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Newsletter "SEI NEWS" 2013

Home > Company Information > SEI WORLD > Back number > Vol.426

[Newsletter "SEI NEWS" Vol.426]

Sumidia Binderless Nano-polycrystalline Diamonds Top all Other Diamonds

Sumitomo Electric’s diamond business has a history of more than 40 years. Making ceaseless efforts to hone its original ultrahigh-pressure technology, the company has provided society with a wide variety of diamond products, including Sumiboron sintered cBN (cubic boron nitride), Sumidia sintered diamonds, and Sumicrystal synthesized single-crystal diamonds. Sumitomo Electric has now developed the ultimate diamond, Sumidia Binderless, which is a nano-polycrystalline diamond. It is superior to single-crystal diamonds in hardness and it has overcome the problem of cleavage to which single-crystal diamonds are susceptible. Hitoshi Sumiya has been committed to research and development of nano-polycrystalline diamonds. Daisuke Murakami and Kazushi Obata applied this new diamond to cutting tools and promoted its development. This story describes the steps these engineers took, integrating their expertise and pioneering the new potentials of a material. Readers will learn how thrilling research and development can be.

Hitoshi Sumiya, Senior Specialist, Inorganic Materials R&D Dept.  Advanced Material Research Department / Kazushi Obata, Assistant Manager, Sales Engineering Department, A.L.M.T. Corp. / Daisuke Murakami, Manager, Round Tool Development Group, Design Development Department, Sumitomo Electric Hardmetal Corp.

* People’s positions and titles are given at the time the authors interviewed them.

Why did Sumitomo Electric start to develop diamonds?

It was around 1970 when Sumitomo Electric started to study synthetic diamonds. The company had already established its position as a manufacturer of ultrahard alloys. Foreseeing the demand for next-generation tool materials that would replace ultrahard alloys, the company had begun to develop new hard materials such as diamonds and cubic boron nitrides (cBN). To manufacture diamonds and cBN, we first needed to develop a technology to reproduce ultrahigh-pressure and ultrahigh-temperature (ultra-HPHT) conditions where natural diamonds are generated deep in the ground. The company started by constructing the basis of the technology from scratch and succeeded in commercializing sintered diamonds and sintered cBN as the materials for products such as cutting tools. Sumitomo Electric applied this ultrahigh-pressure generation technology to synthesizing single-crystal diamonds, and in 1980, it succeeded in developing a synthetic single-crystal diamond of one carat that was about five to six millimeters across. The company succeeded in mass-producing it in a few years, leading the world. In 1982, Sumitomo Electric succeeded in synthesizing a single-crystal diamond of 1.2 carat that was officially registered in Guinness World Records in 1984 as the world’s then largest synthetic diamond.

Sumiya joined Sumitomo Electric around that time. He was initially engaged in research and development of synthetic diamonds, mainly to produce larger synthetic diamonds. In 1990, the research team succeeded in synthesizing a single-crystal diamond as large as nine carats; however, those synthetic diamonds looked yellowish because of impurities (nitrogen).

High-purity large diamond crystal 12 mm in diameter exceeding 10 carats in size

Sumiya proceeded to develop a technology to remove the nitrogen impurities from diamonds. Around the year 2000, his team succeeded in synthesizing a colorless, transparent, highly pure crystal diamond of one to two carats (5 to 6 mm in diameter), and it was commercialized. The team made further efforts to improve diamond quality while enlarging diamonds, and today the company can synthesize large, high-purity diamond crystals 12 mm in diameter, which exceeds ten carats (photograph). Tenacious research and development efforts over 15 years turned one-carat yellowish diamond crystals into around ten-carat colorless, transparent diamond crystals that exceed natural diamonds in quality.

 

Search for materials that would surpass single-crystal diamonds

After succeeding in synthesizing a large colorless and transparent high-purity crystal, Sumiya found a new challenge. Single-crystal diamonds are susceptible to breaking in specific directions (cleavage*) and wear out easily. This could be problematic when being used as cutting tool material. Sintered diamonds, which were already being used as cutting tool material, were largely inferior to single-crystal diamonds in hardness and heat resistance because of the binding material.

“If we can sinter diamonds with extremely small particles without using binding material, the diamond will be sufficiently hard and heat-resistant to avoid the problem of cleavage.” Realizing this, Sumiya first tried to sinter diamond powder without binding material. However, it was impossible to combine extremely hard diamond particles directly. It was known that ultra-HPHT conditions?an environment at a pressure of about a dozen gigapascals and a temperature of two thousand several hundred degrees Celsius?could directly convert non-diamond carbon, such as graphite, into diamond. “If this direct conversion can be used to sinter diamond instantaneously, we could obtain a firmly combined, polycrystalline diamond with no intergranular spaces.” Sumiya decided to start to develop this process method. The technologies available in-house could not achieve such extremely high pressure conditions stably and an ultrahigh pressure facility called a multi-stage multi-anvil system was needed. However, there was strong opposition in the company against introducing facilities that would not be productive.

