Friction as well as all other contact phenomena strongly depend on the size of the real contact area between the mating materials. Due to surface roughness, the real contact area is usually smaller than the apparent contact area by many orders of magnitude. Because of its importance, a huge amount of investigations can be found in literature that aim to measure, calculate or model the size of the real contact area. Never¬theless, for soft polymers with usage above the glass transition temperature, most of these methods and models are not suitable.

Soft polymers like polyethylene, plasticized polyvinylchloride, polyurethane or silicones are widely spread in our everyday life. They are used for packaging and protection or as colour, lacquer and coating on all kinds of substrates. They are lightweight, insulating, can make things water proof or dirt repellent and give interior products and clothes a good look and feel. In all these applications, there will be contacts to other materials, determining important quantities like static and dynamic friction, wear, frictional instabilities like squeak and rattle, adherence between different components of composites, touch haptics or the conduction of electricity and heat.

Compared to harder materials the penetration depth in contacts with soft polymers can be much higher than assumed in typically models for the real contact area. This is the main reason for the inapplicability of most models of contact mechanics for soft polymers. To analyse the consequence of high penetration, atomistic molecular dynamics (MD) simulations have been used in the work to be presented. These simulations are only based on interactions between atoms. So, compared to often used FEM simulations, no information about material properties is required as input parameter. MD simulations allow the calculation of physical quantities that are not accessible through experiments as well as a direct insight into the contact formation at the nanoscale.

For the simulations, the Hertzian contact between a hard sphere and a deformable half space was adopted with semi crystalline polyethylene as representative for a soft polymer above glass transition temperature. Depending on material properties (degree of crystallization and thus stiffness) and loading conditions (indenter radius, normal force, velocity, temperature, contact time) nanoindentation simulations were conducted. The formation of the contact area was observed over the complete range of indentation depths up to the size of the indenter radius and the role of adhesion was considered. Dependencies were analysed and compared to existing models. For an easier usage within practical applications or as a basis for more complex models, approximate equations were derived.