Enhancing the Hardness of Superhard Transition-Metal Borides: Molybdenum-Doped Tungsten Tetraboride

By creation of solid solutions of the recently explored low-cost superhard boride, tungsten tetraboride (WB4), the hardness can be increased. To illustrate this concept, various concentrations of molybdenum (Mo) in WB4, that is, W1-xMoxB4 (x = 0.00-0.50), were systematically synthesized by arc melting from the pure elements. The as-synthesized samples were characterized using energy-dispersive X-ray spectroscopy (EDS) for elemental analysis, powder X-ray diffraction (XRD) for phase identification, Vickers microindentation for hardness testing, and thermal gravimetric analysis for determining the thermal stability limit. While the EDS analysis confirmed the elemental purity of the samples, the XRD results indicated that Mo is completely soluble in WB4 over the entire concentration range studied (0-50 at. %) without forming a second phase. When 3 at. % Mo is added to WB4, Vickers hardness values increased by about 15% from 28.1 +/- 1.4 to 33.4 +/- 0.9 GPa under an applied load of 4.90 N and from 43.3 +/- 2.9 to 50.3 +/- 3.2 GPa under an applied load of 0.49 N. Thermal gravimetric analysis revealed that the powders of this superhard solid solution, W0.97Mo0.03B4, are thermally stable in air up to similar to 400 degrees C. These results indicate that the hardness of superhard transition-metal borides may be enhanced by making solid solutions with small amounts of other transition metals, without introducing a second phase to their structures.