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| Small asperities are destroyed by the normalforce. The real area of contact is enlarged. |
Even a surface which appears to be flat on a millimeter scale may contain micrometer scale asperities i.e. the surface is rough. If we bring two surfaces in contact, only these asperities really touch each other. Friction is due to the interaction between the asperities of the different surfaces and the resulting energy dissipation is due to the interaction of these asperities. The real area of contact is therefore a few orders of magnitude smaller than the apparent area of contact. This important fact has to be taken into account while modelling a friction process.
Wear of the material is one possible reason of friction. Though, if you calculate the rate of wear by means of the mechanical work you have to close that the wheels of a locomotive would be distroyed after a few kilometers of use.
It follows that only a little part of the atoms have been removed from their initial position through this process.
This idea has been dropped as the theory of the (permanent) plastic transformation came up. It says that every transfomation is plastic. The law of Amonton can easily be derived from this theory.
1961 J.F.Archard anew pointed that a permanent plastic deformation destroys every machine within minutes.
Today it is said that the deformations of the surfaces are widely elastic.The back transformation brings the material back to the initial state. But the fast return to the initial state turns on grid vibration inside the solid body which produces temperature.
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