Studying the Function of Surface-active Lubricant Components by Surface Analytical Methods

Lubricant formulations typically contain >95 % of a mineral or synthetic base stock and up to 5 % of lubricant additives. Some of these additives are surface active like corrosion inhibitors, antiwear additives or friction modifiers. Specific base stocks like esters or polyalkylene glycols also are surface active due to its polar character. It is of utmost importance to understand the adsorption as well as the tribological mechanism of these lubricant components on the metal surface to be able to develop an optimized lubricant formulation for each application. Using a non-optimized formulation could cause issues like e.g., increased wear or corrosion in a hydraulic pump due to the antagonistic interaction of antiwear additives and corrosion inhibitors. In this paper we will discuss some surface chemical basics and present relevant physical and chemical surface analytical methods (e.g. XPS, QCM, laser fluorescence microcopy and others). At last, we will present a more in-depth analysis using the example of a friction modifier deposited on an iron oxide surface. To investigate the friction modifier without the influence of the environment and the base oil, a tribometer in vacuum is used. ... mehrAn effusion cell is used to evaporate the friction modifier additive for gas phase deposition on the iron oxide surface. The tribometer is connected to a vacuum transfer system so that the samples can be characterized before and after the friction tests using XPS. Under these conditions, the additive does form droplets whose radii and heights increase with increasing deposition time. To the best of our knowledge, this morphology has not been found previously in other gas phase lubrication studies to date.