Superlubricity is a state of vanishing sliding friction. It has been reported in glycerol/water mixture lubricated contacts, graphene-gold interface, interface in presence of graphene and in other conditions. Increasing number of reports on superlubricity in macroscale is driven by the need of energy costs reduction and to decrease environmental impact.
An international team of researchers from China and France recently explored Nitinol 60-Steel sliding pair under boundary lubrication. As the lubricant, castor oil was used and a new case of superlubricity state was reported. Friction coefficient values even less than 0.001 were recorded. Nitinol-60 is an alloy of Ni and Ti, and is bio compatible, which made it widely spread in medical applications, and particularly in dental industry. Since castor oil is fully bio compatible as well, the described system is very well suited for biotechnology applications.
As it was reported previously, superlubricity can be achieved by various mechanisms. Among those are tuning of atom lattice spacing, use of graphene low shearing properties, generation of hydrogen-bonded layer and use of repulsive van der Waals forces. The combination of two mechanisms was suggested to be responsible for the superlubricity in Nitinol 60-Steel pair with castor oil as a lubricant. It was speculated, that in a sequence of (quite complex) tribochemical reactions, a layered system was formed. First, an -OH terminated surfaces substituted the initial materials ( and ), which in combination with hydrodynamic lubrication made the friction low. As it was discussed in the paper, this was not the only reason for achieving the superlow friction state. It was also found, that the generation of hexanoic acid molecules formed due to oxidation of the castor oil, intercalated within the oxy-hydroxide (of both, Ni and Fe) layers. This was proposed to increase Coulomb repulsion force and decrease the friction to the superlubricity state. The need of two mechanisms simultaneously to achieve the superlubricity was emphasized by the researchers .
The obtained results can be used to promote energy efficiency and environment protection in many industries, including biotechnology.
Further details can be found in the original article: “Green superlubricity of Nitinol 60 alloy against steel in presence of castor oil”, Q. Zeng, G. Dong, J.-M. Martin.