J. MATER. RES. TECHNOL. 2013;2(1):68-74 www.jmrt.com.br Review Article Cubic boron nitride competing with diamond as a superhard engineering material – an overview Sergio Neves Monteiroa,*, Ana Lúcia Diegues Skuryb, Márcia Giardinieri de Azevedob, Guerold Sergueevitch Bobrovnitchiib aInstituto Militar de Engenharia (IME), Materials Science Department, Rio de Janeiro, RJ, Brazil bUniversidade Federal do Norte Fluminense, Laboratory of Superhard Materials (UENF/LAMAV), Campos dos Goytacazes, RJ, Brazil ARTICLE INFO ABSTRACT Article history: Since the synthesis of the boron nitride in its cubic crystallographic structure, cBN, a Received 15 August 2012 material with exceptionally high hardness and special properties was developed for Accepted 28 January 2013 technical engineering applications. For practical use, cBN is, after diamond, the second known hardest material and is today being increasingly used as cutting and drilling tools Keywords: in substitution for diamond-based tools owing to superior thermal stability and chemical Cubic boron nitride inertness. In this study, the advances in the use of the cBN competing with diamond as Superhard materials a superhard industrial tooling for petroleum extraction, automobile manufacture and Industrial tools other engineering applications, was reviewed. The properties of both single crystals and Engineering applications polycrystalline cBN in the form of composites and thin films were assessed to characterize the possibilities of a next generation of superhard materials to replace diamond in tool and other technological devices. © 2013 Brazilian Metallurgical, Materials and Mining Association. Published by Elsevier Editora Ltda. All rights reserved. 1. Introduction Around 5,000 years ago, this role was transferred to steels and remained as such until the last two centuries when diamond, Materials with superior hardness have since the beginning of hard ceramics and cermets became industrially available. our civilization been associated with human achievements. Among the hard materials currently used in industrial Hard stones were first used by our ancestors not only as simple operations related to turning, cutting, drilling, boring and implements such as hammers and anvils, but also as tools grinding, diamond stands as the hardest. In fact, rock drilling to shape and sharpen other stones. This was probably the for petroleum and natural gas extraction is today entirely most important pre-historical developed technology to break, dependent on diamond inserts placed onto the head crown chip, splint and finish less harder stones as well as other of perforation tubes. Without diamond particles incorporated existing natural materials such as wood, bone and horn, for into these inserts it would be practically impossible to extract hunting and defense. At the beginning of the age of metals, the petroleum with the efficiency that the modern world bronze replaced stones as the most efficient tool material. demands. As a wonder material, diamond has outstanding * Corresponding author. E-mail address: [email protected] (S.N. Monteiro). 2238-7854/$ - see front matter © 2013 Brazilian Metallurgical, Materials and Mining Association. Published by Elsevier Editora Ltda. All rights reserved. http://dx.doi.org/10.1016/j.jmrt.2013.03.004http://dx.doi.org/10.1016/j.jmrt.2013.03.012 J. MATER. RES. TECHNOL. 2013;2(1):68-74 69 properties that are significantly contributing to improve our should notice the common regions of thermodynamic stability, society [1,2]. In particular, synthetic diamonds allow today a 1,000 °C and 4 GPa for both diamond and cBN. commercial and relatively low cost offer to many technological applications. Here is relevant to remind that since the turn of the XIX century it was already known that diamond was 3. Research and development in cBN an allotropic form of carbon. By then, it became clear that by applying enough pressure and temperature, it would be In the years following the first synthesis not much interest possible to convert the hexagonal structure of graphite into the was demonstrated by the application of cBN as a diamond cubic of diamond. However, it was only in 1955 that General competing superhard material. However, patents [11-13] were Electric research group in the US succeeded for the first time required for the use of cBN as inserts cutting tools. Only in to synthesize diamond [3,4]. the 70’s considerable attention was paid to the potential of In spite of its remarkable properties, as a carbon-based cBN as a substitute for diamond in industrial applications. material, diamond has serious technological limitation. In particular, it was a strong argument in favor of cBN the Working temperatures are restricted and degradation fact that, contrary to diamond, its structure is stable up to reactions occur with important metals including iron. For 2,000 °C. In addition, the decrease in the hardness of diamond