Indentation variability of natural nanocomposite materials

Small-scale depth-sensing indentation (nanoindentation) is a popular technique for measuring the mechanical properties of a wide range of materials. Contact mechanics solutions used in data analysis are based on the indentation of a homogeneous half-space, but the experiments are frequently conducted on mineralized biological tissues—biocomposite materials with nanometer-scale features—such as bone and dentin. The current study examines the experimental indentation response of bone across orders of magnitude in contact dimension length-scale, from nanometers to micrometers. Scaling arguments are used to establish the need for nanoscale simulations of mineralized tissue indentation. A finite element model of an inhomogeneous contact problem is developed and used to interpret experimental indentation data on bone and dentin. Both experimental data and modeling results demonstrate a convergence in apparent elastic modulus at increasing contact length-scales. Models results are used to estimate a feature size associated with inhomogeneity of the indentation response; for experiments conducted here the characteristic feature size is found to be substantially larger for bone than for dentin, and in both cases larger than for individual nanometer-scale mineral platelets.