Synthesis, microstructure and hardness of bulk ultrahard BN nanocomposites
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M.R. Schwarz Institut für Anorganische Chemie, Technische Universita¨t (TU)-Bergakademie Freiberg, D-09599 Freiberg, Germany; and Chemische Materialwissenschaft, Fachbereich Chemie, Universität Konstanz, D-78457 Konstanz, Germany
T. Barsukova and E. Krokea) Institut für Anorganische Chemie, Technische Universita¨t (TU)-Bergakademie Freiberg, D-09599 Freiberg, Germany
D. Frost, L. Dubrovinsky, and N. Dubrovinskaia Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany (Received 26 April 2007; accepted 3 January 2008)
Ultrahard boron nitride compacts containing nanosized domains of the cubic (c-BN), wurtzitic (w-BN), and hexagonal (h-BN) phase were synthesized at high-pressure/ high-temperature (HP/HT) conditions. Hot-pressed and pyrolytic BN, both containing h-BN as a main component, were used as starting materials. The HP/HT products were investigated by x-ray diffraction via Rietveld and line-profile analysis, as well as high-resolution transmission electron microscopy. c-BN was the dominant phase in all products, complemented by up to 25 wt% w-BN and some remaining “compressed h-BN.” In particular samples, partial crystallographic coherence of adjacent crystallites to x-rays was observed, which has been previously found in superhard transition metal nitride-based nanocomposite coatings. In the BN nanocomposites, the partial coherence of nanocrystallites to x-rays was improved by their strong local preferred orientation, which is made possible by the well-known orientation relationships among h-BN, w-BN, and c-BN phases. The correlation between the weight fraction and the average size of the c-BN crystallites helped to describe the formation of c-BN/(w-BN) nanocomposites from submicron-sized h-BN domains in the starting materials. The Knoop and Vickers hardness of specimens with crystallite sizes ranging from 6 to ∼50 nm was found to be significantly higher than that of c-BN single crystals, despite the presence of residual h-BN. I. INTRODUCTION
The concept of hardness enhancement via nanostructuring was first suggested and demonstrated for superhard multilayers and nanocomposites that are based on transition metal nitrides.1 For somewhat more than a decade, these materials have experienced continuous scientific and technological advancement as well as industrial success as protective coatings, for example, in metal-cutting operations. 2–4 Concomitantly, microstructural features at the nanolevel are increasingly recognized as the source and as a means for further improvement of the traditional high-pressure-derived a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0117 J. Mater. Res., Vol. 23, No. 4, Apr 2008
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superhard bulk materials such as diamond and diamondlike boron nitride polymorphs (sp3-BN). In particular, the synthesis of nanocrystalline forms of diamond via high-pressure/high-temperature (HP/HT) conversion of graphite5,6 and other precursors7 with extreme hardness up to 145
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