Finely dispersed copper nanoparticles were added as an additive to fully-formulated engine oils. The copper additive was in colloidal form, with an inner core of Cu 2+ atoms covered by surfactants to form stable reverse micelles that are completely dispersible in the base oil. The tribological process to form protective films at the metal surface is comprised of three phases. Phase I can be considered a physical process involving the build-up of polar molecules by absorption to produce a friction modifier film, whereas phases II and III have to be treated as mechanochemical processes comprising a combination of redox reactions and a third body formation. The tribological performance was investigated using atomic force microscopy, a microtribometer, a pin-on-disk tribometer in combination with continuous and high-resolution wear measurements with radionuclide technique, and high pressure stressing in a thrust roller bearing test rig. In addition, the nanostructure of the additive was characterized by atomic force microscopy. Finally, the chemical composition of the metal surface was analyzed using photoelectron spectroscopy.
White etching cracks are one of the possible causes of bearing damage, which can lead to the failure of wind turbines. The mechanisms of hydrogen diffusion in bearings are not yet fully understood, but it is assumed under tribological loading that the lubricant degradation releases hydrogen which diffuses into the contacting surfaces (i.e. steel). This leads to hydrogen-induced cracking and damage to the bearings. In order to better understand these mechanisms, it is necessary to investigate the influence of tribological loads and lubricant composition on hydrogen release. For this purpose, a test setup was developed for the analysis of tribologically induced hydrogen permeation by means of a ball-on-plate test. The diffused hydrogen was measured with an electrochemical three-electrode setup. The results showed a direct correlation between the amount of hydrogen released and the applied normal load. Furthermore, an increased velocity resulted in enhanced diffusion of hydrogen through metal. In addition, the tribological contact surface was decisive for hydrogen permeation. The higher velocity and the larger contact area result in an activated nascent iron surface which promotes hydrogen penetration. The results show that lubricants can be classified as critical for the formation of hydrogen embrittlement in tribological contacts.
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