Saiko Aoki

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.

Corrosion Behavior of Three Nanoclay Dispersion Methods of Epoxy/Organoclay Nanocomposites

The aims of this study, firstly, to obtain high degree of clay exfoliation in the epoxy matrix by three dispersion methods such as normal mixing, shear mixing, and high-speed mixing and, secondly, to investigate corrosion behavior of epoxy/organoclay nanocomposite, immersion test, weight change, and penetration behavior were conducted. From the three mixing methods, the high-speed mixing method showed larger clay interlayer distance, smaller clay aggregate, and more homogeneity and expectedly resulted in high anticorrosive properties. Penetration depths of these nanocomposites showed a small difference; however, the most noticeable improvements in anticorrosion performance for epoxy/organoclay nanocomposites under high-speed mixing method were found to reduce penetration and weight uptake which are described via the model of nanoparticulate-filled structure and discussed in corrosion protection mechanism against environmental liquid penetration.

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.

Modelling anisotropic water transport in polymer composite reinforced with aligned triangular bars

This work reports anisotropic water transport in a polymer composite consisting of an epoxy matrix reinforced with aligned triangular bars made of vinyl ester. By gravimetric experiments, water diffusion in resin and polymer composites were characterized. Parameters for Fickian diffusion and polymer relaxation models were determined by least-square curve fitting to the experimental data. Diffusion parameters of epoxy and vinyl ester resin were used as input during development of finite element (FE) model of polymer composite. Through transient FE diffusion analysis, anisotropic water transport in thickness direction of the polymer composite was numerically predicted and validated against experimental results. The case of using impermeable triangular bars was also numerically simulated. The diffusivity of reinforced aligned triangular bars was confirmed to affect anisotropic water transport in the composite. The results of this work suggest possible use of polymer composite for barrier and fluid removal applications.

Effect of Coexistent Additives on the Friction Characteristics and Tribofilm formation of Zinc Dialkyldithiophosphate

The major aim of this study is to investigate the tribofilm formation and friction-speed characteristics of ZnDTP in the presence of other lubricant additives. Simultaneous measurement of friction and electrical conductivity were employed using ZnDTP and several kinds of functionally different additives. Several analyses of friction surfaces were also carried out in order to measure the reaction film thickness and investigate the chemical composition of this film. It was demonstrated that the presence of each additive with ZnDTP prevented the formation of a ZnDTP tribofilm and thereby could provide lower friction than ZnDTP alone.

Elucidation of the Action of Functional Groups in the Coexisting Ashless Compounds on the Tribofilm Formation and Friction Characteristic of Zinc Dialkyldithiophosphate-Formulated Lubricating Oils

ABSTRACT Zinc dialkyldithiophosphate (ZnDTP) is well known to show supreme antiwear performance by forming a thick polyphosphate-based tribofilm on the friction surface. However, it has also been demonstrated that a coexisting dispersant can interfere with the performance of ZnDTP. Because many lubricating oils contain a variety of additives in addition to ZnDTP, the effect of functional groups in the coexisting polar compounds on the tribofilm formation of ZnDTP should be clarified. In this study, several kinds of functionalized polyalkylmethacrylates (PMAs), a dispersant, and friction modifiers were used with ZnDTP, and their effects on the tribofilm formation and the friction characteristic of ZnDTP were discussed. The formulation of polar compounds with different types of functional groups significantly changed the state of ZnDTP compounds in the bulk solutions. Amino groups tended to generate coordination complex compounds and reduced the neutral ZnDTP content in the sample solutions. PMA-OH and glycerol monooleate (GMO) both had hydroxyl groups, and nonfunctionalized PMA 0 did not significantly influence the neutral ZnDTP content. PMA-COOH and stearic acid (StA) containing carboxyl group(s) significantly increased the neutral ZnDTP content. The composition of neutral ZnDTP and basic ZnDTP in the lubricating oil is a key to understanding the influence of coexisting ashless polar compounds on the formation of the ZnDTP-based tribofilms and the friction characteristics of ZnDTP.