Chemical Effects in Friction and Lubrication

To keep an object sliding at a constant speed over a flat surface, one needs to apply a force that is equal to the friction force F, which acts parallel to the surface, in 

The proportionality factor/is called the friction coefficient. Its value depends on mechanical parameters and the presence or not of a lubricant on chemical parameters such as humidity, oxide films, chemical additives and on materials parameters such as hardness, ductility and microstructure. 

The contact area is always smaller than the geometrical surface area A. The pressure acting on the contacting asperities is equal to 

If Pasp exceeds the elastic limit of the material, the asperities undergo plastic deformation. As a consequence, the real contact area increases until the value of Fn/Aj becomes equal to the elastic limit of the material. This is the same situation as in a hardness test the application of a given normal force produces an indentation whose area characterizes the hardness H of the material. We can therefore write  

Sliding continuously creates and breaks asperity junctions having a shear strength T equal to  

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