Nanostructured surfaces are increasingly used in modern miniaturised devices, where nanosized surface features with well-defined geometry and dimensions are incorporated for tailored functionality and properties. It is thus crucially important to understand frictional properties of such nanostructured surfaces.
In this work, lateral force microscopy was used to study the frictional properties between an AFM nanotip and surfaces bearing well-defined nanodomes comprising densely packed prolate spheroids, of diameters ranging from tens to hundreds of nanometers. The results show that the average lateral force varied linearly with applied load, as described by Amontons’ first law of friction, although no direct correlation between the sample topographic properties and their measured friction coefficients was identified.
In order to assess friction data obtained on nanostructured surfaces, scientists have hitherto resorted to the laws of friction described by French physicist Guillaume Amontons in 1699 – particularly the concept of friction coefficient (that is, the ratio between friction and applied load) devised for interpreting the phenomenological macroscopic frictional behaviour of rubbing surfaces.
From violin playing to earthquakes, stick-slip frictional behaviours are widespread in macroscopic phenomena. Using a nanosized AFM (atomic force microscope) tip to scan across a nanodomed surface, the Bristol researchers revealed sustained stick-slip frictional instabilities under all the velocity and load regimes studied. A linear dependence between the amplitude sf of these frictional oscillations and the applied load was found, leading to the definition of the slope as the stick-slip amplitude coefficient (SSAC).
The scientists thus propose that the frictional characteristics of nanotextured surfaces cannot be fully described by the framework of Amontons' laws of friction, and that additional parameters (for examples sf and SSAC) are required when their friction, lubrication and wear properties are important considerations in related nanodevices.