Chiharu Tadokoro

Minimization of Friction at Various Speeds Using Autonomous Viscosity Control of Nematic Liquid Crystal

To demonstrate the concept of the smart lubrication system using liquid crystal (LC) lubricant proposed by Nakano (Tribol Lett 14:17–24, 2003), the following three types of tests were performed: (1) film thickness measurements in pure rolling contacts, (2) friction coefficient measurements in rolling–sliding contacts, and (3) molecular orientation measurements in stationary cells. In all of these types of tests, a nematic LC [4-pentyl-4′-cyanobiphenyl (5CB)] and a carboxylic acid [hexadecanoic acid (HDA)] were used as the base fluid and the additive, respectively. The results of these tests confirm the following mechanism. First, surface films of the HDA additive that spontaneously adsorb onto contact surfaces induce surface anchoring, which has the effect of making 5CB molecules align themselves perpendicular to the surfaces, competing with the flow alignment of 5CB molecules and inducing an increase in the apparent viscosity of 5CB with decreasing entrainment speed. This increase in the apparent viscosity generates a constant friction coefficient region in the Stribeck curve on the left side of the minimum friction coefficient point, resulting in the minimization of friction at various speeds.

Effects of Alkyl Chain Length of Sulfate and Phosphate Anion-Based Ionic Liquids on Tribochemical Reactions

Ionic liquids are expected to become increasingly popular lubricants as they feature a number of attractive properties. This investigation focused on sulfate and phosphate anion-based ionic liquids and the improvement in lubricating performance with the addition of these anions. However, the detailed lubricating mechanism and effect of alkyl chain length on tribochemical reactions are unclear. This study investigates tribochemical reaction processes using a quadrupole mass spectrometer (Q-MS) and X-ray photoelectron spectroscopy. Seven types of ionic liquids: 1-ethyl-3-methylimidazolium hydrogensulfate ([EMIM][HSO4]), 1-ethyl-3-methylimidazolium methylsulfate ([EMIM][MSU]), 1-ethyl-3-methylimidazolium ethylsulfate ([EMIM][ESU]), 1-ethyl-3-methylimidazolium n-octylsulfate ([EMIM][OSU]), 1-ethyl-3-methylimidazolium dimethyl phosphate ([EMIM][DMP]), 1-ethyl-3-methylimidazolium diethyl phosphate ([EMIM][DEP]), and 1-ethyl-3-methylimidazolium dibutyl phosphate ([EMIM][DBP]), were selected as lubricants. The friction coefficient of sulfate anion-based ionic liquids increased as their alkyl chain lengthened. However, wear scar diameter in this case showed the opposite tendency. The friction coefficient and wear scar diameter of phosphate anion-based ionic liquids increased with an increase in the alkyl chain length. Q-MS results indicated that the main outgassing components during sliding were the cation components, whereas the anion remained on the sliding surface and formed a tribofilm. The ionic liquids with short alkyl chains reacted with the sliding surface easily and led to very low friction. However, corrosive wear occurred in the case of the sulfate anion. On the other hand, anions with long alkyl chains underwent gradual tribochemical reactions because that led the mitigation of contact with nascent surface. The phosphate-based ionic liquids with long alkyl chains were unable to cause the lubricating effect due to low reactivity.

Yawing Angular Misalignment Provides Positive Damping to Suppress Frictional Vibration: Basic Applicability to Disc Brake Systems

A novel method for suppressing frictional vibration in sliding systems without using additional devices (e.g., dampers or actuators) is theoretically proposed. This method is based on the principle that yawing angular misalignment provides positive damping to sliding systems. Even if frictional vibration occurs due to a negative slope of the friction versus velocity characteristic, it can be suppressed by applying a misalignment angle φ that is larger than the critical misalignment angle φcr. To examine the basic applicability of this method to disc brake systems, a braking test at a constant normal load was simulated numerically. It was found that yawing angular misalignment can not only suppress frictional vibration but also shorten the braking time as a secondary effect of suppressing frictional vibration.

