WEAR AND FRICTION IN GLASSY-POLYMERS - MICROSCRATCH ON BLENDS OF POLYSTYRENE AND POLY(2,6-DIMETHYL-1,4-PHENYLENE OXIDE)

The microscopic process of abrasive wear and friction in glassy polyme rs was studied by using a special microscratch technique. A miscible b lend of polystyrene (PS) and poly(phenylene oxide) (PPO) was used. It was found that as the composition varies there seems to exist two wear regimes in the blends controlled by different breakdown mechanisms co rresponding to the brittle-ductile transition. Detailed study of the c ontact loads and SEM micrographs indicate that abrasive wear in the gl assy polymers is controlled by microcracking under the asperity contac ts. The critical load tau(c) for initiating microscopic cracks can be linked to the macroscopic wear via a statistical Weibull model where t au(c) is taken to be the mean of a strength distribution function. On the other hand, the friction coefficient was found to be independent o f the composition but to vary strongly with the contact load. It appro aches zero at the extrapolated zero load, but increases rapidly and ev entually levels off with contact load. This behavior can be understood by a simple frictional adhesion model in which the polymer deformatio n during a frictional contact is analyzed by considering the compressi ve plastic ploughing and shearing yielding around the asperity contact . The shear strength S-0 of the polymer/asperity contacts was found to vary with the normal load. The vertical scratch hardness H-upsilon, w hich characterizes the spontaneous indentation yielding on the polymer surface, was found to be independent of scratch length and depth, and indeed can be regarded as a material constant. Although both S-0 and H-upsilon can accurately describe the frictional behavior of the glass y polymers, they bear no correlation to abrasive wear in the same mate rials. (C) 1997 John Wiley & Sons, Inc.