Anti-Shudder Properties of ATFs-Investigation into Tribofilm Composition on Clutch Friction Material and Steel Surfaces and the Link to Frictional Performance

The friction response of an automatic transmission fluid (ATF) should ensure a smooth engagement process with no shudder in the process of power transmission. To study the effect of ATF chemistry on anti-shudder properties, the frictional characteristics of the baseline additive system used in ATFs with various friction modifiers and detergent combinations were assessed using a variable-speed friction test (VSFT) machine. Tests were performed on steel-versus-clutch friction material tribocouples, with the aim of studying the tribofilm formation on both friction couple (steel and friction material) surfaces and to establish the link between the lubricant-surface interactions and anti-shudder performance. An SAE No. 2 test machine was also used to investigate the friction performance of the same ATFs for comparison with the VSFT results. Similar performance rankings were obtained from those two tests. This article focuses on the posttest surface analysis and surface chemistry results of the test samples. Energy dispersive X-ray spectroscopy (EDX) analysis, X-ray photoelectron spectroscopy (XPS), and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) were used to assess the different surface conditions and chemical tribofilm compositions on the friction material and steel counterpart after the test. The mechanisms of the different friction performance provided by additive formulations and the correlation with the composition of tribofilms are discussed. Some adsorbed functional groups on the posttest sample surface, such as OH-, C-N, etc., were suggested to have positive effects on the anti-shudder performance of ATFs.