Wear on a nanoscopical level

Contribution from M. Scherge, IAVF-Antriebstechnik AG

Metals in tribological interaction frequently experience a normal force that is large enough, in order to transfer the first couple of hundred nanometers of both solids into a quasi-viscous or granular state. Each real tribological contact consists of a considerable number of micro- and nanocontacts, which carry the load. These contacts are subject to a recurring temporary granulation, which is accompanied by mechanical intermixing and the introduction of lubricant molecules. Auger electron spectroscopy depth profiles showed that elements such as carbon are introduced into a depth up to 1 µm. Since the mechanical energy input takes place at the microcontacts and since this process is both locally and temporally coincidental, the surfaces of the metals appear in a wavelike structure. One can clarify oneself this process, by imagining falling water drops at the beach. Each impact represents an energy input, which entails an imprint in the sand. The total of all imprints leads to the wavelike appearance. The generation of wear occurs at the wave mountains by squeezing out the granular material and is thus subject to the same randomness like the modification of the topography. In the result the surface wears while maintaining its topography features, whereby the number of the mountains and their height remains almost constant, however, the positions of mountains and valleys constantly change. Experimentally it could be shown that amplitude and frequency of the ripples correlates with the depth of the intermixed zone. Due to the repeating energy inputs the material of the topmost volume changes from the crystalline to an almost amorphous state, as shown by x-ray diffraction. This is especially pronounced when the system experiences a strong running-in procedures. After the running-in the system shows a changed tribo-chemical state that is characterized by lower wear, less heat and lower friction. By means of the radionuclide technique wear of metals was measured continuously with a resolution in the nanometer range. The investigated pairings showed wear velocities in the range of some ten nanometers per hours. The figure shows two wear curves of identical materials. Both tribosystems experienced a running-in, however with different loads. While the system that was subjected to a high load shows increased initial wear the other system showed moderate wear after a low running-in. The insets depict the surface of the metal after high and low running-in. By taking the first derivative of the wear curve one obtains a wear velocity. It can be shown that the system with the high load exhibits a significantly lower wear velocity than the system that experienced a low load running-in.