Distinctive mechanical properties of nano-polycrystalline diamond synthesized by direct conversion sintering under HPHT

doi:10.1016/j.diamond.2011.10.013

Diamond and Related Materials

Volume 24, April 2012, Pages 44-48

https://doi.org/10.1016/j.diamond.2011.10.013 Get rights and content

Abstract

The mechanical properties of nano-polycrystalline diamond (NPD) synthesized by the direct conversion of graphite under high pressure and high temperature have been investigated. Indentation hardness and bending strength tests revealed that NPD has considerably high hardness and high transverse rupture strength (TRS) at high temperature, far surpassing those of conventional polycrystalline diamond (PCD) and single-crystal diamond (SCD). The hardness remained higher than 100 GPa even at 800 °C, while the hardness of SCD sharply decreased to 60 GPa above 300 °C. The TRS remained at about 3 GPa up to 1000 °C, above which it showed a positive temperature dependence, while the TRS of PCD decreased rapidly at about 500 °C. Wear tests using a diamond wheel indicated that the abrasive wear resistance of NPD is equivalent to those of the high wear-resistance directions on SCD, and from 10 to 50 times higher than those of PCD. These results suggest that NPD has outstanding potential for cutting tools.

Highlights

►We investigate mechanical properties of nano-polycrystalline diamond > (NPD). ►NPD is synthesized by the direct conversion of graphite under HPHT. ►NPD has considerably high hardness, TRS and abrasive wear resistance. ►These properties of NPD are far surpassing those of conventional diamonds. ►These results suggest that NPD has outstanding potential for cutting tools.

Introduction

Graphite directly converts to diamond completely under high pressure and high temperature (HPHT) of more than 15 GPa and 2300 °C, and so single-phase (binderless) nano-polycrystalline diamond (NPD), consisting of fine diamond grains of several tens of nanometers, can be obtained [1], [2], [3]. NPD comprises ultra-fine diamond grains with grains bonded directly and solidly to each other to form a dense structure [4], characterized by high hardness surpassing that of single-crystal diamond (SCD) [5]. The Knoop hardness of NPD at room temperature is about 130 GPa, which is equivalent to that on (001) < 100 > of synthetic high purity (type IIa) SCD and obviously higher than those of common type I SCDs [5]. While the indentation hardness of SCD significantly varies in the range of 60–120 GPa depending on the crystallographic planes and directions, NPD has no such anisotropy of hardness. In terms of fracture morphology, transgranular fracture is dominant in NPD, indicating that intergranular bonding strength is extremely high [6]. This high intergranular strength as well as the effect to inhibit the development of plastic deformation and/or microcracking at the grain boundaries leads to the high hardness surpassing that of SCD. As the hardness of conventional polycrystalline diamond (PCD) containing metal binder is around 50 GPa [7], the hardness of NPD is more than double. Also, unlike SCD, NPD has no cleavage features, and has excellent high thermal stability because it has no binder materials or secondary phases. Due to these excellent characteristics, NPD has high potential as a hard material suited for such applications as cutting tools and wear-resistant tools. Currently, we are working on enlarging the size of NPD and developing mass-production technology to apply NPD commercially. Recently, NPD of 8–10 mm in size of satisfactory quality has been obtained.

In this study, we investigated the basic mechanical properties, including high-temperature hardness, high-temperature transverse rupture strength (TRS) and abrasive wear resistance. Hardness and TRS at high temperature are very important properties for a cutting tool, because they are closely related to the resistance of wearing and chipping of the cutting tool. The pin-on-disk abrasion test is particularly useful for evaluating the basic performance of cutting tools. Therefore, it is important to investigate these properties of NPD in order to assess its potential for cutting tool applications, but few such studies have been carried out to date.

Section snippets

Experimental

Several NPD specimens were prepared using high-purity isotropic graphite compacts as a starting material, through direct conversion sintering at ultra-pressures of 16 GPa, temperatures of 2300 °C and retention time of 20 min. The ultra-high pressures and high temperatures were generated using a Kawai-type multi-anvil apparatus [8]. The NPD specimens thus obtained exhibited transparency and consisted of very fine grains of several tens of nanometers as shown in Fig. 1. X-ray diffraction analysis

High-temperature hardness

Knoop hardnesses of NPD at room temperature and 300 °C were 120–135 GPa and 115–130 GPa, respectively. Fig. 3 shows these results compared with Knoop hardnesses in various directions on (001) of synthetic IIa and Ib single-crystal diamond crystals [13]. The hardness of NPD at room temperature is equivalent to that on (001) < 100 > of synthetic type IIa SCD and higher than that of synthetic type Ib SCD (Fig. 3(a)). The hardness on (001) < 110 > of synthetic type IIa SCD is extremely high, and it is

Conclusions

The mechanical properties of NPD derived from direct conversion sintering of graphite under ultra-high pressures and temperatures were evaluated. It was confirmed that NPD exhibits excellent properties far surpassing those of PCD and SCD in terms of high-temperature hardness, TRS and abrasive wear resistance, which are important properties in cutting tool applications, as listed below.

(1)
The hardness of NPD remains higher than 100 GPa even at 800 °C, while the hardness of SCD sharply decreases to 60

Acknowledgments

The research on the synthesis of high-purity polycrystalline diamond was performed in collaboration with Professor Irifune of the Geodynamics Research Center of Ehime University.

References (17)

H. Sumiya et al.
Diam. Relat. Mater.
(2004)
H. Sumiya et al.
Diam. Relat. Mater.
(1997)
H. Sumiya et al.
Diam. Relat. Mater.
(1996)
H. Sumiya
Diam. Relat. Mater.
(2006)
T. Irifune et al.
Nature
(2003)
H. Sumiya et al.
SEI Tech. Rev.
(2005)
H. Sumiya et al.
SEI Tech. Rev.
(2008)
H. Sumiya et al.
J. Mater. Sci.
(2004)

There are more references available in the full text version of this article.

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^☆: Presented at NDNC 2011, the 5th International Conference on New Diamond and Nano Carbons, Suzhou, China.

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Distinctive mechanical properties of nano-polycrystalline diamond synthesized by direct conversion sintering under HPHT☆

Abstract

Highlights

Introduction

Section snippets

Experimental

High-temperature hardness

Conclusions

Acknowledgments

Diam. Relat. Mater.

Diam. Relat. Mater.

Diam. Relat. Mater.

Diam. Relat. Mater.

Nature

SEI Tech. Rev.

SEI Tech. Rev.

J. Mater. Sci.