In recent years, the varnish caused by turbine oil oxidation products has become a serious problem in thermal power generation plants. In our laboratory, we have developed a varnish diagnosis method that uses membrane patch colorimetry for evaluating varnish potential. Membrane patch color is influenced by filtering conditions such as mixing solvent, oil temperature, and incubation period. It is important to understand the influence of these conditions on the patch color and oxidation products to properly evaluate oxidation products using membrane patch colorimetry. Hence, this study aims to investigate these conditions by using two experiments. First, we conducted a filtration test on sample oils that were filtered under different conditions, in which we altered the mixing solvent, oil temperature, and incubation period to investigate their influence on membrane patch color. Then, we investigated the behavior of oxidation products versus heating time, incubation period, and oil temperature using an in-situ analysis of a microscopic fourier transform infrared spectroscopy (FT-IR). In the filtration test, the membrane patch color became darker with increasing incubation period. Additionally, the results also indicated that the membrane patch color became brighter when the sample oil and solvent were mixed and/or the oil temperature was high. From the in-situ analysis using the microscopic FT-IR in the case of a heating process, the peak of 1710 cm-1, which is an absorption band of carboxylic acid (-COOH), shifted to a higher wave number. In the case of a holding process, the peak of 1710 cm-1 shifted to a lower wave number. Overall, the results suggest that the oxidation products became low-molecular owing to cleavage of hydrogen bond and easy solubility in turbine oil when the oil temperature was high, and that the oxidation products became high-molecular by hydrogen bonding and difficult to dissolve in turbine oil when the sample oil was cooled and stored. Moreover, from the behavior of the oxidation products at different temperatures in the in-situ analysis, the cleavage of hydrogen bond of oxidation products in turbine oil ended after reaching at least 60–70°C.
Oil whips might present a risk that is associated with bearing seizures. To conveniently suppress oil whips, a stabilization method using starved lubrication has been proposed. As the oil film becomes thinner, an increase in oil film temperature is expected. In this study, the bearing temperature was studied in the cases of oil whip, transition condition, and starved lubrication. Moreover, the temperature distributions in the transition and starved lubrication conditions were studied using a two-phase flow computational fluid dynamics (CFD) analysis. As a result, it was found that the heat amount due to shear friction was small in the case of the transition condition, whereas the temperature inside the bearing was approximately the same as that of the supply oil, from both experimental and analytical perspectives. In the case of the starved lubrication condition, it was found that the air flowing out of the oil supply groove created a circulated flow, which cooled the side end of the bearing, thereby controlling the temperature at the center of the bearing.
In this paper, the tribological behaviors of 1 μm of tetrahedral amorphous carbon (ta-C) coatings deposited with a different substrate bias of -0 V, -100 V and -300 V are examined under elevated temperature up to 600°C. It is found that wear behavior could be divided into two distinct regions according to testing temperature. The variation trend of the specific wear rate of ta-C coatings in region I at 23, 100 and 200°C are affected by abrasive particles which coincide with sp3 content of ta-C coatings. In region II at 300, 400 and 500°C, the transfer layer play a major role in reduction of wear rate as a function of substrate bias.
Plasma jet sterilization of cutting fluid was performed under atmospheric pressure, and the tribological properties of the resulting fluid were investigated. The mechanisms of atmospheric-pressure plasma jet sterilization were also clarified. The number of bacterial colonies in the sterilized fluid was reduced by more than 90% compared with the untreated fluid. After exposure to ultraviolet light, no comparable reduction was observed. We concluded that the sterilization observed in the study was caused by the reaction of excited N2, N, and Ar. The IR spectra of the plasma-treated cutting fluid were the same as those of the untreated fluid. However, the transmittance of the C=O peak decreased in proportion to the discharge voltage. The treated cutting fluids maintained a low friction coefficient 2.4 times longer than the untreated fluid. The sliding angle and adhesive energy of the untreated cutting fluid were considerably lower than those of the unused cutting fluid. Furthermore, the sliding angle and adhesive energy of the plasma-treated cutting fluid were higher than those of the untreated cutting fluid. Therefore, we consider that the lower friction coefficient of plasma-treated cutting fluid as compared to that of the untreated cutting fluid is related to this result. The study demonstrated that the life of a cutting fluid can be prolonged by this novel plasma treatment, with an associated improvement in tribological properties.
