Experimental measurement and numerical simulation of the plastic strain during indentation and scratch tests on polymeri
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For elastic–plastic contacts, we propose a complete description of the plastic strain field beneath the indenter during indentation and scratch with a spherical indenter (with R, the tip radius), as a function of the testing conditions, defined by the geometrical strain, noted a/R (with a, the contact radius), and the local friction coefficient mloc. The main parameter of the description is the level of the plastic deformation imposed during test into amorphous polymeric surfaces, related in first approximation to the ratio a/R. An equivalent average plastic strain, noted (ep)av, is calculated over a representative plastically deformed volume, both for indentation and scratch tests. The equivalent average plastic strain (ep)av, is observed to increase with the ratio a/R, as predicted by the empirical Tabor’s rule, but also with the local friction coefficient mloc for a given ratio a/R, especially during scratching. The plastic zone dimensions and the plastic strain gradient developed beneath the moving tip are shown to depend both on the geometrical strain a/R and also on the friction coefficient mloc.
I. INTRODUCTION
Because of their much desired properties, polymer thin films and bulk polymeric materials are increasingly used for many applications in industry, for example, as interior and exterior materials in the automobile industry and as replacement for many metals parts. For automotive applications, it is well known that good appearance is a key attribute. However, good appearance is directly related to the scratch and mar resistance of surfaces, and relationships between appearance attributes and surface deformation are poorly understood. Hence, single-probe testing techniques, including indentation and scratch tests, are widely used now in studying tribological and mechanical properties of polymeric materials and coatings. However, these different mechanical tests on polymeric surfaces have been used with limited success and primarily for qualitative comparisons and quality control. Indeed, mechanical behavior of polymers appears to be more complex than metals or ceramics, due to their high sensitivity to temperature (ranging from 20 to 100 C) and to strain rate. Moreover, scratch and mar behavior is not only related to the surface properties but also to the associated loading conditions, such as the tip geometry, scratch velocity, loading rates during indentation, and scratch phases. However, even if the number of indentation and scratch test variables is important, robust methods to identify the effective and local rheological properties at a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0138
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J. Mater. Res., Vol. 24, No. 3, Mar 2009
the micrometer and submicrometer length scales are needed, for a better understanding of the frictional, abrasive, and scratch resistance of polymeric surfaces. The main problem when using a single-probe indentation or scratch system is the determination of the true contact depth hc and then the true contact a
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