The viscosity-related properties (Figs. ) were influenced by the filler addition quite moderately. CIT of all systems slightly increased with filler concentration. This means that both nHAP and μHAP decreased the creep resistance of PLA and PCL polymer matrices. The somewhat surprising decrease in the creep resistance after the addition of inorganic fillers has already been observed and attributed to the soft polymer-filler interface . ηIT of all systems decreased after the filler addition, which documented that the fillers made the materials more difficult to deform (higher EIT and HIT), but the deformation was more permanent (lower ηIT, indicating lower elasticity and higher plasticity). Figure summarizes selected relations between micro-and macroscale mechanical properties. In Fig. , the strong correlation between Martens hardness (HM; determined directly from microindentation data) and indentation hardness (HIT; calculated from microindentation data by means of Oliver and Pharr theory ) indicated that the O&P theory was a relevant approximation in our case of elasto-visco-plastic polymer composites. This is quite important, as the O&P theory was used not only for the calculation of HIT, but also for the calculation of EIT, as mentioned in the Experimental section and discussed below. In Fig. , the correlations between EIT and HIT confirmed the general, albeit approximate, proportionality between the elastic modulus and yield stress of polymer systems. The relation was derived and justified theoretically for amorphous polymers by Struik in the form of approximate Eq. 1. Later it was demonstrated experimentally that the relation holds also for semicrystalline polymers , and even for more complex polymer systems, such as blends and composites . The proportionality constant (30) in Eq. 1 is very approximate; it can change from system to system, but the linear correlation between E and Y for a polymer system containing similar components holds quite well . If we combine the Struik relation (Eq. 1) with the above-mentioned Tabor relation (H  3Y), Oliver&Pharr theory (EIT  E) and correlations between various hardness values (H  HM  HIT), we get the justification of the approximate linear correlation between EIT and HIT in Fig. in the form of Eq. 2: