Tribology Online Vol. 12(2017) No. 5

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.