Observing the adsorption process of friction modifier additives by combination of quartz crystal microbalance (QCM-D) and confocal laser scanning microscopy (CLSM)

Friction modifier additive technologies play a crucial role in minimizing friction and wear of lubricated tribological systems. Present efforts to improve the energy efficiency and new regulations require a fundamental understanding of the adsorption processes. Therefore in-situ and oil compatible methods are needed. This work combines the in-situ measurement of friction modifier additives deposition onto a surface and the resulting viscoelastic properties (using a quartz crystal microbalance with dissipation, QCM-D) with the imaging of the deposition process employing a fluorescence confocal laser scanning microscope CLSM). This approach enabled us to for the first time to observe the deposition in-situ. Contrary to the common assumption present for almost 100 years that friction modifiers form monolayers, our CLSM-study shows that friction modifiers can also forms droplets in the mineral base oil and subsequently stay in the shape of a droplet after deposition on the Fe2O3 surface. The adsorbed droplets have diameters between < 1 µm and 127 µm. Small droplets are not mobile after deposition, but droplets with a certain diameter become mobile and move along the surface to merge with other mostly smaller droplets. We also find that droplets in solution prefer coalescence with already adsorbed droplets over deposition. From distribution and size of the adsorbed droplets the adsorbed amount is calculated and correlated with the values found with QCM-D. Hence, the combination with QCM-D the ongoing processes can be described quantitative and qualitative to give completely new insights in the deposition processes