afry,
The friction coefficient is mathematically independent of the 'apparent' contact area, but the 'real' contact area will influence the friction coefficient. You can see this effect by increasing the contact force during a wear test. As the 'real' contact area increases, more net force can be transmitted through the contacting interface, which results in the 'unlocking' of new energy dissipation mechanisms such as plastic deformation, phase changes, and fracture.
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