The pre-sliding and static friction force behaviour at asperity level between a smooth ball and a smooth flat surface at different normal loads, as well as friction behaviour during full slip has been studied. The normal load dependence of the friction force and the preliminary displacement is discussed when the mean contact pressure is kept under 100 MPa. The theoretical model to calculate the shear stress and the preliminary displacement in the contact is discussed and the experimental data were used to verify the model. The results show that for low applied normal loads the adhesion force has an influence on the friction force measurements. Furthermore, the results for the friction force and preliminary displacement show good agreement with the theoretical trends. The experiments along with the model can be used to analyse the tangential traction in the contact and the behaviour of the stick-slip area. The measurement results along with the model were used to calculate the maximum shear stress at the point of sliding for different applied normal loads. It is also shown that at low applied normal loads the shear stress is not constant as compared to relatively high applied normal loads due to the presence of adhesion force.

The adhesion force due to capillary interaction between two hydrophilic surfaces is strongly dependent on the partial pressure of water and is often calculated using the Kelvin equation. The validity of the Kelvin equation is questionable at low relative humidity (RH) of water, like in high vacuum and dry nitrogen environments, where water is only present as layers of several molecules thick at the surfaces. A model from ordered to bulk form of water has been developed using the Brunauer, Emmett, and Teller adsorption model. The results show that the adhesion force calculated using the Young-Laplace and Kelvin equations at low (5-30 %) RH is underestimated. The total adhesion force shows changes when the RH is changed from 0 to 100 %. In dry conditions, at RH below 10 %, the total adhesion force is contributed by the van der Waals interaction due to solid-solid contact. The total adhesion force then increases and remains constant being equal to the superposition of van der Waals interaction due to solidsolid contact and van der Waals interaction due to adsorbed water layers on the surfaces. The total adhesion force further increases slowly with the increase in RH incorporating capillary forces and then decreases at very high RH due to screening of van der Waals forces. This change in adhesion force occurs from solid-solid interaction to ordered form of water at low RH and from ordered form to bulk form of water at high RH along with the screening effect of van der Waals interaction. The results have been compared with the experiments and it has been seen that at small length scales, the model is in agreement with the existing experimental data. However, at large length scales roughness of the surfaces should be taken into account.

Mechanisms operating in low pressure (vacuum), nitrogen or other special environments are found in many applications. Examples are medical instrumentation, electron microscopes, lithography systems, as well as aviation and space applications. The positioning accuracy and drifts in these mechanisms are strongly influenced by the frictional behaviour of the mating materials. The cause for both drift and positioning accuracy are stick-to-slip and slip-to-stick transitions at asperity level, resulting in a displacement at macrolevel. This paper discusses the experimental setup designed and manufactured to validate a model which relates friction and positioning accuracy for a particular pair of material at asperity level. A simplified form of a single asperity contact is a ball in contact with a flat surface. The experimental setup has been designed to validate the friction and adhesion models. Snap-in, pull-off experiments and friction force measurements can be performed with the resolution of 5 μN. The maximum normal load that can be applied with this system is 100 mN. The setup is capable of working at 10 -6 mbar vacuum level. A 2 Degree of Freedom (DOF) elastic hinge mechanism is the heart of the setup and measures the normal and tangential load with the help of capacitive displacement sensors. In the setup, high precision positioning stages have been used which are capable of moving 20 mm in XYZ with an accuracy of 20 nm. The setup is also able to perform friction measurements with the same accuracy as mentioned above. Design and performance of the setup will be discussed and the results are compared with the theory.