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. 2018 Feb 7;11(2):255.
doi: 10.3390/ma11020255.

On the Stability of c-BN-Reinforcing Particles in Ceramic Matrix Materials

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Free PMC article

On the Stability of c-BN-Reinforcing Particles in Ceramic Matrix Materials

Anne-Kathrin Wolfrum et al. Materials (Basel). .
Free PMC article

Abstract

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.

Keywords: c-BN; composites; cubic boron nitride; hexagonal boron nitride; microstructure; phase transformation.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure A1
Figure A1
Gibbs free energy ΔG° of the c-BN to h-BN transformation.
Figure A2
Figure A2
Temperature dependence of the ratio of the equilibrium concentrations of BN in the melt over c-BN and h-BN calculated based on Equation (A7) and ΔG° (cBN/hBN) (see Figure A1).
Figure 1
Figure 1
FESEM micrographs of the initial (a,b) and the prior oxidized (c,d) cubic boron nitride (c-BN) powders after heat treatment at 1550 °C in Ar.
Figure 2
Figure 2
Raman spectra of the two heat treated c-BN powders compared to the c-BN powder as received (synthesized), showing clear differences in the transformation behavior.
Figure 3
Figure 3
(a) SEM micrograph of the fracture surface of the in the spark plasma sintering (SPS) at 1575 °C sintered Si3N4–c-BN composite without sintering additives and (b) Raman spectra of the as-synthesized c-BN powder and one c-BN grain on the fracture surface of the Si3N4–c-BN composite; (c) Interface between c-BN and sialon matrix-densified at 1575 °C. At the interface, the hexagonal boron nitride (h-BN) grains embedded in the amorphous oxynitride grain boundary phase are clearly visible (light gray phase-sialon, bright phase-oxynitride liquid; dark gray-c-BN/h-BN) and (d) typical EDX data of these phases.
Figure 4
Figure 4
FESEM micrographs of the ion beam polished sections of the glass–c-BN composites heat treated at: (ac): 1400 °C, 1 h; (df): 1300 °C; 10 h and (gi): 1200 °C, 10 h. From left to right: G1-BN, G2-BN, G3-BN.
Figure 5
Figure 5
Calculated partial pressures of gas species over h-BN and c-BN (a) and h-BN/B2O3 (b); (The dotted lines in Figure 5a correspond to c-BN as a solid phase).

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