In situ observation of lubricant films formed with greases was performed with a micro Fourier transform infrared spectrometer (FT-IR) under elastohydrodynamic lubrication (EHL) conditions between a steel ball and a single-crystal silicon disk. Twelve mass percent of lithium stearate or aromatic-type urea compound were added as thickener to polyalphaolefin base oil. Peaks relating to CH, C=O and NH were monitored in the FT-IR analysis. The film thickness and the concentration of thickener were estimated from the absorbance of CH and the ratio of absorbance of C=O and CH. Two-dimensional distribution of the thickness and the concentration around EHL contact was obtained at a resolution of 50 μm. It was found that the concentration of thickener at EHL contact area was dependent on the type of thickener. Although the concentration of Li grease decreased at the EHL contact area, that of urea increased even at low entrainment speed. The thickener of urea grease was concentrated on the ball and the disk surfaces. The film thickness at EHL contact of each grease can be explained by the concentration of thickener at the contact. Lubricating characteristics of urea grease are discussed based on the concentration of thickener at the EHL contact.

No abstract

This study investigates the effects of both, molecular structure and surface roughness, on the tribochemical decomposition of synthetic base oils under high-vacuum conditions. Nascent steel surfaces exhibit high activity to catalyze the decomposition of synthetic oil under lubricating conditions. Decomposition was performed on multiply alkylated cyclopentane (MAC) and alkyl diphenyl ether (ADE) at the nascent surface of bearing steel 52100 and was monitored by a ball-on-disk friction tester in a vacuum chamber attached to a quadrupole mass spectrometer. Although MAC decomposed, ADE did not decompose on the flat steel surface. Steel surfaces used within this experiment underwent procedural abrasion, providing the requisite roughness to generate the nascent steel surface for the friction test. Decomposition of ADE and desorption of benzene were observed in the presence of the nascent steel surface under friction. As the surface morphology of the pretreated roughened surfaces changed during friction, the contact pressure and plasticity of the surface decreased. We concluded that the tribochemical decomposition of ADE was accelerated on rough steel surfaces by the generation of a nascent surface at the point of metal-metal contact. From these results, the critical role of a nascent steel surface on the tribochemical decomposition of syntheticoils is discussed.

The correlation between molecular interaction and traction properties was investigated using a traction tester and in situ observation of elastohydrodynamic lubrication film with a micro-Fourier transform infrared spectrometer. The sample oils used were polypropylene glycols (PPGs) with the end-group of alcohol or ether and a synthetic hydrocarbon oil, poly-α-olefin. From the traction tests, it was found that the traction coefficient of PPG was sensitive to the end-group. PPG with alcohol as the end-group showed a higher traction coefficient than that with the ether group. In situ observation with a micro-Fourier transform infrared was performed in order to investigate the molecular interaction of the lubricant oil. It was found that the hydrogen bonding of hydroxyl groups in PPG was strengthened by high pressure in the Hertzian contact region. These results suggest that the rheological properties in the elastohydrodynamic lubrication contact region were affected by the strengthened hydrogen bonding.

In situ observation of lubricant films formed with greases was carried out at EHL conditions with a micro-FTIR. Three urea compounds were used as a thickener and 12 mass percent of the thickener was added in PAO, respectively. From the IR spectra, CH and NH stretching vibration peaks were observed from urea greases. The film thickness and the concentration of thickener were estimated from the absorbance of CH and the ratio of absorbance of NH and CH. Two-dimensional distribution of the thickness and the concentration around Hertzian contact was obtained at the resolution of 50 μm. The thickener of urea grease was concentrated on ball and disk surfaces. It was found that the concentrate on thickener at Hertzian contact region was dependent on thickener itself. Time dependence of the thickness and the concentration at Hertzian contact were obtained at the resolution of 45 s under shearing conditions. The film thickness of urea greases at Hertzian contact was greatly decreased by shearing and the film thickness and the concentration were recovered when the condition was changed to rolling. This behavior was supposed with detachment and growth of the adhered film of the thickener. Lubricating characteristics of urea greases were discussed based on the concentration of thickener at Hertzian contact.