Creep of the contact with a spherical tip and recovery of the imprint on amorphous polymer surfaces

This paper is devoted to an analysis of the creep and recovery occurring on the surface of an amorphous polymer during indentation, a characteristic of the self-healing performance of the material. Creep and recovery were studied using a home made experimental device which allows one to record in situ the evolution of the imprint created by a probe. The influence of the temperature, the initial imposed strain and the creep duration were analysed. The evolution of the true contact area with time was followed during micro-indentation. We demonstrate the non-linear behaviour of the amorphous polymer studied during the creep and recovery phases. The results obtained allow calculation of the activation energy and activation volume of the polymer. These results are also compared with the predictions of the viscoelastic model of Lee and Radok and viscoplasticity is demonstrated for a reasonable initial mean contact strain as defined by Tabor. Nevertheless, it is still difficult to quantify the recovery and to define the conditions for the appearance of plasticity.