Mechanical Properties of Li-Nb-O System Films
Abstract
Objective: To quantitatively assess the hardness, elasticity, and plasticity, and to determine the influence of structure and substructure on these parameters in Li-Nb-O system films.
Experimental: Li-Nb-O system films with a thickness of ~0.8 μm were grown on non-heated substrates (oxidized single-crystal silicon wafers (SiO₂ layer ~0.4 μm), single-crystal lithium niobate with (0001) orientation) by ion beam sputtering of a lithium niobate target. Thermal annealing of Li-Nb-O films on substrates was performed in air for 10 min (until complete crystallization) at temperatures of 550, 650, 700, 750, 800, and 850 °C. The heterostructures (film/substrate) were cooled with the furnace. The phase composition of the films was investigated by X-ray diffraction (XRD) and selected area electron diffraction (SAED). The substructure was studied by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM, Tecnai G2 30ST) of cross-sectional specimens prepared by ion milling using a Quanta 3D setup. The surface morphology was investigated by scanning electron microscopy (SEM, Teskan Mira) in the topological
contrast mode and atomic force microscopy (AFM, Solver47). Mechanical properties – hardness (H) and Young's modulus (E) – were determined from nanoindentation (NI, NanoHardness Tester CSM Instruments) measurements using a Berkovich diamond indenter under the following conditions: maximum load 10 mN, loading rate 10 mN/min, and unloading rate 15 mN/min.
Results: It was found that thermal annealing in an oxygen-containing atmosphere at 750°C induces crystallization of quasi-amorphous Li-Nb-O films and the synthesis of single-phase LN films with lattice parameters closest to those of stoichiometric single-crystal LN. The most probable mechanisms of irreversible deformation in LN films are: brittle fracture, plastic deformation of crystallites, and grain boundary sliding. LN films synthesized at 650-750°C are most susceptible to brittle fracture. Brittle fracture occurs due to the buildup of macrostresses in the films, resulting from different coefficients of thermal expansion (CTE) of the film and the substrate. The fracture toughness of the films increases significantly when using a substrate with a CTE close to that of the film. The hardness of nano- and microcrystalline LN films is always higher than the hardness of quasi-amorphous Li-Nb-O system films. The decrease in the hardness of а films synthesized at high annealing temperatures is due to a decrease in the concentration of point defects and an increase in the size of the crystallites
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