Zinc dithiophosphates (ZDDP) have been widely applied in automobile industry for over 70 years as a lubricant additive for wear protection. Tribolayers have been described as blue-and brown-colored layers on surfaces observed by microscopical observation or even bare eye presumably as a consequence of layer thickness or chemical composition. However, the reaction pathways of ZDDP tribolayers are still not yet fully understood. In the present study, the difference between the blue-and brown-colored tribolayers has been revealed by high resolution methods in cylindrical roller thrust bearings at relatively high contact pressures of around 1.92 GPa. After running a FE8 standard bearing test with a normal load of 80 kN and a temperature of 60˝C, said tribolayers could be identified on the bearing surfaces. By using Raman spectroscopy, it could be shown that the blue-colored layers are enriched by FeS and ZnS whereas the brown-colored layers show a significant amount of Fe 3 O 4 . This is an interesting finding as it clearly shows a correlation between the color appearance of the films and the chemical composition besides potential film thickness variations. Finally, transmission electron microscopy verified the amorphous nature of the formed tribolayer which is in a good agreement with literature.

Abstract:Laser surface texturing has been successfully applied in the past to various machine elements in order to improve their tribological performance. In this study, direct laser interference patterning was used to produce periodic cross-like surface patterns on commercial cylinder roller bearings. The wear behavior of as-patterned bearings was studied by a modified FE8 test rig under boundary lubrication. A mineral oil (ISO VG 100) as a base oil additivated with 0.02 wt % zinc dialkyldithiophosphate (ZDDP) was used for the tribological tests which were performed under a Hertzian pressure of 1.92 GPa and maintained at 80 • C and 20 rpm for 2 h. The laser-patterned bearings showed a significantly reduced mass loss by two orders of magnitude compared to the unpatterned reference bearings. A closer look at the samples proved the formation of the characteristic blue-colored ZDDP tribofilm on top of the laser-induced topography maximum positions. Due to the higher contact pressure at the laser-induced peaks, the tribofilm formation was preferable at those positions thus protecting against wear. The laser patterns nearly remained unworn compared to the reference samples. A subsequent Raman analysis of the laser-patterned bearings clearly revealed the formation of zinc and iron sulfides as well as phosphates at the peak points.

Within this study, the bearing washer surfaces of cylindrical roller thrust bearings (type 81212, SAE 52100) are laser-patterned with a dot-like pattern (periodicity of 6.5 mm and depth of 1 mm). The wear behavior of the patterned bearings is compared to that of bearings with a ground surface finish. The tribological tests are conducted on an adapted FE-8-test rig using a mineral oil with a low concentration of zinc dialkyldithiophosphate (0.025 wt% P). This study demonstrates an 83% reduction in the wear loss of the laser-patterned bearing. The patterns are able to store wear particles in the topographic minima, thus reducing abrasive wear and therefore the wear loss. Additionally, the formation of a wear inhibiting tribo layer is enhanced as the surface pattern acts as secondary source for the deployed additive.

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The service life of rolling bearings is significantly affected by the film formation in elastohydrodynamic (EHD) contacts, which depends on the operating conditions, like rotational speed or temperature. In grease lubricated EHD contacts, the film formation is determined by the grease consistency and composition, i.e., thickener and base oil type as well as properties of the bleed oil, which is released from the grease during operation. Thus, the film formation of grease lubricated contacts as compared to base oil lubricated contacts can be different. With increasing rolling speed, the film thickness of oil lubricated contacts usually grows. However, in case of grease lubricated contacts, which are not fully flooded, the film thickness remains constant or even decreases with further increasing rotational speed. This effect is referred to as starvation. Since the onset of starvation depends on the grease composition, the film formation of two different grease compositions is investigated in this study. The film thickness measurements are performed on a ball-on-disc tribometer for each grease, as well as the corresponding bleed and pure base oils. Thereby, the characteristic rotational speed leading to the onset of starvation has been identified in dependence of the grease composition and the differences in the lubricating film formation of base oil, bleed oil, and grease lubricated EHD contacts have been discussed. The investigations should help to establish an advanced understanding of the physical mechanisms leading to the onset of starvation to encourage future work with focus on a method to predict the film formation in grease lubricated EHD contacts.