INTRODUCTION: In order to reduce global energy consumption, it is necessary to reduce the fuel consumption of automobile engines1. One effective method for realizing a fuel-efficient engine is the use of low viscosity lubricating oils. However, as the viscosity of oils decrease, the load capacity decreases, so that the gap between the sliding surfaces is narrowed. This leads to a high possibility of transition to mixed lubrication and boundary lubrication. Therefore, in using low viscosity oil, realization of low friction in the boundary lubrication state is a technical problem to be overcome. One method is the use of polymeric additives. Polymeric additives are mainly used to control the temperature characteristics of lubricants. It has been reported that when an adsorption layer is formed on a solid surface, it functions as a boundary film and achieves low friction2. In lubrication utilizing such a polymer adsorption layer as a boundary film, it is necessary for lubrication design to quantify its mechanical properties. Therefore, in this study, we aimed to clarify the viscoelasticity of lubricants with polymer additives confined and sheared in minute sliding gaps. As a model lubricant, we used polyisobutylene added polyalphaolefin. For the viscoelasticity measurement, we used the fiber wobbling method, which we developed in our previous study3.
METHODS: Figure 1 shows the outline of the fiber wobbling method (FWM) used for the viscoelasticity measurement. In FWM, we use a glass fiber having a spherical tip with a diameter of 200 μm. The fiber is about 2 mm in length and its diameter is about 100 μm. A piezo actuator drives this probe in parallel to the substrate, and the lubricant on the substrate is sheared by the spherical tip. The shear resistance force acting on the tip can be quantified by optically detecting the deflection of the fiber. To measure the viscoelasticity of lubricant, we oscillate the fiber sinusoidally and detect the amplitude change and the phase shift of the tip’s oscillation. From these measured values, the complex viscosity, which is η’-iη”, of the lubricant can be determined3. In this study, the lubricant was polyalphaolefin (PAO) containing 2.5 wt% of polyisobutylene (PIB). For comparison, PAO without addition of PIB was also used for the sample. As a solid substrate, a quartz glass substrate coated with a stainless steel thin film was used. The oscillation amplitude in the viscoelasticity measurement was 50 nm, and the frequency was 1 kHz. The initial gap between the tip of the probe and the substrate was set to about 500 nm. While narrowing the gap at a constant rate of about 10 nm/s, viscoelasticity (complex viscosity) was measured until the probe and the substrate were in solid contact.
RESULTS AND DISCUSSION: In the measured complex viscosity coefficient, the real part η’, which represents viscosity, is shown in Fig. 2(a), and the imaginary part η”, which represents elasticity, is shown in Fig. 2(b). Both abscissas represent gaps between the probe tip and the substrate. In both graphs, the red plots are the results of PAO with PIB and the green plots are those of PAO alone. Regardless of the addition of PIB, the viscosity showed a constant value independent of the gap in the relatively wide gap area from 100 to 500 nm. In addition, elasticity in a wider gap range was almost zero. As the gap becomes narrower, both lubricants showed a gentle increase in viscosity in gaps of 100 nm or less. On the other hand, elasticity rapidly increased in the gap less than 200 nm in the case of PAO with PIB, whereas PAO alone showed elasticity increase from about 10 nm or less. From the above results, it was clarified that the effect of adding PIB is