The effect of temperature on lubrication property with MoDTC-containning lubricant —temperature dependence of friction coefficient and tribofilm structure—

The effect of temperature on the lubricity of molybdenum dithiocarbamate (MoDTC) was investigated by a ball on disk tribometer with lubricant oil containing MoDTC and calcium sulfonate (CaSU) at temperatures of 25C, 40C, 60C and 80C. The behavior of friction coefficient was closely dependent on the oil temperature. Friction coefficient decreased after a certain induction period and became to be constant at steady state. At higher temperature, the induction period became to be shorter, and the slope of decreasing friction coefficient became to be steeper. Friction coefficient at steady state decreased at higher temperature, and was changed reversibly when the oil temperature was changed. Oscillatory change in friction was observed when the oil temperature was changed periodically. Chemical analysis of the tribofilm with X-ray Photoelectron Spectroscopy (XPS) revealed that higher content of Mo(IV) as a component of molybdenum disulfide (MoS₂) was observed in the tribofilm formed at higher temperature. Highly oriented MoS₂ layer was observed with High Resolution Transmission Electron Microscopy (HR-TEM) in the tribofilm formed at 80C. It was found that the structure of tribofilm is altered reversibly with oil temperature. A steady state model of tribofilm formation was proposed on the basis of observed results.