Fundamental Studies on ATF Friction, Part II 982670
Interactions between automatic transmission fluid (ATF) components and composite friction materials and their effect on friction system performance continues to be an active area of interest to the automotive industry. A more fundamental understanding is needed of how base fluids, ATF additives, friction materials, and transmission design interact to produce the observed transmission system performance and durability. We herein report results from investigations carried out using a relatively thermo-oxidatively stable polyalphaolefin (PAO) base fluid treated with components representative of several additive types we previously reported to have significant negative effects on frictional performance. Secondly, we investigated a conventionally refined 150 N base oil treated with a calcium sulfonate detergent previously shown to improve friction performance. The additives were tested as single components in each base fluid in a standard and in a high energy version of the 3T40 flat plate SAE#2 test. Lastly, two fully formulated ATFs were run in the same tests for comparison. Changes in steel plate surface chemistry were characterized by Scanning Auger Microscopy (SAM); while, changes in composite friction material topography and surface chemistry were characterized by non-contact profilometry and by Environmental Scanning Electron Microscopy (ESEM). Oxidation of the fluids was monitored by changes in viscosity, total acid number (TAN), total base number (TBN), carbonyl absorbance increase in the infrared, and pentane insolubles. Overall, the dynamic torque levels in both the standard and high energy SAE #2 test procedures were related to the condition of the composite friction plates at the end of test. Loss of porosity, glazing, and surface cracking of the friction plate composite material were observed for tests with poor dynamic torque stability. Visual blackening of the composite surface was not a good indication of the degree of glazing. The depth and chemical composition of reaction layers on the end-of-test steel plates do not appear to be related directly to friction performance. Most interestingly, oxidative change in the test fluids measured by changes in viscosity, TAN, and IR carbonyl absorbance increase had no correlation to the test results. Thus, care should be used when directly relating end-of-test fluid analytical and material surface chemistry to the observed test performance of a transmission clutch system.