Microstructure Characterization of Boron Suboxide, B6O

Microstructure Characterization of Boron Suboxide B6O

Hans-Joachim Kleebe, Stefan Lauterbach, Mathias Herrmann, Jack Sigalas

Hexaboron monoxide (B6O), commonly termed boron suboxide, is known as a superhard material, constituted of the light elements boron and oxygen. The extraordinary hardness of such a low-density material has been attributed to the intrinsic strong and directional covalent bonds between B and O, leading to a tight, three-dimensional network with an excellent resistance towards external shear.

The average Vickers hardness (Hv) of B6O is about 45 GPa, which is next to diamond (Hv: 70-100 GPa) and cubic boron nitride, c-BN (Hv: 45-50 GPa). Similarly, the average fracture toughness of B6O (4.5 MPa m1/2) is higher as compared to c-BN (2.8 MPa m1/2) and comparable to diamond (5.0 MPa m1/2). Due to its strong covalent bonding, B6O materials demonstrate exceptional physical and chemical performance such as high hardness, low density, high thermal conductivity, chemical inertness and good wear resistance. Its thermal stability, even at temperatures above 1000 °C, and its chemical inertness with ferrous alloys makes it in some instances even more suitable for industrial applications as for example diamond. In general, B6O materials have potential applications as abrasives, due to their high hardness, and are also considered as potential candidates for high-temperature semiconductors (with an estimated band gap of approximately 2.4 eV) and for thermo-electrics. Despite various potential applications, densification and processing of B6O materials was shown to be rather cumbersome. Therefore, a detailed microstructure characterization was performed in order to gain insight in the submicron structure that formed upon sintering.

The main objectives of the TEM study were (i) to verify as to whether an amorphous intergranular phase is present at the B6O grain boundaries (doped versus undoped sintered samples), (ii) what type of crystalline grain-boundary phases are present in the sintered, coated B6O sample: Al4B2O9 and/or Al18B4O33, and (iii) the overall defect structure in the materials was to be characterized (in particular, the question whether stacking faults in B6O can be eliminated via long-term annealing was addressed). In case of the undoped sintered B6O sample, the individual B6O grains are typically well faceted with sharp edges. The grain-size distribution in this sample is bimodal with larger grains (1-2 µm) embedded in a fine-grain B6O matrix. All B6O grains typically reveal numerous stacking faults (similar to B4C particles). In the case of the doped, sintered B6O sample, the grain size was greatly increased (by a factor of about 5x). Stacking faults are a common feature of this B6O material even upon long-term anneal.

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Figure 1. ACerS – NIST Phase Equilibria Diagrams, Fig. 02339-System Al2O3-B2O3. P. J. M. Gielisse and W. R. Foster (1962), Nature (London), 195 (4836), pp.69-70.

A secondary phase formation was observed in the Al-containing sample. With respect to the phase diagram, the two aluminium borate compounds that are most likely to form upon sintering are Al4B2O9 and/or Al18B4O33.

Conventional TEM (bright field; Figure 2) and HRTEM (Figure 3) were performed on the crystalline secondary phase in addition to selected area electron diffraction (SAD). The diffraction pattern given in Figure 3can be indexed corresponding to a primitive orthorhombic unit cell (a,b = 0.7617 and c = 0.2827 nm; Pbma) as well as with respect to an A-centered orthorhombic symmetry (Amam). EDS analysis does not allow the distinction between both phases, because this technique is rather insensitive to boron (no quantitative analysis possible). The A-centered phase; however, should follow the extinction rules (k+l=2n). Under the assumption that the rather weak {010}-type reflections visible in the SAD-pattern (Figure 4) are a result from 2nd-order Laue reflections, it is concluded that extinction occurs. Therefore, the secondary crystalline Al-compound observed in this material is the Al18B4O33 phase. It should be noted that the secondary phase shows an identical orientation within an area of 20-50 microns within the sample. This observation indicates that this Al-phase is characterized by a rather low nucleation rate, which is uncommon for most secondary phases.

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Figure 2. TEM image of the secondary Al-phase, Al18B4O33, observed in the doped, sintered B6O sample.

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Figure 3. HRTEM image of the secondary Al-phase present in B6O. Note the presence of stacking faults as well the weak reflections in the SAD pattern (extinction).