition, judging from the fact that boron doped diamond

possesses higher thermal resistance leading to lower reaction with a workpiece compared to

conventional diamond, it is thought that EC-PCD can be used as a cutting tool or a wear part where

diamond has been regarded to be unsuitable.

Authors would like to thank FINE ABRASIVES TAIWAN CO., LTD. who supported our study

by providing the sample.

References

[1] Catalogue from Element Six Co. Ltd.

[2] Y.Seki, K.Suzuki, T.Uematsu, T.Makizaki, F.Nabata, A.Ide: Application of PCP to grinding

wheel - 1st report, Proceedings of ABTEC Conference (1996) pp.349-352. (in Japanese)

[3] K.Suzuki, S.Sano, M.Iwai, W.Pan, A.Sharma, T.Uematsu: A New Application of PCD as a Very

Low Wear Electrode Material for EDM, Proceedings of 2nd International Industrial Diamond

Conference (2007).

[4] K.Suzuki, S.Sano, W.Pan, M.Iwai, S.Ninomiya, T.Uematsu: Precise Profile Forming by

Combining EDM and Grinding with a Same PCD Tool, Proceedings of 2nd International

Industrial Diamond Conference (2007).

[5] Technical data from FACT Co. Ltd.

(a) Surface (b) Edge

Figure 6 Finish cut surface of EC-PCD by wire EDM in oil (12th cut)

Advanced Materials Research Vols. 76-78 689