A comparative nanotribological investigation on amorphous and polycrystalline forms of MoS2

The wear behavior of two amorphous and polycrystalline forms of MoS2 prepared by magnetron sputtering is characterized in a combined nanoindentation and atomic force microscopy study supported by Raman and transmission electron microscopy (TEM) analysis. From the morphology of wear tracks estimated after scratching the surfaces with a Berkovich indenter and a loading force up to 2 mN, we conclude that, on the microscale, both forms follow the Archard’s wear equation, and the wear resistance is about four times higher on the amorphous MoS2. The coefficient of friction is much lower on the worn areas, which is associated to significant smoothing of the surfaces caused by the scratching process. With normal forces in the µN range, the analysis is made difficult by the fact that the linear dimensions of the wear tracks are comparable to those of the smallest surface features. Even if the Archard’s equation looses validity, the wear resistance is considerably larger on amorphous MoS2 also on the nanoscale. Our results concluded that the polycrystalline form of MoS2 has poor tribological properties at the micro/-nanoscale as compared to the amorphous form and hence less suited as a solid coating in ambient conditions.