Tribology studied using atomically smooth surfaces

An investigation of the boundary layer friction resulting from films of nonpolar liquid lubricants, 1 to 6 layers of molecules thick is reported. The liquids were confined between parallel step-free single crystals of muscovite mica. The apparent dynamic viscosity at 1 Hz (shear rate less than 250 sec^-1) was considerably enhanced over that of the isotropic liquids and increased substantially with increasing net normal pressure. A transition to a solid-like response occurred with increasing net normal pressure, without discernible change in thickness. Investigations of the critical shear stress to accomplish sliding showed a buildup of this static friction over times from minutes to hours. For films of hexadecane, the pressure coefficient of the critical shear stress was approximately 2 to 20, depending on the equilibration time allowed at each normal pressure. The tribological behavior of these lubricant films did not appear to reflect material properties of the lubricants as such, but rather to stem from confinement of the lubricants within the tribological contact, where the freedom of molecules to move was restricted. Potential practical implications are discussed.