Mechano-chemical decomposition of organic friction modifiers with multiple reactive centres induces superlubricity of ta-C

Abstract

Superlubricity of tetrahedral amorphous carbon (ta-C) coatings under boundary lubrication with organic friction modifiers is important for industrial applications, but the underlying mechanisms remain elusive. Here, combined experiments and simulations unveil a universal tribochemical mechanism leading to superlubricity of ta-C/ta-C tribopairs. Pin-on-disc sliding experiments show that ultra- and superlow friction with negligible wear can be achieved by lubrication with unsaturated fatty acids or glycerol, but not with saturated fatty acids and hydrocarbons. Atomistic simulations reveal that, due to the simultaneous presence of two reactive centers (carboxylic group and C=C double bond), unsaturated fatty acids can concurrently chemisorb on both ta-C surfaces and bridge the tribogap. Sliding-induced mechanical strain triggers a cascade of molecular fragmentation reactions releasing passivating hydroxyl, keto, epoxy, hydrogen and olefinic groups. Similarly, glycerol’s three hydroxyl groups react simultaneously with both ta-C surfaces, causing the molecule’s complete mechano-chemical fragmentation and formation of aromatic passivation layers with superlow friction.

A review of recent advances in tribology

“…The DLC surface underwent permanent deformation at the nanoscale when subjected to forces as small as tens of nano-Newtons. Recently, Kuwahara et al [22] using sliding experiments of ta-C/ta-C tribopairs showed that superlubricity with negligible wear can be achieved by lubrication with unsaturated fatty acids or glycerol. Zhang et al [23] using high-intensity pulsed ion beam irradiated WC-Ni surface against graphite under water lubrication also got significant friction reduction.…”

Structural lubricity in soft and hard matter systems

“…Notably, ultralow friction can be achieved also in the absence of such lubricants. Thus, amorphous carbon coatings can provide ultralow friction coefficients [10][11][12][13] , and they are already used in industrial applications, including razor blades, magnetic hard drives, microelectromechanical systems and more.…”

Atomistic Insights Into Lubricated Tungsten/Diamond Sliding Contacts

“…This is a good indication that C 4 H 10 and C 16 H 34 will undergo a very similar radicalization process when sheared under high pressure between diamond/W/WO 3 sliding interfaces. Additionally, in a different study (Kuwahara et al, 2019), short C 7 H 12 O 2 and long C 18 H 34 O 2 acid molecules, which have different molecular lengths but the same reactive centers, were investigated using tight binding, which is computationally less expensive than DFT. In that study, it was shown that short cis-heptanoic acid (C 7 H 12 O 2 ) molecules undergo very similar tribochemical reactions as the longer oleic acid (C 18 H 34 O 2 ) molecules during shearing under high pressure between sliding ta-C counter surfaces.…”