Frictional Properties of Automatic Transmission Fluids: Part II-Origins of Friction-Sliding Speed Behavior

A wet clutch in an automatic transmission has very specific and unusual friction characteristics. For the clutch to operate efficiently and without stick-slip, friction must both increase with sliding speed and be high over the whole sliding speed range. This is achieved by the use of a very rough friction material, which inhibits fluid film formation, combined with sophisticated design of the automatic transmission fluid, with separate additives to reduce friction at low speed and increase friction at high speed. There has been much debate in the literature as to the underlying mechanism that allows friction to increase with sliding speed. This article critically discusses the main models that have been suggested, which range from a highly viscous, and thus high friction, hydrodynamic film forming at high sliding speed, to fluid drag supplementing friction as sliding speed increases. The authors then propose an entirely new model for wet clutch friction. This is based on the two principles: (1) the wet clutch morphology ensures that the contact remains in the boundary lubrication regime over the whole sliding speed range and (2) organic friction modifiers form adsorbed boundary films whose friction increases with the logarithm of the sliding speed. The latter behavior has been clearly demonstrated in a number of previous studies and ascribed to an activated shear process. The combination of these two principles means that when an effective organic friction modifier is present, wet clutch friction increases monotonically up to high sliding speeds. Based on friction-sliding speed measurements described in a companion article, Part I (Ingram, et al. (1)) it is then suggested that additives that increase friction do so by partial disruption of the organic friction modifier film, which allows more interpenetration of the films on the opposing surfaces and thus higher friction over the whole speed range. Toward the end of the article, the possible origins of the increase of friction at low sliding speeds observed for base oils and blends without effective organic friction modifiers is discussed.