Lubricants2016, 4(3), 24; doi:10.3390/lubricants4030024 (registering DOI) - published 29 June 2016 Show/Hide Abstract
Abstract: The sliding behaviors of two simplified tribofilms with amorphous structure consisting either of SiO2 molecules or C atoms were simulated by molecular dynamics modeling. The objective was to identify mechanisms explaining the experimentally observed lubricating properties of the two amorphous films. The impacts of layer thickness, normal pressure, temperature and different substrate materials were studied systematically, while the sliding velocity was kept constant at 30 m/s. While the layer thickness was not critical, all the other parameters showed special effects under certain conditions. Normal pressure impeded void formation and could even eliminate voids if applied at high temperature. Stick-slip sliding was changed to smooth sliding at high temperature due to void healing. Considering the carbon film, high friction forces and shearing of the entire film was observed with diamond substrates, whereas interface sliding at low friction forces and an amorphous layer of iron mixed with carbon was observed if the supporting substrates consisted of α-Fe. Both films show a decrease of friction forces and smooth sliding behavior at elevated temperature, corresponding well to the tribological behavior of an advanced nanocomposite sliding against a steel disc under severe stressing conditions when high flash temperatures can be expected.
Abstract: The present concept of being globally “green” puts additional demands on lubricants. They are to be biodegradable and ecofriendly. Therefore, in a search for alternate lubricants meeting the above demands, we have synthesized biodegradable new generation esters using alcohols such as 2,2-dimethyl-1,3-propane diol and 2,2-diethyl-1,3-propane diol and fatty acids like caproic and 2-ethyl caproic in presence of indigenous ion exchange resin catalyst. The synthesized esters were analyzed and characterized for their physico chemical properties. In addition, with a view to finding out the possibility of using these esters as aluminum cold rolling oils, their lubricity characteristics, biodegradability and toxicity were also investigated. The products were found to have good potential for use in biodegradable aluminum cold rolling oils meeting IS: 14385-2002 specification.
Abstract: The development of improved anti-wear additives would enable the use of lower viscosity oils that would lead to improved efficiency. Ionic liquids have the potential to be this type of new anti-wear additive. However, currently the best performing ionic liquids that are miscible in non-polar base oils, the phosphonium phosphates, contain phosphorus on both the cation and anion. Manufacturers are seeking to reduce the presence of phosphorus in oils. Here, as a first step towards phosphorus-free anti-wear additives, we have investigated ionic liquids similar to the phosphonium phosphates but having either a phosphorus-free cation or anion. Two quaternary ammonium phosphates (N6,6,6,14)(BEHP) and (N8,8,8,8)(BEHP) and a phosphonium silyl-sulfonate (P6,6,6,14)(SSi) were compared to a phosphonium phosphate (P6,6,6,14)(BEHP) and a traditional zinc dithiophosphate (ZDDP) as anti-wear additives in mineral oil. The change from a phosphonium to a quaternary ammonium cation drastically reduced the miscibility of the Ionic liquid (IL) in the oil, while the change to a smaller silicon containing anion also resulted in limited miscibility. For the pin-on-disk wear test conditions used here none of the ionic liquids outperformed the ZDDP except the (P6,6,6,14)(BEHP) at a relatively high loading of 0.10 mol·kg−1 (approximately 8 wt%). At a more moderate loading of 0.025 mol·kg−1 the (P6,6,6,14)(SSi) was the best performing ionic liquid by a significant amount, reducing the wear to 44% of the neat mineral oil, while the ZDDP reduced the wear to 25% of the mineral oil value. Electron microscopy and energy dispersive X-ray spectroscopy showed that the presence of a silicon containing tribofilm was responsible for this protective behaviour, suggesting that silicon containing ionic liquids should be further investigated as anti-wear additives for oils.
Abstract: Considering total hip arthroplasty, so-called tribolayers (aka tribomaterial), consist of carbonaceous material from the periprosthetic joint fluid or bovine serum mixed with nanometer size metal and oxide wear particles. Currently, its growth sequence and rate are unknown. Thus, smooth surfaces of low-Carbon (LC-) vs. high-Carbon (HC-)CoCrMo (Cobalt-Chromium-Molybdenum) alloys have been worn in a conforming contact under bovine serum lubrication by means of a pin-on-ball wear tester. These tests were interrupted at certain numbers of cycles in order to weigh the specimens, characterize the topography, and investigate the wear appearances. In addition, after cleaning in ethanol and anionic detergent, before-and-after comparison rendered the weight of the tribomaterial. This revealed that, during run-in, the specimens gained weight by generating tribomaterial. Afterwards the loss of material surpassed the generation of new tribomaterial and a steady weight-loss was measured. Topography measurements were used as input data for contact mechanics calculations. Apparently the incipient, locally high contact stresses accelerated tribochemical reactions. After run-in, the contact situation changes and leads to a much smaller generation rate. This paper provides information about the growth sequence and rate of such tribomaterial formation. It further highlights the significance of highly localized contact stress as an important factor for tribomaterial generation.
Abstract: Self-lubricating polymer compounds are currently used for a wide range of applications such as bearings, gears, and water meters. Under severe conditions such as high pressure, high velocity, and/or high temperatures, the material fails (PV limit). In this study, we investigated the effect of graphite on the tribological properties of polystyrene (PS) with “ball-on-three-plates” tests. Graphite-filled PS plates were produced via an internal mixer and compression molding. Unhardened steel (1.4401) and nylon (PA66) balls were used for the tribological tests. Our results indicate that graphite loading, graphite type, and particle size have a big influence on the friction coefficient, the wear resistance, and the PV limit of PS both against steel and PA66. In particular, primary synthetic graphite performs better than secondary synthetic graphite due to the higher degree of crystallinity.
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 Fe3O4. 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.