Akihito Suzuki

Influence of lubricant additive and surface texture on the sliding friction characteristics of steel under varying speeds ranging from ultralow to moderate

A newly developed tribometer that undergoes significant changes in sliding speed, ranging from ultralow (5 μm/s) to moderate (17 cm/s), was used to study the lubricated friction characteristic of steel. In this study, the friction characteristics of stearic acid-formulated oil were studied to clarify the effects of surface roughness or surface roughness texture on friction. Several kinds of specimens having isotropic and anisotropic surface roughness with different textures were used. For an isotropic surface, a rougher surface resulted in low friction under low-speed conditions. The same surface produced high friction under high-speed conditions, where macroscopic hydrodynamic action was predominant. Remarkably less friction was observed in the transverse than in the longitudinal direction when the specimen had anisotropic roughness. This difference was particularly notable under ultra-low-speed conditions. Two other parameters of skewness and kurtosis of roughness distributions show that low friction was obtained when surface roughness distribution approached normal. It appears that the low friction observed with a rough surface or a transverse roughness direction could be explained by the microscopic hydrodynamic action of fluid together with the lubricity of the adsorbed molecular layer.

Comparison of Sliding Speed Dependency of Friction between Steel Surfaces Lubricated with Several ZnDTPs with Different Hydrocarbon Moieties

Abstract This article describes the friction characteristics of zinc dialkyldithiophosphates (ZnDTPs) with different hydrocarbon moieties. A newly developed tribometer was used for the precise measurement of friction by alteration of the sliding speed from ultra-low to moderate. Four types of ZnDTPs (sec-C3C6, sec-C6, prim-C8, and prim-C12) were used as additives to measure friction between steel surfaces. Several phosphorus compounds were used as references. Auger electron spectroscopy analysis and micro-FT-IR reflection adsorption spec-troscopy analysis were employed to elucidate the chemical composition and chemical structure of tribochemically reacted films on steel surfaces. The results demonstrated that a remarkable difference between friction characteristics was observed among four ZnDTPs with different hydrocarbon moieties and that friction decreased with increasing alkyl chain length of ZnDTPs at lower speeds. These results accounted for the observation that the friction characteristics of ZnDTPs were influenced by their respective hydrocarbon moieties.

High-pressure viscosity prediction of di(2-ethylhexyl) phthalate and tricresyl phosphate binary mixtures using dielectric relaxation data

Abstract Dielectric permittivity and loss in di(2-ethylhexyl) phthalate and tricresyl phosphate binary mixtures were measured over a frequency range from 100 Hz to 1.5 MHz under high pressure. The mixtures showed single dielectric relaxation. The composite plots of the dielectric loss data showed one master curve and time–temperature–pressure superposition were applicable. The prediction of high-pressure viscosity was carried out from the change of dielectric relaxation time with temperature and pressure. The predicted results showed relatively good agreement with viscosity data obtained from a falling-sphere viscometer.

Pressure-dependence of dielectric relaxation time in poly(propylene glycol) and its application to high-pressure viscosity estimation

Dielectric permittivity and loss of poly(propylene glycol) with different molecular weights (400–3000) and terminal groups (OH and CH3) have been measured in the frequency range of 100 Hz to 1.5 MHz. Measurements were conducted over the temperature range 202–293 K under atmospheric pressure and 283–320 K under pressure up to 600 MPa. Two relaxation processes, one with strong absorption in the high-frequency region (α-relaxation) and the other a weak process in the low-frequency region (α′-relaxation), were observed for the OH-terminated samples having molecular weights above 2000 and for all the CH3-terminated samples. Most of the experimental data under high pressure showed a nonlinear decrease in the logarithm of the frequency of maximum dielectric loss with increasing pressure. The pressure-dependence of the dielectric relaxation time of the α-process was analyzed by several models based on the free-volume concept. The regression results of dielectric relaxation time as a function of pressure were applied to the estimation of high-pressure viscosity. The predicted viscosity showed relatively good agreement with viscosity data obtained from a falling-sphere viscometer.

