Macroscale Superlubricity and Polymorphism of Long-Chain n-Alcohols

Abstract

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. File list (2) download file view on ChemRxiv Alcohol Superlubricity paper-revised-clean.pdf (1.44 MiB) download file view on ChemRxiv Supporing Information-final.pdf (1.05 MiB)

Tribochemistry, Mechanical Alloying, Mechanochemistry: What is in a Name?

“…1 GPa is not associated with crystallization, but rather with the formation of a glassy state with reduced mobility. Other examples are known, such as with 1-dodecanol, wherein moderate pressures induce crystallization of polymorphic modifications which favor sliding, and hence increase their lubricating ability: superlubricity ( Reddyhoff et al, 2021 ). Finally, as in 1:1 mixtures of pentane:isopentane ( Liu and Pulham, 2020 ), the pressure-induced solidification of fluids can be also selective and cause only a specific component to crystallize.…”

“…This intermediate state can endure e.g. under continuous mechanical action and provide modified tribological properties ( Prentice et al, 2020 ; Reddyhoff et al, 2021 ), or relax when the mechanical perturbation is released to yield a reaction product ( Boldyrev, 2006 ). Owing to the significant kinetic barriers in solid state reactions, this reaction product may not be always the most thermodynamically stable one ( i.e.…”

Influence of the Heat Transfer Field on Anomalous Lubricant Film Formation in Elastohydrodynamic Lubrication Conditions

“…Further, the in uence of the rheological characteristics of the lubricant within the lubricated area on the lm formation has attracted increasing interest over the last two decades. Signi cant variations in the lm shape were identi ed under rolling/sliding conditions [12][13][14][15][16][17][18][19][20] and opposite sliding conditions at a zero entrainment speed [21,22], and in the cases of high-viscosity oil [23] and liquids with a clear melting point [24][25][26][27][28][29][30]. Variations in the lm shape under rolling/sliding conditions [12][13][14][15][16][17][18][19][20] and opposite sliding conditions [21,22] were concluded to arise from the viscosity wedge action [29], whereas the lm shape variations in the cases of high-viscosity oils [23] and liquids with a clear melting point [24][25][26][27][28][29][30] remain to be fully understood.…”

“…Herein, we present the appearance of anomalous lm shapes in the case of a liquid with a clear melting point [24][25][26][27][28][29][30]. Anomalous lm shapes were discovered at the point contact area between a glass disc and a steel ball lubricated with a fatty alcohol, 1-dodecanol [24][25][26][27].…”

“…The shear rate depends on the liquid type; for example, the shear rates for n-tetradecane and n-hexadecane (alkanes) are ~ 2.0 × 10 6 s − 1 , while that of 1-dodecanol (an alcohol) is ~ 4.0 × 10 5 s − 1 . Reddyhoff et al [30] focused on the transition of the traction behaviour of 1-dodecanol with pressure and ambient temperature.…”