Macroscale Superlubricity and Polymorphism of Long-Chain n-Alcohols

Simple n-alcohols, such as 1-dodecanol, show anomalous film-forming and friction behaviour in the elastohydrodynamic lubrication (EHL) contacts 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 solid polymorphs are formed inside the contact depending on the temperature and pressure conditions. Surprisingly, at moderate temperature and pressure, 1-dodecanol forms a polymorph that exhibits robust macroscale superlubricity. The DAC and DSC experiments show that superlubricity is facilitated by the formation of lamellar, hydrogen-bonded structures of hexagonally close-packed molecules, which promote interlayer sliding. This novel superlubricity mechanism is similar to that proposed for the two-dimensional materials commonly employed as solid lubricants, but it enables the practical advantages of liquid lubricants to be maintained. When the pressure is increased, 1-dodecanol undergoes a polymorphic transformation into a phase that gives higher friction. 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 anomalous film shapes are caused by the formation of rigid orthorhombic crystals inside the contact. The unusual friction and film-formation behaviour of 1-dodecanol result from bulk effects that are insensitive to the surface chemistry or topology. This is the first demonstration of macroscale superlubricity inside EHL contacts using a non-aqueous lubricant. This discovery creates the possibility of implementing superlubricity in a wide range of machine components, which would result in enormous improvements in efficiency and durability.