Macroscale Superlubricity and Polymorphism of Long-Chain n-Alcohols

Anomalous film-formation and friction behaviour has been observed for n-alcohols inside elastohydrodynamic lubrication (EHL) contacts, such as those found inside bearings and gears. Using tribometer, diamond anvil cell (DAC), and differential scanning calorimetry (DSC)

experiments, we show that 1-dodecanol undergoes pressure-induced solidification when entrained into EHL contacts. Different polymorphs are formed depending on the temperature and pressure conditions. At moderate temperature (40 °C) and pressure >A 500 MPa), 1-

dodecanol forms a low-friction polymorph that exhibits robust macroscale superlubricity. Using DAC and DSC experiments, we show that this is due to the formation of lamellar, hydrogen-bonded structures of hexagonally close-packed molecules, which promotes interlayer sliding.

This structural superlubricity mechanism is similar to that proposed for two-dimensional materials, which are commonly employed as solid lubricants. At higher pressure (> 500 MPa at 40 °C), 1-dodecanol undergoes a polymorphic transformation into a high-friction phase. The

DAC and DSC experiments indicate that the high-friction polymorph is an orthorhombic crystal. The polymorphic transformation pressure coincides with the onset of dimple formation in the EHL films, suggesting that the formation of rigid orthorhombic crystals causes the anomalous film shapes. The unusual friction and film-formation behaviour of 1-dodecanol result from bulk effects and are insensitive to the surface chemistry or topology. This is the first demonstration of macroscale superlubricity inside EHL contacts using a non-aqueous liquid lubricant. This creates the possibility of implementing superlubricity in a wide range of machine components, which would lead to enormous efficiency gains.