Semi-crystalline polymers are commonly used in industrial sectors where surfaces undergo many damages like scratches. In order to avoid as much as possible these surface damages, it is necessary to develop mechanical models able to predict such contact mechanical responses. The aim of this work is to study the effect of orientation, through stretching in the solid state, on the contact mechanics for semi-crystalline polymer surfaces.

To that purpose, bulk polymers are uniaxially oriented at various stretching ratio, using hot two-mill rolling process, and are characterized by X-ray analysis. Contact creep and recovery tests were performed on these samples thanks to a home-made experimental device [1]. This apparatus allows an in-situ observation of the contact area during creep step on non-transparent materials. Then, the recovery of the residual imprint is quickly (few seconds after the unloading of the indenter) recorded by non-contact methods. This experiment gives access to the true contact geometry and provides valuable information about the early stage of viscoelastic recovery.

Hence, the influence of surface orientation on the contact response was investigated on a model semi-crystalline polymer: HDPE to identify structural parameters that govern viscoelastic/viscoplastic behavior of the surface. The effects of strain levels and creep duration on viscoelastic behavior were also studied.

As regards the latter parameter, it was shown that creep duration has no major effect on creep step. Nonetheless, for a same imposed strain, residual depths increase with longer creep duration, inducing, in some case, a permanent deformation of the surface. Regarding the effect of orientation, if the contact creep of the non-oriented surface with a spherical indentor displays a circular contact area, the same experiment performed with oriented semi-crystalline polymers shows an elliptical contact area. Two assumptions can be made to explain this contact shape: (a) the polymeric surface displays anisotropic mechanical properties or (b) the sample is isotropic but the contact takes place on a curved surface due to the rolling stage. In order to better understand the in-situ observations of the contact shape during the contact creep and recovery of the residual imprint, numerical modeling of the surface response was performed using MSC MARC® software. The aim is to reproduce, as closely as possible, the contact creep and recovery experiment. To that purpose an axisymmetric numerical model was created, considering the indentation sphere to be infinitely rigid. This model gives access to the true contact radius during the creep phase. First results seem to indicate that the elliptical shape of the contact area is rather govern by the anisotropic mechanical properties of the semi-crystalline polymer surfaces.

Acknowledgement: This research forms part of the research program of the Dutch Polymer Institute (DPI) project #783. The authors would like to acknowledge the funding support "MARMA" from Carnot MICA and HOLO3 (Alsace region) company for developing the instrument.

Reference: [1] : T. Chatel, C. Gauthier, H. Pelletier, V. Le Houérou, D. Favier and R. Schirrer. Journal of Physics D: Applied Physics, 2011, 44, 375-403