Structural and mechanical properties of Al―Mg―B films: Experimental study and first-principles calculations

The Al―Mg―B films were deposited on silicon substrates by direct current magnetron sputtering from the AlMgB14 target at low discharge power and at substrate temperature ranging from 100 to 500 °C. The deposited films have been annealed at 1000 °C in vacuum, and characterized by X-ray diffraction, atomic force microscopy, Fourier transform infra-red spectroscopy, nano- and micro-indentation, and scratch testing. The films exhibit lower hardness than the bulk AlMgB14 material, which is due to their amorphous structure in which the strong intra-icosahedron B―B bonds are almost lacking and the weaker B―O bonds are predominant. After the annealing, a reduction of a number of B―O bonds and a formation of crystallites in the films lead to an increase in the nanohardness and elastic modulus. The as-deposited films exhibit a low coefficient of friction of 0.08-0.12. First-principles studies show that the icosahedra in amorphous AlMgB14-based materials are not fully developed, which is the reason of their lower mechanical performance.