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Superlubricant Effect Explained Using New Friction Force Sensor
Research conducted in the Netherlands has revealed a previously unknown effect in graphite. The discovery was made by Martin Dienwiebel using the Tribolever, a highly-sensitive friction force microscope which he had developed himself. Dienwiebel has termed the effect superlubrication and this effect probably explains why graphite is such a good lubricant.
Graphite had already been extensively studied. German physicist Martin Dienwiebel was therefore extremely surprised when he discovered a completely new effect in this well-known lubricant.
During research into the frictional properties of the material, he discovered that the frictional force almost completely disappeared at a certain moment.
Dienwiebel only intended to test the new friction force microscope he had developed. The Tribolever is a raster microscope which can measure frictional forces of just a few picoNewtons in three spatial dimensions. With his new instrument Dienwiebel first of all studied the frictional properties of graphite.
Graphite consists of carbon atoms arranged in layers one above another. The carbon atoms in a graphite layer form a sort of undulating landscape, which is similar to an egg box. The different layers can slide over each other.
However, resistance can occur during the sliding process if the hills of one layer fit exactly into the valleys of another layer. Yet if the two layers are rotated with respect to each other, there are always points within the contact surface where the hills touch each other.
As a result of this the two layers cannot collapse into each other and the resistance is overcome. The researcher has termed this phenomenon superlubrication.
A spray can with graphite lubricant is full of small graphite flakes. Upon spraying, these flakes land in a totally random manner. Consequently all of the flakes are automatically rotated with respect to each other and can therefore glide over each other with the minimum of resistance. The superlubricant effect discovered by Dienwiebell could be the basis of graphite's outstanding lubricating qualities.
Graphite is not the only material for which the physicist wants to determine the frictional properties. As the properties of most materials change upon being exposed to air (for example, due to corrosion), Dienwiebel has also designed a friction force microscope that can work in an ultrahigh vacuum.
The combination of this method with other microscopic techniques such as raster electron microscopy should make it possible to carry out a complete characterisation of friction in the future.
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