Frictional oscillations in elastomeric sliding

The impetus for this study was the noise generated by an automotive stabilizer bar, rotating in an elastomeric bushing. The focus of the research is to determine the conditions under which noise or stick-slip occurs. Elastomers tested include natural rubber with 0,5,10, 20, and 50 phr (parts per hundred rubber) carbon black, butyl, polydimethyl siloxane, and fluorocarbon. The hard counterfaces include chromium oxide, aluminum, epoxy paint, and glass. Various combinations of these materials were tested in an elastomeric hemisphere-on-flat configuration.

Three regimes of sliding have thus far been identified: 1. Steady state sliding where the sliding friction is constant; 2. A high frequency self-excited oscillation of the test structure superimposed on the mean value of friction; 3. Stick-slip, where the elastomeric specimen goes through alternating periods of no relative and relative motion to the counterface. Friction maps which showed the regimes of stick-slip and steady state sliding as a function of load and velocity were experimentally determined. Stick-slip, the dominant mode of unsteady sliding did not occur below a critical velocity. The effects of surface energy, surface roughness and temperature on the critical velocity for stick-slip were also investigated.

Stick-slip motion of materials has been attributed to a difference in static and kinetic coefficients of friction (typically for metallic systems) or a negative slope of the friction velocity curve. It has also been related to a maximum of tan delta or the loss modulus. Test results show that it is possible to have stick-slip occur in regions of positive or zero slope of the friction velocity curve. While the mechanism of stick-slip is not known for all of the elastomeric specimens, stick-slip of the butyl specimens appear to be related to Schallamach waves traversing the interface. For the natural rubber elastomers the critical velocity for stick-slip is dependent on the amount of carbon black. Decreasing amounts of carbon black decreases the critical velocity. The carbon black greatly increases the stiffness of the specimen while not appreciably shifting the frequency of the viscoelastic maxima. In addition, it was shown that a reduction in stiffness of the elastomeric specimen in the direction of sliding, by hollowing it out caused the critical velocity to decrease. Thus the critical velocity for stick-slip could be changed without changing the viscoelastic properties.