Scratch velocity and wear resistance

Using a micro-scratch tester we have determined the influence of scratch velocity on the sliding wear resistance of several thermoplastics: polystyrene (PS), styrene-acrylonitrile, polyamide 6, polyethersulfone and polysulfone. Variable velocities (from 1 to 15 mm/min) were applied under two different load configurations: progressively increasing loads from 0.03 to 30 N and multiple scratching under constant loads from 2.5 to 10 N. Penetration and residual depth were measured along a 5 mm length. In the case of PS the scanning electron microscopy images of the progressive load scratch reveal a transition from shear yielding to crazing at low velocity. In multiple scratching tests the residual depths obtained at 1 mm/min are substantially higher than those at 15 mm/min for all thermoplastics tested. This result is explained by contact heating at higher speeds resulting in increased chain relaxation capability and thus more viscoelastic recovery. Scanning probe microscopy was used in order to observe surface damages. Except for PS, all other materials at all velocities exhibit asymptotic behaviour of the residual depth as a function of the number of tests.