Paper TR-ThP1
A Vacuum Tribometer to Depict Tribochemical Reactions of Lubricant Additives

Thursday, October 23, 2008, 6:00 pm, Room Hall D

Today, it is well known that surface chemistry plays a key role in tribology and particularly in additives action under boundary lubrication. Tribochemical reactions are very difficult to predict because many parameters occur simultaneously in the tribological system. To simplify and to understand these phenomena, we have developed an analytical vacuum tribometer dedicated to the simulation of boundary lubrication conditions. A pin-on-flat tribometer is installed in a UHV chamber. Friction tests can be run in a wide range of surrounding partial pressures, typically from 10-8 hPa to atmospheric pressure. To study tribochemical reactions, we can introduce either pure gas or mixtures of gases (or vapours) into the chamber. Both pin and flat counterparts are introduced by a Fast Entry Load-Locks (FEL). The temperature of the flat specimen can be varied from room temperature up to 600°C. Before or after friction experiments, the two samples can be transferred without air exposure by using transfer mechanisms from the tribometry chamber into a preparation chamber and then to the analytical chamber. . This chamber is equipped with a hemispherical spectrometer build by ThermoFisher (220i). X-Ray Photoelectron Spectroscopy (XPS) can be done by using a focused monochromatic X-Ray source. A Field emission electron gun (FEG100) with a spot size lower than 1 µm allows Auger Electron Spectroscopy (AES). Ion gun (EXO5) is used for etching surfaces or for performing depth profiles. Imaging facilities are available by using a video camera for optical image, or a secondary electron detector coupled with the scanned FEG for Secondary Electron Microscopy (SEM). Chemical images can be obtained by Scanning Auger Microscopy (SAM) and XPS images by using dedicated lenses. The chemical reactivity of complex additive molecules with solid surfaces is simulated here by using small molecules with the same chemical function but with a lower molecular weight. Friction experiments are performed under variable partial pressures and at different temperatures. Mixtures of gases can be introduced to study synergistic or antagonist effects between chemical functions of additives. A residual gas analyser is installed on the vacuum chamber in order to control the purity of the gases and also to study any friction-induced outgases. We will present representative results with analyses performed inside and outside wear tracks to show chemical changes induced by tribochemical reactions.