Novel superhard nanopolycrystalline materials synthesized by direct conversion sintering under high pressure and high te
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Introduction There is a growing need for cost reduction and high productivity in high-precision machining of hard materials, especially for use in the automobile, aerospace, medical, and electronics equipment manufacturing industries. Accordingly, much harder, tougher, and more durable cutting tool materials than currently available are required. Conventional sintered polycrystalline diamond (PCD) and polycrystalline cubic boron nitride (PcBN) are widely used as superhard tool materials for cutting nonferrous materials and ferrous metals, respectively. They are made by sintering diamond or cubic boron nitride (cBN) powder with binder materials such as cobalt, TiN, and more at high pressure and high temperature (HPHT) of 5–6 GPa and 1300–1500°C. However, binder materials remaining in the diamond or cBN grain boundaries significantly affect the mechanical properties and thermal stability of the sintered compact. To meet market needs, the hardness of PCD and PcBN needs to be increased by reducing the amount of binder materials and the strength needs to be improved by refining the grains. The ideal ultimate target material is a nanopolycrystalline body in which nano-sized grains of diamond or cBN are directly
and tightly bound to one another without any binder. Such a binderless nanopolycrystalline material cannot be created by merely extending conventional technologies. Innovative new processes need to be developed. We succeeded in creating such innovative superhard nanopolycrystalline diamond1–3 and nanopolycrystalline cBN4–6 by developing a new ultra-HPHT technology and a new processing technology that involves direct conversion sintering under ultra-HPHT conditions, enabling commercial production. The materials have been in practical use such as in cutting tools and wear-resistant tools. This article reviews the development of these practical new superhard materials, as well as their features and applications.
Development of innovative binderless sintering process It is difficult to tightly sinter powdered diamond and cBN without using a binder (solid-phase sintering) even at ultraHPHT conditions, because the diamond and cBN particles are extremely hard and rigid. All commercially available PCD and PcBN are currently produced using binder materials. Singlephase polycrystalline diamond7–11 and cBN12–16 consisting of
Hitoshi Sumiya, Advanced Materials Laboratory, Sumitomo Electric Industries, Ltd., Japan; [email protected] doi:10.1557/mrs.2017.212
• VOLUME • OCTOBER • www.mrs.org/bulletin © 2017 Materials Research Society MRS to BULLETIN 42 terms 2017 Downloaded from https://www.cambridge.org/core. University of Liverpool Library, on 11 Oct 2017 at 15:09:44, subject the Cambridge Core of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/mrs.2017.212
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superhard nanopolycrystalline diamond and cBN
fine grains bonded directly have been experimentally prepared by the direct conversion method.7–16 In this method, the normalpressure phase of carbon (graphite) or hexagonal boron nitri
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