Project

Discipline

Nanotribology; Phase transitions; Friction force microscopy; Density functional theory

Lead
Lay summary
The ability to control and manipulate frictional forces at the nanoscale is extremely important for technology, which is closely tied to progress in transportation, manufacturing, energy conversion, and lubricant consumption, impacting on innumerable aspects of our health and environment. In recent years a lot of effort has been spent to gain control of friction at both the macroscopic and microscopic scale. However most of the employed techniques cannot be straightforwardly extended to the nanoscale. A flexible and almost cost-free way to dynamically tune friction force at the nanoscale is still lacking.  The flexibility of some physical property of the sliding bodies, necessary to actuate a dynamical control of friction, might be provided by the occurrence of a phase transition. Recently the possibility to control nanofriction by switching the order parameter of a structural phase transition has been theoretically demonstrated. Friction Force Microscopy (FFM) experiments on a model ferro-distortive substrate have been simulated, showing a non monotonic behavior of friction as a function of the substrate temperature, broadly peaking at the critical temperature. Besides this unusual feature (stick-slip friction of a single contact on ordinary substrates is known to decrease monotonically unless multiple slips occur), below the critical temperature, the frictional response is found to depend strongly on the substrate distortive order parameter: different values of the substrate distortion can give rise to a very different friction force. Acting now with an external stress field the distortive order parameter of the substrate can be changed reversibly and dynamically, increasing or decreasing the frictional properties of the substrate. Contrary to the friction reduction through mechanical vibrations, here the external field needs to be switched on only for a very short time to induce a change in the distortion of the substrate and, upon switching it off, the material will keep the newly induced distortion.   Experimental evidences of this kind of friction control technique to be practically actuated are already present. We propose a joint experimental, theoretical and computational project aimed at designing methods and algorithms to control friction by dynamically driving a phase transition occurring in the substrate underneath the sliding surface. Extending the example outlined above, we  intend  to  address, through experiment and theory,  the widest variety of substrate phase transitions, from a) structural, including  distortive and plastic,  to b) electronic, including superconducting and metal insulator,  to c) magnetic. Building in each case upon long established understanding of the phase transitions, and from exploiting the modest available topographic evidence of domains -- in some case of dissipation too --obtained by tip based tools, we plan to attack these three areas using a well assorted arsenal of tools. Experimentally, force friction microscopy (FFM), contact and noncontact  atomic force microscopy (AFM), and the most recent ultra-sensitive pendulum-type AFM, among others, will be exploited. Conversely, theoretical formulations, classical and quantum modeling, classical Molecular Dynamics (MD)  dynamical simulations of friction and dissipation, and first-principles density functional theory (DFT)  will be used to address these systems.

Direct link to Lay Summary

Last update: 21.02.2013

Self-organised

Communication	Title	Media	Place	Year

Number	Title	Start	Funding scheme

The ability to control and manipulate frictional forces at the nanoscale is extremely important for technology, which is closely tied to progress in transportation, manufacturing, energy conversion, and lubricant consumption, impacting on innumerable aspects of our health and environment. As detailed in section 1.2, in recent years a lot of effort has been spent to gain control of friction at both the macroscopic and microscopic scale. However most of the employed techniques cannot be straightforwardly extended to the nanoscale. A flexible and almost cost-free way to dynamically tune friction force at the nanoscale is still lacking. The flexibility of some physical property of the sliding bodies, necessary to actuate a dynamical control of friction, might be provided by the occurrence of a phase transition. Recently the possibility to control nanofriction by switching the order parameter of a structural phase transition has been theoretically demonstrated (1). Friction Force Microscopy (FFM) experiments on a model ferro-distortive substrate have been simulated, showing a non monotonic behavior of friction as a function of the substrate temperature, broadly peaking at the critical temperature. Besides this unusual feature (stick-slip friction of a single contact on ordinary substrates is known to decrease monotonically unless multiple slips occur), below the critical temperature, the frictional response is found to depend strongly on the substrate distortive order parameter: different values of the substrate distortion can give rise to a very different friction force. Acting now with an external stress field the distortive order parameter of the substrate can be changed reversibly and dynamically, increasing or decreasing the frictional properties of the substrate. Contrary to the friction reduction through mechanical vibrations, here the external field needs to be switched on only for a very short time to induce a change in the distortion of the substrate and, upon switching it off, the material will keep the newly induced distortion.Experimental evidences of this kind of friction control technique to be practically actuated are already present and will be reviewed in section 1.2. We propose a joint experimental, theoretical and computa-tional project aimed at designing methods and algorithms to control friction by dynamically driving a phase transition occurring in the substrate underneath the sliding surface. Extending the example out-lined above, we intend to address, through experiment and theory, the widest variety of substrate phase transitions, from a) structural, including distortive and plastic, to b) electronic, including superconducting and metal insulator, to c) magnetic. Building in each case upon long established understanding of the phase transitions, and from exploiting the modest available topographic evidence of domains -- in some case of dissipation too --obtained by tip based tools, we plan to attack these three areas using a well as-sorted arsenal of tools. Experimentally, FFM, contact and noncontact atomic force microscopy (AFM, ncAFM), and the most recent ultra-sensitive pendulum-type AFM, among others, will be exploited. Conversely, theoretical formulations, classical and quantum modeling, classical Molecular Dynamics (MD) dynamical simulations of friction and dissipation, and first-principles density functional theory (DFT) will be used to address these systems.