Ultra-microindentation at the surface of silk membranes

Ultra-microindentation was used to measure the microhardness and modulus of silk (Bombyx mori) membranes, cast from 20 to 80 °C. The microhardness and modulus were determined from the loading/unloading curves. The membranes exhibit microhardness of about 400 MPa which is larger than the values for most common synthetic polymers (50–300 MPa) implying a greater scratch resistance. The moduli are of the order of those measured by the other means for B. mori silk membranes (5 GPa), and fibers (7–11 GPa). There is some correlation between microhardness and the dimensions of the grains/nanofibrils, but none with surface roughness. The results extend the range of an empirical correlation between microhardness and modulus. The present data together with previous data from other polymers fit the equation, H=0.55 E0.74, with a correlation coefficient of 0.94. Finally, it is shown that elastic recovery of the silk membranes is an increasing function of the maximum load applied.