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, Λ) 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 Λ values ranging from 20 to 50 nm. When the Λ 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 Λ of 150 nm. However, the higher H/E ratio (0.098) and lower friction coefficient (0.059) appeared at a lower Λ value of 40 nm. The lower residual stress (− 1.02 GPa) and the highest critical fracture load (Lmax = 68.8 mN) were also obtained at Λ = 40 nm. The smaller grain size at the lower Λ 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.