Microstructural and Chemical Characterization of the Tribolayer Formation in Highly Loaded Cylindrical Roller Thrust Bearings

Abstract

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.

Enhanced Growth of ZDDP-Based Tribofilms on Laser-Interference Patterned Cylinder Roller Bearings

“…The spectrum measured at P1 shows pronounced peaks related to Zn-S, Fe-S, and P-O at Raman shifts of 351, 386, and 965 cm −1 [28][29][30], whereas P2 reveals peaks related to Fe3O4 at 320 and 670 cm −1 [31]. The peaks assigned to P1 correlate well with the composition of ZDDP, which goes hand in hand with previously published results [10]. This in turn implies that the tribolayer was formed on top of the pattern induced by the increased contact pressure.…”

“…[9]. Furthermore, Gachot et al proved that the blue-colored tribofilm areas contain a high amount of phosphates and zinc sulfides [10]. Even though the mechanism of the ZDDP tribofilm formation and the detailed reaction paths are still not clear and well understood, there is a consensus that the presence of the tribofilm can help to protect the rubbing surface, thus improving the friction and wear performance [11].…”

Synergetic effects of surface texturing and solid lubricants to tailor friction and wear – A review

“…For non-conformal contacts, small contact areas and the possibility of inducing contact fatigue by texturing [58,59] as well as the need to maintain high viscosity inside the contact demand substantially narrower and shallower textures [60]. In boundary lubrication, the main effects of surfaces textures are to increase the lubricant supply [61] and to increase the contact pressure at the textures' edges which activates lubricant additives via stress-induced tribochemical reactions [62,63]. Considering that success or failure of surface texturing depends on contact and operation conditions, the use of surface texturing for different machine elements (seals, gears, roller bearings and piston-ring assemblies in automotive systems) has also been reviewed, providing guidelines for optimal texture design for each application and lubricated condition [19].…”

The Indirect Tribological Role of Carbon Nanotubes Stimulating Zinc Dithiophosphate Anti-Wear Film Formation

“…The first mechanism is the adsorption or, more specifically, chemisorption of additive molecules on lubricated surfaces resulting in anti-wear (AW) film formation; the second mechanism is a chemical reaction of the additive with the material surface forming an extreme pressure (EP) layer. These mechanisms have been studied for many decades and have been extensively described in a large number of publications regarding tribochemistry [ 18 , 19 ], particularly for carbon-based tribofilms [ 20 , 21 , 22 , 23 ]. Under certain conditions, carbon nanostructures show superlubricity effects that result in the near vanishment in friction between the two sliding surfaces [ 24 ].…”