Polarization Observations of Adsorption Behavior of Fatty Acids Using Optical Anisotropy of Liquid Crystal

Chemical interactions and formation of adsorbed films of additives strongly affect the tribological behavior in boundary lubrication. In order to examine the relationship between the adsorption behavior and friction properties of fatty acids, polarization observations using a liquid crystal and friction measurements were performed. A nematic liquid crystal [4-pentyl-4′-cyanobiphenyl (5CB)] and normal fatty acids with 16–22 C chains were base fluid and additive, respectively. The adsorption behavior of the fatty acids was observed using polarization microscopy and analyzing the optical anisotropy of 5CB. The adsorbed film of fatty acids induced a change in the orientation of 5CB from planar to homeotropic. The expansion speed of the homeotropic domain increased as the chain length of the fatty acid decreased. The friction coefficient of the test lubricants was measured in a steel–steel sliding contact using a low-speed reciprocating tribometer. At the first cycle of reciprocating sliding motion, the lower friction coefficient was measured using fatty acids with the shorter alkyl chain length. The friction coefficient at the first cycle showed a strong correlation with the expansion speed of the homeotropic domain. These results show that the shorter-chain fatty acids form an adsorbed film highly oriented perpendicular to the solid surface, and the adsorbed film provides a lower friction coefficient at the first cycle.

The Effects of Surface Texturing on Friction Performance under Reciprocating Sliding Condition

In this research, we focus on the improvement of frictional performance of piston ring achieved by Laser Surface Texturing. Five kinds of samples (non-texture, dimple 100μm, dimple120μm, groove 45 degrees to sliding direction and groove 90 degrees to sliding direction) were tested under reciprocating sliding condition. The results showed that the sample of groove 90 degrees to sliding condition has the maximum decrease on either average friction coefficient (26%) or friction coefficient at end points of stock (33%). And we think that under boundary lubrication regime, the uniformity of distribution of the texture and flow of oil in contact surface are the key factors which affect the friction coefficient.

Observation of the Tribo-Film Formation Derived from ZnDTP by Using FT-IR

To understand the effects of tribo-film formation derived from ZnDTP (zinc dialkyl ditio phosphate) on friction behavior, friction tests were carried out with varying sliding time. The sliding surfaces were analyzed using Fourier transform infrared spectroscopy (FT-IR) and X-ray Photoelectron Spectroscopy (XPS). On the basis of the surface analytical results of the tribo-film, the absorption bands in the region of 1300-1000 cm-1 assigned as polyphosphate appeared in all IR spectra after the sliding. The friction behavior of ZnDTP was related to the intensity change of the IR spectra peak at 1200 cm-1.

Spontaneous Change in Effective Viscosity of a Liquid Crystal by Surface Anchoring in an EHD Contact

The optical film thicknesses of a 0.1 mass% solution of hexadecanoic acid (HDA) in a nematic liquid crystal (i.e., 4-pentyl-4′-cyanobiphenyl (5CB)) were measured in an EHD contact by using ultrathin-film interferometry. Comparing with a 0.1 mass% solution of HDA in an ordinary isotropic liquid (i.e., polyalphaolefin), the optical film thickness of the solution was increased at middle entrainment speeds (i.e., in the mixed lubrication regime). This shows that the effective viscosity of 5CB was increased with decreasing the entrainment speed, by the surface anchoring of 5CB on the boundary films formed by HDA. Using this spontaneous change in the effective viscosity, it is believed that a smart lubrication system can be realized, which optimizes the viscosity of a liquid crystal lubricant autonomously so that the friction coefficient is minimized when the entrainment speed is changed. It should be noted that this method does not need any pretreatments of contact surfaces and any external fields (e.g., an electric field or magnetic field).© 2012 ASME

Anti-Vibration Tribometer Using Viscous Damping Effect Generated by Lateral Slip

This paper proposes a simple and novel principle for suppressing frictional vibration using viscous damping effect generated by “lateral slip” given to a typical sliding system. This principle was applied to the measurement of friction coefficient and an anti-vibration tribometer (AVT) was developed. The AVT suppressed frictional vibration caused by the negative dependence of the friction coefficient on the relative velocity, and it enabled one to measure friction coefficient accurately in the force balance between the friction force and spring force. If using the mean value of oscillating spring force to calculate friction coefficient under frictional vibration, it was found that considerable errors (e.g., approximately 30%) appears.Copyright