Effects of sliding velocity on the coefficients of friction in a model orthodontic system.

Four arch wire alloy products were evaluated against 400- or 600-grit finished stainless steel contact flats at 34 degrees C under prevailing atmospheric conditions. Six relative velocities (10, 1.0, 0.1, 5 x 10(-2), 5 x 10(-3), and 5 x 10(-4) mm/min) were evaluated as many as three times each in order to simulate a range of sliding motion which approaches the mean rate of tooth motion--that of 2.3 x 10(-5) mm/min. Measurements of the static and kinetic coefficients of friction were rather invariant for the stainless steel and nickel titanium arch wire products. In contrast, a slight increase and a definite decrease of both coefficients occurred for the cobalt-chromium and the beta-titanium arch wire products, respectively. On the presumption that tooth motion routinely occurs over a wide range of sliding rates, the stainless steel couple produced the lowest and the most consistent coefficients of friction, whereas the beta-titanium wire on stainless steel flats produced the highest and the most erratic coefficients of friction. These observations should prevail whenever the film layer of saliva breaks down at, for example, the contact points of arch wire and bracket wings.