Sliding friction on particle filled epoxy: Developing a quantitative model for complex coatings

Epoxy resins represent an important class of thermosetting polymers that are extensively used in demanding applications like in scratch resistant coatings. Usually fillers, either hard (inorganic) or soft (rubbery), are added. Here we test hard and soft particle-filled epoxy systems in single asperity sliding friction experiments, and analyze the results with the hybrid numerical-experimental approach presented earlier. The focus is on the detailed modeling of the local deformation processes and it is confirmed that a rate-independent friction model proves appropriate to quantitatively model this complex process. The constitutive framework developed for amorphous thermoplastic polymers adequately describes also these thermoset systems. The materials response during scratching is likewise. Hard fillers decrease the penetration of the indenter into the surface, and consequently enhance scratch resistance; they cause the lateral friction force to decrease, since less material flows in front of the indenter tip. Soft fillers increase the penetration into the surface, according to expectations, but surprisingly also decrease the friction force. Simulations do not predict this, and suggest an alternative explanation. Migration of rubber particles during sample preparation to the surface could have occurred. Adding a thin rubbery layer to the surface makes the model quantitative, but SEM and TEM pictures of the cross-sections do not confirm this phase separation and instead show the presence of a large number of very small voids. Including these voids in the modeling allows to predict the penetration depth into the surface and lateral force quantitative for all sliding speeds.