Determination of the Elastic-plastic Transition of Human Enamel by Nanoindentation

Objectives/Methods: From a materials scientist's perspective, dental materials used for tooth repair should exhibit compatible mechanical properties. Fulfillment of this criterion is complicated by the fact that teeth have a hierarchical structure with changing mechanical behavior at different length scales. In this study, nanoindentation with an 8 μm spherical indenter was used to determine the elastic/plastic transition under contact loading for enamel. Results: The indentation elastic/plastic transition of enamel at the length scale of several hundreds of hydroxyapatite crystallites, which are within one enamel rod, is revealed for the first time. The corresponding penetration depth at the determined indentation yield point of 1.6 GPa and 0.6 % strain is only 7 nm. As a consequence of the small depth it is decisive for the experiment to calibrate the indenter tip radius in this loading regime. The elastic modulus of 123 GPa was evaluated directly by the Hertzian penetration and not by the unloading part of the indentation curve. Significance: We believe these data are also a valuable contribution to understand the mechanical behavior of enamel and to develop nanoscale biomimetic materials

Dielectric breakdown of alumina single crystals

The bulk breakdown behaviour of alumina single crystals with two different crystal orientations, {11-20}-plane (single crystal A) and {0001}-plane (single crystal C), have been studied. Therefor plan-parallel single crystal samples were electrically loaded until dielectric breakdown was achieved. For each crystal orientation, a characteristic breakdown channel direction through the sample could be defined. In C-oriented crystals the breakdown channel originated parallel to the c-axis. For A-oriented crystals however, the breakdown channel crossed the sample in an oblique direction; the angle between crystal surface and breakdown channel was 60°. Here, the breakdown channel crossed the sample along an A-plane. Although the breakdown channel paths of A and C crystals are different, the observed breakdown strength are identical within the scatter range

Investigation of phase boundaries in the system (KxNa1-x)1-yLiy(Nb1-zTaz)O3 using High-Throughput Experimentation (HTE)

A High-Throughput Experimental (HTE) approach starting from dry, fine-grained powders was used to synthesize bulk samples in the system (KxNa1-x)1-yLiy(Nb1-zTaz)O3, a doped variant of the piezoelectric (K0.5Na0.5)NbO3 (KNN). Starting from the system (K0.5Na0.5)1-yLiy(Nb1-zTaz)O3 known from the works of Saito et al. an effort was made toestablish a higher-order phase diagram. Special emphasis was put on expanding the known morphotropic phase boundary that constitutes a region of special interest for electroceramic materials as it features maximum piezoelectric properties. Analyses were performed using a HTE-compatible technique, namely automated powder X-ray diffraction (XRD)

Size-dependent elastic-inelastic behavior of enamel over millimeter and nanometer length scales

The microstructure of enamel like most biological tissues has a hierarchical structure which determines their mechanical behavior. However, current studies of the mechanical behavior of enamel lack a systematic investigation of these hierarchical length scales. In this study, we performed macroscopic uni-axial compression tests and the spherical indentation with different indenter radii to probe enamel’s elastic/inelastic transition over four hierarchical length scales, namely: ‘bulk enamel’ (mm), ‘multiple-rod’ (10’s µm), intra-rod’ (100’s nm with multiple crystallites) and finally ‘single-crystallite’ (10’s nm with an area of approximately one hydroxyapatite crystallite). The enamel’s elastic/inelastic transitions were observed at 0.4-17GPa depending on the length scale and were compared with the values of synthetic hydroxyapatite crystallites. The elastic limit of a material is important as it provides insights into the deformability of the material before fracture. At the smallest investigated length scale (contact radius ~20nm), elastic limit is followed by plastic deformation. At the largest investigated length scale (contact size ~1mm), only elastic then micro-crack induced response was observed. A map of elastic/inelastic regions of enamel from millimeter to nanometer length scale is presented. Possible underlying mechanisms are also discussed