Synthesis, microstructure and hardness of bulk ultrahard BN nanocomposites

• Published in: Journal of materials research 2008-04, Vol.23 (4), p.981-993
• Main Authors: Rafaja, D., Klemm, V., Motylenko, M., Schwarz, M.R., Barsukova, T., Kroke, E., Frost, D., Dubrovinsky, L., Dubrovinskaia, N.
• Format: Article
• Language: English
• Subjects: Applied and Technical Physics; Biomaterials; Hardness; Inorganic Chemistry; Materials Engineering; Materials Science; Microstructure; Nanotechnology; X-ray diffraction (XRD)
• ISSN: 0884-2914; 2044-5326
• DOI: 10.1557/jmr.2008.0117

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.