The present study discusses the effect of micropolar parameters on the stability of symmetric hole-entry hybrid journal bearing system compensated with capillary restrictor. Finite element method has been used to solve the modified Reynolds equation for the flow of micropolar lubricant through the bearing. The linearized and nonlinear equations of motion are solved numerically using fourth order Runge-Kutta Method at each integration time step. Solution of these equations provides the locus of moving journal center and the journal trajectories are plotted with the help of a computer program. A Routh-Hurwitz stability criterion has been applied to calculate the critical mass of the journal to analyze the stability of journal. A comparative analysis for linear and nonlinear models considering Newtonian and micropolar lubricants has been done. The results obtained indicate that micropolar parameters affect the performance as well as the stability margin of the hybrid journal bearing system considerably and nonlinear analysis provides quick response as compared to linear one.

The effects of the molecular structure of zinc dialkyldithiophosphates (ZDDPs) on the tribological properties of a hydrogenated amorphous carbon (a-C:H) film under boundary lubrication were investigated. Friction tests were performed at a-C:H/a-C:H and steel/steel contacts under lubrication with poly-alpha olefin (PAO) and PAO containing five types of ZDDPs with different alkyl groups (primary-C4 ZDDP, secondary-C4 ZDDP, primary-C6 ZDDP, primary-C8 ZDDP and primary-C12 ZDDP). All our results suggest that the structure of the alkyl chain of ZDDPs strongly influence the tribological properties of the a-C:H/a-C:H and steel/steel tribopairs lubricated with ZDDP solutions.

The graphitization process of a tetrahedral amorphous carbon (ta-C) film under lubrication was investigated using a laboratory-built in situ Raman tribometer. The coating was lubricated with poly-alpha-olefin (PAO), PAO with an added friction modifier (FM), glycerol mono-oleate (GMO). Friction tests were carried out using a ball-on-disk setup, in which a 19-mm diameter ta-C-coated ball was loaded and rubbed against a steel disk that was immersed in a lubricant solution. In situ optical microscopy and Raman spectroscopy were used to monitor the graphitization process and wear tracks of the ta-C-coated ball. Raman analysis was conducted on the rubbed surface of the rotating ta-C-coated ball when it was located under an objective lens every three minutes during sliding. In this study, the degree of graphitization of the ta-C surface was estimated by calculating the intensity ratio of the D-peak and G-peak (ID/IG) in the Raman spectra of the ta-C film during friction tests with different lubricants. All our results suggest that the friction modifier inhibits the progression of graphitization of DLC films by reducing friction, in other words, reducing the contact temperature, and the wear progression of DLC films under boundary lubrication can be induced by graphitization of DLC surfaces at the sliding contact.

In the current study, we conducted sensory tests for facial tissues that are commercially available in Japan, and we also tested the friction generated by the rubbing of these tissues on an artificial skin model. Using these results, we investigated the factors affecting the textural characteristics of the facial tissues. The tissue sample which contained a moisturizing agent had the highest evaluation score among the samples. Mean friction coefficient decreased as the sliding velocity decreased for low normal loads. The tissue samples which contained moisturizing agents had the low friction coefficients. From the multiple regression analysis to obtain the highest value of the adjusted coefficient of determination, the mean friction coefficient for a normal load of 0.49 N and a sliding velocity of 1 mm/s was the only parameter surprisingly that could be used to predict the evaluation scores; specifically, there was a negative correlation between the mean friction coefficient and the scores. In order to reduce the friction, one should seek to increase the moisture content of a facial tissue or decrease the tensile strength perpendicular to the fiber direction in dry facial tissues. Increasing the elastic modulus and surface roughness and decreasing the fiber diameter were effective methods for reducing the friction coefficient of facial tissues that did not contain moisturizing agents.

