Nanoindentation of submicron polymeric coating systems

Characterization of the mechanical properties of layered structures is of importance for the applications of nanoscale multilayer materials. Nanoindentation experiments of bilayer structures have been performed with polymeric coatings of 47, 125, 220 and 3000 nm on acrylic substrates of 3 mm. The coatings have higher elastic modulus and flow resistance than the substrates. The reduced relaxed contact modulus decreases with the increase in the indentation load for all the coating systems. A semi-empirical relation is developed to correlate the ratio of the coating modulus to the substrate modulus with the depth of elastic recovery. Using this relationship, the elastic modulus of the polymeric coatings is found to be 4.5 GPa. The flow behavior of the coating systems subjected to a constant indentation load is shear thinning and can be described by a linear relation between the creep depth and the square root of the indentation load with a stress exponent of 1/2 at small indentation loads for the 3000 nm coating system and at large indentation loads for the 47, 125 and 220 nm coating systems.