Influence of Chemical and Physical Contaminants on the Antiwear Performance of Model Automotive Engine Oil

Abstract The antiwear performance of simulated used-engine-oil that contained a chemical contaminant (degraded zinc dialkyldithiophosphate (ZnDTP)) was studied with and without physical contamination (carbon black) using a four-ball tribometer. By reacting with cumene hydroperoxide, sec-C6-ZnDTP was degraded and produced many compounds containing both phosphorous and sulphur. The simulated used-oils were found to promote wear. This wear was considered to be due to corrosive wear by the excess reaction of surfaces with the sulphur contained in the degraded compounds. Carbon black was used to model carbon soot, which is another key substance of degraded engine oils, especially in diesel engines, to study the synergism between chemical contamination (ZnDTP degradation) and physical contamination (carbon soot contamination). Carbon black increased wear irrespective of the level of ZnDTP degradation, and the acceleration was much greater in the degraded oils. The wear acceleration by carbon black was observed even when the antiwear film from ZnDTP was already present on the surface. It was suggested that the wear acceleration by carbon black was due to abrasion.

Experimental study on the effects of sliding velocity and roughness orientation of metal surfaces on the function of lubricant additives in controlling friction in a boundary lubrication regime

We developed a new type of cylinder-on-disk (nominal line contact) tribometer that uses a unique friction data acquisition system to precisely measure friction under boundary lubrication conditions ranging from ultralow (5μm/s) to moderate (17cm/s) sliding velocity. As the tribometer was designed to measure friction at specific positions on a disk surface, fine distinctions in friction characteristics can be evaluated in relation to the surface condition. Two disk specimens were prepared, one with a polished surface and the other with a ground surface. The polished surface had isotropic low roughness (Ra≈0.01 μm) while the ground surface had anisotropic high roughness (Ra≈0.2μm). When the cylindrical specimen slid on the ground surface with anisotropic roughness, the sliding direction changed between transverse and longitudinal as the roughness orientation changed during each disk revolution. Several different base oils and additives were used to examine their performance. When fatty acid was used as an additive, low friction was measured with all the specimens, but remarkably less friction was measured in the transverse direction than in the longitudinal direction or with the isotropic low roughness specimens at the small Λ value region. In addition, friction measured in the transverse direction gradually decreased as sliding velocity decreased throughout the entire range. On the other hand, in the case of longitudinal or isotropic surface roughness, friction increased remarkably as sliding velocity decreased at the very low velocity region. It was also demonstrated that the surface roughness orientation affects friction even at ultra-low sliding velocities, where the hydrodynamic action of the lubricating oils is considered ineffective.

Effect of Ashless Dispersant on Deterioration of Antiwear Characteristics of ZnDTP due to Decomposition during the Oxidation Inhibition Process

A study of the antiwear characteristics of intentionally deteriorated zinc dialkyldithiophosphate (ZnDTP) was carried out. ZnDTPs were degraded to simulate used engine oil by reacting with cumenehydroperoxide (CHP), which models the hydroperoxide present in combustion gas. Sample oils were analyzed by 31 P-NMR, showing that ZnDTP produces various kinds of phosphorus-containing oil-soluble compounds after reacting with CHP. It was found that the antiwear characteristic of ZnDTP deteriorated due to the degradation, and an acceleration of wear was observed in the highly degraded regions. The effect of the ashless dispersant on the antiwear characteristics of both the degraded and the nondegraded ZnDTP was studied. It was found that the dispersant reduced wear in the low-degradation region. The addition of the dispersant, however, had no effect on wear with further degradation, where the dispersant was almost entirely consumed in dispersing Zn-containing solids when the dispersant concentration was low. When the concentration was high, the dispersant showed wear reduction in the region of greater degradation. The antiwear action of the dispersant was discussed herein, based on its adsorption characteristics and micelle formation.

Fundamental study of changes in friction and wear characteristics due to ZnDTP deterioration in simulating engine oil degradation during use

Zinc dialkyldithiophosphate (ZnDTP) is a well-known multi-functional additive for lubricating oils that has superior oxidation inhibition and antiwear performance. All the engine oils for four-stroke automotive engines contain ZnDTP because no additive has been found to have better performance. However, since it was pointed out that ZnDTP poisons the exhaust catalyst, reducing ZnDTP concentrations in engine oil without losing the superior antiwear performance has become an important priority. The friction and wear characteristics of intentionally deteriorated ZnDTP, which can simulate used engine-oil, were studied using a four-ball tribometer. Both see-C6 and prim-C8 ZnDTP were degraded by reacting with cumenehydroperoxide (CHP), which modeled the hydroperoxide present in combustion gas. Several simulated used oils with different degrees of degradation were prepared by changing the molar ratio of [CHP]/[ZnDTP]. Hydro-refined mineral oil was used as a base oil without other additives. Changing the degree of degradation simulated the gradual change in deterioration during actual use. Fresh ZnDTP-containing oil showed lower wear and higher friction than the base oil. However, the anti-wear performance gradually decreased with increasig ZnDTP degradation, and an appreciable wear increase was observed after dissipation of ZnDTP. Beyond this point, further degraded oils showed larger wear and lower friction than those with the additive-free base oil. In other words, simulated used-oils were found to accelerate wear but to decrease friction. ZnDTP production various kinds of accelerated what appeared to be corrosive wear that might have been due to the excess reaction of surfaces with sulfur in the degraded compounds.

