Tribochemistry on Clutch Friction Material Lubricated by Automatic Transmission Fluids and the Link to Frictional Performance

Automatic transmissions (AT) for passenger cars are becoming more popular globally, including some countries that traditionally prefer manual transmissions. Some new friction modifiers for transmission fluid technologies have also emerged due to the downsizing trend of transmissions. In order to study the tribology and tribochemistry effects of some new automatic transmission fluid (ATF) additive formulations, both steel and wet-clutch friction materials were assessed by using surface analysis techniques. A variable speed friction test (VSFT) rig was used to study the antishudder properties in lock-up clutch tests and friction modifying mechanisms of ATFs. A test oil matrix containing basic ATF components was tested. The friction results were analyzed using both the linear-defined multiple parameter spider chart ATF evaluation (LSAE) method (Zhao et al., 2008, “A New Method to Evaluate the Overall Anti-Shudder Property of Automatic Transmission Fluids—Multiple Parameters Spider Chart Evaluation,” Proc. Inst. Mech. Eng., Part J: J. Eng. Tribol., 222(J3), pp. 459–470) and the friction coefficient ratio index method (Zhao et al., 2011, “Understanding Friction Behavior in Automatic Transmission Fluid LVFA Test: A New Positive Curve Parameter to Friction Coefficient Ratio Index Evaluation,” ASME J. Tribol., 133(2), p. 021802) (e.g., μ₁/μ₅₀ on the low-velocity friction apparatus (LVFA) μ-v curve results to compare the overall tribosystem and the snapshot friction performance during the test). Surface analysis results were obtained by using X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectroscopy (ToF-SIMS), and attenuated total reflectance Fourier transformed infrared spectroscopy (ATR FT-IR), and they are presented in this study to investigate the tribofilm compositions formed by different additive formulations. Some organic functional groups were found at the sample surfaces, such as –OH and O–C–O, and their presence is proposed to have a beneficial influence on the ATF friction performance. This paper discusses the surface analysis results of the test sample pieces, the possible links between specific functional groups and friction performance, and the proposed pathways of additive decompositions by using chemical bond dissociation energy comparisons.