Influence of surface roughness and contact load on friction coefficient and scratch behavior of thermoplastic olefins

To study the effects of surface roughness and contact load on the friction behavior and scratch resistance of polymers, a set of model thermoplastic olefins (TPO) systems with various surface roughness (R(a)) levels were prepared and evaluated. It is found that a higher R(a) corresponds to a lower surface friction coefficient (mu(s)). At each level of R(a), mu(s) gets larger as contact load increases, with a greater increase in mu(s) as R(a) level increases. It is also observed that with increasing contact load and increasing R(a), the mu(s) tend to level off. In evaluating TPO scratch resistance, a lower mu(s) would delay the onset of ductile drawing-induced fish-scale surface deformation feature, thereby raising the load required to cause scratch visibility. However, as the contact load is further increased, the mu(s) evolves to become scratch coefficient of friction (SCOF) as significant sub-surface deformation and tip penetration occur and material displacement begins, i.e., ploughing. No dependence of R(a) and mu(s) on the critical load for the onset of ploughing is observed. In this work, the distinction between mu(s) and SCOF will be illustrated. Approaches for improving scratch resistance of polymers via control of R(a) are also discussed. (c) 2008 Elsevier B.V. All rights reserved.