Synthesis and characterization of an alumina forming nanolaminated boride: MoalB

Abstract

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.