[SY-O5] On the formation of superlubricious layers during boundary lubrication of diamond and diamond-like carbon
Invited
Ultralubricity and superlubricity has been observed for diamond and tetrahedral amorphous carbon (ta-C) coatings lubricated with water and unsaturated organic friction modifiers, respectively. Although spectroscopic characterization suggests that lubricity is connected to the formation of oxygen-containing surface layers, the underlying tribo-chemical mechanisms and the detailed structures of these layers remain elusive. This talk presents atomistic calculations on various scales that elucidate the mechano-chemical processes for the formation of oxygen functional groups and aromatic protective layers [1,2].
In the first part of this talk, quantum molecular dynamcis calculations are presented that shed light on friction regimes for diamond lubricated by water and the different characteristic layers in the different regimes [1]. We find four universal friction regimes that are active on all low-index diamond surfaces: cold-welding&amorphisation, ether cold-welding without amorphisation, H/OH-termination, H/OH-termination with water layer coverage. In addition four special regimes are found: aromatic Pandey surface reconstruction for (111), ether/keto passivation for (110) and (001) as well as two mild cold-welding regimes for (110).
In the second part, combined experiments and multiscale simulations are presented that unveil the tribochemical mechanism leading to superlubricity of ta-C/ta-C tribopairs lubricated by unsaturated fatty acids [2]. Experiments show that superlow friction can be achieved by lubrication with unsaturated fatty acids. Atomistic simulations reveal that, due to the simultaneous presence of a carboxylic group and a C=C double bond, unsaturated fatty acids can concurrently chemisorb on both ta-C surfaces and bridge the tribogap. The resulting, sliding-induced mechanical strain triggers a cascade of molecular fragmentation reactions that release passivating hydroxyl, keto, hydrogen and olefinic groups to the ta-C surfaces. These findings provide design principles for novel organic friction modifiers that promote the formation of superlubricious amorphous carbon layers.
[1] T.Kuwahara, G.Moras, M.Moseler, Phys. Rev. Lett. 119 096101 (2017)
[2] T.Kuwahara, P.A.Romero, S.Makowski, V.Weihnacht, G.Moras, M.Moseler, under consideration (2018)
In the first part of this talk, quantum molecular dynamcis calculations are presented that shed light on friction regimes for diamond lubricated by water and the different characteristic layers in the different regimes [1]. We find four universal friction regimes that are active on all low-index diamond surfaces: cold-welding&amorphisation, ether cold-welding without amorphisation, H/OH-termination, H/OH-termination with water layer coverage. In addition four special regimes are found: aromatic Pandey surface reconstruction for (111), ether/keto passivation for (110) and (001) as well as two mild cold-welding regimes for (110).
In the second part, combined experiments and multiscale simulations are presented that unveil the tribochemical mechanism leading to superlubricity of ta-C/ta-C tribopairs lubricated by unsaturated fatty acids [2]. Experiments show that superlow friction can be achieved by lubrication with unsaturated fatty acids. Atomistic simulations reveal that, due to the simultaneous presence of a carboxylic group and a C=C double bond, unsaturated fatty acids can concurrently chemisorb on both ta-C surfaces and bridge the tribogap. The resulting, sliding-induced mechanical strain triggers a cascade of molecular fragmentation reactions that release passivating hydroxyl, keto, hydrogen and olefinic groups to the ta-C surfaces. These findings provide design principles for novel organic friction modifiers that promote the formation of superlubricious amorphous carbon layers.
[1] T.Kuwahara, G.Moras, M.Moseler, Phys. Rev. Lett. 119 096101 (2017)
[2] T.Kuwahara, P.A.Romero, S.Makowski, V.Weihnacht, G.Moras, M.Moseler, under consideration (2018)