Ionic liquids are expected to be used as novel lubricants because of their unique characteristics such as low vapor pressure, low volatility, and high thermal stabilities. Their physical properties compares favorably with conventional lubricants. However, it is well known that the halogen-containing ionic liquids cause corrosion of worn surface, although exhibit high lubricating performance. In order to preclude the corrosion, the application of halogen-free ionic liquids is proposed. The halogen-free ionic liquids such as phosphorus, sulfur, boron and cyano-based ionic liquids are expected as a substitute for the halogen ionic liquids. The phosphorus and sulfur-based ionic liquids show the inferior lubricity to that of halogen-containing ionic liquids. The boron-based ionic liquids exhibit ultralow friction, and these lubricating performance surpass the halogen-containing ionic liquids. The lubricity of cyano-based ionic liquids is improved by changing the cations and anions. In addition, the ultralow friction phenomena is confirmed in the combination of a cyano-based ionic liquid and diamond like carbon (DLC) film as sliding surface. In applying ionic liquids as lubricants, combination with friction materials is also an important subject to be studied.
Two types of oil-soluble polymer as new tribomaterials are introduced based on recent research by the authors. One is polymethylmethacrylate (PMMA) including high concentration methyl group. Film forming properties of polyalkylmethacrylates determined with an optical EHL tester significantly depended upon alkyl group, functional group and molecular weight. Among them, PMMA with high molecular weight showed the greatest oil film thickness under the low rolling conditions due to strong adsorption of the polymer onto the metal surface. As a practical performance of additive, the effect of polymer on the rolling fatigue life of bearings was evaluated with a thrust bearing tester. As a result, the oil with PMMA showed much longer life than the oil without PMMA. Another novel polymer was polylaurylacrylate with hydroxyethyl group (PLA/HEA) recently developed. When the boundary friction-speed behavior was examined, the coefficients of friction for most oils increased with decreasing sliding speed, meanwhile that for PLA/HEA kept low level. It was inferred from the surface analysis that excellent tribological performances of PLA/HEA were brought about by a tribofilm formed based on reaction of hydroxyethyl group with the sliding surface. Then, PLA/HEA was applied to the drawing process of stainless steel AISI304 sheets, in which continuous possible working time was evaluated as the drawability. As a result, PLA/HEA was superior to the other commercially available lubricants except for chlorine-based oil.
This article describes a novel molecular design of synthetic base oils, aimed at developing a new class of base oils with a viscosity index (VI) greater than 200, and the viscosity properties of oligosiloxane-based oils with a VI of 170‒226. The proposed molecular design is derived from the molecular design of liquid crystalline molecules, in which oil molecules consist of a flexible unit and flexible chains, and contain two phenyl rings or less. Based on the molecular design, diphenylbis(alkyldimethylsiloxy)silanes and phenyltris(alkyldimethylsiloxy)silanes have been synthesized, and their structure‒property relationships investigated. The results show that the oligosiloxane oils are a new family of high VI base oils, and that the length of the alkyl chains and the number of the siloxy groups are structural factors that affect the viscosity properties. The kinematic viscosity increases significantly with increasing alkyl chain length, and the VI value increases greatly with increasing siloxy substitution.
Recently the environmentally friendly transformers with natural esters, mainly extracted from soybean, sunflower, rapeseed, have been in operations in wide regions globally. However, the higher viscosity and higher pour point of natural esters restrict their cooling performance and application. Moreover, their oxidation stability generally shows less than mineral oil that is normally used as electrical insulating oil, and therefore lifetime of transformers is concerned. In order to improve these properties, low viscosity esters showing better oxidation stability derived from vegetable oil have been developed as electrical insulating oils. In this review, trends of biodegradable low viscosity esters as electrical insulating oils and their properties are reported.
We introduce technical trend of ester lubricating oils for fluid dynamic bearings, used for HDD (hard disk drive) spindle motors. The particularly important characteristics required for lubricating oil are low viscosity, heat-resistance, high viscosity index and low outgassing. We found that esters derived from fatty acids and 3-Methyl-1, 5-pentanediol show good performance. In this report, we introduce the performance of various 3-Methyl-1, 5-pentanediyl esters.
The friction properties of polymer hydrogels are very complex. Hence, the friction properties of hydrogels have been studied to be applied to various mechanical parts. As a part of such studies, we have measured the dynamic frictional coefficients of high-strength hydrogels such as double network (DN) gel and shape memory gel (SMG). We introduce such experimental results and the project to apply the hydrogels to “Soft-Matter Robotics”.
A thermo-hydrodynamic lubrication (THL) theory is applied to journal bearings that support large-sized high-speed rotary machineries and it is confirmed that the maximum pad surface temperature under operation is below an allowable limit value. The temperature predicted by the traditional THL model is in good agreement with the measurement. However, the predicted journal surface temperature is not comparable with the measurement. In the present paper, it is firstly shown that the amount of heat, which is generated by the shear flow in the oil film and is discharged out of the film, is evaluated appropriately in the film near the pad surface while it is evaluated excessively in the film near the journal surface. Secondly, a new THL model is proposed that the oil film temperature near the pad surface and the temperature near the journal surface are obtained independently, that is, the conventional energy equation for the oil film is solved for the former temperature while the equation described in the relative coordinate system with the journal surface as the reference one is used for the latter temperature. Lastly, it is shown that the THL model can predict the journal surface temperature comparable with the measurement without deteriorating the accuracy of the predicted maximum pad surface temperature.