Synthesis and Characterization of an Alumina Forming Nanolaminated Boride: MoAlB

• Published in: Scientific reports Vol. 6; no. 1; p. 26475
• Main Authors: Kota, Sankalp, Zapata-Solvas, Eugenio, Ly, Alexander, Lu, Jun, Elkassabany, Omar, Huon, Amanda, Lee, William E., Hultman, Lars, May, Steve J., Barsoum, Michel W.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 25.05.2016; Nature Publishing Group
• Subjects: 639/301/1023/1024; 639/301/1023/303; Alloys; Aluminum; Electron microscopy; Humanities and Social Sciences; multidisciplinary; Oxidation; Science; Temperature; Temperature effects; Transmission electron microscopy
• ISSN: 2045-2322; 2045-2322
• DOI: 10.1038/srep26475

The ‘MAlB’ phases are nanolaminated, ternary transition metal borides that consist of a transition metal boride sublattice interleaved by monolayers or bilayers of pure aluminum. However, their synthesis and properties remain largely unexplored. Herein, we synthesized dense, predominantly single-phase samples of one such compound, MoAlB, using a reactive hot pressing method. High-resolution scanning transmission electron microscopy confirmed the presence of two Al layers in between a Mo-B sublattice. Unique among the transition metal borides, MoAlB forms a dense, alumina scale when heated in air. Like other alumina formers, the oxidation kinetics follow a cubic time-dependence. At room temperature, its resistivity is low (0.36–0.49 μΩm) and – like a metal – drops linearly with decreasing temperatures. It is also a good thermal conductor (35 Wm −1 K −1 at 26 °C). In the 25–1300 °C temperature range, its thermal expansion coefficient is 9.5 × 10 −6 K −1 . Preliminary results suggest the compound is stable to at least 1400 °C in inert atmospheres. Moderately low Vickers hardness values of 10.6 ± 0.3 GPa, compared to other transition metal borides, and ultimate compressive strengths up to 1940 ± 103 MPa were measured at room temperature. These results are encouraging and warrant further study of this compound for potential use at high temperatures.