Influence of alkyl group of primary-alkyl zinc dialkyldithiophosphate (ZDDP) on the friction-velocity characteristics was studied using two different kinds of ZDDPs under the boundary lubrication conditions with a pin-on-disk type tribometer. In the test, a variable speed mode was repeatedly conducted at the sliding speed between 0.001m/s and 0.1m/s. As the sliding distance proceeded, the electrical contact resistance rose and the slope of friction-velocity curve changed and then it became stable for both ZDDPs. This indicated that the base layer of the tribofilm played a role in supporting the top layer including adsorbed ZDDP molecules governing the friction. After the tribotest, the surface properties of the tribofilms were studied using an atomic force microscope. The slope of force-distance curve showed that the tribofilm from linear-alkyl ZDDP was much softer than that from branched-alkyl ZDDP. On the phase images, a collection of tiny dots showing light and shade was observed for linear-alkyl ZDDP, while phase difference was uniform over the measured area for branched alkyl ZDDP. The nano-scratch resistance depended upon the scratch speed for linear-alkyl ZDDP. On the other hand, it was independent of the scratch speed for branched-alkyl ZDDP. These suggested viscous properties of tribofilm for linear-alkyl ZDDP while elastic or viscoelastic properties of tribofilm for branched-alkyl ZDDP.
Diamond-Like Carbon (DLC) coating provides low friction properties without lubricants and, with lubricants such as glycerol, should provide ultra low friction. In this study, three kinds of DLC coating, which were hydrogenated amorphous carbon (a-C:H) by the plasma chemical vapor deposition (PCVD) process, tetrahedral amorphous carbon (ta-C(M)) by the arc ion plating process and another type of tetrahedral amorphous carbon (ta-C(T)) by the filtered arc deposition process, were used. The effect of DLC coatings on reducing friction under oleic acid lubrication was evaluated. As a result, it was found that ta-C(T) which had very smooth surface without many droplets provided super low friction. It was suggested that the super low friction property seemed to be caused by the tribochemical reaction after the sliding test.