Improved anti-shudder performance of ATFs—Influence of a new friction modifier and surface chemistry

Abstract

Among OEM strategies for increasing fuel economy and reducing emissions, driveline performance optimisation by using automatic transmissions is particularly important. Technology advances in automatic transmission designs are leading to continued growth of the number of vehicles with automatic transmissions. In fact, over the next five years, it is expected that the market share for automatic transmissions will increase to greater than 60%, with Europe being the largest region for this growth. The performance of automatic transmissions, especially anti-shudder performance, is highly dependent on friction modifiers and other additives in automatic transmission fluids (ATFs). These additives, in a manner similar to engine oil additives, are expected to interact with the steel and clutch material surfaces to form surface films that provide the desired performance. Hence, understanding the tribochemical processes in this automatic transmission system is a very important step towards performance optimisation. While the tribochemistry of engine oil additives and the nature of tribofilms on metallic surfaces are relatively well researched, the tribochemistry of ATFs still remains largely unclear. One reason for this is the level of complexity in analysing the friction materials of automatic transmission systems. Separating lubricant-derived species from the composite clutch friction materials is not trivial. This paper presents a comprehensive look at the tribochemistry of a new friction modifier in ATF systems and assesses the nature of the tribofilms formed on steel and composite friction materials in a variable speed friction test (VSFT) apparatus, which was used to simulate the clutch friction process. The tribofilm evolution is correlated with the tribological response as ageing occurs. Functional groups from the friction modifiers were found on the friction couple surfaces after the test and the mode of their action is discussed. These groups were proposed to form a layer of tribofilm on the top surface after rubbing. The correlation between observed tribochemistry processes and the tribological performance are discussed in detail.