Design of a vacuum based test rig for measuring micro adhesion and friction force

摘要

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.