Effect of Surface Roughening of Substrate Steel on the Improvement of Delamination Strength and Tribological Behavior of Hydrogenated Amorphous Carbon Coating Under Lubricated Conditions

The adhesion strength of diamond-like carbon (DLC) coatings is an obstacle in efforts to improve the reliability of coated products. It is generally believed that the roughening of the substrate surface improves the adhesion between a substrate and coating. The effect of surface roughening of the substrate on the delamination strength of DLC coating and the tribological behavior under lubrication were studied. Five types of roughened substrates were prepared by a wet blast device with differing materials, shapes, and sizes of the shot particles. A hydrogenated DLC film was deposited using plasma-enhanced chemical vapor deposition on the roughened substrates. The tribological properties were investigated under air and lubrication with pure water or n-decane. It was found that the delamination strength of the DLC coating could be improved by roughening the substrate surface, especially by spherical particles. It was also found that slight polishing of either the DLC surface deposited on the rough substrate or the roughened substrate before deposition significantly reduced the wear of the counter surface. The remaining chemical element of alumina particles on the roughened surface affected the delamination strength of the DLC coating.

Fundamental Study of Changes in Friction and Wear Characteristics due to ZnDTP Deterioration in Simulating Engine Oil Degradation during Use (Part 2) -- Influences of the Presence of Peroxide and Dispersed Zn-containing Solids--

Zinc dialkyldithiophosphate (ZnDTP) is a well-known multi-functional additive for engine oils that has superior oxidation inhibition and antiwear performance. However, since it was pointed out that ZnDTP poisons the exhaust catalyst, it has become important to reduce ZnDTP concentrations in engine oil without losing the superior antiwear performance of ZnDTP. A previous study presented at the 30th Leeds-Lyon Symposium on tribology at Lyon examined the friction and wear characteristics of intentionally deteriorated ZnDTP, which can simulate used engine-oil, and this paper is the subsequent written report of that study. Sec-C6 ZnDTP was degraded by cumenehydroperoxide (CHP), which modeled the hydroperoxide present in combustion gas. Changing the degree of degradation simulated the gradual changes in deterioration observed during actual use. ZnDTP produced various types of phosphorous-containing oil-soluble compounds after reacting with CHP. At the same time, Zinc-containing solids were precipitated from the oil during the process of deterioration. In our previous study, the precipitates were removed from the oil, and only the liquid layers were used to study the influence of oil-soluble compounds on performance in terms of friction and wear. It was found that the simulated used-oils accelerated wear, and some of the degradation products from ZnDTP led to the acceleration of wear. In this paper, polyisobutenyl succinimide (PIBSIM) was used as a dispersant in order to study the influence of solid precipitates on wear when they are dispersed in degraded oils as invisible fine particles. It was found that PIBSIM could adsorb on the friction surface, and thereby decreased the wear rate. Zn-containing solids played no role in the control of wear if they were dispersed as invisible fine particles in the oil. PIBSIM was shown to exert no effect on the reduction of the wear rate of the degraded oils when it was used for dispersing the invisible fine particles in the oil. The effect of the trace amounts of CHP, which could be remained in the samples, on antiwear performance of degraded samples was also investigated, and the direct acceleration of wear due to CHP was found not to be the major path showing an acceleration of wear by highly degraded ZnDTP. Disulfide (DS) and some degraded samples showed an initial high-wear region followed by a low-wear region. The initial high level of wear was considered to be corrosive wear due to the excessive reaction with sulfur. ZnDTP produced a surface protective film as soon as the experiment began, whereas in the case of slightly degraded samples and the DS sample, formation of the P-containing films was delayed.