Determination of wear of surfaces by scratch testing

We have formulated a quantitative definition of wear different from the current imprecise definitions. Wear is defined as the unwanted loss of solid material from solid surfaces due to mechanical interaction. The debris method currently used to quantify wear produces results strongly dependent on conditions. We have performed multiple scratch tests for a variety of polymer samples: polypropylene, polytetrafluoroethylene and a polyester. In each of the materials studied, the scratch penetration depths reach a constant value at a given force after 8 scratches or so. Similarly, the scratch recovery (final, healing) depths for a fixed force reach a plateau after a dozen or so scratch tests. Thus, strain hardening by repetitive scratching takes place. A likely explanation is formation of a more ordered phase-as seen before in mechanical tests by Siegmann, Aharoni, Faitelson et al. Given these results we define a measure of wear W(F) for a given indenter geometry and force F as W(F) = lim(n-->infinity) R-h(F) where n is the number of tests performed and Rh is the final (residual, healing) depth after viscoelastic recovery. The present results confirm also our earlier ones that scratch recovery is another useful way to characterize viscoelasticity.