Nanoindentation. Surface mechanical properties were studied with a nano-indentation setup (G200, KLA Inc.) equipped with a Continuous Stiffness Measurement (CSM) module. The indentation modulus E was determined from indentation experiments with a three-sided Berkovich diamond tip (Synton-MDP Inc.). The instrument's frame compliance and area function of the indenter tip were calibrated before the first experiment on a fused silica reference glass (Corning Code 7980, Corning Inc.). On each sample up to 200 indentations with a depth limit of 1 µm were performed at a constant strain-rate of 0.05 s -1 and the analysis performed according to the procedure proposed by Oliver and Pharr.