Detergent nanoparticles facilitate MoDTC-induced tribochemical friction reduction through
shear stress under interfacial confinement

Kazushi Tamura¹, Kenji Sunahara¹, Motoharu Ishikawa¹, Masashi Mizukami², Kazue Kurihara³

¹Lubriacants Research Laboratory, Idemitsu Kosan Co., Ltd., Ichihara, Japan

²Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, Japan

³New Industry Creation Hatchery Center, Tohoku University, Sendai, Japan

INTRODUCTION:  In order to enable efficient energy use in automobiles, mechanical friction in powertrain systems should be reduced. There have been enormous efforts to develop technologies to minimize friction in various mechanical components, such as bearings, gears, chains and piston ring-packs. Lubricant technologies are one of the possible solutions to this problem. Friction interfaces undergo chemical reaction involving solid surfaces and lubricant components, so-called tribochemical reaction. Thus the substances that generate low friction materials on solid surfaces via tribochemical reaction are useful as friction reducer additives for lubricants. Molybdenum dithiocarbamate (MoDTC) is a well-known tribochemical friction reducer used in many applications including engine oils and gear oils. In friction interfaces, MoDTC converts to two-dimensional crystalline flakes of molybdenum disulfide (MoS₂), which shows very low friction coefficient^1,2. Improving efficiency of this reaction is a promising method to achieve less friction and consequently more efficient powertrain systems.

Recent studies showed the evidences supporting that thermal energy generated by shear dominates tribochemical reaction^3,4. Researchers measured the rate of formation of layer-like products of tribochemical reaction (tribofilms) and found that this rate increased exponentially with applied stress or temperature. Hence tribochemical reaction is a stress-augmented thermal activation process⁵. However, the influences of these microscopic processes on macroscale frictional performance are unclear. Lubricants mostly contained not only friction reducers but also many other functional additives. Surface roughness of solids causes spatial heterogeneity of stress and surface separation distance in friction interfaces. We should take into account such a complex context to understand and control mechanisms of tribochemical phenomena.

Here we investigated MoDTC-induced tribochemical friction reduction in the presence of detergents, one of the most common lubricant additives. We found that detergents significantly affected the degree of the friction reduction and this behavior was related to their shear response under interfacial confinement.

METHODS:  We used 5 different overbased detergents and a neutral detergent. We blended each detergent into a paraffinic mineral oil. We examined macroscale friction by using a conventional reciprocating friction tester. Test condition was cylinder-on-disk line contact geometry with mirror-finished bearing steel SUJ-2. Test oils are the detergent-containing oils mixed with MoDTC. We measured the time course of friction coefficient at constant load. We also examined microscale shear response under interfacial confinement by using a surface force apparatus equipped with resonance shear measurement system (SFA-RSM)^6,7. We used freshly-cleaved muscovite mica films with a roughness below 1 nm and optical interferometric determination of surface separation distance based on fringes of equal chromatic order (FECO). We calculated effective viscosity⁸ under interfacial confinement and its dependency on surface separation distance.

RESULTS AND DISCUSSION:  Macroscale friction tests showed that MoDTC rapidly decreased friction coefficient below 0.1 within 5 min after beginning of the test and detergents enhanced MoDTC-induced friction reduction. The degrees of friction reduction varied between the detergents tested. SFA-RSM

revealed the threshold of surface separation distance that drastically changed shear response. When the distance was below this threshold, effective viscosity was more than ten times higher than bulk viscosity measured by a capillary viscometer. This threshold, which we called critical distance, varied between detergents from tens to hundreds of nanometers. We found that critical distance correlated with friction coefficient at 5 min (initial friction coefficient) measured by a friction tester. Dynamic light scattering experiments showed that grain size of detergents was consistent with critical distance. These results suggested that shear stress based on steric effects of detergents facilitated MoDTC-induced tribochemical friction reduction.

To test this hypothesis, we performed same experiments using non-detergent nanoparticles instead of detergents. Detergents were substituted to colloidal silica dispersion or carbon black powder. These nanoparticles showed same tendency as detergents on both macroscale friction and microscale shear response. Therefore steric effects sufficiently explain the influences of detergents on initial friction coefficient and critical distance. Moreover, we decreased shear stress by using an organic friction modifier (OFM). The friction test showed that OFM increased initial friction coefficient. Hence shear stress is considered responsible to MoDTC-induced friction reduction. Figure 1 shows the relationships between initial friction coefficient measured by a macroscale friction tester and critical distance measured by SFA-RSM for all the experimental results in this study. These two values strongly correlated.

These results lead us to the conclusion that detergents facilitate MoDTC-induced friction reduction through shear stress under interfacial confinement. We consider that this concept is applicable for a broader range of tribochemical processes. Nanoparticle-based control of these processes may be beneficial for next-generation design of lubricant formulation.

Figure 1 – Relationships between critical distance D_c measured by SFA-RSM and initial friction coefficient μ_ini measured by a macroscale friction tester.

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