Fundamental wear mechanism of metals

Friction and wear are strongly-coupled dynamic processes. The nominal tribological contact comprises a considerable number of asperities which are the location of highest contact pressure. At each asperity the introduced energy is frequently high enough to transfer the material into a fluid-like state. Depth profiles have shown that elements like carbon are brought in up to a depth of approximately 1 μm. This process is not diffusion-controlled but involves mechanical intermixing of the entire asperity. Since the mechanical energy input—or in terms of friction the energy dissipation—occurs at the asperities and since this process is stochastic with respect to time and location the surfaces adopt a wave-like structure. The wear mechanism resembles a squeezing process that removes the flowed material. As a result the surface wears with retention of its topography features in which the number of asperities and their height almost remain constant but the location of hills and valleys continually changes. Experiments showed that amplitude and frequency of the microwaviness correlate with the depth of the mixed zone. This zone changes its metallurgical properties, thereby changing its frictional behavior in the course of the experiment.