instance, the oxidation of diamond initiates at 600 °C while its conversion to graphite takes place above 900 °C [5]. The contact of diamond with a ferrous alloy, like steel, or a nickel superalloy deteriorates its properties by carbide formation [6]. a) As consequences of these limitations, a diamond containing tool can only be used at moderate temperatures and its application is restricted to non-metallic materials as well as Transformation a range of non-ferrous alloys. Another synthetic superhard material, the cubic boron nitride (cBN), is the solution for the adverse diamond limitations. b) Transformation 2. Genesis of cBN Contrary to diamond, the allotropic cubic structure of boron nitride does not exist in a natural form, only synthetically Fig. 1 – Structural transformation at high pressure and high made. Historically, the synthesis of cBN holds parallel temperature for (a) G → D, and (b) hexagonal structure of facts to that of diamond. In 1956 [7], one year after the first boron nitride → cBN. synthesis of diamond [3,4], Wentorf Jr., also a GE researcher at the same laboratory in Schenectady, state of New York, a) 10 USA, transformed for the first time a cubic crystalline lattice compound from mixtures containing boron and nitrogen inside 8 a metal capsule heated by electrical resistance while subjected to high pressure in a belt-type equipment. The announcement 6 Diamond of the cBN discovery in 1957 [8] revealed that the lowest 4 pressure and temperatures then used were 62,000 atmospheres Pressure, GPa Pressure, (6.2 GPa) and 1,350 °C. The name of ‘Borazon’ was proposed 2 Graphite for the cBN and Wentorf reported that it was hard enough to scratch diamond [8]. Following the discovery, other articles 0 0 500 1000 1500 2000 2500 3000 by the GE group [9,10] added information on the synthesis, Temperature (°C) properties and characteristics of cBN. It was reported that high pressure and high temperature (HPHT) conditions similar to b) 10 those effective for diamond synthesis were also suitable for the 8 hexagonal structure of boron nitride (hBN) to be converted into Cubic the ‘diamond-like’ cubic structure of cBN. Moreover, both direct 6 boron and catalyst-assisted transformations were found appropriate nitride for HPHT conversion of the ‘graphite-like’, stacked sheets of 4 (cBN) Pressure, GPa Pressure, hBN into the zinc-blend cubic structure of cBN. Fig. 1 compares Hexagonal 2 the crystalline similarities between the graphite to diamond boron nitride (hBN) (G → D) synthesis and that of hBN → cBN. 0 Coincidences exist not only in the crystallographic 0 500 1000 1500 2000 2500 3000 Temperature (°C) transformation illustrated in Fig.1 but also in the synthesis conditions. Fig. 2 shows the pressure-temperature (P, T) Fig. 2– Pressure vs. temperature diagrams for (a) carbon and diagram for both carbon and boron nitride. In this figure one (b) boron nitride. 70 J. MATER. RES. TECHNOL. 2013;2(1):68-74 Table 1 – Properties of abrasive materials. Material Density Knoop hardness Compressive strength Thermal conductivity Thermal expansion (g · cm–3) (GPa) (GPa) (W/m.K) (10–6 · K–1) cBN 3.45 45 5.33 740 1.2 Diamond 3.51 57-104 8.68 600-2,000 1.5-4.8 WC 14.7 13 4.5 100 5.4 SiC 3.20 32 3.9 120 4.0 is accentuated beyond 500 °C becoming inferior to that of cBN at 800 °C and above. Furthermore, it was realized that cBN stayed inert in contact with steels, cast irons, and superalloys at operational conditions for which diamond would react and loose machinability. Despite its technological advantages, the cost of cBN up to the 80’s was comparatively high enough to discourage a prompt substitution for diamond. Meanwhile, R&D efforts gave a significant contribution to more efficient synthesis process and motivated the expansion of composite products based on cBN. In the last decade, the industrial forms on productivity and cost reduction, created an increasing interest for high performance materials associated with long life tools. In this period, the fabrication of cBN and related sintered composites has grown more than 20 times [14]. From its creation, investigations on the properties and Fig. 3 – cBN crystal synthesized at 6.0 GPa and 1,500 °C with characteristics of cBN permitted to amass relevant data for 4 wt.% of Mg as catalyst/solvent. its possible industrial applications. While the disclosure of information up to the 90’s on cBN in both US [11-13,15-18] and the UK [19-21] was done mainly through patents, Russians [22-29] and Japaneses [30-40] also contributed with to synthesize cBN carried out by Wentorf [9] showed that publications in journals and conferences. In recent years, more alkali and alkali-earth metals as well as their nitrides were than 40 patents and hundreds of articles including those from effective catalysts for HPHT conversion of hBN → cBN. Up to Chinese [41-47] and Brazilian [48-50] researchers have also now, according to a very recent work by Guo et al.