Seeing the start of shear
Drs. Wilfred T. Tysoe & Nicholas D. Spencer | TLT Cutting Edge June 2010
Measuring the evolution of the real area of contact over short timescales provides insights into the onset of sliding.
We previously reported in this column on an elegant method for measuring the real area of a static contact that was developed by professor Jay Fineberg and his group at the Hebrew University of Jerusalem in Israel. The interface between two contacting Plexiglas ® blocks was illuminated with a laser beam. The light was reflected from regions that were not in contact but was transmitted at the asperity-asperity contacts, thereby enabling the real area of contact to be measured from the amount of light transmitted.
The Fineberg group has more recently refined the experiment to address the much more complicated issue of what happens to the interface when a shear force is applied. Now the real area of contact is imaged in real-time using a fast video camera. An additional refinement is that displacements of the block are simultaneously measured by focusing another laser beam onto grids attached to the side of the moving block. Motion is detected by monitoring the diffracted beam using a precise position-sensitive detector.
An experiment is carried out using blocks of poly (methyl methacrylate) (PMMA) with a root-mean-square roughness of about 1 micrometer. A normal load is applied to the blocks, and a lateral force is then slowly applied using a load cell. The contact is imaged and the block displacement simultaneously monitored. Increasing the lateral shear force leads to the appearance of a series of detachment fronts that propagate from the trailing edge of the contact (where the force is applied). Macroscopic slip finally occurs when the detachment front has reached the leading edge of the block.
Insights into the sequence of events that occur during slip are obtained by simultaneously measuring the time dependence of the real area of contact and the displacement. Four distinct regimes are identified. In the first regime, lasting a few microseconds, there is an approximately 20% reduction in the real area of contact, but no net slip takes place. It is suggested that this is when interfacial fracture occurs. The energy, thus dissipated, causes a significant temperature rise that weakens the PMMA-PMMA interface, which has a glass transition temperature of about 110 C.
A second phase is observed, which lasts about 60 microseconds, where the contact area remains constant, but rapid slip takes place at sliding velocities between 5 and 20 cm/s. This is proposed to occur because of a weakening of the strength of the contact due to the fracture-induced temperature rise and the characteristic time for this phase being that required for the interface to cool to ambient temperature.
A third, slower phase is then seen, where the slip velocities are between 0.1 and 2 cm/s, which lasts for a few hundred microseconds. The real area of contact still remains constant, and this region is ascribed to interfacial slip, which is slowed because of the presence of stronger contacts at lower temperatures. Slip ceases completely after this, and the contacts then reform.
While the authors acknowledge that the interpretation of the various regimes found for PMMA sliding against PMMA is strictly only relevant to glassy materials, they conjecture that a similar fracture-induced weakening mechanism might also apply to other systems such as brittle materials like granite where, in this case, rather than thermal softening of PMMA, weakening might occur by the crushing of interlocking asperities.
FOR FURTHER READING:
Den-David, O., Rubinstein, S.M., and Fineberg, J. (2010) “Slip-Stick and the Evolution of Friction Strength,” Nature, 463 (7277), pp. 76-79.
Tysoe, W., and Spencer, N. (2007) “The Contact Conundrum Cracked,” TLT, 63 (2), p. 64.
Eddy Tysoe is a Distinguished Professor of Physical Chemistry at the University of Wisconsin-Milwaukee. You can reach him at wtt@uwm.edu.
Nic Spencer is professor of surface science and technology at the ETH Zurich, Switzerland. Both serve as editors-in-chief of STLE-affiliated Tribology Letters journal. You can reach him at nspencer@ethz.ch.