This study evaluated the potential use of three polyol esters (POEs) derived from palm oil as a lubricant in bentonite suspension drilling fluid. Three different POEs were investigated, which are pentaerythritol ester (PEE), trimethylolpropane ester (TMPE), and neopentylglycol ester (NPGE). It was revealed that POEs reduced the coefficient of friction (COF) in bentonite suspension by more than 80%. However, TMPE and NPGE undesirably caused foaming and altered the gel texture of the suspension. PEE showed the best performance as it produced the highest lubricity and the lowest effect on the suspension texture. The decrease of COF might increase the drilling efficiency and prevent the lubricity-related drilling problems. The findings of this investigation will be used for future applications of POE from palm oil in wider mud formulation.

In the material designing process of footwear outer-soles, the rubber grip property under lubricated conditions is important. Generally, friction behaviors of rubber under such conditions are influenced by contact states, which can be changed by various factors, i.e. viscosity, sliding velocity, normal force, surface roughness and wettability between two substrates. To discuss the influences of each parameter, the contact conditions are investigated. The purpose of this study is to establish a new method to measure the distributions of real contact area and film thickness between two substrates based on the decay behavior of an evanescent field in a total reflection method. In conclusion, film thickness was measured with an accuracy of 1 nm in a range of clearance of less than 800 nm.

In this work, the influence of ink composition on the tribological properties of ballpoint pens has been examined with a view to improve writing smoothness and pen lifetime. Friction experiments were performed using a ball on plate tribometer in order to simulate the sliding contact of a ballpoint pen. Ballpoint pen ink is composed of various solvents, resins, friction modifiers, colorants. To determine the basic tribological properties of ballpoint pen ink, this study focuses on the investigating the influence of different ink compositions: solvents, resins, and phosphate on the tribological behavior. The worn surfaces of specimens from tribological tests using different ink compositions were analyzed by using Fourier-transform infrared spectroscopy. Tribological tests were performed on eight different lubricant compositions including: i) the solvents without additives ii) the solvents with the resins and iii) the solvents with the friction modifiers. The results showed that for Oil A the tribological properties were affected by the molecular weight of the resin and the addition of phosphate reduced the coefficients of friction. Similar results were not observed for Oil B. In addition, it was found that the ratio of the components of the base oil affects the tribological properties of ballpoint pen ink.

The lubricant film thickness behaviors of lithium (Li) thickener type greases and their base oil were investigated for the contact between a glass disk and a smooth/dented steel ball using an optical interferometry. The main focus is to understand grease film behaviors including high speed history and contact surface conditions. A Li stearate thickened grease showed similar behaviors to the base oil itself and the thickener particles were not observed on the contact. Thickener particle entrainments were observed in greases with Li 12-hydroxy stearate and Li complex thickeners. Track patterns formed in the downstream of the contacts correlated with the thickener particle entrainments to the contacts. The Li stearate thickened grease showed quick starvation phenomenon without a scoop which maintains fully flooded conditions. The particle entrainments of Li 12-hydroxy stearate and Li complex thickened greases were less observed after measurements at high speeds. On the dented surfaces, the particle entrainments of the Li 12-hydroxy stearate grease were more frequent than those of Li complex grease. These results suggest that thickener chemical structures have influence on the grease film formation. The thickener with higher polarity could promote the entrainment of greases into contact.

It was shown in previous reports by authors that friction coefficients of the level of 0.0001 were achieved when ZrO₂ (Y-PSZ: yttria partially stabilized zirconia) pins were slid against diamond-like carbon (DLC) films in H₂-gas environment under heavy applied load of 63.7 N (friction fade-out, FFO). It was also shown that FFO continued long when the main H₂-gas flow was mixed with a small amount of aqueous-alcohol vapor. A tribofilm was formed at the contact area of ZrO₂ surface, and it was considered that the aqueous-alcohol vapor made the tribofilm strong. In the present research, the run-in process for achieving more stable FFO is investigated by varying the run-in pattern consisting of load step-up and aqueous-alcohol vapor addition, and the stable FFO continuing several hours is realized reproducibly. Then, before and after the FFO onset, the contact area on ZrO₂ surface is observed by SEM, and nano-indentation hardness of tribofilm is measured. It will be shown that a substance made of hydrocarbons with low melting temperature, low electrical conductivity and low nano-indentation hardness such as hydrocarbon polymers is formed on the tribofilm, suggesting the evolution of hydrocarbon gas at the sliding surface.