Abstract
To clarify the effect of precipitates on low friction, we conducted the friction test and in-situ observation of precipitates on contact point by reflectance spectroscopy. From the reflectance spectrum measured by reflectance spectroscopy, we observed and estimated the thickness of precipitates. From the viscosity measurement of lubricating oil, it was observed that the lubricating oil containing palmitic acid becomes turbid and rapidly increases in viscosity below a certain temperature. Friction tests showed that the lubricating effect was reduced when the temperature was below the melting point of palmitic acid. Oil film analysis revealed an abundant presence of palmitic acid in the frictional area. Moreover, the thickness of the oil film increased compared to the theoretical value, suggesting that the deposition of palmitic acid contributed to the increase in film thickness. Consequently, the deposition of palmitic acid led to an increase in oil film thickness, suppressing direct contact and reducing friction. A correlation was found between the thickness of the palmitic acid layer and the friction coefficient, with negative correlation at 30°C (below the cloud point) and positive correlation at 50°C (above the cloud point), suggesting the possibility of different friction reduction mechanisms depending on the temperature.
