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.
Highlights
Superlubricity of tetrahedral amorphous carbon coatings under boundary lubrication with organic friction modifiers is important for industrial applications, but the underlying mechanisms remain elusive
Friction and wear experiments combined with extensive quantum and classical molecular dynamics (QMD and CMD) simulations show for the first time that the formation of a superlubricious layer on tetrahedral amorphous carbon (ta-C) coatings under boundary lubrication conditions in the presence of fatty acids and glycerol is caused by a mechanochemical process that relies on the presence of multiple reactive centres in the lubricants
Quantum molecular statics (QMS) simulations of five different C7 molecules pressurized between ta-C surfaces are performed to study differences in the bonding of these lubricants to ta-C. These simulations show that, at pressure values typical of the experimental conditions, only the unsaturated fatty acids can bridge the tribogap by chemisorption of their two reactive centres on the two ta-C surfaces
Summary
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. Accompanying atomistic simulations suggested that H/OH groups originating from glycerol can passivate the ta-C surfaces These observations indicate that a complete decomposition of the lubricant allows passivation layers to form on top of ta-C, which is a necessary precondition for its superlubricity and nearly wearless sliding[6,7]. Despite these important investigations into the structural details of the lubrication mechanisms, the tribochemical processes underlying the lubricant’s fragmentation and the formation of passivating functional groups on ta-C remain elusive. Friction and wear experiments combined with extensive quantum and classical molecular dynamics (QMD and CMD) simulations show for the first time that the formation of a superlubricious layer on ta-C coatings under boundary lubrication conditions in the presence of fatty acids and glycerol is caused by a mechanochemical process that relies on the presence of multiple reactive centres in the lubricants
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