Accordingly, Sumiya started a joint research project with Ehime University who were studying the underground mantle, which is under ultra-HPHT conditions. As a result of this joint research, a high-purity polycrystalline diamond was obtained, although it was small at 1 mm. He brought it back to the company, examined it, and found that extremely fine diamond particles of a few dozen nanometers had constituted a closely-bound structure, and that it was harder than single-crystal diamond. He published the joint development of this nano-polycrystalline diamond (NPD) with Ehime University in a paper that was subsequently covered by many newspapers and attracted public attention.

* Cleavage: Splitting in a specific direction, which is specific to single crystals

 

A new diamond surpassing all other diamonds in the world

In 2005, the team succeeded in synthesizing an NPD 1 cm in diameter, and in November, this NPD development was selected by Japan’s New Energy and Industrial Technology Development Organization (NEDO) as one of the projects to subsidize for commercialization. The company recognized the NPD project as an official development project and accelerated its development as a business.

Nano-polycrystalline diamond (NPD)

There were more challenges in commercializing NPD: further enlarging sample size, building a large-capacity system capable of generating stable ultra-HPHT conditions, and developing mass-production technologies. In addition, the practical performance of NPD as a tool material was insufficient and stabilizing the quality of the material itself was a difficult challenge. Development team members committed themselves to producing larger-sized samples using multi-stage, multi-anvil devices, and to developing technologies to mass-produce NPD on an industrial scale. The technologies and experience gained through the development of single-crystal diamonds were useful in solving these challenges. Furthermore, by improving the starting materials, optimizing the synthesizing conditions, and controlling the fine structure, the team succeeded in building technologies that could provide improved and stabilized NPD characteristics, and this enabled industrial production of NPD that was good enough to use as a cutting tool material. Thus the team succeeded in developing NPD (photograph), a new diamond that is harder than single-crystal diamond and that offers a solution to the weaknesses of sintered diamonds, which are cleavage and low heat resistance.

 

Application of nano-polycrystalline diamond to cutting tools

While committing himself to these research and development efforts, Sumiya had contact with potential business partners, including Obata, who were developing and manufacturing diamond cutting tools, to find out if NPD could be used as a material for precision cutting tools.

“It sounds an interesting material.” Their first response did not sound very enthusiastic. This was only reasonable, because at that time NPD varied in shape and color, and had problems such as the production of some damaged areas on the processed material. In addition, the superior characteristics of the material, which were extreme hardness and wear resistance, meant that processing it into the shape of a cutting tool was difficult. There were many challenges left unsolved in its use as a cutting tool.

 

“We want tools of nano-polycrystalline diamond!”

On the other hand, potential customers who heard about NPD through Sumiya’s newspaper articles and conference presentations, started to ask about the new material: “We would like a chance to evaluate it, and we want you to produce a cutting tool.” In response to these strong requests, Obata, Murakami and others started to try to produce a cutting tool from NPD: “Even if the processing takes time, let’s use it in a tool.”

Soon after they started this attempt, they found that NPD processing was more difficult than expected. The strength and hardness of single-crystal diamonds are different depending on the direction and it is usually possible to process them from a soft direction. However, NPD was hard in all directions, and machining it using conventional processing methods was extremely time-consuming. “Engineers at the processing site tried many methods to establish new processing techniques,” Murakami recollects.

The cutting tools thus created with great effort were tried by customers and highly evaluated, as expected. “This material is wonderful!” The development of cutting tools using NPD started full-scale. In January 2012, fine-machining ball end mills and ultrahigh-precision processing cutting tools with a cutting edge of Sumidia Binderless NPD made their commercial debut.

Ultra-high precision machining cutting tools (A.L.M.T. Corp.)

 

Micromachining ball end mills (Sumitomo Electric)

 

A revolution in the processing of ultrahard alloys is underway

Industrial products are requiring even higher-precision and finer processing these days, and extremely hard materials that are difficult to cut are becoming common. Tools with NPD can enable processing that used be impossible. In processing ultrahard alloys, the edges of single-crystal diamonds tended to chip, and sintered diamonds with low-precision edges easily wore out, resulting in low precision and a short service life. NPD cutting tools, which have sharp cutting edges and superior resistance to chipping and wear, have enabled ultrahigh-precision processing such as direct cutting and mirror finishing.

 

Research and development will continue

Currently, NPD has found application for special uses, such as die processing, but the team hopes to make it useful in a wide variety of applications. To do so, it is necessary not only to make improvements to the material but also to develop processing technologies. While developing new processing technologies, Sumiya and his partners intend to continue to develop new tool products using NPD and bring in more varied options, aiming at widening the range of applications.

・Sumidia, Sumiboron, and Sumicrystal are trademarks or registered trademarks of Sumitomo Electric Industries, Ltd.

 
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