The molecular interaction of oleic acid in different base oils at Elastohydrodynamic lubrication (EHL) contact was investigated by means of in-situ observation with micro-Fourier Transform Infrared Spectrometer (FTIR). Moreover, the effect of hydrostatic pressure was confi rmed using a diamond-anvil cell (DAC). Poly-α-olefi n (PAO) with non-polar, and Polypropylene glycol and Polybutylene glycol (PPG and PBG) which have ether group were used as base oil. When PAO was used, oleic acid interacts with itself to form a dimer. On the other hand, oleic acid interacts with PPG and PBG and exists mainly as a monomeric moelcule.The IR peak of C = O stretching mode of oleic acid dimmer was shifted to lower wavenumber at EHL contact region. The peak shift was not obvious in the case of the monomer. This result means that the hydrogen bonding of the dimer was stabilized under high pressure, but the hydrogen bonding of monomer was not.

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In the present work, the stabilisation of hydrogen bonding of polypropylene glycol at elastohydrodynamic lubrication (EHL) contact was investigated by in situ observation with a micro-Fourier transform infrared spectrometer, and the effect of hydrostatic pressure on infrared (IR) spectra was confi rmed using a diamond anvil cell. Polypropylene glycols with different molecular weights were used as sample oils. Polypropylene glycol molecules interact with themselves by hydrogen bonding with their hydroxyl groups. Stability of the hydrogen bonding is observed by IR peak shift of the O-H stretching mode (3400-3700 cm −1 ). In the case of the dynamic condition under EHL, the C-H stretching mode shifted to a higher wavenumber, whereas the O-H stretching mode shifted to a lower wavenumber at the Hertzian contact region. This result means that the hydrogen bonding of polypropylene glycol was stabilised under high pressure, and the stabilisation of hydrogen bonding by pressure was dependent on the molecular weight of polypropylene glycol.