Humidity effects on in situ vapor phase lubrication with n-pentanol

The effect of water vapor on n-pentanol vapor phase lubrication (VPL) was studied with a microelectromechanical system (MEMS) side-wall tribometer, a pinon- disc tribometer, and attenuated total reflection infrared (ATR-IR) spectroscopy. The n-pentanol vapor pressure was fixed at 50 % relative to its saturation vapor pressure (Psat = ̃2.2 Torr at room temperature), which is sufficient to maintain a monolayer of n-pentanol on a SiO2 surface in a dry Ar environment. As the relative humidity (RH) was increased from zero to 30 %, ATR-IR measurements showed that the water adsorption on the surface increases and the adsorbed pentanol thickness decreases by 60 %. These changes in the adsorption isotherm were manifested as higher, and more scattered friction coefficients observed during the MEMS tribometer operation. The maximum RH tolerance appeared to be 25-30 % RH above which the MEMS tribometer failed to operate reliably. In contrast, the n-pentanol VPL efficiency was not affected significantly during the macro-scale pin-on-disc tribometer tests. These results imply that the friction behavior of the asperity contacts in MEMS is more susceptible to co-adsorption of water than the friction behavior of macro-scale contacts.