This literature study is based on the research work conducted in the field of erosion behavior of steel alloys. The focus is on the surface modification of the steel alloys by heat treatment, surface hardening and surface coating methods. Laser cladding and laser hardening processes are also explained. The influence of tribological parameters on the erosion is explained. Steel alloys are chosen because of a wide number of applications due to their excellent properties. Efforts have been made to understand the cause and the remedial action towards the improvements in wear performance. Suitable surface treatment method on the steel alloys may definitely do this work but there should be a need of proper control over the process parameters. The environmental conditions of the laboratory are differing from the real working environment. Hence, the results which are satisfactory at the laboratory are not doing well in real place of application.
Star PFPE (perfluoropolyether) polymers based on an aromatic benzene core and a non-aromatic cyclotriphosphazene (CTP) core are investigated as boundary lubricant films on rigid magnetic media. The effect of the benzene and CTP cores on head-disk spacing are measured by the changes in the acoustic emission signal as a function of head-disk spacing at similar lubricant film thicknesses. The earlier head-disk contact observed with the CTP core is attributed in part to its larger size perpendicular to the plane of the disk surface. The adhesion of the benzene and CTP cores to the underlying carbon film is investigated by ab initio quantum chemistry. The non-aromatic nature of the CTP ring and the lack of steric accessibility to the CTP core nitrogen atoms prevent good adhesive interactions with the underlying carbon film. Conversely, the π-electrons of aromatic benzene create a quadrupole moment in the direction perpendicular to the plane of the benzene that can interact with the underlying carbon surface and hence provide weak adhesion. Electron-withdrawing and -donating substituents on the benzene ring can be used to exert a “push-pull” effect on the π-electrons to alter the strength of the intermolecular interactions to specific functional groups of the underlying carbon surface. These effects are visualized by mapping the electrostatic potential of the benzene core as a function of electron-withdrawing and -donating substituents and computing model dimer optimized geometries and intermolecular interaction energies. Finally, the thermal stability of the star PFPE polymers are quantified by measuring the evaporation rate of thin films (11 Å) as a function of time at the HDD temperature (~ 60℃). Thermogravimetric analyses (TGA) of the neat polymers also provide direction for improvement of the synthetic lubricants.
The effect of temperature on the lubricity of molybdenum dithiocarbamate (MoDTC) was investigated by a ball on disk tribometer with lubricant oil containing MoDTC and calcium sulfonate (CaSU) at temperatures of 25˚C, 40˚C, 60˚C and 80˚C. The behavior of friction coefficient was closely dependent on the oil temperature. Friction coefficient decreased after a certain induction period and became to be constant at steady state. At higher temperature, the induction period became to be shorter, and the slope of decreasing friction coefficient became to be steeper. Friction coefficient at steady state decreased at higher temperature, and was changed reversibly when the oil temperature was changed. Oscillatory change in friction was observed when the oil temperature was changed periodically. Chemical analysis of the tribofilm with X-ray Photoelectron Spectroscopy (XPS) revealed that higher content of Mo(IV) as a component of molybdenum disulfide (MoS2) was observed in the tribofilm formed at higher temperature. Highly oriented MoS2 layer was observed with High Resolution Transmission Electron Microscopy (HR-TEM) in the tribofilm formed at 80˚C. It was found that the structure of tribofilm is altered reversibly with oil temperature. A steady state model of tribofilm formation was proposed on the basis of observed results.
In this paper, combined effects of piezo-viscous dependency and surface roughness on the squeeze film characteristics of non-Newtonian micropolar fluid in conical bearings are presented. On the basis of Christensen’s theory, two types of one dimensional structure, the longitudinal roughness and transverse patterns are considered. The stochastic modified Reynolds equation for these two types of roughness patterns is derived for micropolar fluid by taking into account variation of viscosity with pressure. Through a small perturbation technique, the analytical approximate solution for the mean fluid film pressure, load carrying capacity and squeeze film time is obtained. According to the results, the combined effects of non-Newtonian and viscosity pressure dependency provide an enhancement in the load carrying capacity and lengthen the response time for both types of roughness patterns as compared to the classical iso-viscous Newtonian lubricant case. On the whole, the squeeze film characteristics of conical bearings is improved especially for higher values of coupling parameter and viscosity parameter.
Investigations on tribological behaviors of Diamond-like Carbon (DLC) coating against other materials are strongly needed for scientific and industrial purposes. Recently, carbon diffusion has been found to play a key role in the wear of hydrogenated amorphous carbon (a-C:H) DLC coating when it is rubbed against steel counterpart material in boundary lubrication. Chromium has been reported to decrease carbon diffusion due to increase of the activation energy. In this research, we investigated on the effects of carbon diffusion on friction and wear properties of a-C:H coating when rubbed against Cr plating material. Results show that compared to steel counterpart, a-C:H coating exhibits lower friction coefficient, lower wear rate, and lower carbon diffusion rate when rubbed against Cr plating counterpart.