The Elastic and Frictional Properties of Nanoscale Coatings Based on Molybdenum Disulfide at Micro and Nano Levels

Abstract

Nanoindentation at temperatures of 23 and 150°C is used to study the coating of molybdenum disulfide doped with silver and calcium fluoride. The Nanoscan-4D scanning nanohardness tester was used for the experiments. A method is presented for determining the elastic properties of a coating from elastic indentation curves, taking into account the real shape of the indenter head, which is determined by optical profilometry. The elasticity modulus of the coating is determined based on the exact solution of the contact problem for a two-layered elastic foundation, taking into account the calculated compliance of the measurement system. Newton’s method is used for the inverse problem solution. The input parameters of the problem, in addition to the geometry of the indenter head and the load, are the elastic properties of the head and substrate materials. The elastic type of indentation was provided at maximal load of 10 mN for both temperatures. The loading–unloading curves at room temperature and at 150°C turned out to be close (within the experimental error), which proves the stability of the elastic properties in the considered temperature range. The calculated elastic modulus of the coating was 326 GPa. Using the same device, equipped with a lateral force sensor, the sliding friction coefficient of the coating was determined under different loads (5, 10, and 20 mN). Such a study can be considered as a physical model of the contact of the coating with a single asperity. The experiments were carried out on straight tracks 1 mm long at a speed of 11 µm/s. It is shown that the coatings are antifrictional (with friction coefficients in the range 0.033—0.078). The coefficient of friction increases with increasing load, which may be due to the dissipation of energy for plastic deformation of the coating material at relatively high loads. The conclusion about the presence of plastic deformation is based on the results of optical profilometry, which showed plastically deformed and pushed material along the edges of the friction track under relatively high loads. At low loads, this phenomenon is not observed. This coating can be used in sliding friction units that require one or two applications with a low friction coefficient.