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On the stability of c-BN-reinforcing particles in ceramic matrix materials

: Wolfrum, Anne-Kathrin; Matthey, Björn; Michaelis, Alexander; Herrmann, Mathias

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Materials 11 (2018), No.2, Art. 255, 17 pp.
ISSN: 1996-1944
Journal Article, Electronic Publication
Fraunhofer IKTS ()
cubic boron nitride (cBN); c-BN Batteriewerkstoff; composite; phase transformation; hexagonal boron nitride; microstructure

Cubic boron nitride (c-BN) composites produced at high pressures and temperatures are widely used as cutting tool materials. The advent of new, effective pressure-assisted densification methods, such as spark plasma sintering (SPS), has stimulated attempts to produce these composites at low pressures. Under low-pressure conditions, however, transformation of c-BN to the soft hexagonal BN (h-BN) phase can occur, with a strong deterioration in hardness and wear. In the present work, the influence of secondary phases (B2O3, Si3N4, and oxide glasses) on the transformation of c-BN was studied in the temperature range between 1100 °C and 1575 °C. The different heat treated c-BN particles and c-BN composites were analyzed by SEM, X-ray diffraction, and Raman spectroscopy. The transformation mechanism was found to be kinetically controlled solution–diffusion–precipitation. Given a sufficiently low liquid phase viscosity, the transformation could be observed at temperatures as low as 1200 °C for the c-BN–glass composites. In contrast, no transformation was found at temperatures up to 1575 °C when no liquid oxide phase is present in the composite. The results were compared with previous studies concerning the c-BN stability and the c-BN phase diagram.