Self-organized columnar Zr0.7Ta0.3B1.5 core/shell-nanostructure thin films

We recently showed that Zr1-xTaxBy thin films have columnar nanostructure in which column boundaries are B-rich for x < 0.2, while Ta-rich for x >= 0.2. Layers with x >= 0.2 exhibit higher hardness and, simultaneously, enhanced toughness. Here, we determine the atomic-scale nanostructure of sputter-deposited columnar Zr0.7Ta0.3B1.5 thin films. The columns, 95 +/- 17 angstrom, are core/shell nanostructures in which 80 +/- 15-angstrom cores are crystalline hexagonal-AlB2-structure Zr-rich stoichiometric Zr1-xTaxB2. The shell structure is a narrow dense, disordered region that is Ta-rich and highly B-deficient. The cores are formed under intense ion mixing via preferential Ta segregation, due to the lower formation enthalpy of TaB2 than ZrB2, in response to the chemical driving force to form a stoichiometric compound. The films with unique combination of nanosized crystalline cores and dense metallic-glass-like shells provide excellent mechanical properties.