Enhanced mechanical properties of yttrium doped ZnO nanoparticles as determined by instrumented indentation technique

Yttrium doped (1, 3 and 5 wt%) zinc oxide nanoparticles were synthesized via sol-gel process. The phase, structural and mechanical properties were investigated using X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy and micro hardness based on indentation technique. The lattice parameters and grain sizes of the samples were calculated from the XRD data. As the lattice parameters increased, the grain sizes decreased dramatically, resulting in more grain boundaries and strong grain connectivity in the ZnO microstructure. Load-depth curves were obtained by applying indentation loads in the range from 400 to 2000 mN at room temperature. As the Y concentration increased, a significant increase was observed in the hardness values computed from loading-unloading curves using the Oliver and Pharr method. The indentation modulus of the samples reached a saturation value for 3% Y and then decreased as the doping rate increased. Moreover, the crack formation around the indent on the sample surface was examined by electron microscopy and was identified as radial/median type. The fracture toughness of the samples was calculated using the Vickers indentation fracture method. Increased fracture toughness values confirm that ZnO nanoparticles are mechanically strengthened by Y doping.