Effects of aluminum addition on microstructures and mechanical properties of NbTiVZr high-entropy alloy nitride films

In this study, the effects of Al addition on the phase evolution, morphology and mechanical properties of (NbTiVZrN)100-xAlx (x = 0, 0.7, 2.3, 3.6, 4.0) high-entropy alloy nitride films (HEANFs) were investigated. The films were prepared by co-sputtering of NbTiVZr alloy target and Al target with a constant N2/Ar flow ratio. The XRD patterns revealed FCC structure and the preferred orientation transferred from TiN(200) to (Nb, Zr)N(111) as the Al content increased. The results of the corresponding selected-area electron diffraction patterns agreed well with the XRD results. Cross-sectional SEM and TEM observations exhibited ultra-fine columnar structure. The nanoindentation test was performed to acquire the mechanical properties of the films. The hardness, reduced modulus and fracture toughness of nitride film increased with the increasing Al content and reached the maximum values of 28.1 GPa, 253 GPa and 2.08 MPa x m0.5, respectively, at x = 3.6. The significant improvement could be ascribed to solid solution strengthening, inverse Hall-Petch effect and the replacement of dominant nitride compounds. As the Al content increased from 0 to 4 at.%, the coefficient of friction decreased from 0.082 to 0.065. The relationship among the composition, structure and mechanical properties was studied for the (NbTiVZrN)100-xAlx HEANFs in this work. Comparisons with (NbTiVZr)100-xAlx HEAFs without nitridation and other nitride films were also made.