Friction between heterogeneous surfaces

One of the possible origins of static friction in macroscopic systems is the presence of third bodies between the solids (67). These typically include wear debris, adsorbed atmospheric molecules, dust, and thin lubricant films. Like the thin films considered here, third bodies have more degrees of freedom than crystalline surfaces. This allows the combined system of bounding surfaces and third bodies to lock into local energy minima, and can produce static friction. Chemical heterogeneity, surface roughness and length scale can also lead to static friction (67), and more work is needed to establish which effects dominate in different limits. 

Taking into account the discreteness of the friction contact, heterogeneity of the friction surfaces, micro-inhomogeneity of the metal structure, local plastic deformations and difference in energy between the states of fresh surface and areas covered with oxide films, the friction pair can be treated as an intricate multielectrode system. 

Pull-off force experiments and friction mode AFM with unmodified tips can be utilized to measure differences on chemically heterogeneous surfaces and have been shown to differentiate between chemically distinct phases in phase separated LB films, etc. Flowever, the tip functionality cannot be varied and therefore, this approach is inherently limited [148]. 

These tests allow the following analyses based on in situ and ex situ measurements, like the determination of the mean and local coefficient of friction, the identification of and study of the interactions between surfaces and environment, the nature of the mechanical-chemical coupling, the electrochemical or galvanic coupling due to a heterogeneous structure, the shape and location of rubbed and non-rubbed areas, or the establishment of local wear laws and their spatial distribution on the surface in view of a modelling of wear aiming at a future predictive approach. 

Lubricants can be active as they either are reduce the melt viscosity of the polymer or reduce the heterogeneous friction of the polymer to the static and moving walls of the processing equipment. A lubricant makes the surface of a final resin product composition smooth so as to improve its processability. Both an external lubricant and an internal lubricant can be used. The internal lubricant resides inside the polymer to reduce the viscosity of the resin itself, thereby improving flowability. The external lubricant reduces extrusion load between the polymer melt and metal surface in an extruder (2). 

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