Friction coefficient of solid lubricating coating as a function of contact pressure: experimental results and microscale modeling

Abstract

The paper presents experimental analysis of relation between friction coefficient and contact pressure of $$\hbox {MoS}_2$$ MoS 2 film deposited on $$\hbox {Ti}_6\hbox {Al}_4\hbox {V}$$ Ti 6 Al 4 V substrate in contact with sapphire ball during reciprocating sliding motion. It is shown that the value of friction coefficient decreases with increasing contact pressure. A microscale modeling approach is next developed to mimic the experimental observations. Representative volume element is defined based on the actual topography of outer surface of $$\hbox {MoS}_2$$ MoS 2 film. Assuming thermo-elastic material properties, the calculations on the asperity level are performed in two steps. Firstly, the mechanical contact between two surfaces is calculated. As a result, the relation between the global load and micro-stress distribution is obtained. Secondly, for a given stress load, thermal analysis is performed providing temperature fluctuation within simplified conical asperity. By assuming relation between friction coefficient and temperature on the microscale, it is possible to obtain macroscopic friction coefficient as a function of contact pressure. In the end, model results are compared with experimental data. The novel aspects of presented approach lie in the selection of three main factors on a micro-level defining macroscopic friction. They are actual surface topography, microscopic temperature and microscopic friction-temperature relation.

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