Centuries-old Amontons’ laws of friction challenged
30 Nov 2013
The frictional characteristics of nano-textured surfaces cannot be fully described by the framework of Amontons' laws of friction, according to new research from the University of Bristol, published in ACS Nano.
Nano-structured surfaces are increasingly used in modern miniaturised devices, where nano-sized 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 nano-structured surfaces.
In order to assess friction data obtained on nano-structured 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 nano-domed surface, the Bristol researchers revealed sustained stick-slip frictional instabilities under all the velocity and load regimes studied.
A linear dependence between the amplitude sfof 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 nano-textured 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 nano-devices.
The research, partly supported by a University of Bristol Building Global Engagements in Research (BGER) grant, was carried out by PhD students Benoit Quignon and Georgia Pilkington in Dr Wuge Briscoe's group, in collaboration with Professor Mike Ashfold and Dr Sean Davis in Bristol's School of Chemistry and colleagues at the University of Bath and the Royal Institute of Technology (KTH) in Stockholm.
