The influence of modulation periods on the evolution of microstructure and mechanical properties of nanoscale HfN/HfB2 multilayers

A series of the HfN/HfB2 nanomultilayers with different modulation periods (bilayer thickness, Lambda) were synthesized via a magnetron sputtering system. The X-ray diffraction (XRD) and cross-sectional scanning electron microscope (SEM) measurements indicated that all the HfN/HfB2 multilayers showed a lower crystallization and a columnar microstructure at the lower Lambda values ranging from 20 to 50 nm. When the Lambda varied from 90 to 150 nm, the multilayers presented the strong polycrystalline and fine-grained microstructure. Owing to the existence of excess B elements measured by X-ray photoelectron spectroscopy (XPS) in the HfB2 layer that prevented grain boundary sliding, the highest hardness (42.58 GPa) and elastic modulus (519.27 GPa) values were reached at Lambda of 150 nm. However, the higher H/E ratio (0.098) and lower friction coefficient (0.059) appeared at a lower Lambda value of 40 nm. The lower residual stress (-1.02 GPa) and the highest critical fracture load (L-max = 68.8 mN) were also obtained at Lambda = 40 nm. The smaller grain size at the lower Lambda should be one of the main contributions to the improved mechanical properties. So, the modulation period might be a key parameter to control the microstructure and the mechanical properties of the HfN/HfB2 multilayers. (C) 2016 Elsevier B.V. All rights reserved.