Microstructure, mechanical properties and corrosion resistance of high-level hard Nb-Ta-W and Nb-Ta-W-Hf multi-principal element alloy thin films

In the present work, a series of Nb-Ta-W and Nb-Ta-W-Hf refractory multi-principal element alloy thin films were fabricated by multi-target direct current magnetron co-sputtering. The ternary Nb-Ta-W thin films displayed a single BCC solid solution structure with the flake-like surface morphologies and a double-layer structure for cross sections with a columnar upper layer and an amorphous layer near the substrate. The average grain size was 16–30 nm estimated by Williamson-Hall mode and the surface roughness Ra value was 5–7 nm. These ternary thin films had excellent mechanical properties with the hardness values of 25–29 GPa and elastic modulus of 294–325 GPa. After Hf was introduced into the equiatomic NbTaW thin film, the plane view morphology of the NbTaWHf0.6 thin film changed into a fine granular structure, but with a small amount of round particles on the surface. With the further increase of Hf content, the surfaces of the thin films kept being granular structure, but round particles disappeared on the surface of NbTaWHf film, and appeared again on the surfaces of high Hf content films with the increasing number. The cross sections of all Hf-contained thin films were smooth, dense and featureless. Meanwhile, the Ra of Nb-Ta-W-Hf thin films was significantly reduced, ranging from 0.92 to 1.47 nm. All Hf-contained thin films formed amorphous structure. The hardness of Nb-Ta-W-Hf thin films exceeded 19 GPa, and the NbTaWHf thin film had the highest hardness of 22.1 ± 1.3 GPa and elastic modulus of 239.3 ± 6.5 GPa, respectively. Due to the amorphous structure, Nb-Ta-W-Hf thin films exhibited excellent corrosion resistance. The corrosion current density was about two orders of magnitude lower than that of 304 stainless steel, and the polarization resistance was much higher than that of 304 stainless steel.