Liquid superlubricity under boundary lubrication (BL) can be defined by the occurrence of extremely low friction coefficients (below 0.01) under contact pressures above 100 MPa in a wide range of temperature and it is associated with lambda values well below unity. Wear is also very low so that boundary lubrication regime is kept till the end of the friction test. Importantly, at the opposite of structural superlubricity, frictionless sliding is not expected under liquid superlubricity.

In these conditions, the number of lubricant molecules entering the contact zone is very limited (theoretical film thickness calculated of the order of molecular size) so that fluid flow and viscosity effects are certainly negligible. More importantly, complex tribochemical reactions take place under the combined effects of pressure and shear creating new interface materials (tribofilms) and new surface terminations both leading to superlubricity.

Liquid superlubricity using the steel/steel combination is very difficult to reach although not impossible (in this case generally not lasting a long time). The situation is much better when using hydrogen-free DLC coatings and Si-based ceramic materials (SiC and Si3N4 for example) associated with green lubricants like vegetable oils (esters of polyols), unsaturated fatty acids and water-based systems. Symmetric pairs can be used but steel/DLC or steel/ceramic asymmetric pairs can also work very well.

The identification of triboreaction products created in the interface (and responsible of superlow friction) needs the use of specific techniques because the tribofilm thickness is typically below 20 nm. Surface analyses (XPS/AES/SIMS) and analytical FIB-TEM are powerful methods but computer simulation is perfectly well adapted to the situation when associated with such surface analysis techniques.

Three mechanisms have been identified which are not exclusive: (i) in situ synthesis of lamellar compounds (graphene, carbon nitride, hydroxides, etc.) [1], (ii) hydrogen-bond network interface nanofilm coupled with H/OH terminations [2] and (iii) oligomerization of carbon surfaces [3].

[1] Y. Long et al, ACS Applied Nano Materials, 4 (3) 2721 (2021).

[2] Y. Long et al, Scientific Report, 9 (1) 1-13 (2019).

[3] T. Kuwahara et al, Coatings, 11 (9) 1069 (2021).