Exploring High Pressure Behavior of Superhard Transition Metal Borides

Seminar series
Physical Chemistry Seminar
Thu, May 30 12:00pm
2033 Young Hall
Speaker Miao Xie
University of California, Los Angeles
Dept. of Chemistry & Biochemistry

Abstract: Dense transition metal borides are members of a new and growing class of superhard materials that can be synthesized at ambient pressure. These compounds may find use in cutting tools and protective coatings. A prime example of this group of superhard solids is tungsten tetraboride (WB4). WB4, which can be synthesized by arc melting at ambient pressure, possesses a Vickers microindentation hardness of 43 ± 2.9 GPa under an applied load of 0.49 N. To understand the high hardness and to examine the lattice behavior of WB4 under elevated pressure, we have measured elastic and plastic deformation of WB4 using in situ high pressure x-ray diffraction under both hydrostatic and uniaxial loading. We show that WB4 has a high bulk modulus of 326 GPa and a high yield strength, supporting up to 16 GPa of differential load at a pressure of 50 GPa.  WB4 appears to show these remarkable mechanical properties because it contains a three-dimensional, almost isotropic, rigid covalently boron network.  Interesting, this rigidity also results in a second-order high pressure phase transition that is not observed in other, softer transition metal borides. To further enhance the hardness of WB4, we add tantalum (Ta), manganese (Mn), and chromium (Cr) to form solid solutions. With the high pressure x-ray diffraction technique, we identify the role of electronic effects in determining the strength and hardness of the solid solutions.