The present study aims to investigate the orientation-dependent mechanical behaviors of ZnO single crystals under nanoindentation by molecular dynamics simulation. The load–indentation depth curves, atomic displacement, shear strain and dislocations for the c-plane, m-plane and a-plane ZnO single crystals were analyzed in detail. The simulation results showed that the elastic deformation stage of the loading curves for the three oriented ZnO single crystals can be described well by the Herz elastic contact model. The Young modulus values for the c-plane, m-plane and a-plane ZnO were calculated to be 122.5 GPa, 158.3 GPa and 170.5 GPa, respectively. The onset of plastic deformation occurred first in a-plane ZnO, then in m-plane ZnO, and lastly in c-planeZnO. The atomic displacement vectors in the three oriented ZnO single crystals were in good agreement with the primary activated slip systems predicted by the maximum Schmid factor. For the c-plane ZnO, the activated pyramidal {$11\overline{2}2$}<$11\overline{2}3$> slip system led to a complex dislocation pattern surrounding the indenter. A U-shaped prismatic half-loop was formed in the [$2\overline{11}0$] direction, confirming the activation of the prismatic {$10\overline{1}0$}<$11\overline{2}0$> slip system. For the m-plane ZnO, the activated prismatic {$10\overline{1}0$}<$11\overline{2}0$> slip system led to the preferential nucleation of dislocations along the $\left[\overline{11}20\right]$ and [$\overline{2}110$] directions. A prismatic loop was formed and emitted along the [$\overline{2}110$] direction, governed by a confined glide on {$10\overline{1}0$} planes. For the a-plane ZnO, the activated prismatic {$10\overline{1}0$}<$11\overline{2}0$> slip system led to dislocations concentrated in the [$\overline{1}\overline{1}20$] direction beneath the indentation pit, emitting a prismatic loop along this direction. Perfect dislocation (with a Burgers vector of 1/3 <$1\overline{2}10$>) is the dominant dislocation in the three oriented ZnO single crystals. The findings are expected to deepen insights into the anisotropic mechanical properties of ZnO single crystals, offering guidance for the development and applications of ZnO-based devices.

SiO₂ thin films are widely used in micro-electro-mechanical systems, integrated circuits and optical thin film devices. Tremendous efforts have been devoted to studying the preparation technology and optical properties of SiO₂ thin films, but little attention has been paid to their mechanical properties. Herein, the surface morphology of the 500-nm-thick, 1000-nm-thick and 2000-nm-thick SiO₂ thin films on the Si substrates was observed by atomic force microscopy. The hardnesses of the three SiO₂ thin films with different thicknesses were investigated by nanoindentation technique, and the dependence of the hardness of the SiO₂ thin film with its thickness was analyzed. The results showed that the average grain size of SiO₂ thin film increased with increasing film thickness. For the three SiO₂ thin films with different thicknesses, the same relative penetration depth range of ~0.4–0.5 existed, above which the intrinsic hardness without substrate influence can be determined. The average intrinsic hardness of the SiO₂ thin film decreased with the increasing film thickness and average grain size, which showed the similar trend with the Hall-Petch type relationship.

The present study aimed to experimentally evaluate the mechanical properties of Pb(In_1/2Nb_1/2)O₃–Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ (PIN–PMN–PT) bulk single crystals with different crystallographic directions using the nanoindentation technique. The load–indentation depth curves, elastic and plastic deformations, hardnesses, and Young’s moduli of [100]- and [110]-oriented 0.28PIN–0.43PMN–0.29PT bulk single crystals were investigated. Our results show that with an increase in the maximum indentation depth h_max, the plastic residual percentage increased for both the [100]- and the [110]-oriented single crystals. At each h_max, the plastic residual percentage of the [100]-oriented PIN–PMN–PT single crystals was less than that of the [110]-oriented PIN–PMN–PT single crystals. At h_max from 500 nm to 2000 nm, the plastic deformation was larger than the elastic deformation, and the plastic residual percentage was larger than 50% for both the [100]- and the [110]-oriented single crystals. This means that the plastic deformation dominated in the indentation process of PIN–PMN–PT single crystals. The indentation size effect on the hardness of the PIN–PMN–PT single crystals was apparent in the nanoindentation process. Both the hardness and the Young’s modulus of the [100]-PIN–PMN–PT single crystals were greater than those of the [110]-PIN–PMN–PT single crystals, which indicates that the PIN–PMN–PT single crystals had anisotropic mechanical characteristics.