Prior to nanoindentation, a portion of each sample (area of ca. 25 mm 2 ) was embedded in a cold curing acrylic mounting resin, and polished down to a surface roughness of 50 nm using a sequence of SiC papers and diamond suspensions (for 15 -0.25 µm from ATM GMBH, Germany, and for 0.05 µm from Buehler, Germany). In order to ensure that no influence of the embedding medium and substrate could affect the nanoindentation measurements, the thickness of the tested samples was at least 500 times larger than the indentation depth. The mechanical tests were performed in an Agilent Nano Indenter G200 (Agilent, USA) system using the continuous stiffness measurement method, with a constant strain target of 0.05 s -1 , a harmonic displacement target of 2 nm and a harmonic frequency of 45 Hz.