Some properties and cutting performance of polycrystalline cubic boron nitride without additives

Tadakazu Ohashi, Hidetoshi Nakajima, Yoichi Hamada, Katsuyoshi Omino and Kazuo Yamamoto

Mitsubishi Materials Corporation Central Research Institute

Abstract

The cubic Boron Nitride (cBN) is the second hardest material next to the diamond, and possesses many excellent physical and chemical properties similar to those of the diamond. The cBN-ceramics or metal composites produced under pressures form about 4 to 6GPa and at temperatures from around 1300 to 1800K have superior mechanical properties, and cutting tools made from them are being widely used for cutting hardened steels, cast iron and others. In order to improve cutting performance of cBN’s tool tips, tips made from products of different cBN contents and different crystalline grain sizes, and different kinds of binders are being produced by several manufactures. Mechanical properties of cBN are better than those of binder phases. Therefore, it is expected that products containing higher volume fraction of cBN will show better cutting performance. In this regard, polycrystalline cBN solids with no additives have been developed by many techniques, but their cutting performance is not well evaluated yet [1]-[4]. Recently we have developed a manufacturing technique of polycrystalline cBN with no additives in a disk form. In this process, disks of pBN (stable form at ambient temperature) were processed under high pressures and at high temperatures, and products obtained had different volume fractions of pBN and cBN depending on process conditions (named as PCBN’s).

In this study, effects of cBN contents for cutting performance of PCBN’s were examined by comparing the cutting performance of chips made from PCBN’s with those of chips made from conventional cBN-ceramics or metal compacts. The starting pBN used was prepared by the chemical vapor deposition (CVD) method. PCBN samples were prepared under a pressure of 6.8GPa and at temperatures of 2073K, 2373K, 2573K and 2773K respectively, and the heat-treatment time was 30min. The size of starting disks was 1.5mm, and the high-pressure press used in this study was developed by NIRIM (National Institute for Research in Inorganic Materials). After the production of disks, their upper and lower surface were ground by a diamond grind wheel (#400), and then rapped by a diamond paste of particle size below 2μm. Compositions of all samples were analyzed by a x ray diffractometer (XRD), and their fractured surface and rapped surface etched in a molten NaOH for 1min were examined by a scanning electronic microscope (SEM) and a transmission electron microscope (TEM) respectively, and their Vickers hardness was also measured. A diffraction peak corresponding to that of compressed BN (a type of BN phase trapped in PCBN under a high pressure [2]) was found in the sample prepared at 2073K. Above 2373K, only the cBN phase alone was produced. The SEM observation showed that the sample prepared at 2373K is a unique phase, i.e., the grain size is blow 1μm. In the sample prepared at 2573, a partial grain growth was seen, and grains grow remarkably larger at 2753K. The TEM observation showed that twinned grains were created as grains grew. Few defects were seen before the grain growth, but some defects were also induced in cBN grains after the grain growth. Vickers hardness of this sample is 52GPa (reduction from 60GPa). Based on these results above, the sample prepared at 2373K was selected for cutting tests.

The results of cutting tests were as follows; PCBN shows better performance in cutting Ni based super alloys, Co based super alloys and gray cast iron than conventional cBN-ceramics or metal composite. On the other hand, cutting performance of PCBN for hardened steel is worse than that of cBN-ceramics or metal composites.

Reference

[1] M. Wakatsuki, K. Ichinose and T. Aoki, Mater. Res. Bull., 7 (1972) 999

[2] F. R. Corrigan, in “High pressure science and technology”, Vol.1, edited by K. D. Timmerhaus and M.S. Barber (Plenum, New York, 1979) 994

[3] A. Onodera, K. Inoue, H. Yoshihara, H. Nakae, T. Matsuda and T. Hirai, J. Mater. Sci. 25 (1990) 4279

[4] M. Akaishi, T. Satoh, M. Ishii, T. Taniguchi and S. Yamaoka, J. Mater. Sci. Letters, 12 (1993) 1883

The corresponding author: Tadakazu Ohashi, 1-297 Kitabukuro-cho, Omiya, Saitama 330-8508, Japan. E-mail: Ohashi@mmc.co.jp. Tel: 81-48-642-0514. Fax: 81-48-649-7767.

Keywords: cBN, property, cutting, super-alloy, cast-iron.