Correlation between Tribofilm Formation and Friction Coefficient in Continuously Variable Transmission at the Initial Stage of Rubbing

To improve the transmission efficiency of a continuously variable transmission (CVT) system, it is effective to form a tribofilm with higher friction coefficient. Therefore, understanding of the correlation between the tribofilm formation and friction coefficient in a CVT fluid has been a great deal. In this study, the tribofilm formation on a bearing steel ball surface under the simulated additive oil was investigated. We used ball-on-disk tribological test and optical microscopy imaging to observe the temporal changes in the tribofilm growth. Atomic force microscopy and energy-dispersive X-ray spectrometry analyses revealed that the tribofilm consisted of pad-like structures derived by tricresyl phosphate and calcium sulfonate additives. A binary imaging analysis showed that the total area of the pad-like structure was a key parameter determining the friction coefficient. The pads were repeatedly desorbed and reformed during the sliding test. These results improve the design of future CVT systems and lubricants.