Yingxi Zhu

Modification of Boundary Lubrication by Oil-Soluble Friction Modifier Additives

The molecular-level function of model and commercial friction modifier additives in lubricants of the type used at the wet clutch interface in automatic transmissions has been studied using a surface forces apparatus (SFA) modified for oscillatory shear. The nanorheological properties of tetradecane with and without a model friction modifier additive (1-hexadecylamine) were examined in the boundary lubrication regime and compared to a fully-formulated automatic transmission fluid (ATF). 1-Hexadecylamine adsorbed as a single layer on the sliding surfaces, reduced the static frictional force and the limiting shear stress, and eliminated the stick–slip transition that exists in pure tetradecane. The ATF, which contains commercial-grade friction modifiers, showed nanorheological properties similar to those observed for tetradecane containing 0.1–0.2 wt% 1-hexadecylamine.

Healing of confined polymer films following deformation at high shear rate

Recovery of equilibrated linear viscoelastic response of confined polymer melts, following cessation of large-amplitude shear in a surface forces apparatus, was found to be a single exponential process. The most extensive experiments concerned a polydimethylsiloxane of narrow molecular weight distribution and weight-average molecular weight Mw=8330 g mol−1, for which recovery times were in the range 2–12 h when the film thickness (D) was D/RG=0.5–6 (RG is radius of gyration). Initially, to produce the deformed state, the films were sheared with effective shear rate ≈104 s−1. Recovery was probed by the subsequent application of small-amplitude sinusoidal shear forces at 256 Hz. Surprisingly, the nonlinear and linear shear moduli evaluated at the input frequency nearly coincided just before and just after cessation of large-amplitude shear. Recovery time constants, τR, increased linearly with the prior shear rate at a given thickness (D). But at a given shear rate and variable D, τR passed through a maximum...