Influence of W content on microstructure and surface morphology of hard Ni-W films fabricated by magnetron co-sputtering

Abstract

Due to their unique mechanical, tribological, thermal, and anticorrosion properties, nickel-tungsten (Ni-W) alloy films have become indispensable for many industrial applications. The present study investigates the impact of W content on the microstructure and mechanical properties of Ni-W thin films. By co-sputtering of Ni and W on silicon wafers coated with a thin buffer layer (∼20 nm) of titanium (Ti), six Ni-W coatings were fabricated, ranging from pure Ni to pure W. The samples were characterized using energy dispersive spectroscopy, x-ray diffraction, scanning electron microscopy, atomic force microscopy, and microindentation. The results show that hardness of the Ni-W films is primarily a function of the W content, which changes the microstructure and surface morphology of the samples. When W concentration is smaller than 40 at. %, the Ni-rich samples have a face-centered cubic structure and the hardness increases with the W content. For the samples having 40 < W < 55 at. %, the sensitivity of the hardness to the W content becomes markedly low, which could be due to the presence of an amorphous phase. Finally, the impact of W addition on the hardness of the samples containing 55–80 at. % W is two times greater than that of W < 40 at. %. The extra hardening effect could be attributed to the dominancy of a solid solution hardened body-centered cubic W phase and electronic interaction between two transition metals. This sharp increase in the hardness leads to obtaining a high hardness of 21.9 ± 2.0 GPa for the Ni-79 at. % W film. The findings of this study show that solid solution strengthening could be considered the main hardening mechanism of these films.